Accepted Manuscript

Brazing techniques for the fabrication of biocompatible carbon-based electronicdevices

Nicholas V Apollo Desmond Lau Arman Ahnood Alastair Stacey KumaraveluGanesan Samantha G Lichter Kate Fox Javad Foroughi Hamish Meffin Gordon GWallace Ray Baughman Steven Prawer David J Garrett

PII S0008-6223(16)30413-4

DOI 101016jcarbon201605047

Reference CARBON 11011

To appear in Carbon

Received Date 12 February 2016

Revised Date 18 May 2016

Accepted Date 20 May 2016

Please cite this article as NV Apollo D Lau A Ahnood A Stacey K Ganesan SG Lichter KFox J Foroughi H Meffin GG Wallace R Baughman S Prawer DJ Garrett Brazing techniquesfor the fabrication of biocompatible carbon-based electronic devices Carbon (2016) doi 101016jcarbon201605047

This is a PDF file of an unedited manuscript that has been accepted for publication As a service toour customers we are providing this early version of the manuscript The manuscript will undergocopyediting typesetting and review of the resulting proof before it is published in its final form Pleasenote that during the production process errors may be discovered which could affect the content and alllegal disclaimers that apply to the journal pertain

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Brazing techniques for the fabrication of biocompatible carbon-based electronic devices Nicholas V Apollo1 Desmond Lau2 Arman Ahnood1 Alastair Stacey1 Kumaravelu Ganesan1 Samantha G Lichter1 Kate Fox3 Javad Foroughi4 Hamish Meffin5 Gordon G Wallace4 Ray Baughman6 Steven Prawer1 David J Garrett1 1 School of Physics The University of Melbourne Victoria 3010 Australia 2 School of Applied Sciences RMIT 3 Centre for Additive Manufacturing School of Engineering RMIT University Melbourne Australia 4 Intelligent Polymer Research Institute ARC Centre of Excellence for Electromaterials Science AIIM Facility Innovation Campus University of Wollongong Australia 5 National Vision Research Institute Australian College of Optometry Victoria 3053 and Department of Optometry and Vision Science University of Melbourne Victoria 3010 Australia 6 Alan G MacDiarmid NanoTech Institute University of Texas at Dallas Richardson TX 75083 USA Corresponding author David Garrett e-mail dgarrettunimelbeduau

Abstract

Prototype electronic devices have been critical to the discovery and demonstration of the

unique properties of new materials including composites based on carbon nanotubes (CNT)

and graphene However these devices are not typically constructed with durability or

biocompatibility in mind relying on conductive polymeric adhesives mechanical clamps or

crimps or solders for electrical connections In this paper two key metallization techniques

are presented that employ commercially-available brazing alloys to fabricate electronic

devices based on diamond and carbonaceous wires Investigation of the carbonmdashalloy

interfacial interactions was utilized to guide device fabrication The interplay of both chemical

(ldquoadhesiverdquo) and mechanical (ldquocohesiverdquo) forces at the interface of different forms of carbon

was exploited to fabricate either freestanding or substrate-fixed carbonaceous electronic

devices Elemental analysis in conjunction with scanning electron microscopy of the carbonmdash

alloy interface revealed the chemical nature of the Ag alloy bond and the mechanical nature of

the Au alloy bond Electrical characterization revealed the non-rectifying nature of the

carbonmdashAu alloy interconnects Finally electronic devices were fabricated including a Au

circuit structure embedded in a polycrystalline diamond substrate

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1 Introduction

Owing to their unique combination of biological mechanical electrochemical electrical

thermal and optical properties carbon-based materials such as graphene carbon nanotubes

(CNT) carbon fiber (CF) and diamond have attracted significant scientific and industrial

interest Exploiting CNT and graphene properties in micro- and macroscale structures is of

significant interest for engineering research and product development [1-3] For example

high strength and high conductivity fabrics based on these materials could enable a variety of

applications from lightweight body armor to space elevators [4 5] Several solutions to the

problem of scale have been demonstrated for both graphene and CNT films Spinning CVD-

grown CNT forests into a continuous yarn has enabled conductive high-strength structures

with micro-scale diameters and macro-scale lengths [3] Control of mechanical strength and

electricalthermal conductivity of CNT yarns is established by adjusting component CNT

lengths as well as the spinning angles of the fibers [6 7] Twisted andor coiled structures of

CNT yarns have been used to fabricate electrochemical as well as electrolyte-free CNT

torsional actuators (artificial muscles) with large-stroke and high work-capacity [8 9] Cell

culture dishes containing CNT yarns have shown that neurons grow and respond to electrical

stimulation delivered by the yarn [10] As bulk carriers of electricity and data CNT yarns

have been evaluated as replacements for standard Cu and Al wires due to their light weight

(alleviating stress on joint connections) ballistic conduction (lack of scattering reduces risk of

Joule heating) and capacity to handle high frequencies [11] Expanding upon the application

of long-distance electricity transfer CNT yarns have been used to build wireless data transfer

networks that were mechanically-resilient and displayed frequency-independent resistive

behavior [12] As a demonstration of their favorable electrochemical characteristics CNT

yarns have been used as an alternative to the standard CF electrochemical sensor for

neurotransmitters due to an intrinsic ability to resist surface fouling [13]

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3

All of the aforementioned research and much that was not mentioned was performed using

prototype devices containing silver epoxy gold paste solder or mechanical clamps to create

electrical connections to carbonaceous wires While these connections served their respective

experimental purposes their use in fabrication does not generally yield scalable or integrated

electronic devices nor are these connection methods resilient or biocompatible enough for

consideration as materials for biomedical devices There have been several methods

developed for bonding a single CNT to metals as well as joining a single CNT to another

CNT end-to-end [14] However making electrical connections to larger (micro rather than

nano) CNT composites and yarns is a different challenge Current approaches include

mechanical clamps Ag- and Au-based epoxy adhesives carbon solder ultrasonic welding

and vacuum brazing [15 16] A recent very exciting approach involves use of transition

metal soldering alloys to join carbon wires using standard solder conditions (eg 350 degC in

air) [17] While it is a technique that will certainly revolutionize the utility of carbonaceous

wires the Cu- and Sn-based alloys are unlikely to display biocompatibility due to the known

cytotoxicity of these metals [18 19] We have recently developed a technique to create a

hermetic diamond capsule using biocompatible gold alloys [20] Additionally we have

demonstrated a method for the construction of hermetic biocompatible feedthroughs for a

retinal prosthesis device based on conductive nitrogen-doped diamond electrodes [21] In the

present paper we extend our previous work and describe the construction of diamond-based

electronic circuit substrates with embedded Ag- or Au-based interconnects and soldering pads

Finally we describe a method to metallize the aforementioned carbonaceous wires (CNT yarn

CF and graphene fiber) with the option to incorporate them into diamond circuit substrates

The methods we describe constitute an important toolkit to allow for CNT-based and other

similar carbonaceous ldquosuper materialsrdquo to be incorporated into electronic devices and

integrated with traditional surface-mount electronics with specific focus on biocompatible

systems for implantation into the body

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4

11 Brazing to carbon

Active brazing a variation of vacuum brazing utilizes an alloy containing an active

component that reacts chemically with a relatively inert surface such as diamond or ceramics

[22 23] Transition metals are of particular interest due to electron vacancies in their d-

orbitals with greater numbers of vacancies leading to greater reactivity (overlap) with

carbonrsquos p-orbitals [24] In the case of carbon substrates the active componentmdashtypically Ti

V or Crmdashforms a carbide interface layer that acts as a surfactant to enable the wetting of

other filler metals and produce non-rectifying electrical contacts [25 26] A Ag-based active

brazing alloy (ABA) has been used previously to make electrical contact to a CNT bundle

and formation of the TiC interface layer was confirmed with X-ray photon scattering (XPS)

though the electrical and mechanical properties were not fully characterized [16] The

formation of interfacial TiC particles has also been shown to significantly increase the tensile

strength of carbon composites brazed with Ti-based alloys containing CNT reinforcements

[27] In the present work commercially-available ABAs were investigated to determine their

suitability to make electronic connections to several carbonaceous materials including

polycrystalline diamond (PCD) CNT yarns graphene oxide (GO) fibers and PAN-based

carbon fiber (CF) to enable various types of electronic devices (Fig 1)

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5

Fig 1 - ABAs were used to fabricate two basic electronic device platforms (a) a circuit board embedded into a polycrystalline diamond (PCD) substrate and (b) a freestanding carbonaceous wire with electrical connections on one or both ends The aforementioned technologies had lead to the fabrication of medical devices containing active carbon electrodes such as (c) conductive diamond electrodes and (d) carbonaceous wire electrodes

In the first technique described here ABAs were used to create embedded circuit boards in

PCD wherein the solidified ABA forms the interconnects and contact pads following

mechanical polishing The second metallization system involves making free-standing Au

contacts to carbonaceous fibers using a graphite ldquolift-offrdquo method which exploits the balance

of cohesive and adhesive forces of the liquid metalgraphite interface (Equation 1)

cos

1131

Where θ is the contact angle between the substrate and liquid metal WA is the work of

adhesion (adhesion force) between the substrate and the liquid metal and σL is the cohesion

force (liquid-vapour surface tension) within the liquid metal droplet [28] Equation 1

describes the balance of forces at the liquid metalsolid substrate interface wherein liquid

metals (intrinsically very high cohesion force materials) tend to create large contact angles on

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6

most substrates unless σL lt WA where contact angles lt90˚ are considered ldquowettingrdquo In one

facet of this work we use active brazing to modify WA and create robust electrical

connections to carbonaceous materials such as PCD and CF In another technique we exploit

the lack of adhesion and wetting between graphitic carbon and Au as well as the cohesion

forces within the liquid metal droplet to create a freestanding carbonaceous wire with

metallic connections Broadly speaking the techniques described here are applicable to the

development of devices in which a strong biocompatible and non-rectifying connection to

carbon-based materials is required

2 Experimental

21 Surface interactions brazing alloys and carbonaceous materials

211 Sample preparation and wettability studies

Thermal grade polycrystalline diamond (PCD) substrates (TM-250 Element Six) with a

thickness of 250 microm were laser-cut into 5 times 5 mm squares with a 532 nm NdYAG laser

(Oxford Lasers) Approximately 65 mg of either TiCuNi Ag ABA or Au ABA paste were

placed ontop of either the PCD substrate the PCD substrate with a carbonaceous wire or a

graphite substrate with a carbonaceous wire The elemental composition and melting points of

all braze alloys tested is listed in Table 1

ABA (Manufacturer)

Ag () Au () Cu () Al () Ti () Ni () MP (degC)

Ag-ABA (Wesgo)

9275 - 5 1 125 - 912

Au-ABA (Morgan)

- 964 - - 06 3 1003

TiCuNi (Wesgo)

- - 15 - 70 15 960

Table 1 Elemental composition and melting point (MP) of the braze alloys used in this study

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7

Samples were placed in a brazing furnace under vacuum and heated until the braze was

visibly melted and then held at the melting temperature for 1 minute Upon cooling samples

were imaged using either an optical microscope or scanning electron microscope (SEM)

Contact angles were measured using the low-bond axisymmetric drop shape analysis in the

Droplet-Analysis package in ImageJ image processing software [29 30]

212 Elemental analysis of brazemdashcarbon interface

Some diamond samples were laser milled with deep grooves prior to brazing enabling

visualization and characterization of the interface with both SEM and energy-dispersive X-ray

spectroscopy (EDX)(Supplementary Information Fig S1) The metalcarbonaceous wire

interfaces were studied by embedding brazed samples in epoxy resin (Araldite) and

mechanically-polishing them to reveal the cross-section (Fig 2)

Fig 2 ndash Embedding and polishing method utilized to visualize the carbonmdashbraze interface during both SEM and EDX analysis

SEM images were acquired and EDX was used to identify elemental composition at the

interface of both Ag-ABACF and Au-ABACF joints

213 Raman spectroscopy

Raman spectroscopy (inVia Raman Renishaw) was used to investigate the effects of high

temperature brazing on the microstructure of CF bundles CNT yarns and reduced liquid

crystalline graphene oxide fiber (rLCGO) A 514 nm green laser with a power of 50 (15

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8

mW) was used for CF bundles while a power of 5 (15 mW) was used for 15 microm CNT

yarns and 40 microm rLCGO fiber to avoid ablation damage Samples were scanned before and

after high temperature brazing (1003-1030˚C) The laser was focused on the center of the

sample and an exposure time of 60 seconds was used Acquired signals were processed in

Matlab using a linear baseline correction procedure followed by Gaussian fitting to resolve

peaks

22 Device fabrication and electrical characterization

221 PCD circuit substrates

Thermal grade PCD substrates (TM-500 Element Six) with a thickness of 500 microm were laser

engraved using a 532 nm NdYAG laser (Oxford lasers) at a pulse frequency of 10 kHz A

boiling solution of sodium nitrate and sulfuric acid was used to remove debris from the

engraved pattern followed by 5 minutes of ultrasonication in acetone The PCD substrates

were rinsed with isopropanol and deionized water then dried with nitrogen Groove width

was correlated with laser milling power using scanning electron microscopy (SEM) images

ABA paste was spread over the PCD substrate Samples were transferred to a 10 times 10 mm

graphite wafer placed inside an electron beam deposition system (Thermionics) and

evacuated to 1 times 10-6 Pa The electron beam was focused onto the graphite substrate rather

than the diamond sample and current was increased until both the graphite and PCD were

glowing red ultimately leading to the ABA melting and spreading over the substrate and into

the laser-milled patterns Current was decreased over the course of 1 minute and the samples

were cooled over the course of 1 hour in a N2 environment The brazed samples were

removed from the deposition chamber and mechanical polishing was performed with a

diamond-embedded polishing plate (Struers Inc) using deionized water as a coolant The

samples were polished until embedded Ag ABA or Au ABA tracks were revealed and

residual metals between patterns removed ensuring no shorting between interconnects

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9

Resistance between isolated interconnects was measured to ensure no shorting (eg gtMΩ

resistance) between unconnected tracks If shorting did occur due to residual unpolished

metal mechanical polishing was continued until it was removed The fabrication of AuPCD

circuit boards requires an additional brazing step to enable Au wetting (Fig 3)

Fig 3 - Construction of embedded-in-diamond Au ABA circuit structures using a Ag ABA adhesion layer (a-b) circuit patterns were laser cut into the diamond substrate and the surface treated with a combination of sulfuric acid and sodium nitrate (c) Ag-ABA paste was applied to the surface melted in an electron beam deposition system (d) The electron beam was continuously applied until a majority of Ag evaporated from the surface leaving behind a conformal film for Au adhesion (e) A Au-based paste was applied to the surface and melted with the aforementioned method (f) (g) The substrate was mechanically-polished revealing Au circuit structures (h) Finally connections or components were attached with laser welding soldering or wirebonding if necessary

222 Freestanding carbonaceous wires

MWNT forest was synthesized by catalytic chemical vapor deposition using acetylene gas as

the carbon source Carbon nanotubes in the 400 um tall forests typically had diameters of 10

nm The CNT yarns were drawn from the forest by pulling and twisting as described by

Zhang et al [31] Graphene oxide (GO) was prepared from intercalated graphene flakes The

liquid crystalline state of graphene oxide was exploited to wet-spin conductive fibers as

described previously [32] Unsized CF bundles having filament diameters of 7 microm were

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10

purchased from Goodfellow On a graphite substrate two mounds of ABA paste were placed

on opposite sides of a 10 times 10 mm graphite wafer Either a bundle of CFs (Goodfellow) a

single CNT yarn (15 microm) or a graphene oxide fiber (50 microm) was placed across the mounds

and pressed into them Additionally one PtIr wire (75 microm Goodfellow) was placed into each

of the ABA paste mounds The graphite substrate was transferred to an electron beam

deposition system (Thermionics) and evacuated to 1 times 10-6 Pa The electron beam was

focused onto the graphite substratemdashas described above for PCD circuit board constructionmdash

and the current was increased until the substrate was glowing red The aforementioned

process is illustrated in Fig 4

Fig 4 - (a) Fabrication of freestanding metalABACNT yarn wires Mounds of ABA paste were placed onto a graphite substrate and either CNT yarns CFs or graphene oxide fibers were pressed into them PtIr wires were pressed into the end for testing (b) The graphite substrate was heated until the Au formed beads on the surface (c) Upon cooling the entire wiremetal structure was easily lifted from the surface

Once melting of the ABA was observed the current was ramped down over 1 minute and the

samples were cooled for 30 minutes in an N2 environment Au-ABA samples were easily

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11

removed from the graphite substrate due to lack of reaction while considerable force was

needed to remove the Ag ABA and TiCuNi samples

223 Integrated PCDcarbonaceous electrode devices

Our group has previously published a method for integrating PCD substrates with conductive

diamond electrodes to produce hermetic feedthroughs [21] Integrating carbonaceous wire

electrodes in this case 7 microm diameter CF was done manually as depicted in Fig 5

Fig 5 - Method for placing individual CF into laser cut groove in PCD substrate and securing the CF with braze paste The braze paste is then ldquoscreen-printedrdquo with a squeegee to remove the excess braze from the surface of the PCD to minimize shorting upon braze melting

PCD substrates were laser machined as described above to create grooves for CF filaments to

rest in Next individual CF filaments were manually placed into the grooves and Ag ABA

paste was spread over the entire surface The surface was wiped with a mini squeegee

(cleanroom cloth wrapped around syringe needle) to remove excess braze paste from the

surface Finally the sample was heated to melt the braze as described above

224 Electrical characterization of devices

The resistance of the Au ABA interconnects embedded in the polished PCD substrates were

measured using a 2-way probe station and a multimeter (Keithley Instruments) Transmission

line devices were fabricated to correct for contact resistance The electrical behavior of the

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12

freestanding PtIrAuCNT yarn structures was characterized by performing voltage sweeps (-1

to +1 V) with a potentiostat (eDAQ) while measuring current

3 Results and Discussion

31 Embedded-in-diamond circuit substrates

311 Investigating PCD-braze interactions

Three different commercially-available active brazing alloy (ABA) pastes were assessed for

their ability to wet and bond to polycrystalline diamond (PCD) substrates The elemental

composition of these brazes is included in Table 1 of the Experimental section Ag ABA was

found to spread and adhere strongly to PCD which was likely facilitated by the formation of

an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig

6a) SEM and EDX elemental analysis of the PCDAg ABA interface reveals a Ti-rich region

of ~34 microm adjacent to the Ag ABA joint (Supplementary Information Fig S1) Conversely

Au ABA did not spread on the PCD surface upon melting instead forming a droplet with a

111deg contact angle on the surface (Fig 6b)

Fig 6 - (a) Ag ABA wets PCD and bonds strongly (b) Au ABA does not wet PCD forming a droplet that makes ~111deg contact angle with the surface (c) TiCuNi braze wets the PCD surface but does not bond The cooled TiCuNi droplet is easily pulled from the PCD surface with forceps (d) Au ABA wets the PCD following a previous Ag ABA brazing step implemented to create a conformal adhesion layer

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13

Though the manufacturers have included Ti as the active component in the Au ABA this

braze is unable to spread on carbonaceous materials Considering the elemental composition

of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack

of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active

component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in

the TiC-Au melt [33]

rarr + 132

Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which

is unsurprising considering the large difference in electronegativity (X) between Au

(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic

is a possibility but the electronegativity differences are less pronounced between Ti and Ag

(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this

reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based

medical devices [35] wetted the PCD surface upon heating but did not bond and was easily

removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and

bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)

This finding is consistent with what has been shown previously for TiCuNirsquos interaction with

carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding

between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between

the braze alloy and the material The stress induced by thermal mismatch (σTM) can be

described theoretically by Equation 3 [37]

13 ∙ ∆ (3)

Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the

braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the

temperature differential approximately 1000 K for the difference between brazing

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14

temperature and room temperature According to manufacturer data (Element Six) the PCD

used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates

have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203

ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the

TiCuNiPCD system is warranted though beyond the scope of the present work In

consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate

compatible with standard joining technologies (ie wirebonding laser welding reflow

soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system

a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a

subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If

a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded

thick- film conductors or contact pads required for the embedded-in-diamond circuit

structures described in the following sections Additionally the ubiquity of Au contacts in

semiconductor and electronics fabrication makes the AuPCD system a practical device

substrate

312 Device fabrication and characterization

The entire fabrication process for these devices is illustrated above in the Experimental

section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate

prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished

devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far

with this technique range between 6-15 microm in width Lower milling powers lead to less

reproducibility perhaps due to issues with focusing on the surface which would be more

pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are

likely possible following more extensive laser parameter optimization including adjustment

of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices

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15

were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au

ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled

structures having three dimensions which demonstrates a significant advantage over

conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing

process eliminates the extra annealing step to form TiC at the interface [40] The resistance of

Au ABA interconnects embedded in PCD was measured using a transmission line set-up to

correct for contact resistance The relationship between the resistance of a 500 microm length Au

interconnect and laser power utilized to mill the track is shown in Fig 7d

Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found

The 3D nature of the interconnect leads to a significant decrease in electrical resistance

compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is

material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In

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16

addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal

for thermal management applications due to diamondrsquos extremely high thermal conductivity

(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining

technologies is paramount Both varieties of braze are easily joined via laser welding

wirebonding or reflow soldering (Supplementary Information Fig S3)

32 Freestanding carbonaceous wires

321 Investigating braze-wire interactions

The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not

been studied as far as we are aware As a result the surface interaction of these materials was

investigated initially using PCD as a substrate PCD was originally chosen due to its low

electrical conductivity making it an adequate support structure for electrical characterization

However the interaction between PCD and Au though non-wetting is significant enough to

prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an

individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either

material though some bonding between Au ABA and PCD occurs A similar effect was

observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of

the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting

behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact

angle of ~404deg (Fig 8c)

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17

Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)

In the interest of creating freestanding carbonaceous wires with metallic contacts we

investigated whether the lack of wetting between a graphite substrate and braze alloy could be

exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate

(Fig 9) It has been observed previously that surface roughness can lead to large increases in

contact angle during brazing making graphite a reasonable choice to perhaps further diminish

the wetting behavior [42]

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18

Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)

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19

A graphite substrate was laser milled in an attempt to control separation distance between the

contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate

a number of geometries are possible for the freestanding wires Ag ABA was able to both wet

and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag

ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig

9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact

with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction

with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure

to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a

seal around the bundle effectively compressing the filaments together (Fig 9h) However if

enough force is applied the Au contact can be removed from the CF bundle though this was

not observed for CNT yarn devices This effect is likely proportional to the ratio of wire

diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of

the bond due to a rise in the contact surface area

322 Device fabrication and characterization

A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or

AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred

because it does not form a strong bond to the graphite substrate following a melting cycle so

the entire wire can be lifted off of the substrate with the contacts intact preventing damage to

the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing

facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent

lift-off was attempted Au does not bond to the graphite for reasons explained above When

the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing

both the carbonaceous fiber and a metallic wire within it (Fig 10a)

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20

Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample

The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig

10c) This technique is a potential improvement over mechanical clamps especially

considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from

clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a

woven or spun material such as CNT yarn prevents unwinding and generally helps to keep

the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead

to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)

were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a

linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the

CNT yarns in this work was comparable to those of similar size and length measured by

Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn

structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While

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21

preliminary measurements suggest that contact resistances are negligible further investigation

is needed to estimate the contact resistance through the contact It was of interest to determine

whether this process modified the carbon structure of the wire materials To investigate this

phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire

following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)

Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1

The before and after Raman spectra of CFs reveals very little change in the microstructure

aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This

peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a

prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor

graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears

quite prominent in both CF spectra presented here though there appears to have been some

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22

moderate increase in graphitization following brazing Additionally the loss of the D-peak

overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to

increased graphitization However it is important to note that the graphitization process

during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to

create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several

peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)

Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1

(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible

that the loss of these peaks is the result of the loss of a surface functionality as a result of high

temperature treatment such as a C-O group However further investigation is required to

acquire a full understanding of the structural changes occurring not only as a result of the high

temperature processes in the presence of various metals In an attempt to improve the thermal

and electrical properties of CNTs and composites gamma irradiation and high temperature

treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease

in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in

the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal

treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that

the vacuum brazing processes described here do not have deleterious effects on carbonaceous

wires

33 Integrating diamond substrates with carbonaceous electrodes

Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)

nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective

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23

processes for integrating these materials with a suitable substrate differ considerably For N-

UNCD stimulating electrodes our group has demonstrated a method to grow the conductive

diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]

This method produces a hermetic electrode array for implantable neural prosthetic

applications However the integration of this array with standard electronic components

requires a metallic interface utilizing the embedded circuit board technology described here

A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze

interconnects and contact pads utilized to contact conductive diamond electrodes (dotted

circle) in a full electrode array device (Fig 12a)

Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes

Incorporating carbonaceous wire electrodes into electronic devices is less elegant and

currently requires the manual placement of individual wire electrodes into designated laser-

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24

milled grooves in the PCD substrate Next the suitable brazing paste is applied though a

screen-printing technique is required to bypass the mechanical polishing step Avoiding

mechanical polishing is important for delicate small and fragile electrode materials such as

those described in this work However because the brazing pastes are comprised of both

metal beads and a polymeric binder the functional volume of the interconnect is reduced

upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed

to produce a CF array embedded in diamond (Fig 12c) We are currently investigating

methods for protecting the CFs and other carbonaceous wires during mechanical polishing to

enable more complex robust device architectures

4 Conclusion

Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires

(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all

components paramount For PCD electronics Ag ABA is used due to its wetting and covalent

bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in

physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial

layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the

AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit

boards are compatible with commercially-available solders and are easily laser-welded for

solder-free joining The non-wetting method of enclosing carbonaceous wires was used to

create strong low-resistance and non-rectifying connections between PtIr wires and

carbonaceous wires Structural and interfacial examinations of these structures lend insight

into the forces at play during both the wetting and non-wetting fabrication of carbonaceous

devices We exploit surface tension effects of two commercially-available active brazing

alloys to create two different device architectures

(1) The wetting technique was used in conjunction with mechanical polishing to create

robust circuit substrates with embedded Ag or Au interconnects and contact pads The option

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25

to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate

illustrates the utility of the method for enabling carbon-based electrode arrays

(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or

AuPtIr wire contacts This technique is interesting for the creation of not only free-standing

carbonaceous conductors but also for neural probes or sensors for both central and peripheral

nervous system integration We have previously described methods to fabricate neural

stimulation and recording electrodes from carbonaceous wires described here such as rLCGO

fiber which demonstrated both in vitro and in vivo functionality [47]

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for

insightful technical conversations NVA is supported by a MMI-CSIRO Material Science

PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was

supported by the Australian Research Council (ARC) through its Special Research Initiative

(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is

supported by the Australian Research Council under Discovery Early Career Researcher

award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and

support through the ARC Centre of Excellence for Electromaterials Science

Supplementary Information Available Figures S1-S4 This material is available free of charge online References

MANUSCRIP

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26

[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

MANUSCRIP

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27

[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

MANUSCRIP

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ACCEPTED MANUSCRIPT

28

[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

MANUSCRIP

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ACCEPTED MANUSCRIPT

2

1 Introduction

Owing to their unique combination of biological mechanical electrochemical electrical

thermal and optical properties carbon-based materials such as graphene carbon nanotubes

(CNT) carbon fiber (CF) and diamond have attracted significant scientific and industrial

interest Exploiting CNT and graphene properties in micro- and macroscale structures is of

significant interest for engineering research and product development [1-3] For example

high strength and high conductivity fabrics based on these materials could enable a variety of

applications from lightweight body armor to space elevators [4 5] Several solutions to the

problem of scale have been demonstrated for both graphene and CNT films Spinning CVD-

grown CNT forests into a continuous yarn has enabled conductive high-strength structures

with micro-scale diameters and macro-scale lengths [3] Control of mechanical strength and

electricalthermal conductivity of CNT yarns is established by adjusting component CNT

lengths as well as the spinning angles of the fibers [6 7] Twisted andor coiled structures of

CNT yarns have been used to fabricate electrochemical as well as electrolyte-free CNT

torsional actuators (artificial muscles) with large-stroke and high work-capacity [8 9] Cell

culture dishes containing CNT yarns have shown that neurons grow and respond to electrical

stimulation delivered by the yarn [10] As bulk carriers of electricity and data CNT yarns

have been evaluated as replacements for standard Cu and Al wires due to their light weight

(alleviating stress on joint connections) ballistic conduction (lack of scattering reduces risk of

Joule heating) and capacity to handle high frequencies [11] Expanding upon the application

of long-distance electricity transfer CNT yarns have been used to build wireless data transfer

networks that were mechanically-resilient and displayed frequency-independent resistive

behavior [12] As a demonstration of their favorable electrochemical characteristics CNT

yarns have been used as an alternative to the standard CF electrochemical sensor for

neurotransmitters due to an intrinsic ability to resist surface fouling [13]

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3

All of the aforementioned research and much that was not mentioned was performed using

prototype devices containing silver epoxy gold paste solder or mechanical clamps to create

electrical connections to carbonaceous wires While these connections served their respective

experimental purposes their use in fabrication does not generally yield scalable or integrated

electronic devices nor are these connection methods resilient or biocompatible enough for

consideration as materials for biomedical devices There have been several methods

developed for bonding a single CNT to metals as well as joining a single CNT to another

CNT end-to-end [14] However making electrical connections to larger (micro rather than

nano) CNT composites and yarns is a different challenge Current approaches include

mechanical clamps Ag- and Au-based epoxy adhesives carbon solder ultrasonic welding

and vacuum brazing [15 16] A recent very exciting approach involves use of transition

metal soldering alloys to join carbon wires using standard solder conditions (eg 350 degC in

air) [17] While it is a technique that will certainly revolutionize the utility of carbonaceous

wires the Cu- and Sn-based alloys are unlikely to display biocompatibility due to the known

cytotoxicity of these metals [18 19] We have recently developed a technique to create a

hermetic diamond capsule using biocompatible gold alloys [20] Additionally we have

demonstrated a method for the construction of hermetic biocompatible feedthroughs for a

retinal prosthesis device based on conductive nitrogen-doped diamond electrodes [21] In the

present paper we extend our previous work and describe the construction of diamond-based

electronic circuit substrates with embedded Ag- or Au-based interconnects and soldering pads

Finally we describe a method to metallize the aforementioned carbonaceous wires (CNT yarn

CF and graphene fiber) with the option to incorporate them into diamond circuit substrates

The methods we describe constitute an important toolkit to allow for CNT-based and other

similar carbonaceous ldquosuper materialsrdquo to be incorporated into electronic devices and

integrated with traditional surface-mount electronics with specific focus on biocompatible

systems for implantation into the body

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4

11 Brazing to carbon

Active brazing a variation of vacuum brazing utilizes an alloy containing an active

component that reacts chemically with a relatively inert surface such as diamond or ceramics

[22 23] Transition metals are of particular interest due to electron vacancies in their d-

orbitals with greater numbers of vacancies leading to greater reactivity (overlap) with

carbonrsquos p-orbitals [24] In the case of carbon substrates the active componentmdashtypically Ti

V or Crmdashforms a carbide interface layer that acts as a surfactant to enable the wetting of

other filler metals and produce non-rectifying electrical contacts [25 26] A Ag-based active

brazing alloy (ABA) has been used previously to make electrical contact to a CNT bundle

and formation of the TiC interface layer was confirmed with X-ray photon scattering (XPS)

though the electrical and mechanical properties were not fully characterized [16] The

formation of interfacial TiC particles has also been shown to significantly increase the tensile

strength of carbon composites brazed with Ti-based alloys containing CNT reinforcements

[27] In the present work commercially-available ABAs were investigated to determine their

suitability to make electronic connections to several carbonaceous materials including

polycrystalline diamond (PCD) CNT yarns graphene oxide (GO) fibers and PAN-based

carbon fiber (CF) to enable various types of electronic devices (Fig 1)

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5

Fig 1 - ABAs were used to fabricate two basic electronic device platforms (a) a circuit board embedded into a polycrystalline diamond (PCD) substrate and (b) a freestanding carbonaceous wire with electrical connections on one or both ends The aforementioned technologies had lead to the fabrication of medical devices containing active carbon electrodes such as (c) conductive diamond electrodes and (d) carbonaceous wire electrodes

In the first technique described here ABAs were used to create embedded circuit boards in

PCD wherein the solidified ABA forms the interconnects and contact pads following

mechanical polishing The second metallization system involves making free-standing Au

contacts to carbonaceous fibers using a graphite ldquolift-offrdquo method which exploits the balance

of cohesive and adhesive forces of the liquid metalgraphite interface (Equation 1)

cos

1131

Where θ is the contact angle between the substrate and liquid metal WA is the work of

adhesion (adhesion force) between the substrate and the liquid metal and σL is the cohesion

force (liquid-vapour surface tension) within the liquid metal droplet [28] Equation 1

describes the balance of forces at the liquid metalsolid substrate interface wherein liquid

metals (intrinsically very high cohesion force materials) tend to create large contact angles on

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6

most substrates unless σL lt WA where contact angles lt90˚ are considered ldquowettingrdquo In one

facet of this work we use active brazing to modify WA and create robust electrical

connections to carbonaceous materials such as PCD and CF In another technique we exploit

the lack of adhesion and wetting between graphitic carbon and Au as well as the cohesion

forces within the liquid metal droplet to create a freestanding carbonaceous wire with

metallic connections Broadly speaking the techniques described here are applicable to the

development of devices in which a strong biocompatible and non-rectifying connection to

carbon-based materials is required

2 Experimental

21 Surface interactions brazing alloys and carbonaceous materials

211 Sample preparation and wettability studies

Thermal grade polycrystalline diamond (PCD) substrates (TM-250 Element Six) with a

thickness of 250 microm were laser-cut into 5 times 5 mm squares with a 532 nm NdYAG laser

(Oxford Lasers) Approximately 65 mg of either TiCuNi Ag ABA or Au ABA paste were

placed ontop of either the PCD substrate the PCD substrate with a carbonaceous wire or a

graphite substrate with a carbonaceous wire The elemental composition and melting points of

all braze alloys tested is listed in Table 1

ABA (Manufacturer)

Ag () Au () Cu () Al () Ti () Ni () MP (degC)

Ag-ABA (Wesgo)

9275 - 5 1 125 - 912

Au-ABA (Morgan)

- 964 - - 06 3 1003

TiCuNi (Wesgo)

- - 15 - 70 15 960

Table 1 Elemental composition and melting point (MP) of the braze alloys used in this study

MANUSCRIP

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7

Samples were placed in a brazing furnace under vacuum and heated until the braze was

visibly melted and then held at the melting temperature for 1 minute Upon cooling samples

were imaged using either an optical microscope or scanning electron microscope (SEM)

Contact angles were measured using the low-bond axisymmetric drop shape analysis in the

Droplet-Analysis package in ImageJ image processing software [29 30]

212 Elemental analysis of brazemdashcarbon interface

Some diamond samples were laser milled with deep grooves prior to brazing enabling

visualization and characterization of the interface with both SEM and energy-dispersive X-ray

spectroscopy (EDX)(Supplementary Information Fig S1) The metalcarbonaceous wire

interfaces were studied by embedding brazed samples in epoxy resin (Araldite) and

mechanically-polishing them to reveal the cross-section (Fig 2)

Fig 2 ndash Embedding and polishing method utilized to visualize the carbonmdashbraze interface during both SEM and EDX analysis

SEM images were acquired and EDX was used to identify elemental composition at the

interface of both Ag-ABACF and Au-ABACF joints

213 Raman spectroscopy

Raman spectroscopy (inVia Raman Renishaw) was used to investigate the effects of high

temperature brazing on the microstructure of CF bundles CNT yarns and reduced liquid

crystalline graphene oxide fiber (rLCGO) A 514 nm green laser with a power of 50 (15

MANUSCRIP

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8

mW) was used for CF bundles while a power of 5 (15 mW) was used for 15 microm CNT

yarns and 40 microm rLCGO fiber to avoid ablation damage Samples were scanned before and

after high temperature brazing (1003-1030˚C) The laser was focused on the center of the

sample and an exposure time of 60 seconds was used Acquired signals were processed in

Matlab using a linear baseline correction procedure followed by Gaussian fitting to resolve

peaks

22 Device fabrication and electrical characterization

221 PCD circuit substrates

Thermal grade PCD substrates (TM-500 Element Six) with a thickness of 500 microm were laser

engraved using a 532 nm NdYAG laser (Oxford lasers) at a pulse frequency of 10 kHz A

boiling solution of sodium nitrate and sulfuric acid was used to remove debris from the

engraved pattern followed by 5 minutes of ultrasonication in acetone The PCD substrates

were rinsed with isopropanol and deionized water then dried with nitrogen Groove width

was correlated with laser milling power using scanning electron microscopy (SEM) images

ABA paste was spread over the PCD substrate Samples were transferred to a 10 times 10 mm

graphite wafer placed inside an electron beam deposition system (Thermionics) and

evacuated to 1 times 10-6 Pa The electron beam was focused onto the graphite substrate rather

than the diamond sample and current was increased until both the graphite and PCD were

glowing red ultimately leading to the ABA melting and spreading over the substrate and into

the laser-milled patterns Current was decreased over the course of 1 minute and the samples

were cooled over the course of 1 hour in a N2 environment The brazed samples were

removed from the deposition chamber and mechanical polishing was performed with a

diamond-embedded polishing plate (Struers Inc) using deionized water as a coolant The

samples were polished until embedded Ag ABA or Au ABA tracks were revealed and

residual metals between patterns removed ensuring no shorting between interconnects

MANUSCRIP

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9

Resistance between isolated interconnects was measured to ensure no shorting (eg gtMΩ

resistance) between unconnected tracks If shorting did occur due to residual unpolished

metal mechanical polishing was continued until it was removed The fabrication of AuPCD

circuit boards requires an additional brazing step to enable Au wetting (Fig 3)

Fig 3 - Construction of embedded-in-diamond Au ABA circuit structures using a Ag ABA adhesion layer (a-b) circuit patterns were laser cut into the diamond substrate and the surface treated with a combination of sulfuric acid and sodium nitrate (c) Ag-ABA paste was applied to the surface melted in an electron beam deposition system (d) The electron beam was continuously applied until a majority of Ag evaporated from the surface leaving behind a conformal film for Au adhesion (e) A Au-based paste was applied to the surface and melted with the aforementioned method (f) (g) The substrate was mechanically-polished revealing Au circuit structures (h) Finally connections or components were attached with laser welding soldering or wirebonding if necessary

222 Freestanding carbonaceous wires

MWNT forest was synthesized by catalytic chemical vapor deposition using acetylene gas as

the carbon source Carbon nanotubes in the 400 um tall forests typically had diameters of 10

nm The CNT yarns were drawn from the forest by pulling and twisting as described by

Zhang et al [31] Graphene oxide (GO) was prepared from intercalated graphene flakes The

liquid crystalline state of graphene oxide was exploited to wet-spin conductive fibers as

described previously [32] Unsized CF bundles having filament diameters of 7 microm were

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10

purchased from Goodfellow On a graphite substrate two mounds of ABA paste were placed

on opposite sides of a 10 times 10 mm graphite wafer Either a bundle of CFs (Goodfellow) a

single CNT yarn (15 microm) or a graphene oxide fiber (50 microm) was placed across the mounds

and pressed into them Additionally one PtIr wire (75 microm Goodfellow) was placed into each

of the ABA paste mounds The graphite substrate was transferred to an electron beam

deposition system (Thermionics) and evacuated to 1 times 10-6 Pa The electron beam was

focused onto the graphite substratemdashas described above for PCD circuit board constructionmdash

and the current was increased until the substrate was glowing red The aforementioned

process is illustrated in Fig 4

Fig 4 - (a) Fabrication of freestanding metalABACNT yarn wires Mounds of ABA paste were placed onto a graphite substrate and either CNT yarns CFs or graphene oxide fibers were pressed into them PtIr wires were pressed into the end for testing (b) The graphite substrate was heated until the Au formed beads on the surface (c) Upon cooling the entire wiremetal structure was easily lifted from the surface

Once melting of the ABA was observed the current was ramped down over 1 minute and the

samples were cooled for 30 minutes in an N2 environment Au-ABA samples were easily

MANUSCRIP

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11

removed from the graphite substrate due to lack of reaction while considerable force was

needed to remove the Ag ABA and TiCuNi samples

223 Integrated PCDcarbonaceous electrode devices

Our group has previously published a method for integrating PCD substrates with conductive

diamond electrodes to produce hermetic feedthroughs [21] Integrating carbonaceous wire

electrodes in this case 7 microm diameter CF was done manually as depicted in Fig 5

Fig 5 - Method for placing individual CF into laser cut groove in PCD substrate and securing the CF with braze paste The braze paste is then ldquoscreen-printedrdquo with a squeegee to remove the excess braze from the surface of the PCD to minimize shorting upon braze melting

PCD substrates were laser machined as described above to create grooves for CF filaments to

rest in Next individual CF filaments were manually placed into the grooves and Ag ABA

paste was spread over the entire surface The surface was wiped with a mini squeegee

(cleanroom cloth wrapped around syringe needle) to remove excess braze paste from the

surface Finally the sample was heated to melt the braze as described above

224 Electrical characterization of devices

The resistance of the Au ABA interconnects embedded in the polished PCD substrates were

measured using a 2-way probe station and a multimeter (Keithley Instruments) Transmission

line devices were fabricated to correct for contact resistance The electrical behavior of the

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12

freestanding PtIrAuCNT yarn structures was characterized by performing voltage sweeps (-1

to +1 V) with a potentiostat (eDAQ) while measuring current

3 Results and Discussion

31 Embedded-in-diamond circuit substrates

311 Investigating PCD-braze interactions

Three different commercially-available active brazing alloy (ABA) pastes were assessed for

their ability to wet and bond to polycrystalline diamond (PCD) substrates The elemental

composition of these brazes is included in Table 1 of the Experimental section Ag ABA was

found to spread and adhere strongly to PCD which was likely facilitated by the formation of

an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig

6a) SEM and EDX elemental analysis of the PCDAg ABA interface reveals a Ti-rich region

of ~34 microm adjacent to the Ag ABA joint (Supplementary Information Fig S1) Conversely

Au ABA did not spread on the PCD surface upon melting instead forming a droplet with a

111deg contact angle on the surface (Fig 6b)

Fig 6 - (a) Ag ABA wets PCD and bonds strongly (b) Au ABA does not wet PCD forming a droplet that makes ~111deg contact angle with the surface (c) TiCuNi braze wets the PCD surface but does not bond The cooled TiCuNi droplet is easily pulled from the PCD surface with forceps (d) Au ABA wets the PCD following a previous Ag ABA brazing step implemented to create a conformal adhesion layer

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13

Though the manufacturers have included Ti as the active component in the Au ABA this

braze is unable to spread on carbonaceous materials Considering the elemental composition

of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack

of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active

component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in

the TiC-Au melt [33]

rarr + 132

Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which

is unsurprising considering the large difference in electronegativity (X) between Au

(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic

is a possibility but the electronegativity differences are less pronounced between Ti and Ag

(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this

reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based

medical devices [35] wetted the PCD surface upon heating but did not bond and was easily

removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and

bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)

This finding is consistent with what has been shown previously for TiCuNirsquos interaction with

carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding

between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between

the braze alloy and the material The stress induced by thermal mismatch (σTM) can be

described theoretically by Equation 3 [37]

13 ∙ ∆ (3)

Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the

braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the

temperature differential approximately 1000 K for the difference between brazing

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14

temperature and room temperature According to manufacturer data (Element Six) the PCD

used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates

have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203

ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the

TiCuNiPCD system is warranted though beyond the scope of the present work In

consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate

compatible with standard joining technologies (ie wirebonding laser welding reflow

soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system

a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a

subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If

a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded

thick- film conductors or contact pads required for the embedded-in-diamond circuit

structures described in the following sections Additionally the ubiquity of Au contacts in

semiconductor and electronics fabrication makes the AuPCD system a practical device

substrate

312 Device fabrication and characterization

The entire fabrication process for these devices is illustrated above in the Experimental

section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate

prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished

devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far

with this technique range between 6-15 microm in width Lower milling powers lead to less

reproducibility perhaps due to issues with focusing on the surface which would be more

pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are

likely possible following more extensive laser parameter optimization including adjustment

of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices

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15

were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au

ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled

structures having three dimensions which demonstrates a significant advantage over

conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing

process eliminates the extra annealing step to form TiC at the interface [40] The resistance of

Au ABA interconnects embedded in PCD was measured using a transmission line set-up to

correct for contact resistance The relationship between the resistance of a 500 microm length Au

interconnect and laser power utilized to mill the track is shown in Fig 7d

Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found

The 3D nature of the interconnect leads to a significant decrease in electrical resistance

compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is

material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In

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16

addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal

for thermal management applications due to diamondrsquos extremely high thermal conductivity

(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining

technologies is paramount Both varieties of braze are easily joined via laser welding

wirebonding or reflow soldering (Supplementary Information Fig S3)

32 Freestanding carbonaceous wires

321 Investigating braze-wire interactions

The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not

been studied as far as we are aware As a result the surface interaction of these materials was

investigated initially using PCD as a substrate PCD was originally chosen due to its low

electrical conductivity making it an adequate support structure for electrical characterization

However the interaction between PCD and Au though non-wetting is significant enough to

prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an

individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either

material though some bonding between Au ABA and PCD occurs A similar effect was

observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of

the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting

behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact

angle of ~404deg (Fig 8c)

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17

Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)

In the interest of creating freestanding carbonaceous wires with metallic contacts we

investigated whether the lack of wetting between a graphite substrate and braze alloy could be

exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate

(Fig 9) It has been observed previously that surface roughness can lead to large increases in

contact angle during brazing making graphite a reasonable choice to perhaps further diminish

the wetting behavior [42]

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18

Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)

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19

A graphite substrate was laser milled in an attempt to control separation distance between the

contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate

a number of geometries are possible for the freestanding wires Ag ABA was able to both wet

and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag

ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig

9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact

with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction

with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure

to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a

seal around the bundle effectively compressing the filaments together (Fig 9h) However if

enough force is applied the Au contact can be removed from the CF bundle though this was

not observed for CNT yarn devices This effect is likely proportional to the ratio of wire

diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of

the bond due to a rise in the contact surface area

322 Device fabrication and characterization

A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or

AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred

because it does not form a strong bond to the graphite substrate following a melting cycle so

the entire wire can be lifted off of the substrate with the contacts intact preventing damage to

the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing

facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent

lift-off was attempted Au does not bond to the graphite for reasons explained above When

the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing

both the carbonaceous fiber and a metallic wire within it (Fig 10a)

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20

Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample

The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig

10c) This technique is a potential improvement over mechanical clamps especially

considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from

clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a

woven or spun material such as CNT yarn prevents unwinding and generally helps to keep

the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead

to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)

were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a

linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the

CNT yarns in this work was comparable to those of similar size and length measured by

Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn

structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While

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21

preliminary measurements suggest that contact resistances are negligible further investigation

is needed to estimate the contact resistance through the contact It was of interest to determine

whether this process modified the carbon structure of the wire materials To investigate this

phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire

following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)

Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1

The before and after Raman spectra of CFs reveals very little change in the microstructure

aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This

peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a

prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor

graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears

quite prominent in both CF spectra presented here though there appears to have been some

MANUSCRIP

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22

moderate increase in graphitization following brazing Additionally the loss of the D-peak

overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to

increased graphitization However it is important to note that the graphitization process

during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to

create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several

peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)

Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1

(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible

that the loss of these peaks is the result of the loss of a surface functionality as a result of high

temperature treatment such as a C-O group However further investigation is required to

acquire a full understanding of the structural changes occurring not only as a result of the high

temperature processes in the presence of various metals In an attempt to improve the thermal

and electrical properties of CNTs and composites gamma irradiation and high temperature

treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease

in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in

the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal

treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that

the vacuum brazing processes described here do not have deleterious effects on carbonaceous

wires

33 Integrating diamond substrates with carbonaceous electrodes

Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)

nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective

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23

processes for integrating these materials with a suitable substrate differ considerably For N-

UNCD stimulating electrodes our group has demonstrated a method to grow the conductive

diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]

This method produces a hermetic electrode array for implantable neural prosthetic

applications However the integration of this array with standard electronic components

requires a metallic interface utilizing the embedded circuit board technology described here

A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze

interconnects and contact pads utilized to contact conductive diamond electrodes (dotted

circle) in a full electrode array device (Fig 12a)

Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes

Incorporating carbonaceous wire electrodes into electronic devices is less elegant and

currently requires the manual placement of individual wire electrodes into designated laser-

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24

milled grooves in the PCD substrate Next the suitable brazing paste is applied though a

screen-printing technique is required to bypass the mechanical polishing step Avoiding

mechanical polishing is important for delicate small and fragile electrode materials such as

those described in this work However because the brazing pastes are comprised of both

metal beads and a polymeric binder the functional volume of the interconnect is reduced

upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed

to produce a CF array embedded in diamond (Fig 12c) We are currently investigating

methods for protecting the CFs and other carbonaceous wires during mechanical polishing to

enable more complex robust device architectures

4 Conclusion

Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires

(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all

components paramount For PCD electronics Ag ABA is used due to its wetting and covalent

bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in

physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial

layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the

AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit

boards are compatible with commercially-available solders and are easily laser-welded for

solder-free joining The non-wetting method of enclosing carbonaceous wires was used to

create strong low-resistance and non-rectifying connections between PtIr wires and

carbonaceous wires Structural and interfacial examinations of these structures lend insight

into the forces at play during both the wetting and non-wetting fabrication of carbonaceous

devices We exploit surface tension effects of two commercially-available active brazing

alloys to create two different device architectures

(1) The wetting technique was used in conjunction with mechanical polishing to create

robust circuit substrates with embedded Ag or Au interconnects and contact pads The option

MANUSCRIP

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25

to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate

illustrates the utility of the method for enabling carbon-based electrode arrays

(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or

AuPtIr wire contacts This technique is interesting for the creation of not only free-standing

carbonaceous conductors but also for neural probes or sensors for both central and peripheral

nervous system integration We have previously described methods to fabricate neural

stimulation and recording electrodes from carbonaceous wires described here such as rLCGO

fiber which demonstrated both in vitro and in vivo functionality [47]

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for

insightful technical conversations NVA is supported by a MMI-CSIRO Material Science

PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was

supported by the Australian Research Council (ARC) through its Special Research Initiative

(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is

supported by the Australian Research Council under Discovery Early Career Researcher

award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and

support through the ARC Centre of Excellence for Electromaterials Science

Supplementary Information Available Figures S1-S4 This material is available free of charge online References

MANUSCRIP

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26

[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

MANUSCRIP

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27

[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

MANUSCRIP

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ACCEPTED MANUSCRIPT

28

[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

3

All of the aforementioned research and much that was not mentioned was performed using

prototype devices containing silver epoxy gold paste solder or mechanical clamps to create

electrical connections to carbonaceous wires While these connections served their respective

experimental purposes their use in fabrication does not generally yield scalable or integrated

electronic devices nor are these connection methods resilient or biocompatible enough for

consideration as materials for biomedical devices There have been several methods

developed for bonding a single CNT to metals as well as joining a single CNT to another

CNT end-to-end [14] However making electrical connections to larger (micro rather than

nano) CNT composites and yarns is a different challenge Current approaches include

mechanical clamps Ag- and Au-based epoxy adhesives carbon solder ultrasonic welding

and vacuum brazing [15 16] A recent very exciting approach involves use of transition

metal soldering alloys to join carbon wires using standard solder conditions (eg 350 degC in

air) [17] While it is a technique that will certainly revolutionize the utility of carbonaceous

wires the Cu- and Sn-based alloys are unlikely to display biocompatibility due to the known

cytotoxicity of these metals [18 19] We have recently developed a technique to create a

hermetic diamond capsule using biocompatible gold alloys [20] Additionally we have

demonstrated a method for the construction of hermetic biocompatible feedthroughs for a

retinal prosthesis device based on conductive nitrogen-doped diamond electrodes [21] In the

present paper we extend our previous work and describe the construction of diamond-based

electronic circuit substrates with embedded Ag- or Au-based interconnects and soldering pads

Finally we describe a method to metallize the aforementioned carbonaceous wires (CNT yarn

CF and graphene fiber) with the option to incorporate them into diamond circuit substrates

The methods we describe constitute an important toolkit to allow for CNT-based and other

similar carbonaceous ldquosuper materialsrdquo to be incorporated into electronic devices and

integrated with traditional surface-mount electronics with specific focus on biocompatible

systems for implantation into the body

MANUSCRIP

T

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4

11 Brazing to carbon

Active brazing a variation of vacuum brazing utilizes an alloy containing an active

component that reacts chemically with a relatively inert surface such as diamond or ceramics

[22 23] Transition metals are of particular interest due to electron vacancies in their d-

orbitals with greater numbers of vacancies leading to greater reactivity (overlap) with

carbonrsquos p-orbitals [24] In the case of carbon substrates the active componentmdashtypically Ti

V or Crmdashforms a carbide interface layer that acts as a surfactant to enable the wetting of

other filler metals and produce non-rectifying electrical contacts [25 26] A Ag-based active

brazing alloy (ABA) has been used previously to make electrical contact to a CNT bundle

and formation of the TiC interface layer was confirmed with X-ray photon scattering (XPS)

though the electrical and mechanical properties were not fully characterized [16] The

formation of interfacial TiC particles has also been shown to significantly increase the tensile

strength of carbon composites brazed with Ti-based alloys containing CNT reinforcements

[27] In the present work commercially-available ABAs were investigated to determine their

suitability to make electronic connections to several carbonaceous materials including

polycrystalline diamond (PCD) CNT yarns graphene oxide (GO) fibers and PAN-based

carbon fiber (CF) to enable various types of electronic devices (Fig 1)

MANUSCRIP

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5

Fig 1 - ABAs were used to fabricate two basic electronic device platforms (a) a circuit board embedded into a polycrystalline diamond (PCD) substrate and (b) a freestanding carbonaceous wire with electrical connections on one or both ends The aforementioned technologies had lead to the fabrication of medical devices containing active carbon electrodes such as (c) conductive diamond electrodes and (d) carbonaceous wire electrodes

In the first technique described here ABAs were used to create embedded circuit boards in

PCD wherein the solidified ABA forms the interconnects and contact pads following

mechanical polishing The second metallization system involves making free-standing Au

contacts to carbonaceous fibers using a graphite ldquolift-offrdquo method which exploits the balance

of cohesive and adhesive forces of the liquid metalgraphite interface (Equation 1)

cos

1131

Where θ is the contact angle between the substrate and liquid metal WA is the work of

adhesion (adhesion force) between the substrate and the liquid metal and σL is the cohesion

force (liquid-vapour surface tension) within the liquid metal droplet [28] Equation 1

describes the balance of forces at the liquid metalsolid substrate interface wherein liquid

metals (intrinsically very high cohesion force materials) tend to create large contact angles on

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6

most substrates unless σL lt WA where contact angles lt90˚ are considered ldquowettingrdquo In one

facet of this work we use active brazing to modify WA and create robust electrical

connections to carbonaceous materials such as PCD and CF In another technique we exploit

the lack of adhesion and wetting between graphitic carbon and Au as well as the cohesion

forces within the liquid metal droplet to create a freestanding carbonaceous wire with

metallic connections Broadly speaking the techniques described here are applicable to the

development of devices in which a strong biocompatible and non-rectifying connection to

carbon-based materials is required

2 Experimental

21 Surface interactions brazing alloys and carbonaceous materials

211 Sample preparation and wettability studies

Thermal grade polycrystalline diamond (PCD) substrates (TM-250 Element Six) with a

thickness of 250 microm were laser-cut into 5 times 5 mm squares with a 532 nm NdYAG laser

(Oxford Lasers) Approximately 65 mg of either TiCuNi Ag ABA or Au ABA paste were

placed ontop of either the PCD substrate the PCD substrate with a carbonaceous wire or a

graphite substrate with a carbonaceous wire The elemental composition and melting points of

all braze alloys tested is listed in Table 1

ABA (Manufacturer)

Ag () Au () Cu () Al () Ti () Ni () MP (degC)

Ag-ABA (Wesgo)

9275 - 5 1 125 - 912

Au-ABA (Morgan)

- 964 - - 06 3 1003

TiCuNi (Wesgo)

- - 15 - 70 15 960

Table 1 Elemental composition and melting point (MP) of the braze alloys used in this study

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7

Samples were placed in a brazing furnace under vacuum and heated until the braze was

visibly melted and then held at the melting temperature for 1 minute Upon cooling samples

were imaged using either an optical microscope or scanning electron microscope (SEM)

Contact angles were measured using the low-bond axisymmetric drop shape analysis in the

Droplet-Analysis package in ImageJ image processing software [29 30]

212 Elemental analysis of brazemdashcarbon interface

Some diamond samples were laser milled with deep grooves prior to brazing enabling

visualization and characterization of the interface with both SEM and energy-dispersive X-ray

spectroscopy (EDX)(Supplementary Information Fig S1) The metalcarbonaceous wire

interfaces were studied by embedding brazed samples in epoxy resin (Araldite) and

mechanically-polishing them to reveal the cross-section (Fig 2)

Fig 2 ndash Embedding and polishing method utilized to visualize the carbonmdashbraze interface during both SEM and EDX analysis

SEM images were acquired and EDX was used to identify elemental composition at the

interface of both Ag-ABACF and Au-ABACF joints

213 Raman spectroscopy

Raman spectroscopy (inVia Raman Renishaw) was used to investigate the effects of high

temperature brazing on the microstructure of CF bundles CNT yarns and reduced liquid

crystalline graphene oxide fiber (rLCGO) A 514 nm green laser with a power of 50 (15

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8

mW) was used for CF bundles while a power of 5 (15 mW) was used for 15 microm CNT

yarns and 40 microm rLCGO fiber to avoid ablation damage Samples were scanned before and

after high temperature brazing (1003-1030˚C) The laser was focused on the center of the

sample and an exposure time of 60 seconds was used Acquired signals were processed in

Matlab using a linear baseline correction procedure followed by Gaussian fitting to resolve

peaks

22 Device fabrication and electrical characterization

221 PCD circuit substrates

Thermal grade PCD substrates (TM-500 Element Six) with a thickness of 500 microm were laser

engraved using a 532 nm NdYAG laser (Oxford lasers) at a pulse frequency of 10 kHz A

boiling solution of sodium nitrate and sulfuric acid was used to remove debris from the

engraved pattern followed by 5 minutes of ultrasonication in acetone The PCD substrates

were rinsed with isopropanol and deionized water then dried with nitrogen Groove width

was correlated with laser milling power using scanning electron microscopy (SEM) images

ABA paste was spread over the PCD substrate Samples were transferred to a 10 times 10 mm

graphite wafer placed inside an electron beam deposition system (Thermionics) and

evacuated to 1 times 10-6 Pa The electron beam was focused onto the graphite substrate rather

than the diamond sample and current was increased until both the graphite and PCD were

glowing red ultimately leading to the ABA melting and spreading over the substrate and into

the laser-milled patterns Current was decreased over the course of 1 minute and the samples

were cooled over the course of 1 hour in a N2 environment The brazed samples were

removed from the deposition chamber and mechanical polishing was performed with a

diamond-embedded polishing plate (Struers Inc) using deionized water as a coolant The

samples were polished until embedded Ag ABA or Au ABA tracks were revealed and

residual metals between patterns removed ensuring no shorting between interconnects

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9

Resistance between isolated interconnects was measured to ensure no shorting (eg gtMΩ

resistance) between unconnected tracks If shorting did occur due to residual unpolished

metal mechanical polishing was continued until it was removed The fabrication of AuPCD

circuit boards requires an additional brazing step to enable Au wetting (Fig 3)

Fig 3 - Construction of embedded-in-diamond Au ABA circuit structures using a Ag ABA adhesion layer (a-b) circuit patterns were laser cut into the diamond substrate and the surface treated with a combination of sulfuric acid and sodium nitrate (c) Ag-ABA paste was applied to the surface melted in an electron beam deposition system (d) The electron beam was continuously applied until a majority of Ag evaporated from the surface leaving behind a conformal film for Au adhesion (e) A Au-based paste was applied to the surface and melted with the aforementioned method (f) (g) The substrate was mechanically-polished revealing Au circuit structures (h) Finally connections or components were attached with laser welding soldering or wirebonding if necessary

222 Freestanding carbonaceous wires

MWNT forest was synthesized by catalytic chemical vapor deposition using acetylene gas as

the carbon source Carbon nanotubes in the 400 um tall forests typically had diameters of 10

nm The CNT yarns were drawn from the forest by pulling and twisting as described by

Zhang et al [31] Graphene oxide (GO) was prepared from intercalated graphene flakes The

liquid crystalline state of graphene oxide was exploited to wet-spin conductive fibers as

described previously [32] Unsized CF bundles having filament diameters of 7 microm were

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10

purchased from Goodfellow On a graphite substrate two mounds of ABA paste were placed

on opposite sides of a 10 times 10 mm graphite wafer Either a bundle of CFs (Goodfellow) a

single CNT yarn (15 microm) or a graphene oxide fiber (50 microm) was placed across the mounds

and pressed into them Additionally one PtIr wire (75 microm Goodfellow) was placed into each

of the ABA paste mounds The graphite substrate was transferred to an electron beam

deposition system (Thermionics) and evacuated to 1 times 10-6 Pa The electron beam was

focused onto the graphite substratemdashas described above for PCD circuit board constructionmdash

and the current was increased until the substrate was glowing red The aforementioned

process is illustrated in Fig 4

Fig 4 - (a) Fabrication of freestanding metalABACNT yarn wires Mounds of ABA paste were placed onto a graphite substrate and either CNT yarns CFs or graphene oxide fibers were pressed into them PtIr wires were pressed into the end for testing (b) The graphite substrate was heated until the Au formed beads on the surface (c) Upon cooling the entire wiremetal structure was easily lifted from the surface

Once melting of the ABA was observed the current was ramped down over 1 minute and the

samples were cooled for 30 minutes in an N2 environment Au-ABA samples were easily

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11

removed from the graphite substrate due to lack of reaction while considerable force was

needed to remove the Ag ABA and TiCuNi samples

223 Integrated PCDcarbonaceous electrode devices

Our group has previously published a method for integrating PCD substrates with conductive

diamond electrodes to produce hermetic feedthroughs [21] Integrating carbonaceous wire

electrodes in this case 7 microm diameter CF was done manually as depicted in Fig 5

Fig 5 - Method for placing individual CF into laser cut groove in PCD substrate and securing the CF with braze paste The braze paste is then ldquoscreen-printedrdquo with a squeegee to remove the excess braze from the surface of the PCD to minimize shorting upon braze melting

PCD substrates were laser machined as described above to create grooves for CF filaments to

rest in Next individual CF filaments were manually placed into the grooves and Ag ABA

paste was spread over the entire surface The surface was wiped with a mini squeegee

(cleanroom cloth wrapped around syringe needle) to remove excess braze paste from the

surface Finally the sample was heated to melt the braze as described above

224 Electrical characterization of devices

The resistance of the Au ABA interconnects embedded in the polished PCD substrates were

measured using a 2-way probe station and a multimeter (Keithley Instruments) Transmission

line devices were fabricated to correct for contact resistance The electrical behavior of the

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12

freestanding PtIrAuCNT yarn structures was characterized by performing voltage sweeps (-1

to +1 V) with a potentiostat (eDAQ) while measuring current

3 Results and Discussion

31 Embedded-in-diamond circuit substrates

311 Investigating PCD-braze interactions

Three different commercially-available active brazing alloy (ABA) pastes were assessed for

their ability to wet and bond to polycrystalline diamond (PCD) substrates The elemental

composition of these brazes is included in Table 1 of the Experimental section Ag ABA was

found to spread and adhere strongly to PCD which was likely facilitated by the formation of

an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig

6a) SEM and EDX elemental analysis of the PCDAg ABA interface reveals a Ti-rich region

of ~34 microm adjacent to the Ag ABA joint (Supplementary Information Fig S1) Conversely

Au ABA did not spread on the PCD surface upon melting instead forming a droplet with a

111deg contact angle on the surface (Fig 6b)

Fig 6 - (a) Ag ABA wets PCD and bonds strongly (b) Au ABA does not wet PCD forming a droplet that makes ~111deg contact angle with the surface (c) TiCuNi braze wets the PCD surface but does not bond The cooled TiCuNi droplet is easily pulled from the PCD surface with forceps (d) Au ABA wets the PCD following a previous Ag ABA brazing step implemented to create a conformal adhesion layer

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13

Though the manufacturers have included Ti as the active component in the Au ABA this

braze is unable to spread on carbonaceous materials Considering the elemental composition

of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack

of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active

component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in

the TiC-Au melt [33]

rarr + 132

Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which

is unsurprising considering the large difference in electronegativity (X) between Au

(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic

is a possibility but the electronegativity differences are less pronounced between Ti and Ag

(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this

reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based

medical devices [35] wetted the PCD surface upon heating but did not bond and was easily

removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and

bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)

This finding is consistent with what has been shown previously for TiCuNirsquos interaction with

carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding

between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between

the braze alloy and the material The stress induced by thermal mismatch (σTM) can be

described theoretically by Equation 3 [37]

13 ∙ ∆ (3)

Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the

braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the

temperature differential approximately 1000 K for the difference between brazing

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14

temperature and room temperature According to manufacturer data (Element Six) the PCD

used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates

have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203

ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the

TiCuNiPCD system is warranted though beyond the scope of the present work In

consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate

compatible with standard joining technologies (ie wirebonding laser welding reflow

soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system

a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a

subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If

a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded

thick- film conductors or contact pads required for the embedded-in-diamond circuit

structures described in the following sections Additionally the ubiquity of Au contacts in

semiconductor and electronics fabrication makes the AuPCD system a practical device

substrate

312 Device fabrication and characterization

The entire fabrication process for these devices is illustrated above in the Experimental

section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate

prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished

devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far

with this technique range between 6-15 microm in width Lower milling powers lead to less

reproducibility perhaps due to issues with focusing on the surface which would be more

pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are

likely possible following more extensive laser parameter optimization including adjustment

of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices

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15

were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au

ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled

structures having three dimensions which demonstrates a significant advantage over

conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing

process eliminates the extra annealing step to form TiC at the interface [40] The resistance of

Au ABA interconnects embedded in PCD was measured using a transmission line set-up to

correct for contact resistance The relationship between the resistance of a 500 microm length Au

interconnect and laser power utilized to mill the track is shown in Fig 7d

Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found

The 3D nature of the interconnect leads to a significant decrease in electrical resistance

compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is

material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In

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16

addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal

for thermal management applications due to diamondrsquos extremely high thermal conductivity

(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining

technologies is paramount Both varieties of braze are easily joined via laser welding

wirebonding or reflow soldering (Supplementary Information Fig S3)

32 Freestanding carbonaceous wires

321 Investigating braze-wire interactions

The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not

been studied as far as we are aware As a result the surface interaction of these materials was

investigated initially using PCD as a substrate PCD was originally chosen due to its low

electrical conductivity making it an adequate support structure for electrical characterization

However the interaction between PCD and Au though non-wetting is significant enough to

prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an

individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either

material though some bonding between Au ABA and PCD occurs A similar effect was

observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of

the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting

behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact

angle of ~404deg (Fig 8c)

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17

Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)

In the interest of creating freestanding carbonaceous wires with metallic contacts we

investigated whether the lack of wetting between a graphite substrate and braze alloy could be

exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate

(Fig 9) It has been observed previously that surface roughness can lead to large increases in

contact angle during brazing making graphite a reasonable choice to perhaps further diminish

the wetting behavior [42]

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18

Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)

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19

A graphite substrate was laser milled in an attempt to control separation distance between the

contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate

a number of geometries are possible for the freestanding wires Ag ABA was able to both wet

and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag

ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig

9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact

with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction

with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure

to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a

seal around the bundle effectively compressing the filaments together (Fig 9h) However if

enough force is applied the Au contact can be removed from the CF bundle though this was

not observed for CNT yarn devices This effect is likely proportional to the ratio of wire

diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of

the bond due to a rise in the contact surface area

322 Device fabrication and characterization

A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or

AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred

because it does not form a strong bond to the graphite substrate following a melting cycle so

the entire wire can be lifted off of the substrate with the contacts intact preventing damage to

the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing

facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent

lift-off was attempted Au does not bond to the graphite for reasons explained above When

the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing

both the carbonaceous fiber and a metallic wire within it (Fig 10a)

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20

Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample

The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig

10c) This technique is a potential improvement over mechanical clamps especially

considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from

clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a

woven or spun material such as CNT yarn prevents unwinding and generally helps to keep

the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead

to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)

were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a

linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the

CNT yarns in this work was comparable to those of similar size and length measured by

Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn

structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While

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21

preliminary measurements suggest that contact resistances are negligible further investigation

is needed to estimate the contact resistance through the contact It was of interest to determine

whether this process modified the carbon structure of the wire materials To investigate this

phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire

following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)

Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1

The before and after Raman spectra of CFs reveals very little change in the microstructure

aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This

peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a

prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor

graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears

quite prominent in both CF spectra presented here though there appears to have been some

MANUSCRIP

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22

moderate increase in graphitization following brazing Additionally the loss of the D-peak

overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to

increased graphitization However it is important to note that the graphitization process

during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to

create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several

peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)

Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1

(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible

that the loss of these peaks is the result of the loss of a surface functionality as a result of high

temperature treatment such as a C-O group However further investigation is required to

acquire a full understanding of the structural changes occurring not only as a result of the high

temperature processes in the presence of various metals In an attempt to improve the thermal

and electrical properties of CNTs and composites gamma irradiation and high temperature

treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease

in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in

the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal

treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that

the vacuum brazing processes described here do not have deleterious effects on carbonaceous

wires

33 Integrating diamond substrates with carbonaceous electrodes

Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)

nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective

MANUSCRIP

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23

processes for integrating these materials with a suitable substrate differ considerably For N-

UNCD stimulating electrodes our group has demonstrated a method to grow the conductive

diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]

This method produces a hermetic electrode array for implantable neural prosthetic

applications However the integration of this array with standard electronic components

requires a metallic interface utilizing the embedded circuit board technology described here

A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze

interconnects and contact pads utilized to contact conductive diamond electrodes (dotted

circle) in a full electrode array device (Fig 12a)

Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes

Incorporating carbonaceous wire electrodes into electronic devices is less elegant and

currently requires the manual placement of individual wire electrodes into designated laser-

MANUSCRIP

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24

milled grooves in the PCD substrate Next the suitable brazing paste is applied though a

screen-printing technique is required to bypass the mechanical polishing step Avoiding

mechanical polishing is important for delicate small and fragile electrode materials such as

those described in this work However because the brazing pastes are comprised of both

metal beads and a polymeric binder the functional volume of the interconnect is reduced

upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed

to produce a CF array embedded in diamond (Fig 12c) We are currently investigating

methods for protecting the CFs and other carbonaceous wires during mechanical polishing to

enable more complex robust device architectures

4 Conclusion

Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires

(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all

components paramount For PCD electronics Ag ABA is used due to its wetting and covalent

bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in

physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial

layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the

AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit

boards are compatible with commercially-available solders and are easily laser-welded for

solder-free joining The non-wetting method of enclosing carbonaceous wires was used to

create strong low-resistance and non-rectifying connections between PtIr wires and

carbonaceous wires Structural and interfacial examinations of these structures lend insight

into the forces at play during both the wetting and non-wetting fabrication of carbonaceous

devices We exploit surface tension effects of two commercially-available active brazing

alloys to create two different device architectures

(1) The wetting technique was used in conjunction with mechanical polishing to create

robust circuit substrates with embedded Ag or Au interconnects and contact pads The option

MANUSCRIP

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25

to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate

illustrates the utility of the method for enabling carbon-based electrode arrays

(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or

AuPtIr wire contacts This technique is interesting for the creation of not only free-standing

carbonaceous conductors but also for neural probes or sensors for both central and peripheral

nervous system integration We have previously described methods to fabricate neural

stimulation and recording electrodes from carbonaceous wires described here such as rLCGO

fiber which demonstrated both in vitro and in vivo functionality [47]

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for

insightful technical conversations NVA is supported by a MMI-CSIRO Material Science

PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was

supported by the Australian Research Council (ARC) through its Special Research Initiative

(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is

supported by the Australian Research Council under Discovery Early Career Researcher

award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and

support through the ARC Centre of Excellence for Electromaterials Science

Supplementary Information Available Figures S1-S4 This material is available free of charge online References

MANUSCRIP

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ACCEPTED MANUSCRIPT

26

[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

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27

[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

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28

[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

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4

11 Brazing to carbon

Active brazing a variation of vacuum brazing utilizes an alloy containing an active

component that reacts chemically with a relatively inert surface such as diamond or ceramics

[22 23] Transition metals are of particular interest due to electron vacancies in their d-

orbitals with greater numbers of vacancies leading to greater reactivity (overlap) with

carbonrsquos p-orbitals [24] In the case of carbon substrates the active componentmdashtypically Ti

V or Crmdashforms a carbide interface layer that acts as a surfactant to enable the wetting of

other filler metals and produce non-rectifying electrical contacts [25 26] A Ag-based active

brazing alloy (ABA) has been used previously to make electrical contact to a CNT bundle

and formation of the TiC interface layer was confirmed with X-ray photon scattering (XPS)

though the electrical and mechanical properties were not fully characterized [16] The

formation of interfacial TiC particles has also been shown to significantly increase the tensile

strength of carbon composites brazed with Ti-based alloys containing CNT reinforcements

[27] In the present work commercially-available ABAs were investigated to determine their

suitability to make electronic connections to several carbonaceous materials including

polycrystalline diamond (PCD) CNT yarns graphene oxide (GO) fibers and PAN-based

carbon fiber (CF) to enable various types of electronic devices (Fig 1)

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5

Fig 1 - ABAs were used to fabricate two basic electronic device platforms (a) a circuit board embedded into a polycrystalline diamond (PCD) substrate and (b) a freestanding carbonaceous wire with electrical connections on one or both ends The aforementioned technologies had lead to the fabrication of medical devices containing active carbon electrodes such as (c) conductive diamond electrodes and (d) carbonaceous wire electrodes

In the first technique described here ABAs were used to create embedded circuit boards in

PCD wherein the solidified ABA forms the interconnects and contact pads following

mechanical polishing The second metallization system involves making free-standing Au

contacts to carbonaceous fibers using a graphite ldquolift-offrdquo method which exploits the balance

of cohesive and adhesive forces of the liquid metalgraphite interface (Equation 1)

cos

1131

Where θ is the contact angle between the substrate and liquid metal WA is the work of

adhesion (adhesion force) between the substrate and the liquid metal and σL is the cohesion

force (liquid-vapour surface tension) within the liquid metal droplet [28] Equation 1

describes the balance of forces at the liquid metalsolid substrate interface wherein liquid

metals (intrinsically very high cohesion force materials) tend to create large contact angles on

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6

most substrates unless σL lt WA where contact angles lt90˚ are considered ldquowettingrdquo In one

facet of this work we use active brazing to modify WA and create robust electrical

connections to carbonaceous materials such as PCD and CF In another technique we exploit

the lack of adhesion and wetting between graphitic carbon and Au as well as the cohesion

forces within the liquid metal droplet to create a freestanding carbonaceous wire with

metallic connections Broadly speaking the techniques described here are applicable to the

development of devices in which a strong biocompatible and non-rectifying connection to

carbon-based materials is required

2 Experimental

21 Surface interactions brazing alloys and carbonaceous materials

211 Sample preparation and wettability studies

Thermal grade polycrystalline diamond (PCD) substrates (TM-250 Element Six) with a

thickness of 250 microm were laser-cut into 5 times 5 mm squares with a 532 nm NdYAG laser

(Oxford Lasers) Approximately 65 mg of either TiCuNi Ag ABA or Au ABA paste were

placed ontop of either the PCD substrate the PCD substrate with a carbonaceous wire or a

graphite substrate with a carbonaceous wire The elemental composition and melting points of

all braze alloys tested is listed in Table 1

ABA (Manufacturer)

Ag () Au () Cu () Al () Ti () Ni () MP (degC)

Ag-ABA (Wesgo)

9275 - 5 1 125 - 912

Au-ABA (Morgan)

- 964 - - 06 3 1003

TiCuNi (Wesgo)

- - 15 - 70 15 960

Table 1 Elemental composition and melting point (MP) of the braze alloys used in this study

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7

Samples were placed in a brazing furnace under vacuum and heated until the braze was

visibly melted and then held at the melting temperature for 1 minute Upon cooling samples

were imaged using either an optical microscope or scanning electron microscope (SEM)

Contact angles were measured using the low-bond axisymmetric drop shape analysis in the

Droplet-Analysis package in ImageJ image processing software [29 30]

212 Elemental analysis of brazemdashcarbon interface

Some diamond samples were laser milled with deep grooves prior to brazing enabling

visualization and characterization of the interface with both SEM and energy-dispersive X-ray

spectroscopy (EDX)(Supplementary Information Fig S1) The metalcarbonaceous wire

interfaces were studied by embedding brazed samples in epoxy resin (Araldite) and

mechanically-polishing them to reveal the cross-section (Fig 2)

Fig 2 ndash Embedding and polishing method utilized to visualize the carbonmdashbraze interface during both SEM and EDX analysis

SEM images were acquired and EDX was used to identify elemental composition at the

interface of both Ag-ABACF and Au-ABACF joints

213 Raman spectroscopy

Raman spectroscopy (inVia Raman Renishaw) was used to investigate the effects of high

temperature brazing on the microstructure of CF bundles CNT yarns and reduced liquid

crystalline graphene oxide fiber (rLCGO) A 514 nm green laser with a power of 50 (15

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T

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8

mW) was used for CF bundles while a power of 5 (15 mW) was used for 15 microm CNT

yarns and 40 microm rLCGO fiber to avoid ablation damage Samples were scanned before and

after high temperature brazing (1003-1030˚C) The laser was focused on the center of the

sample and an exposure time of 60 seconds was used Acquired signals were processed in

Matlab using a linear baseline correction procedure followed by Gaussian fitting to resolve

peaks

22 Device fabrication and electrical characterization

221 PCD circuit substrates

Thermal grade PCD substrates (TM-500 Element Six) with a thickness of 500 microm were laser

engraved using a 532 nm NdYAG laser (Oxford lasers) at a pulse frequency of 10 kHz A

boiling solution of sodium nitrate and sulfuric acid was used to remove debris from the

engraved pattern followed by 5 minutes of ultrasonication in acetone The PCD substrates

were rinsed with isopropanol and deionized water then dried with nitrogen Groove width

was correlated with laser milling power using scanning electron microscopy (SEM) images

ABA paste was spread over the PCD substrate Samples were transferred to a 10 times 10 mm

graphite wafer placed inside an electron beam deposition system (Thermionics) and

evacuated to 1 times 10-6 Pa The electron beam was focused onto the graphite substrate rather

than the diamond sample and current was increased until both the graphite and PCD were

glowing red ultimately leading to the ABA melting and spreading over the substrate and into

the laser-milled patterns Current was decreased over the course of 1 minute and the samples

were cooled over the course of 1 hour in a N2 environment The brazed samples were

removed from the deposition chamber and mechanical polishing was performed with a

diamond-embedded polishing plate (Struers Inc) using deionized water as a coolant The

samples were polished until embedded Ag ABA or Au ABA tracks were revealed and

residual metals between patterns removed ensuring no shorting between interconnects

MANUSCRIP

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9

Resistance between isolated interconnects was measured to ensure no shorting (eg gtMΩ

resistance) between unconnected tracks If shorting did occur due to residual unpolished

metal mechanical polishing was continued until it was removed The fabrication of AuPCD

circuit boards requires an additional brazing step to enable Au wetting (Fig 3)

Fig 3 - Construction of embedded-in-diamond Au ABA circuit structures using a Ag ABA adhesion layer (a-b) circuit patterns were laser cut into the diamond substrate and the surface treated with a combination of sulfuric acid and sodium nitrate (c) Ag-ABA paste was applied to the surface melted in an electron beam deposition system (d) The electron beam was continuously applied until a majority of Ag evaporated from the surface leaving behind a conformal film for Au adhesion (e) A Au-based paste was applied to the surface and melted with the aforementioned method (f) (g) The substrate was mechanically-polished revealing Au circuit structures (h) Finally connections or components were attached with laser welding soldering or wirebonding if necessary

222 Freestanding carbonaceous wires

MWNT forest was synthesized by catalytic chemical vapor deposition using acetylene gas as

the carbon source Carbon nanotubes in the 400 um tall forests typically had diameters of 10

nm The CNT yarns were drawn from the forest by pulling and twisting as described by

Zhang et al [31] Graphene oxide (GO) was prepared from intercalated graphene flakes The

liquid crystalline state of graphene oxide was exploited to wet-spin conductive fibers as

described previously [32] Unsized CF bundles having filament diameters of 7 microm were

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10

purchased from Goodfellow On a graphite substrate two mounds of ABA paste were placed

on opposite sides of a 10 times 10 mm graphite wafer Either a bundle of CFs (Goodfellow) a

single CNT yarn (15 microm) or a graphene oxide fiber (50 microm) was placed across the mounds

and pressed into them Additionally one PtIr wire (75 microm Goodfellow) was placed into each

of the ABA paste mounds The graphite substrate was transferred to an electron beam

deposition system (Thermionics) and evacuated to 1 times 10-6 Pa The electron beam was

focused onto the graphite substratemdashas described above for PCD circuit board constructionmdash

and the current was increased until the substrate was glowing red The aforementioned

process is illustrated in Fig 4

Fig 4 - (a) Fabrication of freestanding metalABACNT yarn wires Mounds of ABA paste were placed onto a graphite substrate and either CNT yarns CFs or graphene oxide fibers were pressed into them PtIr wires were pressed into the end for testing (b) The graphite substrate was heated until the Au formed beads on the surface (c) Upon cooling the entire wiremetal structure was easily lifted from the surface

Once melting of the ABA was observed the current was ramped down over 1 minute and the

samples were cooled for 30 minutes in an N2 environment Au-ABA samples were easily

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11

removed from the graphite substrate due to lack of reaction while considerable force was

needed to remove the Ag ABA and TiCuNi samples

223 Integrated PCDcarbonaceous electrode devices

Our group has previously published a method for integrating PCD substrates with conductive

diamond electrodes to produce hermetic feedthroughs [21] Integrating carbonaceous wire

electrodes in this case 7 microm diameter CF was done manually as depicted in Fig 5

Fig 5 - Method for placing individual CF into laser cut groove in PCD substrate and securing the CF with braze paste The braze paste is then ldquoscreen-printedrdquo with a squeegee to remove the excess braze from the surface of the PCD to minimize shorting upon braze melting

PCD substrates were laser machined as described above to create grooves for CF filaments to

rest in Next individual CF filaments were manually placed into the grooves and Ag ABA

paste was spread over the entire surface The surface was wiped with a mini squeegee

(cleanroom cloth wrapped around syringe needle) to remove excess braze paste from the

surface Finally the sample was heated to melt the braze as described above

224 Electrical characterization of devices

The resistance of the Au ABA interconnects embedded in the polished PCD substrates were

measured using a 2-way probe station and a multimeter (Keithley Instruments) Transmission

line devices were fabricated to correct for contact resistance The electrical behavior of the

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12

freestanding PtIrAuCNT yarn structures was characterized by performing voltage sweeps (-1

to +1 V) with a potentiostat (eDAQ) while measuring current

3 Results and Discussion

31 Embedded-in-diamond circuit substrates

311 Investigating PCD-braze interactions

Three different commercially-available active brazing alloy (ABA) pastes were assessed for

their ability to wet and bond to polycrystalline diamond (PCD) substrates The elemental

composition of these brazes is included in Table 1 of the Experimental section Ag ABA was

found to spread and adhere strongly to PCD which was likely facilitated by the formation of

an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig

6a) SEM and EDX elemental analysis of the PCDAg ABA interface reveals a Ti-rich region

of ~34 microm adjacent to the Ag ABA joint (Supplementary Information Fig S1) Conversely

Au ABA did not spread on the PCD surface upon melting instead forming a droplet with a

111deg contact angle on the surface (Fig 6b)

Fig 6 - (a) Ag ABA wets PCD and bonds strongly (b) Au ABA does not wet PCD forming a droplet that makes ~111deg contact angle with the surface (c) TiCuNi braze wets the PCD surface but does not bond The cooled TiCuNi droplet is easily pulled from the PCD surface with forceps (d) Au ABA wets the PCD following a previous Ag ABA brazing step implemented to create a conformal adhesion layer

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13

Though the manufacturers have included Ti as the active component in the Au ABA this

braze is unable to spread on carbonaceous materials Considering the elemental composition

of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack

of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active

component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in

the TiC-Au melt [33]

rarr + 132

Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which

is unsurprising considering the large difference in electronegativity (X) between Au

(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic

is a possibility but the electronegativity differences are less pronounced between Ti and Ag

(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this

reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based

medical devices [35] wetted the PCD surface upon heating but did not bond and was easily

removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and

bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)

This finding is consistent with what has been shown previously for TiCuNirsquos interaction with

carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding

between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between

the braze alloy and the material The stress induced by thermal mismatch (σTM) can be

described theoretically by Equation 3 [37]

13 ∙ ∆ (3)

Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the

braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the

temperature differential approximately 1000 K for the difference between brazing

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14

temperature and room temperature According to manufacturer data (Element Six) the PCD

used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates

have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203

ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the

TiCuNiPCD system is warranted though beyond the scope of the present work In

consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate

compatible with standard joining technologies (ie wirebonding laser welding reflow

soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system

a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a

subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If

a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded

thick- film conductors or contact pads required for the embedded-in-diamond circuit

structures described in the following sections Additionally the ubiquity of Au contacts in

semiconductor and electronics fabrication makes the AuPCD system a practical device

substrate

312 Device fabrication and characterization

The entire fabrication process for these devices is illustrated above in the Experimental

section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate

prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished

devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far

with this technique range between 6-15 microm in width Lower milling powers lead to less

reproducibility perhaps due to issues with focusing on the surface which would be more

pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are

likely possible following more extensive laser parameter optimization including adjustment

of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices

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15

were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au

ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled

structures having three dimensions which demonstrates a significant advantage over

conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing

process eliminates the extra annealing step to form TiC at the interface [40] The resistance of

Au ABA interconnects embedded in PCD was measured using a transmission line set-up to

correct for contact resistance The relationship between the resistance of a 500 microm length Au

interconnect and laser power utilized to mill the track is shown in Fig 7d

Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found

The 3D nature of the interconnect leads to a significant decrease in electrical resistance

compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is

material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In

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16

addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal

for thermal management applications due to diamondrsquos extremely high thermal conductivity

(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining

technologies is paramount Both varieties of braze are easily joined via laser welding

wirebonding or reflow soldering (Supplementary Information Fig S3)

32 Freestanding carbonaceous wires

321 Investigating braze-wire interactions

The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not

been studied as far as we are aware As a result the surface interaction of these materials was

investigated initially using PCD as a substrate PCD was originally chosen due to its low

electrical conductivity making it an adequate support structure for electrical characterization

However the interaction between PCD and Au though non-wetting is significant enough to

prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an

individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either

material though some bonding between Au ABA and PCD occurs A similar effect was

observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of

the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting

behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact

angle of ~404deg (Fig 8c)

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17

Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)

In the interest of creating freestanding carbonaceous wires with metallic contacts we

investigated whether the lack of wetting between a graphite substrate and braze alloy could be

exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate

(Fig 9) It has been observed previously that surface roughness can lead to large increases in

contact angle during brazing making graphite a reasonable choice to perhaps further diminish

the wetting behavior [42]

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T

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18

Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)

MANUSCRIP

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19

A graphite substrate was laser milled in an attempt to control separation distance between the

contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate

a number of geometries are possible for the freestanding wires Ag ABA was able to both wet

and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag

ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig

9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact

with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction

with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure

to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a

seal around the bundle effectively compressing the filaments together (Fig 9h) However if

enough force is applied the Au contact can be removed from the CF bundle though this was

not observed for CNT yarn devices This effect is likely proportional to the ratio of wire

diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of

the bond due to a rise in the contact surface area

322 Device fabrication and characterization

A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or

AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred

because it does not form a strong bond to the graphite substrate following a melting cycle so

the entire wire can be lifted off of the substrate with the contacts intact preventing damage to

the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing

facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent

lift-off was attempted Au does not bond to the graphite for reasons explained above When

the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing

both the carbonaceous fiber and a metallic wire within it (Fig 10a)

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ACCEPTED MANUSCRIPT

20

Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample

The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig

10c) This technique is a potential improvement over mechanical clamps especially

considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from

clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a

woven or spun material such as CNT yarn prevents unwinding and generally helps to keep

the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead

to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)

were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a

linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the

CNT yarns in this work was comparable to those of similar size and length measured by

Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn

structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While

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21

preliminary measurements suggest that contact resistances are negligible further investigation

is needed to estimate the contact resistance through the contact It was of interest to determine

whether this process modified the carbon structure of the wire materials To investigate this

phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire

following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)

Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1

The before and after Raman spectra of CFs reveals very little change in the microstructure

aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This

peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a

prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor

graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears

quite prominent in both CF spectra presented here though there appears to have been some

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22

moderate increase in graphitization following brazing Additionally the loss of the D-peak

overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to

increased graphitization However it is important to note that the graphitization process

during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to

create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several

peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)

Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1

(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible

that the loss of these peaks is the result of the loss of a surface functionality as a result of high

temperature treatment such as a C-O group However further investigation is required to

acquire a full understanding of the structural changes occurring not only as a result of the high

temperature processes in the presence of various metals In an attempt to improve the thermal

and electrical properties of CNTs and composites gamma irradiation and high temperature

treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease

in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in

the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal

treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that

the vacuum brazing processes described here do not have deleterious effects on carbonaceous

wires

33 Integrating diamond substrates with carbonaceous electrodes

Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)

nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective

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23

processes for integrating these materials with a suitable substrate differ considerably For N-

UNCD stimulating electrodes our group has demonstrated a method to grow the conductive

diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]

This method produces a hermetic electrode array for implantable neural prosthetic

applications However the integration of this array with standard electronic components

requires a metallic interface utilizing the embedded circuit board technology described here

A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze

interconnects and contact pads utilized to contact conductive diamond electrodes (dotted

circle) in a full electrode array device (Fig 12a)

Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes

Incorporating carbonaceous wire electrodes into electronic devices is less elegant and

currently requires the manual placement of individual wire electrodes into designated laser-

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24

milled grooves in the PCD substrate Next the suitable brazing paste is applied though a

screen-printing technique is required to bypass the mechanical polishing step Avoiding

mechanical polishing is important for delicate small and fragile electrode materials such as

those described in this work However because the brazing pastes are comprised of both

metal beads and a polymeric binder the functional volume of the interconnect is reduced

upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed

to produce a CF array embedded in diamond (Fig 12c) We are currently investigating

methods for protecting the CFs and other carbonaceous wires during mechanical polishing to

enable more complex robust device architectures

4 Conclusion

Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires

(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all

components paramount For PCD electronics Ag ABA is used due to its wetting and covalent

bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in

physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial

layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the

AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit

boards are compatible with commercially-available solders and are easily laser-welded for

solder-free joining The non-wetting method of enclosing carbonaceous wires was used to

create strong low-resistance and non-rectifying connections between PtIr wires and

carbonaceous wires Structural and interfacial examinations of these structures lend insight

into the forces at play during both the wetting and non-wetting fabrication of carbonaceous

devices We exploit surface tension effects of two commercially-available active brazing

alloys to create two different device architectures

(1) The wetting technique was used in conjunction with mechanical polishing to create

robust circuit substrates with embedded Ag or Au interconnects and contact pads The option

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25

to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate

illustrates the utility of the method for enabling carbon-based electrode arrays

(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or

AuPtIr wire contacts This technique is interesting for the creation of not only free-standing

carbonaceous conductors but also for neural probes or sensors for both central and peripheral

nervous system integration We have previously described methods to fabricate neural

stimulation and recording electrodes from carbonaceous wires described here such as rLCGO

fiber which demonstrated both in vitro and in vivo functionality [47]

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for

insightful technical conversations NVA is supported by a MMI-CSIRO Material Science

PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was

supported by the Australian Research Council (ARC) through its Special Research Initiative

(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is

supported by the Australian Research Council under Discovery Early Career Researcher

award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and

support through the ARC Centre of Excellence for Electromaterials Science

Supplementary Information Available Figures S1-S4 This material is available free of charge online References

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26

[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

MANUSCRIP

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27

[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

MANUSCRIP

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ACCEPTED MANUSCRIPT

28

[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

MANUSCRIP

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ACCEPTED MANUSCRIPT

5

Fig 1 - ABAs were used to fabricate two basic electronic device platforms (a) a circuit board embedded into a polycrystalline diamond (PCD) substrate and (b) a freestanding carbonaceous wire with electrical connections on one or both ends The aforementioned technologies had lead to the fabrication of medical devices containing active carbon electrodes such as (c) conductive diamond electrodes and (d) carbonaceous wire electrodes

In the first technique described here ABAs were used to create embedded circuit boards in

PCD wherein the solidified ABA forms the interconnects and contact pads following

mechanical polishing The second metallization system involves making free-standing Au

contacts to carbonaceous fibers using a graphite ldquolift-offrdquo method which exploits the balance

of cohesive and adhesive forces of the liquid metalgraphite interface (Equation 1)

cos

1131

Where θ is the contact angle between the substrate and liquid metal WA is the work of

adhesion (adhesion force) between the substrate and the liquid metal and σL is the cohesion

force (liquid-vapour surface tension) within the liquid metal droplet [28] Equation 1

describes the balance of forces at the liquid metalsolid substrate interface wherein liquid

metals (intrinsically very high cohesion force materials) tend to create large contact angles on

MANUSCRIP

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6

most substrates unless σL lt WA where contact angles lt90˚ are considered ldquowettingrdquo In one

facet of this work we use active brazing to modify WA and create robust electrical

connections to carbonaceous materials such as PCD and CF In another technique we exploit

the lack of adhesion and wetting between graphitic carbon and Au as well as the cohesion

forces within the liquid metal droplet to create a freestanding carbonaceous wire with

metallic connections Broadly speaking the techniques described here are applicable to the

development of devices in which a strong biocompatible and non-rectifying connection to

carbon-based materials is required

2 Experimental

21 Surface interactions brazing alloys and carbonaceous materials

211 Sample preparation and wettability studies

Thermal grade polycrystalline diamond (PCD) substrates (TM-250 Element Six) with a

thickness of 250 microm were laser-cut into 5 times 5 mm squares with a 532 nm NdYAG laser

(Oxford Lasers) Approximately 65 mg of either TiCuNi Ag ABA or Au ABA paste were

placed ontop of either the PCD substrate the PCD substrate with a carbonaceous wire or a

graphite substrate with a carbonaceous wire The elemental composition and melting points of

all braze alloys tested is listed in Table 1

ABA (Manufacturer)

Ag () Au () Cu () Al () Ti () Ni () MP (degC)

Ag-ABA (Wesgo)

9275 - 5 1 125 - 912

Au-ABA (Morgan)

- 964 - - 06 3 1003

TiCuNi (Wesgo)

- - 15 - 70 15 960

Table 1 Elemental composition and melting point (MP) of the braze alloys used in this study

MANUSCRIP

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7

Samples were placed in a brazing furnace under vacuum and heated until the braze was

visibly melted and then held at the melting temperature for 1 minute Upon cooling samples

were imaged using either an optical microscope or scanning electron microscope (SEM)

Contact angles were measured using the low-bond axisymmetric drop shape analysis in the

Droplet-Analysis package in ImageJ image processing software [29 30]

212 Elemental analysis of brazemdashcarbon interface

Some diamond samples were laser milled with deep grooves prior to brazing enabling

visualization and characterization of the interface with both SEM and energy-dispersive X-ray

spectroscopy (EDX)(Supplementary Information Fig S1) The metalcarbonaceous wire

interfaces were studied by embedding brazed samples in epoxy resin (Araldite) and

mechanically-polishing them to reveal the cross-section (Fig 2)

Fig 2 ndash Embedding and polishing method utilized to visualize the carbonmdashbraze interface during both SEM and EDX analysis

SEM images were acquired and EDX was used to identify elemental composition at the

interface of both Ag-ABACF and Au-ABACF joints

213 Raman spectroscopy

Raman spectroscopy (inVia Raman Renishaw) was used to investigate the effects of high

temperature brazing on the microstructure of CF bundles CNT yarns and reduced liquid

crystalline graphene oxide fiber (rLCGO) A 514 nm green laser with a power of 50 (15

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8

mW) was used for CF bundles while a power of 5 (15 mW) was used for 15 microm CNT

yarns and 40 microm rLCGO fiber to avoid ablation damage Samples were scanned before and

after high temperature brazing (1003-1030˚C) The laser was focused on the center of the

sample and an exposure time of 60 seconds was used Acquired signals were processed in

Matlab using a linear baseline correction procedure followed by Gaussian fitting to resolve

peaks

22 Device fabrication and electrical characterization

221 PCD circuit substrates

Thermal grade PCD substrates (TM-500 Element Six) with a thickness of 500 microm were laser

engraved using a 532 nm NdYAG laser (Oxford lasers) at a pulse frequency of 10 kHz A

boiling solution of sodium nitrate and sulfuric acid was used to remove debris from the

engraved pattern followed by 5 minutes of ultrasonication in acetone The PCD substrates

were rinsed with isopropanol and deionized water then dried with nitrogen Groove width

was correlated with laser milling power using scanning electron microscopy (SEM) images

ABA paste was spread over the PCD substrate Samples were transferred to a 10 times 10 mm

graphite wafer placed inside an electron beam deposition system (Thermionics) and

evacuated to 1 times 10-6 Pa The electron beam was focused onto the graphite substrate rather

than the diamond sample and current was increased until both the graphite and PCD were

glowing red ultimately leading to the ABA melting and spreading over the substrate and into

the laser-milled patterns Current was decreased over the course of 1 minute and the samples

were cooled over the course of 1 hour in a N2 environment The brazed samples were

removed from the deposition chamber and mechanical polishing was performed with a

diamond-embedded polishing plate (Struers Inc) using deionized water as a coolant The

samples were polished until embedded Ag ABA or Au ABA tracks were revealed and

residual metals between patterns removed ensuring no shorting between interconnects

MANUSCRIP

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9

Resistance between isolated interconnects was measured to ensure no shorting (eg gtMΩ

resistance) between unconnected tracks If shorting did occur due to residual unpolished

metal mechanical polishing was continued until it was removed The fabrication of AuPCD

circuit boards requires an additional brazing step to enable Au wetting (Fig 3)

Fig 3 - Construction of embedded-in-diamond Au ABA circuit structures using a Ag ABA adhesion layer (a-b) circuit patterns were laser cut into the diamond substrate and the surface treated with a combination of sulfuric acid and sodium nitrate (c) Ag-ABA paste was applied to the surface melted in an electron beam deposition system (d) The electron beam was continuously applied until a majority of Ag evaporated from the surface leaving behind a conformal film for Au adhesion (e) A Au-based paste was applied to the surface and melted with the aforementioned method (f) (g) The substrate was mechanically-polished revealing Au circuit structures (h) Finally connections or components were attached with laser welding soldering or wirebonding if necessary

222 Freestanding carbonaceous wires

MWNT forest was synthesized by catalytic chemical vapor deposition using acetylene gas as

the carbon source Carbon nanotubes in the 400 um tall forests typically had diameters of 10

nm The CNT yarns were drawn from the forest by pulling and twisting as described by

Zhang et al [31] Graphene oxide (GO) was prepared from intercalated graphene flakes The

liquid crystalline state of graphene oxide was exploited to wet-spin conductive fibers as

described previously [32] Unsized CF bundles having filament diameters of 7 microm were

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10

purchased from Goodfellow On a graphite substrate two mounds of ABA paste were placed

on opposite sides of a 10 times 10 mm graphite wafer Either a bundle of CFs (Goodfellow) a

single CNT yarn (15 microm) or a graphene oxide fiber (50 microm) was placed across the mounds

and pressed into them Additionally one PtIr wire (75 microm Goodfellow) was placed into each

of the ABA paste mounds The graphite substrate was transferred to an electron beam

deposition system (Thermionics) and evacuated to 1 times 10-6 Pa The electron beam was

focused onto the graphite substratemdashas described above for PCD circuit board constructionmdash

and the current was increased until the substrate was glowing red The aforementioned

process is illustrated in Fig 4

Fig 4 - (a) Fabrication of freestanding metalABACNT yarn wires Mounds of ABA paste were placed onto a graphite substrate and either CNT yarns CFs or graphene oxide fibers were pressed into them PtIr wires were pressed into the end for testing (b) The graphite substrate was heated until the Au formed beads on the surface (c) Upon cooling the entire wiremetal structure was easily lifted from the surface

Once melting of the ABA was observed the current was ramped down over 1 minute and the

samples were cooled for 30 minutes in an N2 environment Au-ABA samples were easily

MANUSCRIP

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11

removed from the graphite substrate due to lack of reaction while considerable force was

needed to remove the Ag ABA and TiCuNi samples

223 Integrated PCDcarbonaceous electrode devices

Our group has previously published a method for integrating PCD substrates with conductive

diamond electrodes to produce hermetic feedthroughs [21] Integrating carbonaceous wire

electrodes in this case 7 microm diameter CF was done manually as depicted in Fig 5

Fig 5 - Method for placing individual CF into laser cut groove in PCD substrate and securing the CF with braze paste The braze paste is then ldquoscreen-printedrdquo with a squeegee to remove the excess braze from the surface of the PCD to minimize shorting upon braze melting

PCD substrates were laser machined as described above to create grooves for CF filaments to

rest in Next individual CF filaments were manually placed into the grooves and Ag ABA

paste was spread over the entire surface The surface was wiped with a mini squeegee

(cleanroom cloth wrapped around syringe needle) to remove excess braze paste from the

surface Finally the sample was heated to melt the braze as described above

224 Electrical characterization of devices

The resistance of the Au ABA interconnects embedded in the polished PCD substrates were

measured using a 2-way probe station and a multimeter (Keithley Instruments) Transmission

line devices were fabricated to correct for contact resistance The electrical behavior of the

MANUSCRIP

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12

freestanding PtIrAuCNT yarn structures was characterized by performing voltage sweeps (-1

to +1 V) with a potentiostat (eDAQ) while measuring current

3 Results and Discussion

31 Embedded-in-diamond circuit substrates

311 Investigating PCD-braze interactions

Three different commercially-available active brazing alloy (ABA) pastes were assessed for

their ability to wet and bond to polycrystalline diamond (PCD) substrates The elemental

composition of these brazes is included in Table 1 of the Experimental section Ag ABA was

found to spread and adhere strongly to PCD which was likely facilitated by the formation of

an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig

6a) SEM and EDX elemental analysis of the PCDAg ABA interface reveals a Ti-rich region

of ~34 microm adjacent to the Ag ABA joint (Supplementary Information Fig S1) Conversely

Au ABA did not spread on the PCD surface upon melting instead forming a droplet with a

111deg contact angle on the surface (Fig 6b)

Fig 6 - (a) Ag ABA wets PCD and bonds strongly (b) Au ABA does not wet PCD forming a droplet that makes ~111deg contact angle with the surface (c) TiCuNi braze wets the PCD surface but does not bond The cooled TiCuNi droplet is easily pulled from the PCD surface with forceps (d) Au ABA wets the PCD following a previous Ag ABA brazing step implemented to create a conformal adhesion layer

MANUSCRIP

T

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13

Though the manufacturers have included Ti as the active component in the Au ABA this

braze is unable to spread on carbonaceous materials Considering the elemental composition

of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack

of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active

component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in

the TiC-Au melt [33]

rarr + 132

Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which

is unsurprising considering the large difference in electronegativity (X) between Au

(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic

is a possibility but the electronegativity differences are less pronounced between Ti and Ag

(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this

reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based

medical devices [35] wetted the PCD surface upon heating but did not bond and was easily

removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and

bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)

This finding is consistent with what has been shown previously for TiCuNirsquos interaction with

carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding

between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between

the braze alloy and the material The stress induced by thermal mismatch (σTM) can be

described theoretically by Equation 3 [37]

13 ∙ ∆ (3)

Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the

braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the

temperature differential approximately 1000 K for the difference between brazing

MANUSCRIP

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14

temperature and room temperature According to manufacturer data (Element Six) the PCD

used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates

have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203

ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the

TiCuNiPCD system is warranted though beyond the scope of the present work In

consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate

compatible with standard joining technologies (ie wirebonding laser welding reflow

soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system

a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a

subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If

a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded

thick- film conductors or contact pads required for the embedded-in-diamond circuit

structures described in the following sections Additionally the ubiquity of Au contacts in

semiconductor and electronics fabrication makes the AuPCD system a practical device

substrate

312 Device fabrication and characterization

The entire fabrication process for these devices is illustrated above in the Experimental

section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate

prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished

devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far

with this technique range between 6-15 microm in width Lower milling powers lead to less

reproducibility perhaps due to issues with focusing on the surface which would be more

pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are

likely possible following more extensive laser parameter optimization including adjustment

of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices

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15

were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au

ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled

structures having three dimensions which demonstrates a significant advantage over

conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing

process eliminates the extra annealing step to form TiC at the interface [40] The resistance of

Au ABA interconnects embedded in PCD was measured using a transmission line set-up to

correct for contact resistance The relationship between the resistance of a 500 microm length Au

interconnect and laser power utilized to mill the track is shown in Fig 7d

Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found

The 3D nature of the interconnect leads to a significant decrease in electrical resistance

compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is

material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In

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16

addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal

for thermal management applications due to diamondrsquos extremely high thermal conductivity

(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining

technologies is paramount Both varieties of braze are easily joined via laser welding

wirebonding or reflow soldering (Supplementary Information Fig S3)

32 Freestanding carbonaceous wires

321 Investigating braze-wire interactions

The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not

been studied as far as we are aware As a result the surface interaction of these materials was

investigated initially using PCD as a substrate PCD was originally chosen due to its low

electrical conductivity making it an adequate support structure for electrical characterization

However the interaction between PCD and Au though non-wetting is significant enough to

prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an

individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either

material though some bonding between Au ABA and PCD occurs A similar effect was

observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of

the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting

behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact

angle of ~404deg (Fig 8c)

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17

Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)

In the interest of creating freestanding carbonaceous wires with metallic contacts we

investigated whether the lack of wetting between a graphite substrate and braze alloy could be

exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate

(Fig 9) It has been observed previously that surface roughness can lead to large increases in

contact angle during brazing making graphite a reasonable choice to perhaps further diminish

the wetting behavior [42]

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18

Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)

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19

A graphite substrate was laser milled in an attempt to control separation distance between the

contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate

a number of geometries are possible for the freestanding wires Ag ABA was able to both wet

and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag

ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig

9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact

with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction

with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure

to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a

seal around the bundle effectively compressing the filaments together (Fig 9h) However if

enough force is applied the Au contact can be removed from the CF bundle though this was

not observed for CNT yarn devices This effect is likely proportional to the ratio of wire

diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of

the bond due to a rise in the contact surface area

322 Device fabrication and characterization

A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or

AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred

because it does not form a strong bond to the graphite substrate following a melting cycle so

the entire wire can be lifted off of the substrate with the contacts intact preventing damage to

the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing

facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent

lift-off was attempted Au does not bond to the graphite for reasons explained above When

the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing

both the carbonaceous fiber and a metallic wire within it (Fig 10a)

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20

Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample

The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig

10c) This technique is a potential improvement over mechanical clamps especially

considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from

clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a

woven or spun material such as CNT yarn prevents unwinding and generally helps to keep

the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead

to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)

were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a

linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the

CNT yarns in this work was comparable to those of similar size and length measured by

Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn

structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While

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21

preliminary measurements suggest that contact resistances are negligible further investigation

is needed to estimate the contact resistance through the contact It was of interest to determine

whether this process modified the carbon structure of the wire materials To investigate this

phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire

following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)

Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1

The before and after Raman spectra of CFs reveals very little change in the microstructure

aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This

peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a

prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor

graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears

quite prominent in both CF spectra presented here though there appears to have been some

MANUSCRIP

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22

moderate increase in graphitization following brazing Additionally the loss of the D-peak

overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to

increased graphitization However it is important to note that the graphitization process

during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to

create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several

peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)

Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1

(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible

that the loss of these peaks is the result of the loss of a surface functionality as a result of high

temperature treatment such as a C-O group However further investigation is required to

acquire a full understanding of the structural changes occurring not only as a result of the high

temperature processes in the presence of various metals In an attempt to improve the thermal

and electrical properties of CNTs and composites gamma irradiation and high temperature

treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease

in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in

the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal

treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that

the vacuum brazing processes described here do not have deleterious effects on carbonaceous

wires

33 Integrating diamond substrates with carbonaceous electrodes

Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)

nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective

MANUSCRIP

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23

processes for integrating these materials with a suitable substrate differ considerably For N-

UNCD stimulating electrodes our group has demonstrated a method to grow the conductive

diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]

This method produces a hermetic electrode array for implantable neural prosthetic

applications However the integration of this array with standard electronic components

requires a metallic interface utilizing the embedded circuit board technology described here

A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze

interconnects and contact pads utilized to contact conductive diamond electrodes (dotted

circle) in a full electrode array device (Fig 12a)

Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes

Incorporating carbonaceous wire electrodes into electronic devices is less elegant and

currently requires the manual placement of individual wire electrodes into designated laser-

MANUSCRIP

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24

milled grooves in the PCD substrate Next the suitable brazing paste is applied though a

screen-printing technique is required to bypass the mechanical polishing step Avoiding

mechanical polishing is important for delicate small and fragile electrode materials such as

those described in this work However because the brazing pastes are comprised of both

metal beads and a polymeric binder the functional volume of the interconnect is reduced

upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed

to produce a CF array embedded in diamond (Fig 12c) We are currently investigating

methods for protecting the CFs and other carbonaceous wires during mechanical polishing to

enable more complex robust device architectures

4 Conclusion

Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires

(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all

components paramount For PCD electronics Ag ABA is used due to its wetting and covalent

bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in

physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial

layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the

AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit

boards are compatible with commercially-available solders and are easily laser-welded for

solder-free joining The non-wetting method of enclosing carbonaceous wires was used to

create strong low-resistance and non-rectifying connections between PtIr wires and

carbonaceous wires Structural and interfacial examinations of these structures lend insight

into the forces at play during both the wetting and non-wetting fabrication of carbonaceous

devices We exploit surface tension effects of two commercially-available active brazing

alloys to create two different device architectures

(1) The wetting technique was used in conjunction with mechanical polishing to create

robust circuit substrates with embedded Ag or Au interconnects and contact pads The option

MANUSCRIP

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25

to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate

illustrates the utility of the method for enabling carbon-based electrode arrays

(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or

AuPtIr wire contacts This technique is interesting for the creation of not only free-standing

carbonaceous conductors but also for neural probes or sensors for both central and peripheral

nervous system integration We have previously described methods to fabricate neural

stimulation and recording electrodes from carbonaceous wires described here such as rLCGO

fiber which demonstrated both in vitro and in vivo functionality [47]

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for

insightful technical conversations NVA is supported by a MMI-CSIRO Material Science

PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was

supported by the Australian Research Council (ARC) through its Special Research Initiative

(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is

supported by the Australian Research Council under Discovery Early Career Researcher

award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and

support through the ARC Centre of Excellence for Electromaterials Science

Supplementary Information Available Figures S1-S4 This material is available free of charge online References

MANUSCRIP

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26

[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

MANUSCRIP

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27

[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

MANUSCRIP

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28

[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

MANUSCRIP

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6

most substrates unless σL lt WA where contact angles lt90˚ are considered ldquowettingrdquo In one

facet of this work we use active brazing to modify WA and create robust electrical

connections to carbonaceous materials such as PCD and CF In another technique we exploit

the lack of adhesion and wetting between graphitic carbon and Au as well as the cohesion

forces within the liquid metal droplet to create a freestanding carbonaceous wire with

metallic connections Broadly speaking the techniques described here are applicable to the

development of devices in which a strong biocompatible and non-rectifying connection to

carbon-based materials is required

2 Experimental

21 Surface interactions brazing alloys and carbonaceous materials

211 Sample preparation and wettability studies

Thermal grade polycrystalline diamond (PCD) substrates (TM-250 Element Six) with a

thickness of 250 microm were laser-cut into 5 times 5 mm squares with a 532 nm NdYAG laser

(Oxford Lasers) Approximately 65 mg of either TiCuNi Ag ABA or Au ABA paste were

placed ontop of either the PCD substrate the PCD substrate with a carbonaceous wire or a

graphite substrate with a carbonaceous wire The elemental composition and melting points of

all braze alloys tested is listed in Table 1

ABA (Manufacturer)

Ag () Au () Cu () Al () Ti () Ni () MP (degC)

Ag-ABA (Wesgo)

9275 - 5 1 125 - 912

Au-ABA (Morgan)

- 964 - - 06 3 1003

TiCuNi (Wesgo)

- - 15 - 70 15 960

Table 1 Elemental composition and melting point (MP) of the braze alloys used in this study

MANUSCRIP

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7

Samples were placed in a brazing furnace under vacuum and heated until the braze was

visibly melted and then held at the melting temperature for 1 minute Upon cooling samples

were imaged using either an optical microscope or scanning electron microscope (SEM)

Contact angles were measured using the low-bond axisymmetric drop shape analysis in the

Droplet-Analysis package in ImageJ image processing software [29 30]

212 Elemental analysis of brazemdashcarbon interface

Some diamond samples were laser milled with deep grooves prior to brazing enabling

visualization and characterization of the interface with both SEM and energy-dispersive X-ray

spectroscopy (EDX)(Supplementary Information Fig S1) The metalcarbonaceous wire

interfaces were studied by embedding brazed samples in epoxy resin (Araldite) and

mechanically-polishing them to reveal the cross-section (Fig 2)

Fig 2 ndash Embedding and polishing method utilized to visualize the carbonmdashbraze interface during both SEM and EDX analysis

SEM images were acquired and EDX was used to identify elemental composition at the

interface of both Ag-ABACF and Au-ABACF joints

213 Raman spectroscopy

Raman spectroscopy (inVia Raman Renishaw) was used to investigate the effects of high

temperature brazing on the microstructure of CF bundles CNT yarns and reduced liquid

crystalline graphene oxide fiber (rLCGO) A 514 nm green laser with a power of 50 (15

MANUSCRIP

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8

mW) was used for CF bundles while a power of 5 (15 mW) was used for 15 microm CNT

yarns and 40 microm rLCGO fiber to avoid ablation damage Samples were scanned before and

after high temperature brazing (1003-1030˚C) The laser was focused on the center of the

sample and an exposure time of 60 seconds was used Acquired signals were processed in

Matlab using a linear baseline correction procedure followed by Gaussian fitting to resolve

peaks

22 Device fabrication and electrical characterization

221 PCD circuit substrates

Thermal grade PCD substrates (TM-500 Element Six) with a thickness of 500 microm were laser

engraved using a 532 nm NdYAG laser (Oxford lasers) at a pulse frequency of 10 kHz A

boiling solution of sodium nitrate and sulfuric acid was used to remove debris from the

engraved pattern followed by 5 minutes of ultrasonication in acetone The PCD substrates

were rinsed with isopropanol and deionized water then dried with nitrogen Groove width

was correlated with laser milling power using scanning electron microscopy (SEM) images

ABA paste was spread over the PCD substrate Samples were transferred to a 10 times 10 mm

graphite wafer placed inside an electron beam deposition system (Thermionics) and

evacuated to 1 times 10-6 Pa The electron beam was focused onto the graphite substrate rather

than the diamond sample and current was increased until both the graphite and PCD were

glowing red ultimately leading to the ABA melting and spreading over the substrate and into

the laser-milled patterns Current was decreased over the course of 1 minute and the samples

were cooled over the course of 1 hour in a N2 environment The brazed samples were

removed from the deposition chamber and mechanical polishing was performed with a

diamond-embedded polishing plate (Struers Inc) using deionized water as a coolant The

samples were polished until embedded Ag ABA or Au ABA tracks were revealed and

residual metals between patterns removed ensuring no shorting between interconnects

MANUSCRIP

T

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ACCEPTED MANUSCRIPT

9

Resistance between isolated interconnects was measured to ensure no shorting (eg gtMΩ

resistance) between unconnected tracks If shorting did occur due to residual unpolished

metal mechanical polishing was continued until it was removed The fabrication of AuPCD

circuit boards requires an additional brazing step to enable Au wetting (Fig 3)

Fig 3 - Construction of embedded-in-diamond Au ABA circuit structures using a Ag ABA adhesion layer (a-b) circuit patterns were laser cut into the diamond substrate and the surface treated with a combination of sulfuric acid and sodium nitrate (c) Ag-ABA paste was applied to the surface melted in an electron beam deposition system (d) The electron beam was continuously applied until a majority of Ag evaporated from the surface leaving behind a conformal film for Au adhesion (e) A Au-based paste was applied to the surface and melted with the aforementioned method (f) (g) The substrate was mechanically-polished revealing Au circuit structures (h) Finally connections or components were attached with laser welding soldering or wirebonding if necessary

222 Freestanding carbonaceous wires

MWNT forest was synthesized by catalytic chemical vapor deposition using acetylene gas as

the carbon source Carbon nanotubes in the 400 um tall forests typically had diameters of 10

nm The CNT yarns were drawn from the forest by pulling and twisting as described by

Zhang et al [31] Graphene oxide (GO) was prepared from intercalated graphene flakes The

liquid crystalline state of graphene oxide was exploited to wet-spin conductive fibers as

described previously [32] Unsized CF bundles having filament diameters of 7 microm were

MANUSCRIP

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10

purchased from Goodfellow On a graphite substrate two mounds of ABA paste were placed

on opposite sides of a 10 times 10 mm graphite wafer Either a bundle of CFs (Goodfellow) a

single CNT yarn (15 microm) or a graphene oxide fiber (50 microm) was placed across the mounds

and pressed into them Additionally one PtIr wire (75 microm Goodfellow) was placed into each

of the ABA paste mounds The graphite substrate was transferred to an electron beam

deposition system (Thermionics) and evacuated to 1 times 10-6 Pa The electron beam was

focused onto the graphite substratemdashas described above for PCD circuit board constructionmdash

and the current was increased until the substrate was glowing red The aforementioned

process is illustrated in Fig 4

Fig 4 - (a) Fabrication of freestanding metalABACNT yarn wires Mounds of ABA paste were placed onto a graphite substrate and either CNT yarns CFs or graphene oxide fibers were pressed into them PtIr wires were pressed into the end for testing (b) The graphite substrate was heated until the Au formed beads on the surface (c) Upon cooling the entire wiremetal structure was easily lifted from the surface

Once melting of the ABA was observed the current was ramped down over 1 minute and the

samples were cooled for 30 minutes in an N2 environment Au-ABA samples were easily

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11

removed from the graphite substrate due to lack of reaction while considerable force was

needed to remove the Ag ABA and TiCuNi samples

223 Integrated PCDcarbonaceous electrode devices

Our group has previously published a method for integrating PCD substrates with conductive

diamond electrodes to produce hermetic feedthroughs [21] Integrating carbonaceous wire

electrodes in this case 7 microm diameter CF was done manually as depicted in Fig 5

Fig 5 - Method for placing individual CF into laser cut groove in PCD substrate and securing the CF with braze paste The braze paste is then ldquoscreen-printedrdquo with a squeegee to remove the excess braze from the surface of the PCD to minimize shorting upon braze melting

PCD substrates were laser machined as described above to create grooves for CF filaments to

rest in Next individual CF filaments were manually placed into the grooves and Ag ABA

paste was spread over the entire surface The surface was wiped with a mini squeegee

(cleanroom cloth wrapped around syringe needle) to remove excess braze paste from the

surface Finally the sample was heated to melt the braze as described above

224 Electrical characterization of devices

The resistance of the Au ABA interconnects embedded in the polished PCD substrates were

measured using a 2-way probe station and a multimeter (Keithley Instruments) Transmission

line devices were fabricated to correct for contact resistance The electrical behavior of the

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12

freestanding PtIrAuCNT yarn structures was characterized by performing voltage sweeps (-1

to +1 V) with a potentiostat (eDAQ) while measuring current

3 Results and Discussion

31 Embedded-in-diamond circuit substrates

311 Investigating PCD-braze interactions

Three different commercially-available active brazing alloy (ABA) pastes were assessed for

their ability to wet and bond to polycrystalline diamond (PCD) substrates The elemental

composition of these brazes is included in Table 1 of the Experimental section Ag ABA was

found to spread and adhere strongly to PCD which was likely facilitated by the formation of

an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig

6a) SEM and EDX elemental analysis of the PCDAg ABA interface reveals a Ti-rich region

of ~34 microm adjacent to the Ag ABA joint (Supplementary Information Fig S1) Conversely

Au ABA did not spread on the PCD surface upon melting instead forming a droplet with a

111deg contact angle on the surface (Fig 6b)

Fig 6 - (a) Ag ABA wets PCD and bonds strongly (b) Au ABA does not wet PCD forming a droplet that makes ~111deg contact angle with the surface (c) TiCuNi braze wets the PCD surface but does not bond The cooled TiCuNi droplet is easily pulled from the PCD surface with forceps (d) Au ABA wets the PCD following a previous Ag ABA brazing step implemented to create a conformal adhesion layer

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13

Though the manufacturers have included Ti as the active component in the Au ABA this

braze is unable to spread on carbonaceous materials Considering the elemental composition

of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack

of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active

component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in

the TiC-Au melt [33]

rarr + 132

Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which

is unsurprising considering the large difference in electronegativity (X) between Au

(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic

is a possibility but the electronegativity differences are less pronounced between Ti and Ag

(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this

reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based

medical devices [35] wetted the PCD surface upon heating but did not bond and was easily

removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and

bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)

This finding is consistent with what has been shown previously for TiCuNirsquos interaction with

carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding

between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between

the braze alloy and the material The stress induced by thermal mismatch (σTM) can be

described theoretically by Equation 3 [37]

13 ∙ ∆ (3)

Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the

braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the

temperature differential approximately 1000 K for the difference between brazing

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14

temperature and room temperature According to manufacturer data (Element Six) the PCD

used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates

have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203

ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the

TiCuNiPCD system is warranted though beyond the scope of the present work In

consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate

compatible with standard joining technologies (ie wirebonding laser welding reflow

soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system

a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a

subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If

a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded

thick- film conductors or contact pads required for the embedded-in-diamond circuit

structures described in the following sections Additionally the ubiquity of Au contacts in

semiconductor and electronics fabrication makes the AuPCD system a practical device

substrate

312 Device fabrication and characterization

The entire fabrication process for these devices is illustrated above in the Experimental

section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate

prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished

devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far

with this technique range between 6-15 microm in width Lower milling powers lead to less

reproducibility perhaps due to issues with focusing on the surface which would be more

pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are

likely possible following more extensive laser parameter optimization including adjustment

of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices

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15

were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au

ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled

structures having three dimensions which demonstrates a significant advantage over

conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing

process eliminates the extra annealing step to form TiC at the interface [40] The resistance of

Au ABA interconnects embedded in PCD was measured using a transmission line set-up to

correct for contact resistance The relationship between the resistance of a 500 microm length Au

interconnect and laser power utilized to mill the track is shown in Fig 7d

Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found

The 3D nature of the interconnect leads to a significant decrease in electrical resistance

compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is

material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In

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16

addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal

for thermal management applications due to diamondrsquos extremely high thermal conductivity

(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining

technologies is paramount Both varieties of braze are easily joined via laser welding

wirebonding or reflow soldering (Supplementary Information Fig S3)

32 Freestanding carbonaceous wires

321 Investigating braze-wire interactions

The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not

been studied as far as we are aware As a result the surface interaction of these materials was

investigated initially using PCD as a substrate PCD was originally chosen due to its low

electrical conductivity making it an adequate support structure for electrical characterization

However the interaction between PCD and Au though non-wetting is significant enough to

prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an

individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either

material though some bonding between Au ABA and PCD occurs A similar effect was

observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of

the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting

behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact

angle of ~404deg (Fig 8c)

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17

Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)

In the interest of creating freestanding carbonaceous wires with metallic contacts we

investigated whether the lack of wetting between a graphite substrate and braze alloy could be

exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate

(Fig 9) It has been observed previously that surface roughness can lead to large increases in

contact angle during brazing making graphite a reasonable choice to perhaps further diminish

the wetting behavior [42]

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18

Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)

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19

A graphite substrate was laser milled in an attempt to control separation distance between the

contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate

a number of geometries are possible for the freestanding wires Ag ABA was able to both wet

and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag

ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig

9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact

with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction

with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure

to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a

seal around the bundle effectively compressing the filaments together (Fig 9h) However if

enough force is applied the Au contact can be removed from the CF bundle though this was

not observed for CNT yarn devices This effect is likely proportional to the ratio of wire

diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of

the bond due to a rise in the contact surface area

322 Device fabrication and characterization

A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or

AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred

because it does not form a strong bond to the graphite substrate following a melting cycle so

the entire wire can be lifted off of the substrate with the contacts intact preventing damage to

the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing

facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent

lift-off was attempted Au does not bond to the graphite for reasons explained above When

the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing

both the carbonaceous fiber and a metallic wire within it (Fig 10a)

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20

Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample

The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig

10c) This technique is a potential improvement over mechanical clamps especially

considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from

clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a

woven or spun material such as CNT yarn prevents unwinding and generally helps to keep

the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead

to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)

were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a

linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the

CNT yarns in this work was comparable to those of similar size and length measured by

Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn

structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While

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21

preliminary measurements suggest that contact resistances are negligible further investigation

is needed to estimate the contact resistance through the contact It was of interest to determine

whether this process modified the carbon structure of the wire materials To investigate this

phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire

following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)

Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1

The before and after Raman spectra of CFs reveals very little change in the microstructure

aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This

peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a

prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor

graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears

quite prominent in both CF spectra presented here though there appears to have been some

MANUSCRIP

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22

moderate increase in graphitization following brazing Additionally the loss of the D-peak

overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to

increased graphitization However it is important to note that the graphitization process

during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to

create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several

peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)

Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1

(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible

that the loss of these peaks is the result of the loss of a surface functionality as a result of high

temperature treatment such as a C-O group However further investigation is required to

acquire a full understanding of the structural changes occurring not only as a result of the high

temperature processes in the presence of various metals In an attempt to improve the thermal

and electrical properties of CNTs and composites gamma irradiation and high temperature

treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease

in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in

the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal

treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that

the vacuum brazing processes described here do not have deleterious effects on carbonaceous

wires

33 Integrating diamond substrates with carbonaceous electrodes

Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)

nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective

MANUSCRIP

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23

processes for integrating these materials with a suitable substrate differ considerably For N-

UNCD stimulating electrodes our group has demonstrated a method to grow the conductive

diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]

This method produces a hermetic electrode array for implantable neural prosthetic

applications However the integration of this array with standard electronic components

requires a metallic interface utilizing the embedded circuit board technology described here

A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze

interconnects and contact pads utilized to contact conductive diamond electrodes (dotted

circle) in a full electrode array device (Fig 12a)

Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes

Incorporating carbonaceous wire electrodes into electronic devices is less elegant and

currently requires the manual placement of individual wire electrodes into designated laser-

MANUSCRIP

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24

milled grooves in the PCD substrate Next the suitable brazing paste is applied though a

screen-printing technique is required to bypass the mechanical polishing step Avoiding

mechanical polishing is important for delicate small and fragile electrode materials such as

those described in this work However because the brazing pastes are comprised of both

metal beads and a polymeric binder the functional volume of the interconnect is reduced

upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed

to produce a CF array embedded in diamond (Fig 12c) We are currently investigating

methods for protecting the CFs and other carbonaceous wires during mechanical polishing to

enable more complex robust device architectures

4 Conclusion

Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires

(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all

components paramount For PCD electronics Ag ABA is used due to its wetting and covalent

bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in

physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial

layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the

AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit

boards are compatible with commercially-available solders and are easily laser-welded for

solder-free joining The non-wetting method of enclosing carbonaceous wires was used to

create strong low-resistance and non-rectifying connections between PtIr wires and

carbonaceous wires Structural and interfacial examinations of these structures lend insight

into the forces at play during both the wetting and non-wetting fabrication of carbonaceous

devices We exploit surface tension effects of two commercially-available active brazing

alloys to create two different device architectures

(1) The wetting technique was used in conjunction with mechanical polishing to create

robust circuit substrates with embedded Ag or Au interconnects and contact pads The option

MANUSCRIP

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25

to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate

illustrates the utility of the method for enabling carbon-based electrode arrays

(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or

AuPtIr wire contacts This technique is interesting for the creation of not only free-standing

carbonaceous conductors but also for neural probes or sensors for both central and peripheral

nervous system integration We have previously described methods to fabricate neural

stimulation and recording electrodes from carbonaceous wires described here such as rLCGO

fiber which demonstrated both in vitro and in vivo functionality [47]

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for

insightful technical conversations NVA is supported by a MMI-CSIRO Material Science

PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was

supported by the Australian Research Council (ARC) through its Special Research Initiative

(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is

supported by the Australian Research Council under Discovery Early Career Researcher

award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and

support through the ARC Centre of Excellence for Electromaterials Science

Supplementary Information Available Figures S1-S4 This material is available free of charge online References

MANUSCRIP

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26

[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

MANUSCRIP

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27

[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

MANUSCRIP

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ACCEPTED MANUSCRIPT

28

[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

MANUSCRIP

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7

Samples were placed in a brazing furnace under vacuum and heated until the braze was

visibly melted and then held at the melting temperature for 1 minute Upon cooling samples

were imaged using either an optical microscope or scanning electron microscope (SEM)

Contact angles were measured using the low-bond axisymmetric drop shape analysis in the

Droplet-Analysis package in ImageJ image processing software [29 30]

212 Elemental analysis of brazemdashcarbon interface

Some diamond samples were laser milled with deep grooves prior to brazing enabling

visualization and characterization of the interface with both SEM and energy-dispersive X-ray

spectroscopy (EDX)(Supplementary Information Fig S1) The metalcarbonaceous wire

interfaces were studied by embedding brazed samples in epoxy resin (Araldite) and

mechanically-polishing them to reveal the cross-section (Fig 2)

Fig 2 ndash Embedding and polishing method utilized to visualize the carbonmdashbraze interface during both SEM and EDX analysis

SEM images were acquired and EDX was used to identify elemental composition at the

interface of both Ag-ABACF and Au-ABACF joints

213 Raman spectroscopy

Raman spectroscopy (inVia Raman Renishaw) was used to investigate the effects of high

temperature brazing on the microstructure of CF bundles CNT yarns and reduced liquid

crystalline graphene oxide fiber (rLCGO) A 514 nm green laser with a power of 50 (15

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8

mW) was used for CF bundles while a power of 5 (15 mW) was used for 15 microm CNT

yarns and 40 microm rLCGO fiber to avoid ablation damage Samples were scanned before and

after high temperature brazing (1003-1030˚C) The laser was focused on the center of the

sample and an exposure time of 60 seconds was used Acquired signals were processed in

Matlab using a linear baseline correction procedure followed by Gaussian fitting to resolve

peaks

22 Device fabrication and electrical characterization

221 PCD circuit substrates

Thermal grade PCD substrates (TM-500 Element Six) with a thickness of 500 microm were laser

engraved using a 532 nm NdYAG laser (Oxford lasers) at a pulse frequency of 10 kHz A

boiling solution of sodium nitrate and sulfuric acid was used to remove debris from the

engraved pattern followed by 5 minutes of ultrasonication in acetone The PCD substrates

were rinsed with isopropanol and deionized water then dried with nitrogen Groove width

was correlated with laser milling power using scanning electron microscopy (SEM) images

ABA paste was spread over the PCD substrate Samples were transferred to a 10 times 10 mm

graphite wafer placed inside an electron beam deposition system (Thermionics) and

evacuated to 1 times 10-6 Pa The electron beam was focused onto the graphite substrate rather

than the diamond sample and current was increased until both the graphite and PCD were

glowing red ultimately leading to the ABA melting and spreading over the substrate and into

the laser-milled patterns Current was decreased over the course of 1 minute and the samples

were cooled over the course of 1 hour in a N2 environment The brazed samples were

removed from the deposition chamber and mechanical polishing was performed with a

diamond-embedded polishing plate (Struers Inc) using deionized water as a coolant The

samples were polished until embedded Ag ABA or Au ABA tracks were revealed and

residual metals between patterns removed ensuring no shorting between interconnects

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9

Resistance between isolated interconnects was measured to ensure no shorting (eg gtMΩ

resistance) between unconnected tracks If shorting did occur due to residual unpolished

metal mechanical polishing was continued until it was removed The fabrication of AuPCD

circuit boards requires an additional brazing step to enable Au wetting (Fig 3)

Fig 3 - Construction of embedded-in-diamond Au ABA circuit structures using a Ag ABA adhesion layer (a-b) circuit patterns were laser cut into the diamond substrate and the surface treated with a combination of sulfuric acid and sodium nitrate (c) Ag-ABA paste was applied to the surface melted in an electron beam deposition system (d) The electron beam was continuously applied until a majority of Ag evaporated from the surface leaving behind a conformal film for Au adhesion (e) A Au-based paste was applied to the surface and melted with the aforementioned method (f) (g) The substrate was mechanically-polished revealing Au circuit structures (h) Finally connections or components were attached with laser welding soldering or wirebonding if necessary

222 Freestanding carbonaceous wires

MWNT forest was synthesized by catalytic chemical vapor deposition using acetylene gas as

the carbon source Carbon nanotubes in the 400 um tall forests typically had diameters of 10

nm The CNT yarns were drawn from the forest by pulling and twisting as described by

Zhang et al [31] Graphene oxide (GO) was prepared from intercalated graphene flakes The

liquid crystalline state of graphene oxide was exploited to wet-spin conductive fibers as

described previously [32] Unsized CF bundles having filament diameters of 7 microm were

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10

purchased from Goodfellow On a graphite substrate two mounds of ABA paste were placed

on opposite sides of a 10 times 10 mm graphite wafer Either a bundle of CFs (Goodfellow) a

single CNT yarn (15 microm) or a graphene oxide fiber (50 microm) was placed across the mounds

and pressed into them Additionally one PtIr wire (75 microm Goodfellow) was placed into each

of the ABA paste mounds The graphite substrate was transferred to an electron beam

deposition system (Thermionics) and evacuated to 1 times 10-6 Pa The electron beam was

focused onto the graphite substratemdashas described above for PCD circuit board constructionmdash

and the current was increased until the substrate was glowing red The aforementioned

process is illustrated in Fig 4

Fig 4 - (a) Fabrication of freestanding metalABACNT yarn wires Mounds of ABA paste were placed onto a graphite substrate and either CNT yarns CFs or graphene oxide fibers were pressed into them PtIr wires were pressed into the end for testing (b) The graphite substrate was heated until the Au formed beads on the surface (c) Upon cooling the entire wiremetal structure was easily lifted from the surface

Once melting of the ABA was observed the current was ramped down over 1 minute and the

samples were cooled for 30 minutes in an N2 environment Au-ABA samples were easily

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11

removed from the graphite substrate due to lack of reaction while considerable force was

needed to remove the Ag ABA and TiCuNi samples

223 Integrated PCDcarbonaceous electrode devices

Our group has previously published a method for integrating PCD substrates with conductive

diamond electrodes to produce hermetic feedthroughs [21] Integrating carbonaceous wire

electrodes in this case 7 microm diameter CF was done manually as depicted in Fig 5

Fig 5 - Method for placing individual CF into laser cut groove in PCD substrate and securing the CF with braze paste The braze paste is then ldquoscreen-printedrdquo with a squeegee to remove the excess braze from the surface of the PCD to minimize shorting upon braze melting

PCD substrates were laser machined as described above to create grooves for CF filaments to

rest in Next individual CF filaments were manually placed into the grooves and Ag ABA

paste was spread over the entire surface The surface was wiped with a mini squeegee

(cleanroom cloth wrapped around syringe needle) to remove excess braze paste from the

surface Finally the sample was heated to melt the braze as described above

224 Electrical characterization of devices

The resistance of the Au ABA interconnects embedded in the polished PCD substrates were

measured using a 2-way probe station and a multimeter (Keithley Instruments) Transmission

line devices were fabricated to correct for contact resistance The electrical behavior of the

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12

freestanding PtIrAuCNT yarn structures was characterized by performing voltage sweeps (-1

to +1 V) with a potentiostat (eDAQ) while measuring current

3 Results and Discussion

31 Embedded-in-diamond circuit substrates

311 Investigating PCD-braze interactions

Three different commercially-available active brazing alloy (ABA) pastes were assessed for

their ability to wet and bond to polycrystalline diamond (PCD) substrates The elemental

composition of these brazes is included in Table 1 of the Experimental section Ag ABA was

found to spread and adhere strongly to PCD which was likely facilitated by the formation of

an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig

6a) SEM and EDX elemental analysis of the PCDAg ABA interface reveals a Ti-rich region

of ~34 microm adjacent to the Ag ABA joint (Supplementary Information Fig S1) Conversely

Au ABA did not spread on the PCD surface upon melting instead forming a droplet with a

111deg contact angle on the surface (Fig 6b)

Fig 6 - (a) Ag ABA wets PCD and bonds strongly (b) Au ABA does not wet PCD forming a droplet that makes ~111deg contact angle with the surface (c) TiCuNi braze wets the PCD surface but does not bond The cooled TiCuNi droplet is easily pulled from the PCD surface with forceps (d) Au ABA wets the PCD following a previous Ag ABA brazing step implemented to create a conformal adhesion layer

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13

Though the manufacturers have included Ti as the active component in the Au ABA this

braze is unable to spread on carbonaceous materials Considering the elemental composition

of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack

of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active

component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in

the TiC-Au melt [33]

rarr + 132

Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which

is unsurprising considering the large difference in electronegativity (X) between Au

(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic

is a possibility but the electronegativity differences are less pronounced between Ti and Ag

(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this

reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based

medical devices [35] wetted the PCD surface upon heating but did not bond and was easily

removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and

bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)

This finding is consistent with what has been shown previously for TiCuNirsquos interaction with

carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding

between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between

the braze alloy and the material The stress induced by thermal mismatch (σTM) can be

described theoretically by Equation 3 [37]

13 ∙ ∆ (3)

Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the

braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the

temperature differential approximately 1000 K for the difference between brazing

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14

temperature and room temperature According to manufacturer data (Element Six) the PCD

used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates

have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203

ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the

TiCuNiPCD system is warranted though beyond the scope of the present work In

consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate

compatible with standard joining technologies (ie wirebonding laser welding reflow

soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system

a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a

subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If

a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded

thick- film conductors or contact pads required for the embedded-in-diamond circuit

structures described in the following sections Additionally the ubiquity of Au contacts in

semiconductor and electronics fabrication makes the AuPCD system a practical device

substrate

312 Device fabrication and characterization

The entire fabrication process for these devices is illustrated above in the Experimental

section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate

prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished

devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far

with this technique range between 6-15 microm in width Lower milling powers lead to less

reproducibility perhaps due to issues with focusing on the surface which would be more

pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are

likely possible following more extensive laser parameter optimization including adjustment

of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices

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15

were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au

ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled

structures having three dimensions which demonstrates a significant advantage over

conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing

process eliminates the extra annealing step to form TiC at the interface [40] The resistance of

Au ABA interconnects embedded in PCD was measured using a transmission line set-up to

correct for contact resistance The relationship between the resistance of a 500 microm length Au

interconnect and laser power utilized to mill the track is shown in Fig 7d

Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found

The 3D nature of the interconnect leads to a significant decrease in electrical resistance

compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is

material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In

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16

addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal

for thermal management applications due to diamondrsquos extremely high thermal conductivity

(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining

technologies is paramount Both varieties of braze are easily joined via laser welding

wirebonding or reflow soldering (Supplementary Information Fig S3)

32 Freestanding carbonaceous wires

321 Investigating braze-wire interactions

The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not

been studied as far as we are aware As a result the surface interaction of these materials was

investigated initially using PCD as a substrate PCD was originally chosen due to its low

electrical conductivity making it an adequate support structure for electrical characterization

However the interaction between PCD and Au though non-wetting is significant enough to

prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an

individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either

material though some bonding between Au ABA and PCD occurs A similar effect was

observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of

the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting

behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact

angle of ~404deg (Fig 8c)

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17

Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)

In the interest of creating freestanding carbonaceous wires with metallic contacts we

investigated whether the lack of wetting between a graphite substrate and braze alloy could be

exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate

(Fig 9) It has been observed previously that surface roughness can lead to large increases in

contact angle during brazing making graphite a reasonable choice to perhaps further diminish

the wetting behavior [42]

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18

Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)

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19

A graphite substrate was laser milled in an attempt to control separation distance between the

contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate

a number of geometries are possible for the freestanding wires Ag ABA was able to both wet

and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag

ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig

9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact

with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction

with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure

to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a

seal around the bundle effectively compressing the filaments together (Fig 9h) However if

enough force is applied the Au contact can be removed from the CF bundle though this was

not observed for CNT yarn devices This effect is likely proportional to the ratio of wire

diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of

the bond due to a rise in the contact surface area

322 Device fabrication and characterization

A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or

AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred

because it does not form a strong bond to the graphite substrate following a melting cycle so

the entire wire can be lifted off of the substrate with the contacts intact preventing damage to

the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing

facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent

lift-off was attempted Au does not bond to the graphite for reasons explained above When

the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing

both the carbonaceous fiber and a metallic wire within it (Fig 10a)

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20

Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample

The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig

10c) This technique is a potential improvement over mechanical clamps especially

considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from

clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a

woven or spun material such as CNT yarn prevents unwinding and generally helps to keep

the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead

to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)

were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a

linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the

CNT yarns in this work was comparable to those of similar size and length measured by

Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn

structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While

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21

preliminary measurements suggest that contact resistances are negligible further investigation

is needed to estimate the contact resistance through the contact It was of interest to determine

whether this process modified the carbon structure of the wire materials To investigate this

phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire

following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)

Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1

The before and after Raman spectra of CFs reveals very little change in the microstructure

aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This

peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a

prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor

graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears

quite prominent in both CF spectra presented here though there appears to have been some

MANUSCRIP

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22

moderate increase in graphitization following brazing Additionally the loss of the D-peak

overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to

increased graphitization However it is important to note that the graphitization process

during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to

create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several

peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)

Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1

(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible

that the loss of these peaks is the result of the loss of a surface functionality as a result of high

temperature treatment such as a C-O group However further investigation is required to

acquire a full understanding of the structural changes occurring not only as a result of the high

temperature processes in the presence of various metals In an attempt to improve the thermal

and electrical properties of CNTs and composites gamma irradiation and high temperature

treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease

in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in

the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal

treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that

the vacuum brazing processes described here do not have deleterious effects on carbonaceous

wires

33 Integrating diamond substrates with carbonaceous electrodes

Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)

nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective

MANUSCRIP

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23

processes for integrating these materials with a suitable substrate differ considerably For N-

UNCD stimulating electrodes our group has demonstrated a method to grow the conductive

diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]

This method produces a hermetic electrode array for implantable neural prosthetic

applications However the integration of this array with standard electronic components

requires a metallic interface utilizing the embedded circuit board technology described here

A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze

interconnects and contact pads utilized to contact conductive diamond electrodes (dotted

circle) in a full electrode array device (Fig 12a)

Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes

Incorporating carbonaceous wire electrodes into electronic devices is less elegant and

currently requires the manual placement of individual wire electrodes into designated laser-

MANUSCRIP

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24

milled grooves in the PCD substrate Next the suitable brazing paste is applied though a

screen-printing technique is required to bypass the mechanical polishing step Avoiding

mechanical polishing is important for delicate small and fragile electrode materials such as

those described in this work However because the brazing pastes are comprised of both

metal beads and a polymeric binder the functional volume of the interconnect is reduced

upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed

to produce a CF array embedded in diamond (Fig 12c) We are currently investigating

methods for protecting the CFs and other carbonaceous wires during mechanical polishing to

enable more complex robust device architectures

4 Conclusion

Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires

(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all

components paramount For PCD electronics Ag ABA is used due to its wetting and covalent

bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in

physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial

layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the

AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit

boards are compatible with commercially-available solders and are easily laser-welded for

solder-free joining The non-wetting method of enclosing carbonaceous wires was used to

create strong low-resistance and non-rectifying connections between PtIr wires and

carbonaceous wires Structural and interfacial examinations of these structures lend insight

into the forces at play during both the wetting and non-wetting fabrication of carbonaceous

devices We exploit surface tension effects of two commercially-available active brazing

alloys to create two different device architectures

(1) The wetting technique was used in conjunction with mechanical polishing to create

robust circuit substrates with embedded Ag or Au interconnects and contact pads The option

MANUSCRIP

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25

to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate

illustrates the utility of the method for enabling carbon-based electrode arrays

(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or

AuPtIr wire contacts This technique is interesting for the creation of not only free-standing

carbonaceous conductors but also for neural probes or sensors for both central and peripheral

nervous system integration We have previously described methods to fabricate neural

stimulation and recording electrodes from carbonaceous wires described here such as rLCGO

fiber which demonstrated both in vitro and in vivo functionality [47]

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for

insightful technical conversations NVA is supported by a MMI-CSIRO Material Science

PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was

supported by the Australian Research Council (ARC) through its Special Research Initiative

(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is

supported by the Australian Research Council under Discovery Early Career Researcher

award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and

support through the ARC Centre of Excellence for Electromaterials Science

Supplementary Information Available Figures S1-S4 This material is available free of charge online References

MANUSCRIP

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26

[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

MANUSCRIP

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ACCEPTED MANUSCRIPT

27

[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

28

[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

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8

mW) was used for CF bundles while a power of 5 (15 mW) was used for 15 microm CNT

yarns and 40 microm rLCGO fiber to avoid ablation damage Samples were scanned before and

after high temperature brazing (1003-1030˚C) The laser was focused on the center of the

sample and an exposure time of 60 seconds was used Acquired signals were processed in

Matlab using a linear baseline correction procedure followed by Gaussian fitting to resolve

peaks

22 Device fabrication and electrical characterization

221 PCD circuit substrates

Thermal grade PCD substrates (TM-500 Element Six) with a thickness of 500 microm were laser

engraved using a 532 nm NdYAG laser (Oxford lasers) at a pulse frequency of 10 kHz A

boiling solution of sodium nitrate and sulfuric acid was used to remove debris from the

engraved pattern followed by 5 minutes of ultrasonication in acetone The PCD substrates

were rinsed with isopropanol and deionized water then dried with nitrogen Groove width

was correlated with laser milling power using scanning electron microscopy (SEM) images

ABA paste was spread over the PCD substrate Samples were transferred to a 10 times 10 mm

graphite wafer placed inside an electron beam deposition system (Thermionics) and

evacuated to 1 times 10-6 Pa The electron beam was focused onto the graphite substrate rather

than the diamond sample and current was increased until both the graphite and PCD were

glowing red ultimately leading to the ABA melting and spreading over the substrate and into

the laser-milled patterns Current was decreased over the course of 1 minute and the samples

were cooled over the course of 1 hour in a N2 environment The brazed samples were

removed from the deposition chamber and mechanical polishing was performed with a

diamond-embedded polishing plate (Struers Inc) using deionized water as a coolant The

samples were polished until embedded Ag ABA or Au ABA tracks were revealed and

residual metals between patterns removed ensuring no shorting between interconnects

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9

Resistance between isolated interconnects was measured to ensure no shorting (eg gtMΩ

resistance) between unconnected tracks If shorting did occur due to residual unpolished

metal mechanical polishing was continued until it was removed The fabrication of AuPCD

circuit boards requires an additional brazing step to enable Au wetting (Fig 3)

Fig 3 - Construction of embedded-in-diamond Au ABA circuit structures using a Ag ABA adhesion layer (a-b) circuit patterns were laser cut into the diamond substrate and the surface treated with a combination of sulfuric acid and sodium nitrate (c) Ag-ABA paste was applied to the surface melted in an electron beam deposition system (d) The electron beam was continuously applied until a majority of Ag evaporated from the surface leaving behind a conformal film for Au adhesion (e) A Au-based paste was applied to the surface and melted with the aforementioned method (f) (g) The substrate was mechanically-polished revealing Au circuit structures (h) Finally connections or components were attached with laser welding soldering or wirebonding if necessary

222 Freestanding carbonaceous wires

MWNT forest was synthesized by catalytic chemical vapor deposition using acetylene gas as

the carbon source Carbon nanotubes in the 400 um tall forests typically had diameters of 10

nm The CNT yarns were drawn from the forest by pulling and twisting as described by

Zhang et al [31] Graphene oxide (GO) was prepared from intercalated graphene flakes The

liquid crystalline state of graphene oxide was exploited to wet-spin conductive fibers as

described previously [32] Unsized CF bundles having filament diameters of 7 microm were

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10

purchased from Goodfellow On a graphite substrate two mounds of ABA paste were placed

on opposite sides of a 10 times 10 mm graphite wafer Either a bundle of CFs (Goodfellow) a

single CNT yarn (15 microm) or a graphene oxide fiber (50 microm) was placed across the mounds

and pressed into them Additionally one PtIr wire (75 microm Goodfellow) was placed into each

of the ABA paste mounds The graphite substrate was transferred to an electron beam

deposition system (Thermionics) and evacuated to 1 times 10-6 Pa The electron beam was

focused onto the graphite substratemdashas described above for PCD circuit board constructionmdash

and the current was increased until the substrate was glowing red The aforementioned

process is illustrated in Fig 4

Fig 4 - (a) Fabrication of freestanding metalABACNT yarn wires Mounds of ABA paste were placed onto a graphite substrate and either CNT yarns CFs or graphene oxide fibers were pressed into them PtIr wires were pressed into the end for testing (b) The graphite substrate was heated until the Au formed beads on the surface (c) Upon cooling the entire wiremetal structure was easily lifted from the surface

Once melting of the ABA was observed the current was ramped down over 1 minute and the

samples were cooled for 30 minutes in an N2 environment Au-ABA samples were easily

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11

removed from the graphite substrate due to lack of reaction while considerable force was

needed to remove the Ag ABA and TiCuNi samples

223 Integrated PCDcarbonaceous electrode devices

Our group has previously published a method for integrating PCD substrates with conductive

diamond electrodes to produce hermetic feedthroughs [21] Integrating carbonaceous wire

electrodes in this case 7 microm diameter CF was done manually as depicted in Fig 5

Fig 5 - Method for placing individual CF into laser cut groove in PCD substrate and securing the CF with braze paste The braze paste is then ldquoscreen-printedrdquo with a squeegee to remove the excess braze from the surface of the PCD to minimize shorting upon braze melting

PCD substrates were laser machined as described above to create grooves for CF filaments to

rest in Next individual CF filaments were manually placed into the grooves and Ag ABA

paste was spread over the entire surface The surface was wiped with a mini squeegee

(cleanroom cloth wrapped around syringe needle) to remove excess braze paste from the

surface Finally the sample was heated to melt the braze as described above

224 Electrical characterization of devices

The resistance of the Au ABA interconnects embedded in the polished PCD substrates were

measured using a 2-way probe station and a multimeter (Keithley Instruments) Transmission

line devices were fabricated to correct for contact resistance The electrical behavior of the

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12

freestanding PtIrAuCNT yarn structures was characterized by performing voltage sweeps (-1

to +1 V) with a potentiostat (eDAQ) while measuring current

3 Results and Discussion

31 Embedded-in-diamond circuit substrates

311 Investigating PCD-braze interactions

Three different commercially-available active brazing alloy (ABA) pastes were assessed for

their ability to wet and bond to polycrystalline diamond (PCD) substrates The elemental

composition of these brazes is included in Table 1 of the Experimental section Ag ABA was

found to spread and adhere strongly to PCD which was likely facilitated by the formation of

an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig

6a) SEM and EDX elemental analysis of the PCDAg ABA interface reveals a Ti-rich region

of ~34 microm adjacent to the Ag ABA joint (Supplementary Information Fig S1) Conversely

Au ABA did not spread on the PCD surface upon melting instead forming a droplet with a

111deg contact angle on the surface (Fig 6b)

Fig 6 - (a) Ag ABA wets PCD and bonds strongly (b) Au ABA does not wet PCD forming a droplet that makes ~111deg contact angle with the surface (c) TiCuNi braze wets the PCD surface but does not bond The cooled TiCuNi droplet is easily pulled from the PCD surface with forceps (d) Au ABA wets the PCD following a previous Ag ABA brazing step implemented to create a conformal adhesion layer

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13

Though the manufacturers have included Ti as the active component in the Au ABA this

braze is unable to spread on carbonaceous materials Considering the elemental composition

of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack

of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active

component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in

the TiC-Au melt [33]

rarr + 132

Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which

is unsurprising considering the large difference in electronegativity (X) between Au

(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic

is a possibility but the electronegativity differences are less pronounced between Ti and Ag

(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this

reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based

medical devices [35] wetted the PCD surface upon heating but did not bond and was easily

removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and

bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)

This finding is consistent with what has been shown previously for TiCuNirsquos interaction with

carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding

between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between

the braze alloy and the material The stress induced by thermal mismatch (σTM) can be

described theoretically by Equation 3 [37]

13 ∙ ∆ (3)

Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the

braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the

temperature differential approximately 1000 K for the difference between brazing

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14

temperature and room temperature According to manufacturer data (Element Six) the PCD

used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates

have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203

ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the

TiCuNiPCD system is warranted though beyond the scope of the present work In

consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate

compatible with standard joining technologies (ie wirebonding laser welding reflow

soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system

a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a

subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If

a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded

thick- film conductors or contact pads required for the embedded-in-diamond circuit

structures described in the following sections Additionally the ubiquity of Au contacts in

semiconductor and electronics fabrication makes the AuPCD system a practical device

substrate

312 Device fabrication and characterization

The entire fabrication process for these devices is illustrated above in the Experimental

section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate

prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished

devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far

with this technique range between 6-15 microm in width Lower milling powers lead to less

reproducibility perhaps due to issues with focusing on the surface which would be more

pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are

likely possible following more extensive laser parameter optimization including adjustment

of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices

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15

were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au

ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled

structures having three dimensions which demonstrates a significant advantage over

conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing

process eliminates the extra annealing step to form TiC at the interface [40] The resistance of

Au ABA interconnects embedded in PCD was measured using a transmission line set-up to

correct for contact resistance The relationship between the resistance of a 500 microm length Au

interconnect and laser power utilized to mill the track is shown in Fig 7d

Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found

The 3D nature of the interconnect leads to a significant decrease in electrical resistance

compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is

material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In

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16

addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal

for thermal management applications due to diamondrsquos extremely high thermal conductivity

(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining

technologies is paramount Both varieties of braze are easily joined via laser welding

wirebonding or reflow soldering (Supplementary Information Fig S3)

32 Freestanding carbonaceous wires

321 Investigating braze-wire interactions

The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not

been studied as far as we are aware As a result the surface interaction of these materials was

investigated initially using PCD as a substrate PCD was originally chosen due to its low

electrical conductivity making it an adequate support structure for electrical characterization

However the interaction between PCD and Au though non-wetting is significant enough to

prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an

individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either

material though some bonding between Au ABA and PCD occurs A similar effect was

observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of

the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting

behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact

angle of ~404deg (Fig 8c)

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17

Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)

In the interest of creating freestanding carbonaceous wires with metallic contacts we

investigated whether the lack of wetting between a graphite substrate and braze alloy could be

exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate

(Fig 9) It has been observed previously that surface roughness can lead to large increases in

contact angle during brazing making graphite a reasonable choice to perhaps further diminish

the wetting behavior [42]

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18

Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)

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19

A graphite substrate was laser milled in an attempt to control separation distance between the

contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate

a number of geometries are possible for the freestanding wires Ag ABA was able to both wet

and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag

ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig

9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact

with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction

with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure

to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a

seal around the bundle effectively compressing the filaments together (Fig 9h) However if

enough force is applied the Au contact can be removed from the CF bundle though this was

not observed for CNT yarn devices This effect is likely proportional to the ratio of wire

diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of

the bond due to a rise in the contact surface area

322 Device fabrication and characterization

A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or

AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred

because it does not form a strong bond to the graphite substrate following a melting cycle so

the entire wire can be lifted off of the substrate with the contacts intact preventing damage to

the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing

facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent

lift-off was attempted Au does not bond to the graphite for reasons explained above When

the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing

both the carbonaceous fiber and a metallic wire within it (Fig 10a)

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20

Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample

The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig

10c) This technique is a potential improvement over mechanical clamps especially

considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from

clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a

woven or spun material such as CNT yarn prevents unwinding and generally helps to keep

the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead

to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)

were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a

linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the

CNT yarns in this work was comparable to those of similar size and length measured by

Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn

structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While

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21

preliminary measurements suggest that contact resistances are negligible further investigation

is needed to estimate the contact resistance through the contact It was of interest to determine

whether this process modified the carbon structure of the wire materials To investigate this

phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire

following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)

Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1

The before and after Raman spectra of CFs reveals very little change in the microstructure

aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This

peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a

prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor

graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears

quite prominent in both CF spectra presented here though there appears to have been some

MANUSCRIP

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22

moderate increase in graphitization following brazing Additionally the loss of the D-peak

overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to

increased graphitization However it is important to note that the graphitization process

during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to

create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several

peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)

Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1

(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible

that the loss of these peaks is the result of the loss of a surface functionality as a result of high

temperature treatment such as a C-O group However further investigation is required to

acquire a full understanding of the structural changes occurring not only as a result of the high

temperature processes in the presence of various metals In an attempt to improve the thermal

and electrical properties of CNTs and composites gamma irradiation and high temperature

treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease

in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in

the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal

treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that

the vacuum brazing processes described here do not have deleterious effects on carbonaceous

wires

33 Integrating diamond substrates with carbonaceous electrodes

Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)

nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective

MANUSCRIP

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23

processes for integrating these materials with a suitable substrate differ considerably For N-

UNCD stimulating electrodes our group has demonstrated a method to grow the conductive

diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]

This method produces a hermetic electrode array for implantable neural prosthetic

applications However the integration of this array with standard electronic components

requires a metallic interface utilizing the embedded circuit board technology described here

A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze

interconnects and contact pads utilized to contact conductive diamond electrodes (dotted

circle) in a full electrode array device (Fig 12a)

Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes

Incorporating carbonaceous wire electrodes into electronic devices is less elegant and

currently requires the manual placement of individual wire electrodes into designated laser-

MANUSCRIP

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24

milled grooves in the PCD substrate Next the suitable brazing paste is applied though a

screen-printing technique is required to bypass the mechanical polishing step Avoiding

mechanical polishing is important for delicate small and fragile electrode materials such as

those described in this work However because the brazing pastes are comprised of both

metal beads and a polymeric binder the functional volume of the interconnect is reduced

upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed

to produce a CF array embedded in diamond (Fig 12c) We are currently investigating

methods for protecting the CFs and other carbonaceous wires during mechanical polishing to

enable more complex robust device architectures

4 Conclusion

Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires

(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all

components paramount For PCD electronics Ag ABA is used due to its wetting and covalent

bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in

physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial

layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the

AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit

boards are compatible with commercially-available solders and are easily laser-welded for

solder-free joining The non-wetting method of enclosing carbonaceous wires was used to

create strong low-resistance and non-rectifying connections between PtIr wires and

carbonaceous wires Structural and interfacial examinations of these structures lend insight

into the forces at play during both the wetting and non-wetting fabrication of carbonaceous

devices We exploit surface tension effects of two commercially-available active brazing

alloys to create two different device architectures

(1) The wetting technique was used in conjunction with mechanical polishing to create

robust circuit substrates with embedded Ag or Au interconnects and contact pads The option

MANUSCRIP

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25

to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate

illustrates the utility of the method for enabling carbon-based electrode arrays

(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or

AuPtIr wire contacts This technique is interesting for the creation of not only free-standing

carbonaceous conductors but also for neural probes or sensors for both central and peripheral

nervous system integration We have previously described methods to fabricate neural

stimulation and recording electrodes from carbonaceous wires described here such as rLCGO

fiber which demonstrated both in vitro and in vivo functionality [47]

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for

insightful technical conversations NVA is supported by a MMI-CSIRO Material Science

PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was

supported by the Australian Research Council (ARC) through its Special Research Initiative

(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is

supported by the Australian Research Council under Discovery Early Career Researcher

award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and

support through the ARC Centre of Excellence for Electromaterials Science

Supplementary Information Available Figures S1-S4 This material is available free of charge online References

MANUSCRIP

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ACCEPTED MANUSCRIPT

26

[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

MANUSCRIP

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27

[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

MANUSCRIP

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ACCEPTED MANUSCRIPT

28

[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

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9

Resistance between isolated interconnects was measured to ensure no shorting (eg gtMΩ

resistance) between unconnected tracks If shorting did occur due to residual unpolished

metal mechanical polishing was continued until it was removed The fabrication of AuPCD

circuit boards requires an additional brazing step to enable Au wetting (Fig 3)

Fig 3 - Construction of embedded-in-diamond Au ABA circuit structures using a Ag ABA adhesion layer (a-b) circuit patterns were laser cut into the diamond substrate and the surface treated with a combination of sulfuric acid and sodium nitrate (c) Ag-ABA paste was applied to the surface melted in an electron beam deposition system (d) The electron beam was continuously applied until a majority of Ag evaporated from the surface leaving behind a conformal film for Au adhesion (e) A Au-based paste was applied to the surface and melted with the aforementioned method (f) (g) The substrate was mechanically-polished revealing Au circuit structures (h) Finally connections or components were attached with laser welding soldering or wirebonding if necessary

222 Freestanding carbonaceous wires

MWNT forest was synthesized by catalytic chemical vapor deposition using acetylene gas as

the carbon source Carbon nanotubes in the 400 um tall forests typically had diameters of 10

nm The CNT yarns were drawn from the forest by pulling and twisting as described by

Zhang et al [31] Graphene oxide (GO) was prepared from intercalated graphene flakes The

liquid crystalline state of graphene oxide was exploited to wet-spin conductive fibers as

described previously [32] Unsized CF bundles having filament diameters of 7 microm were

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10

purchased from Goodfellow On a graphite substrate two mounds of ABA paste were placed

on opposite sides of a 10 times 10 mm graphite wafer Either a bundle of CFs (Goodfellow) a

single CNT yarn (15 microm) or a graphene oxide fiber (50 microm) was placed across the mounds

and pressed into them Additionally one PtIr wire (75 microm Goodfellow) was placed into each

of the ABA paste mounds The graphite substrate was transferred to an electron beam

deposition system (Thermionics) and evacuated to 1 times 10-6 Pa The electron beam was

focused onto the graphite substratemdashas described above for PCD circuit board constructionmdash

and the current was increased until the substrate was glowing red The aforementioned

process is illustrated in Fig 4

Fig 4 - (a) Fabrication of freestanding metalABACNT yarn wires Mounds of ABA paste were placed onto a graphite substrate and either CNT yarns CFs or graphene oxide fibers were pressed into them PtIr wires were pressed into the end for testing (b) The graphite substrate was heated until the Au formed beads on the surface (c) Upon cooling the entire wiremetal structure was easily lifted from the surface

Once melting of the ABA was observed the current was ramped down over 1 minute and the

samples were cooled for 30 minutes in an N2 environment Au-ABA samples were easily

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11

removed from the graphite substrate due to lack of reaction while considerable force was

needed to remove the Ag ABA and TiCuNi samples

223 Integrated PCDcarbonaceous electrode devices

Our group has previously published a method for integrating PCD substrates with conductive

diamond electrodes to produce hermetic feedthroughs [21] Integrating carbonaceous wire

electrodes in this case 7 microm diameter CF was done manually as depicted in Fig 5

Fig 5 - Method for placing individual CF into laser cut groove in PCD substrate and securing the CF with braze paste The braze paste is then ldquoscreen-printedrdquo with a squeegee to remove the excess braze from the surface of the PCD to minimize shorting upon braze melting

PCD substrates were laser machined as described above to create grooves for CF filaments to

rest in Next individual CF filaments were manually placed into the grooves and Ag ABA

paste was spread over the entire surface The surface was wiped with a mini squeegee

(cleanroom cloth wrapped around syringe needle) to remove excess braze paste from the

surface Finally the sample was heated to melt the braze as described above

224 Electrical characterization of devices

The resistance of the Au ABA interconnects embedded in the polished PCD substrates were

measured using a 2-way probe station and a multimeter (Keithley Instruments) Transmission

line devices were fabricated to correct for contact resistance The electrical behavior of the

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12

freestanding PtIrAuCNT yarn structures was characterized by performing voltage sweeps (-1

to +1 V) with a potentiostat (eDAQ) while measuring current

3 Results and Discussion

31 Embedded-in-diamond circuit substrates

311 Investigating PCD-braze interactions

Three different commercially-available active brazing alloy (ABA) pastes were assessed for

their ability to wet and bond to polycrystalline diamond (PCD) substrates The elemental

composition of these brazes is included in Table 1 of the Experimental section Ag ABA was

found to spread and adhere strongly to PCD which was likely facilitated by the formation of

an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig

6a) SEM and EDX elemental analysis of the PCDAg ABA interface reveals a Ti-rich region

of ~34 microm adjacent to the Ag ABA joint (Supplementary Information Fig S1) Conversely

Au ABA did not spread on the PCD surface upon melting instead forming a droplet with a

111deg contact angle on the surface (Fig 6b)

Fig 6 - (a) Ag ABA wets PCD and bonds strongly (b) Au ABA does not wet PCD forming a droplet that makes ~111deg contact angle with the surface (c) TiCuNi braze wets the PCD surface but does not bond The cooled TiCuNi droplet is easily pulled from the PCD surface with forceps (d) Au ABA wets the PCD following a previous Ag ABA brazing step implemented to create a conformal adhesion layer

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13

Though the manufacturers have included Ti as the active component in the Au ABA this

braze is unable to spread on carbonaceous materials Considering the elemental composition

of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack

of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active

component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in

the TiC-Au melt [33]

rarr + 132

Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which

is unsurprising considering the large difference in electronegativity (X) between Au

(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic

is a possibility but the electronegativity differences are less pronounced between Ti and Ag

(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this

reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based

medical devices [35] wetted the PCD surface upon heating but did not bond and was easily

removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and

bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)

This finding is consistent with what has been shown previously for TiCuNirsquos interaction with

carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding

between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between

the braze alloy and the material The stress induced by thermal mismatch (σTM) can be

described theoretically by Equation 3 [37]

13 ∙ ∆ (3)

Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the

braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the

temperature differential approximately 1000 K for the difference between brazing

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14

temperature and room temperature According to manufacturer data (Element Six) the PCD

used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates

have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203

ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the

TiCuNiPCD system is warranted though beyond the scope of the present work In

consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate

compatible with standard joining technologies (ie wirebonding laser welding reflow

soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system

a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a

subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If

a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded

thick- film conductors or contact pads required for the embedded-in-diamond circuit

structures described in the following sections Additionally the ubiquity of Au contacts in

semiconductor and electronics fabrication makes the AuPCD system a practical device

substrate

312 Device fabrication and characterization

The entire fabrication process for these devices is illustrated above in the Experimental

section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate

prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished

devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far

with this technique range between 6-15 microm in width Lower milling powers lead to less

reproducibility perhaps due to issues with focusing on the surface which would be more

pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are

likely possible following more extensive laser parameter optimization including adjustment

of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices

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15

were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au

ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled

structures having three dimensions which demonstrates a significant advantage over

conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing

process eliminates the extra annealing step to form TiC at the interface [40] The resistance of

Au ABA interconnects embedded in PCD was measured using a transmission line set-up to

correct for contact resistance The relationship between the resistance of a 500 microm length Au

interconnect and laser power utilized to mill the track is shown in Fig 7d

Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found

The 3D nature of the interconnect leads to a significant decrease in electrical resistance

compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is

material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In

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16

addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal

for thermal management applications due to diamondrsquos extremely high thermal conductivity

(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining

technologies is paramount Both varieties of braze are easily joined via laser welding

wirebonding or reflow soldering (Supplementary Information Fig S3)

32 Freestanding carbonaceous wires

321 Investigating braze-wire interactions

The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not

been studied as far as we are aware As a result the surface interaction of these materials was

investigated initially using PCD as a substrate PCD was originally chosen due to its low

electrical conductivity making it an adequate support structure for electrical characterization

However the interaction between PCD and Au though non-wetting is significant enough to

prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an

individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either

material though some bonding between Au ABA and PCD occurs A similar effect was

observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of

the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting

behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact

angle of ~404deg (Fig 8c)

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17

Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)

In the interest of creating freestanding carbonaceous wires with metallic contacts we

investigated whether the lack of wetting between a graphite substrate and braze alloy could be

exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate

(Fig 9) It has been observed previously that surface roughness can lead to large increases in

contact angle during brazing making graphite a reasonable choice to perhaps further diminish

the wetting behavior [42]

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18

Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)

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19

A graphite substrate was laser milled in an attempt to control separation distance between the

contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate

a number of geometries are possible for the freestanding wires Ag ABA was able to both wet

and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag

ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig

9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact

with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction

with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure

to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a

seal around the bundle effectively compressing the filaments together (Fig 9h) However if

enough force is applied the Au contact can be removed from the CF bundle though this was

not observed for CNT yarn devices This effect is likely proportional to the ratio of wire

diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of

the bond due to a rise in the contact surface area

322 Device fabrication and characterization

A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or

AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred

because it does not form a strong bond to the graphite substrate following a melting cycle so

the entire wire can be lifted off of the substrate with the contacts intact preventing damage to

the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing

facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent

lift-off was attempted Au does not bond to the graphite for reasons explained above When

the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing

both the carbonaceous fiber and a metallic wire within it (Fig 10a)

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20

Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample

The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig

10c) This technique is a potential improvement over mechanical clamps especially

considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from

clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a

woven or spun material such as CNT yarn prevents unwinding and generally helps to keep

the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead

to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)

were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a

linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the

CNT yarns in this work was comparable to those of similar size and length measured by

Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn

structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While

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21

preliminary measurements suggest that contact resistances are negligible further investigation

is needed to estimate the contact resistance through the contact It was of interest to determine

whether this process modified the carbon structure of the wire materials To investigate this

phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire

following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)

Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1

The before and after Raman spectra of CFs reveals very little change in the microstructure

aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This

peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a

prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor

graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears

quite prominent in both CF spectra presented here though there appears to have been some

MANUSCRIP

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22

moderate increase in graphitization following brazing Additionally the loss of the D-peak

overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to

increased graphitization However it is important to note that the graphitization process

during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to

create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several

peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)

Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1

(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible

that the loss of these peaks is the result of the loss of a surface functionality as a result of high

temperature treatment such as a C-O group However further investigation is required to

acquire a full understanding of the structural changes occurring not only as a result of the high

temperature processes in the presence of various metals In an attempt to improve the thermal

and electrical properties of CNTs and composites gamma irradiation and high temperature

treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease

in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in

the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal

treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that

the vacuum brazing processes described here do not have deleterious effects on carbonaceous

wires

33 Integrating diamond substrates with carbonaceous electrodes

Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)

nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective

MANUSCRIP

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23

processes for integrating these materials with a suitable substrate differ considerably For N-

UNCD stimulating electrodes our group has demonstrated a method to grow the conductive

diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]

This method produces a hermetic electrode array for implantable neural prosthetic

applications However the integration of this array with standard electronic components

requires a metallic interface utilizing the embedded circuit board technology described here

A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze

interconnects and contact pads utilized to contact conductive diamond electrodes (dotted

circle) in a full electrode array device (Fig 12a)

Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes

Incorporating carbonaceous wire electrodes into electronic devices is less elegant and

currently requires the manual placement of individual wire electrodes into designated laser-

MANUSCRIP

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24

milled grooves in the PCD substrate Next the suitable brazing paste is applied though a

screen-printing technique is required to bypass the mechanical polishing step Avoiding

mechanical polishing is important for delicate small and fragile electrode materials such as

those described in this work However because the brazing pastes are comprised of both

metal beads and a polymeric binder the functional volume of the interconnect is reduced

upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed

to produce a CF array embedded in diamond (Fig 12c) We are currently investigating

methods for protecting the CFs and other carbonaceous wires during mechanical polishing to

enable more complex robust device architectures

4 Conclusion

Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires

(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all

components paramount For PCD electronics Ag ABA is used due to its wetting and covalent

bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in

physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial

layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the

AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit

boards are compatible with commercially-available solders and are easily laser-welded for

solder-free joining The non-wetting method of enclosing carbonaceous wires was used to

create strong low-resistance and non-rectifying connections between PtIr wires and

carbonaceous wires Structural and interfacial examinations of these structures lend insight

into the forces at play during both the wetting and non-wetting fabrication of carbonaceous

devices We exploit surface tension effects of two commercially-available active brazing

alloys to create two different device architectures

(1) The wetting technique was used in conjunction with mechanical polishing to create

robust circuit substrates with embedded Ag or Au interconnects and contact pads The option

MANUSCRIP

T

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25

to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate

illustrates the utility of the method for enabling carbon-based electrode arrays

(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or

AuPtIr wire contacts This technique is interesting for the creation of not only free-standing

carbonaceous conductors but also for neural probes or sensors for both central and peripheral

nervous system integration We have previously described methods to fabricate neural

stimulation and recording electrodes from carbonaceous wires described here such as rLCGO

fiber which demonstrated both in vitro and in vivo functionality [47]

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for

insightful technical conversations NVA is supported by a MMI-CSIRO Material Science

PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was

supported by the Australian Research Council (ARC) through its Special Research Initiative

(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is

supported by the Australian Research Council under Discovery Early Career Researcher

award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and

support through the ARC Centre of Excellence for Electromaterials Science

Supplementary Information Available Figures S1-S4 This material is available free of charge online References

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26

[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

27

[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

28

[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

MANUSCRIP

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ACCEPTED MANUSCRIPT

10

purchased from Goodfellow On a graphite substrate two mounds of ABA paste were placed

on opposite sides of a 10 times 10 mm graphite wafer Either a bundle of CFs (Goodfellow) a

single CNT yarn (15 microm) or a graphene oxide fiber (50 microm) was placed across the mounds

and pressed into them Additionally one PtIr wire (75 microm Goodfellow) was placed into each

of the ABA paste mounds The graphite substrate was transferred to an electron beam

deposition system (Thermionics) and evacuated to 1 times 10-6 Pa The electron beam was

focused onto the graphite substratemdashas described above for PCD circuit board constructionmdash

and the current was increased until the substrate was glowing red The aforementioned

process is illustrated in Fig 4

Fig 4 - (a) Fabrication of freestanding metalABACNT yarn wires Mounds of ABA paste were placed onto a graphite substrate and either CNT yarns CFs or graphene oxide fibers were pressed into them PtIr wires were pressed into the end for testing (b) The graphite substrate was heated until the Au formed beads on the surface (c) Upon cooling the entire wiremetal structure was easily lifted from the surface

Once melting of the ABA was observed the current was ramped down over 1 minute and the

samples were cooled for 30 minutes in an N2 environment Au-ABA samples were easily

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11

removed from the graphite substrate due to lack of reaction while considerable force was

needed to remove the Ag ABA and TiCuNi samples

223 Integrated PCDcarbonaceous electrode devices

Our group has previously published a method for integrating PCD substrates with conductive

diamond electrodes to produce hermetic feedthroughs [21] Integrating carbonaceous wire

electrodes in this case 7 microm diameter CF was done manually as depicted in Fig 5

Fig 5 - Method for placing individual CF into laser cut groove in PCD substrate and securing the CF with braze paste The braze paste is then ldquoscreen-printedrdquo with a squeegee to remove the excess braze from the surface of the PCD to minimize shorting upon braze melting

PCD substrates were laser machined as described above to create grooves for CF filaments to

rest in Next individual CF filaments were manually placed into the grooves and Ag ABA

paste was spread over the entire surface The surface was wiped with a mini squeegee

(cleanroom cloth wrapped around syringe needle) to remove excess braze paste from the

surface Finally the sample was heated to melt the braze as described above

224 Electrical characterization of devices

The resistance of the Au ABA interconnects embedded in the polished PCD substrates were

measured using a 2-way probe station and a multimeter (Keithley Instruments) Transmission

line devices were fabricated to correct for contact resistance The electrical behavior of the

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12

freestanding PtIrAuCNT yarn structures was characterized by performing voltage sweeps (-1

to +1 V) with a potentiostat (eDAQ) while measuring current

3 Results and Discussion

31 Embedded-in-diamond circuit substrates

311 Investigating PCD-braze interactions

Three different commercially-available active brazing alloy (ABA) pastes were assessed for

their ability to wet and bond to polycrystalline diamond (PCD) substrates The elemental

composition of these brazes is included in Table 1 of the Experimental section Ag ABA was

found to spread and adhere strongly to PCD which was likely facilitated by the formation of

an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig

6a) SEM and EDX elemental analysis of the PCDAg ABA interface reveals a Ti-rich region

of ~34 microm adjacent to the Ag ABA joint (Supplementary Information Fig S1) Conversely

Au ABA did not spread on the PCD surface upon melting instead forming a droplet with a

111deg contact angle on the surface (Fig 6b)

Fig 6 - (a) Ag ABA wets PCD and bonds strongly (b) Au ABA does not wet PCD forming a droplet that makes ~111deg contact angle with the surface (c) TiCuNi braze wets the PCD surface but does not bond The cooled TiCuNi droplet is easily pulled from the PCD surface with forceps (d) Au ABA wets the PCD following a previous Ag ABA brazing step implemented to create a conformal adhesion layer

MANUSCRIP

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13

Though the manufacturers have included Ti as the active component in the Au ABA this

braze is unable to spread on carbonaceous materials Considering the elemental composition

of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack

of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active

component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in

the TiC-Au melt [33]

rarr + 132

Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which

is unsurprising considering the large difference in electronegativity (X) between Au

(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic

is a possibility but the electronegativity differences are less pronounced between Ti and Ag

(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this

reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based

medical devices [35] wetted the PCD surface upon heating but did not bond and was easily

removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and

bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)

This finding is consistent with what has been shown previously for TiCuNirsquos interaction with

carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding

between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between

the braze alloy and the material The stress induced by thermal mismatch (σTM) can be

described theoretically by Equation 3 [37]

13 ∙ ∆ (3)

Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the

braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the

temperature differential approximately 1000 K for the difference between brazing

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14

temperature and room temperature According to manufacturer data (Element Six) the PCD

used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates

have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203

ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the

TiCuNiPCD system is warranted though beyond the scope of the present work In

consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate

compatible with standard joining technologies (ie wirebonding laser welding reflow

soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system

a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a

subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If

a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded

thick- film conductors or contact pads required for the embedded-in-diamond circuit

structures described in the following sections Additionally the ubiquity of Au contacts in

semiconductor and electronics fabrication makes the AuPCD system a practical device

substrate

312 Device fabrication and characterization

The entire fabrication process for these devices is illustrated above in the Experimental

section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate

prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished

devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far

with this technique range between 6-15 microm in width Lower milling powers lead to less

reproducibility perhaps due to issues with focusing on the surface which would be more

pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are

likely possible following more extensive laser parameter optimization including adjustment

of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices

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15

were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au

ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled

structures having three dimensions which demonstrates a significant advantage over

conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing

process eliminates the extra annealing step to form TiC at the interface [40] The resistance of

Au ABA interconnects embedded in PCD was measured using a transmission line set-up to

correct for contact resistance The relationship between the resistance of a 500 microm length Au

interconnect and laser power utilized to mill the track is shown in Fig 7d

Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found

The 3D nature of the interconnect leads to a significant decrease in electrical resistance

compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is

material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In

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16

addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal

for thermal management applications due to diamondrsquos extremely high thermal conductivity

(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining

technologies is paramount Both varieties of braze are easily joined via laser welding

wirebonding or reflow soldering (Supplementary Information Fig S3)

32 Freestanding carbonaceous wires

321 Investigating braze-wire interactions

The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not

been studied as far as we are aware As a result the surface interaction of these materials was

investigated initially using PCD as a substrate PCD was originally chosen due to its low

electrical conductivity making it an adequate support structure for electrical characterization

However the interaction between PCD and Au though non-wetting is significant enough to

prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an

individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either

material though some bonding between Au ABA and PCD occurs A similar effect was

observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of

the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting

behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact

angle of ~404deg (Fig 8c)

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17

Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)

In the interest of creating freestanding carbonaceous wires with metallic contacts we

investigated whether the lack of wetting between a graphite substrate and braze alloy could be

exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate

(Fig 9) It has been observed previously that surface roughness can lead to large increases in

contact angle during brazing making graphite a reasonable choice to perhaps further diminish

the wetting behavior [42]

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18

Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)

MANUSCRIP

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19

A graphite substrate was laser milled in an attempt to control separation distance between the

contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate

a number of geometries are possible for the freestanding wires Ag ABA was able to both wet

and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag

ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig

9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact

with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction

with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure

to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a

seal around the bundle effectively compressing the filaments together (Fig 9h) However if

enough force is applied the Au contact can be removed from the CF bundle though this was

not observed for CNT yarn devices This effect is likely proportional to the ratio of wire

diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of

the bond due to a rise in the contact surface area

322 Device fabrication and characterization

A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or

AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred

because it does not form a strong bond to the graphite substrate following a melting cycle so

the entire wire can be lifted off of the substrate with the contacts intact preventing damage to

the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing

facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent

lift-off was attempted Au does not bond to the graphite for reasons explained above When

the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing

both the carbonaceous fiber and a metallic wire within it (Fig 10a)

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20

Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample

The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig

10c) This technique is a potential improvement over mechanical clamps especially

considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from

clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a

woven or spun material such as CNT yarn prevents unwinding and generally helps to keep

the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead

to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)

were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a

linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the

CNT yarns in this work was comparable to those of similar size and length measured by

Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn

structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While

MANUSCRIP

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21

preliminary measurements suggest that contact resistances are negligible further investigation

is needed to estimate the contact resistance through the contact It was of interest to determine

whether this process modified the carbon structure of the wire materials To investigate this

phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire

following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)

Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1

The before and after Raman spectra of CFs reveals very little change in the microstructure

aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This

peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a

prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor

graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears

quite prominent in both CF spectra presented here though there appears to have been some

MANUSCRIP

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22

moderate increase in graphitization following brazing Additionally the loss of the D-peak

overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to

increased graphitization However it is important to note that the graphitization process

during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to

create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several

peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)

Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1

(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible

that the loss of these peaks is the result of the loss of a surface functionality as a result of high

temperature treatment such as a C-O group However further investigation is required to

acquire a full understanding of the structural changes occurring not only as a result of the high

temperature processes in the presence of various metals In an attempt to improve the thermal

and electrical properties of CNTs and composites gamma irradiation and high temperature

treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease

in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in

the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal

treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that

the vacuum brazing processes described here do not have deleterious effects on carbonaceous

wires

33 Integrating diamond substrates with carbonaceous electrodes

Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)

nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective

MANUSCRIP

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23

processes for integrating these materials with a suitable substrate differ considerably For N-

UNCD stimulating electrodes our group has demonstrated a method to grow the conductive

diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]

This method produces a hermetic electrode array for implantable neural prosthetic

applications However the integration of this array with standard electronic components

requires a metallic interface utilizing the embedded circuit board technology described here

A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze

interconnects and contact pads utilized to contact conductive diamond electrodes (dotted

circle) in a full electrode array device (Fig 12a)

Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes

Incorporating carbonaceous wire electrodes into electronic devices is less elegant and

currently requires the manual placement of individual wire electrodes into designated laser-

MANUSCRIP

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24

milled grooves in the PCD substrate Next the suitable brazing paste is applied though a

screen-printing technique is required to bypass the mechanical polishing step Avoiding

mechanical polishing is important for delicate small and fragile electrode materials such as

those described in this work However because the brazing pastes are comprised of both

metal beads and a polymeric binder the functional volume of the interconnect is reduced

upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed

to produce a CF array embedded in diamond (Fig 12c) We are currently investigating

methods for protecting the CFs and other carbonaceous wires during mechanical polishing to

enable more complex robust device architectures

4 Conclusion

Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires

(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all

components paramount For PCD electronics Ag ABA is used due to its wetting and covalent

bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in

physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial

layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the

AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit

boards are compatible with commercially-available solders and are easily laser-welded for

solder-free joining The non-wetting method of enclosing carbonaceous wires was used to

create strong low-resistance and non-rectifying connections between PtIr wires and

carbonaceous wires Structural and interfacial examinations of these structures lend insight

into the forces at play during both the wetting and non-wetting fabrication of carbonaceous

devices We exploit surface tension effects of two commercially-available active brazing

alloys to create two different device architectures

(1) The wetting technique was used in conjunction with mechanical polishing to create

robust circuit substrates with embedded Ag or Au interconnects and contact pads The option

MANUSCRIP

T

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ACCEPTED MANUSCRIPT

25

to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate

illustrates the utility of the method for enabling carbon-based electrode arrays

(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or

AuPtIr wire contacts This technique is interesting for the creation of not only free-standing

carbonaceous conductors but also for neural probes or sensors for both central and peripheral

nervous system integration We have previously described methods to fabricate neural

stimulation and recording electrodes from carbonaceous wires described here such as rLCGO

fiber which demonstrated both in vitro and in vivo functionality [47]

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for

insightful technical conversations NVA is supported by a MMI-CSIRO Material Science

PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was

supported by the Australian Research Council (ARC) through its Special Research Initiative

(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is

supported by the Australian Research Council under Discovery Early Career Researcher

award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and

support through the ARC Centre of Excellence for Electromaterials Science

Supplementary Information Available Figures S1-S4 This material is available free of charge online References

MANUSCRIP

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26

[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

27

[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

28

[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

MANUSCRIP

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11

removed from the graphite substrate due to lack of reaction while considerable force was

needed to remove the Ag ABA and TiCuNi samples

223 Integrated PCDcarbonaceous electrode devices

Our group has previously published a method for integrating PCD substrates with conductive

diamond electrodes to produce hermetic feedthroughs [21] Integrating carbonaceous wire

electrodes in this case 7 microm diameter CF was done manually as depicted in Fig 5

Fig 5 - Method for placing individual CF into laser cut groove in PCD substrate and securing the CF with braze paste The braze paste is then ldquoscreen-printedrdquo with a squeegee to remove the excess braze from the surface of the PCD to minimize shorting upon braze melting

PCD substrates were laser machined as described above to create grooves for CF filaments to

rest in Next individual CF filaments were manually placed into the grooves and Ag ABA

paste was spread over the entire surface The surface was wiped with a mini squeegee

(cleanroom cloth wrapped around syringe needle) to remove excess braze paste from the

surface Finally the sample was heated to melt the braze as described above

224 Electrical characterization of devices

The resistance of the Au ABA interconnects embedded in the polished PCD substrates were

measured using a 2-way probe station and a multimeter (Keithley Instruments) Transmission

line devices were fabricated to correct for contact resistance The electrical behavior of the

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12

freestanding PtIrAuCNT yarn structures was characterized by performing voltage sweeps (-1

to +1 V) with a potentiostat (eDAQ) while measuring current

3 Results and Discussion

31 Embedded-in-diamond circuit substrates

311 Investigating PCD-braze interactions

Three different commercially-available active brazing alloy (ABA) pastes were assessed for

their ability to wet and bond to polycrystalline diamond (PCD) substrates The elemental

composition of these brazes is included in Table 1 of the Experimental section Ag ABA was

found to spread and adhere strongly to PCD which was likely facilitated by the formation of

an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig

6a) SEM and EDX elemental analysis of the PCDAg ABA interface reveals a Ti-rich region

of ~34 microm adjacent to the Ag ABA joint (Supplementary Information Fig S1) Conversely

Au ABA did not spread on the PCD surface upon melting instead forming a droplet with a

111deg contact angle on the surface (Fig 6b)

Fig 6 - (a) Ag ABA wets PCD and bonds strongly (b) Au ABA does not wet PCD forming a droplet that makes ~111deg contact angle with the surface (c) TiCuNi braze wets the PCD surface but does not bond The cooled TiCuNi droplet is easily pulled from the PCD surface with forceps (d) Au ABA wets the PCD following a previous Ag ABA brazing step implemented to create a conformal adhesion layer

MANUSCRIP

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13

Though the manufacturers have included Ti as the active component in the Au ABA this

braze is unable to spread on carbonaceous materials Considering the elemental composition

of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack

of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active

component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in

the TiC-Au melt [33]

rarr + 132

Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which

is unsurprising considering the large difference in electronegativity (X) between Au

(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic

is a possibility but the electronegativity differences are less pronounced between Ti and Ag

(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this

reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based

medical devices [35] wetted the PCD surface upon heating but did not bond and was easily

removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and

bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)

This finding is consistent with what has been shown previously for TiCuNirsquos interaction with

carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding

between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between

the braze alloy and the material The stress induced by thermal mismatch (σTM) can be

described theoretically by Equation 3 [37]

13 ∙ ∆ (3)

Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the

braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the

temperature differential approximately 1000 K for the difference between brazing

MANUSCRIP

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14

temperature and room temperature According to manufacturer data (Element Six) the PCD

used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates

have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203

ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the

TiCuNiPCD system is warranted though beyond the scope of the present work In

consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate

compatible with standard joining technologies (ie wirebonding laser welding reflow

soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system

a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a

subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If

a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded

thick- film conductors or contact pads required for the embedded-in-diamond circuit

structures described in the following sections Additionally the ubiquity of Au contacts in

semiconductor and electronics fabrication makes the AuPCD system a practical device

substrate

312 Device fabrication and characterization

The entire fabrication process for these devices is illustrated above in the Experimental

section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate

prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished

devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far

with this technique range between 6-15 microm in width Lower milling powers lead to less

reproducibility perhaps due to issues with focusing on the surface which would be more

pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are

likely possible following more extensive laser parameter optimization including adjustment

of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices

MANUSCRIP

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15

were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au

ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled

structures having three dimensions which demonstrates a significant advantage over

conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing

process eliminates the extra annealing step to form TiC at the interface [40] The resistance of

Au ABA interconnects embedded in PCD was measured using a transmission line set-up to

correct for contact resistance The relationship between the resistance of a 500 microm length Au

interconnect and laser power utilized to mill the track is shown in Fig 7d

Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found

The 3D nature of the interconnect leads to a significant decrease in electrical resistance

compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is

material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In

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16

addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal

for thermal management applications due to diamondrsquos extremely high thermal conductivity

(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining

technologies is paramount Both varieties of braze are easily joined via laser welding

wirebonding or reflow soldering (Supplementary Information Fig S3)

32 Freestanding carbonaceous wires

321 Investigating braze-wire interactions

The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not

been studied as far as we are aware As a result the surface interaction of these materials was

investigated initially using PCD as a substrate PCD was originally chosen due to its low

electrical conductivity making it an adequate support structure for electrical characterization

However the interaction between PCD and Au though non-wetting is significant enough to

prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an

individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either

material though some bonding between Au ABA and PCD occurs A similar effect was

observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of

the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting

behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact

angle of ~404deg (Fig 8c)

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17

Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)

In the interest of creating freestanding carbonaceous wires with metallic contacts we

investigated whether the lack of wetting between a graphite substrate and braze alloy could be

exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate

(Fig 9) It has been observed previously that surface roughness can lead to large increases in

contact angle during brazing making graphite a reasonable choice to perhaps further diminish

the wetting behavior [42]

MANUSCRIP

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18

Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)

MANUSCRIP

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19

A graphite substrate was laser milled in an attempt to control separation distance between the

contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate

a number of geometries are possible for the freestanding wires Ag ABA was able to both wet

and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag

ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig

9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact

with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction

with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure

to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a

seal around the bundle effectively compressing the filaments together (Fig 9h) However if

enough force is applied the Au contact can be removed from the CF bundle though this was

not observed for CNT yarn devices This effect is likely proportional to the ratio of wire

diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of

the bond due to a rise in the contact surface area

322 Device fabrication and characterization

A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or

AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred

because it does not form a strong bond to the graphite substrate following a melting cycle so

the entire wire can be lifted off of the substrate with the contacts intact preventing damage to

the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing

facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent

lift-off was attempted Au does not bond to the graphite for reasons explained above When

the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing

both the carbonaceous fiber and a metallic wire within it (Fig 10a)

MANUSCRIP

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20

Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample

The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig

10c) This technique is a potential improvement over mechanical clamps especially

considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from

clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a

woven or spun material such as CNT yarn prevents unwinding and generally helps to keep

the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead

to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)

were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a

linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the

CNT yarns in this work was comparable to those of similar size and length measured by

Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn

structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While

MANUSCRIP

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21

preliminary measurements suggest that contact resistances are negligible further investigation

is needed to estimate the contact resistance through the contact It was of interest to determine

whether this process modified the carbon structure of the wire materials To investigate this

phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire

following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)

Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1

The before and after Raman spectra of CFs reveals very little change in the microstructure

aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This

peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a

prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor

graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears

quite prominent in both CF spectra presented here though there appears to have been some

MANUSCRIP

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22

moderate increase in graphitization following brazing Additionally the loss of the D-peak

overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to

increased graphitization However it is important to note that the graphitization process

during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to

create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several

peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)

Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1

(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible

that the loss of these peaks is the result of the loss of a surface functionality as a result of high

temperature treatment such as a C-O group However further investigation is required to

acquire a full understanding of the structural changes occurring not only as a result of the high

temperature processes in the presence of various metals In an attempt to improve the thermal

and electrical properties of CNTs and composites gamma irradiation and high temperature

treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease

in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in

the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal

treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that

the vacuum brazing processes described here do not have deleterious effects on carbonaceous

wires

33 Integrating diamond substrates with carbonaceous electrodes

Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)

nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective

MANUSCRIP

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23

processes for integrating these materials with a suitable substrate differ considerably For N-

UNCD stimulating electrodes our group has demonstrated a method to grow the conductive

diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]

This method produces a hermetic electrode array for implantable neural prosthetic

applications However the integration of this array with standard electronic components

requires a metallic interface utilizing the embedded circuit board technology described here

A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze

interconnects and contact pads utilized to contact conductive diamond electrodes (dotted

circle) in a full electrode array device (Fig 12a)

Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes

Incorporating carbonaceous wire electrodes into electronic devices is less elegant and

currently requires the manual placement of individual wire electrodes into designated laser-

MANUSCRIP

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24

milled grooves in the PCD substrate Next the suitable brazing paste is applied though a

screen-printing technique is required to bypass the mechanical polishing step Avoiding

mechanical polishing is important for delicate small and fragile electrode materials such as

those described in this work However because the brazing pastes are comprised of both

metal beads and a polymeric binder the functional volume of the interconnect is reduced

upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed

to produce a CF array embedded in diamond (Fig 12c) We are currently investigating

methods for protecting the CFs and other carbonaceous wires during mechanical polishing to

enable more complex robust device architectures

4 Conclusion

Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires

(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all

components paramount For PCD electronics Ag ABA is used due to its wetting and covalent

bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in

physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial

layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the

AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit

boards are compatible with commercially-available solders and are easily laser-welded for

solder-free joining The non-wetting method of enclosing carbonaceous wires was used to

create strong low-resistance and non-rectifying connections between PtIr wires and

carbonaceous wires Structural and interfacial examinations of these structures lend insight

into the forces at play during both the wetting and non-wetting fabrication of carbonaceous

devices We exploit surface tension effects of two commercially-available active brazing

alloys to create two different device architectures

(1) The wetting technique was used in conjunction with mechanical polishing to create

robust circuit substrates with embedded Ag or Au interconnects and contact pads The option

MANUSCRIP

T

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ACCEPTED MANUSCRIPT

25

to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate

illustrates the utility of the method for enabling carbon-based electrode arrays

(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or

AuPtIr wire contacts This technique is interesting for the creation of not only free-standing

carbonaceous conductors but also for neural probes or sensors for both central and peripheral

nervous system integration We have previously described methods to fabricate neural

stimulation and recording electrodes from carbonaceous wires described here such as rLCGO

fiber which demonstrated both in vitro and in vivo functionality [47]

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for

insightful technical conversations NVA is supported by a MMI-CSIRO Material Science

PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was

supported by the Australian Research Council (ARC) through its Special Research Initiative

(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is

supported by the Australian Research Council under Discovery Early Career Researcher

award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and

support through the ARC Centre of Excellence for Electromaterials Science

Supplementary Information Available Figures S1-S4 This material is available free of charge online References

MANUSCRIP

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26

[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

MANUSCRIP

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ACCEPTED MANUSCRIPT

27

[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

28

[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

MANUSCRIP

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12

freestanding PtIrAuCNT yarn structures was characterized by performing voltage sweeps (-1

to +1 V) with a potentiostat (eDAQ) while measuring current

3 Results and Discussion

31 Embedded-in-diamond circuit substrates

311 Investigating PCD-braze interactions

Three different commercially-available active brazing alloy (ABA) pastes were assessed for

their ability to wet and bond to polycrystalline diamond (PCD) substrates The elemental

composition of these brazes is included in Table 1 of the Experimental section Ag ABA was

found to spread and adhere strongly to PCD which was likely facilitated by the formation of

an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig

6a) SEM and EDX elemental analysis of the PCDAg ABA interface reveals a Ti-rich region

of ~34 microm adjacent to the Ag ABA joint (Supplementary Information Fig S1) Conversely

Au ABA did not spread on the PCD surface upon melting instead forming a droplet with a

111deg contact angle on the surface (Fig 6b)

Fig 6 - (a) Ag ABA wets PCD and bonds strongly (b) Au ABA does not wet PCD forming a droplet that makes ~111deg contact angle with the surface (c) TiCuNi braze wets the PCD surface but does not bond The cooled TiCuNi droplet is easily pulled from the PCD surface with forceps (d) Au ABA wets the PCD following a previous Ag ABA brazing step implemented to create a conformal adhesion layer

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13

Though the manufacturers have included Ti as the active component in the Au ABA this

braze is unable to spread on carbonaceous materials Considering the elemental composition

of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack

of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active

component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in

the TiC-Au melt [33]

rarr + 132

Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which

is unsurprising considering the large difference in electronegativity (X) between Au

(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic

is a possibility but the electronegativity differences are less pronounced between Ti and Ag

(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this

reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based

medical devices [35] wetted the PCD surface upon heating but did not bond and was easily

removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and

bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)

This finding is consistent with what has been shown previously for TiCuNirsquos interaction with

carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding

between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between

the braze alloy and the material The stress induced by thermal mismatch (σTM) can be

described theoretically by Equation 3 [37]

13 ∙ ∆ (3)

Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the

braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the

temperature differential approximately 1000 K for the difference between brazing

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T

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14

temperature and room temperature According to manufacturer data (Element Six) the PCD

used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates

have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203

ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the

TiCuNiPCD system is warranted though beyond the scope of the present work In

consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate

compatible with standard joining technologies (ie wirebonding laser welding reflow

soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system

a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a

subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If

a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded

thick- film conductors or contact pads required for the embedded-in-diamond circuit

structures described in the following sections Additionally the ubiquity of Au contacts in

semiconductor and electronics fabrication makes the AuPCD system a practical device

substrate

312 Device fabrication and characterization

The entire fabrication process for these devices is illustrated above in the Experimental

section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate

prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished

devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far

with this technique range between 6-15 microm in width Lower milling powers lead to less

reproducibility perhaps due to issues with focusing on the surface which would be more

pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are

likely possible following more extensive laser parameter optimization including adjustment

of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices

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15

were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au

ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled

structures having three dimensions which demonstrates a significant advantage over

conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing

process eliminates the extra annealing step to form TiC at the interface [40] The resistance of

Au ABA interconnects embedded in PCD was measured using a transmission line set-up to

correct for contact resistance The relationship between the resistance of a 500 microm length Au

interconnect and laser power utilized to mill the track is shown in Fig 7d

Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found

The 3D nature of the interconnect leads to a significant decrease in electrical resistance

compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is

material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In

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16

addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal

for thermal management applications due to diamondrsquos extremely high thermal conductivity

(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining

technologies is paramount Both varieties of braze are easily joined via laser welding

wirebonding or reflow soldering (Supplementary Information Fig S3)

32 Freestanding carbonaceous wires

321 Investigating braze-wire interactions

The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not

been studied as far as we are aware As a result the surface interaction of these materials was

investigated initially using PCD as a substrate PCD was originally chosen due to its low

electrical conductivity making it an adequate support structure for electrical characterization

However the interaction between PCD and Au though non-wetting is significant enough to

prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an

individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either

material though some bonding between Au ABA and PCD occurs A similar effect was

observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of

the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting

behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact

angle of ~404deg (Fig 8c)

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17

Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)

In the interest of creating freestanding carbonaceous wires with metallic contacts we

investigated whether the lack of wetting between a graphite substrate and braze alloy could be

exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate

(Fig 9) It has been observed previously that surface roughness can lead to large increases in

contact angle during brazing making graphite a reasonable choice to perhaps further diminish

the wetting behavior [42]

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18

Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)

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19

A graphite substrate was laser milled in an attempt to control separation distance between the

contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate

a number of geometries are possible for the freestanding wires Ag ABA was able to both wet

and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag

ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig

9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact

with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction

with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure

to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a

seal around the bundle effectively compressing the filaments together (Fig 9h) However if

enough force is applied the Au contact can be removed from the CF bundle though this was

not observed for CNT yarn devices This effect is likely proportional to the ratio of wire

diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of

the bond due to a rise in the contact surface area

322 Device fabrication and characterization

A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or

AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred

because it does not form a strong bond to the graphite substrate following a melting cycle so

the entire wire can be lifted off of the substrate with the contacts intact preventing damage to

the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing

facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent

lift-off was attempted Au does not bond to the graphite for reasons explained above When

the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing

both the carbonaceous fiber and a metallic wire within it (Fig 10a)

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20

Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample

The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig

10c) This technique is a potential improvement over mechanical clamps especially

considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from

clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a

woven or spun material such as CNT yarn prevents unwinding and generally helps to keep

the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead

to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)

were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a

linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the

CNT yarns in this work was comparable to those of similar size and length measured by

Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn

structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While

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21

preliminary measurements suggest that contact resistances are negligible further investigation

is needed to estimate the contact resistance through the contact It was of interest to determine

whether this process modified the carbon structure of the wire materials To investigate this

phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire

following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)

Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1

The before and after Raman spectra of CFs reveals very little change in the microstructure

aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This

peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a

prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor

graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears

quite prominent in both CF spectra presented here though there appears to have been some

MANUSCRIP

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22

moderate increase in graphitization following brazing Additionally the loss of the D-peak

overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to

increased graphitization However it is important to note that the graphitization process

during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to

create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several

peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)

Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1

(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible

that the loss of these peaks is the result of the loss of a surface functionality as a result of high

temperature treatment such as a C-O group However further investigation is required to

acquire a full understanding of the structural changes occurring not only as a result of the high

temperature processes in the presence of various metals In an attempt to improve the thermal

and electrical properties of CNTs and composites gamma irradiation and high temperature

treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease

in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in

the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal

treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that

the vacuum brazing processes described here do not have deleterious effects on carbonaceous

wires

33 Integrating diamond substrates with carbonaceous electrodes

Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)

nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective

MANUSCRIP

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23

processes for integrating these materials with a suitable substrate differ considerably For N-

UNCD stimulating electrodes our group has demonstrated a method to grow the conductive

diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]

This method produces a hermetic electrode array for implantable neural prosthetic

applications However the integration of this array with standard electronic components

requires a metallic interface utilizing the embedded circuit board technology described here

A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze

interconnects and contact pads utilized to contact conductive diamond electrodes (dotted

circle) in a full electrode array device (Fig 12a)

Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes

Incorporating carbonaceous wire electrodes into electronic devices is less elegant and

currently requires the manual placement of individual wire electrodes into designated laser-

MANUSCRIP

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24

milled grooves in the PCD substrate Next the suitable brazing paste is applied though a

screen-printing technique is required to bypass the mechanical polishing step Avoiding

mechanical polishing is important for delicate small and fragile electrode materials such as

those described in this work However because the brazing pastes are comprised of both

metal beads and a polymeric binder the functional volume of the interconnect is reduced

upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed

to produce a CF array embedded in diamond (Fig 12c) We are currently investigating

methods for protecting the CFs and other carbonaceous wires during mechanical polishing to

enable more complex robust device architectures

4 Conclusion

Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires

(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all

components paramount For PCD electronics Ag ABA is used due to its wetting and covalent

bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in

physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial

layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the

AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit

boards are compatible with commercially-available solders and are easily laser-welded for

solder-free joining The non-wetting method of enclosing carbonaceous wires was used to

create strong low-resistance and non-rectifying connections between PtIr wires and

carbonaceous wires Structural and interfacial examinations of these structures lend insight

into the forces at play during both the wetting and non-wetting fabrication of carbonaceous

devices We exploit surface tension effects of two commercially-available active brazing

alloys to create two different device architectures

(1) The wetting technique was used in conjunction with mechanical polishing to create

robust circuit substrates with embedded Ag or Au interconnects and contact pads The option

MANUSCRIP

T

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25

to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate

illustrates the utility of the method for enabling carbon-based electrode arrays

(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or

AuPtIr wire contacts This technique is interesting for the creation of not only free-standing

carbonaceous conductors but also for neural probes or sensors for both central and peripheral

nervous system integration We have previously described methods to fabricate neural

stimulation and recording electrodes from carbonaceous wires described here such as rLCGO

fiber which demonstrated both in vitro and in vivo functionality [47]

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for

insightful technical conversations NVA is supported by a MMI-CSIRO Material Science

PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was

supported by the Australian Research Council (ARC) through its Special Research Initiative

(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is

supported by the Australian Research Council under Discovery Early Career Researcher

award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and

support through the ARC Centre of Excellence for Electromaterials Science

Supplementary Information Available Figures S1-S4 This material is available free of charge online References

MANUSCRIP

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ACCEPTED MANUSCRIPT

26

[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

MANUSCRIP

T

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27

[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

MANUSCRIP

T

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28

[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

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13

Though the manufacturers have included Ti as the active component in the Au ABA this

braze is unable to spread on carbonaceous materials Considering the elemental composition

of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack

of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active

component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in

the TiC-Au melt [33]

rarr + 132

Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which

is unsurprising considering the large difference in electronegativity (X) between Au

(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic

is a possibility but the electronegativity differences are less pronounced between Ti and Ag

(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this

reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based

medical devices [35] wetted the PCD surface upon heating but did not bond and was easily

removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and

bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)

This finding is consistent with what has been shown previously for TiCuNirsquos interaction with

carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding

between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between

the braze alloy and the material The stress induced by thermal mismatch (σTM) can be

described theoretically by Equation 3 [37]

13 ∙ ∆ (3)

Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the

braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the

temperature differential approximately 1000 K for the difference between brazing

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14

temperature and room temperature According to manufacturer data (Element Six) the PCD

used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates

have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203

ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the

TiCuNiPCD system is warranted though beyond the scope of the present work In

consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate

compatible with standard joining technologies (ie wirebonding laser welding reflow

soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system

a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a

subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If

a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded

thick- film conductors or contact pads required for the embedded-in-diamond circuit

structures described in the following sections Additionally the ubiquity of Au contacts in

semiconductor and electronics fabrication makes the AuPCD system a practical device

substrate

312 Device fabrication and characterization

The entire fabrication process for these devices is illustrated above in the Experimental

section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate

prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished

devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far

with this technique range between 6-15 microm in width Lower milling powers lead to less

reproducibility perhaps due to issues with focusing on the surface which would be more

pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are

likely possible following more extensive laser parameter optimization including adjustment

of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices

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15

were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au

ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled

structures having three dimensions which demonstrates a significant advantage over

conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing

process eliminates the extra annealing step to form TiC at the interface [40] The resistance of

Au ABA interconnects embedded in PCD was measured using a transmission line set-up to

correct for contact resistance The relationship between the resistance of a 500 microm length Au

interconnect and laser power utilized to mill the track is shown in Fig 7d

Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found

The 3D nature of the interconnect leads to a significant decrease in electrical resistance

compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is

material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In

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16

addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal

for thermal management applications due to diamondrsquos extremely high thermal conductivity

(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining

technologies is paramount Both varieties of braze are easily joined via laser welding

wirebonding or reflow soldering (Supplementary Information Fig S3)

32 Freestanding carbonaceous wires

321 Investigating braze-wire interactions

The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not

been studied as far as we are aware As a result the surface interaction of these materials was

investigated initially using PCD as a substrate PCD was originally chosen due to its low

electrical conductivity making it an adequate support structure for electrical characterization

However the interaction between PCD and Au though non-wetting is significant enough to

prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an

individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either

material though some bonding between Au ABA and PCD occurs A similar effect was

observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of

the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting

behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact

angle of ~404deg (Fig 8c)

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17

Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)

In the interest of creating freestanding carbonaceous wires with metallic contacts we

investigated whether the lack of wetting between a graphite substrate and braze alloy could be

exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate

(Fig 9) It has been observed previously that surface roughness can lead to large increases in

contact angle during brazing making graphite a reasonable choice to perhaps further diminish

the wetting behavior [42]

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18

Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)

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19

A graphite substrate was laser milled in an attempt to control separation distance between the

contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate

a number of geometries are possible for the freestanding wires Ag ABA was able to both wet

and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag

ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig

9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact

with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction

with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure

to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a

seal around the bundle effectively compressing the filaments together (Fig 9h) However if

enough force is applied the Au contact can be removed from the CF bundle though this was

not observed for CNT yarn devices This effect is likely proportional to the ratio of wire

diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of

the bond due to a rise in the contact surface area

322 Device fabrication and characterization

A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or

AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred

because it does not form a strong bond to the graphite substrate following a melting cycle so

the entire wire can be lifted off of the substrate with the contacts intact preventing damage to

the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing

facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent

lift-off was attempted Au does not bond to the graphite for reasons explained above When

the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing

both the carbonaceous fiber and a metallic wire within it (Fig 10a)

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20

Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample

The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig

10c) This technique is a potential improvement over mechanical clamps especially

considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from

clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a

woven or spun material such as CNT yarn prevents unwinding and generally helps to keep

the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead

to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)

were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a

linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the

CNT yarns in this work was comparable to those of similar size and length measured by

Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn

structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While

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21

preliminary measurements suggest that contact resistances are negligible further investigation

is needed to estimate the contact resistance through the contact It was of interest to determine

whether this process modified the carbon structure of the wire materials To investigate this

phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire

following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)

Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1

The before and after Raman spectra of CFs reveals very little change in the microstructure

aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This

peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a

prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor

graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears

quite prominent in both CF spectra presented here though there appears to have been some

MANUSCRIP

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22

moderate increase in graphitization following brazing Additionally the loss of the D-peak

overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to

increased graphitization However it is important to note that the graphitization process

during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to

create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several

peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)

Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1

(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible

that the loss of these peaks is the result of the loss of a surface functionality as a result of high

temperature treatment such as a C-O group However further investigation is required to

acquire a full understanding of the structural changes occurring not only as a result of the high

temperature processes in the presence of various metals In an attempt to improve the thermal

and electrical properties of CNTs and composites gamma irradiation and high temperature

treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease

in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in

the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal

treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that

the vacuum brazing processes described here do not have deleterious effects on carbonaceous

wires

33 Integrating diamond substrates with carbonaceous electrodes

Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)

nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective

MANUSCRIP

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23

processes for integrating these materials with a suitable substrate differ considerably For N-

UNCD stimulating electrodes our group has demonstrated a method to grow the conductive

diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]

This method produces a hermetic electrode array for implantable neural prosthetic

applications However the integration of this array with standard electronic components

requires a metallic interface utilizing the embedded circuit board technology described here

A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze

interconnects and contact pads utilized to contact conductive diamond electrodes (dotted

circle) in a full electrode array device (Fig 12a)

Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes

Incorporating carbonaceous wire electrodes into electronic devices is less elegant and

currently requires the manual placement of individual wire electrodes into designated laser-

MANUSCRIP

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24

milled grooves in the PCD substrate Next the suitable brazing paste is applied though a

screen-printing technique is required to bypass the mechanical polishing step Avoiding

mechanical polishing is important for delicate small and fragile electrode materials such as

those described in this work However because the brazing pastes are comprised of both

metal beads and a polymeric binder the functional volume of the interconnect is reduced

upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed

to produce a CF array embedded in diamond (Fig 12c) We are currently investigating

methods for protecting the CFs and other carbonaceous wires during mechanical polishing to

enable more complex robust device architectures

4 Conclusion

Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires

(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all

components paramount For PCD electronics Ag ABA is used due to its wetting and covalent

bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in

physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial

layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the

AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit

boards are compatible with commercially-available solders and are easily laser-welded for

solder-free joining The non-wetting method of enclosing carbonaceous wires was used to

create strong low-resistance and non-rectifying connections between PtIr wires and

carbonaceous wires Structural and interfacial examinations of these structures lend insight

into the forces at play during both the wetting and non-wetting fabrication of carbonaceous

devices We exploit surface tension effects of two commercially-available active brazing

alloys to create two different device architectures

(1) The wetting technique was used in conjunction with mechanical polishing to create

robust circuit substrates with embedded Ag or Au interconnects and contact pads The option

MANUSCRIP

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25

to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate

illustrates the utility of the method for enabling carbon-based electrode arrays

(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or

AuPtIr wire contacts This technique is interesting for the creation of not only free-standing

carbonaceous conductors but also for neural probes or sensors for both central and peripheral

nervous system integration We have previously described methods to fabricate neural

stimulation and recording electrodes from carbonaceous wires described here such as rLCGO

fiber which demonstrated both in vitro and in vivo functionality [47]

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for

insightful technical conversations NVA is supported by a MMI-CSIRO Material Science

PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was

supported by the Australian Research Council (ARC) through its Special Research Initiative

(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is

supported by the Australian Research Council under Discovery Early Career Researcher

award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and

support through the ARC Centre of Excellence for Electromaterials Science

Supplementary Information Available Figures S1-S4 This material is available free of charge online References

MANUSCRIP

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26

[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

MANUSCRIP

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ACCEPTED MANUSCRIPT

27

[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

MANUSCRIP

T

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ACCEPTED MANUSCRIPT

28

[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

MANUSCRIP

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14

temperature and room temperature According to manufacturer data (Element Six) the PCD

used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates

have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203

ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the

TiCuNiPCD system is warranted though beyond the scope of the present work In

consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate

compatible with standard joining technologies (ie wirebonding laser welding reflow

soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system

a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a

subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If

a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded

thick- film conductors or contact pads required for the embedded-in-diamond circuit

structures described in the following sections Additionally the ubiquity of Au contacts in

semiconductor and electronics fabrication makes the AuPCD system a practical device

substrate

312 Device fabrication and characterization

The entire fabrication process for these devices is illustrated above in the Experimental

section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate

prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished

devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far

with this technique range between 6-15 microm in width Lower milling powers lead to less

reproducibility perhaps due to issues with focusing on the surface which would be more

pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are

likely possible following more extensive laser parameter optimization including adjustment

of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices

MANUSCRIP

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15

were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au

ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled

structures having three dimensions which demonstrates a significant advantage over

conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing

process eliminates the extra annealing step to form TiC at the interface [40] The resistance of

Au ABA interconnects embedded in PCD was measured using a transmission line set-up to

correct for contact resistance The relationship between the resistance of a 500 microm length Au

interconnect and laser power utilized to mill the track is shown in Fig 7d

Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found

The 3D nature of the interconnect leads to a significant decrease in electrical resistance

compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is

material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In

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16

addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal

for thermal management applications due to diamondrsquos extremely high thermal conductivity

(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining

technologies is paramount Both varieties of braze are easily joined via laser welding

wirebonding or reflow soldering (Supplementary Information Fig S3)

32 Freestanding carbonaceous wires

321 Investigating braze-wire interactions

The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not

been studied as far as we are aware As a result the surface interaction of these materials was

investigated initially using PCD as a substrate PCD was originally chosen due to its low

electrical conductivity making it an adequate support structure for electrical characterization

However the interaction between PCD and Au though non-wetting is significant enough to

prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an

individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either

material though some bonding between Au ABA and PCD occurs A similar effect was

observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of

the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting

behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact

angle of ~404deg (Fig 8c)

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17

Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)

In the interest of creating freestanding carbonaceous wires with metallic contacts we

investigated whether the lack of wetting between a graphite substrate and braze alloy could be

exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate

(Fig 9) It has been observed previously that surface roughness can lead to large increases in

contact angle during brazing making graphite a reasonable choice to perhaps further diminish

the wetting behavior [42]

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18

Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)

MANUSCRIP

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19

A graphite substrate was laser milled in an attempt to control separation distance between the

contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate

a number of geometries are possible for the freestanding wires Ag ABA was able to both wet

and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag

ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig

9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact

with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction

with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure

to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a

seal around the bundle effectively compressing the filaments together (Fig 9h) However if

enough force is applied the Au contact can be removed from the CF bundle though this was

not observed for CNT yarn devices This effect is likely proportional to the ratio of wire

diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of

the bond due to a rise in the contact surface area

322 Device fabrication and characterization

A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or

AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred

because it does not form a strong bond to the graphite substrate following a melting cycle so

the entire wire can be lifted off of the substrate with the contacts intact preventing damage to

the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing

facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent

lift-off was attempted Au does not bond to the graphite for reasons explained above When

the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing

both the carbonaceous fiber and a metallic wire within it (Fig 10a)

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20

Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample

The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig

10c) This technique is a potential improvement over mechanical clamps especially

considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from

clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a

woven or spun material such as CNT yarn prevents unwinding and generally helps to keep

the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead

to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)

were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a

linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the

CNT yarns in this work was comparable to those of similar size and length measured by

Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn

structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While

MANUSCRIP

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21

preliminary measurements suggest that contact resistances are negligible further investigation

is needed to estimate the contact resistance through the contact It was of interest to determine

whether this process modified the carbon structure of the wire materials To investigate this

phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire

following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)

Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1

The before and after Raman spectra of CFs reveals very little change in the microstructure

aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This

peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a

prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor

graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears

quite prominent in both CF spectra presented here though there appears to have been some

MANUSCRIP

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22

moderate increase in graphitization following brazing Additionally the loss of the D-peak

overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to

increased graphitization However it is important to note that the graphitization process

during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to

create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several

peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)

Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1

(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible

that the loss of these peaks is the result of the loss of a surface functionality as a result of high

temperature treatment such as a C-O group However further investigation is required to

acquire a full understanding of the structural changes occurring not only as a result of the high

temperature processes in the presence of various metals In an attempt to improve the thermal

and electrical properties of CNTs and composites gamma irradiation and high temperature

treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease

in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in

the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal

treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that

the vacuum brazing processes described here do not have deleterious effects on carbonaceous

wires

33 Integrating diamond substrates with carbonaceous electrodes

Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)

nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective

MANUSCRIP

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23

processes for integrating these materials with a suitable substrate differ considerably For N-

UNCD stimulating electrodes our group has demonstrated a method to grow the conductive

diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]

This method produces a hermetic electrode array for implantable neural prosthetic

applications However the integration of this array with standard electronic components

requires a metallic interface utilizing the embedded circuit board technology described here

A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze

interconnects and contact pads utilized to contact conductive diamond electrodes (dotted

circle) in a full electrode array device (Fig 12a)

Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes

Incorporating carbonaceous wire electrodes into electronic devices is less elegant and

currently requires the manual placement of individual wire electrodes into designated laser-

MANUSCRIP

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24

milled grooves in the PCD substrate Next the suitable brazing paste is applied though a

screen-printing technique is required to bypass the mechanical polishing step Avoiding

mechanical polishing is important for delicate small and fragile electrode materials such as

those described in this work However because the brazing pastes are comprised of both

metal beads and a polymeric binder the functional volume of the interconnect is reduced

upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed

to produce a CF array embedded in diamond (Fig 12c) We are currently investigating

methods for protecting the CFs and other carbonaceous wires during mechanical polishing to

enable more complex robust device architectures

4 Conclusion

Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires

(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all

components paramount For PCD electronics Ag ABA is used due to its wetting and covalent

bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in

physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial

layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the

AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit

boards are compatible with commercially-available solders and are easily laser-welded for

solder-free joining The non-wetting method of enclosing carbonaceous wires was used to

create strong low-resistance and non-rectifying connections between PtIr wires and

carbonaceous wires Structural and interfacial examinations of these structures lend insight

into the forces at play during both the wetting and non-wetting fabrication of carbonaceous

devices We exploit surface tension effects of two commercially-available active brazing

alloys to create two different device architectures

(1) The wetting technique was used in conjunction with mechanical polishing to create

robust circuit substrates with embedded Ag or Au interconnects and contact pads The option

MANUSCRIP

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25

to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate

illustrates the utility of the method for enabling carbon-based electrode arrays

(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or

AuPtIr wire contacts This technique is interesting for the creation of not only free-standing

carbonaceous conductors but also for neural probes or sensors for both central and peripheral

nervous system integration We have previously described methods to fabricate neural

stimulation and recording electrodes from carbonaceous wires described here such as rLCGO

fiber which demonstrated both in vitro and in vivo functionality [47]

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for

insightful technical conversations NVA is supported by a MMI-CSIRO Material Science

PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was

supported by the Australian Research Council (ARC) through its Special Research Initiative

(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is

supported by the Australian Research Council under Discovery Early Career Researcher

award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and

support through the ARC Centre of Excellence for Electromaterials Science

Supplementary Information Available Figures S1-S4 This material is available free of charge online References

MANUSCRIP

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26

[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

MANUSCRIP

T

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27

[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

28

[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

15

were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au

ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled

structures having three dimensions which demonstrates a significant advantage over

conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing

process eliminates the extra annealing step to form TiC at the interface [40] The resistance of

Au ABA interconnects embedded in PCD was measured using a transmission line set-up to

correct for contact resistance The relationship between the resistance of a 500 microm length Au

interconnect and laser power utilized to mill the track is shown in Fig 7d

Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found

The 3D nature of the interconnect leads to a significant decrease in electrical resistance

compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is

material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In

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16

addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal

for thermal management applications due to diamondrsquos extremely high thermal conductivity

(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining

technologies is paramount Both varieties of braze are easily joined via laser welding

wirebonding or reflow soldering (Supplementary Information Fig S3)

32 Freestanding carbonaceous wires

321 Investigating braze-wire interactions

The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not

been studied as far as we are aware As a result the surface interaction of these materials was

investigated initially using PCD as a substrate PCD was originally chosen due to its low

electrical conductivity making it an adequate support structure for electrical characterization

However the interaction between PCD and Au though non-wetting is significant enough to

prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an

individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either

material though some bonding between Au ABA and PCD occurs A similar effect was

observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of

the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting

behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact

angle of ~404deg (Fig 8c)

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17

Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)

In the interest of creating freestanding carbonaceous wires with metallic contacts we

investigated whether the lack of wetting between a graphite substrate and braze alloy could be

exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate

(Fig 9) It has been observed previously that surface roughness can lead to large increases in

contact angle during brazing making graphite a reasonable choice to perhaps further diminish

the wetting behavior [42]

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18

Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)

MANUSCRIP

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19

A graphite substrate was laser milled in an attempt to control separation distance between the

contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate

a number of geometries are possible for the freestanding wires Ag ABA was able to both wet

and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag

ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig

9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact

with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction

with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure

to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a

seal around the bundle effectively compressing the filaments together (Fig 9h) However if

enough force is applied the Au contact can be removed from the CF bundle though this was

not observed for CNT yarn devices This effect is likely proportional to the ratio of wire

diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of

the bond due to a rise in the contact surface area

322 Device fabrication and characterization

A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or

AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred

because it does not form a strong bond to the graphite substrate following a melting cycle so

the entire wire can be lifted off of the substrate with the contacts intact preventing damage to

the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing

facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent

lift-off was attempted Au does not bond to the graphite for reasons explained above When

the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing

both the carbonaceous fiber and a metallic wire within it (Fig 10a)

MANUSCRIP

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20

Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample

The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig

10c) This technique is a potential improvement over mechanical clamps especially

considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from

clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a

woven or spun material such as CNT yarn prevents unwinding and generally helps to keep

the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead

to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)

were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a

linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the

CNT yarns in this work was comparable to those of similar size and length measured by

Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn

structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While

MANUSCRIP

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21

preliminary measurements suggest that contact resistances are negligible further investigation

is needed to estimate the contact resistance through the contact It was of interest to determine

whether this process modified the carbon structure of the wire materials To investigate this

phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire

following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)

Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1

The before and after Raman spectra of CFs reveals very little change in the microstructure

aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This

peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a

prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor

graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears

quite prominent in both CF spectra presented here though there appears to have been some

MANUSCRIP

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22

moderate increase in graphitization following brazing Additionally the loss of the D-peak

overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to

increased graphitization However it is important to note that the graphitization process

during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to

create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several

peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)

Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1

(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible

that the loss of these peaks is the result of the loss of a surface functionality as a result of high

temperature treatment such as a C-O group However further investigation is required to

acquire a full understanding of the structural changes occurring not only as a result of the high

temperature processes in the presence of various metals In an attempt to improve the thermal

and electrical properties of CNTs and composites gamma irradiation and high temperature

treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease

in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in

the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal

treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that

the vacuum brazing processes described here do not have deleterious effects on carbonaceous

wires

33 Integrating diamond substrates with carbonaceous electrodes

Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)

nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective

MANUSCRIP

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23

processes for integrating these materials with a suitable substrate differ considerably For N-

UNCD stimulating electrodes our group has demonstrated a method to grow the conductive

diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]

This method produces a hermetic electrode array for implantable neural prosthetic

applications However the integration of this array with standard electronic components

requires a metallic interface utilizing the embedded circuit board technology described here

A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze

interconnects and contact pads utilized to contact conductive diamond electrodes (dotted

circle) in a full electrode array device (Fig 12a)

Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes

Incorporating carbonaceous wire electrodes into electronic devices is less elegant and

currently requires the manual placement of individual wire electrodes into designated laser-

MANUSCRIP

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24

milled grooves in the PCD substrate Next the suitable brazing paste is applied though a

screen-printing technique is required to bypass the mechanical polishing step Avoiding

mechanical polishing is important for delicate small and fragile electrode materials such as

those described in this work However because the brazing pastes are comprised of both

metal beads and a polymeric binder the functional volume of the interconnect is reduced

upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed

to produce a CF array embedded in diamond (Fig 12c) We are currently investigating

methods for protecting the CFs and other carbonaceous wires during mechanical polishing to

enable more complex robust device architectures

4 Conclusion

Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires

(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all

components paramount For PCD electronics Ag ABA is used due to its wetting and covalent

bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in

physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial

layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the

AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit

boards are compatible with commercially-available solders and are easily laser-welded for

solder-free joining The non-wetting method of enclosing carbonaceous wires was used to

create strong low-resistance and non-rectifying connections between PtIr wires and

carbonaceous wires Structural and interfacial examinations of these structures lend insight

into the forces at play during both the wetting and non-wetting fabrication of carbonaceous

devices We exploit surface tension effects of two commercially-available active brazing

alloys to create two different device architectures

(1) The wetting technique was used in conjunction with mechanical polishing to create

robust circuit substrates with embedded Ag or Au interconnects and contact pads The option

MANUSCRIP

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25

to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate

illustrates the utility of the method for enabling carbon-based electrode arrays

(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or

AuPtIr wire contacts This technique is interesting for the creation of not only free-standing

carbonaceous conductors but also for neural probes or sensors for both central and peripheral

nervous system integration We have previously described methods to fabricate neural

stimulation and recording electrodes from carbonaceous wires described here such as rLCGO

fiber which demonstrated both in vitro and in vivo functionality [47]

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for

insightful technical conversations NVA is supported by a MMI-CSIRO Material Science

PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was

supported by the Australian Research Council (ARC) through its Special Research Initiative

(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is

supported by the Australian Research Council under Discovery Early Career Researcher

award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and

support through the ARC Centre of Excellence for Electromaterials Science

Supplementary Information Available Figures S1-S4 This material is available free of charge online References

MANUSCRIP

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26

[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

MANUSCRIP

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ACCEPTED MANUSCRIPT

27

[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

28

[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

16

addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal

for thermal management applications due to diamondrsquos extremely high thermal conductivity

(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining

technologies is paramount Both varieties of braze are easily joined via laser welding

wirebonding or reflow soldering (Supplementary Information Fig S3)

32 Freestanding carbonaceous wires

321 Investigating braze-wire interactions

The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not

been studied as far as we are aware As a result the surface interaction of these materials was

investigated initially using PCD as a substrate PCD was originally chosen due to its low

electrical conductivity making it an adequate support structure for electrical characterization

However the interaction between PCD and Au though non-wetting is significant enough to

prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an

individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either

material though some bonding between Au ABA and PCD occurs A similar effect was

observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of

the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting

behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact

angle of ~404deg (Fig 8c)

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

17

Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)

In the interest of creating freestanding carbonaceous wires with metallic contacts we

investigated whether the lack of wetting between a graphite substrate and braze alloy could be

exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate

(Fig 9) It has been observed previously that surface roughness can lead to large increases in

contact angle during brazing making graphite a reasonable choice to perhaps further diminish

the wetting behavior [42]

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

18

Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

19

A graphite substrate was laser milled in an attempt to control separation distance between the

contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate

a number of geometries are possible for the freestanding wires Ag ABA was able to both wet

and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag

ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig

9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact

with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction

with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure

to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a

seal around the bundle effectively compressing the filaments together (Fig 9h) However if

enough force is applied the Au contact can be removed from the CF bundle though this was

not observed for CNT yarn devices This effect is likely proportional to the ratio of wire

diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of

the bond due to a rise in the contact surface area

322 Device fabrication and characterization

A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or

AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred

because it does not form a strong bond to the graphite substrate following a melting cycle so

the entire wire can be lifted off of the substrate with the contacts intact preventing damage to

the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing

facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent

lift-off was attempted Au does not bond to the graphite for reasons explained above When

the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing

both the carbonaceous fiber and a metallic wire within it (Fig 10a)

MANUSCRIP

T

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20

Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample

The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig

10c) This technique is a potential improvement over mechanical clamps especially

considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from

clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a

woven or spun material such as CNT yarn prevents unwinding and generally helps to keep

the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead

to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)

were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a

linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the

CNT yarns in this work was comparable to those of similar size and length measured by

Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn

structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

21

preliminary measurements suggest that contact resistances are negligible further investigation

is needed to estimate the contact resistance through the contact It was of interest to determine

whether this process modified the carbon structure of the wire materials To investigate this

phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire

following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)

Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1

The before and after Raman spectra of CFs reveals very little change in the microstructure

aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This

peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a

prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor

graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears

quite prominent in both CF spectra presented here though there appears to have been some

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

22

moderate increase in graphitization following brazing Additionally the loss of the D-peak

overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to

increased graphitization However it is important to note that the graphitization process

during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to

create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several

peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)

Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1

(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible

that the loss of these peaks is the result of the loss of a surface functionality as a result of high

temperature treatment such as a C-O group However further investigation is required to

acquire a full understanding of the structural changes occurring not only as a result of the high

temperature processes in the presence of various metals In an attempt to improve the thermal

and electrical properties of CNTs and composites gamma irradiation and high temperature

treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease

in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in

the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal

treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that

the vacuum brazing processes described here do not have deleterious effects on carbonaceous

wires

33 Integrating diamond substrates with carbonaceous electrodes

Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)

nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective

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23

processes for integrating these materials with a suitable substrate differ considerably For N-

UNCD stimulating electrodes our group has demonstrated a method to grow the conductive

diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]

This method produces a hermetic electrode array for implantable neural prosthetic

applications However the integration of this array with standard electronic components

requires a metallic interface utilizing the embedded circuit board technology described here

A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze

interconnects and contact pads utilized to contact conductive diamond electrodes (dotted

circle) in a full electrode array device (Fig 12a)

Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes

Incorporating carbonaceous wire electrodes into electronic devices is less elegant and

currently requires the manual placement of individual wire electrodes into designated laser-

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24

milled grooves in the PCD substrate Next the suitable brazing paste is applied though a

screen-printing technique is required to bypass the mechanical polishing step Avoiding

mechanical polishing is important for delicate small and fragile electrode materials such as

those described in this work However because the brazing pastes are comprised of both

metal beads and a polymeric binder the functional volume of the interconnect is reduced

upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed

to produce a CF array embedded in diamond (Fig 12c) We are currently investigating

methods for protecting the CFs and other carbonaceous wires during mechanical polishing to

enable more complex robust device architectures

4 Conclusion

Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires

(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all

components paramount For PCD electronics Ag ABA is used due to its wetting and covalent

bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in

physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial

layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the

AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit

boards are compatible with commercially-available solders and are easily laser-welded for

solder-free joining The non-wetting method of enclosing carbonaceous wires was used to

create strong low-resistance and non-rectifying connections between PtIr wires and

carbonaceous wires Structural and interfacial examinations of these structures lend insight

into the forces at play during both the wetting and non-wetting fabrication of carbonaceous

devices We exploit surface tension effects of two commercially-available active brazing

alloys to create two different device architectures

(1) The wetting technique was used in conjunction with mechanical polishing to create

robust circuit substrates with embedded Ag or Au interconnects and contact pads The option

MANUSCRIP

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25

to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate

illustrates the utility of the method for enabling carbon-based electrode arrays

(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or

AuPtIr wire contacts This technique is interesting for the creation of not only free-standing

carbonaceous conductors but also for neural probes or sensors for both central and peripheral

nervous system integration We have previously described methods to fabricate neural

stimulation and recording electrodes from carbonaceous wires described here such as rLCGO

fiber which demonstrated both in vitro and in vivo functionality [47]

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for

insightful technical conversations NVA is supported by a MMI-CSIRO Material Science

PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was

supported by the Australian Research Council (ARC) through its Special Research Initiative

(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is

supported by the Australian Research Council under Discovery Early Career Researcher

award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and

support through the ARC Centre of Excellence for Electromaterials Science

Supplementary Information Available Figures S1-S4 This material is available free of charge online References

MANUSCRIP

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26

[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

MANUSCRIP

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27

[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

MANUSCRIP

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28

[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

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17

Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)

In the interest of creating freestanding carbonaceous wires with metallic contacts we

investigated whether the lack of wetting between a graphite substrate and braze alloy could be

exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate

(Fig 9) It has been observed previously that surface roughness can lead to large increases in

contact angle during brazing making graphite a reasonable choice to perhaps further diminish

the wetting behavior [42]

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18

Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)

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19

A graphite substrate was laser milled in an attempt to control separation distance between the

contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate

a number of geometries are possible for the freestanding wires Ag ABA was able to both wet

and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag

ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig

9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact

with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction

with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure

to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a

seal around the bundle effectively compressing the filaments together (Fig 9h) However if

enough force is applied the Au contact can be removed from the CF bundle though this was

not observed for CNT yarn devices This effect is likely proportional to the ratio of wire

diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of

the bond due to a rise in the contact surface area

322 Device fabrication and characterization

A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or

AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred

because it does not form a strong bond to the graphite substrate following a melting cycle so

the entire wire can be lifted off of the substrate with the contacts intact preventing damage to

the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing

facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent

lift-off was attempted Au does not bond to the graphite for reasons explained above When

the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing

both the carbonaceous fiber and a metallic wire within it (Fig 10a)

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20

Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample

The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig

10c) This technique is a potential improvement over mechanical clamps especially

considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from

clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a

woven or spun material such as CNT yarn prevents unwinding and generally helps to keep

the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead

to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)

were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a

linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the

CNT yarns in this work was comparable to those of similar size and length measured by

Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn

structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While

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21

preliminary measurements suggest that contact resistances are negligible further investigation

is needed to estimate the contact resistance through the contact It was of interest to determine

whether this process modified the carbon structure of the wire materials To investigate this

phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire

following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)

Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1

The before and after Raman spectra of CFs reveals very little change in the microstructure

aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This

peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a

prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor

graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears

quite prominent in both CF spectra presented here though there appears to have been some

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22

moderate increase in graphitization following brazing Additionally the loss of the D-peak

overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to

increased graphitization However it is important to note that the graphitization process

during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to

create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several

peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)

Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1

(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible

that the loss of these peaks is the result of the loss of a surface functionality as a result of high

temperature treatment such as a C-O group However further investigation is required to

acquire a full understanding of the structural changes occurring not only as a result of the high

temperature processes in the presence of various metals In an attempt to improve the thermal

and electrical properties of CNTs and composites gamma irradiation and high temperature

treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease

in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in

the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal

treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that

the vacuum brazing processes described here do not have deleterious effects on carbonaceous

wires

33 Integrating diamond substrates with carbonaceous electrodes

Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)

nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective

MANUSCRIP

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23

processes for integrating these materials with a suitable substrate differ considerably For N-

UNCD stimulating electrodes our group has demonstrated a method to grow the conductive

diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]

This method produces a hermetic electrode array for implantable neural prosthetic

applications However the integration of this array with standard electronic components

requires a metallic interface utilizing the embedded circuit board technology described here

A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze

interconnects and contact pads utilized to contact conductive diamond electrodes (dotted

circle) in a full electrode array device (Fig 12a)

Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes

Incorporating carbonaceous wire electrodes into electronic devices is less elegant and

currently requires the manual placement of individual wire electrodes into designated laser-

MANUSCRIP

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24

milled grooves in the PCD substrate Next the suitable brazing paste is applied though a

screen-printing technique is required to bypass the mechanical polishing step Avoiding

mechanical polishing is important for delicate small and fragile electrode materials such as

those described in this work However because the brazing pastes are comprised of both

metal beads and a polymeric binder the functional volume of the interconnect is reduced

upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed

to produce a CF array embedded in diamond (Fig 12c) We are currently investigating

methods for protecting the CFs and other carbonaceous wires during mechanical polishing to

enable more complex robust device architectures

4 Conclusion

Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires

(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all

components paramount For PCD electronics Ag ABA is used due to its wetting and covalent

bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in

physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial

layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the

AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit

boards are compatible with commercially-available solders and are easily laser-welded for

solder-free joining The non-wetting method of enclosing carbonaceous wires was used to

create strong low-resistance and non-rectifying connections between PtIr wires and

carbonaceous wires Structural and interfacial examinations of these structures lend insight

into the forces at play during both the wetting and non-wetting fabrication of carbonaceous

devices We exploit surface tension effects of two commercially-available active brazing

alloys to create two different device architectures

(1) The wetting technique was used in conjunction with mechanical polishing to create

robust circuit substrates with embedded Ag or Au interconnects and contact pads The option

MANUSCRIP

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ACCEPTED MANUSCRIPT

25

to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate

illustrates the utility of the method for enabling carbon-based electrode arrays

(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or

AuPtIr wire contacts This technique is interesting for the creation of not only free-standing

carbonaceous conductors but also for neural probes or sensors for both central and peripheral

nervous system integration We have previously described methods to fabricate neural

stimulation and recording electrodes from carbonaceous wires described here such as rLCGO

fiber which demonstrated both in vitro and in vivo functionality [47]

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for

insightful technical conversations NVA is supported by a MMI-CSIRO Material Science

PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was

supported by the Australian Research Council (ARC) through its Special Research Initiative

(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is

supported by the Australian Research Council under Discovery Early Career Researcher

award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and

support through the ARC Centre of Excellence for Electromaterials Science

Supplementary Information Available Figures S1-S4 This material is available free of charge online References

MANUSCRIP

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26

[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

MANUSCRIP

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ACCEPTED MANUSCRIPT

27

[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

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28

[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

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18

Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)

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19

A graphite substrate was laser milled in an attempt to control separation distance between the

contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate

a number of geometries are possible for the freestanding wires Ag ABA was able to both wet

and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag

ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig

9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact

with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction

with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure

to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a

seal around the bundle effectively compressing the filaments together (Fig 9h) However if

enough force is applied the Au contact can be removed from the CF bundle though this was

not observed for CNT yarn devices This effect is likely proportional to the ratio of wire

diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of

the bond due to a rise in the contact surface area

322 Device fabrication and characterization

A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or

AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred

because it does not form a strong bond to the graphite substrate following a melting cycle so

the entire wire can be lifted off of the substrate with the contacts intact preventing damage to

the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing

facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent

lift-off was attempted Au does not bond to the graphite for reasons explained above When

the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing

both the carbonaceous fiber and a metallic wire within it (Fig 10a)

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20

Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample

The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig

10c) This technique is a potential improvement over mechanical clamps especially

considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from

clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a

woven or spun material such as CNT yarn prevents unwinding and generally helps to keep

the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead

to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)

were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a

linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the

CNT yarns in this work was comparable to those of similar size and length measured by

Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn

structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While

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21

preliminary measurements suggest that contact resistances are negligible further investigation

is needed to estimate the contact resistance through the contact It was of interest to determine

whether this process modified the carbon structure of the wire materials To investigate this

phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire

following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)

Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1

The before and after Raman spectra of CFs reveals very little change in the microstructure

aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This

peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a

prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor

graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears

quite prominent in both CF spectra presented here though there appears to have been some

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22

moderate increase in graphitization following brazing Additionally the loss of the D-peak

overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to

increased graphitization However it is important to note that the graphitization process

during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to

create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several

peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)

Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1

(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible

that the loss of these peaks is the result of the loss of a surface functionality as a result of high

temperature treatment such as a C-O group However further investigation is required to

acquire a full understanding of the structural changes occurring not only as a result of the high

temperature processes in the presence of various metals In an attempt to improve the thermal

and electrical properties of CNTs and composites gamma irradiation and high temperature

treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease

in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in

the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal

treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that

the vacuum brazing processes described here do not have deleterious effects on carbonaceous

wires

33 Integrating diamond substrates with carbonaceous electrodes

Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)

nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective

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23

processes for integrating these materials with a suitable substrate differ considerably For N-

UNCD stimulating electrodes our group has demonstrated a method to grow the conductive

diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]

This method produces a hermetic electrode array for implantable neural prosthetic

applications However the integration of this array with standard electronic components

requires a metallic interface utilizing the embedded circuit board technology described here

A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze

interconnects and contact pads utilized to contact conductive diamond electrodes (dotted

circle) in a full electrode array device (Fig 12a)

Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes

Incorporating carbonaceous wire electrodes into electronic devices is less elegant and

currently requires the manual placement of individual wire electrodes into designated laser-

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24

milled grooves in the PCD substrate Next the suitable brazing paste is applied though a

screen-printing technique is required to bypass the mechanical polishing step Avoiding

mechanical polishing is important for delicate small and fragile electrode materials such as

those described in this work However because the brazing pastes are comprised of both

metal beads and a polymeric binder the functional volume of the interconnect is reduced

upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed

to produce a CF array embedded in diamond (Fig 12c) We are currently investigating

methods for protecting the CFs and other carbonaceous wires during mechanical polishing to

enable more complex robust device architectures

4 Conclusion

Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires

(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all

components paramount For PCD electronics Ag ABA is used due to its wetting and covalent

bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in

physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial

layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the

AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit

boards are compatible with commercially-available solders and are easily laser-welded for

solder-free joining The non-wetting method of enclosing carbonaceous wires was used to

create strong low-resistance and non-rectifying connections between PtIr wires and

carbonaceous wires Structural and interfacial examinations of these structures lend insight

into the forces at play during both the wetting and non-wetting fabrication of carbonaceous

devices We exploit surface tension effects of two commercially-available active brazing

alloys to create two different device architectures

(1) The wetting technique was used in conjunction with mechanical polishing to create

robust circuit substrates with embedded Ag or Au interconnects and contact pads The option

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25

to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate

illustrates the utility of the method for enabling carbon-based electrode arrays

(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or

AuPtIr wire contacts This technique is interesting for the creation of not only free-standing

carbonaceous conductors but also for neural probes or sensors for both central and peripheral

nervous system integration We have previously described methods to fabricate neural

stimulation and recording electrodes from carbonaceous wires described here such as rLCGO

fiber which demonstrated both in vitro and in vivo functionality [47]

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for

insightful technical conversations NVA is supported by a MMI-CSIRO Material Science

PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was

supported by the Australian Research Council (ARC) through its Special Research Initiative

(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is

supported by the Australian Research Council under Discovery Early Career Researcher

award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and

support through the ARC Centre of Excellence for Electromaterials Science

Supplementary Information Available Figures S1-S4 This material is available free of charge online References

MANUSCRIP

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26

[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

MANUSCRIP

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27

[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

MANUSCRIP

T

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28

[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

MANUSCRIP

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ACCEPTED MANUSCRIPT

19

A graphite substrate was laser milled in an attempt to control separation distance between the

contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate

a number of geometries are possible for the freestanding wires Ag ABA was able to both wet

and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag

ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig

9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact

with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction

with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure

to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a

seal around the bundle effectively compressing the filaments together (Fig 9h) However if

enough force is applied the Au contact can be removed from the CF bundle though this was

not observed for CNT yarn devices This effect is likely proportional to the ratio of wire

diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of

the bond due to a rise in the contact surface area

322 Device fabrication and characterization

A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or

AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred

because it does not form a strong bond to the graphite substrate following a melting cycle so

the entire wire can be lifted off of the substrate with the contacts intact preventing damage to

the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing

facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent

lift-off was attempted Au does not bond to the graphite for reasons explained above When

the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing

both the carbonaceous fiber and a metallic wire within it (Fig 10a)

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20

Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample

The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig

10c) This technique is a potential improvement over mechanical clamps especially

considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from

clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a

woven or spun material such as CNT yarn prevents unwinding and generally helps to keep

the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead

to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)

were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a

linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the

CNT yarns in this work was comparable to those of similar size and length measured by

Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn

structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While

MANUSCRIP

T

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21

preliminary measurements suggest that contact resistances are negligible further investigation

is needed to estimate the contact resistance through the contact It was of interest to determine

whether this process modified the carbon structure of the wire materials To investigate this

phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire

following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)

Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1

The before and after Raman spectra of CFs reveals very little change in the microstructure

aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This

peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a

prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor

graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears

quite prominent in both CF spectra presented here though there appears to have been some

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

22

moderate increase in graphitization following brazing Additionally the loss of the D-peak

overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to

increased graphitization However it is important to note that the graphitization process

during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to

create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several

peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)

Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1

(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible

that the loss of these peaks is the result of the loss of a surface functionality as a result of high

temperature treatment such as a C-O group However further investigation is required to

acquire a full understanding of the structural changes occurring not only as a result of the high

temperature processes in the presence of various metals In an attempt to improve the thermal

and electrical properties of CNTs and composites gamma irradiation and high temperature

treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease

in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in

the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal

treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that

the vacuum brazing processes described here do not have deleterious effects on carbonaceous

wires

33 Integrating diamond substrates with carbonaceous electrodes

Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)

nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

23

processes for integrating these materials with a suitable substrate differ considerably For N-

UNCD stimulating electrodes our group has demonstrated a method to grow the conductive

diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]

This method produces a hermetic electrode array for implantable neural prosthetic

applications However the integration of this array with standard electronic components

requires a metallic interface utilizing the embedded circuit board technology described here

A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze

interconnects and contact pads utilized to contact conductive diamond electrodes (dotted

circle) in a full electrode array device (Fig 12a)

Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes

Incorporating carbonaceous wire electrodes into electronic devices is less elegant and

currently requires the manual placement of individual wire electrodes into designated laser-

MANUSCRIP

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24

milled grooves in the PCD substrate Next the suitable brazing paste is applied though a

screen-printing technique is required to bypass the mechanical polishing step Avoiding

mechanical polishing is important for delicate small and fragile electrode materials such as

those described in this work However because the brazing pastes are comprised of both

metal beads and a polymeric binder the functional volume of the interconnect is reduced

upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed

to produce a CF array embedded in diamond (Fig 12c) We are currently investigating

methods for protecting the CFs and other carbonaceous wires during mechanical polishing to

enable more complex robust device architectures

4 Conclusion

Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires

(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all

components paramount For PCD electronics Ag ABA is used due to its wetting and covalent

bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in

physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial

layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the

AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit

boards are compatible with commercially-available solders and are easily laser-welded for

solder-free joining The non-wetting method of enclosing carbonaceous wires was used to

create strong low-resistance and non-rectifying connections between PtIr wires and

carbonaceous wires Structural and interfacial examinations of these structures lend insight

into the forces at play during both the wetting and non-wetting fabrication of carbonaceous

devices We exploit surface tension effects of two commercially-available active brazing

alloys to create two different device architectures

(1) The wetting technique was used in conjunction with mechanical polishing to create

robust circuit substrates with embedded Ag or Au interconnects and contact pads The option

MANUSCRIP

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ACCEPTED MANUSCRIPT

25

to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate

illustrates the utility of the method for enabling carbon-based electrode arrays

(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or

AuPtIr wire contacts This technique is interesting for the creation of not only free-standing

carbonaceous conductors but also for neural probes or sensors for both central and peripheral

nervous system integration We have previously described methods to fabricate neural

stimulation and recording electrodes from carbonaceous wires described here such as rLCGO

fiber which demonstrated both in vitro and in vivo functionality [47]

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for

insightful technical conversations NVA is supported by a MMI-CSIRO Material Science

PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was

supported by the Australian Research Council (ARC) through its Special Research Initiative

(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is

supported by the Australian Research Council under Discovery Early Career Researcher

award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and

support through the ARC Centre of Excellence for Electromaterials Science

Supplementary Information Available Figures S1-S4 This material is available free of charge online References

MANUSCRIP

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26

[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

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27

[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

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28

[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

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20

Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample

The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig

10c) This technique is a potential improvement over mechanical clamps especially

considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from

clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a

woven or spun material such as CNT yarn prevents unwinding and generally helps to keep

the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead

to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)

were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a

linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the

CNT yarns in this work was comparable to those of similar size and length measured by

Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn

structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While

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21

preliminary measurements suggest that contact resistances are negligible further investigation

is needed to estimate the contact resistance through the contact It was of interest to determine

whether this process modified the carbon structure of the wire materials To investigate this

phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire

following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)

Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1

The before and after Raman spectra of CFs reveals very little change in the microstructure

aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This

peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a

prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor

graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears

quite prominent in both CF spectra presented here though there appears to have been some

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22

moderate increase in graphitization following brazing Additionally the loss of the D-peak

overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to

increased graphitization However it is important to note that the graphitization process

during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to

create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several

peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)

Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1

(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible

that the loss of these peaks is the result of the loss of a surface functionality as a result of high

temperature treatment such as a C-O group However further investigation is required to

acquire a full understanding of the structural changes occurring not only as a result of the high

temperature processes in the presence of various metals In an attempt to improve the thermal

and electrical properties of CNTs and composites gamma irradiation and high temperature

treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease

in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in

the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal

treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that

the vacuum brazing processes described here do not have deleterious effects on carbonaceous

wires

33 Integrating diamond substrates with carbonaceous electrodes

Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)

nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective

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23

processes for integrating these materials with a suitable substrate differ considerably For N-

UNCD stimulating electrodes our group has demonstrated a method to grow the conductive

diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]

This method produces a hermetic electrode array for implantable neural prosthetic

applications However the integration of this array with standard electronic components

requires a metallic interface utilizing the embedded circuit board technology described here

A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze

interconnects and contact pads utilized to contact conductive diamond electrodes (dotted

circle) in a full electrode array device (Fig 12a)

Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes

Incorporating carbonaceous wire electrodes into electronic devices is less elegant and

currently requires the manual placement of individual wire electrodes into designated laser-

MANUSCRIP

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24

milled grooves in the PCD substrate Next the suitable brazing paste is applied though a

screen-printing technique is required to bypass the mechanical polishing step Avoiding

mechanical polishing is important for delicate small and fragile electrode materials such as

those described in this work However because the brazing pastes are comprised of both

metal beads and a polymeric binder the functional volume of the interconnect is reduced

upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed

to produce a CF array embedded in diamond (Fig 12c) We are currently investigating

methods for protecting the CFs and other carbonaceous wires during mechanical polishing to

enable more complex robust device architectures

4 Conclusion

Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires

(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all

components paramount For PCD electronics Ag ABA is used due to its wetting and covalent

bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in

physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial

layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the

AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit

boards are compatible with commercially-available solders and are easily laser-welded for

solder-free joining The non-wetting method of enclosing carbonaceous wires was used to

create strong low-resistance and non-rectifying connections between PtIr wires and

carbonaceous wires Structural and interfacial examinations of these structures lend insight

into the forces at play during both the wetting and non-wetting fabrication of carbonaceous

devices We exploit surface tension effects of two commercially-available active brazing

alloys to create two different device architectures

(1) The wetting technique was used in conjunction with mechanical polishing to create

robust circuit substrates with embedded Ag or Au interconnects and contact pads The option

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

25

to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate

illustrates the utility of the method for enabling carbon-based electrode arrays

(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or

AuPtIr wire contacts This technique is interesting for the creation of not only free-standing

carbonaceous conductors but also for neural probes or sensors for both central and peripheral

nervous system integration We have previously described methods to fabricate neural

stimulation and recording electrodes from carbonaceous wires described here such as rLCGO

fiber which demonstrated both in vitro and in vivo functionality [47]

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for

insightful technical conversations NVA is supported by a MMI-CSIRO Material Science

PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was

supported by the Australian Research Council (ARC) through its Special Research Initiative

(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is

supported by the Australian Research Council under Discovery Early Career Researcher

award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and

support through the ARC Centre of Excellence for Electromaterials Science

Supplementary Information Available Figures S1-S4 This material is available free of charge online References

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

26

[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

MANUSCRIP

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27

[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

MANUSCRIP

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28

[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

MANUSCRIP

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21

preliminary measurements suggest that contact resistances are negligible further investigation

is needed to estimate the contact resistance through the contact It was of interest to determine

whether this process modified the carbon structure of the wire materials To investigate this

phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire

following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)

Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1

The before and after Raman spectra of CFs reveals very little change in the microstructure

aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This

peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a

prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor

graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears

quite prominent in both CF spectra presented here though there appears to have been some

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

22

moderate increase in graphitization following brazing Additionally the loss of the D-peak

overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to

increased graphitization However it is important to note that the graphitization process

during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to

create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several

peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)

Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1

(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible

that the loss of these peaks is the result of the loss of a surface functionality as a result of high

temperature treatment such as a C-O group However further investigation is required to

acquire a full understanding of the structural changes occurring not only as a result of the high

temperature processes in the presence of various metals In an attempt to improve the thermal

and electrical properties of CNTs and composites gamma irradiation and high temperature

treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease

in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in

the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal

treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that

the vacuum brazing processes described here do not have deleterious effects on carbonaceous

wires

33 Integrating diamond substrates with carbonaceous electrodes

Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)

nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

23

processes for integrating these materials with a suitable substrate differ considerably For N-

UNCD stimulating electrodes our group has demonstrated a method to grow the conductive

diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]

This method produces a hermetic electrode array for implantable neural prosthetic

applications However the integration of this array with standard electronic components

requires a metallic interface utilizing the embedded circuit board technology described here

A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze

interconnects and contact pads utilized to contact conductive diamond electrodes (dotted

circle) in a full electrode array device (Fig 12a)

Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes

Incorporating carbonaceous wire electrodes into electronic devices is less elegant and

currently requires the manual placement of individual wire electrodes into designated laser-

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

24

milled grooves in the PCD substrate Next the suitable brazing paste is applied though a

screen-printing technique is required to bypass the mechanical polishing step Avoiding

mechanical polishing is important for delicate small and fragile electrode materials such as

those described in this work However because the brazing pastes are comprised of both

metal beads and a polymeric binder the functional volume of the interconnect is reduced

upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed

to produce a CF array embedded in diamond (Fig 12c) We are currently investigating

methods for protecting the CFs and other carbonaceous wires during mechanical polishing to

enable more complex robust device architectures

4 Conclusion

Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires

(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all

components paramount For PCD electronics Ag ABA is used due to its wetting and covalent

bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in

physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial

layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the

AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit

boards are compatible with commercially-available solders and are easily laser-welded for

solder-free joining The non-wetting method of enclosing carbonaceous wires was used to

create strong low-resistance and non-rectifying connections between PtIr wires and

carbonaceous wires Structural and interfacial examinations of these structures lend insight

into the forces at play during both the wetting and non-wetting fabrication of carbonaceous

devices We exploit surface tension effects of two commercially-available active brazing

alloys to create two different device architectures

(1) The wetting technique was used in conjunction with mechanical polishing to create

robust circuit substrates with embedded Ag or Au interconnects and contact pads The option

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

25

to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate

illustrates the utility of the method for enabling carbon-based electrode arrays

(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or

AuPtIr wire contacts This technique is interesting for the creation of not only free-standing

carbonaceous conductors but also for neural probes or sensors for both central and peripheral

nervous system integration We have previously described methods to fabricate neural

stimulation and recording electrodes from carbonaceous wires described here such as rLCGO

fiber which demonstrated both in vitro and in vivo functionality [47]

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for

insightful technical conversations NVA is supported by a MMI-CSIRO Material Science

PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was

supported by the Australian Research Council (ARC) through its Special Research Initiative

(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is

supported by the Australian Research Council under Discovery Early Career Researcher

award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and

support through the ARC Centre of Excellence for Electromaterials Science

Supplementary Information Available Figures S1-S4 This material is available free of charge online References

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

26

[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

MANUSCRIP

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27

[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

MANUSCRIP

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28

[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

MANUSCRIP

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22

moderate increase in graphitization following brazing Additionally the loss of the D-peak

overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to

increased graphitization However it is important to note that the graphitization process

during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to

create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several

peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)

Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1

(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible

that the loss of these peaks is the result of the loss of a surface functionality as a result of high

temperature treatment such as a C-O group However further investigation is required to

acquire a full understanding of the structural changes occurring not only as a result of the high

temperature processes in the presence of various metals In an attempt to improve the thermal

and electrical properties of CNTs and composites gamma irradiation and high temperature

treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease

in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in

the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal

treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that

the vacuum brazing processes described here do not have deleterious effects on carbonaceous

wires

33 Integrating diamond substrates with carbonaceous electrodes

Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)

nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

23

processes for integrating these materials with a suitable substrate differ considerably For N-

UNCD stimulating electrodes our group has demonstrated a method to grow the conductive

diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]

This method produces a hermetic electrode array for implantable neural prosthetic

applications However the integration of this array with standard electronic components

requires a metallic interface utilizing the embedded circuit board technology described here

A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze

interconnects and contact pads utilized to contact conductive diamond electrodes (dotted

circle) in a full electrode array device (Fig 12a)

Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes

Incorporating carbonaceous wire electrodes into electronic devices is less elegant and

currently requires the manual placement of individual wire electrodes into designated laser-

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

24

milled grooves in the PCD substrate Next the suitable brazing paste is applied though a

screen-printing technique is required to bypass the mechanical polishing step Avoiding

mechanical polishing is important for delicate small and fragile electrode materials such as

those described in this work However because the brazing pastes are comprised of both

metal beads and a polymeric binder the functional volume of the interconnect is reduced

upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed

to produce a CF array embedded in diamond (Fig 12c) We are currently investigating

methods for protecting the CFs and other carbonaceous wires during mechanical polishing to

enable more complex robust device architectures

4 Conclusion

Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires

(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all

components paramount For PCD electronics Ag ABA is used due to its wetting and covalent

bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in

physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial

layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the

AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit

boards are compatible with commercially-available solders and are easily laser-welded for

solder-free joining The non-wetting method of enclosing carbonaceous wires was used to

create strong low-resistance and non-rectifying connections between PtIr wires and

carbonaceous wires Structural and interfacial examinations of these structures lend insight

into the forces at play during both the wetting and non-wetting fabrication of carbonaceous

devices We exploit surface tension effects of two commercially-available active brazing

alloys to create two different device architectures

(1) The wetting technique was used in conjunction with mechanical polishing to create

robust circuit substrates with embedded Ag or Au interconnects and contact pads The option

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

25

to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate

illustrates the utility of the method for enabling carbon-based electrode arrays

(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or

AuPtIr wire contacts This technique is interesting for the creation of not only free-standing

carbonaceous conductors but also for neural probes or sensors for both central and peripheral

nervous system integration We have previously described methods to fabricate neural

stimulation and recording electrodes from carbonaceous wires described here such as rLCGO

fiber which demonstrated both in vitro and in vivo functionality [47]

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for

insightful technical conversations NVA is supported by a MMI-CSIRO Material Science

PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was

supported by the Australian Research Council (ARC) through its Special Research Initiative

(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is

supported by the Australian Research Council under Discovery Early Career Researcher

award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and

support through the ARC Centre of Excellence for Electromaterials Science

Supplementary Information Available Figures S1-S4 This material is available free of charge online References

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

26

[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

27

[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

28

[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

23

processes for integrating these materials with a suitable substrate differ considerably For N-

UNCD stimulating electrodes our group has demonstrated a method to grow the conductive

diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]

This method produces a hermetic electrode array for implantable neural prosthetic

applications However the integration of this array with standard electronic components

requires a metallic interface utilizing the embedded circuit board technology described here

A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze

interconnects and contact pads utilized to contact conductive diamond electrodes (dotted

circle) in a full electrode array device (Fig 12a)

Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes

Incorporating carbonaceous wire electrodes into electronic devices is less elegant and

currently requires the manual placement of individual wire electrodes into designated laser-

MANUSCRIP

T

ACCEPTED

ACCEPTED MANUSCRIPT

24

milled grooves in the PCD substrate Next the suitable brazing paste is applied though a

screen-printing technique is required to bypass the mechanical polishing step Avoiding

mechanical polishing is important for delicate small and fragile electrode materials such as

those described in this work However because the brazing pastes are comprised of both

metal beads and a polymeric binder the functional volume of the interconnect is reduced

upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed

to produce a CF array embedded in diamond (Fig 12c) We are currently investigating

methods for protecting the CFs and other carbonaceous wires during mechanical polishing to

enable more complex robust device architectures

4 Conclusion

Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires

(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all

components paramount For PCD electronics Ag ABA is used due to its wetting and covalent

bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in

physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial

layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the

AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit

boards are compatible with commercially-available solders and are easily laser-welded for

solder-free joining The non-wetting method of enclosing carbonaceous wires was used to

create strong low-resistance and non-rectifying connections between PtIr wires and

carbonaceous wires Structural and interfacial examinations of these structures lend insight

into the forces at play during both the wetting and non-wetting fabrication of carbonaceous

devices We exploit surface tension effects of two commercially-available active brazing

alloys to create two different device architectures

(1) The wetting technique was used in conjunction with mechanical polishing to create

robust circuit substrates with embedded Ag or Au interconnects and contact pads The option

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to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate

illustrates the utility of the method for enabling carbon-based electrode arrays

(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or

AuPtIr wire contacts This technique is interesting for the creation of not only free-standing

carbonaceous conductors but also for neural probes or sensors for both central and peripheral

nervous system integration We have previously described methods to fabricate neural

stimulation and recording electrodes from carbonaceous wires described here such as rLCGO

fiber which demonstrated both in vitro and in vivo functionality [47]

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for

insightful technical conversations NVA is supported by a MMI-CSIRO Material Science

PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was

supported by the Australian Research Council (ARC) through its Special Research Initiative

(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is

supported by the Australian Research Council under Discovery Early Career Researcher

award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and

support through the ARC Centre of Excellence for Electromaterials Science

Supplementary Information Available Figures S1-S4 This material is available free of charge online References

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[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

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[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

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[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

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milled grooves in the PCD substrate Next the suitable brazing paste is applied though a

screen-printing technique is required to bypass the mechanical polishing step Avoiding

mechanical polishing is important for delicate small and fragile electrode materials such as

those described in this work However because the brazing pastes are comprised of both

metal beads and a polymeric binder the functional volume of the interconnect is reduced

upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed

to produce a CF array embedded in diamond (Fig 12c) We are currently investigating

methods for protecting the CFs and other carbonaceous wires during mechanical polishing to

enable more complex robust device architectures

4 Conclusion

Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires

(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all

components paramount For PCD electronics Ag ABA is used due to its wetting and covalent

bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in

physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial

layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the

AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit

boards are compatible with commercially-available solders and are easily laser-welded for

solder-free joining The non-wetting method of enclosing carbonaceous wires was used to

create strong low-resistance and non-rectifying connections between PtIr wires and

carbonaceous wires Structural and interfacial examinations of these structures lend insight

into the forces at play during both the wetting and non-wetting fabrication of carbonaceous

devices We exploit surface tension effects of two commercially-available active brazing

alloys to create two different device architectures

(1) The wetting technique was used in conjunction with mechanical polishing to create

robust circuit substrates with embedded Ag or Au interconnects and contact pads The option

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to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate

illustrates the utility of the method for enabling carbon-based electrode arrays

(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or

AuPtIr wire contacts This technique is interesting for the creation of not only free-standing

carbonaceous conductors but also for neural probes or sensors for both central and peripheral

nervous system integration We have previously described methods to fabricate neural

stimulation and recording electrodes from carbonaceous wires described here such as rLCGO

fiber which demonstrated both in vitro and in vivo functionality [47]

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for

insightful technical conversations NVA is supported by a MMI-CSIRO Material Science

PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was

supported by the Australian Research Council (ARC) through its Special Research Initiative

(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is

supported by the Australian Research Council under Discovery Early Career Researcher

award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and

support through the ARC Centre of Excellence for Electromaterials Science

Supplementary Information Available Figures S1-S4 This material is available free of charge online References

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[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

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[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

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[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

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to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate

illustrates the utility of the method for enabling carbon-based electrode arrays

(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or

AuPtIr wire contacts This technique is interesting for the creation of not only free-standing

carbonaceous conductors but also for neural probes or sensors for both central and peripheral

nervous system integration We have previously described methods to fabricate neural

stimulation and recording electrodes from carbonaceous wires described here such as rLCGO

fiber which demonstrated both in vitro and in vivo functionality [47]

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for

insightful technical conversations NVA is supported by a MMI-CSIRO Material Science

PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was

supported by the Australian Research Council (ARC) through its Special Research Initiative

(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is

supported by the Australian Research Council under Discovery Early Career Researcher

award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and

support through the ARC Centre of Excellence for Electromaterials Science

Supplementary Information Available Figures S1-S4 This material is available free of charge online References

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[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

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[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

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[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

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[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93

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[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

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[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

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[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92

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[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9

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[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9