vertically aligned graphene layer arrays from chromonic liquid crystal precursors

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Vertically Aligned Graphene Layer Arrays from Chromonic Liquid Crystal Precursors

Fei Guo , Amartya Mukhopadhyay , Brian W. Sheldon , and Robert H. Hurt *

Progress in graphene synthesis has led to increased interest in the assembly of graphene into superstructures, and to the fabri-cation of novel ordered materials from graphene or engineered graphenic molecular building blocks. [ 1–8 ] Graphene monolayers typically deposit fl at on substrates or associate face-to-face to form horizontal stacked papers or multilayer coatings. [ 8–11 ] The opposite structure, vertically aligned graphene layer arrays on substrates ( Figure 1 A ), are more diffi cult to fabricate, but are expected to show a range of unique properties and behaviors. Their high concentration of edge-sites at the top surface would allow functionalization at high density for superhydrophobic/philic coatings, high-redox-activity electrode surfaces, or chemi-cally patterned surfaces for cell adhesion and guidance. The vertical layer orientation would allow rapid intercalation and deintercalation of lithium in high-discharge-rate thin fi lm bat-teries, [ 12 ] and would provide high Z -directional thermal/elec-trical conductivity. If also ordered in a second dimension, such arrays would be anisotropic in the substrate plane and provide unidirectional in-plane heat spreading or optical polarization. If the heights of arrays can be limited to below 50 nm, they may fi nd application as transparent conductive fi lms, [ 10 , 13 ] or as graphene nanoribbons where the ribbon width is set by the array height. Here we use chromonic liquid crystal precursors to fabricate vertically aligned graphene layer arrays (VAGLAs) on substrates and also demonstrate a method to achieve full two-dimensional order, which we defi ne as further control of graphene layer orientational patterns within the substrate plane using local shear-forces. We also demonstrate one example of a unique property of these arrays – the ability to etch Z -directional nanopores by catalytic hydrogenation, in which cobalt nanoparticles tunnel vertically into and through the arrays as they track vertically receding edge-plane surfaces.

A promising route to the desired vertical graphene array structure is one based on polyaromatic precursors, [ 2 ] which can adopt edge-on orientation on substrates. [ 5 , 14 , 15 ] Many poly-aromatic compounds, however, do not retain supramolecular alignment upon carbonization, and also their solution or vapor deposition typically gives only short-range order in-plane. [ 2 ] Liquid crystal phases offer long-range order, but most discotic phases are high-temperature viscous liquids that are diffi cult

© 2011 WILEY-VCH Verlag Gmwileyonlinelibrary.com

F. Guo , Dr. A. Mukhopadhyay , Prof. B. W. Sheldon , Prof. R. H. Hurt School of Engineering & Institute for Molecular and Nanoscale Innovation (IMNI) Brown University Providence, Rhode Island 02912, USA E-mail: [email protected]

DOI: 10.1002/adma.201003158

to process and their supramolecular order can also be unstable during carbonization. [ 1 ] A promising solution to these chal-lenges are so-called “chromonic liquid crystals” (CLCs), which combine graphenic disk assembly with water solubility. CLCs are formed from water soluble organic dyes which form mas-sive π -stacks in aqueous solution that act as supramolecular rods. These structures then order into nematic liquid crystal phases through rod self-avoidance at high concentration [ 1 , 16–18 ] (Figure 1 ). The isotropic-nematic (I–N) phase transition occurs when the decrease in orientational entropy upon ordering is more than offset by the increase in packing entropy at high rod density in concentrated solutions. [ 19 , 20 ] Parallel orientation of the supramolecular rods at substrate surfaces produces a set of vertically aligned disks, which may be converted to graphene layers with retention of the vertical order. [ 1 , 17 ] There is a signifi -cant literature on the physics of chromonic liquid crystals, [ 21 , 22 ] but very little information on their conversion to carbon. [ 1 , 17 ] CLCs may become important precursors to engineered graph-enic superstructures if the conversion can be better understood through systematic study.

The present work fabricates VAGLAs from a set of fi ve mole-cular precursors (Figure 1B ), which are sulfonated polyaromatic dyes with a hydrophilic periphery and a hydrophobic core. For this set of precursors, we fi rst used controlled drying and dilu-tion under the polarizing microscope to fi nd and observe the I–N transition. Figure 1C shows the transition concentrations, which are sensitive to molecular structure – the larger polyaro-matic cores correlate with lower I–N transition concentrations implying improved hydrophobic stacking for the larger disks. At the transition, in a narrow concentration range ( Δ C < 1 wt%), is an apparent two-phase behavior in the form of dispersed liquid crystalline droplets (Figure 1D ). [ 23 ] The bulk nematic phase at high concentration shows Schlieren textures with two and four brush defects [ 24 ] between crossed polarizers (Figure 1E ). The Onsager hard rod theory [ 19 , 20 ] can be used to estimate rod length L at the I–N transition point through:

LD

= cDLC

DLC C

molecule

solution −1

(1)

where D is the diameter of the disk-like molecules, estimated from the molecular structure; ρ is the true material density, C is the concentration (w/w) of the CLC solution at the I–N tran-sition, and c is a dimensionless constant estimated to be 4 by Vroege et al. [ 19 ] Results for rod lengths in Figure 1C range from 14 to 80 nm corresponding to 40–240 disks per rod at 0.34 nm spacing.

Figure 2 shows example results of VAGLAs fabricated through the chromonic liquid crystal route. To obtain uniform array heights we needed to apply the precursor solutions at

bH & Co. KGaA, Weinheim Adv. Mater. 2011, 23, 508–513


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Figure 1 . Molecular assembly process leading to the formation of vertically aligned graphene layer arrays (VAGLAs). (A) Assembly principle involving hydrophobic stacking of amphiphilic disks, liquid crystal ordering and fl ow alignment, drying, covalent capture, and carbonization. (B) Molecular structures of the polyaromatic cores used in chromonic liquid crystals. (C) Characterization of the chromonic liquid crystal phases formed by sulfona-tion of 1–5. (D,E) Example optical microscopy images of chromonic liquid crystalline phases under crossed polars: (D) Liquid crystalline droplets seen in two-phase mixture occurring in narrow range ( + /−1%) around the I–N transition point (precursor 4), (E) bulk nematic phase for with Schlieren textures (precursor 5).

(A)

(1) (2)

(B)

ColorStock Solution Concentration

(w/w %)

I-N PhaseTransition(w/w %)

Molecular Core

Diameter (nm)

Onsager Rod

Length(nm)

1 Blue 5.7 ~7 1.3 74

2 Violet 2.8 ~7 1.4 80

3 Bordeaux 2.5 ~6 1.2 80

4 Yellow 30 ~18 1 22

(C)

HN

NH

OO

OO N NO

NN O

N NO

NO N

NN N N

(3) (4) (5)5 Clear 44 ~31 0.9 14

(D) (E)

N

uniform fi lm thickness and in the presence of shear or elonga-tional fl ow to align the supramolecular rods. In the Mayer-bar application technique, the shear and elongational fl ow in the gaps at the interface of the substrate and bar aligns the rod-like aggregates parallel to the bar rubbing direction, which in turn aligns the disk-like building blocks with their molecular planes perpendicular to both the substrate and rubbing direction. For a coating velocity of 2 cm s − 1 and a dimension of the negative curvature space between wires of 40 μ m, the shear rate through the gaps is of order 500 s − 1 . Under these conditions, the Rey-nolds number, Re = Dv l/: is estimated to be < ∼ 25, which is

© 2011 WILEY-VCH Verlag GmAdv. Mater. 2011, 23, 508–513

far below the onset of turbulence, thus ensuring a laminar fl ow that produces ordered (not chaotic) rod alignment patterns in the wake. The CLC fi lms were then dried into solid crystal fi lms at room temperature and carbonized by directly heating in nitrogen at 700 ° C for 30 min. “During the carbonization process, the orientation of the disk-like molecules on sub-strates becomes stabilized through edge-to-edge polymeriza-tion reactions, in which neighboring molecular disks crosslink and merge into vertical graphene layers (see Figure 1A , 2 ). Stearic hindrance in the close-packed molecular fi lm together with bending energy of graphene sheets prevents the same

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Figure 2 . The morphology, structure, and graphene orientational patterns in vertically aligned graphene layer arrays (VAGLAs). (A) HRTEM image showing the full fringe fi eld in Z-axis projection, indicating vertical graphene layer alignment. VAGLAs fabricated on quartz-nickel substrate. (B, C) Polarized light images of backlit arrays showing the effect of local shear on layer orientation. (B) An optically imbedded VAGLAs pattern on quartz (25 × 75 mm) with a background ordered in one direction by full Mayer-bar, and stripes ordered in the orthogonal direction by the microshear technique; (C) Polarization-active letters “IMNI” of VAGLAs written on quartz by the microshear technique. The arrow indicates the polarization direction and the circle indicates unpolarized light. All example VAGLAs shown here originate from precursor 2. (D) Theoretical relationship between the VAGLA height (carbon fi lm thickness) and the CLC starting concentration and Mayer-bar selection. The calculation is based on a 40% carbonization yield and ignores carbonization shrinkage, which is small.

Organic Film Carbonized VAGLAs

0 2 4 6 8 10

10

100

1000

VA

GL

As H

eig

ht

(nm

)CLC Starting Concentration (w/w %)

Calibrated Thickness

of Wet Film

6.86 μm

18.29 μm

45.72 μm

(A) (B)

(C) (D)

cross-linking reactions from occurring between disks stacked in the same rod. [ 25 ] After heating above 700 ° C, the products can be classifi ed as carbon thin fi lms, but we choose to use the term VAGLAs to emphasize their unique graphene assembly pattern and crystal structure, which are quite distinct from conventional carbon fi lms. Each of the fi ve chromonic precur-sors produced vertically aligned graphene layer arrays with full 2D order by this technique. We found, however, that when the starting concentration is below the I–N transition (precursors 1 – 3 ), that multiple Mayer-bar strokes were necessary to achieve strong anisotropy. In these systems, the I–N transition occurs during fi lm drying and the simultaneous occurrence of nematic order and shear fl ow appears necessary to achieve in-plane disk alignment. In some cases, we observe only a narrow window between the I–N phase transition and the point at which the viscosity, μ , increases to the point where the fl at liquid fi lm is not restored in the Mayer bar wake, and if this time window is missed, residual Mayer-bar tracks remain in the array.

The vertical orientation of the precursor organic disks and the graphene layer arrays is indicated by both polarized light microscopy (Figure 2B , 2C ) and HRTEM (Figure 2A ). The TEM


image shown was obtained with the e-beam on the VAGLA Z-axis following back thinning of the quartz-nickel substrate. On quartz alone the lattice fringes are all visible in the Z -axis projection, confi rming vertical layer alignment. These ordered regions are widespread but there are local statistical fl uc-tuations (not shown) which are consistent with liquid crystal assembly mode. [ 2 ] The addition of the nickel layer improves the length and perfection of the lattice fringes (Figure 2A ), probably through a Ni–C surface or interdiffusional interaction. [ 26 , 27 ]

The VAGLA height can be tuned by selecting the precursor concentration and the Mayer-bar wire dimension. According to geometry and mass-conservation relations, the height is:

h =

(1

2− B

8

)DLCmolecule

DLCderivedcarbonDYC

(2)

Where D is wire diameter of the Mayer-bar winding; Y is the carbonization yield of CLC; C is the mass concentration (w/w) of the precursor solution and the bracketed term arises from the convex geometry of the Mayer-bar gap. The ratio of material densities, ρ , accounts for carbonization shrinkage. We



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measured the 700 ° C carbonization yield for all 5 precursors and found them similar (40% ± 3%). The behavior of Equation 2 is shown in Figure 2D , which shows the potential to create nano-scale arrays ( h < 100 nm) by the use of dilute solutions. In the present work, the VAGLAs have been fabricated with heights as low as 50 nm. Above 800 nm in height, VAGLAs often detached from the substrate, likely due to stress associated with differen-tial thermal expansion and carbonization shrinkage.

The Mayer technique can be miniaturized by winding stainless steel wire on short rod segments and used in pen writing mode to make orientational micropatterns. Figure 2 demonstrates the ability to create polarization-active lettering (Figure 2C ) and to make imbedded “codes” consisting of polarization-active regions in an isotropic carbon background (Figure 2B ). These codes are nearly invisible under normal illumination, but are

200 nm

50 nm

top view

(channel entry)

VAGLAs

50 nm

bottom view

(channel exit)

quartz200 nm

500 nm

(A) (B)

(C) graphite foil

(D) glassy carbon

Figure 3 . Example VAGLA application in the form of Z-directional nanopore fabrication. (A) Cobalt nanoparticles deposited on VAGLAs surface prior to reaction; (B) Z-directional nanopores formed by 2 h catalytic hydrogenation at 900 ° C in 1% H 2 /He showing the channel entry cavity (upper image) and channel exit cavity (lower image). The Co particles continue to channel after reaching the substrate, but are then constrained to horizontal (X,Y) motion. The example VAGLAs was from precursor 2 with the height of 150 nm. (C,D) Comparison to conventional carbon materials that show surface channeling at steps in the basal plane due to horizontal graphene layer orientation (C, graphite foil) or no catalytic reaction under the same conditions due to low edge site availability (D, glassy carbon).

clearly revealed under polarized light. The selective use of local shear forces is a pow-erful tool for achieving full 2D orientational patterns VAGLAs.

Of the many potential uses for vertically aligned graphene nanolayer arrays, we chose to demonstrate one – the synthesis of Z -directional nanopores by catalytic hydro-genation. Nanopore membranes are of great interest in the selective molecular transport, sorting, and detection, an example appli-cation being electrical detection of single nucleotides traversing the nanopore for high-throughput DNA or RNA sequencing. [ 28–30 ] It is desirable to fabricate nanopores with fl exi-bility in size and shape, robustness in var-ious chemical environments, and with elec-trical conductivity for device integration. [ 30 , 31 ] Existing nanopore membrane fabrication techniques typically require transmission electron microscope beams or focused ion beams. [ 31–33 ] For many practical applications, it is desirable to develop simple and effi cient methods to fabricate nanopores that trans-verse solid fi lm structures.

It has long been known that the reactions between carbon and gas are strongly catalyzed by metals such as iron, nickel and cobalt. [ 34–36 ] The gasifi cation reactions occur naturally in impure or doped carbon materials with metallic impurities, which in result of cata-lytic pitting and channeling. [ 37–41 ] Recently, these solid-catalyzed solid-gas reactions have been explored as a method of carbon mate-rials patterning. [ 42–44 ] Many studies have shown that catalytic nanoparticles are usu-ally active on edge-plane surfaces, forming channels starting at graphite step edges on the basal plane, and often follow well-defi ned crystallographic directions. [ 45–47 ] Recent advances in nanoparticle synthesis provide a wide range of monodisperse particle sys-tems that could be used to form nanopores by controlled catalytic etching. Z-directional


nanopores are diffi cult to fabricate, however, due to the unfa-vorable crystal structures of conventional carbon materials. Natural graphite, pyrolytic graphite, and CVD carbon fi lms all consistent of graphene layers lying statistically parallel to the substrate, which leads to surface channeling initiated at step defects, rather than Z-directional “burrowing”.

We hypothesized that VAGLAs would guide catalytic nano-particles by offering active edge sites at the top surface and inducing Z-directional motion as the particles adhere to and follow the vertically receding edge planes. Our experiments used metallic cobalt nanoparticles ( < 50 nm) deposited from a 10 ppm suspension in ethanol and dried ( Figure 3 A ). Catalyst loaded samples were then placed in a fused silica tube furnace, and heated at 900 ° C for 2 h under a high purity gas fl ow con-taining H 2 (1%) and He (99%).



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Figure 3 shows the results for CLC-derived VAGLAs (Figure 3B ) and for glassy carbon (an isotropic material with randomly ordered layers and few edge sites) and graphite foil (with hori-zontally ordered graphene planes) as reference samples. In this study, gasifi cation etching does not occur at temperatures below 600 ° C, but is seen to be signifi cant at 700 ° C, and a 30 min reaction time is suffi cient to produce long channels that defi ne the reaction direction. For graphite foil (Figure 3C ), hydrogena-tion produced no pits but rather X–Y surface channels, as has been observed often. [ 42 , 46 , 47 ] Cobalt particles are observed at the leading edge of each channel tip with the diameter similar to the channel width. It is expected that these conventionally ordered graphenic carbons would have a very low Z -directional reactivity. On the glassy carbon, 700 ° C hydrogenation pro-duced no visible pits or channels (Figure 3D ), likely due to the low availability of active edge sites. Under the same conditions, VAGLAs produce Z -directional nanopores of sizes similar to the Co nanoparticles (Figure 3B ). As anticipated, the Co nanoparti-cles burrow into the fi lm moving with a signifi cant directional component perpendicular to the substrate plane consistent with edge-site attack. To confi rm that the Co nanoparticles have already have penetrated and reached the bottom of the fi lm, the sample was removed from quartz using double-sided black electrical tape, and then the bottom side of the carbon fi lm was examined by SEM. Figure 3B (lower image) clearly shows the pores at the bottom side, which indicates that Co nanoparticles have penetrated the fi lm transversely following the Z -direction, creating a carbon-based membrane with individual permeating nanopores. Detailed examination of the channels shows they some are not strictly perpendicular to the substrate, but are angled with a signifi cant perpendicular component. It is inter-esting that some nanoparticles continued etching after reaching the quartz bottom, and then moved horizontally parallel and in close proximity to the quartz substrate. This behavior has not been explored in detail, but we expect reaction and motion at this three-phase interface to be driven by both crystallographic plane selection in carbon and by the confi ning effect of the substrate.

Overall, we have shown that a range of chromonic liquid crys-talline precursor molecules can be used to fabricate vertically aligned graphene layer arrays with heights from 50 to 800 nm and with full 2D order in-plane set by depositing the solution precursor under shear fl ows of order 500 s − 1 . Local shear fl ow “writing” can be used to make polarization-active patterns or imbedded lettering apparent only under polarized light. Of the potential applications, we demonstrate the synthesis of Z -directional nanopores using cobalt-catalyzed hydrogen gasifi -cation. Under the same reaction conditions, conventional carbon materials give no reaction or only surface channels rather than the transverse pores desired for membrane applications.

Experimental Section A set of chromonic liquid crystal precursors was acquired from the display technology fi rm Optiva (South San Francisco, CA), in the form of aqueous solutions at the mass concentrations given in Figure 1 . The precursors were concentrated or diluted in steps with deionized water to identify the I–N phase transition by optical microscopy. Solid crystals were obtained by drying on a hot plate at 250 ° C overnight.


Carbonization yield was determined by the mass loss of solid crystals after carbonization which was carried out carried out at 700 ° C in fl owing nitrogen for 30 min.

Quartz plates (25 × 75 mm) were washed with water and ethanol, and heated in air at 500 ° C for 15 min to improve their hydrophilicity prior to applying the aqueous phase CLCs. The quartz-nickel substrate included a 200 nm thick nickel layer deposited by electron beam physical vapor deposition. A 20 nm titanium layer, also deposited using the same technique, was used as an inter-layer between nickel and quartz to improve adhesion. About 20 μ L CLC precursor was applied to the quartz and drawn into a uniform fi lm over using Mayer-bars (RD Specialties Inc., Webster, NY): RDS03, RDS08, and RDS20, with the diameter of 0.08 mm, 0.20 mm, and 0.51 mm, respectively. The thickness of the carbon fi lms (VAGLAs) were measured by profi lometry (Dektak, Veeco) and confi rmed by FIB (FEI Helios Dual Beam).

Cobalt nanoparticles with d < 50 nm (Strem Chemicals) were suspended in ethanol at approximately 10 ppm. After 15 min of bath sonication, the suspension was deposited on the VAGLAs, or on graphite foil (Alfa Aesar), or glassy carbon surfaces (SPI-Glas) as reference materials. The dried samples were then heated at 900 ° C for 2 h under fl owing high-purity H 2 (1%) in He(99%). Morphology and crystal structures of the samples were characterized by polarized light microscopy, fi eld emission scanning electron microscopy (LEO 1530 at 5 kV below 10 − 5 Torr) and HRTEM (JEOL JEM-2010 operated at 200 kV) following back thinning of the substrate by mechanical grinding and ion milling at 5 kV (691 PIPS, GATAN).

Acknowledgements The authors would also like to thank Dr. Indrek Kulaots from Brown University for the technical assistance with the fused silica tube furnace. Financial support was provided by National Science Foundation, through the Brown University MRSEC (DMR0520651), and also by the China Scholarship Council (CSC).

Received: August 31, 2010 Revised: October 12, 2010

Published online: November 22, 2010

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vertically aligned graphene layer arrays from chromonic liquid crystal precursors

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