proton transport through two-dimensional materials
TRANSCRIPT
Proton transport through
two-dimensional materials
A THESIS
SUBMITTED TO THE UNIVERSIY OF MANCHESTR
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
IN THE FACULTY OF ENGINEERING AND PHYSICAL SCIENCES
Sheng Hu
SCHOOL OF PHYSICS AND ASTRONOMY
2014
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CONTENT
List of Figures 5
List of Tables 9
Abstract 11
Declaration 13
Copyright 15
Acknowledgements 17
Introduction 19
1 Fundamentals of graphene and 2D materials............................................................................24
1.1 Fundamentals of graphene .......................................................................................................... 24
1.1.1 Carbon materials ................................................................................................................... 24
1.1.2 Graphene lattice and band structure ................................................................................... 26
1.1.3 Basic electronic properties in graphene ............................................................................... 29
1.2 Two dimensional materials beyond graphene ............................................................................ 32
1.2.1 Boron nitride ......................................................................................................................... 33
1.2.2 Molybdenum disulfide .......................................................................................................... 34
1.3 Matter transport properties of 2D materials ............................................................................... 36
2 Experimental techniques and device fabrication .......................................................................38
2.1 Fabrication of layered materials by mechanical cleavage ........................................................... 38
2.2 Assembly of 2D membranes based proton conductive devices .................................................. 45
2.2.1 Substrate fabrication ............................................................................................................ 46
2.2.2 Transfer techniques .............................................................................................................. 53
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2.3 Proton transport device fabrication ............................................................................................ 56
3 Proton transport through 2D materials in Nafion based solid systems ...................................... 59
3.1 Introduction ................................................................................................................................. 59
3.2 Solid proton medium and source ................................................................................................ 60
3.3 Sample fabrication and measurement system ............................................................................ 62
3.4 Results.......................................................................................................................................... 64
3.5 Conclusion ................................................................................................................................... 70
4 Pt catalyzed proton transport through 2D materials ................................................................. 71
4.1 Introduction ................................................................................................................................. 71
4.2 Device fabrication ........................................................................................................................ 72
4.3 Results.......................................................................................................................................... 74
4.4 Proton transport introduced H2 flow ........................................................................................... 79
4.5 Conclusion ................................................................................................................................... 83
5 Proton transport through 2D materials in liquids ...................................................................... 85
5.1 Introduction ................................................................................................................................. 85
5.2 Liquid/liquid interface method .................................................................................................... 86
5.2.1 Immiscible interface ............................................................................................................. 86
5.2.2 Bipolar cell ............................................................................................................................ 88
5.3 Sample fabrication and measurements ....................................................................................... 89
5.4 Results.......................................................................................................................................... 92
5.5 Conclusion ................................................................................................................................. 102
5.6 Supplementary experiments ..................................................................................................... 102
6 Summary .............................................................................................................................. 106
6.1 Conclusion ................................................................................................................................. 106
6.2 Outlook ...................................................................................................................................... 107
Reference 110
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LIST OF FIGURES
1 Carbon allotropes of different dimensionalities .......................................................................... 21
1.1 Graphene orbital hybridization .................................................................................................... 25
1.2 Graphene lattice and Brilouin zone .............................................................................................. 25
1.3 Electronic band structure of graphene ........................................................................................ 28
1.4 Electric field effect in graphene ................................................................................................... 30
1.5 Boron nitride lattice structure...................................................................................................... 34
1.6 Lattice and band structure of molybdenum disulfide .................................................................. 35
2.1 Procedures of producing graphene flakes ................................................................................... 39
2.2 Optical image of thin graphene flakes ......................................................................................... 40
2.3 Large scale graphene production methods .................................................................................. 42
2.4 Optical images of boron nitride and molybdenum disulfide ....................................................... 43
2.5 AFM images of various 2D materials ........................................................................................... 44
2.6 Raman spectra of graphene ........................................................................................................ 45
2.7 Process of photolithography and development ........................................................................... 48
2.8 Reactive ion etching process ........................................................................................................ 50
2.9 KOH wet etching ........................................................................................................................... 52
2.10 Polymer based wet transfer ......................................................................................................... 54
2.11 Procedures of dry transfer ........................................................................................................... 55
2.12 Proton transport device fabrication procedures ......................................................................... 58
3.1 Nafion and its proton conductive mechanism ............................................................................. 60
3.2 Unit cell of metallic palladium with octahedral sites occupied by H atoms ................................ 61
3.3 Diagrams of Nafion contacted proton transport devices ............................................................ 63
3.4 Schematics of experimental system and proton transport mechanism in Nafion sample .......... 64
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3.5 Proton transport I-V characteristics of 2D materials ................................................................... 65
3.6 Electron density within 2D materials hexagonal rings before and during proton transition ...... 66
3.7 Proton conductance across thin BN flakes .................................................................................. 67
3.8 Proton conductance histogram of various 2D materials ............................................................. 68
3.9 Nafion proton conductivity on 2µm-in-diameter hole ................................................................ 69
4.1 Electron beam evaporation system ............................................................................................. 73
4.2 Diagrams of Pt catalyzed devices ................................................................................................. 74
4.3 Proton conductance histogram of various 2D materials with Pt catalyst.................................... 75
4.4 Proton conductive I-V characteristics of BN thin flakes with Pt catalyst ..................................... 76
4.5 Ar blister AFM on single layer graphene with and without Pt catalyst ....................................... 77
4.6 Ar leak rate of single layer graphene with and without Pt catalyst ............................................. 79
4.7 Optical images of H2 bubbles trapped between metal film and graphene flakes ....................... 80
4.8 Metal contact deposition and lift off ........................................................................................... 81
4.9 Diagrams of hydrogen flow measurements: devices fabrication and measurement system ..... 81
4.10 Hydrogen flow rate as a function of current across graphene flakes .......................................... 82
5.1 Proton transport current across DCE/aqueous immiscible interface .......................................... 87
5.2 Diagrams of bipolar cell system ................................................................................................... 88
5.3 Procedures of PDMS mask fabrication......................................................................................... 90
5.4 Mechanical transfer of PDMS mask ............................................................................................. 91
5.5 Schematic diagram of proton transport through 2D materials at L/L interface .......................... 92
5.6 Histogram of proton conductance through various 2D materials in HCl aqueous interface ...... 93
5.7 Graphene proton transport devices geometry and images......................................................... 94
5.8 I-V characteristics of various thicknesses graphene at the aqueous/aqueous interface ............ 96
5.9 Chemical structure of 1,2-DCE and 1-[Bis(trifluoromethanesulfonyl)methyl]-2,3,4,5,6-
pentafluorobenzene. ................................................................................................................... 98
5.10 Proton transport I-V characteristics of graphene at the DCE/aqueous interface with various
acid concentration ...................................................................................................................... 98
5.11 I-V characteristics of various thicknesses graphene flakes at the DCE/aqueous interface ......... 99
5.12 I-V characteristics of single layer MoS2 at both aqueous/aqueous and DCE/aqueous
interface .................................................................................................................................... 100
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5.13 I-V characteristics of various thicknesses BN flakes in HCl solution .......................................... 101
5.14 Water electrolysis at various thicknesses graphene flakes ....................................................... 102
5.15 Proton transport through single layer graphene at the DCE/aqueous interface with H+ at the
aqueous phase ........................................................................................................................... 103
5.16 Proton transport I-V of single layer graphene at acetonitrile/aqueous miscible interface ....... 104
6.1 Electron transport through graphene in bipolar cell system ..................................................... 108
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LIST OF TABLES
Table 2.1 Spin coating parameters of photoresist 48
Table 2.2 Parameters of RIE etching recipes 50
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ABSTRACT
Two-dimensional (2D) materials, referring to materials being just one atom thick, prove to
be attractive not only in fundamental research but also in applications. Graphene, a single
layer of carbon atoms arranged in hexagonal rings, is just the first among other materials
(including hexagonal boron nitride and molybdenum disulfide) that could be isolated into
mono-atomic layers.
The presented thesis investigates proton transport through atomically thin two-dimensional
materials. While the electronic, optical and mechanical properties of graphene and other 2D
materials have been intensely researched over the past decade, much less is known on the
interaction of these crystals with protons. It has been reported that most of the defect free
two dimensional materials are impermeable to nearly all gases, molecules and ions.
Whether proton, the smallest positively charged ion, could transport through two
dimensional materials at a low energy level remains unknown.
This work investigates proton transport through 2D materials, including graphene,
hexagonal boron nitride and molybdenum disulfide, in two different systems: Nafion/Pd
solid system and liquid/liquid interface system, both of which provided consistent results.
Our results suggest that proton can transport through the interatomic spacings in the lattice
of single layer BN and graphene, while single layer MoS2 is impermeable to protons. Single
layer BN is the most conductive to protons among the 2D materials investigated in this
thesis. Lower proton conductance of graphene is due to its delocalized π electrons while
proton impermeability of MoS2 is due to the three atomic layers structure.
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Moreover, proton transfer is greatly facilitated by the deposition of platinum nanoparticles
on the proton conductive 2D membranes to such a degree that platinum decorated BN
seems to present negligible resistance to the transfer of protons through its lattice.
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DECLARATION
The University of Manchester
PhD by published work Candidate Declaration
Candidate Name: Sheng Hu
Faculty: Engineering and Physical Science
Thesis Title: Proton transport through two dimensional materials
Declaration to be completed by the candidate:
I declare that no portion of this work referred to in this thesis has been submitted in
support of an application for another degree of qualification of this or any other university
or other institute of learning.
Signed: Date:
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COPYRIGHT
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owns any copyright in it (the "Copyright")1and s/he has given The University of Manchester
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ACKNOWLEDGEMENTS
First of all, I would like to acknowledge my supervisor Prof. Sir Andre Geim for accepting me
as his student and giving me a chance of working in his world class Lab and research group. I
have learnt a lot from him. He is kind to give his expert opinion to help with any problems I
have faced during my three years PhD study while his unique ideas and wide knowledge
have inspired me a lot.
I am thankful to my Advisor Dr. Fredrik Schedin who provided me with tremendous help of
operating clean room facilities and suggestions for sample fabrication. I also thank Prof.
Robert Dryfe for his help in introducing me into a deep insight to electrochemistry with
patience.
Sincere thanks goes to Dr Artem Mishchenko, Dr. Thanasis Georgiou, Dr. Matěj Velický for
proof reading this thesis. I want to give my special acknowledgements to Danil Bukhvalov
and Mikhail. I. Katsnelson for their help with electron density simulation.
Many thanks to all my group members especially Dr. Rashid Jalil, Dr. Alexander Zhuhov,
Marcelo Lozada-Hidalgo, Dr. Yang Cao, Dr Axel Ackman, Tu Jhih-Sian, Dr Branson Belle, Dr.
Ernie Hill, Dr Pete Blake, Yu Geliang, for their help and sharing their experimental skills.
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INTRODUCTION
Proton transport through membranes is fundamental to many biological and technical
processes ranging from ATP synthesis [1] to proton exchange membrane fuel cells (PEMFC)
[2]. The concept of proton transport in water by hopping from one water molecule to the
next via the Grotthuss mechanism [3] has got theoretical and simulation supports [4, 5, 6].
Inspired from this, artificial materials such as Nafion [7], in which protons hopping along a
sulfur chain, were made and they are widely used in PEMFCs – in fact PEMFCs is the most
important application of a proton exchange membrane. After first being applied in USA’s
space craft in 1960s, PEMFCs were quickly applied as the main power for cars, submarines,
and even power plants. Due to its pollution free and friendly working environment
properties, PEMFCs becomes competitive in the area of future energy. The global PEMFC
market is predicted to be 16 billion by 2016 [8]. However, as proton conductors based on
the Grotthuss or “vehicle” mechanism [9, 10], these materials have to work in aqueous
phase. The high cost of membrane fabrication is a problem as well. The limitations spark a
run of looking for new materials and new proton transport mechanisms.
In 2004, Novoselov et.al from the University of Manchester shocked the scientific world. An
atomically thin carbon sheet with carbon atoms arranging themselves in hexagonal rings,
called graphene, was mechanically exfoliated from bulk graphite and electrically
characterized [11]. As the first truly isolated two dimensional material, graphene made the
last missing piece of the carbon materials family: three dimensional graphite known as a
conductive material for centuries, while zero-dimensional fullerenes and one-dimensional
carbon nanotubes were synthesized in 1980s and 1990s, respectively [12, 13] – actually
graphene can be wrapped up into 0D fullerense, rolled up into 1D carbon nanotubes or
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stacked into 3D graphite, from a sense of which graphene is a fundamental building block of
all the graphitic materials [14].
Mechanical exfoliation of graphene with a piece of adhesive tape [11] facilitated graphene
research as it is an easy accessible approach of fabricating graphene, promoting researchers
to investigate this new and amazing material. This leads graphene to be the new star in the
2010 Physics Noble Prize competition. During years after graphene’s first isolation, unveiling
its properties has been one of the focuses in the scientific world: graphene has unique
electronic properties because of its two dimensional nature [11, 14-19]; it is the strongest
material ever measured in the Universe [20] but still remains to be highly stretchable [21,
22]; it has extraordinary thermal [23] and electrical [24] conductivity; it adsorbs 2.3% light
[25] while being just one atom thick. The study of electronic properties of graphene reveals
that it has linear energy dispersion near the charge neutrality point and its carriers can be
described as massless Dirac fermions [15, 26]. Graphene shows an ambipolar field effect,
that its carriers can be tuned continuously from electrons to holes by applying an electric
field, and the carrier concentration can be tuned up to 1013cm-2 by varying gate voltage [15].
All of these fascinating properties make graphene a promising and interesting material for
future technology applications.
Perhaps the most intriguing feature of graphene is its transport properties. Electronic
transport in graphene, both planar and vertical, attracted much of the initial interest due to
graphene’s unique conical electronic dispersion [17, 27]. In line with miniaturization
requests in modern electronic fabrication, graphene’s unusual electronic properties paved
the way for using it in sub-micron electronic devices fabrication.
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Fig.1 Graphene is the basic building block of graphitic materials. It can be wrapped into 0D fullerene,
rolled up into 1D carbon nanotube or stacked into 3D graphite. Adapted from [14].
Soon after, molecular, ionic and gas transport were all investigated, revealing that despite
just being one atom thick, graphene’s hexagonal lattice is fully populated with delocalized
electrons, making such particles, including helium, transport through graphene membrane
impossible. This allowed ways of applying graphene of acting as compliant membrane
pressure sensors and barriers of two or more distinct phases [22].
On the other hand, there are efforts concentrating on utilizing graphene’s impermeability by
defining nano-pores on its surface. Such nano-pores have been investigated to transport
gases, ions, and even translocate DNA molecules through them [28, 29, 30, 31].
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While graphene is the first isolated two-dimensional material, there are other layered
materials with van-der-Waals force in between layers. As a direct consequence, they can be
mechanically exfoliated from their bulk crystals to have thin, even monolayer, membranes,
showing intriguing properties quite different from their bulk materials. These materials
include hexagonal boron nitride (h-BN) [33] and molybdenum disulfide (MoS2) [34], each
having its own unique features: BN is a wide gap insulator while MoS2 is a semiconductor.
Such two dimensional materials, although different from graphene, show interesting
properties that can be used in competitive applications. While boron nitride is widely used
as substrate for high mobility heterostructures fabrication because of its insulating nature
and atomic flatness [35], molybdenum disulfide single layer transistors show promising
characteristics in digital electronics [36-38], sensors [39, 40] and photocatalysts [41].
The basic but amazing character of two dimensional materials being in atomic thickness has
been somewhat neglected by initial researches. This result in the fact that other than
electrons, particles have received less attention as regards transport through 2D membranes
– especially 2D materials other than graphene . While thinner materials could effectively
lower the transport barrier, two dimensional materials become promising in transporting
matters selectively through them without damaging their lattice – if the particles are smaller
than their hexagonal rings.
Transport of protons, the smallest positively charged ions with bare size of 0.8fm (isolated)
[42], along one dimensional water chain through water filled one dimensional carbon
nanotubes was reported [43]. High proton conductivity in bulk graphite oxide, graphene
oxide/proton hybrid, and graphene oxide has been proved recently, with the mechanism
that, rather than protons penetrating through, water associated protons flow through
tunnels between layered materials [44]. Though protons transport directly through
graphene has been researched both theoretically [45, 46] and experimentally of high energy
(several mega electron volts) protons irradiation [47, 48], low energy of protons transferring
through two dimensional materials without damaging their lattices has not been
systematically studied.
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Indeed, protons transporting through graphene and other two dimensional materials, as we
will see later in this thesis, is possible after overcoming the electron cloud introduced barrier,
and protons physically penetrate through atomically thin 2D sheets. Here the mechanism is
completely different compare to other proton conducting materials. This highlights two
dimensional materials’ new unique physical property which has not yet been studied and it
paves the way for them to be incorporated in fields where a proton semi-permeable
membrane is required.
The outline of the thesis is as follows:
In Chapter 1, we discuss fundamentals of graphene, including its electronic properties and
mass transport properties. We then proceed to present other two dimensional materials,
mainly h-BN and MoS2.
In Chapter 2, we present the experimental techniques used in this thesis, the mechanical
transfer methods and device fabrication procedures.
In Chapter 3, we demonstrate proton conductivity measurement in Nafion based solid
system. We show evidence of proton transport through two dimensional materials including
thin graphene and h-BN.
In Chapter 4, we show the performance improvements of proton conductivity with the Pt
catalyzed 2D materials. Proton transport introduced H2 gas flow through 2D materials
experiment is presented as well.
In Chapter 5, we introduce the liquid/liquid interface method as another method of
investigating proton transport through two dimensional materials.
In Chapter 6, we provide conclusions and view for future work.
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Fundamentals of graphene and
2D materials
In this chapter we discuss fundamental properties of graphene, starting from its atomic
structure and band structure, then continuing talking about its mass transport properties.
We also discuss properties of other 2D materials, namely h-BN and MoS2.
1.1 Fundamentals of graphene
1.1.1 Carbon materials
Graphene’s remarkable properties, particularly electronic properties, are the results of its
atomic structure. Carbon atoms form a 2D plane hexagonal lattice structure with each
carbon atom 1.42 apart from its three nearest neighbors. Free carbon atoms have four
outer electrons, namely 2s, 2px, 2py, 2pz (Fig 1.1a, top). In graphene, a carbon atom’ one 2s
orbit interacts with its two in plane p orbits, 2px and 2py (Fig 1.1a, bottom), forming strong
sp2 covalent bond, planar σ bond, with its three nearest neighbors that provide strength to
graphene honeycomb lattice. The fourth orbital is a π orbital, oriented in the z-direction,
perpendicular to the σ bond plane (Fig 1.1b). As electrons in z-direction are not paired, they
are responsible for the high electric conductivity in graphene.
Chapter ONE
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Fig 1.1 (a) Electronic orbital occupation in carbon element (top) and in graphene (bottom). (b)
Graphene orbital hybridization. Three σ bonds in plane with perpendicular π bond.
Fig 1.2 (a) Graphene honeycomb lattice structure. Unit cell consists of two non-equivalent sublattice
sites A and B. The lattice unit vectors are presented as and . , and are the three
nearest neighbors with lattice constant 1.42 . (b) Reciprocal lattice vectors b1 and b2 of the first
Brillouin zone of graphene with Γ as the center while K and K’ as corners.
a b
a b
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1.1.2 Graphene lattice and band structure
The structure of graphene can be seen as a hexagonal lattice with a basis of two carbon
atoms, A and B, per unit cell. The basis vectors and in real space can be used to
represent any lattice point in graphene structure as the lattice translation symmetry (Fig
1.2a).
The lattice vectors can be written as
where = = = ≈ 2.46Å. = 1.42Å is the length of C-C σ bond. Each carbon
atom’s three nearest neighbors can be given by:
with = = = .
The basis vectors of reciprocal lattice and (Fig 1.2b) can be represented as:
(1.1)
(1.2)
(1.3)
(1.4)
(1.5)
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Where = = b =
The first Brillouin Zone (BZ) is shown in Fig 1.2b. The center of the BZ is the Г point while K
and K’ are two non-equivalent points among the six corners of the BZ. M is the middle point
of the sides. Their positions in reciprocal space are given by
The graphene electronic band structure can be derived and described by applying the tight
binding model to the π orbital formed from the pz orbit electrons. As mentioned before pz
electron is responsible for graphene’s electronic transport properties, this approximation
works well by considering only a carbon atom and its three nearest neighboring atoms wave
functions. In 1947 P. R. Wallace [49] has used the tight binding approach and obtained the
energy dispersion relation of graphene, which is given by
(1.6)
(1.7)
(1.8)
(1.10)
(1.9)
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where is the hopping energy between nearest neighbors with a typical value 2.9eV [17]. kx
and ky are the x and y component of the electronic momentum. The plus and minus sign in
the expression represent the valence band π and conduction band π*, respectively.
By expanding above equation near the K or K’ points, one obtains
where is the momentum taken at the K or K’ point, ћ is the reduced Planck constant, is
the Fermi velocity with ≈ 106 m/s for the charge carriers in graphene [15]. This equation
reveals that near the points where valence and conduction band meets (often referred to as
Dirac point), there is a linear dependence of energy on momentum (Fig 1.3). The linear
energy dispersion relation near the low energy range reveals that charge carriers in
graphene behave as ultrarelativistic particles with zero effective mass, obeying the Dirac
relation.
Fig 1.3 Electronic band structure of graphene. Full band structure with zooming in near Dirac point.
Adapted from [17].
(1.11)
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In graphene, the Fermi energy locates at where the valence and conduction band meets and
the density of states vanishes. Graphene is not the same as traditional semiconductors
which have a finite band gap where the Fermi level lies in between conduction and valence
bands.
It is worth noting that graphene has a quite different band structure compared to its bilayer
and trilayer counterparts. Bilayer graphene is a zero gap semiconductor with a parabolic
energy dispersion relation near the K/K’ point. In graphene the electronic structure varies
rapidly with the number of layers, approaching the 3D graphite limit at around 10 layers [14].
1.1.3 Basic electronic properties in graphene
One of the landmark characteristics of graphene is its possibility of tuning charge carriers
continuously between electrons and holes, which is known as ambipolar field effect [11, 15].
Undoped graphene has its Fermi level exactly located at the Dirac point where density of
states vanishes.
In a typical graphene device, graphene is separated by a thin dielectric layer (usually several
hundreds of nanometers silicon oxide) from its conductive substrate which enables gate
voltage to be applied (Fig 1.4a). Fig 1.4a can be simplified to a parallel plane capacitor. With
gate voltage Vg applied, the capacitance can be deduced:
So that
1.12
1.13
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where , d represents permittivity of free space and relative permittivity, and the
dielectric spacer thickness, respectively. and for a silicon oxide spacer,
. n is carrier concentration.
Fig 1.4 Electric field effect of graphene. (a) Schematic of applying electric field on graphene. (b)
Optical image of graphene Hall bar device. Adapted from [15]. (c) Ambipolar field effect in single
layer graphene. Insert of the figure shows graphene conical electron spectrum with Fermi level
changes at different gate voltage. Adapted from [14]. (d) Graphene conductance as a function of gate
voltage at 10K. Adapted from [15].
(a) (b)
(c) (d)
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Carriers’ type corresponds to gate voltage sign so that positive Vg induce electrons while
negative Vg induce holes.
Noting that equation 1.13 refers to ideal graphene, in real devices graphene is easily doped
from environment [50] so that its Ef shifts into either the valence or conduction band, in
which case one must subtract the voltage V0 to reach its neutrality point:
In order to investigate graphene’s electronic properties, graphene based devices need to be
designed in a well-defined geometry to simplify the physics picture. A typical graphene
device is etched into a multi-terminal Hall bar geometry (Fig 1.4b), according to which
resistivity can be deduced:
where w and l are the width and length of Hall bar channel, respectively, while R is
measured from device IV characteristics. The conductivity . The carrier mobility, μ,
defined as how fast a charge carrier can move through its media in an electric field, can be
determined by the Drude model in which charges are treated classically:
where n is the charge carrier density and e is the charge of electron. Using this relation
mobility of both electrons and holes can be determined. The resistivity of graphene changes
with applied electric field applied and reaches the maximum where as conductivity reaches
its minimum (Fig 1.4c). The conductivity changes linearly with gate voltage Vg and reaches its
minimum at zero Vg with undoped graphene (Fig 1.4d).
1.14
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When a non-quantizing magnetic field is applied to graphene devices, the Hall coefficient
is inversely proportional to gate voltage with
. The mobility can also be
deduced from the Hall coefficient:
Defects, impurities and ripples in graphene limit the mobility [51, 52, 53].The mobility μ is
found to be weakly dependent on temperature showing that the scattering from surface
charge impurities [54], phonons [55] and surface roughness [56] are the main reasons
limiting the charge carriers’ mobility. Graphene mobility on SiO2 at room temperature
ranges from 5,000 to 15,000 cm2/Vs [14]. While mobility limiting factors originate from the
substrate, suspended graphene devices are fabricated with mobility reaching up to
1,000,000 cm2/Vs [57]. Recently on boron nitride substrates, as a consequence of BN’s inert
nature and atomic flatness, the mobility of graphene exceeds 100,000 cm2/Vs [35, 58].
The study of electronic transport also reveals other interesting character of graphene: it has
quantum Hall effect even at room temperature [59]; it exhibits a minimum conductivity in
the limit of vanishing carrier concentration [15]; it exhibits Klein tunneling through potential
barriers [60, 61].
1.2 Two dimensional materials beyond graphene
Although graphene is the first isolated two-dimensional material, there are plenty of layered
materials that can be mechanically exfoliated. Indeed, these layered materials are stacked
up from their single layer counterparts with van-der-Waals forces in between layers. This
gives these materials the possibility to be isolated into two dimensional individual sheets.
The simplest layered materials are atomically thin hexagonal sheets of graphene and
hexagonal boron nitride (h-BN), while other layered materials, for example transition metal
dichalcogenides (TMDs), have more complicated crystal structures. Their electric properties
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vary from insulating (e.g. hexagonal boron nitride) to semiconducting (e.g. MoS2). In addition
to graphene, these other two dimensional nano sheets also need further investigation. In
this section we discuss fundamentals of layered materials boron nitride (BN) and
molybdenum disulfide (MoS2).
1.2.1 Boron nitride
Boron nitride does not exist naturally but can be chemically synthesized typically from boric
acid or boron trioxide. Its single layer shares the same hexagonal lattice structure with
graphene but with a lattice constant 1.7% larger than graphene and it contains one boron
atom and one nitrogen atom in its unit cell. Boron nitride is AA stacking (Fig 1.5) with same
atoms from different layers exactly aligned on top of each other.
The lack of interest in a few layers of boron nitride down to a single layer is due to the
unavailability of high quality bulk boron nitride, until recently the synthesis methods enables
high grades boron nitride [62, 63]. A high temperature, high pressure growth was reported
where millimeter sized single crystal boron nitride can be obtained. CVD growth of single
and few layers boron nitride has also provoked researchers’ interests in this material [64, 65].
The TEM and conductive AFM studies suggest that there are no pinholes in this material that
boron nitride is ideally used as insulators [66].
Though single layer boron nitride is sp2-bonded, it is a wide band gap semiconductor with
energy gap of 5.97eV [63], resulting from the fact that pz orbitals in boron atoms are vacant
while in nitrogen atoms they are occupied by paired electrons. As electrons are localized in
boron nitride, one can expect that compare to graphene, electron density is lower within
boron nitride hexagonal rings. The importance will be clear later in this thesis.
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Fig 1.5 Boron nitride planar and crystal structure.
Boron nitride is an ideal dielectric substrate that can be used to improve the quality of
graphene based devices. As it is inert, h-BN is expected to be free of dangling bonds and less
surface charges [35]. An atomically planar substrate surface could avoid rippling in graphene
[56, 67], while BN has similar dielectric constant with that of silicon dioxide which allowing
BN to be applied as an alternative gate dielectric [68]. Planar electric transport in BN can not
be measured with typical transport measurements, but with BN on a conductive substrate or
sandwiched in between two single layers of graphene, transport properties through thin BN
have been reported [27, 66, 69].
1.2.2 Molybdenum disulfide
Molybdenum disulfide belongs to TMDs family consisting of a plane of molybdenum atoms
sandwiched between two planes of sulfur atoms (Fig 1.6c). MoS2 crystals are naturally
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35
appearing as centimeter sized crystalline materials similar to graphite. It is traditionally used
as lubricant and surface protectors [70, 71].
Fig 1.6 Band structure and lattice structure of MoS2. (a) Single layer MoS2 with its direct band gap
and (b) bulk MoS2 with its indirect band gap. Adapted from [73] (c) Lattice structure of MoS2. The
distance between layers is 6.5 . Adapted from [38].
Bulk MoS2 turned out to be an in-direct gap semiconductor (Fig 1.6b) from bulk of 1.29eV up
to 1.9eV with thickness decreasing [72] while its single layer sheet is theoretically shown
(c)
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36
transferred to be a direct gap semiconductor (Fig1.6a) [73]. It gives MoS2 new possibilities
for photo-current devices [74]. As sulfur atoms stay above and below molybdenum atoms,
when referred as single layer MoS2, it contains three atomic layers.
1.3 Matter transport properties of 2D materials
Graphene’s electronic transport properties have firstly attracted researchers’ attention. But
as graphene is the ultimate limit of a membrane – it is only one atom thick – an interesting
question is whether such a thin membrane would be permeable to particles other than
electrons – such as ions, molecules including gases and liquids.
In order to probe its matter transport properties, J. S. Bunch et al [22] prepared suspended
graphene over predefined wells. By applying a pressure difference between the input and
output of a micro-chamber, it was found that the gas leaking rate is independent on
graphene thickness, between 1 and 75 layers, suggesting that gases including helium cannot
go through graphene. It has been further investigated that for graphene with defects,
helium transport barrier reduced exponentially with defect size [75]. Graphene’s
impermeability to ions in liquids has been reported as well [31]. Although it is only one atom
thick, graphene’s π-orbital forms a dense, delocalized electron cloud that blocks the gap
within its hexagonal rings [76].
While by utilizing its impermeable properties, graphene, with an electron beam drilled nano-
size pore through it, is an ideal substrate for a nanopore based single DNA molecule detector
[77, 78]. With chemically modified nanopores, it is possible to realize selective passage of
specific ions and molecules in graphene [28, 79, 80]. Graphene microchamber for TEM
imaging [81] and graphene protective coating [82, 83] has been reported as well due to its
impermeability.
On the other hand, research on graphene derivatives – graphene oxide paper shows
permeation of water through it [32]. Water molecules are driven by capillary force and
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37
transport through pathways between graphene oxide layers. Gas and ionic transport
through one-dimensional carbon nanotube has also been reported [84, 85].
When ion irradiation is applied to a graphene sheet, ions can easily travel through graphene,
which is reasonable since ions are accelerated to several MeVs energy [86]. Interestingly,
proton, the smallest positively charged ion, is the only one that can go through single layer
graphene from irradiation without damaging graphene’s lattice [47, 48].
Besides graphene, salt rejection and water transport through nano-size boron nitride
nanotubes using molecular dynamics simulations has been reported recently [87, 88], while
mass transport through other 2D materials were barely investigated.
2D materials have fascinating unique properties all of which mainly due to their electronic
structures. As we will examine later in this thesis, electron cloud distribution within 2D
materials lattices strongly affects the barrier of mass transport through 2D materials.
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38
Experimental techniques and
Device fabrication
In this chapter we review the experimental procedures for producing few layers 2D
materials from their bulk crystals, followed by introducing reactive ion dry etching and KOH
wet etching techniques for drilling through-holes on Si/SiNx substrate. The flake transfer
methods will be discussed as well.
2.1 Fabrication of layered materials by mechanical cleavage
Substrate cleaning
2D flakes are usually produced on a oxidized silicon substrate. The reason for using a silicon
oxide covered wafer as a substrate (which will be referred as SiO2/Si later) is to easily enable
the identification of graphene flakes with an optical microscope. While graphene absorbs
only small amount of light (2.3%), oxidized silicon layer works as an extra reflection layer,
enhancing the contrast up to 18% for single layer graphene [89].
Chapter Two
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39
The SiO2/Si wafer used in this thesis is purchased from IDB Technologies Ltd. arrived in 4
inches wafers. The thicknesses of the silicon oxide layers are 70nm, 90nm or 290nm. After
wafers being cut into a suitable size (typically 1 inch square), they are cleaned with acetone
and isopropanol in an ultrasonic bath for 10 minutes each, followed by blowing them dry
with filtered nitrogen. This cleaning step is to remove particles and chemicals adsorbed on
wafer surface.
Subsequently, SiO2/Si substrate is exposed to oxygen plasma at low pressure as oxygen
plasma reacts with hydrocarbons and contaminants adsorbed on the surface. After plasma
cleaning, the plasma chamber is filled with pure nitrogen to avoid recontamination from the
air. Typical time for plasma cleaning is 10 minutes.
Fig 2.1 Procedures of producing thin graphene flakes. (a) Graphite on a sticky tape. (b) Bulk graphite
repeatedly peeled on the tape. (c) Thin graphite pressed on a pre-cleaned substrate. (d) Tape
removed, with thin graphene flakes on the substrate.
(a) (b) (c) (d)
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40
Graphene exfoliation
Mechanical cleavage of graphene starts with natural graphite. A piece of natural graphite
(single crystal) is pressed onto a piece of adhesive tape (Fig 2.1a) and then peeled repeatedly
(Fig 2.1b). Due to the layered structure of graphite, the repeatedly peeling produces thinner
graphite flakes on the adhesive tape. Thin graphite flakes are then pressed on a SiO2/Si
substrate as soon as the substrate is removed from oxygen plasma treatment (Fig 2.1c). The
adhesive tape is then chemically removed from the substrate (Fig 2.1d). After tape removal,
the substrate is treated in isopropanol to minimize contamination from the tape, followed
by baking on a hotplate at 130⁰C for 10 minutes to give better adhesion between graphite
and substrate. A second peeling is applied afterwards by pressing a piece of adhesion tape
on the substrate and then peeling it off with an angled peeling method to remove thick
graphite and leave only thin graphene behind.
Fig 2.2 Optical image of single layer graphene combined with two layers, three layers and four layers
thin flakes. The scale bar is 50um.
two layers
three layers
single layer
four layers
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41
Accurate identification of the thicknesses of graphene is critical in this thesis. Optical
microscopy is a frequent and quick method to identify graphene flakes.
Other methods of producing graphene
Mechanical exfoliation is the primary method of producing graphene flakes for scientific
research. While limited by graphite crystal size, it’s not suitable for large scale and mass
production. Instead, there are some other ways of producing graphene each of which has its
specific application.
Chemical vapor deposition (CVD)
CVD is a very important and perhaps the most investigated method of producing large scale
graphene. During the CVD process, gas species (e.g. CH4) as carbon source pass through a
hot zone (up to 1000⁰C), where hydrocarbons decompose to carbon radicals at a metal
substrate surface, then forming graphene. The thickness of graphene could be controlled by
the growth environment [90]. In 2009, Li et al. reported large area CVD graphene growth up
to a centimeter size on copper foil [91]. Though recently developed, CVD graphene can be
produced as large as a square meter size based on roll-to-roll basis (Fig 2.3a) [92]. CVD
graphene is expected to have commercial value, especially be promising in application area
for flexible, transparent electronic devices such as touch screens.
Liquid exfoliation
This method belongs to physical exfoliation while the key is to choose a suitable liquid
whose surface tension is prefers to extend graphite crystallites into larger surface area. With
proper solution, liquid exfoliation can be used to exfoliate not only graphene [93], but also
many other 2D materials (Fig 2.3b) [94]. It can reach mass production up to grams. Though
isolated flakes size are relatively small (few microns), liquid exfoliation is a promising
candidate in conductive ink and ink printable electronic circuit applications.
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42
Epitaxial growth
Epitaxial growth of graphene based on high temperature and low pressure at which silicon
carbide (SiC) substrate reduces to graphene. It was first reported by Berger et al. [95].
Fig 2.3 (a) CVD graphene transferred on to PET substrate with size exceeding 30 inch. Adapted from
[92]. (b) Liquid exfoliated a variety of layered materials. Adapted from [94].
Exfoliation of other 2D materials
Boron nitride
BN thin flakes are mechanically exfoliated and identified on substrates of 70nm silicon oxide
coated silicon wafers, since 70nm silicon dioxide would give the best contrast for single layer
BN [96]. Limited by the bulk crystal size (Fig 2.4a), the lateral size of single layer BN is
typically 10μm (Fig 2.4b).
Molybdenum disulfide
MoS2 flakes are prepared on PMGI (Polymethylglutarimide)/PMMA (Polymethyl
methacrylate) double layer resists substrate. The resists are spin coated on a plain silicon
(a) (b)
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43
wafer. This double layer is used in a dry transfer technique which will be introduced later in
this chapter. Typical spin coating speed is 4000rpm and 7000rpm for PMGI and PMMA,
Fig 2.4 Optical images of (a) bulk hexagonal BN, (b) single layer BN on 70nm silicon oxide wafer with
its bilayer and trilayer counterparts, (c) bulk MoS2, and (d) single layer MoS2 on PMMA/PMGI double
layer substrate. The scale bar is (a) 100μm, (b) 10μm (c) 1cm and (d) 50μm, respectively.
respectively. PMGI/PMMA dual layer resists is baked on a hotplate for 10 minutes at 130⁰C.
Subsequently, thin MoS2 crystal on a piece of adhesive tape is pressed onto the resists
substrate. Tape is subsequently peeled off with an angled peeling method.
1L
3L 2L
(a) (b)
(d)
1L
(c)
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MoS2 thin flakes could be produced on the SiO2/Si substrate as well, while lateral size of
single layer MoS2 produced on the resists substrate (typically 50μm, Fig 2.4c and d) is
generally larger than produced on the SiO2/Si substrate.
Characterization techniques for 2D materials identification
In order to distinguish between graphene single layer and its few layers counterparts, atomic
force microscope (AFM) could be used to measure the step height of these flakes from the
Fig 2.5 AFM images of various single layer 2D materials. (a) Single layer graphene. Adapted from [11].
(b) Single layer BN and its few layers. Adapted from [96]. (c) Single layer MoS2. Adapted from [38].
(a) (b)
(c)
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substrate (Fig 2.5a). The step height for single layer graphene is usually different (higher)
from the expected inter layer spacing of graphite (0.334nm). This is due to the roughness
from the substrate (e.g. 290nm SiO2/Si substrate roughness about 1nm). Therefore, the step
height between a monolayer and bilayer on the same substrate is checked as well.
AFM step height analysis is equally applicable for other 2D materials as well (Fig 2.5b and
2.5c). Apart from AFM, Raman spectroscopy is widely used as a quick characterization
technique to distinguish between single and few layers graphene.
Fig 2.6 Raman spectroscopy of graphene. (a) Raman spectra of bulk graphite and single layer
graphene at 514nm. (b) Evolution of the Raman spectra at 633nm with the number of layers.
Adapted from [160].
Raman spectroscopy of graphite and graphene consist of two main peaks, namely G peak at
~1580 cm-1 and 2D peak at ~2700 cm-1. Fig 2.6a shows the Raman spectroscopy of graphene
and graphite in which the 2D peaks of the two are rescaled to be the same height. The
relative height of G and 2D peak is one clue of distinguishing thin graphene membranes. The
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46
other feature from graphene Raman spectra is that the line shape of its 2D peak varies with
the number of graphene layers. Fig 2.6b shows the 2D peaks of different number of
graphene layers. For monolayer graphene, its 2D peak is a single Lorentzian peak while with
the number of layer increasing from 2 layers to graphite, the shape of 2D peak becomes
broader and is finally split into two components. The position of bilayer graphene 2D peak is
upshifted compare to single layer and split into four small features. Therefore, Raman
spectroscopy of graphene layers can be successfully used to identify single layer graphene to
its bilayer and few layers.
2.2 Assembly of 2D membranes based proton conductive
devices
The availability of two dimensional materials production with mechanical peeling method
and microscope identification provides an easy way to have 2D materials thin flakes. As to
investigate the possibility of protons going through 2D materials: (1) the final device has to
have a substrate with a straight through hole that protons with its medium can access at the
surface of 2D flakes from both sides, and (2) 2D flakes have to be transferred from the
original substrate (where they were produced) to the device substrate. The methodology
consists of two parts: substrate fabrication and flake transfer techniques.
2.2.1 Substrate fabrication
The main method of defining micro size holes is photolithography while a combination of
reactive ion etching and KOH wet etching is used to drill holes straight through on a 500μm
silicon substrate with a polished layer of 500nm silicon nitride coating on both sides.
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Photolithography
Photolithography is a technique transferring a geometric pattern from photo mask to light
sensitive chemical, usually called photoresist. Initially, photolithography had depended on
photo masks to transfer pre-designed pattern, until 1987, when the first laser writing system
came out [97]. This technique is based on a focused laser shining directly onto target
photoresist with a software designed pattern. This direct laser writing system provides quick
and accurate exposure with sub-micron resolution.
A Microtech LW405 laser writer with a 405nm GaN laser is used for doing photolithography
for devices fabrication presented in this thesis. The resolution of the LW405 is 500nm.
A layer of Microposit S1813 (positive tone optical resist) is spun onto a SiNx/Si wafer with a
spin speed of 3000rpm, followed by hotplate baking at 115˚C for 3 minutes. The thickness of
S1813 resist is about 1.3μm.
After exposure with the LaserWriter, the samples are developed in Microchem MF319 (a
solution of 2.5% Tetramethylammonium hydroxide in water) for 1 minute in order to
remove resists from exposed regions (Fig 2.7). De-ionized water rinse is required afterwards
to stop further development.
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48
Fig.2.7 Process of photolithography with laser writing and development.
Table 2.1 Spin coating parameters of photoresist used in this thesis
Photoresist Frequency
(Revolutions
per minute)
Time
(s)
Soft baking
Temperature
(˚C)
Baking Time
(s)
Thickness
@3000rpm
(µm)
S1813 3000 60 115 180 1.3
S1805 3000 60 110 60 0.5
Su-8 2025 3000 60 95 300 25
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Etching techniques
In order to etch through double sides polished, 500nm silicon nitride layer coated Si
substrate, a combination of reactive ion etching and potassium hydroxide wet etching is
used.
Reactive Ion etching
Reactive Ion etching (RIE) is a micro-fabrication technique using plasma to remove materials
presented on a target wafer. It has been rapidly developed due to (1) achievement of
directional etching without depending on crystal orientation unlike the case of silicon wet
etching which will be discussed later; (2) good transferring of a photolithographically defined
pattern into underlying layer and (3) easy cleanliness with gas pumping processes [98].
RIE usually uses fluorine gas plasma to etch the target substrate. During the RIE process,
high energy gas species from plasma, which is generated at low pressure by an
electromagnetic field, attack the wafer surface, and react with the surface material to
generate gas products.
A silicon nitride RIE etching recipe with sulfur hexafluoride, SF6 [99], is used in this thesis.
With this recipe, a glow discharge from SF6 is generated to have an etching environment
containing neutrals, electrons, photons, radicals (F*), positive (SF5+) and negative (F-) ions.
Radicals diffuse from the bulk plasma while positive ions are driven by DC bias to the wafer
surface to assist the etching:
Si3N4 (s) + 3 SF6 => 3 SiF4 (g) +3 SF2 (g) + 2 N2 (g)
Gas products are pumped out.
RIE etching rate of SF6 silicon nitride recipe is 200nm per minute. In order to completely
remove a 500nm silicon nitride layer, typical etching time is 3 minutes. Unexposed S1813
photoresist works as an etching mask. RIE etching selectivity, defined as etch rate ratio of
etch mask compared to target material, of S1813 and silicon nitride is 1:30 (Fig 2.8). After
RIE etching, S1813 photoresist is removed in acetone.
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50
Fig 2.8 The RIE etching process. S1813 works as an etch mask and is being etched simultaneously
with silicon nitride layer requiring sufficient thickness of the S1813 layer.
Table 2.2 Recipes of RIE etching recipes used in this thesis
Recipe Plasma gases Target material Etching rate
(nm/s)
Selectivity
(target : S1813
mask)
SiNx recipe SF6 Silicon nitride 4 1:15
Bosch process
recipe
SF6/CHF3 Si 45 1:30
SiO2 recipe CHF3/Ar Silicon dioxide 1 1:20
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KOH wet etching
Wet etching is a fabrication method eliminating material by its dissolution in an aqueous
etching solution. To etch a material there is a variety of chemical agent available. It is worth
considering that, however, the solution used in the etching process must be etching
selectively, without affecting the etch mask and underlying materials. Hence, etching
chemical chosen must have a high selectivity, defined as the etching rate ratio of target
materials and each mask in the same solution.
Generally, wet etching provides a higher etch rate compared to the RIE etching. (etching rate
of silicon: 2.7µm per minute for RIE dry etching, 6µm per minutes for wet etching at 90˚C,
measured in this thesis). Modification of etchant can affect selectivity as well. The etch rate
highly depends on reactant diffusing to the material surface, surface reaction, and products
diffusing away from the surface, all of which can be speed up by increasing the reaction
temperature. Other effects for example gas products trapped at the surface could affect the
reaction as well.
There are two different types of etching: (1) isotropic etching, while etching rate is the same
at all directions, and (2) anisotropic etching, when etching rate has a preference in one or
some directions (usually crystal orientation). In this thesis, potassium hydroxide (KOH) is
used for anisotropic wet etching of the silicon substrate:
Si + 2OH- + 2H2O => Si(OH)4 2- + 2H2(g)
Silicon has a diamond cubic crystal structure. The etching rate of KOH strongly depends on
the crystallographic orientation [100]. The KOH etching process is much slower on {111}
planes compared to {100} and {110} surface with an etching rate ratio up to 1:500 (Fig2.9a).
The dependence of the etching rate on the crystal orientation could be explained as the
differences of the atomic lattice packing density and available bonds in the crystallographic
planes [101]. However, the mechanism of the dependence remains inconclusive.
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52
Fig 2.9 KOH wet etching with a SiNx mask. The Zetch is proportional to the Wmask. (b) Etching goes
straight through the whole Si substrate. (c) Etching stops with a sharp triangular profiler.
Due to the slow etching rate at the silicon {111} surface, if considering plane {111} as a non-
etching plane, the final width can be calculated as (Fig 2.9b)
where is the angle between the planes {111} and {100}.
20% weight percentage KOH water solution is used in this thesis. At temperature 90°C, the
etching rate is about 6μm/min. The etching process lasts about 1.5 to 2 hours to etch
through 500μm silicon substrate. After KOH wet etching, repeated cycles of de-ionized water
rinses are required to minimize KOH residues.
(a) (b)
(c)
{100} {111}
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53
2.2.2 Transfer techniques
Flakes from the original SiO2/Si wafer have to be transferred on to target substrate with
transfer techniques discussed here. Transfer methodology consists of mechanically lifting
flakes up with polymer support and aligning them to a target substrate. Two different
transfer techniques are used in this thesis, each has its benefits.
Etching based wet transfer
The so called wet transfer [102] is the mostly used transfer method in this thesis. This
transfer technique works with potassium hydroxide etchant to etch away silicon and its
oxide which was originally the substrate for mechanical exfoliation of layered materials. The
flakes to be transferred are firstly spin coated with PMMA at spin speed 3000rpm, followed
by hotplate baking at PMMA glass transition temperature of 130˚C for 10 minutes (Fig 2.10a).
Subsequently the wafer is immersed into low concentration KOH solution (0.1mol/L) to etch
away silicon oxide and silicon slowly (Fig 2.10b) until the PMMA membrane with flakes
attached is floating in the solution (Fig 2.10c). Typical etching time for this process is four
hours.
The floated PMMA membrane is carefully washed with de-ionized water and then ready to
be transferred to a target substrate. It is worth noting that one can accelerate KOH etching
process by using higher concentrated KOH solution at higher temperature (but less than
50˚C to avoid tape melting). Although flake-PMMA bilayer can float up in 10 minutes, this
condition is typically avoided due to the more KOH contamination and more stress flakes
experienced in this condition. This process is called wet transfer as flakes are in touch with
solution during the etching process.
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Fig 2.10 Procedure of polymer based wet transfer. (a) PMMA is spin onto graphene, and then (b)
immersed into KOH solution to etch SiO2 underneath to have (c) free standing graphene/PMMA layer.
The graph uses graphene as an example, but also available for other 2D membranes.
Dry transfer
Dry transfer [103], as referred by its name, is a technique by which the side of the flake to be
transferred doesn't get wet before it contacts with target substrate. This method depends
on a bilayer resist substrate system. First, a PMGI/PMMA dual layer resist is spin on a
chemical cleaned Si wafer with spin speed 4000rpm and 7000rpm, respectively, followed by
hotplate baking at 130˚C for 10 minutes. Thickness of the double layer is a few hundred of
nanometers. 2D crystals on the adhesive tape are then pressed onto double resists layer to
exfoliate thin flakes (Fig 2.11a).
As PMGI/PMMA double layer is an extra layer to reflect light, similar as for silicon oxide, the
contrast of flakes can be modified by the thicknesses of the resist layer which is controlled
by the resists spin speed. This enables identification of graphene and other materials
exfoliated on PMMA.
After identifying flakes to be transferred, a scratch circle of about 5mm in diameter on
bilayer resist is made, with flakes in the middle. PMGI layer is subsequently dissolved by its
developer MF-319, leaving the PMMA layer unaffected (Fig 2.11b). The PMMA – flake
(a) (b) (c)
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55
bilayer is then carefully transferred into de-ionized water (Fig 2.9c). Due to PMMA’s
hydrophobic property, PMMA membrane floats in the water, with flakes non-wet on top of
PMMA, while MF-319 residue underneath PMMA can be cleaned away.
Fig 2.11 Procedures of dry transfer. (a) PMGI/PMMA double-layer is spin on to substrate with
mechanical exfoliated graphene on top. After (b) PMGI dissolved with MF319, (c) graphene/PMMA
layer is cleaned in DI water, followed by (d) optical alignment and then transferred onto target
substrate.
Optical alignment
The PMMA- flake membrane, from both wet and dry transfer techniques, is carefully stuck
onto a micromechanical stage allowing independent motion control in x, y, and z directions.
Target substrate is fixed by vacuum on a bottom stage with temperature control (Fig 2.9d).
(a) (b)
(c) (d)
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56
After optical microscope alignment, the top stage with PMMA moves down until PMMA and
target substrate has intimate contact. Target substrate warming up could enhance the
PMMA/substrate adhesion.
2.3 Proton transport device fabrication
The proton transport devices were fabricated with a combination of the techniques
described above. Double sides polished, 500nm silicon nitride coated 500μm silicon wafer
was cut into 16.5mm by 16.5mm size. Noting that silicon nitride KOH wet etching rate is
negligibly slow, it works as a silicon wet etching mask.
After cleaning in acetone and isopropanol, the SiNX/Si substrate was spin coated with
MicroPosit S1813 at 3000 rpm on both sides of the substrate, then hot plate soft baked at
115˚C for 3 minutes. Subsequently, the bottom side (could be either side from the two,
defined as bottom while the other one is top) S1813 was exposed by LW 405 laser writer
and developed to open a 1000 microns square window on the S1813 layer in the center of
substrate (Fig 2.12a). This was followed by RIE silicon nitride SF6 etching for 3 minutes to
completely remove the bare silicon nitride (Fig 2.12b). The bottom layer S1813 acted as a
dry etching mask to etch the silicon nitride layer, while top S1813 layer was to protect the
top side silicon nitride from scratching during the whole process.
The S1813 layers were removed on both sides in acetone for 10 minutes after RIE etching.
The cleaned substrate was immersed into 20% weight percentage potassium
hydroxide/water solution at 90⁰C. Etching starts at the silicon that is unprotected by the
silicon nitride mask, with etching rate 6μm per minutes. This process stops when the KOH
etchant reaches the top side silicon nitride stop layer and leaves a 300μm x 300μm silicon
nitride (500nm thick) suspended membrane (Fig 2.12c). De-ionized water is used to remove
KOH residue afterwards, followed by isopropanol rinse and filtered nitrogen blow dry.
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57
Again, with a combination of photolithography and RIE, a straight through hole with
designed micron-size is drilled on the suspended top side silicon nitride membrane (Fig
2.12d and e).
The flakes of 2D materials with PMMA support are then transferred to cover the through
hole (Fig 2.12f), followed by hotplate baked at 130˚C for 10 minutes to have better adhesion
between flakes and SiNX/Si substrate. PMMA is removed in acetone. The next step is rinse in
hexane. Hexane is reported to have a low surface tension so that suspended 2D membranes
will not be ruptured when removed from liquids [104].
Fig 2.12a to f demonstrated the whole procedures, while Fig 2.12g shows the device diagram
of a transferred 2D membrane on SiNX/Si substrate with a straight through hole. Although it
is not yet a measureable device, all samples are further fabricated from this structure. As
there are a few different types of devices, further device fabrication steps will be specified in
each chapter.
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58
Fig 2.12 (a-f) Processes of device fabrication and (g) prepared sample geometry.
a b
c d
e f
g
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59
Proton transport through 2D materials in
Nafion based solid systems
Proton transport through 2D materials is barely investigated previously. Therefore, new
device fabrication procedure and measurement methods need to be developed. In this
chapter we review fabrication of devices for proton transport through Nafion based solid
systems. We then demonstrate proton conductivity of various 2D materials including BN,
graphene and MoS2.
3.1 Introduction
Matter transport through two dimensional materials has attracted much interest. Electronic
transport in graphene, both planar and vertical, attracted much of the initial attention due
to graphene’s unique conical electronic dispersion [17, 27]. Soon after, molecular, ionic and
gas transport were all investigated [21, 22, 31]. Despite being just one atom thick,
graphene’s hexagonal carbon lattice is densely populated by delocalized electrons, making
transport of such particles impossible [22].
Chapter Three
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60
Instead, there are efforts concentrated on utilizing graphene’s impermeability by defining
nano-pore on its surface. Such nano-pores have been reported to transport chemicals, ions,
and even translocate DNA molecules through it [28,29,31].
In this work, we study transport of protons, the smallest positively charged ions, through a
mechanically exfoliated pristine monolayer graphene and other 2D materials, namely
hexagonal boron nitride and molybdenum disulfide. While there are studies reporting
proton transport through graphene with proton irradiation (proton source with energy level
several MeV) [47, 48], low energy proton transport through 2D materials without damaging
their lattices has not been systematically studied.
3.2 Solid proton medium and source
Proton conductive medium – Nafion
Nafion refers to a positive charged ion (cation) conducting polymer that was first developed
in the late 1960s by DuPont. It consists of a tetrafluoroethylene backbone with
perfluorovinyl ether groups terminated with sulfonate groups incorporation (Fig 3.1a).
Nafion has been paid a considerable amount of attention as a highly conductive proton
Fig 3.1 (a) Nafion chemical structure and (b) proton conductive mechanism in Nafion
a b
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61
conductor in proton exchange membrane fuel cells, owing to its good thermal and
mechanical stability [105].
Nafion exhibits excellent proton conductivity, but only when soaked with water which works
as proton transport medium. In Nafion, the proton conduction mechanism could be easily
described as protons on the sulfonic acid (SO3H) group “hopping” from one acid site to
another. Water is confined into nanometer dimensions domains in Nafion polymer [106]. In
one proton hopping, proton dissociates from the acidic site, followed by transferring into the
confined water medium (Fig 3.1b).
The proton diffuses in the water domain until it associates with another acid site. In this way,
protons transports through the polymer matrix with the help of water. With an external
potential (electric field), the protons can move in a certain direction.
Fig 3.2 Unit cell of metallic palladium with octahedral sites occupied by hydrogen in β palladium
hydride, with Pd: H = 1: 1. Note that the same sites are occupied by hydrogen in its α form but with
lower Pd: H ratio. Adapted from [107].
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62
Proton source – palladium hydride
Despite its name, palladium hydride (PdHx, where x is the H: Pd ratio) is not an ionic hydride
but a palladium metal with hydrogen molecules adsorbed inside the palladium crystal lattice
with an expanded lattice constant (Fig 3.2).
At room temperature, there are two phases in the PdHx system. With x<0.03, it is the so
called α phase at which the lattice constant is close to palladium metal. As more hydrogen
dissolves in palladium, the lattice constant continuously increase until the β phase where x is
approximately 0.6 and the lattice constant expansion about 6%. It is worth noting that the
same sites are occupied by hydrogen in both the α and β phases but as the lattice expands,
more vacancies are created, thus providing more available adsorption sites [107].
Hydrogen atoms can be injected into the Pd lattice either by immersing Pd in hydrogen gas
[108,109] or electrically charged in acidic solution [110,111]. Hydration of palladium is
reversible and therefore it has been investigated as a possible material for hydrogen storage
[112].
3.3 Sample fabrication and measurement system
Sample fabrication
Graphene and boron nitride are mechanically exfoliated on 290nm and 70nm silicon oxide
coated silicon wafers, respectively, while MoS2 is peeled on a PMGI/PMMA dual layer
substrate. Subsequently flakes are transferred onto silicon nitride substrate with a 2μm
straight through-hole by the wet transfer technique for graphene and BN and by dry transfer
for MoS2.
Palladium foil (99.9%, 0.5mm thickness from Sigma-Aldrich) was cut into 2mm by 8mm
pieces, followed by immersion into NaBH4 water solution (3mol/L), and stirred overnight (>8
`
63
hours) to achieve hydration [113]. A rinse in de-ionized water of the palladium hydride is
required to minimize chemical residue.
Fig 3.3 (a) Three dimensional presentation and (b) vertical profile of solid Nafion proton transport
sample.
Droplets of Nafion 117 solution (~5% in a mixture of lower aliphatic alcohols and water, from
Sigma-Aldrich) were applied on both sides of suspended two dimensional material
membrane, followed by oven baking at 130⁰C for 20 minutes in a water vapor environment
to allow water soaked into the Nafion membrane. Pre-hydrated palladium film strips were
attached onto both sides of the Nafion solid membrane before the Nafion had completely
dried (Fig 3.3a and b).
Measurement system
The technique of injecting protons directly from a hydrated palladium electrode into a solid
membrane was used [114]. The measurement was performed in a pre-pumped vacuum
a b
`
64
chamber (10-3 mbar) followed by insertion of a gas mixture of hydrogen (10% H2, 90% Argon)
and water vapor (in order to keep the Nafion wet during measurements) (Fig 3.4a). While
the hydrogen concentration is not strictly controlled, this is not a limiting part to proton
transport in this thesis, which will be discussed later.
A Keithley 2636 SourceMeter was used for the measurements. Voltage bias was applied
between two PdHX electrodes (Fig 3.4b). All the results were measured at room temperature.
Fig 3.4 (a) Schematics of experimental system and (b) mechanism of proton transfer in solid Nafion
samples measurements.
3.4 Results
When dissolved in the palladium lattice, the hydrogen is almost completely ionized, with its
electron going into the Pd d shell [115]. According to reaction [116]:
3.1
a b
`
65
-2 0 2
-15
0
15 S-BN
S-Gr
S-Mo
Curr
ent
(nA
)
Bias voltage (V)
linear region
During measurement, a hydrogen atom loses one electron at the Pd electrode to produce a
proton. With electric field applied, the proton transports through Nafion and through the 2D
sheet to the other Pd electrode where it is re-combined with an electron to generate a
hydrogen atom. The hydrogen atom is subsequently absorbed by the palladium (Fig 3.4b).
Fig 3.5 Proton transport I-V characteristics of 2D membranes. The proton conductive current curves
as a function of bias voltage for single layer boron nitride (S-BN, red), single layer graphene (S-Gr,
blue), and single layer MoS2 (S-Mo, green) are recorded at voltages from -2V to +2V. The curves are
linear at low biases.
`
66
Fig 3.5 shows the current-voltage measurements for three different materials used as a
barrier: single layer BN (S-BN), single layer graphene (S-Gr), and single layer MoS2 (S-Mo).
Here voltages within ±1V is defined as linear region since out of this region, the I-V curve
responds in a non-linear behavior, which is possibly due to chemical reaction in Nafion. The
performance of devices will decrease if they are kept in the non-linear region for a long time
(typically 30 minutes), hence all measurement results will from this point be presented in
the defined linear region.
Fig 3.6 (a) Electron density of single layer BN (S-BN), single layer graphene (S-Gr) and single layer
MoS2 (S-Mo) from top view. Electron density decreases from black to white. (b) Electron density of
proton transport for single layer graphene, adapted from [46]. Left: initial state. Right: transition
state. Red indicates electron accumulation while blue indicates electron depletion.
S-BN S-Gr S-Mo
a
b
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67
0 5 10 150
10
20
30
Con
du
cta
nce
(nS
)
Hole size in area (m2)
-1 0 1
-20
0
20
1m
2m
3m
4m
Curr
ent
(nA
)Voltage (V)
-1.0 -0.5 0.0 0.5 1.0
-5
0
5
Cu
rre
nt
(nA
)
Bias voltage (V)
S-BN
Bi-BN
Tr-BN
-1 0 1
-0.5
0.0
0.5
Cu
rre
nt
(nA
)
S-Gr
S-BN
Voltage (V)
S-BN and S-Gr displays a linear proton conductive current response at low bias while S-BN is
one order of magnitude more conductive to protons compared to S-Gr. Single layer MoS2
exhibits a current which remains indistinguishable above the noise current level (pA). The
result is not surprising, since as shown in Fig 3.6a, electrons in single layer BN are localized
so that S-BN has a lower electron density within its hexagonal ring which provides easier
proton transport through it. Protons were found not transport through S-Mo, which is
consistent with its three atomic layers structure.
Fig 3.7 (a) Proton conductive I-V characteristics of single layer BN (S-BN, red), two layers BN (Bi-BN,
green), and three layers BN (Tr-BN, blue). Insert: comparison of proton conductive current for single
layer graphene (S-Gr, red) and Bi-BN (blue). Their respective proton conductance is comparable. (b)
Conductance as a function of hole sizes in linear region. Single layer flakes of BN covering holes sizes
of 1µm (blue), 2µm (pink), 3µm (green), 4µm (yellow) in diameter were measured. Insert: Linear IV
characteristic of single layer BN for different hole-diameters.
S-BN
a b
`
68
0.1
1
0.02
Tr-BNS-MoBi-BNS-Gr
Conducta
nce(nS
)
bare hole
S-BN
150
Fig 3.6b shows the electron density for the initial (Fig.2b left) and transition state (Fig.2b
right) of proton penetration in the single layer graphene system. In the initial state with one
proton on top of the graphene lattice, graphene has a trend of screening the positive charge
in the proton by depleting electrons from the unit cells around it (Fig.2b left). In the
transition state, the proton interacts differently with each carbon atom that it is closer to
one of the carbon atoms than the others (Fig.2b right). This indicates that instead of
penetrating from the center of the hexagonal ring, the proton experiences bond formation
and breakage with the carbon atoms [44].
Fig 3.8 Proton conductance statistics of single layer BN (red), single layer graphene (green), bi-layer
BN (blue) and their few layers counterparts. Single layer MoS2 (cyan) and tri-layer BN (pink)
conductance are under noise level (shadowed area). Nafion covered 2µm in-diameter hole
conductance is about 150nS, noted as bare hole in the graph. Insert: SEM image of single layer
graphene covering a 2μm through hole. Scale bar: 500nm.
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69
-2 0 2
-200
0
200
Curr
ent (n
A)
Bias voltage (V)
Bare hole
S-BN
Proton conductivity of single layer BN and graphene and their few layers are shown in Fig
3.7a. As BN is AA stacking, bi-layer BN (Bi-BN) has a proton conductivity comparable with
single layer graphene (Fig 3.7a insert). Fig 3.7b shows linear proton conductance as a
function of through-hole’s area with single layer BN covering the holes.
The conductance statistics presented in Figure 3.8 confirms the result that single layer boron
nitride has a proton conductance of more than 5nS, one order of magnitude higher than for
single layer graphene. Noting that bi-layer BN has comparable proton conductivity with S-Gr,
it follows that even in bilayer BN, electron clouds allow space for protons travelling through.
Fig 3.9 I-V behavior of Nafion membrane over a 2μm diameter hole (blue curve). The conductance is
two orders of magnitude greater than for single layer BN (red curve).
`
70
The proton conductance of a Nafion membrane on a bare 2µm hole was measured and the
result is presented in Fig 3.9, showing that compared with Nafion, S-BN is two orders of
magnitude less conductive and that 2D materials proton conductivity is not comparable with
a commercialized Nafion membrane.
3.5 Conclusion
In conclusion, the existence of a new, unique property has been added to the range of
unusual physical properties of two dimensional materials. It was shown that single layer
graphene and boron nitride is permeable for protons with low energy. Monolayer boron
nitride was found to be the most conductive one due to its localized electronic structure.
Graphene is less conductive than BN due to its delocalized π electrons, while MoS2 was not
found to conduct as a result of its three atomic layers structure.
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71
Pt catalyzed proton transport through
2D materials
For the purpose of improving the performance of 2D materials proton conductance, in this
chapter we investigate the effect of a Pt catalyst on the proton transport conductivity of 2D
materials. A Pt evaporation technique and sample fabrication procedures are reviewed,
followed by presenting electrical measurements of proton transport through Pt catalyzed 2D
membranes. We also demonstrate proton transport introduced hydrogen gas flow through
2D materials.
4.1 Introduction
Proton transport through various 2D materials, namely single layer BN and graphene, has
been observed in a solid Nafion system. It not only reveals a new, unique property of 2D
materials, but also opens up the possibility of 2D materials being used in the field of proton
selective membrane. However, 2D materials proton conductivity is orders of magnitudes
lower than commercialized Nafion, making them less attractive.
On the other hand, platinum is widely used as catalyst in chemical reactions, especially in
hydrogen oxidization in fuel cells. Platinum nanoparticles with porous carbon support (Pt/C)
electrodes are widely applied in Nafion based proton exchange membrane fuel cells (PEMFC)
Chapter Four
`
72
[117]. Pt/C electrode could not only enlarge surface areas of electrode, in the sense that Pt
could adsorb hydrogen on its surface [118], and nanoparticles have larger surface area, but
also catalyze both hydrogen oxidization and oxygen reduction.
Pt catalyst supported by carbon nanotubes [119, 120] as well as single wall carbon
nanohorns [121] for fuel cell applications were investigated and have displayed reasonable
performance. The high proton conductive performance of a fuel cell system with a Pt
catalyst layer sandwiched between two Nafion layers has been reported as well [122, 123].
Since Pt works as a catalyst in Nafion based proton transport fuel cells, the effect of Pt on
proton transport through 2D materials are investigated in this chapter.
4.2 Device fabrication
Metal evaporation and lift-off
Electron beam evaporation techniques were used for depositing metal films with controlled
thicknesses on a target surface, here either 2D materials flakes or the silicon oxide surface.
As the name indicates, a focused electron beam was used to evaporate target metal. The
electron beam is emitted from a hot e-gun filament via an applied high voltage, followed by
magnetic deflection and acceleration to be focused on the target metal. The metal vaporizes
as a result of the energy delivered by the electron beam. The evaporation procedure was
performed in high vacuum so that the evaporated metal particles can reach the target
substrate directly with minimized collisions with background gases (Fig 4.1a).
In order to evaporate platinum, chromium and gold by electron beam evaporation, the
Moorfield MiniLab e-gun evaporation system (Fig 4.1b) was used in this thesis. Typical
acceleration DC voltage was 9kV and the vacuum in the evaporation chamber was 10-8 mbar.
`
73
Fig 4.1 (a) Schematic view of e-beam evaporation system. (b) Moorfield e-beam deposition system.
Sample fabrication
Graphene and boron nitride flakes were mechanically exfoliated on 290nm and 70nm silicon
oxide coated silicon substrates, respectively. Subsequently, flakes were transferred onto a
2µm (in diameter) straight through hole on the SiNX/Si substrate with a wet transfer
technique.
2nm platinum was e-beam evaporated on suspended graphene or BN flakes, followed by
Nafion 117 solution drops applied on both sides. After an oven bake in a water vapor
environment at 130˚C for 20min, PdHX films are formed onto both sides of the Nafion (Fig
4.2a). Fig 4.2b illustrates Pt directly evaporated on Nafion, which will be referred as a bare
hole in this chapter.
a b
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74
Fig 4.2 (a) Profiler of Pt evaporated solid Nafion device. (b) Pt evaporated directly on Nafion, referred
as “bare hole:” in this chapter.
4.3 Results
Proton transport I-V characteristics
Measurements are basically the same as in Chapter 3. Briefly, measurements were
performed in a H2 (10% H2 and 90% Ar) and H2O mixed gas environment. Water vapor was
introduced to keep the Nafion film wet and conductive to protons while PdHX films worked
as a proton source due to adsorption of H atoms (from H2). Bias potential was applied
between two PdHX electrodes. All the measurements were performed at room temperature.
The proton conductive behavior of 2D materials with 2nm Pt film catalyst has a linear I-V
characteristic at low bias (Fig 4.3 insert). Interestingly, single layer BN with Pt has
comparable proton conductance to Nafion (Fig 4.3). It is worth noting that S-Gr and Bi-BN
has similar proton conductivity, which is consistent with the previous experiment result in
Chapter 3.
a b
`
75
1
10
Conducta
nce (
nS
)
Nafion
S-BN S-Gr Bi-BN
20
-0.2 0.0 0.2-5
0
5 Nafion
S-BN
S-Gr
Bi-BN
Bias voltage (V)
Cu
rre
nt
(nA
)
0.2
Fig 4.3 Proton conductive histogram of single layer BN (S-BN, red), single layer graphene (S-Gr, blue)
and two layers BN (Bi-BN, green). Single layer BN has a comparable conductivity with Nafion. Insert:
linear I-V characteristic of 2D materials.
With 2nm Pt evaporated, the proton transport current of single layer BN is significantly
increased (Fig 4.4a) than S-BN without Pt by a factor of 5. The Pt modifies the proton
transport system by lowering the 2D materials’ proton transport barrier. Both S-BN and S-
graphene have less proton resistivity with Pt evaporated.
Pt exhibits a strong interaction with graphene [124, 125, 126]. As the pz orbital in graphene is
hybridized with the Pt d-orbital, the π electrons are localized to Pt, effectively lowering the
proton transport barrier through the hexagonal lattice of graphene. A similar interaction
between Pt and boron nitride has been reported as well [127].
`
76
-1 0 1
0
1
Cu
rre
nt
(A
)
Bias voltage (V)
Nafion
S-BN
Bi-BN
-0.2 -0.1 0.0 0.1 0.2
-5
0
5
Cu
rre
nt
(nA
)
Bias voltage (V)
no Pt
with Pt
S-BN on 2m hole
Fig 4.4 (a) I-V characteristic of single layer BN on a 2µm hole with Nafion/Pd contacts on both sides.
With 2nm Pt evaporated (red), the proton transport current is greatly increased compared to S-BN
without Pt (black). (b) I-V characteristics of 2nm Pt evaporated on a 2µm hole covered with Nafion
membrane (black), single layer BN (red), and two layers BN (blue). With the bias voltage further
being increased to ±2V, the I-V curve shows a nonlinear behavior while the current of S-BN and
Nafion is comparable.
With the applied voltage further increased (Fig 4.4b), the Nafion I-V curve presents nonlinear
behavior even without graphene (Fig 4.4b, black), a behavior which is different with the
linear characteristics described in Chapter 3. At higher voltage, the proton current of a
Nafion bare hole, single- and two layers BN, remains in the same orders of magnitude,
indicating that conductivity of the system is limited by Nafion.
It is important that the evaporated Pt film is discontinuous as a continuous film has trapped
H2 bubbles between flakes and Pt, preventing further transport (Fig 4.7).
a b
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77
Fig 4.5 (a) Inflated pristine single layer graphene across over a micro-well, (c) its AFM image, and (e)
its curvature reduction as a function of time. (b) Inflated 2nm Pt deposited single layer graphene
across over a micro-well and (d) its AFM image. Scale bar: 500nm.
a b
c d
e
`
78
In order to exclude the possibility that defects were introduced into 2D materials during Pt
evaporation processes, a pressured blister test [128] with atomic force microscope (AFM) is
used to measure the leak rate of Ar gas transport through graphene with 2nm Pt catalyst
film.
2µm in diameter, 200nm in depth wells were etched into 290nm SiO2/Si substrate with a
combination of photolithography and RIE dry etching. The etching rate of SiO2 is 1nm/s.
Suspended single layer graphene were fabricated by mechanical exfoliation and were wet
transferred onto predesigned micro-wells. Suspended graphene flakes are impermeable to
standard gases [22] and were clamped on the SiO2 substrate with surface forces [129].
However, gas molecules were able to enter and exit through micro-cavity in the substrate by
slow diffusion.
On the other hand, it has been reported that gas leaking rate is significantly increased
through defected single layer graphene than through pristine single layer graphene [28, 130].
With single layer graphene covered micro-well filled with pressed Ar, gas diffused through
defected single layer graphene within a minute, while for pristine graphene, Ar blister were
sustained for more than 1 day [128].
Here, suspended graphene covered micro-wells were filled with Ar by placing the sample in
a chamber pressurized with 2bar Ar above atmosphere pressure for 4 days. After being
removed from the pressed chamber, due to pressure difference in and out of the wells, the
graphene bulged up , which curvature could be monitored with AFM.
After being removed from the Ar pressed chamber, mechanical peeled pristine single layer
graphene was inflated to be a balloon with maximum height 120nm (Fig 4.5a and c). After
evaporated with 2nm Pt film, the sample was put back into the Ar pressurized chamber for 4
days. Inflated graphene was observed once more on Pt evaporated samples (Fig 4.5 b and d).
Comparable Ar leaking rate (slope in Fig 4.6) indicates that Pt evaporation did not introduce
extra defects into pristine graphene.
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79
Fig 4.6 Maximum deflection of single layer graphene blister with 2nm Pt (blue, pink and green) and
pristine single layer graphene blister (black and red). Different color represents different sample.
Noting that the maximum height of the balloon for Pt evaporated sample is lower than for
pristine graphene, we attribute this as that Pt particles block the micro-cavities for gas
diffusion on SiO2.
4.4 Proton transport introduced H2 flow
Proton transport through 2D materials has been well investigated above, and with Pt
working as a catalyst, the proton conductivity of single layer BN is comparable with, if not
better than, Nafion. All experiments described depend on the reaction:
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80
where two H atoms combine to form a H2 molecule.
With a continuous metal film covering the 2D flakes, after applying high bias potential (up to
2V), bubbles could be observed clearly under an optical microscope, indicating H2 gases
formation (Fig 4.7).
Fig 4.7 Single layer graphene covered with continuous Pt/Au film (5nm Pt + 40nm Au). (a) Before
proton transport measurements. (b) After measurements. H2 bubbles were produced between
graphene and metal film. Scale bar: 10µm.
As the proton transport current is stable, the H2 flow should also be continuous and possible
to be detected with a leak detector.
Sample preparation and measurement system
To detected H2 molecules, the 2D membranes were contacted with Nafion/Pd on one side
while a chromium (Cr) /gold (Au) contact provided the contact on the other side. A Cr/Au
contact was fabricated as shown in Fig 4.8. After photolithography (Fig 4.8a), Cr/Au is
deposited over the entire substrate by e-beam evaporation, including the area masked by
photoresist and areas where exposed photoresist is removed during development. In order
to achieve the metal contact only in the developed area, the remaining metal deposited on
a b
S-Gr
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81
top of the photoresist mask has to be removed by dissolving the underlying photoresist (Fig
4.8b).
Fig 4.8 Fabrication of Cr/Au contact. (a) Substrate is masked with S1813. Contact area is opened by
photolithography and development. Scale bar: 300μm. (b) Cr/Au contact evaporated on developed
area. Scale bar: 300μm.
Fig 4.9 H2 flow sample diagram and measurement system. (a) Single layer graphene is over a 50µm
in-diameter through hole, separating chamber A filled with H2/H2O and chamber B connected to H2
leak detector. (b) Measurement system image.
a b
Chamber A Chamber B
a b
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82
0.0 0.1 0.2 0.3
0
2
4
6
8
H2 flo
w r
ate
(10
-5b
ar
cm
3/s
)
Current (mA)
s1
s2
s3
theory
0 500 1000 1500 2000
6
8
10
12
H2 flo
w r
ate
(10
-5b
ar
cm
3/s
)
Time (s)
A typical recipe for Cr/Au contact is 3nm Cr and 40nm Au where Cr is used for better
adhesion on the SiNX/Si substrate. After evaporation, the sample is left in acetone until most
of the metal in masked area has been removed. Blowing with a pipette can be used to
accelerate this process. Subsequently, the sample is rinsed in IPA for 5 minutes before drying
in filtered N2.
Single layer graphene was transferred over a 50µm (in diameter) hole through the SiNX/Si
substrate, and 2nm Pt was evaporated on top (Fig 4.9a). The measurement was done in a
helium tight system (He leaking rate less than 1 x 10-8 bar·cm3/s, Fig 4.9b).
Fig 4.10 H2 flow rate through graphene flakes with Pt catalyst. (a) H2 flow rate with various bias
applied: a potential bias from 0V to 20V was applied. H2 flow rate could be controlled with voltage
applied between Au and PdHX electrodes. (b) Flow rate as a function of current across the graphene.
Red line: theoretical calculation. S1 to S3 represents different samples.
a b
0V
3V
10V
15V
20V
15V
10V
3V
0V
`
83
After the whole system is pumped down, chamber A (Fig 4.9a) is filled with a H2 + H2O gas
mixture up to 600mbar pressure while chamber B remains under vacuum (H2 leak rate < 1 x
10-6 bar·cm3/s).
The bias voltage was applied between the PdHX and Cr/Au electrodes. Both the current
across the two electrodes and the H2 leak rate were recorded.
With bias voltage applied from 0V to 20V, the H2 flow rate is controlled (Fig 4.10a) and
proportional to proton transport current (Fig 4.10b).
The Red line in Fig 4.10b is the theoretical values calculated according to the following
equation,
where is the H2 leak rate, i is the current through Pd and Cr/Au electrodes, NA is the
Avogadro constant. T is the temperature. Factor 2 arises because the two H atoms in one H2
molecule. Here we assume that all the current measured was from proton transport:
Experimental results match well with the theory calculation in Fig 4.10b, indicating that the
current measured is mainly from proton transport.
As a reference, 2nm gold was evaporated on graphene. The corresponding H2 leak rate was
below the noise level.
4.5 Conclusion
In summary, with a discontinuous Pt catalyst layer evaporated onto the 2D flakes, the
proton conductivity is greatly increased. Single layer BN is unique in the sense that it has
comparable proton conductivity with Nafion, while two layers BN and single layer graphene
`
84
conduct protons as well, which is consistent with results from Chapter 3. Further
investigation is limited by the Nafion conductivity. Proton transport introduced H2 gas flow
was measured as well and the flow rate could be controlled with a potential bias across the
2D membranes.
`
85
Proton transport through 2D materials
In liquids
In this chapter we review fabrication procedures of 2D materials devices that are suitable for
proton transport measurements in liquids. We start measuring proton transport through 2D
materials’ I-V characteristics at aqueous/aqueous interface, with 2D membranes separating
two aqueous phases at the interface. We also applied organic/aqueous interface, including
1,2 – dichloroethane (DCE)/aqueous immiscible interface and acetonitrile/aqueous miscible
interface, with graphene separation at the interface.
5.1 Introduction
The liquid/liquid (L/L) interface method of investigating charge transfer, including transport
of ions and electrons between species in solution, is well investigated since many important
phenomena in chemistry and biology involve processes that occur at L/L interfaces [131].
Among these processes, the proton is important as many reactions in organic chemistry
have acidic catalysts. Similarly, enzymes and proteins also utilize proton exchange processes
at various interfaces [132]. So proton transfer through the L/L interface has been reported
extensively [133, 134, 135].
Chapter Five
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86
Since proton transport through 2D materials has been discussed in previous chapters, the
investigation of proton transport through L/L interface with 2D membranes separation could
be interesting not only as it could confirm proton transport through 2D materials, but also
give information about the liquid/2D materials interface.
5.2 Liquid/liquid interface method
In contrast to conventional electrochemical methods, where a solid electrode is immersed
into an electrolyte solution and charge transfer across the solid/liquid interface is studied,
liquid/liquid electrochemistry employs another liquid phase to replace the solid electrode.
Therefore the charge transfer occurs across the liquid/liquid interface. The two liquids must
be immiscible, or at least have restricted miscibility, or be separated physically, and suitable
electrolyte must also exist for both liquids in order to support the current flow in both
phases. This results in a system with a polarizable Interface between Two Immiscible
Electrolyte Solutions (ITIES) [136, 137, 138].
5.2.1 Immiscible interface
Typically, a liquid/liquid system contains an aqueous phase and an organic solution phase.
The most commonly used system is the water/1,2-dichloroethane interface system. It is
widely used in investigating electron transfer and ion transfer (including proton) across the
ITIES [135, 139, 140].
To carry out electrochemistry in a L/L system, electrolytes are needed in both liquid phases
to facilitate charge transport. LiCl or MgSO4 is commonly used as the aqueous phase
electrolyte due to their high conductivity in aqueous solution and their high hydrophilic
nature. However, in the organic phase, since the organic solution is a non-polar phase, an
organic electrolyte with a high dissociation constant is needed to provide conduction in the
organic phase. Generally, bis(triphenylphosporanylidene) ammonium (BTPPA+) is applied as
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87
-0.5 0.0
-5
0
5
10
Cu
rre
nt (n
A)
Bias voltage (V)
without H+
5mM H+
the cation and tetraphenylborate (TPB-) or tetrakis(4-chlorophenyl) borate (TPBCl4-) as the
anion in the organic electrolyte.
An electric double layer can be established near the L/L interface, similar to the solid/liquid
interface case. The layer is usually a few nanometers thick, and contains ions (cation or
anion) in two phases depending on the polarity of the external electric field. Each ion has a
certain transfer potential for transfer across the L/L interface, which dictates the choice of
electrolyte. For a highly hydrophilic electrolyte in the aqueous phase and a highly lipophilic
electrolytes in the organic phase, respectively, there is a potential region without any
electrolyte ion transport; hence no net current flows across the interface. In this region
Fig 5.1 I-V characteristic of protons transporting through the 1,2-dichloroethane/water interface.
5mM H+ was introduced into the organic phase (Red curve). Blue curve: blank potential window.
H+ current
`
88
defined as a potential window, one can therefore analyze other processes, for example ion
transfer. As indicated above, different electrolytes yield different potential windows [141].
Transport of an ion, whose transport potential lies within the potential window, results in
the detection of current corresponding to the ion transferring from one phase to the other.
A comparison between Current-Voltage (CV) measurements of a blank potential window
(only organic/aqueous solution with their electrolytes) and proton (H+) transfer across the
interface is presented in Fig 5.1.
5.2.2 Bipolar cell
A bipolar electrode system employs a middle electrode separating two redox couples
located in separate phases. The electrical connection, or electron transfer, is controlled by a
potential bias across the two compartments of a cell. As two phases are physically not in
direct contact, mass transport is prevented while electron transport dominates the current
Fig 5.2 (a) Bipolar cell electron change system and (b) bipolar cell equilibrium single cell system. The
dashed square in (a) can be considered as a single electrode in (b).
a b Central electrode
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89
response (Fig 5.2a) [142]. In this system, the potential drops almost entirely at the two
interfaces between solution and bipolar electrode when two ideal non-polarized electrodes
(the potential of which will not change from its equilibrium potential with a large current
density. The reason for this behavior is that the electrode reaction is extremely fast) are
used. To simplify the electron transfer at the interface, the concentration of redox couple in
one phase is in excess compared to the other phase so that charge transfer can be treated as
being limited by the low concentration phase only while the excess phase works as a solid
electrode (Fig 5.2b).
5.3 Sample fabrication and measurements
Fabrication of proton transport devices in liquids has to take liquid leakage into
consideration. The measurement system is a combination of immiscible L/L interface and
bipolar cell system.
PDMS mask fabrication
To avoid liquid leakage between the 2D flakes and the Si/SiNX substrate,
polydimethylsiloxane (PDMS) mask was designed and used for appropriate sealing.
PDMS is a soft and transparent solid organic polymer which is widely used in chemical and
biological micro-fluidic cells for adequate sealing [143, 144] due to its hydrophobic nature
and suitable mechanical properties [145]. PDMS can stick onto polished SiNx surface to avoid
liquids leakage while its transparency makes it possible to be transferred onto graphene
using optical alignment.
`
90
Fig 5.3 Procedures of PDMS mask fabrication. (a) Si plasma etching with a Su-8 photoresist mask. (b)
After etching, a 50µm tall pillar is etched on the Si substrate. (c) PDMS is spun onto the silicon
substrate, with the PDMS layer thickness less than the pillar height to generate a 30µm hole in the
PDMS. (d) Typical PDMS mask geometry.
To make the PDMS mask, a plain silicon wafer was spin coated with Su-8 2025 (negative
tone photoresist from MicroChem) at 2000rpm (resist thickness about 25μm) and prebaked
at 95⁰C for 5 minutes. After photolithography with a laser writer, the resist was softbaked at
95⁰C for 5 minutes and developed in Microposit EC solvent for 5 minutes, followed by RIE
silicon Bosch etching recipe for 20 minutes (2.7μm per minutes, Fig 5.3a), to have a 30μm in
a b
c d
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91
diameter and 50μm tall pillar on the silicon wafer which will later be referred to as the
silicon master (Fig 5.3b).
Sylgard 184 PDMS (viscous liquid) was mixed with its curing agent (liquid) at a ratio of 10:1.
Subsequently, the mixture was placed into a desiccator for 20 minutes to free air bubbles
trapped in PDMS during mixing. Next, PDMS was spin coated onto silicon master at a spin
speed of 3000 rpm, at which speed the thickness of the PDMS layer is thinner than silicon
pillar (Fig 5.3c). A nitrogen blow technique could be used at this step [146] to blow the
PDMS prepolymer away from the Si pillars. After cure baking at 150⁰C for 15 minutes, the
PDMS was peeled off from the silicon master to produce a 10μm-thick PDMS membrane
with a 30μm diameter through hole (which is the size of the silicon pillar, Fig 5.3d).
Sample fabrication and measurements
Graphene and BN flakes were prepared by standard mechanical exfoliation on 290nm and
Fig 5.4 (a) Optical transfer of the PDMS mask on to 2D flakes. (b) Transferred sample.
70nm SiO2/Si wafer, respectively, while MoS2 flakes were prepared on a PMGI/PMMA
bilayer resist substrate. We use the “wet transfer” technique to transfer graphene and BN
a b
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92
flakes and “dry transfer” to transfer MoS2 flakes onto straight through holes. The PDMS
mask was optically aligned and mechanically transferred on top of the flakes (Fig 5.4).
Fig 5.5 (a) Measurement fluidic cell and (b) schematic diagram.
Measurements were performed using an in-house made fluidic cell (Fig 5.5a). The potential
bias was applied by a Kelthley 2636 SourceMeter between the two Ag/AgCl electrodes in the
two phases. Samples were amount into the cell to separate two liquid phases (Fig 5.5b). All
the measurements were done with-in a Faraday cage.
5.4 Results
In the liquid-liquid experiments, the results are consistent with previous experiments, that
BN is the most conductive for protons among the 2D materials investigated in this thesis,
while single layer graphene and two layers BN are conductive to protons as well (Fig 5.6).
Interestingly, the different materials behave differently in liquid environment. So in this
Chapter, graphene, BN and MoS2 will be presented separately.
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93
0.01
0.1
1
Conducta
nce (
nS
)
S-BN S-Gr Bi-BN Bi-Gr S-Mo Tr-Gr
0.02
Fig 5.6 Proton transport in liquids. Histogram of the proton conductivity of various 2D materials in
0.1M HCl aqueous solution. Single layer BN (S-BN, red) is the best proton conductor while single layer
graphene (S-Gr, green) and two layers BN (Bi-BN, blue) have a comparable conductivity. Current
responses of two layers graphene (Bi-Gr, cyan), single layer MoS2 (S-Mo, pink) and three layers
graphene (Tr-Gr, yellow) stay within the noise level (shadowed area in the figure). All the 2D flakes
are over 2µm in diameter straight through hole in silicon nitride substrate.
Graphene
The lateral size of typical graphene flakes is above 100µm. To better investigate graphene’s
behavior in liquids, the sample geometry is modified with graphene flakes over a 10µm (in
diameter) straight through hole, as shown in Fig 5.7a.
0.1M HCl 0.1M HCl
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94
Fig 5.7 Sample geometry and images. (a) Structure and geometry of a graphene sample. (b) Optical
microscope image of a graphene device at 20x magnification. Insert: 10x magnification (top left) and
5x magnification (top right). (c) SEM imaging of suspended single layer graphene. (d) Stressed
suspended single layer graphene with a nano-hole on the edge. The spikes in the middle are
contamination spots burnt by the electron beam. Insert: Zoom in of the graphene hole on the edge
(dashed circle area).
Aqueous/aqueous interface
To determine the height of the activation barrier for proton transport through graphene,
two aqueous 0.1M HCl (H+ as proton source) symmetric environment separated by graphene
were initially described.
a b
c d
`
95
A symmetric system is important since the open circuit potential difference depends on the
proton concentration gradient, which is 59 mV per unit pH according to the Nernst equation
at room temperature:
Where E0 is the standard cell potential at room temperature, z is the number of moles of
charges transferred, aRed and aOx is the chemical activity of the reduced and oxidized species,
respectively.
A bias voltage was applied between the two compartments in the cell and the current was
measured. Typically the current was left to stabilize over a few minutes (Fig 5.8 insert) in
order to establish a steady state across the diffusion layer, which refers to the proton
concentration gradient between the graphene surface and the bulk solution.
Figure 5.8 shows the current response as a function of bias voltage for graphene and its few
layer counterparts. As the experiment was performed in a symmetric system, only a positive
potential window of 0V to 2V is presented. With single layer graphene, no net current more
than the leakage level was observed until 0.5V was applied, where a pronounced increase in
current starts to occur (Fig 5.8 blue curve). As the graphene blocks direct mass transport
between the two compartment, no ion transfer and hence no current should be observed.
Therefore the current which significantly rises at 1.2V, is attributed to proton transport
through graphene.
In order to exclude the possibility of nanometer sized holes in single layer graphene allowing
ion transport, flakes were characterized using Raman spectroscopy and SEM. No holes or D
peak were observed, indicating that graphene remains continuous and rigid after the
measurements.
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96
0 1 2
0.0
0.5
Curr
ent
(nA
)
Bias voltage (V)
monolayer
bilayer
trilayer
graphite
Fig 5.8 Proton transport through various thicknesses graphene at the aqueous/aqueous interface. A
typical potential window of 0V to 2V is demonstrated, with the voltage stepped in increments of 0.1
V. The steady current (Insert) were plotted with its bias voltage. Proton conductive current was
measured within 0.1M HCl with the separation of single layer graphene (blue), unique compared
against the response seen for bilayer (green), trilayer (red), and thick graphite (black). Insert: Steady-
state current measured as a function of applied bias voltage.
To further examine the transport through graphene, we calculated the diffusion current
limited by the 10μm hole. This is given by [147]:
0 50 100 150 200
0.0
0.5
1.0
1.5
Cu
rre
nt (n
A)
Time (s)
0.7V
1V
1.2V
1.5V
1.8V
(1)
`
97
where n is the charge number of the transferring species (1 in the case of proton), F is the
Faraday constant, D and C0 are the diffusion coefficient (9.31 x 10-9 m2/s) and the bulk
concentration of H+ in water, respectively, and r is the radius of the membrane which here is
the size of the graphene exposed to solution.
According to Equation (1), the diffusion-limited current should be 100nA, more than two
orders of magnitude larger than the limiting current measured from the experiment. This
indicates that the current is not limited by the diffusive flux of protons, rather by graphene.
We suggest that the current reduction is due to graphene being covered with adsorbed
organic contamination making the effective area much smaller than the area exposed in the
solution. These adsorbates are sufficient to block the proton transfer. Poor contact of
associated protons (H3O+) with the graphene surface is another problem since the proton is
separated by its water shell, about 0.6nm away from graphene surface [148].
The result in Fig 5.8 shows that a single layer membrane was unique since bilayer and
thicker samples displayed a current response which remained within the leakage current
level over the same potential range. Noting that as all thicknesses graphite are good electron
conductors, the result indicates there is no distinguishable electron transport current
through our samples in the potential window we applied, and monolayer graphene is the
only proton conductor among its few layer counterparts.
1,2-Dichloroethane (DCE)/aqueous interface
To investigate the proton transport in more details, the 1,2-Dichloroethane(DCE, Fig
5.9a)/aqueous interface was used. This immiscible solvent system has been very well
investigated for proton transport [135]. Here, 1-[Bis(trifluoromethanesulfonyl)methyl]-
2,3,4,5,6-pentafluorobenzene (H+A- , Tokyo Chemical Institute, 95%, 100mg, Fig 5.9b) was
used as the organic phase proton source since it has a high proton dissociation constant (pKa
about -14) in DCE [149]. 0.1mM LiCl and 0.1mM BTPPA+TPBCl4- are the electrolytes in
aqueous and organic phase, respectively.
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98
0 1
0
1
2
Curr
ent
(nA
)
Bias voltage (V)
5mM H+
2mM H+
1mM H+
DCE only
Fig 5.9 Chemical structure of (a) 1,2-Dichloroethane and
(b) 1-[Bis(trifluoromethanesulfonyl)methyl]-2,3,4,5,6-pentafluorobenzene.
Fig 5.10 Current response as a function of applied bias with graphene separating DCE/aqueous
interface for various acid concentration. Insert: sub-linear dependence of saturating current and acid
concentration.
0 2 4
0
1
Sa
tura
tio
n C
urr
en
t (n
A)
Acid concentration (mM)
`
99
0 1
0
1
2
Curr
ent
(nA
)
Bias voltage (V)
5 layers
bilayer
single layer
Fig 5.10 shows the limiting current as a function of proton concentration in DCE/water with
graphene separating the two liquids at the interface. Protons transport from DCE phase into
aqueous phase. As the response current with negative bias voltage applied was within the
noise level, only I-V curves at positive bias voltage are presented in Fig 5.10. Graphene is
sensitive to H+A- concentration down to 1mM.
By applying Eq (1), with the 1 mM proton concentration, the diffusion limited current is 1nA,
comparable with the saturated current experimentally measured (Fig 5.10). The insert of Fig
5.10 shows the limiting currents as a function of acid concentrations. The linear dependence
implies that in the low H+ concentration range, the current is limited by the proton diffusion.
At the DCE/aqueous interface, bilayer or thicker graphene flakes could not transport protons
across (Fig 5.11), which is consistent with our previous experiment.
Fig 5.11 Proton transport current response with various thicknesses of graphene flakes in the
DCE/water system.
`
100
0 1 2
0.0
0.5
Bias voltage (V)
Cu
rre
nt
(nA
)
Graphene
MoS2
Aqueous/aqueous
0 1 2
0
1
2
Cu
rre
nt
(nA
)
Bias voltage (V)
Graphene
MoS2
DCE/aqueous
Molybdenum disulfide and Boron nitride
Single layer molybdenum disulfide (MoS2) and single layer boron nitride (BN) were studied in
liquids as well, as they are also mechanically stable to be measured in liquids. However, with
MoS2, the current response deviated from the behavior seen with single layer graphene in
the same potential window, in both aqueous/aqueous (Fig 5.12a) and DCE/aqueous (Fig
5.12b) cases.
Fig 5.12 Proton transport current responses of single layer MoS2 and single layer graphene. (a) 0.1M
HCl aqueous/aqueous interface. (b) DCE/aqueous interface with 5mM H+A- in DCE phase.
Boron nitride as a proton conductor was well investigated in the previous chapters, but its
proton conducting behavior is investigate in an aqueous/aqueous interface system with BN
separation as well. Limited by the lateral size of single layer BN flakes (typically 10µm,
occasionally up to 30µm), the sample geometry is modified so that the through hole size is
2µm in diameter while the PDMS mask hole size is 20µm.
a b
`
101
0 1
0
4
Curr
ent
(nA
)
Bias voltage (V)
S-BN
Bi-BN
Tr-BN
0 1 2
0.0
0.2
0.4 Bi-BN
S-Gr
Cu
rre
nt (n
A)
Potential(V)
Boron nitride proton conductivity measurements were carried out in two 0.1M HCl aqueous
solutions separated by a BN membrane. The current response of single layer BN and its few
layers counterparts as a function of applied bias voltage is presented in Figure 5.13. Thin BN
membranes demonstrate a significant proton current, with a single layer BN proton
conductance of 5nS, one order of magnitude better than its double layer, while flakes
thicker than three layers have current response within the noise level.
Fig.5.13 I-V characteristics of single layer boron nitride (black plots) and its counterparts bilayer
BN(red plots) and three layers BN (green plots) in 0.1M HCl. Insert: zoom in of bilayer and trilayer BN.
Noting that BN exhibits a linear proton current vs voltage dependence, which follows Ohm’s
law, the result indicates that the transport barrier for protons penetrating through BN in
`
102
liquids is kept at the thermal turbulence level. Since hexagonal boron nitride is AA stacking,
bilayer BN shows same current trend.
5.5 Conclusion
In conclusion, in L/L experiments, the proton conductance results are consistent with the
solid Nafion experiments described in previous chapters in that single layer BN is the most
conductive material among the 2D materials investigated. Single layer graphene is
conductive to protons in liquids but presents different I-V behaviors in aqueous/aqueous
and DCE/aqueous systems.
We attribute difference of proton transport behavior in liquids than in solid Nafion to
medium effects on transport barrier. Though, liquid behavior of graphene is not completely
clear at this stage. Further investigation is needed.
5.6 Supplementary experiments
0 1 2
0
1
2
Curr
en
t (n
A)
Bias voltage (V)
2.4 0 1 2
0.0
0.5
1.0
Curr
en
t (n
A)
Bias voltage (V)
2.4 0 1 2
0.0
0.5
Curr
en
t (n
A)
Bias voltage (V)
2.4
a b c
`
103
0 1
0
1
Curr
ent
(nA
)
Bias voltage (V)
10mM
2mM
1mM
Fig 5.14 Water electrolysis at various thicknesses graphene membranes. (a) Single layer graphene. (b)
Double layers graphene. (c) Three layers graphene. Potential of water electrolysis are at 2.4V.
In the aqueous/aqueous interface system (0.1M HCl/Graphene/0.1M HCl system with Pt
electrodes), graphene behaves as a centre electrode, like in a bipolar cell system in which
water electrolysis would occur at a potential of 2.4V, and chlorine oxidation 2.6V. This can
be compared with the single cell potential for water electrolysis of 1.23V and chlorine
oxidation of 1.3V, respectively.
Fig 5.14 shows that water electrolysis occurs for various graphene thicknesses at around
2.4V, indicating that our graphene works quite well as a centre electrode in the double cell
system. Potential window is ±2.4V.
.
0 5 100.4
0.8
Satu
ration c
urr
ent (n
A)
HCl concentration (mM)
`
104
0 1
0.0
1.5
3.0
Curr
ent
(nA
)
Bias voltage (V)
5mM
2mM
1mM
Fig 5.15 Current response at various acid concentrations as a function of applied bias for the
DCE/aqueous interface with protons on the aqueous side. Na+ A- was as electrolyte in DCE since A-
behaves as proton accepter.
Transport of protons from the aqueous phase to the organic phase (DCE, with Na+A- as the
organic phase electrolyte) phase was measured and is shown in Fig 5.15. Since protons are
preferentially solvated by the water phase, the transport potential (around 1V) is higher
than the transport potential of protons from DCE to water (see Fig 5.8, about 0.5V).
Fig 5.16 Current response at various acid (H+A-) concentrations as a function of applied bias at the
acetonitrile/aqueous interface with graphene separation. H+ was in the organic phase. Insert: limiting
current as a function of acid concentration.
2 4
0
1
2
Sa
tura
tio
n C
urr
en
t (n
A)
Acid concentration (mM)
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105
Since DCE and water separated into two mutually-saturated phases, there is an interface
even without introducing graphene. To avoid the possibility that recorded data are due to
proton transport through the DCE/water interface with broken graphene membranes, a
miscible interface was investigated as well. Fig 5.16 shows the results obtained from the
acetonitrile/aqueous miscible interface with graphene separation. The same chemicals were
used as in the DCE/aqueous experiments. H+A- was in acetonitrile phase. Proton transport
current with a linear proton concentration dependence were observed for this system as
well. It is worth noting that without introducing graphene at the acetonitrile/aqueous
interface, no regular I-V characteristics could be measured since the two phases are miscible
with each other.
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106
Summary
6.1 Conclusion
This thesis presents a study of proton transport through two dimensional materials: boron
nitride, graphene and molybdenum disulfide. Proton conductivity is investigated in both a
Nafion based solid system and a liquid/liquid interface based system. By electrically
measuring the I-V characteristics, single layer boron nitride is proved to be the best proton
conductor among the two dimensional membranes studied in this thesis as a consequence
of its localized electron density. Single layer graphene and bilayer boron nitride are
conductive to protons as well and have comparable proton conductivity to each other. Other
thin membranes including single layer MoS2 and thicker graphene and BN flakes are not
permeable to protons.
The proton transport efficiency is greatly increased with a Pt catalyst evaporated on the two
dimensional membranes. Single layer boron nitride reaches a proton conductance as good
as, if not better than, Nafion, while the performance of other two dimensional membranes is
improved as well. Proton transport introduced hydrogen flow through graphene was
Chapter Six
`
107
detected and the flow rate could be controlled by the voltage applied, yet another evidence
of proton transport.
Graphene is specially studied at the liquid/liquid interface since it behaves differently
compared to the Nafion system as a result from a different proton diffusion mechanism. An
aqueous/aqueous symmetric interface and an 1,2-dichloroethane/aqueous immiscible
interface were studied, showing that graphene is sensitive to proton concentration down to
1mM. However, further investigations are necessary in the liquid/liquid interface
experiment.
The study in this thesis reveals a new, unique property of two dimensional materials, namely
that 2D materials including single layer BN and single layer graphene are permeable to
protons. This paves the way for them to be incorporated in fields where a proton semi-
permeable membrane is required.
6.2 Outlook
Electron transport through graphene in liquids
As the liquid/liquid interface measurement system for investigating charge transfer through
graphene has been set up, one extension is studying electron transfer between two redox
species in solutions separated by graphene. The electron transfer rate with various
thicknesses of graphene could be investigated as well.
Graphite has been used as a battery electrode material for ages – weather in normal AA
batteries or Li-ion batteries. With a high conductivity comparable with metal, graphite is
more stable, both chemically and thermally, which makes it a more useful electrode than
metals in applications. Recently, graphite has been reported as an alternative electrode
material in electrochemical capacitors [150,151] and in various sensors in solar cells [152,
153]. Nanostructuring of electrodes with carbon nanotubes have been reported as well
[154,155].
`
108
1.2 1.4 1.6
0.4
0.6
0.8
Curr
ent
(nA
)
Voltage (V)
Graphene, as graphite’s single layer counterpart, attracted enormous interests because of its
unique transport properties. With its faster electron exchange rate compared to graphite,
graphene is considered as an electrode material as well, including graphene based
supercapacitors[156], and the use of graphene as a transparent electrodes in solar cells
[157]. Optimization of such applications needs a complete understanding about electron
transport properties of graphene. Planar electrochemical electron transfer kinetics of
graphene has been reported recently [158, 159] while vertical electron transport properties
were barely investigated.
With liquid measurement system established in Chapter 5, 0.5mM (NH4)2IrCl6 and 5mM
(NH4)2IrCl6 and 5mM (NH4)3IrCl6, both of which were buffed in 0.1M NaCl aqueous solution,
was separated into two phases with single layer graphene supported with 10μm in diameter
through hole Si/SiNX substrate. PDMS mask was applied to avoid liquids leaking (Fig 6.1a).
Fig 6.1 Electron transport through graphene. (a) Measurement geometry and mechanism. (b) IV
characteristic of electron transport between Ir3+ and Ir4+.
a b
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109
Fig 6.1b shows initial experiment result of electron transport (bias voltage scan rate:
10mV/s). At bias voltage 1.3V, a significant current rise occurred indicating electron
transport current is measurable. According to Eq. (1) in Chapter 5:
where Irn+ diffusion constant D=9 x 10-10 m2/s, diffusion limiting current is 0.8nA against
0.4nA measured experimentally. Noting that as in double cell system, Iridium
oxidization/reduction potential was doubled from 0.8V against standard hydrogen electrode
to 1.3V.
Further experiments, including graphene thickness and scan rate influences to electron
transport rate, will be performed to have more information of electron transport efficiency
in electrochemistry.
Proton permeable membrane
Another obvious extension is employing two dimensional materials as proton permeable
membranes. But limited by the 2D membrane size and mechanical stability, it is not yet
competitive in areas such as fuel cells and so on where a macro –size membrane is required.
Though, two-dimensional materials are competitive in proton exchange between protein,
where the proteins need to be separated by membranes of suitable size and thickness
without the membrane being chemically or physically damaged.
Hydrogen collection is another interesting area of future research. As demonstrated in
Chapter 4, hydrogen was collected from a 10% hydrogen and 90% argon gas mixture.
Weather we can harvest hydrogen from other gas mixtures, even air, with adequate
efficiency is a big challenge ahead.
`
110
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