A Growth Mechanism for Free-Standing Vertical Graphene - Nano

Letter pubs.acs.org/NanoLett

A Growth Mechanism for Free-Standing Vertical Graphene Jiong Zhao,†,‡ Mehrdad Shaygan,§ Jürgen Eckert,†,∥ M. Meyyappan,⊥ and Mark H. Rümmeli*,‡,¶,□ †

IFW Dresden, Institute of Complex Materials, P.O. Box 270116, D-01171 Dresden, Germany IBS Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Daejon 305-701, Republic of Korea § Division of IT Convergence Engineering, Pohang University of Science and Technology, Pohang, Republic of Korea ∥ TU Dresden, Institute of Materials Science and the Center for Advancing Electronics Dresden (cfaed), 01062 Dresden, Germany ⊥ NASA Ames Research Center, Moffett Field, California 94035, United States ¶ Department of Energy Science and □Department of Physics, Sungkyunkwan University, Suwon 440-746, Republic of Korea ‡

ABSTRACT: We propose a detailed mechanism for the growth of vertical graphene by plasma-enhanced vapor deposition. Different steps during growth including nucleation, growth, and completion of the free-standing two-dimensional structures are characterized and analyzed by transmission electron microscopy. The nucleation of vertical graphene growth is either from the buffer layer or from the surface of carbon onions. A continuum model based on the surface diffusion and moving boundary (mass flow) is developed to describe the intermediate states of the steps and the edges of graphene. The experimentally observed convergence tendency of the steps near the top edge can be explained by this model. We also observed the closure of the top edges that can possibly stop the growth. This two-dimensional vertical growth follows a self-nucleated, step-flow mode, explained for the first time. KEYWORDS: Vertical graphene, nucleation, step-flow, edge closure

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study on vertical growth of 2D structures will promote the realization of more types of vertical structures and that an understanding of the underlying mechanisms will enable nanostructures for various applications. It is well-known that sp2 carbon, which is the most stable elementary form of carbon at room temperature, can lead to various kinds of layered structures. Among these structures, graphene is a true 2D material8 with the large anisotropy between the in-plane and out-of-plane directions providing possibilities to manage the 2D growth. As reported in numerous publications, the growth of high quality graphene (monolayer or few layers) can be carried out by chemical vapor deposition (CVD) on different substrates such as Cu, Ni, SiC, and so forth.9−12 Even before the emergence of these atomically thin carbon layers, there has already been some work focusing on carbon nanosheets produced by plasma enhanced CVD (PECVD). The PECVD method proceeds at a lower temperature and has better control in nanostructure ordering and higher purity which can easily produce very dense carbon nanoflakes or carbon nanowalls (CNWs).13−25 The gas precursors (mainly hydrocarbons) undergo inelastic collisions with the electrons in the plasma to form free radicals, ions, and other reactive species. In the presence of metallic catalytic

ingle crystal growth is an old topic typically including two stages, nucleation, and growth.1 The nucleate has to first overcome an energy barrier caused by the interfacial mismatch under supersaturation and then act as the subsequent growth template. Crystal growth issues have come to the forefront with the advance of nanotechnology and growth of nanomaterials for use in the so-called bottom-up scheme.2 The growth mechanism is clear when discussing the one-dimensional (1D) growth like whiskers,3 nanowires,4 and nanotubes,5 and they are often free-standing (self-supported). These growths can be initiated by atomic clusters, either by the same material (homogeneous nucleation) or different materials (heterogeneous nucleation or catalysts). The surface energy anisotropy or even surface decoration anisotropy plays an important role in 1D growth, according to the so-called L−S (liquid−solid) or V−S (vapor−solid) mechanism;2−5 in the case of nucleation by catalysts, the growth might probably continue at the interface between the catalyst and the nanowire (nanotube), according to the VLS (vapor−liquid−solid) or VSS (vapor−solid−solid) mechanism.4−6 In contrast, two-dimensional (2D) growth normally requires additional support (substrate) and the asgrown thin film is parallel with the substrate.7 The growth mechanism of vertical free-standing 2D structures has not been discussed much compared to various techniques for thin film deposition, which have been well developed. In this report, we use vertical graphene as an example to investigate its specific bottom-up growth mechanism. We believe that the present © XXXX American Chemical Society

Received: January 3, 2014 Revised: April 23, 2014

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dx.doi.org/10.1021/nl501039c | Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. Vertical graphene (VG) grown by PECVD. (a) The cross-sectional SEM image of an as-grown vertical graphene sample on the substrate. Red and blue triangles indicate two interfaces. (b) TEM image of the upper layer of VG, which stems from the carbon onion. Red dashed circle highlights the position of the carbon onion. (c) TEM image of the bottom layer of VG, which stems from the substrate and buffer layer.

nanotubes, but there are more possibilities for 2D growth: at the contact line between the graphene and substrate, at the free edges of the graphene (growing larger), as well as at active steps/edges on the two sides of the graphene wall (growing thicker). The samples employed in this study were fabricated by an RF (radio frequency)-PECVD method. The plasma was ignited by using a mixture of methane and hydrogen at an RF power of 900 W for a fixed deposition time of 20 min. Different substrates, including Si and Si covered with a 150 nm film of Au (thermally deposited) were used. The temperature of the sample stage ranged from 600 to 1100 °C at growth pressures between 10−3 Torr and atmospheric pressure. The effects of the different parameters (temperature, pressure, substrate, RF power) have been discussed elsewhere.18 A review can be found in ref 17 on how these parameters influence the final products. However, all variations induced by the growth parameters do not alter the major growth mechanism proposed here but only create different amount of defects or lead to different growth rates, sizes, and thicknesses. Hence, in the following discussion we mainly focus on the microscopic features in the process of vertical graphene growth, disregarding specific growth conditions. The scanning electron microscopy (SEM) image in Figure 1a shows a cross-section view of the as-grown vertical graphene. At the bottom of the SEM image one can see lines or wrinkle-like formations on the substrate. These are in effect the tops of vertical graphene sheets that have emerged perpendicularly to the VG sheets above the substrate. Above the substrate two distinct regions or layers can be observed; one layer of VG sheets sits immediately above the substrate (indicated by a blue triangle) while another rests on top of the initial layer with its interface indicated by a red triangle. As we will show below, the upper layer VG sheets nucleate from carbon onions. Our detailed investigation is based on low-voltage aberrationcorrected transmission electron microscopy (LVACTEM). A JEOL 2010F TEM retrofitted with CEOS aberration correctors working with a 80 kV accelerating voltage was used.30 The TEM specimen was prepared by mechanically scratching the surface of the vertical graphene samples and transferring to TEM grids. The TEM copper grid is put directly onto the surface of the sample, pressed and scratched, for the manual mechanical cleavage of the samples. After this process, the upper layer of vertical graphene can be peeled off and separated during scratching while the bottom layer of vertical graphene remains on the substrate. A comparison of the SEM images (Figure 1a) and TEM images (Figure 1b,c) reveals that the morphology and size of the vertical graphene samples did not change. Also, because the top edges of these vertical graphene

particles, the dissociation of the feedstock gas in the plasma is not necessary in principle; rather, carbon is produced from dissociation at the metal catalyst surface as in most cases of carbon nanotube growth with PECVD. But the carbon nanowalls can also grow without the use of catalysts, which means precursor dissociation by the plasma plays an important role. Various substrates have been shown to enable carbon nanowall growth, including Si, SiO2, Si3N4, Cu, steel, and so forth.13,18 The as-grown CNWs usually have tapered structures, typically 1−3 graphene layers at the top and several layers at the bottom, standing perpendicular to the substrates. Hence they are also called vertical graphene (VG), first to indicate the few layer graphitic nature, and second to differentiate this type of graphene from graphene deposited parallel to a substrate by thermal CVD. The vertical graphene can be useful in fuel cells as catalyst support and as electrodes in energy storage devices such as lithium ion batteries and supercapacitors. As-grown structures directly on metal substrates without the aid of catalysts eliminate the need for binding materials commonly used to make pastes of reduced graphene oxide for electrode preparation. The binding materials add to the gravimetric weight estimation but do not enhance the conductivity and thus, as-grown VG can be advantageous for intercalation of other materials useful in batteries and supercapacitors Compared to the widely studied graphene synthesis by thermal CVD, the microscopic growth mechanism of vertical graphene is still not well understood. The nucleation site for the vertical structure is the first puzzle. Some evidence has been presented for the formation of a thin carbon film on the substrate before the onset of vertical growth,14 and the upward curling force can lead to the takeoff of some top carbon layers at the grain boundaries that continue to grow perpendicular to the substrate.26 The electric field in the plasma is also suggested to assist in aligned vertical growth,26,27 as in the case of PECVD of carbon nanotubes.28 The plasma appears to play several roles in nanostructure growth.27 The plasma can produce a relatively larger chemical potential gradient near the surface through ion focusing effects and fast delivery of precursors. The plasma exposure can increase the temperature of the top surface layer. The diffusion barrier may be reduced by the electric-field induced polarization effects that in turn will reduce the adhesion/bonding energy of atoms to the surface. The nonuniform electric field on the surface can effectively increase the diffusion coefficient. The complex interplay of such effects, while providing unique opportunities, makes the understanding of mechanisms difficult. Second, how are these tapered structures formed? And where are the active growth sites for these 2D structures? There are tip growth2 and root growth29 models for 1D structures such as nanowires and carbon B


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Figure 2. Nucleation mechanisms and schemes for the VG growth. (a) Low-magnification TEM image for the VG stemmed from substrate and buffer layer (bottom layer VG). (b) HRTEM image of the buffer layer shows the buffer layer consists of amorphous carbon and graphitic layer on the surface. There are some mismatches where VG is initialized. (c) TEM image for the VG stemmed from carbon onions (upper layer VG). (d) HRTEM of the carbon onion that shows the mismatch of graphitic layers at the surface, which initialize the growth of VG. (e) The scheme for nucleation at curved areas on the buffer layers as well as on the surface carbon onions, the arrows show the growth direction. (f) The scheme for growth of 1D and 2D nanostructures where the arrows represent the mass supply for growth. (g) The scheme for the nearly completed vertical graphene with some seamless (straight) edges and some open (nonstraight) edges. The growth history is presented by the dashed lines.

surface. Some mismatches in the graphitic layers can also be observed. The hypothesis that the vertical graphene stems from the mismatches and curved areas of graphitic layers31 can be confirmed (Figure 2e), especially referring to the large concentration of defects due to the plasma. The amorphous carbon can be formed due to the catalytic effect of the Au layer, because metals can act as catalytic decomposition sites for the carbon sources and easily form amorphous carbon by PECVD.28 The amorphous carbon is formed because the carbon deposition rate is greatly enhanced by catalysts and both the time and temperature are insufficient for the crystallization of carbon. However, in the regions away from the substrate, the growth of sp2 carbon still dominates, which is a noncatalyst growth process. In contrast, the upper layer of vertical graphene nucleates from the carbon onions (Figure 2c). The formation of onions by PECVD has been reported by many groups.32,33 Similar to the buffer layer, the onions also serve as active

samples are closed or seamless (to be discussed later) between each neighboring wall that provides more mechanical stability, the sample preparation process is unlikely to introduce any folding or damage. Figure 1b shows the TEM image of the upper layer of vertical graphene which is peeled off during TEM sample preparation. This kind of vertical graphene has a bloomed flower shape that originates from a carbon onion (highlighted by the dashed circle in Figure 1b). In Figure 1c, the vertical graphene sheets are firmly connected with the substrate. The nucleation mechanisms for vertical graphene are discussed below. There is a ∼20 nm thick buffer layer (highlighted in blue in the image in Figure 2a) between the substrate (150 nm thick Au film) and the vertical graphene, consistent with the SEM analysis by Wang et al.13 The HRTEM of the area for this buffer layer (Figure 2b) shows that it is composed of amorphous carbon and graphitic layering on the C


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Figure 3. (a) Atomistic model of a curved vertical graphene with active growing edges (highlighted with colors). (b) TEM image of a growing vertical graphene sheet with many curved steps and terraces (highlighted in different colors) on the surfaces. The two wings are curved and overlapped by itself in the viewing direction. (c) Finite element simulation of the carbon adatom diffusion on the surfaces. The isolines for adatom concentrations and the mass flowchart (marked by the red arrows) are presented. The scale bar (from white to black) on the right indicates the value of C0 (higher to lower concentrations of free carbon atoms). The conditions for the simulation are C0 = 10, μ = 1, d = 1, g = 1 and the lateral size of the graphene is approximately 1, all in arbitrary units. Changes in the absolute values of these parameters do not change the shapes of the isolines and the mass flow directions. (d) For convenient comparison between the experimental result (panel b) and simulation (panel c), the two wings in the simulation are folded.

the products. In Figure 3b, there is a typical vertical graphene during growth, while in Figure 3a the atomic model highlights the active edges and the curved nature. A series of steps can be clearly identified on the surfaces, and the terraces are highlighted in different colors. The steps are not straight but in a round shape and arch upward. These steps play the same role as the active edges that mediate the growth of vertical graphene in the upward direction. Here we can give further explanation of the step morphologies and positions. The decomposition rate of the precursor and the deposition rate of free carbon (adatoms) onto the substrate as well as the growth rates are quite fast in PECVD, evidenced by the thick buffer layer. Hence we assume that the attachment and detachment of carbon adatoms on the active steps or edges are the rate controlling processes for graphene growth. Within this quasi-stationary boundary condition (the diffusion rate is an order of magnitude larger than the growth rate at the active sites), the concentration of carbon adatoms on the terraces (surface) of vertical graphene will be governed by the common 2D diffusion equation

nucleation sites here for the growth of vertical graphene. The HRTEM (Figure 2d) shows the graphitic layering mismatches at the outmost six layers of the onions. The graphene layers then grow in the tangential direction, naturally leading to a flower shaped vertical graphene structure. The active edges or steps will play important roles after nucleation (vertical growth initiation). For graphene grown by CVD on the substrates, the growing edges are normally zigzag edges, having a hexagonal polygon structure34 or sometimes even fractal shapes.35 The growth mode for carbon nanotube (1D) can be either top or bottom modes (Figure 1f), and the vertical 2D growth has also several possibilities: growth at the top edge, on the surface (in the lateral direction), or at the bottom (Figure 1f). Normally, the presence of the catalyst particle and its position provide evidence for the growth mode, top36 or bottom,37 as clarified by in situ experiments.38 If we just inspect the final vertical graphene products, for example, in Figure 1c, it is difficult to tell the mechanism as there is no catalyst left behind, and the edges at the top and at the bottom are both completely smooth. The folded structures of graphene39,40 are also one possibility. However, these straight edges have been previously proved to be closed (seamless) edges of two monolayer graphene layers18 and a few other reports also indicate the tendency of edge closure of two adjacent graphene layers under high temperatures.41−43 The growth will cease naturally at a closed edge (schematically shown in Figure 1g). Nevertheless, we are able to find some intermediate states of the growing vertical graphene among all

D∇2 C − g C + d = 0

(1)

where C is the local carbon adatom concentration, d is the carbon deposition rate which is homogeneous over the whole surface, g is the carbon consuming rate due to growth which is simplified to be proportional to the local carbon concentration, and D is the diffusion constant. Consider our subject as a D


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Figure 4. (a) The schematic model for the vertical graphene in a cross section view. The steps from the top edge are numbered X0, X1, X2...and so forth. (b) The simulated result for a moving boundary problem using eq 4. The conditions for the simulations are the same as in the last simulation in Figure 2c. The initial interval between nearest steps is assumed to be equal (time = 10) and at time = 27, the intervals between nearest steps (terrace lengths) as a function of their number (counted from the top edge) are presented in the inset. (c,d) Cross section views of two growing vertical graphene cases. The morphologies are highlighted by red dashed lines. These experimental terrace lengths of growing edges for vertical graphene (red square in the inset of panel b) match well with the simulated terrace lengths (green circle in the inset of panel b), showing a tendency of step convergence.

features of the concentration distribution. The shape of the experimentally observed steps in Figure 3b matches well with the contour lines of the carbon adatom concentration map in Figure 3c. This can be understood readily by a simple linear relationship between the step growth velocity and the local carbon adatom concentration (also the supersaturation),44

similar shape to the real vertical graphene in Figure 3b (see Figure 3c), and one side connecting to the substrate has a constant adatom concentration (C0), (Dirichlet boundary condition, C = C0, much like contacting to a big carbon supply tank). The other growing edges obey the Neumann boundary condition, (∂C/∂x)·n = g0 where n is unit vector in the normal direction at the boundary and g0 is the net mass flux (growth rate at the edges) at the boundary. The 2D diffusion equation can be numerically solved using a finite element method or other appropriate techniques. Following this scheme, Matlab Software was used and the results are presented in Figure 3c. Before simulation, eq 1 can be recast into a dimensionless form, where C* = gC/d, and eq 1 will become 2

w∇ C * − C * + 1 = 0

v = K(C − Ceq )

(3)

where Ceq is the equilibrium carbon adatom concentration, which is determined by the temperature as well as the formation energy of carbon adatoms (or vancancies) (E) on the graphene steps, ⎛ E⎞ Ceq ∼ exp⎜ ⎟ ⎝ kT ⎠

(2)

where w = D/gl2 and l is a characteristic length scale taken as √D/g. Diffusion becomes negligible for small values of w (w ∼ 0) when concentration becomes constant and the growth will have no direction. This limiting case is certainly not applicable in our work. The other limiting condition of large w (w ∼ ∞) corresponds to the diffusion limit wherein the carbon consumption due to growth and deposition of carbon directly on the surface of graphene become negligible. This is not also appropriate in our case and therefore, intermediate w values (w ∼ 1) would be meaningful to study here. Because we expect C* = gC/d > 1 (to be discussed later), the boundary concentration C0 would be large. We discuss a case where values for D, d, and g were chosen as C0 = 10, D = 1, d = 1, and g = 1. A careful analysis revealed that other values of w did not change the main

(4)

During growth, the shapes of the steps located along the concentration isolines are stable. Deviations from these isolines which cause either faster or slower motion of the steps can lead to negative feedback (because ∂C/∂x < 0, the growth speed is slower at a faster growing location) and keep the shapes of the steps. The carbon concentration on the two wings of the vertical graphene is lower, therefore the step formation is slower than in the center, hence leading to a round shape for the steps. (Note in Figure 3a, the vertical graphene sheet is curved and partially overlapped in the viewing direction, therefore the sides are folded over, as schematically shown in Figure 3b.) We assume that d < gC in our analysis above. If d were larger, then the carbon adatom flow would be reversed, and the carbon E


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Figure 5. TEM images of some VG edges. (a) The seamless edge has two layers, forming a corner (lower inset, atomic structure of a double layer seamless edge; upper inset, multislice TEM image simulation for this structure. There is also one incomplete graphene layer on the surface, similar to what is present in the TEM image. (b) The 12 layer flat seamless edges with a nearly 90° angle corner (lower inset, atomic structure of a 12 layer seamless edge; upper inset, TEM image simulation for this structure. There are some differences between the image and simulation at the corner because the VG is not perfect at the corner.) (c) The tapered seamless edge, black triangles indicate the edge closure. (e,f) The nonstraight open edges, which are in the growing processes.

The above kinetic model and experimental observations demonstrate that the growth of vertical graphene can be considered as a step-flow process45 on the basis of nucleation at the bottom. According to this mechanism, the thickness of graphene will depend on the number of layers which nucleates from the bottom. Regarding the size of the vertical graphene, there are at least two limitations. The first limitation is that two graphene layers nearby can naturally connect with each other to form a sealed edge. The seamless edges that have double graphene layers and 12 layers are shown in Figure 5a,b, respectively. The atomic structures and multislice TEM image simulations are presented in the insets. The formation of this closed structure can lead to the “magic angle” (the integer times of 30°) that has been reported previously and such sealed edges are usually zigzag or armchair.18 The tapered edges are more frequently observed because the neighboring layers can naturally form a lip−lip connection during growth and the growth can cease (Figure 5c). In contrast, the growing edges are normally nonstraight (Figure 5d−f). The bending curvature of the whole vertical graphene can also affect their sizes. Any local curvatures in sp2 carbon may possibly cause more defects (like heptagons and pentagons) during growth and these defects will disturb the diffusion of free carbon adatoms on the surfaces. In other words, the diffusion length on surfaces of curved graphene is shorter than on flat graphene. Therefore, the growth speed at the edges for curved vertical graphene (Figure 1b) will be slower than for flat graphene (Figure 1c). Eventually the flat graphene without curvature can be larger than the highly curved ones under the same growth conditions, which is confirmed by our TEM observations. Finally, these seamless edges as well as folded structures, which are common in few-layered grapheme,46 provide the mechanical support for the free-standing vertical

concentration would be larger at the two wings of the graphene sheet. In this case, the shapes of the steps are unstable, deviation from the isolines can be positive feedback (∂C/∂x > 0) and hence enlarge the deviations. Therefore, we can exclude the possibility that d > gC. The kinetics of steps can be further elaborated if a time lapse is included into the boundary conditions. For the simplified 1D model (cross section view at the center line in the vertical direction of graphene, Figure 4a) for vertical graphene, the moving boundary conditions are set and the governing equations are ⎧ ∂ 2C(x , t ) ⎪D − gC + d = 0 ∂x 2 ⎪ ⎪ dX n ⎪ = KC(X n , t ) ⎨ dt ⎪ C(0, t ) = C0 ⎪ ⎪ ∂C(x = X 0) ⎪ =0 ⎩ ∂x

(5)

where Xn are the positions for each step, and X0 stands for the position of the top edge. With the initial terraces (decided by nucleation) assumed to be equally distributed, and using a finite element method to simulate this growth, the step positions as a function of time can be plotted in Figure 4b. Here, time is divided into finite periods and the diffusion equation is then solved under stationary assumptions. The inset of Figure 4b also shows the different terrace lengths (Ln =Xn − Xn+1) after some growth. This linear relationship of the terrace length and the terrace number implies a convergence tendency of all the steps which starts from the top, and this tendency matches well with our experimental HRTEM observations (Figure 4c,d). F


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M.; Shiraishi, M.; Meyyappan, M.; Büchner, B.; Roche, S.; Cuniberti, G. Graphene: Piecing it together. Adv. Mater. 2011, 23, 4471−4490. (12) Mattevi, C.; Kim, H.; Chhowalla, M. A review of chemical vapour deposition of graphene on copper. J. Mater. Chem. 2011, 21, 3324−3334. (13) Wang, J.; Zhu, M.; Outlaw, R. A.; Zhao, X.; Manos, D. M.; Holloway, B. C. Synthesis of carbon nanosheets by inductively coupled radio-frequency plasma enhanced chemical vapor deposition. Carbon 2004, 42, 2867−2872. (14) Zhu, M.; Wang, J.; Holloway, B. C.; Outlaw, R. A.; Zhao, X.; Hou, K.; Shutthanandan, V.; Manos, D. M. A mechanism for carbon nanosheet formation. Carbon 2007, 45, 2229−2234. (15) Lisi, N.; Giorgi, R.; Re, M.; Dikonimos, T.; Giorgi, L.; Salernitano, E.; Gagliardi, S.; Tatti, F. Carbon nanowall growth on carbon paper by hot filament chemical vapour deposition and its microstructure. Carbon 2011, 49, 2134−2140. (16) Dikonimos, T.; Giorgi, L.; Giorgi, R.; Lisi, N.; Salernitano, E.; Rossi, R. DC plasma enhanced growth of oriented carbon nanowall films by HFCVD. Diamond Relat. Mater. 2007, 16, 1240−1243. (17) Bo, Z.; Yang, Y.; Chen, J.; Yu, K.; Yan, J.; Cen, K. Plasmaenhanced chemical vapor deposition synthesis of vertically oriented graphene nanosheets. Nanoscale 2013, 5, 5180−5204. (18) Davami, K.; Shaygan, M.; Kheirabi, N.; Zhao, J.; Kovalenko, D. A.; Rümmeli, M. H.; Opitz, J.; Cuniberti, G.; Lee, J. S.; Meyyappan, M. Synthesis and characterization of vertical graphene on different substrates by radio frequency plasma enhanced chemical vapor deposition. Carbon 2014, 72, 372−380. (19) Kondo, S.; Kawai, S.; Takeuchi, W.; Yamakawa, K.; Den, S.; Kano, H.; Hiramatsu, M.; Hori, M. Initial growth process of carbon nanowalls synthesized by radical injection plasma-enhanced chemical vapor deposition. J. Appl. Phys. 2009, 106, 094302. (20) Chatei, H.; Belmahi, M.; Assouar, M. B.; Le Brizoual, L.; Bourson, P.; Bougdira, J. Growth and characterisation of carbon nanostructures obtained by MPACVD system using CH4/CO2 gas mixture. Diamond. Relat. Mater. 2006, 15, 1041−1046. (21) Vizireanu, S.; Stoica, S. D.; Mitu, B.; Husanu, M. A.; Galca, A.; Nistor, L.; Dinescu, G. Radio frequency plasma beam deposition of various forms of carbon based thin films and their characterization. Appl. Surf. Sci. 2009, 255, 5378−5381. (22) Teii, K.; Shimada, S.; Nakashima, M.; Chuang, A. T. H. Synthesis and electrical characterization of n-type carbon nanowalls. J. Appl. Phys. 2009, 106, 084303. (23) Malesevic, A.; Vitchev, R.; Schouteden, K.; Volodin, A.; Zhang, L.; Van Tendeloo, G.; Vanhulsel, A.; Van Haesendonck, C. Nanotechnology 2008, 19, 305604. (24) Hiramatsu, M.; Hori, M. Fabrication of carbon nanowalls using novel plasma processing. Jpn. J. Appl. Phys. 2006, 45, 5522−5527. (25) French, B. L.; Wang, J. J.; Zhu, M. Y.; Holloway, B. C. Evolution of structure and morphology during plasma-enhanced chemical vapor deposition of carbon nanosheets. Thin Solid Films 2006, 494, 105− 109. (26) Warner, J. H.; Schaffel, F.; Rümmeli, M.; Bachmatiuk, A. Graphene: Fundamentals and emergent applications; Springer: New York, 2012. (27) Ostrikov, K.; Neyts, E. C.; Meyyappan, M. Plasma Nanoscience: from Nano-solids in Plasmas to Nano-plasmas in Solids. Adv. Phys. 2013, 62, 113−224. (28) Meyyappan, M.; Delzeit, L.; Cassell, A.; Hash, D. Carbon nanotube growth by PECVD: a review. Plasma Sources Sci. Technol. 2003, 12, 205−216. (29) Gavillet, J.; Loiseau, A.; Journet, C.; Willaime, F.; Ducastelle, F.; Charlier, J. C. Root-growth mechanism for single-wall carbon nanotubes. Phys. Rev. Lett. 2001, 87, 275504. (30) Börrnert, F.; Bachmatiuk, A.; Gorantla, S.; Wolf, D.; Lubk, A.; Büchner, B.; Rümmeli, M. H. Retro-fitting an older (S)TEM with two Cs aberration correctors for 80 kV and 60 kV operation. J. Microscopy 2013, 249, 87−92.

alignment; otherwise, these large ultrathin sheets would easily bend or scroll. The closure of edges can greatly suppress the interlayer shear which could occur for open edge graphene. The bending rigidity for the closed edges (similar to nanotubes) is at least 1 order of magnitude larger than the open edges.47 In conclusion, our TEM observations reveal the growth mechanisms for vertical graphene grown by PECVD. The nucleation occurs at the mismatch of graphitic carbon layers (surface at buffer layer on the substrate as well as carbon onions) and the active sites for growth can be the surface steps as well as the edges at the top. The diffusion of carbon adatoms on the surfaces of vertical graphene supplies the materials needed for the growth while the rate controlling step is the attachment of carbon atoms at the active sites. We developed a moving boundary model that can describe the motion of the steps with respect to time. The experimental observations of the step morphologies and the relative positions of the steps can thus be explained. Finally, the closure of open edges determines the final size of the vertical graphene and together with the overall curvature provide the necessary mechanical support for vertical alignment.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.Z. thanks the DAAD. Support from the excellence cluster (CFAED) is gratefully acknowledged. This work was supported by the Institute of Basic Science (IBS) Korea (EM1304). We also thank Ms. Y. Woo and Mr. J. Ju for their support with the TEM facilities at SKKU.

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REFERENCES

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