CHEMPHYSCHEM COMMUNICATIONS DOI: 10.1002/cphc.201402633
The Influence of Cu Lattices on the Structure and Electrical Properties of Graphene Domains during Low-Pressure Chemical Vapor Deposition Dae Woo Kim,[a] Seon Joon Kim,[a] Jae Sung Kim,[b] Minju Shin,[b] Gyu-Tae Kim,[b] and HeeTae Jung*[a] The influence of various Cu lattices on the texturing of graphene domains during low-pressure chemical vapor deposition was investigated in a large area. The results show that the sizes and shapes of graphene domains grown on Cu(111) substrates match well with those of the underlying Cu(111) domains, which seem to be quasi-single-crystalline. In contrast, on other Cu substrates such as (100) and more intermediate domains, graphene islands with poly-domains (ca. 85 %) are significantly nucleated, eventually merging into polycrystalline graphene. Within the overall channel-length range, graphene from a Cu foil shows a higher resistance compared to graphene from a Cu(111) domain, with the extracted average channel resistances being 34.51 W mm 1 for Cu(111) and 66.17 W mm 1 for the Cu foil. Owing to the fact that it enables the successive synthesis of monolayer graphene films with large surface areas on the order of several square meters, the chemical vapor deposition (CVD) method has opened new frontiers in the practical applications of graphene.[1a–c] CVD-grown monolayer graphene exhibits good electrical conductivity and mechanical stability during folding and stretching processes.[1c] Also, this material is highly optically transparent (98 % in monolayer sheets).[2] As a result, monolayer graphene films are destined to replace presently used transparent conducting films in flexible and foldable optoelectronic devices. However, the intense efforts that have been conducted in this field thus far have only enabled the development of monolayer graphene that have electronic properties below ideal values.[3a–c] The results of recent studies suggest that as is the case with other common two-dimensional crystals, the non-optimal properties are largely a consequence of the incomplete structures and morphologies of CVD-grown graphenes, including domain size and boundaries,[4a,b] ripples[5a,b] and impurities[6] .
[a] D. W. Kim, S. J. Kim, Prof. H.-T. Jung Department of Chemical and Biomolecular Eng. (BK-21 plus) Korea Advanced Institute of Science and Technology Daejeon 305-701 (Republic of Korea) E-mail: [email protected] [b] J. S. Kim, M. Shin, Prof. G.-T. Kim School of Electrical Engineering, Korea University Anam-Dong 5–1 Seongbuk-Gu, Seoul 136-713 (Republic of Korea) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201402633. An invited contribution to a Special Issue on Organic Electronics
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Thus, an important goal that remains elusive in this area is the development of practical methods for the synthesis of large single crystals of graphene that do not contain structural defects. Despite the great effort on this issue, only a limited amount of information has been revealed about the nature of polycrystalline graphene formation.[7a–h] Especially, the interaction between graphene domains and underlying Cu domains and its effects on the epitaxy growth of graphene are yet to be confirmed.[8a–d] The origin of current controversies appears to be a consequence of the fact that all previous investigations of the structure and morphology of graphene are focused on small and local areas. Importantly, a detailed comparison of various Cu domains has been only made on facets of (111), (100), and (010) because of the difficulties in preparing intermediate facets.[8a, 9] Furthermore, ultrahigh-vacuum growth studies widely reported in previous works may not precisely reflect the actual results under common conditions, which use low-pressure CVD. To resolve the questions that have arisen, studies need to be designed to assess graphene growth under common conditions and domain variations in various Cu domains with large areas. Here, we have utilized a combination of optical birefringence from aligned liquid crystals (LCs),[7f] electron backscattered scattering diffraction (EBSD), and transmission electron microscopy (TEM) with selected-area electron diffraction (SAED) to reveal the structures of crystalline graphene grown on Cu(111), Cu(100), and other intermediate Cu domains. The results demonstrate that the average crystalline orientation of the graphene domains is highly influenced by the underlying Cu domains. Especially, the domains in graphene match those of the underlying Cu(111) domains. In contrast, highly polycrystalline graphene domains aligned towards particular directions are observed in other Cu domains, such as (100), (101), and high-index facets. This phenomenon can be explained by the tendency that single-crystal graphene islands with a preferred direction easily grow on Cu(111), and graphene islands with multi-domains are commonly synthesized in other Cu domains. Thus, our observation can answer the reason why the growth of high-quality graphene has been achieved using Cu(111) in previous works.[10a,b] Moreover, in the overall range of channel length, graphene from a Cu foil shows a higher resistance compared to graphene from Cu(111) domains, with the extracted average channel resistances being 34.51 W mm 1 for Cu(111) and 66.17 W mm 1 for the Cu foil. Figure 1 a shows a typical scanning electron microscope (SEM) image of a Cu foil (ca. 25 mm thick, Goodfellow) after ChemPhysChem 0000, 00, 1 – 7
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Graphene films were transferred to a SiO2/Si substrate, and the graphene domains were visualized by observing aligned liquid crystal (LC) molecules on the graphene surface.[7f] Thermal annealing at 400 8C under Ar gas was conducted to remove residual poly(methyl methacrylate) (PMMA) and produce a relatively clean graphene surface. The common nematic LC, 4-pentyl4’-cyanobiphenyl: 5CB used for this purpose was spin-coated at 5000 rpm on the graphene surface. The polarized optical microscope (POM) image of the resulting surface, shown in Figure 1 c, was found to contain graphene regions that display both uniform and relatively non-uniform color regions. The results of a number of repeated observations reveal that the birefringent colors of the CVD-grown graphenes on Cu(111) or (111) containing domains (marked with blue dots, Figure 1 c) are relatively uniform as compared to those of the other Cu domains, suggestFigure 1. Influence of the Cu crystal structure on the domain structure of as-grown graphene on each Cu domain. ing that the order and orientaa) Typical scanning electron microscopy (SEM) image of a Cu foil after graphene growth using CH4 at 1030 8C. b) Electron-backscatter diffraction (EBSD) image of a Cu foil corresponding to (a). The inset is the EBSD legend. tion of graphene domains The EBSD data shows the underlying Cu crystal structure. While the size of the Cu domains is usually larger than almost exactly match those of several hundred micrometers, the Cu foil consists of randomly oriented Cu domains. c) Visualized domain structhe underlying Cu domains ture of graphene transferred from the Cu foil to a glass substrate. The graphene domains from Cu(111) or (111) [Cu(111: blue), Figure 1 b]. In containing facets are marked with blue dots. Visualization is achieved by observing the aligned liquid crystals (5CB) on the surface of graphene domains. d) Magnified images of graphene from Cu(100) and Cu(111) domains. contrast, the graphene surface Each domain is rotated counterclockwise by 408 for clear observation of the graphene domains. While graphene regions corresponding to other from Cu(111) shows homogeneous alignments of LCs, many small graphene domains are observed in graphene Cu domains, such as (101: green from Cu(100). in Figure 1 b), (100: red in Figure 1 b), other high-index facets and small Cu domains (Figure S2), display relatively non-unigrowth of a graphene monolayer at 1030 8C for 30 min using form colors when observed by using POM, indicating the presa mixture of 8 sccm H2 and 24 sccm CH4 (Figure S1, Supporting ence of polycrystalline-type graphene, possibly containing Information).[1b] Various Cu domains with relatively large sizes a number of small domains (Figures 1 b,c and S2). Magnified of several hundreds of mm were commonly observed after graPOM images of graphene grown on the Cu(100) and Cu(111) phene growth. The order and orientations of the underlying facets show that significant differences exist in the domain Cu crystal domains seen in Figure 1 a were identified by EBSD structures (Figure 1 d). By rotating the graphene specimens to (Figure 1 b). Previous studies have concluded that the Cu(111) induce changes in the birefringence color, the graphene dofacet is dominant in the crystallographic map owing to the mains on the Cu(111) facet appear to be composed of single fact that Cu surface energy is lowest at this facet.[11] In contrast crystals, which have large areas ranging up to several hunto these reports, the Cu foil used in our work is comprised of dreds of micrometers and a very uniform alignment of the corcrystallographically diverse Cu domain orientations composed responding LCs on the graphene surface. In contrast, graphene of Cu (100: red), Cu (101: green), Cu (111: blue) and various ingrown on the Cu(100) facet is comprised of many small pieces termediate domains. that have discretely different birefringence colors, indicating To explore the effects of the order and orientation of the Cu the existence of a number of small domains.[12] A similar bedomains on the properties of grown graphene, an investigation was carried out to determine if a correlation exists behavior (i.e. the formation of polycrystalline graphene) was also tween the domains of the Cu crystal and those of graphene. observed in other Cu domains (Figure S3). To guarantee the 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 2. Crystalline structure of graphene islands from various Cu domains. a,b) Visualized domain structure of graphene islands grown for 30 s at 1040 8C from Cu(100) and (111), respectively. The insets are optical images of graphene islands corresponding to the POM images. The shape of the graphene islands is visualized by oxidizing the underlying Cu at 160 8C for 5 min. Domain structures of graphene islands are obtained by observing aligned LCs on graphene islands through POM. Single-crystal graphene islands are observed on Cu(111); on the other hand, polycrystalline graphene islands are observed on Cu(100). c– e) The number of counts for graphene islands with single and multi domains on Cu(100), Cu(111), and Cu(111) (with polishing), respectively. Graphene islands grown for 30 s are counted by observing around 2500 graphene islands for each sample after coating the LCs.
validity of these results, the observations were performed repeatedly and the same results were obtained. The major conclusion drawn from these experiments is that the domain nature of graphene grown on the Cu(111) facet closely mimics the shape and size of the underlying Cu domain, and that little variation occurs at the Cu boundary (Figure S4). To determine if a relationship between the domain structure of Cu and the corresponding grown graphene exists at an early stage, following nucleation on Cu, the domain structures of monolayer graphene islands (Figure S5) grown for 30 s at 1030 8C on Cu(100) and Cu (111) were investigated by using the optical birefringence method. The insets in Figure 2 are optical images of graphene islands corresponding to the POM images (Figure 2 a,b), in which the bright and brown colors represent graphene and oxidized Cu regions, respectively. In fact, non-oxidized Cu regions, protected by CVD grown graphene, exhibit relatively bright colors as compared to those of oxidized Cu regions. Thus, to improve the contrast of the optical image between the graphene regions, Cu associated with the graphene islands was thermally oxidized at 160 8C for 5 min.[13] Viewing the improved images shows that graphene islands grown on the Cu(100) facet contain several branches of less than 10 mm domain sizes (inset of Figure 2 a). 5CB was then directly applied so that the domain structure of the graphene islands could be visualized. The results demonstrate that the domain structure of graphene islands grown on the 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Cu(100) facet displays several different colors, indicating the presence of various domain orientations of the LCs aligned with the graphene islands. The color variations (e.g. black and gray) corresponding to the LCs on the graphene islands indicate that the graphene domains in a graphene island are aligned in different directions. These observations indicate that the graphene islands grown on the Cu(100) facet are mostly composed of poly-domains with a distribution of different shapes and sizes (Figure 2 a). In addition, similar phenomena were observed when all the other Cu domains including highindex Cu facets except Cu(111) were probed (Figure S6). In contrast to these observations, a very uniform optical birefringent color is observed in POM images of graphene islands grown on the Cu(111) facet (Figure 2 b). Moreover, the density of graphene islands grown on this facet is higher than that on the Cu(100) facet, indicating that a preference exists for nucleation of graphene seeds on the Cu(111) facet (inset of Figure 2 b). After coating with LC, all the graphene islands display a yellow color, indicating that they have a homogenous single-crystalline nature over the entire region with a preferred growth direction. To quantitatively estimate the tendency toward domain order and orientation of graphene islands CVD-grown on each Cu surface, the birefringent color distribution of about 2500 graphene islands grown for 30 s on Cu(100) (Figure 2 c), Cu(111) (Figure 2 d), and electro-polished Cu(111) (Figure 2 e) ChemPhysChem 0000, 00, 1 – 7
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CHEMPHYSCHEM COMMUNICATIONS were determined. It is interesting that 85 % of the graphene islands grown on Cu(100) had a poly-domain nature, whereas 90 % of the graphene islands grown on Cu(111) were single crystalline. Additionally, because the minor amount (10 %) of poly-domain structure of the graphene islands on non-polished Cu(111) results from surface defects caused by the rough Cu surface[14] (Figure S7). The Cu(111) domain was electro-polished to transform the wavy surface to a flat one, (Figure S8) which increased the single-crystal content of graphene islands to 98 %. The same conclusion was further derived from TEM and SAED observations, see Figure 3. Graphene islands grown to the micrometer scale on a Cu foil and on Cu(111) for 30 s were transferred to a holey carbon TEM grid (Figures 3 a,c). SAED
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Figure 4. Schematic illustration of graphene growth depending on the Cu domains.
only a single-crystalline property and that they are oriented in the same direction. In contrast, graphene islands grown on normal polycrystalline Cu commonly show multiple hexagonal sets with a nearly 308 angular rotation, indicating the presence of poly-domains in a graphene island. The evolution of the crystal structure of graphene during CVD is illustrated in Figure 4. Considering the results above, it is likely that epitaxy between the nanoscale graphene domain after nucleation and the underlying Cu structure is highly important for the control of the preferential orientations of the graphene islands and for the reduction of the growth of poly-domains[15a–c] . On Cu(111), single-crystal graphene islands with a preferred growth orientation nucleate and merge into a graphene film during the growth period, and the merged graphene film resembles a quasi-single crystal. Since the size of the graphene islands is much smaller than that of the Cu(111) domain, the resulting graphene film mimics the shape of the Cu(111) domains. On the other hand, poly-crystalline graphene islands commonly grow on the other Cu Figure 3. TEM analysis of graphene islands from typical poly-Cu and Cu(111) domains. facets, resulting in the polycrystalline nature of the a,c) TEM image of graphene islands transferred on a TEM grid from a normal Cu foil and merged graphene film. However, the orientations of a Cu(111) domain, respectively. b,d) Selected-area electron diffraction (SAED) of the numbered area in (a) and (c). The blue circles in (a) and (c) mark the investigated regions by polycrystalline graphene domains are not totally ranSAED. The SAED patterns of the graphene islands from Cu(111) usually present single domly distributed, but also aligned along particular hexagonal dot sets without relative angular rotation, indicating not only the single-crysdirections depending on the Cu domains. This indital property of the graphene islands, but also the presence of oriented graphene docates that a dominant alignment still exists among mains in a particular direction. Instead, several hexagonal sets are observed in a graphene island from a normal Cu foil, indicating poly-domains. the random polycrystalline domains, which is the reason why polycrystalline textures with small domains are observed at high magnification, while the measurements were carried out in the regions marked with graphene domains resemble the Cu domains at low magnificaa blue line, as shown in Figures 3 a,c, which correspond to the tion, even on the other Cu domains. Whether the polycrystalnumbered regions shown in Figures 3 b and 3 d, respectively. line nature of these islands is introduced at the beginning of The SAED patterns of graphene islands on Cu(111) are usually nucleation is not clear, but it seems that polycrystalline gracomprised of a single hexagonal dot set that does not display phene islands begin to emerge at the early stages of growth, a significant relative angular rotation, and all the hexagonal rather than being distorted during lateral expansion. While the sets from the graphene islands are aligned with a similar orienexact mechanism, the driving forces of theses phenomena, tation. These observations indicate that graphene islands have and the strength of the interactions between the graphene is 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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CHEMPHYSCHEM COMMUNICATIONS lands and the Cu facets are not clear at this stage, the degree of strain, catalytic interactions, and lattice mismatches between the graphene islands and the Cu domain may can be important factors.[16a,b] To emphasize the importance of employing a flat Cu(111) domain in constructing large-area, high-performance, quasisingle-crystalline graphene films, the electronic properties of graphene films grown on various Cu substrates, including a polycrystalline Cu foil and single-crystalline Cu(111) with polishing, are shown in Figure 5. Since the Cu(111) single domains
Figure 5. a) EBSD image of a Cu(111) domain grown up to several millimeters during thermal annealing. b) POM image of graphene from large Cu(111) after LC coating. The POM image indicates that the graphene domain is highly oriented along a single direction, like a single crystal. c) Resistance variation of defined graphene channels from Cu(111) and a Cu foil as the channel length increases from 10 to 50 mm at a fixed width (25 mm). The graphene channels were defined by conventional photolithography. 20 devices were used for each length. The insets are optical images of defined graphene channels and fabricated Ti/Au electrodes. While the extracted resistances are higher than that of as-prepared graphene, possibly owing to damage or absorbed polymer during the photolithography procedure, obvious lower resistances are observed for Cu(111) compared to a normal Cu foil. The extracted channel resistance is 34.51 W mm 1 for Cu(111) and 66.17 W mm 1 for the Cu foil.
in commercial Cu foils can extend up to several centimeters when annealed for several hours at high temperatures (ca. 1000 8C) in a hydrogen atmosphere, the commercial Cu foils in this study were annealed for 5 h at 1030 8C in a hydrogen atmosphere.[17] Then, additional electropolishing was conducted to make the surface of Cu flat by removing the microscale grooves introduced during the manufacturing process of the Cu foil. EBSD mapping of extended Cu(111) in the annealed foil shows a highly uniform blue color over the entire large-area 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org without any Cu boundaries. Also, the inset of the inverse pole figure map shows that Cu(111) is dominant, indicating that a large-area single-crystalline Cu(111) domain is present (Figure 5 a). While graphene grown on a Cu foil usually displays a mosaic-like image when observed using LCs and POM, graphene grown on flat, single-crystalline Cu(111) displays a highly uniform birefringence color in a large region up to the millimeter scale depending on the size of the Cu(111) domain in Figure 5 b. These observations indicate that while it is difficult to consider the domain of graphene on Cu(111) as a perfect single-crystalline graphene, the orientation of the graphene domain is highly oriented along a single direction, like a quasi-single crystal. The electrical properties of the obtained large-area, quasisingle-crystal graphene was compared to that of graphene from a typical Cu foil in Figure 5 c. The insets of Figure 5 c show the optical image of a defined graphene channel between Ti(3 nm)/Au(4 nm) electrodes, obtained by conventional photolithography, where the channel length was increased from 10 to 50 mm in increments of 10 mm at a fixed width (20 mm). 20 devices were used for each length and the extracted resistance for each channel is marked with individual dots in Figure 5 c. All measurements were conducted at room temperature and in air. In the overall range of channel lengths, the graphene from the Cu foil shows a higher resistance compared to graphene from the Cu(111) domain, with the extracted average channel resistances being 34.51 W mm 1 for Cu(111) and 66.17 W mm 1 for the Cu foil. In the graphene channel with 10 mm length, the graphenes from both Cu(111) and the Cu foil show a similar resistance and distribution because both graphene channels do not have significant boundaries considering the domain size of polycrystalline graphene with about 10 mm. In contrast, as the channel length is increased, the graphene channels from the Cu foil show a broad distribution of channel resistances compared to the graphene channels from Cu(111) at each length. These higher resistances with broad distribution can be explained by the fact that the influence of the channel resistance from the graphene boundaries becomes more significant in graphene channels derived from a common Cu foil as the channel length increases. The results of the investigation described above demonstrate that the use of a Cu(111) plane with a flat surface is critical for producing high-quality graphene with ideal electronic properties. The observations suggest that epitaxy between the graphene islands after nucleation and the underlying Cu structure is responsible for the formation of the structures of the graphene domains. As the CVD process is conducted at about 1000 8C, graphene islands with single-crystalline structure become compactly aligned on the surface of the Cu(111) domains. In contrast, highly polycrystalline graphene domains aligned along particular directions are observed in other Cu domains, such as (100), (101) and high-index facets. The highly flat surface of Cu(111) not only inhibits the nucleation of polycrystalline graphene seeds, but also eliminates the rotational disorder of graphene islands during graphene growth at defect sites. We believe that this investigation has revealed the importance of the Cu grain structure in controlling graphene ChemPhysChem 0000, 00, 1 – 7
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CHEMPHYSCHEM COMMUNICATIONS domains during CVD and has provided insight into how highquality, single-crystalline graphene can be prepared.
Experimental Section Materials The Cu foil (99.9 % purity, 25 mm thick) was purchased from Goodfellow, London, England.
Synthesis and Transfer of Graphene Graphene was synthesized by the CVD method. In the first step, a Cu foil was inserted into a quartz tube, which was then heated to 1030 8C with a H2 flow at a rate of 8 sccm and a pressure of 90 mtorr. After reaching 1030 8C, the Cu foil was annealed for 3 hour at the same H2 flow rate and pressure. This resulted in an increase of the Cu grain size to about 400 mm and a reduction of the defect-like structure of Cu. Then, CH4 and H2 were flown through the tube at rates of 24 and 8 sccm, respectively, for 30 min each. Finally, the Cu foil was rapidly cooled to room temperature with a H2 flow at a pressure of 90 mtorr. After growth, PMMA was spin-coated on the graphene surface for generation of supporting layers as graphene was transferred to secondary substrates. The Cu foil was removed using FeCl3 as an aqueous etchant and the graphene/PMMA film was rinsed to remove any residual etchant and transferred to SiO2/Si. After drying, PMMA was removed using acetone. To remove any PMMA residues or other contaminants from the graphene surface, an additional thermal treatment was conducted at 400 8C under an Ar atmosphere.
Electropolishing of the Cu Foils Cu foils were electrically polished using an electro polisher (LcetroPol-5, Struers). The electrolyte used for electropolishing was prepared by mixing 250 mL phosphoric acid, 500 mL distilled water, 250 mL ethanol, 50 mL propanol, and 5 g urea.
Visualization of Graphene Domains by Optical Microscopy 4-Pentyl-4’-cyanobiphenyl (5CB) was purchased from Sigma Aldrich and typically spin-coated at 5000 rpm, resulting in a thin film of 5CB below 2 mm. Oxidization of Cu was conducted at 160 8C for 5 min under an air atmosphere.
Characterization The surfaces of the Cu domains and EBSD textures were visualized by field-emission scanning electron microscopy, FE-SEM (Nova230, FEI company). The textures of the LC molecules aligned on the surface of graphene were determined in situ using a POM (LV100POL, Nikon), equipped with a hot stage. Field-emission transmission electron microscopy (FE-TEM, Tecnai G2 F30) was applied to observe the SAED of graphene. After coating the PMMA supporting film on graphene/Cu foil, the Cu foil was removed using 1 m FeCl3 and the resulting graphene was rinsed with distilled water and then transferred to a holey carbon TEM grid (300 Mesh, purchased from Electron Microscopy Science). The PMMA layer was removed by immersing the TEM grid in acetone. Raman spectroscopy (514 nm excitation) (high-resolution dispersive Raman microscope, LabRAM HR UV/Vis/NIR) was applied to monitor the quality and thickness of the graphene. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org Acknowledgements This research was financially supported by a grant from the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (MSIP) (no. 2012R1A2A1A01003537), and by the Center for Advanced Soft Electronics funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project (2014M3A6A5060937). Keywords: Cu domains · chemical vapor deposition · epitaxy · graphene · single crystals
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CHEMPHYSCHEM COMMUNICATIONS [14] Y. Wu, Y. Hao, H. Y. Jeong, Z. Lee, S. Chen, W. Jiang, Q. Wu, R. D. Piner, J. Kang, R. S. Ruoff, Adv. Mater. 2013, 25, 6744 – 6751. [15] a) G. H. Han, F. Gunes, J. J. Bae, E. S. Kim, S. J. Chae, H.-J. Shin, J.-Y. Choi, D. Pribat, Y. H. Lee, Nano Lett. 2011, 11, 4144 – 4148; b) H. Hu, H. Ago, Y. Ito, K. Kawahara, M. Tsuji, E. Magome, K. Sumitani, N. Mizuta, K.-I. Ikeda, S. Mizuno, Carbon 2012, 50, 57 – 65; c) J. M. Wofford, E. Starodub, A. L. Walter, S. Nie, A. Bostwick, N. C. Bartelt, K. Thurmer, E. Rotenberg, K. F. McCarty, O. D. Dubon, New J. Phys. 2012, 14, 053008. [16] a) J. Cho, L. Gao, H. Cao, W. Wu, Q. Yu, E. N. Yitamben, B. Fisher, J. R. Guest, Y. P. Chen, N. P. Guisinger, ACS Nano 2011, 5, 3607 – 3613; b) X.
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www.chemphyschem.org Zhang, Z. Xu, L. Hui, J. Xin, F. Ding, J. Phys. Chem. Lett. 2012, 3, 2822 – 2827. [17] D. P. Field, L. T. Bradford, M. M. Nowell, T. M. Lillo, Acta Mater. 2007, 55, 4233 – 4241.
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COMMUNICATIONS D. W. Kim, S. J. Kim, J. S. Kim, M. Shin, G.-T. Kim, H.-T. Jung* && – && The Influence of Cu Lattices on the Structure and Electrical Properties of Graphene Domains during LowPressure Chemical Vapor Deposition
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The influence of various Cu lattices on the texturing of graphene domains during low-pressure chemical vapor deposition is investigated in a large area. While single-crystal graphene islands are formed on Cu(111), multidomain graphene islands nucleate on other Cu facets. The domains of graphene are highly aligned depending on the Cu domains.
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The Influence of Cu Lattices on the Structure and Electrical Properties of Graphene Domains during Low-Pressure Chemical Vapor Deposition Dae Woo Kim,[a] Seon Joon Kim,[a] Jae Sung Kim,[b] Minju Shin,[b] Gyu-Tae Kim,[b] and HeeTae Jung*[a] The influence of various Cu lattices on the texturing of graphene domains during low-pressure chemical vapor deposition was investigated in a large area. The results show that the sizes and shapes of graphene domains grown on Cu(111) substrates match well with those of the underlying Cu(111) domains, which seem to be quasi-single-crystalline. In contrast, on other Cu substrates such as (100) and more intermediate domains, graphene islands with poly-domains (ca. 85 %) are significantly nucleated, eventually merging into polycrystalline graphene. Within the overall channel-length range, graphene from a Cu foil shows a higher resistance compared to graphene from a Cu(111) domain, with the extracted average channel resistances being 34.51 W mm 1 for Cu(111) and 66.17 W mm 1 for the Cu foil. Owing to the fact that it enables the successive synthesis of monolayer graphene films with large surface areas on the order of several square meters, the chemical vapor deposition (CVD) method has opened new frontiers in the practical applications of graphene.[1a–c] CVD-grown monolayer graphene exhibits good electrical conductivity and mechanical stability during folding and stretching processes.[1c] Also, this material is highly optically transparent (98 % in monolayer sheets).[2] As a result, monolayer graphene films are destined to replace presently used transparent conducting films in flexible and foldable optoelectronic devices. However, the intense efforts that have been conducted in this field thus far have only enabled the development of monolayer graphene that have electronic properties below ideal values.[3a–c] The results of recent studies suggest that as is the case with other common two-dimensional crystals, the non-optimal properties are largely a consequence of the incomplete structures and morphologies of CVD-grown graphenes, including domain size and boundaries,[4a,b] ripples[5a,b] and impurities[6] .
[a] D. W. Kim, S. J. Kim, Prof. H.-T. Jung Department of Chemical and Biomolecular Eng. (BK-21 plus) Korea Advanced Institute of Science and Technology Daejeon 305-701 (Republic of Korea) E-mail: [email protected] [b] J. S. Kim, M. Shin, Prof. G.-T. Kim School of Electrical Engineering, Korea University Anam-Dong 5–1 Seongbuk-Gu, Seoul 136-713 (Republic of Korea) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201402633. An invited contribution to a Special Issue on Organic Electronics
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Thus, an important goal that remains elusive in this area is the development of practical methods for the synthesis of large single crystals of graphene that do not contain structural defects. Despite the great effort on this issue, only a limited amount of information has been revealed about the nature of polycrystalline graphene formation.[7a–h] Especially, the interaction between graphene domains and underlying Cu domains and its effects on the epitaxy growth of graphene are yet to be confirmed.[8a–d] The origin of current controversies appears to be a consequence of the fact that all previous investigations of the structure and morphology of graphene are focused on small and local areas. Importantly, a detailed comparison of various Cu domains has been only made on facets of (111), (100), and (010) because of the difficulties in preparing intermediate facets.[8a, 9] Furthermore, ultrahigh-vacuum growth studies widely reported in previous works may not precisely reflect the actual results under common conditions, which use low-pressure CVD. To resolve the questions that have arisen, studies need to be designed to assess graphene growth under common conditions and domain variations in various Cu domains with large areas. Here, we have utilized a combination of optical birefringence from aligned liquid crystals (LCs),[7f] electron backscattered scattering diffraction (EBSD), and transmission electron microscopy (TEM) with selected-area electron diffraction (SAED) to reveal the structures of crystalline graphene grown on Cu(111), Cu(100), and other intermediate Cu domains. The results demonstrate that the average crystalline orientation of the graphene domains is highly influenced by the underlying Cu domains. Especially, the domains in graphene match those of the underlying Cu(111) domains. In contrast, highly polycrystalline graphene domains aligned towards particular directions are observed in other Cu domains, such as (100), (101), and high-index facets. This phenomenon can be explained by the tendency that single-crystal graphene islands with a preferred direction easily grow on Cu(111), and graphene islands with multi-domains are commonly synthesized in other Cu domains. Thus, our observation can answer the reason why the growth of high-quality graphene has been achieved using Cu(111) in previous works.[10a,b] Moreover, in the overall range of channel length, graphene from a Cu foil shows a higher resistance compared to graphene from Cu(111) domains, with the extracted average channel resistances being 34.51 W mm 1 for Cu(111) and 66.17 W mm 1 for the Cu foil. Figure 1 a shows a typical scanning electron microscope (SEM) image of a Cu foil (ca. 25 mm thick, Goodfellow) after ChemPhysChem 0000, 00, 1 – 7
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Graphene films were transferred to a SiO2/Si substrate, and the graphene domains were visualized by observing aligned liquid crystal (LC) molecules on the graphene surface.[7f] Thermal annealing at 400 8C under Ar gas was conducted to remove residual poly(methyl methacrylate) (PMMA) and produce a relatively clean graphene surface. The common nematic LC, 4-pentyl4’-cyanobiphenyl: 5CB used for this purpose was spin-coated at 5000 rpm on the graphene surface. The polarized optical microscope (POM) image of the resulting surface, shown in Figure 1 c, was found to contain graphene regions that display both uniform and relatively non-uniform color regions. The results of a number of repeated observations reveal that the birefringent colors of the CVD-grown graphenes on Cu(111) or (111) containing domains (marked with blue dots, Figure 1 c) are relatively uniform as compared to those of the other Cu domains, suggestFigure 1. Influence of the Cu crystal structure on the domain structure of as-grown graphene on each Cu domain. ing that the order and orientaa) Typical scanning electron microscopy (SEM) image of a Cu foil after graphene growth using CH4 at 1030 8C. b) Electron-backscatter diffraction (EBSD) image of a Cu foil corresponding to (a). The inset is the EBSD legend. tion of graphene domains The EBSD data shows the underlying Cu crystal structure. While the size of the Cu domains is usually larger than almost exactly match those of several hundred micrometers, the Cu foil consists of randomly oriented Cu domains. c) Visualized domain structhe underlying Cu domains ture of graphene transferred from the Cu foil to a glass substrate. The graphene domains from Cu(111) or (111) [Cu(111: blue), Figure 1 b]. In containing facets are marked with blue dots. Visualization is achieved by observing the aligned liquid crystals (5CB) on the surface of graphene domains. d) Magnified images of graphene from Cu(100) and Cu(111) domains. contrast, the graphene surface Each domain is rotated counterclockwise by 408 for clear observation of the graphene domains. While graphene regions corresponding to other from Cu(111) shows homogeneous alignments of LCs, many small graphene domains are observed in graphene Cu domains, such as (101: green from Cu(100). in Figure 1 b), (100: red in Figure 1 b), other high-index facets and small Cu domains (Figure S2), display relatively non-unigrowth of a graphene monolayer at 1030 8C for 30 min using form colors when observed by using POM, indicating the presa mixture of 8 sccm H2 and 24 sccm CH4 (Figure S1, Supporting ence of polycrystalline-type graphene, possibly containing Information).[1b] Various Cu domains with relatively large sizes a number of small domains (Figures 1 b,c and S2). Magnified of several hundreds of mm were commonly observed after graPOM images of graphene grown on the Cu(100) and Cu(111) phene growth. The order and orientations of the underlying facets show that significant differences exist in the domain Cu crystal domains seen in Figure 1 a were identified by EBSD structures (Figure 1 d). By rotating the graphene specimens to (Figure 1 b). Previous studies have concluded that the Cu(111) induce changes in the birefringence color, the graphene dofacet is dominant in the crystallographic map owing to the mains on the Cu(111) facet appear to be composed of single fact that Cu surface energy is lowest at this facet.[11] In contrast crystals, which have large areas ranging up to several hunto these reports, the Cu foil used in our work is comprised of dreds of micrometers and a very uniform alignment of the corcrystallographically diverse Cu domain orientations composed responding LCs on the graphene surface. In contrast, graphene of Cu (100: red), Cu (101: green), Cu (111: blue) and various ingrown on the Cu(100) facet is comprised of many small pieces termediate domains. that have discretely different birefringence colors, indicating To explore the effects of the order and orientation of the Cu the existence of a number of small domains.[12] A similar bedomains on the properties of grown graphene, an investigation was carried out to determine if a correlation exists behavior (i.e. the formation of polycrystalline graphene) was also tween the domains of the Cu crystal and those of graphene. observed in other Cu domains (Figure S3). To guarantee the 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 2. Crystalline structure of graphene islands from various Cu domains. a,b) Visualized domain structure of graphene islands grown for 30 s at 1040 8C from Cu(100) and (111), respectively. The insets are optical images of graphene islands corresponding to the POM images. The shape of the graphene islands is visualized by oxidizing the underlying Cu at 160 8C for 5 min. Domain structures of graphene islands are obtained by observing aligned LCs on graphene islands through POM. Single-crystal graphene islands are observed on Cu(111); on the other hand, polycrystalline graphene islands are observed on Cu(100). c– e) The number of counts for graphene islands with single and multi domains on Cu(100), Cu(111), and Cu(111) (with polishing), respectively. Graphene islands grown for 30 s are counted by observing around 2500 graphene islands for each sample after coating the LCs.
validity of these results, the observations were performed repeatedly and the same results were obtained. The major conclusion drawn from these experiments is that the domain nature of graphene grown on the Cu(111) facet closely mimics the shape and size of the underlying Cu domain, and that little variation occurs at the Cu boundary (Figure S4). To determine if a relationship between the domain structure of Cu and the corresponding grown graphene exists at an early stage, following nucleation on Cu, the domain structures of monolayer graphene islands (Figure S5) grown for 30 s at 1030 8C on Cu(100) and Cu (111) were investigated by using the optical birefringence method. The insets in Figure 2 are optical images of graphene islands corresponding to the POM images (Figure 2 a,b), in which the bright and brown colors represent graphene and oxidized Cu regions, respectively. In fact, non-oxidized Cu regions, protected by CVD grown graphene, exhibit relatively bright colors as compared to those of oxidized Cu regions. Thus, to improve the contrast of the optical image between the graphene regions, Cu associated with the graphene islands was thermally oxidized at 160 8C for 5 min.[13] Viewing the improved images shows that graphene islands grown on the Cu(100) facet contain several branches of less than 10 mm domain sizes (inset of Figure 2 a). 5CB was then directly applied so that the domain structure of the graphene islands could be visualized. The results demonstrate that the domain structure of graphene islands grown on the 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Cu(100) facet displays several different colors, indicating the presence of various domain orientations of the LCs aligned with the graphene islands. The color variations (e.g. black and gray) corresponding to the LCs on the graphene islands indicate that the graphene domains in a graphene island are aligned in different directions. These observations indicate that the graphene islands grown on the Cu(100) facet are mostly composed of poly-domains with a distribution of different shapes and sizes (Figure 2 a). In addition, similar phenomena were observed when all the other Cu domains including highindex Cu facets except Cu(111) were probed (Figure S6). In contrast to these observations, a very uniform optical birefringent color is observed in POM images of graphene islands grown on the Cu(111) facet (Figure 2 b). Moreover, the density of graphene islands grown on this facet is higher than that on the Cu(100) facet, indicating that a preference exists for nucleation of graphene seeds on the Cu(111) facet (inset of Figure 2 b). After coating with LC, all the graphene islands display a yellow color, indicating that they have a homogenous single-crystalline nature over the entire region with a preferred growth direction. To quantitatively estimate the tendency toward domain order and orientation of graphene islands CVD-grown on each Cu surface, the birefringent color distribution of about 2500 graphene islands grown for 30 s on Cu(100) (Figure 2 c), Cu(111) (Figure 2 d), and electro-polished Cu(111) (Figure 2 e) ChemPhysChem 0000, 00, 1 – 7
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CHEMPHYSCHEM COMMUNICATIONS were determined. It is interesting that 85 % of the graphene islands grown on Cu(100) had a poly-domain nature, whereas 90 % of the graphene islands grown on Cu(111) were single crystalline. Additionally, because the minor amount (10 %) of poly-domain structure of the graphene islands on non-polished Cu(111) results from surface defects caused by the rough Cu surface[14] (Figure S7). The Cu(111) domain was electro-polished to transform the wavy surface to a flat one, (Figure S8) which increased the single-crystal content of graphene islands to 98 %. The same conclusion was further derived from TEM and SAED observations, see Figure 3. Graphene islands grown to the micrometer scale on a Cu foil and on Cu(111) for 30 s were transferred to a holey carbon TEM grid (Figures 3 a,c). SAED
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Figure 4. Schematic illustration of graphene growth depending on the Cu domains.
only a single-crystalline property and that they are oriented in the same direction. In contrast, graphene islands grown on normal polycrystalline Cu commonly show multiple hexagonal sets with a nearly 308 angular rotation, indicating the presence of poly-domains in a graphene island. The evolution of the crystal structure of graphene during CVD is illustrated in Figure 4. Considering the results above, it is likely that epitaxy between the nanoscale graphene domain after nucleation and the underlying Cu structure is highly important for the control of the preferential orientations of the graphene islands and for the reduction of the growth of poly-domains[15a–c] . On Cu(111), single-crystal graphene islands with a preferred growth orientation nucleate and merge into a graphene film during the growth period, and the merged graphene film resembles a quasi-single crystal. Since the size of the graphene islands is much smaller than that of the Cu(111) domain, the resulting graphene film mimics the shape of the Cu(111) domains. On the other hand, poly-crystalline graphene islands commonly grow on the other Cu Figure 3. TEM analysis of graphene islands from typical poly-Cu and Cu(111) domains. facets, resulting in the polycrystalline nature of the a,c) TEM image of graphene islands transferred on a TEM grid from a normal Cu foil and merged graphene film. However, the orientations of a Cu(111) domain, respectively. b,d) Selected-area electron diffraction (SAED) of the numbered area in (a) and (c). The blue circles in (a) and (c) mark the investigated regions by polycrystalline graphene domains are not totally ranSAED. The SAED patterns of the graphene islands from Cu(111) usually present single domly distributed, but also aligned along particular hexagonal dot sets without relative angular rotation, indicating not only the single-crysdirections depending on the Cu domains. This indital property of the graphene islands, but also the presence of oriented graphene docates that a dominant alignment still exists among mains in a particular direction. Instead, several hexagonal sets are observed in a graphene island from a normal Cu foil, indicating poly-domains. the random polycrystalline domains, which is the reason why polycrystalline textures with small domains are observed at high magnification, while the measurements were carried out in the regions marked with graphene domains resemble the Cu domains at low magnificaa blue line, as shown in Figures 3 a,c, which correspond to the tion, even on the other Cu domains. Whether the polycrystalnumbered regions shown in Figures 3 b and 3 d, respectively. line nature of these islands is introduced at the beginning of The SAED patterns of graphene islands on Cu(111) are usually nucleation is not clear, but it seems that polycrystalline gracomprised of a single hexagonal dot set that does not display phene islands begin to emerge at the early stages of growth, a significant relative angular rotation, and all the hexagonal rather than being distorted during lateral expansion. While the sets from the graphene islands are aligned with a similar orienexact mechanism, the driving forces of theses phenomena, tation. These observations indicate that graphene islands have and the strength of the interactions between the graphene is 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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CHEMPHYSCHEM COMMUNICATIONS lands and the Cu facets are not clear at this stage, the degree of strain, catalytic interactions, and lattice mismatches between the graphene islands and the Cu domain may can be important factors.[16a,b] To emphasize the importance of employing a flat Cu(111) domain in constructing large-area, high-performance, quasisingle-crystalline graphene films, the electronic properties of graphene films grown on various Cu substrates, including a polycrystalline Cu foil and single-crystalline Cu(111) with polishing, are shown in Figure 5. Since the Cu(111) single domains
Figure 5. a) EBSD image of a Cu(111) domain grown up to several millimeters during thermal annealing. b) POM image of graphene from large Cu(111) after LC coating. The POM image indicates that the graphene domain is highly oriented along a single direction, like a single crystal. c) Resistance variation of defined graphene channels from Cu(111) and a Cu foil as the channel length increases from 10 to 50 mm at a fixed width (25 mm). The graphene channels were defined by conventional photolithography. 20 devices were used for each length. The insets are optical images of defined graphene channels and fabricated Ti/Au electrodes. While the extracted resistances are higher than that of as-prepared graphene, possibly owing to damage or absorbed polymer during the photolithography procedure, obvious lower resistances are observed for Cu(111) compared to a normal Cu foil. The extracted channel resistance is 34.51 W mm 1 for Cu(111) and 66.17 W mm 1 for the Cu foil.
in commercial Cu foils can extend up to several centimeters when annealed for several hours at high temperatures (ca. 1000 8C) in a hydrogen atmosphere, the commercial Cu foils in this study were annealed for 5 h at 1030 8C in a hydrogen atmosphere.[17] Then, additional electropolishing was conducted to make the surface of Cu flat by removing the microscale grooves introduced during the manufacturing process of the Cu foil. EBSD mapping of extended Cu(111) in the annealed foil shows a highly uniform blue color over the entire large-area 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org without any Cu boundaries. Also, the inset of the inverse pole figure map shows that Cu(111) is dominant, indicating that a large-area single-crystalline Cu(111) domain is present (Figure 5 a). While graphene grown on a Cu foil usually displays a mosaic-like image when observed using LCs and POM, graphene grown on flat, single-crystalline Cu(111) displays a highly uniform birefringence color in a large region up to the millimeter scale depending on the size of the Cu(111) domain in Figure 5 b. These observations indicate that while it is difficult to consider the domain of graphene on Cu(111) as a perfect single-crystalline graphene, the orientation of the graphene domain is highly oriented along a single direction, like a quasi-single crystal. The electrical properties of the obtained large-area, quasisingle-crystal graphene was compared to that of graphene from a typical Cu foil in Figure 5 c. The insets of Figure 5 c show the optical image of a defined graphene channel between Ti(3 nm)/Au(4 nm) electrodes, obtained by conventional photolithography, where the channel length was increased from 10 to 50 mm in increments of 10 mm at a fixed width (20 mm). 20 devices were used for each length and the extracted resistance for each channel is marked with individual dots in Figure 5 c. All measurements were conducted at room temperature and in air. In the overall range of channel lengths, the graphene from the Cu foil shows a higher resistance compared to graphene from the Cu(111) domain, with the extracted average channel resistances being 34.51 W mm 1 for Cu(111) and 66.17 W mm 1 for the Cu foil. In the graphene channel with 10 mm length, the graphenes from both Cu(111) and the Cu foil show a similar resistance and distribution because both graphene channels do not have significant boundaries considering the domain size of polycrystalline graphene with about 10 mm. In contrast, as the channel length is increased, the graphene channels from the Cu foil show a broad distribution of channel resistances compared to the graphene channels from Cu(111) at each length. These higher resistances with broad distribution can be explained by the fact that the influence of the channel resistance from the graphene boundaries becomes more significant in graphene channels derived from a common Cu foil as the channel length increases. The results of the investigation described above demonstrate that the use of a Cu(111) plane with a flat surface is critical for producing high-quality graphene with ideal electronic properties. The observations suggest that epitaxy between the graphene islands after nucleation and the underlying Cu structure is responsible for the formation of the structures of the graphene domains. As the CVD process is conducted at about 1000 8C, graphene islands with single-crystalline structure become compactly aligned on the surface of the Cu(111) domains. In contrast, highly polycrystalline graphene domains aligned along particular directions are observed in other Cu domains, such as (100), (101) and high-index facets. The highly flat surface of Cu(111) not only inhibits the nucleation of polycrystalline graphene seeds, but also eliminates the rotational disorder of graphene islands during graphene growth at defect sites. We believe that this investigation has revealed the importance of the Cu grain structure in controlling graphene ChemPhysChem 0000, 00, 1 – 7
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CHEMPHYSCHEM COMMUNICATIONS domains during CVD and has provided insight into how highquality, single-crystalline graphene can be prepared.
Experimental Section Materials The Cu foil (99.9 % purity, 25 mm thick) was purchased from Goodfellow, London, England.
Synthesis and Transfer of Graphene Graphene was synthesized by the CVD method. In the first step, a Cu foil was inserted into a quartz tube, which was then heated to 1030 8C with a H2 flow at a rate of 8 sccm and a pressure of 90 mtorr. After reaching 1030 8C, the Cu foil was annealed for 3 hour at the same H2 flow rate and pressure. This resulted in an increase of the Cu grain size to about 400 mm and a reduction of the defect-like structure of Cu. Then, CH4 and H2 were flown through the tube at rates of 24 and 8 sccm, respectively, for 30 min each. Finally, the Cu foil was rapidly cooled to room temperature with a H2 flow at a pressure of 90 mtorr. After growth, PMMA was spin-coated on the graphene surface for generation of supporting layers as graphene was transferred to secondary substrates. The Cu foil was removed using FeCl3 as an aqueous etchant and the graphene/PMMA film was rinsed to remove any residual etchant and transferred to SiO2/Si. After drying, PMMA was removed using acetone. To remove any PMMA residues or other contaminants from the graphene surface, an additional thermal treatment was conducted at 400 8C under an Ar atmosphere.
Electropolishing of the Cu Foils Cu foils were electrically polished using an electro polisher (LcetroPol-5, Struers). The electrolyte used for electropolishing was prepared by mixing 250 mL phosphoric acid, 500 mL distilled water, 250 mL ethanol, 50 mL propanol, and 5 g urea.
Visualization of Graphene Domains by Optical Microscopy 4-Pentyl-4’-cyanobiphenyl (5CB) was purchased from Sigma Aldrich and typically spin-coated at 5000 rpm, resulting in a thin film of 5CB below 2 mm. Oxidization of Cu was conducted at 160 8C for 5 min under an air atmosphere.
Characterization The surfaces of the Cu domains and EBSD textures were visualized by field-emission scanning electron microscopy, FE-SEM (Nova230, FEI company). The textures of the LC molecules aligned on the surface of graphene were determined in situ using a POM (LV100POL, Nikon), equipped with a hot stage. Field-emission transmission electron microscopy (FE-TEM, Tecnai G2 F30) was applied to observe the SAED of graphene. After coating the PMMA supporting film on graphene/Cu foil, the Cu foil was removed using 1 m FeCl3 and the resulting graphene was rinsed with distilled water and then transferred to a holey carbon TEM grid (300 Mesh, purchased from Electron Microscopy Science). The PMMA layer was removed by immersing the TEM grid in acetone. Raman spectroscopy (514 nm excitation) (high-resolution dispersive Raman microscope, LabRAM HR UV/Vis/NIR) was applied to monitor the quality and thickness of the graphene. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org Acknowledgements This research was financially supported by a grant from the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (MSIP) (no. 2012R1A2A1A01003537), and by the Center for Advanced Soft Electronics funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project (2014M3A6A5060937). Keywords: Cu domains · chemical vapor deposition · epitaxy · graphene · single crystals
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Received: September 13, 2014 Published online on && &&, 0000
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COMMUNICATIONS D. W. Kim, S. J. Kim, J. S. Kim, M. Shin, G.-T. Kim, H.-T. Jung* && – && The Influence of Cu Lattices on the Structure and Electrical Properties of Graphene Domains during LowPressure Chemical Vapor Deposition
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
The influence of various Cu lattices on the texturing of graphene domains during low-pressure chemical vapor deposition is investigated in a large area. While single-crystal graphene islands are formed on Cu(111), multidomain graphene islands nucleate on other Cu facets. The domains of graphene are highly aligned depending on the Cu domains.
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