Catalytic Growth of Graphene - American Chemical Society

Perspective pubs.acs.org/JPCL

Catalytic Growth of Graphene: Toward Large-Area Single-Crystalline Graphene Hiroki Ago,*,†,‡ Yui Ogawa,‡ Masaharu Tsuji,†,‡ Seigi Mizuno,‡ and Hiroki Hibino⊥ †

Institute for Materials Chemistry and Engineering and ‡Graduate School of Engineering Sciences, Kyushu University, Fukuoka 81 6-8580, Japan ⊥ NTT Basic Research Laboratories, NTT Corporation, Kanagawa 243-0198, Japan ABSTRACT: For electronic applications, synthesis of large-area, single-layer graphene with high crystallinity is required. One of the most promising and widely employed methods is chemical vapor deposition (CVD) using Cu foil/film as the catalyst. However, the CVD graphene is generally polycrystalline and contains a significant amount of domain boundaries that limit intrinsic physical properties of graphene. In this Perspective, we discuss the growth mechanism of graphene on a Cu catalyst and review recent development in the observation and control of the domain structure of graphene. We emphasize the importance of the growth condition and crystallinity of the Cu catalyst for the realization of large-area, single-crystalline graphene.

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raphene, a single atomic sheet of sp2-hybridized carbon atoms, shows unique physical properties, such as extremely high carrier mobility, high optical transparency, and superior mechanical flexibility.1−4 On the basis of these properties, enormous research efforts have been devoted to develop electronic applications of graphene sheets.5−7 Various electronic applications that include use either as a metal (transparent electrodes for touch panels8 and solar cells9) or semiconductors (integrated circuits,10 high-frequency transistors,11 and sensors12) have been demonstrated. In addition, flexible, bendable, and/or stretchable electronics have been fabricated with graphene sheets, which are difficult to achieve in modern Si electronics.8,13,14 These electronic devices inevitably require large-area graphene with high crystallinity. Mechanical exfoliation from graphite, originally used to investigate physical properties of graphene, gives highly crystalline graphene flakes. The exfoliated single-layer graphene shows high carrier mobility of ∼10 000 cm2/(V s) on a Si wafer and 200 000 cm2/(V s) when suspended,15,16 but the flake size is limited, and it lacks uniformity in the number of layers. Among other methods to prepare graphene films, such as thermal decomposition of SiC,17,18 chemical reduction of graphene oxide (GO) film,19,20 and chemical vapor deposition (CVD) growth, the CVD growth in the presence of a metal film catalyst has become one of the most promising methods because the CVD gives transferrable high-quality graphene films with large area whose sizes are limited only by the metal film and furnace size. The catalytic growth of single-layer graphene was initially studied on Ni21,22 and Cu23 before the first report of mechanical exfoliation.15 At that time, graphene was studied mainly from the viewpoint of basic surface science. In the recent CVD growth, various metals, such as Ni,24−27 Co,27−29 Cu,30−33 Ru,34,35 Ir,36 and Pd,37 have been reported to catalyze the growth of graphene. In particular, Cu metal is the most widely used because the low carbon solubility of Cu enables the preferential growth of single-layer graphene, which is explained by a self-limiting mechanism.30,33 In addition, © 2012 American Chemical Society

availability of large-area substrates and the low cost of Cu foil are advantageous for practical applications of single-layer graphene.

Considering influences of graphene’s domain boundaries on the electrical, mechanical, and thermal properties, the synthesis of “single-crystalline graphene” free from domain boundaries is an important and challenging issue. However, the single-layer graphene grown on Cu foil is polycrystalline with a number of domain boundaries38,39 (in this Perspective, “grain” and “domain” are used for Cu metal and a graphene sheet, respectively, to distinguish the structures of Cu and graphene). This is ascribed to the polycrystallinity and rough surface of Cu foil but is also related to the growth kinetics of graphene. Domain boundaries present in the graphene influence the electrical, mechanical, and thermal properties. The CVD-grown single-layer graphene usually shows lower mobility (ranges from several hundred to 4000 cm2/(V s) at room temperature) than mechanically exfoliated graphene. This difference has been mainly explained by the presence of the domain boundaries.40 Mechanical properties are also influenced by the domain boundaries.41 Considering influences of graphene’s domain boundaries on the electrical, mechanical, and thermal properties, the synthesis of “singleReceived: May 30, 2012 Accepted: July 26, 2012 Published: July 26, 2012 2228

dx.doi.org/10.1021/jz3007029 | J. Phys. Chem. Lett. 2012, 3, 2228−2236

The Journal of Physical Chemistry Letters

Perspective

the orientation of the graphene domain can correlate with the in-plane orientation of the Cu grains. Figure 2 shows the

crystalline graphene” free from domain boundaries is an important and challenging issue. In this Perspective, we review the recent development of the understanding of the growth mechanism of graphene on Cu metals by introducing the observation and control of domain structures. Future directions for single-crystalline graphene and related issues with graphene growth are also discussed. Growth Mechanism on Cu. Graphene growth is widely carried out in the presence of Cu foil/film because Cu metal enables selective growth of single-layer graphene due to relatively low C solubility (0.001−0.008 wt % at 1084 °C) compared with that of other catalyst metals like Ni and Co (∼0.6 wt % for Ni at 1326 °C and ∼0.9 wt % for Co at 1320 °C).33 The low C solubility of Cu greatly limits the C atom diffusion into the bulk, thus simplifying the growth model described only at the Cu surface.30,33 On the other hand, Ni and Co films generally give nonuniform graphene films whose thicknesses range from single- to multilayers24−26 because the number of carbon atoms dissolved in the metal film is difficult to control due to their high C solubility and enhanced precipitation at grain boundaries. The growth mechanism of single-layer graphene on the Cu surface is illustrated in Figure 1. There are many surface

Figure 2. Structures of Cu foil (Alpha Aesar) used for graphene growth. Optical microscope image (a) and crystallographic orientation (b) taken after the CVD at 1000 °C. (c) Distribution of inclined angles from the Cu(100) plane. Adapted from ref 45 with permission.

characteristics of the Cu foil (Alpha Aesar, 99.8% purity) used for the graphene growth.45 The crystallinity and surface structure depend on the supplier, but here, we show the data of the Cu foil purchased from Alpha Aesar because this is frequently employed for the graphene CVD.30,38,39,43,45 The surface of the Cu foil shows lines that are caused by the metal rolling process during production (Figure 2a). Despite the rolling lines, graphene can grow in a carpet-like fashion, covering the Cu surface.46 The crystallographic orientation of the Cu foil measured by electron back scatter diffraction (EBSD) is displayed in Figure 2b. The Cu foil shows red shades corresponding to the Cu(100) plane, but the shades have various contrasts due to slightly inclined Cu grains (the inclined angle distributes from 0 to 10° (see Figure 2c)). The Cu(100) plane tends to appear after the high-temperature metal rolling process,47,48 although the (100) plane is not the closest packing plane. The estimated Cu grain size ranges from 10 to several 100 μm, and the rolling lines are not directly related with the EBSD data. We note that the Cu(100) plane has square lattice with four-fold symmetry that does not match with the six-fold symmetry of graphene, which will be discussed later. Observation of Graphene Domains. Cu foil has polycrystalline, multigrain structure with a number of grain boundaries. How about the graphene grown on Cu foil? Does the graphene have domain boundaries as well? Recent advances in the direct/ indirect observations of domain structures are depicted in Figure 3. We note that such experimental observation of the domain structure is a first step toward the domain boundaryfree graphene growth. Dark-field transmission electron microscopy (DF-TEM) was successfully applied to analyze the size and orientation of graphene domains.38,39 Electron diffraction spots from graphene domains having different orientations (Figure 3b) are converted to the real space (Figure 3a). Here, each color indicates the graphene domain with different orientation, and one can see that the domain size is smaller than several micrometers. The TEM is also applicable to atomic-scale imaging of the domain boundaries, but it sometimes suffers from contamination and damage during the graphene transfer to a TEM grid. The second method is low-energy electron microscopy (LEEM). Panels c and d−f of Figure 3 show the bright-field (BF) and dark-field (DF) LEEM images, respectively.45 This method does not require a graphene transfer process, thus

Figure 1. Schematic of single-layer graphene growth on a Cu surface. Black, red, and brown particles indicate C, H, and Cu atoms, respectively. Brown lines represent grain boundaries of Cu metal. See the text for details.

reactions and processes involved in the graphene growth: (1) catalytic decomposition of CH4 feedstock, which gives C atoms adsorbed on the Cu surface; (2) surface diffusion of the adsorbed C atoms. This diffusion is thermally activated and driven by a concentration gradient of C atoms; (3) nucleation of graphene domains; here, grain boundaries and the rough surface of Cu are known to stimulate the graphene nucleation;42,43 (4) growth of graphene domains, which probably accompanies edge reconstruction; (5) etching of graphene domains by H2 gas; this is the reverse reaction of the CH4 decomposition (1) and may be assisted by Cu surface; (6) coalescence of neighboring graphene domains into a continuous graphene film; and (7) during the CVD, evaporation of Cu atoms from the Cu surface also occurs because the growth temperature (1000−1080 °C) is close to the melting temperature of Cu (1085 °C); this is more severe in vacuum CVD (10−1000 mTor) compared with the ambient-pressure CVD, and sometimes it contaminates the growth chamber with evaporated Cu. Not only the presence of grain boundaries but also the crystallinity of the Cu catalyst is important in the catalytic graphene growth.42,44 This is because (i) the graphene nucleation is associated with the Cu’s grain boundary and (ii) 2229



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enables study of the growth dynamics,51 which was initially used for carbon nanotube growth.52 Methane flow is repeatedly switched from 13CH4 to 12CH4, and the spatial distribution of 13 C- and 12C-labeled graphene is analyzed using the Raman Gband. Panels j and k of Figure 3 highlight the distributions of 13 C and 12C domains, respectively. These images show that the graphene domains evolve laterally with time and that their size reaches approximately 5−10 μm. Microscopic methods, such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM), also offer information on the local orientation of graphene domains,46,53−57 but they are not suitable for the large-area statistical domain analysis.

One main strategy for the synthesis of large single-crystalline graphene is to make domains as large as possible.

Domain Structure and Size. One main strategy for the synthesis of large single-crystalline graphene is to make domains as large as possible. For such purpose, we need to reduce the number of nucleation sites and gradually increase the domain size without forming additional nuclei. The challenge is to reduce the number of nucleation sites to as low as possible, and ideally, one nucleation site on the whole Cu surface is desired. There are different nucleation sites on catalyst metals caused by grain boundaries, surface impurities, local defects, and surface roughness.58 Grain boundaries frequently cause the nucleation of graphene,33 probably because the carbon diffusion as well as carbon solubility is enhanced at the metal grain boundaries. A rough Cu surface also increases the density of graphene nuclei because electrochemical polishing is shown to be effective in reducing the density of the nuclei.43 We should note that not only the catalyst metal but also the CVD condition strongly influences the quality of graphene. To reduce the nucleation density, a very low CH4 concentration is favored because catalytic dissociation of CH4 occurs more frequently under high CH4 concentrations. To reduce the CH4 supply, vacuum CVD is easier for handling the growth, but at the same time, the vacuum condition at high temperature stimulates thermal evaporation of the Cu catalyst, making the Cu surface rough. The growth at ambient pressure also offers the benefit of scalability of the CVD setup suitable for industrial development. The lateral size of graphene domains can be analyzed by terminating the growth before covering the whole Cu surface. Interestingly, various structures of graphene domains/islands, such as four-lobed, hexagonal, square, and other fractal shapes, are observed on Cu foils depending on the growth conditions (temperature, pressure, CH4 concentration, and H2/CH4 ratio). Their size varies from several to 500 μm. Table 1 summarizes the recent results obtained on the Cu catalyst. In the vacuum CVD, four-lobed graphene structures frequently appear on Cu foil, as shown in Figure 4a.60 This four-lobed, four-foldsymmetric island is related to the Cu(100) square lattice. There is some discussion on the domain structure of these four-lobed islands; these islands are expected to be single-domain based on the symmetrical shape, but a LEEM study reveals that the four-

Figure 3. Domain structures of single-layer graphene grown on Cu foil. (a) DF-TEM image of the transferred graphene.38 Colors represent the graphene domains with different hexagon orientations, which are constructed from the electron diffraction (b). (c−f) BF- and DF-LEEM images of the as-grown graphene on Cu foil.45 Numbers, 1, 2, and 3 of the BF image (c) show different Cu grains, while DF images (d−f) indicate spatial distribution of graphene domains in each Cu grain. (g,h) Graphene domains imaged by POM using the surfaceadsorbed liquid crystal molecules and the schematic of the arranged molecules.50 (i−k) Raman mapping images of the G-band intensity of 13 C-rich (j) and 12C-rich (k) graphene areas, which are grown by the isotope-labeled CH4 feedstocks.51 (i) is the integrated intensity of (j) and (k). The spatial distribution of 13C- and 12C-graphene reflects the lateral development of graphene domains. Reprinted from refs 38, 45, and 50 with permission.

enabling us to measure the grain structure of Cu, as labeled with 1−3 in Figure 3c. The spatial distribution of the number of graphene layers can be determined from the electron reflectivity as a function of accelerating voltage in the BF-LEEM.49 The DF-LEEM images give information on the distribution of graphene domains by selecting the diffraction conditions. One can see from Figure 3d−f that many small graphene domains with different orientations are present even in one Cu grain. There are other methods to observe the graphene domains indirectly. Self-assembly of liquid crystal molecules on graphene is utilized to visualize the domain structure.50 The orientation of adsorbed molecules that follows the orientation of a graphene domain is easily observed by a polarized optical microscope (POM), as shown in Figure 3g,h. Changing the 13 C/12C isotope-labeled CH4 feedstock during the growth 2230



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Table 1. Shape and Size of Graphene Domains/Islands Observed on Cu Catalysts catalyst

pressure

temperature (°C)

shape

size (μm)

ref

Cu foil Cu foil Cu foil Cu foil Cu foil Cu foil Cu foil Cu foil Cu foil Cu/sapphire

160−460 mTorr UHVb 50 mTorr ambient ambient ambientc ambient ambient 200 mTorr ambient

985−1035 790−842 1035 1050 1000 1000 na 1045 1000 1075

four-lobed four-lobed hexagonal with dendric edges hexagon hexagon hexagon hexagon square hexagonal flower-shaped hexagon

∼30a ∼10 ∼500 ∼10 ∼3 ∼20 5−75 400 ∼100 ∼100

59 60 61 62 63 64 65 66 67 unpublished

a The increase of the island size is realized by the second-step CVD, but first-step CVD data are indicated in this table. bUltra high vacuum (actual pressure is not specified). cLow vacuum (below 600 mTorr) was also studied.

Figure 4. Graphene domains/islands appearing on Cu foils. (a) Four-lobed island (UHV) imaged by LEEM.60 (b) SEM image of hexagonal domains (ambient pressure) and (c) the Raman D-band mapping for the adjacent hexagonal domains.62 Note that the orientations of the three domains are different in (b). (d) Atomic models of hexagonal graphene domains having zigzag and armchair edges. (e) Square domains (ambient pressure) and (f) the size evolution with growth time.66 (g) Hexagonal domains with fractal edges (50 mTorr).61 (h) Flower-shaped domains with fractal edges (200 mTorr).67 Reprinted with permission from ref 62.

Figure 5. Dependence of the H2 partial pressure on the size and shape of graphene domains. CVD growth was performed at 1000 °C under ambient pressure. Adapted from ref 64.

lobed island shown in Figure 4a is polycrystalline with multiple domains.60 On the other hand, the ambient-pressure CVD tends to give hexagonal graphene structures, as seen in the SEM image shown in Figure 4b. These islands are confirmed to be singlecrystal domains from the electron beam diffraction.62 There are two possible edges in the hexagonal graphene domains, zigzag

and armchair edges, as indicated in Figure. 4d. Electron diffraction and Raman analysis verifies that the hexagonal domains have zigzag edges.62,65 It is quite interesting to observe the zigzag edge in the graphene domains because molecular orbital calculation suggests the presence of a nonbonding molecular orbital at the zigzag edges, which generally destabilizes the molecule.68 Preferential growth of the zigzag 2231



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On Cu(111), the epitaxial growth of graphene ([10]graphene// [101]Cu(111)) and the slight domain rotation by 7° are reported,53 while Cu(100) gives a variety of rotational angles of graphene domains.54−56 To grow high-quality graphene on a well-defined Cu crystal, we have developed the heteroepitaxial approach, as depicted in Figure 6a.28,45,74,75 Different catalyst metals, Cu,45,74−76 Co,28

edge has been discussed based on theoretical calculations incorporating the effects of the Cu surface,65,69 but the mechanism still needs more study. The zigzag edges are known to appear after metal-assisted anisotropic etching of graphene and hydrogen plasma etching.70−73 These etching results might be related to the observed selectivity of the zigzag edge in the ambient-pressure CVD, suggesting superior thermal stability of the zigzag edge compared to the armchair edge. We should note that all of these phenomena involve the reaction with hydrogen. Other domain structures, square and hexagonal with fractal edges, are reported (see Figure 4e,g,h).61,66,67 The square domain seems to correlate with the four-fold symmetry of Cu(100) of the Cu foil,66 and this might relate to interaction between the Cu surface and growing graphene domains. On the other hand, hexagonal domains are frequently observed on polycrystalline Cu foil by ambient-pressure CVD, suggesting weak Cu−graphene interaction at high temperature. Thus, it is difficult to understand the reason why square domains can grow on the Cu foil at the moment. The domain size increases with reaction time, reaching ∼400 μm only after 16 min of reaction (Figure 4f). Unique, fractal, or flower-like domains with distorted six-fold symmetry are mainly observed in the low-pressure CVD.61,67 This is attributed to the severe Cu evaporation that perturbs the development of highly faceted graphene domains. In addition to the CH4 concentration and the Cu crystallinity, the H2 concentration is very important in the graphene growth. Shown in Figure 5 is the effect of H2 partial pressure on the structure of graphene domains grown by ambient-pressure CVD.64 Not only the size but also the shape of the graphene domains changes considerably by changing the H2 concentration. This is ascribed to the multiple roles of H2 gas in CVD growth, cleaning and chemical reduction of the Cu surface, edge reconstruction, and etching of graphene domains, as well as removal of surface-adsorbed C atoms (see Figure 1). Raman mapping analysis of the two coalesced hexagonal domains (Figure 4c) shows a clear disorder-induced D-band at the boundary. This image strongly suggests a possibility that domain boundaries cannot be atomically connected. This may be due to the different hexagonal orientations in the neighboring domains. In the next section, we discuss the orientation of graphene hexagons with respect to the underlying Cu lattice.

Atomic connection of adjacent domains is another possible approach for the growth of large single-crystal graphene.

Figure 6. (a) Schematics of the CVD growth of graphene over Cu films deposited on sapphire c-plane substrates. (b) Crystallographic orientation of Cu on c-plane sapphire.45 (c) Relative orientations of graphene formed on the heteroepitaxial Cu(111) film.45 Orientations of graphene hexagons grown on Cu(100) (d) and Cu(111) lattices (e). DF-LEEM images of graphene/Cu(100) (f) and graphene/ Cu(111) (g) whose colors represent the orientation of graphene domains.74 Reprinted from ref 45 with permission.

Epitaxy in CVD Graphene. Atomic connection of adjacent domains is another possible approach for the growth of large single-crystal graphene. In this case, the orientation of graphene domains should be identical. Therefore, understanding the interplay between the atomic structure of the Cu lattice and the graphene domains is important in terms of epitaxial graphene growth. STM is used to study the single-layer graphene grown on single-crystalline Cu(111) and Cu(100), and moiré patterns are used to the determine the relative orientations of graphene.53−56 The STM studies show that the graphene has an epitaxial relationship with the underlying Cu lattice and that the graphene domain can extend over the Cu grain boundary.

Ni,77 Ru,35,78 and Ir,79 are epitaxially deposited on singlecrystalline substrates, such as sapphire and MgO, usually at high temperature. The crystallographic orientation of Cu on c-plane sapphire measured by EBSD (Figure 6b) clearly shows the preparation of a single-crystal Cu(111) film,45 with great contrast to the Cu foil (see Figure 2b). Shown in Figure 6c is the experimentally verified relative orientation of graphene on Cu(111)/sapphire.45 The heteroepitaxial approach offers much more practical means to grow graphene than the single-crystal 2232



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Another approach for large-area, single-crystalline graphene is to use a heteroepitaxial Cu(111) film as a template.45,74−76 In this case, the seamless connection of neighboring domains is necessary. Because Cu(111) and graphene have 4% lattice mismatch (Cu−Cu of Cu(111) and C−C bonds of graphene are 0.148 and 0.142 nm, respectively), the neighboring hexagons in two domains cannot meet exactly at the same position, although the directions of hexagons are the same for all of the domains. However, because the growth temperature is high and a Cu catalyst is present between the domains, there is still a chance to atomically connect the domain boundary on Cu(111). The dynamics of domain boundaries should be closely related with the growth kinetics of domains as well as the diffusion and chemical reactions occurring on the exposed Cu surface. In this epitaxial approach, the quantity of consumed Cu (a 0.5 μm thick film) is much lower than that of Cu foil (typically 25 μm), but we may need to recycle the singlecrystalline substrates, as demonstrated previously.77 In this Perspective, we mainly focus on the graphene domains, but other issues should be also addressed for future applications. One is wrinkle formation during the CVD growth, which is formed due to different thermal expansion coefficients of Cu and graphene.82 The wrinkles can also be formed during the transfer process, which involves polymer coating and removal processes.82 Because the wrinkles act as scattering sites and lead to inferior electrical conduction properties,83 wrinklefree growth can be another challenge in the graphene growth. Further development requires the metal-free, direct graphene growth on insulating substrates at low temperature. Highquality graphene is achieved on sapphire, but a growth temperature of higher than 1500 °C is necessarry.84 Therefore, low-temperature growth on insulating substrates will become a key technique in commercialization of graphene. For semiconductor applications, band gap opening is an essential issue while maintaining the high carrier mobility because single-layer graphene has no band gap.3,6,7 Due to this zero-gap nature, the graphene transistors have a drawback of high off current that gives a low on/off ratio and high energy consumption. Applying a vertical electric field to double-layer graphene is suggested to open the band gap with 0.2−0.3 eV.85 This requires selective growth of AB-stacked double-layer graphene. Although the CVD growth of double-layer graphene is demonstrated on Cu foil,86 it is not well-established, and the layer stacking is not so ordered like exfoliated graphene based on the Raman 2D band shape. The low C solubility of Cu is suitable for single-layer graphene growth, but it is not suitable for double-layer graphene growth. This is because once the first layer graphene covers the Cu surface, the second layer cannot grow in between the graphene−Cu interface. Graphene nanoribbons (GNRs) are also expected to open the band gap due to quantum confinement in the one-dimensional structure. Several methods based on the top-down approach using lithography and successive etching87,88 and the unzipping carbon nanotubes by chemical oxidation or plasma treatment89,90 are used to obtain GNRs. These methods, however, suffer from the damage to the nanoribbons especially at the ribbon edges, which strongly influences the electronic structure. Therefore, direct CVD growth of GNRs is required. Recently, we demonstrated the graphene nanoribbon formation along the steps of a metal catalyst,91 implying the preferential segregation at the step edges. Further study to reduce the GNR width to less than 10 nm as well as to control the edge structure is a future challenge.

Cu(111) substrate because the latter is expensive and the available size is limited so that applying the metal-etchingassisted transfer process is not realistic. Moreover, the heteroepitaxial approach allows us to define the orientation of graphene, either the zigzag or armchair direction, by the crystallographic orientation of the original substrate, which is difficult in exfoliated graphene flakes. The ambient-pressure CVD using Cu(111)/sapphire results in large hexagonal graphene domains (∼100 μm), and we find that the hexagonal domains have the same orientation with the underlying Cu(111) lattice (unpublished result, Table 1). We investigated the domain structure using the heteroepitaxial Cu(111) and Cu(100) films deposited on MgO(111) and MgO(100), respectively.74 After the ambient-pressure CVD at 1000 °C, the ex situ LEEM measurement was performed for the single-layer graphene grown on Cu(111) and Cu(100). The DF-LEEM images are displayed in Figure 6f,g, in which the domain orientation is highlighted with different colors. On Cu(100), the multidomain structure is observed with two main orientations rotated by 30°. These two orientations are depicted in Figure 6d, where the C−C bond is aligned parallel to one of the Cu−Cu bonds. The Raman Dband is weakly observed along the boundaries for the graphene transferred from Cu(100).74 In contrast, Cu(111) gives singlelayer graphene whose hexagon orientation is highly uniform, and the orientation is consistent with the Cu(111) lattice for areas over 1 mm2 (Figure 6g, scanned LEEM and LEED data are not shown here). This is due to the symmetry match of graphene and Cu(111), as shown in Figure 6e. The Raman Dband was not observed for the graphene from Cu(111) except for wrinkles. However, it is still unclear whether the adjacent graphene domains can connect atomically during the growth because the LEEM gives information on the domain structure but not with atomic resolution. Therefore, the future development of reliable and facile observation of graphene domain boundaries on the atomic scale is expected. One promising approach is high-resolution TEM imaging. For example, formation of a pentagon−heptagon pair at the boundary is reported.38,39 Such atomic-scale investigation allows us to study dynamic phenomena, such as migration of domain boundaries, as reported very recently.80 Direct observation of the growth dynamics of CVD graphene would give us an important clue for the domain boundary-free graphene growth. Future Challenges. How large of a domain size can we make? The domain size is limited by the nucleation density and the growth speed. Nucleation density can be reduced by removing grain boundaries, surface roughness, and impurities from the Cu catalyst. The evolution of a graphene domain needs a constant supply of C atoms adsorbed on the Cu surface, and these C atoms have to travel a long distance when the nuclei density is very low. The diffusion constant of surface-adsorbed C atoms can be increased by increasing the CVD temperature, but the relatively low melting temperature (1083 °C) becomes a bottleneck for the enhanced diffusion because the Cu evaporation makes the Cu surface rough and sometimes damages the resultant graphene domains. Also, as the domain size increases, more C atoms are necessary to keep the domain growing. Therefore, increasing of the CH4 concentration with the reaction time (or second-step growth) might be a rational approach to increase the domain size.59,81 However, the current domain size is still smaller than the wafer size, and further study is necessary. 2233



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ACKNOWLEDGMENTS This work was supported by the JSPS Funding Program for Next Generation World-Leading Researchers (NEXT Program).

Graphene is a leading material of layered materials that show unique physical properties. Further understanding and control of graphene growth as well as integration with other layered materials is intriguing and will open a door for a new world of layered artificial materials that are expected to develop in various fields.

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REFERENCES

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Further understanding and control of graphene growth as well as integration with other layered materials is intriguing and will open a door for a new world of layered artificial materials that are expected to develop in various fields.

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Perspective

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Hiroki Ago received his Ph.D. from Kyoto University in 1997. After spending one year and a half at the Cavendish Laboratory at Cambridge University, he became a researcher of AIST at Tsukuba. In 2003, he joined the Institute for Materials Chemistry and Engineering at Kyushu University as an associate professor. His current research focuses on the controlled growth of graphene and nanotubes for future carbon electronics. http://nano.cm.kyushu-u.ac.jp/ago. Yui Ogawa is a Ph.D. student at Kyushu University. She received her B.S. in material science and engineering in 2009 from Kumamoto University. Her current research interests are controlling the structure of nanocarbon materials, such as graphene and carbon nanotubes, and investigating their physical properties related with the structure. Masaharu Tsuji has been a professor at the Institute for Materials Chemistry and Engineering, Kyushu University in Japan since 2002. He received his Ph.D. in 1978 from Kyushu University. His lab is focused on the area of syntheses and application of nanomaterials, including new carbons and metallic nanoparticles. http://nano.cm. kyushu-u.ac.jp/tsuji/ Seigi Mizuno has been a professor at Kyushu University since 2011. He became a researcher at Hokkaido University in 1990 and moved to Kyushu University as an associate professor. His research focuses on surface structure determination using low-energy electron diffraction. http://www.mm.kyushu-u.ac.jp/lab_01/. Hiroki Hibino received his B.S. (1987) and M.S. (1989) in physics from the University of Tokyo and Ph.D. (2006) in Pure and Applied Physics from Waseda University. He joined NTT Basic Research Laboratories in 1989. He is currently Executive Manager of the Materials Research Laboratory and Group Leader of the Lowdimensional Nanomaterials Research Group at NTT Basic Research Laboratories. 2234



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