Growth Dynamics of Single-Layer Graphene on Epitaxial Cu Surfaces

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Chemistry of Materials

Growth Dynamics of Single-Layer Graphene on Epitaxial Cu Surfaces. Hiroki Ago,†,‡,§,* Yujiro Ohta,‡ Hiroki Hibino,|| Daisuke Yoshimura,┴ Rina Takizawa,‡ Yuki Uchida,‡ Masaharu Tsuji†,‡ Toshihiro Okajima,┴ Hisashi Mitani,¶ and Seigi Mizuno‡ †

Institute for Materials Chemistry and Engineering (IMCE), Kyushu University, Fukuoka 816-8580, Japan

‡

Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Fukuoka 816-8580, Japan

§

PRESTO, Japan Science and Technology Agency (JST), Saitama 332-0012, Japan NTT Basic Research Laboratories, NTT Corporation, Kanagawa 243-0198, and Kwansei Gakuin University, Hyogo 6691337, Japan ||

┴

Kyushu Synchrotron Light Research Center, Saga 841-0005, Japan

¶

Fukuoka University of Education, Fukuoka 811-4192, Japan

KEYWORDS: graphene, chemical vapor deposition, epitaxy, surface reaction, epitaxial Cu ABSTRACT: The growth of single-layer graphene on Cu metal by chemical vapor deposition (CVD) is a versatile method to synthesize high-quality, large-area graphene. It is known that high CVD temperatures, close to the Cu melting temperature (1083 ºC), are effective for the growth of large graphene domains, but the growth dynamics of graphene over the high-temperature Cu surface is not clearly understood. Here, we investigated the surface dynamics of the single-layer graphene growth by using heteroepitaxial Cu(111) and Cu(100) films. At relatively lower temperatures, 900~1030 ºC, the as-grown graphene showed the identical orientation with the underlying Cu(111) lattice. However, when the graphene was grown above 1040 ºC a new stable configuration of graphene with 3.4º-rotation became dominant. This slight rotation is interpreted by the enhanced graphene-Cu interaction due to the formation of long-range ordered structure. Further increase of the CVD temperature gave the graphene which is rotated with a wide angle distributions, suggesting the enhanced thermal fluctuation of the Cu lattice. The band structures of CVD graphene grown at different temperatures are well correlated with the observed structural change of the graphene. The strong impact of high CVD temperature on a Cu catalyst was further confirmed by the structural conversion of a Cu(100) film to Cu(111) which occurred during the high temperature CVD process. Our work presents important insight on the growth dynamics of CVD graphene, which can be developed to high quality graphene for future high-performance electronic and photonic devices.

■ INTRODUCTION Since the first preparation of graphene by mechanical exfoliation in 2004,1 there have been increased interest in singlelayer graphene and its related materials due to their unique and excellent electronic, optical, thermal, and mechanical properties.2-7 These properties promise electronic applications in many devices, such as field-effect transistors, integrated circuits, high-frequency transistors, sensors, actuators, transparent electrodes, and touch panels.7-14 The large-area synthesis of single-layer graphene can be now available by chemical vapor deposition (CVD) mainly on Cu catalyst.7,8 The CVD growth has been widely studied using Cu with different morphologies and crystallinity, such as Cu foils,7,8,15 polycrystalline Cu films,16 and Cu(111) films.17-21 The graphene CVD has been generally carried out at temperatures around 1000 ºC7,8,15-20, but recently higher temperatures above 1050 or 1070 ºC or even higher than the melting temperature of Cu (1083 ºC)21-25 is found to be preferable for the growth of large-

domain graphene. This is accounted for by the enhanced surface diffusion of C species on the Cu surface, which reduces the nucleation density of graphene and also increases the growth rate.23,26 However, such high growth temperatures induce unavoidable Cu evaporation due to the relatively low melting temperature of Cu metal. This is more severe for the low-pressure CVD, but even in the ambient pressure CVD the Cu evaporation occurs, and at least the surface lattice of the Cu metal is strongly suffered from the thermal fluctuation. It was reported that such high temperature graphene growth process can also bring surface reconstruction of Cu surface greatly affecting the surface morphology.28 Therefore, it is important and interesting to understand the mechanism of graphene growth on such dynamic Cu surface. In the previous work the epitaxial graphene growth on Cu(111) surface has been achieved, in which the hexagonal lattice of graphene is highly oriented in the same direction of the underlying Cu(111) lattice.17-19,29-32 Large hexagonal do-

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mains of graphene was also found to show the uniform orientation with their zigzag edges parallel to the [1 10]Cu direction.23 We note that it is surprising that the graphene lattice follows the Cu(111) lattice even on the thermally activated Cu surface which is accompanied with the step-terrace formation and Cu evaporation. It is also noted that several other orientations were reported on the Cu(111) surface, such as 7º,29 810º,33 and even random orientations,34 but the relationship with the growth condition is not clearly understood. Although the growth temperature is known to strongly influence the structure of graphene,26,35 little is known on the temperature dependence of graphene’s orientation. In addition, the growth dynamics on the Cu surface at the high temperatures close to the melting temperature is unclear. Here, we investigated the influence of the growth temperature on the orientation of the hexagonal lattice as well as the domain size and density of the single-layer graphene grown by ambient pressure CVD. We used epitaxial Cu(111) and Cu(100) films deposited on single-crystalline spinel(111) and spinel(100) substrates, respectively, as catalyst. Using the Cu(111) film, we present that the orientation hexagonal lattice of graphene is consistent with the orientation of the underlying Cu(111) lattice at relatively low temperatures. We found a new stable configuration of graphene for the temperature above 1040 ºC. Further increase of the CVD temperature induced the rotation of graphene lattice with wider angles. These results are discussed based on the domain size and growth rate of graphene as well as the surface dynamics of the Cu catalyst. Moreover, the Cu(111) film was found to reduce the nucleation density of graphene when compared with the previous works using Cu foils. Band structure of the graphene grown on Cu(111) was also investigated to elucidate the influence of the growth temperature. Finally, dramatic structure reconstruction of Cu(100) film deposited on spinel(100) is also demonstrated, indicating the strong impact of growth temperature in the CVD growth.

■ RESULTS AND DISCUSSION 1. Graphene growth and characterization of Cu catalysts. For the growth of single-layer graphene, we used a 500 nm-thick epitaxial Cu(111) film deposited on a singlecrystalline spinel(111) substrate (see Figure 1a inset for the structure). Here, spinel is used instead of sapphire, in order to avoid the formation of twins in the Cu(111) film.17 Also, the surface of spinel is more stable than that of MgO(111) in ambient condition, as the MgO surface is facile to be hydrized in air. The ambient pressure CVD was performed using the Cu(111)/spinel(111) substrate in a tubular furnace. After reducing and cleaning the Cu surface in H2/Ar flow at 1000 ºC, the furnace temperature was changed to a target temperature (900-1080 ºC) in the same H2/Ar flow. Then, diluted CH4 gas (10 and 40 ppm for the temperatures above 1010 ºC and below 1000 ºC, respectively) was introduced with 2.3% of H2. The growth time was set either 20 min or 60-90 min for the synthesis of isolated graphene domains or a continuous graphene sheet, respectively. The growth time (60-90 min) was adjusted to fully cover the Cu surface with graphene, because the growth rate is dependent on the growth temperature, as we discuss later. After the reaction with CH4, the substrate was rapidly taken out from the heating zone.

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Figure 1a,b shows x-ray diffraction (XRD) and electron backscatter diffraction (EBSD) data of the Cu catalyst. The XRD profile indicates that the thin Cu film has a highly crystallized face-centered cubic (fcc) structure whose (111) plane is normal the substrate surface. The uniform blue contrast of the crystallographic EBSD image (Figure 1b) proves that the Cu(111) plane is highly uniform and it is completely free from a twin structure. In addition, the crystallographic orientation in the lateral direction of the Cu(111) was confirmed to be uniform (see Figure S1 of Supporting Information for additional EBSD images). These results support the formation of single-crystalline Cu(111) thin film on the spinel(111) substrate. The CVD graphene grown at 1070 ºC was transferred onto Si wafer with 300 nm-thick SiO2 layer to check the quality of the graphene. Raman spectra taken at three different points are shown in Figure 1c. The spectra present clear 2Dand G-bands with relative intensity ratios (I2D/IG ratio) of ~2.02.5. The ratio indicates the growth of uniform single-layer graphene.36 In addition, D-band which is related to structural defects and graphene edges was almost negligible, indicating the growth of high-quality graphene. The morphological change of the Cu catalyst was studied by atomic force microscope (AFM). The surfaces of assputtered Cu (Figure 1d), and those after H2 annealing up to 1000 ºC (Figure 1e) and 1070 ºC (Figure 1f) were studied. The as-sputtered Cu showed a rather rough surface with many small pores (Figure 1d). The surface roughness, root-meansquare (RMS), is estimated to be 4.42 nm from the AFM image. This rough and porous structure was greatly improved by the H2 annealing, as can be seen in Figure 1e,f. The RMS is significantly reduced after heating to 1000 ºC (1.02 nm) and 1070 ºC (0.76 nm) in a H2/Ar mixed flow. These surfaces correspond to the Cu surface just before introducing CH4 gas at each growth temperature. The higher temperature was found to give a flatter surface, indicating the surface reconstruction of the Cu film during H2 annealing process. Thus, in the CVD process, the Cu atoms at least those located on the surface are rearranged to form a denser film with atomically flat surface. Furthermore, in Figure 1e,f we can see atomic steps oriented in three directions. The Fourier transformation image (Figure 1f inset) clearly shows the three-fold symmetry of the Cu(111) surface. These steps are related with the crystallographic directions of the fcc(111) plane which has threefold surface symmetry.37 2. Orientation of graphene lattice. The orientation of CVD graphene grown on the epitaxial Cu(111) by ambient pressure CVD was investigated by low-energy electron diffraction (LEED) measurements. Here, an electron beam with a ~1 mm spot size was used for the uniform single-layer graphene covering the whole Cu surface. We found that the Cu surface needs to be fully covered with graphene in order to obtain clear LEED patterns, because the electrically conducting substrate, in our case metallic Cu surface, is necessary for the LEED measurement (uncovered Cu surface can be easily oxidized during the transfer to the LEED chamber). Therefore, the long CVD time, 60-90 min, was used to make fully covered graphene sheet on Cu surface. As shown in Figure 2a-d, we found that the orientation of graphene considerably changes with the growth temperature. More detailed temperature dependence of LEED patterns is presented in Figure S2. At relatively low temperatures, 1010 and 1030 ºC (Figure 2a,b)


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the orientation of CVD graphene well matched with the Cu(111) lattice. The inset of Figure 2e shows slightly overlapping two diffraction spots originated from Cu(111) and graphene. This structure is consistent with our previous work for the single-layer graphene grown on Cu(111) at 1000 ºC.17,19,37 This structure has been also reported by other groups.29-32 The atomic structure of the graphene lattice on Cu(111) lattice is illustrated in Figure 2e. Due to 4% mismatch of the lattice constants of graphene (0.246 nm) and Cu (0.256 nm), a moiré pattern with long periodicity (6.6 nm) was observed. We further studied the lower temperatures, 9001000 ºC. To grow graphene at these temperatures, we increased the CH4 concentration from 10 ppm to 40 ppm, to compensate the reduced catalytic activity of the Cu at lower temperatures. As shown in Figure S3, clear six diffraction spots were also observed even at 900-1000 ºC. Thus, under the wide low temperature range, 900-1030 ºC, our as-grown graphene has the hexagonal graphene lattice whose orientation is perfectly consistent with that of the underlying Cu(111) lattice. This infers the epitaxy between graphene and the Cu(111). Interestingly, when we increased the growth temperature up to 1040 ºC or 1050 ºC, a new diffraction pattern appeared, as seen in Figures 2c and S2. From the magnified image of the LEED pattern (Figure 2f inset) we see that the diffraction spots from graphene are slightly rotated against those of the Cu(111) lattice. In addition, several new satellite spots were also seen, which are originated in a moiré pattern. We analyzed this moiré pattern by simulating the diffraction patterns including both single and double electron diffractions. The result of the LEED pattern simulation is depicted in Figure S4. From the analysis, we conclude that the most plausible structure is a (14×14) structure which is rotated from the Cu(111) lattice by 3.4º. We note that this super-structure has never been reported so far, thus indicating a new stable orientation appeared only at the high temperature growth condition. The corresponding atomic structure with the 3.4º-rotation is illustrated in Figure 2f. In the real space structure, it is seen that the graphene grown at 1050 ºC has denser moiré pattern than that grown at 1010/1030 ºC (see Figure 2e). The energy gain due to the orbital interaction between C atoms of graphene and the surface Cu atoms is enhanced by rotating the graphene hexagonal lattice, resulting in the more energetically stable configuration. There are two possible reasons for the long-range periodicity with 3.4º-rotational angle observed for the graphene grown above 1040 ºC. One reason is the different domain size which is strongly dependent on the growth temperature. As discussed later, the average domain size of graphene increases with increasing the growth temperature. For example, the graphene grown at 1050 ºC is more than three times larger than that of 1010 ºC, meaning that the domain area is more than nine times bigger. This also indicates that the growth rate is higher for the higher CVD temperature. It is highly likely that the orientation of graphene domain is determined at the initial growth stage when the domain size is rather small. The difference in the domain size (or growth rate) at the initial stage might significantly influence the domain orientation considering the unit size of moiré pattern. In addition, the rotation angle of 3.4º is relatively small. Thus, the higher growth temperatures increase the contact area between the graphene and Cu lattice at the initial growth stage, which helps

to have a more energetically stable, 3.4º-rotated, orientation (see Figure 2f) compared with the non-rotated configuration (Figure 2e). Another reason is thermal fluctuation of the Cu lattice due to high-temperature CVD, which is close to the melting temperature of Cu (1083 ºC). As can be seen in the AFM images (Figure 1e,f), the surface flatness of the Cu film improves with increasing the temperature. This indicates that Cu atoms on the surface move dynamically during the CVD process.28 Such enhanced lattice vibration is believed to assist a small graphene nucleus formed in the very early growth stage to orient in the more stable configuration. We think that both the reasons, increasing the growth rate of graphene and thermal fluctuation-assisted reorientation, explains the observed 3.4ºrotated structure seen in Figure 2c. On the other hand, at relatively low temperatures, 9001030 ºC, the domain size is small (low growth rate) and the Cu surface is more static. Therefore, the graphene tends to have the sub-stable structure (Figure 2a,b) which is the locally stable configuration in a very small scale like 1 nm. The moiré pattern seen in the LEED of the 1050 ºC sample (Figure 2c) was also observed in the LEED of the graphene grown at 1060 and 1070 ºC (Figures 2d and S2). However, with increasing the growth temperature, diffractions from other orientations appeared in the LEED patterns. In addition, at 1070 ºC, the moiré pattern became slightly unclear. These results infer that the too high temperature close to the melting temperature of Cu gives partially disordered orientation, most likely due to enhanced lattice fluctuation and surface melting effect of the Cu catalyst. 3. Domain structures of graphene. Next, we investigated the shape, size, and density of domains of single-layer graphene grown on Cu(111). Here, short reaction time, 20 min, with 10 ppm CH4 was used to visualize the graphene domain structure. Figure 3a-h shows the scanning electron microscope (SEM) images of graphene domains grown on the Cu surface. It is seen that the size of graphene domains increases with increasing the growth temperature. In addition, distorted domains at lower temperatures (1010-1040 ºC) changed into hexagonal shape at higher temperatures (1050-1080 ºC). In particular, at 1070 and 1080 ºC, the large and well-faceted hexagonal graphene domains whose lateral sizes are 50-70 µm were obtained (see Figure 3g,h). The well-faceted structure of graphene domains seen at high temperatures can be explained by two effects. One is enhanced surface diffusion of carbon intermediates (CHx (x=0-3)) which enables the isotropic development of graphene domains. At the same time, hydrogen etching occurs which assists the zigzag edge formation. Thus, combining these effects, well-faceted and hexagonal domains are created at the high growth temperatures. Moreover, these hexagonal domains are likely to orient in the almost the same direction, which is defined by the underlying Cu(111) lattice. The zigzag direction of graphene is aligned parallel to the [1 10] direction of Cu(111).23 This is consistent with the LEED data, but it is difficult to distinguish 0º- and 3.4º-rotations from the SEM images. It is noted that a thin Cu film on spinel(111) sometimes disappeared when the crystallinity of sputtered Cu was low due to evaporation during the CVD at 1080 ºC, since this temperature is very close to the melting temperature of Cu (1083 ºC). Thus, 1080 ºC was the critical temperature for our



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ambient pressure CVD, and this result also suggests the graphene growth occurs in dynamic environment accompanying with the Cu evaporation. Holes seen in the graphene domains grown at 1080 ºC (Figure 3h) might be originated in unintentional oxidation after terminating CH4 gas flow. The temperature dependences of the domain size and density are plotted in Figure 3i,j. It is noteworthy that our domain density is much lower than the previously reported density observed on Cu foils. At 1030 ºC, our sample possesses ~1,600 nuclei per 1 mm2 (Figure 3j). On the contrary, in the previous works, the densities of ~500,000 nuclei (1000 ºC)35 and ~100,000 nuclei (1025 ºC) 38 per 1 mm2 were reported for the ambient pressure CVD using polycrystalline Cu foils. The observed reduction of the nucleation density can be ascribed to the growth condition and the quality of the Cu catalyst. Compared with conventional Cu foils there are two advantages in our epitaxial Cu(111) film. As seen in Figures 1a,b and S1, the Cu(111) has high crystallinity while Cu foil is polycrystalline with a number of grain boundaries.19 The other advantage is that the surface of the Cu(111) film is highly flat (see Figure 1e,f) which also suppresses the unwanted graphene nucleation, because the surface roughness or surface impurities are known to act as nucleation sites. In addition, the different CVD condition, such as CH4 concentration and flow rate, may also have contributed to the observed reduced nucleation density. Both plots (Figure 3i,j) are well fitted by Arrhenius plots, and we obtained the similar activation energies (Ea) for the domain size (3.8 eV) and density (3.9 eV). In the CVD growth on Cu, several chemical reactions and surface processes are involved; (i) chemisorption of hydrocarbon feedstock (CH4) via dissociative adsorption, (ii) diffusion of carbon intermediate CHx (x=0-3) on Cu surface, (iii) graphene nucleation which occurs when the C concentration exceeds the critical density, (iv) development of the graphene domains by attaching additional carbon atoms which is accompanied by dissociation of C-H bonds in the CHx species, and finally (v) coalescence of the graphene domains to form a uniform sheet.35,38,39 At the same time, hydrogen-assisted etching38 and Cu evaporation also occurs. The observed Ea value (~4 eV) should relate at least one of the processes listed above. The process (i) is not likely a rate-limiting step, because the surface coverage of graphene did not increase rapidly. The coverage increases from ca. 60% to ca. 70-80% when the temperature was increased from 1010 to 1080 ºC (see Figure 3a-h). In the previous work, the barrier for the dissociative adsorption of CH4 was estimated to below 2 eV,40,41 which is lower than our experimental Ea. The surface diffusion (ii) can be also ruled out because the Ea for the diffusion of carbon intermediate on Cu was only less than 1 eV.42 In addition, as can be seen in Figure 3, since the distance between the neighboring domains becomes shorter with temperature, the diffusion length required to grow graphene becomes shorter with increasing the CVD temperature. As we are focusing on the isolated domains, steps (v) is not related to the obtained activation energy. Therefore, we conclude that processes (iii) and (iv), nucleation and development of graphene domains, are the essential step of the graphene growth which determined the observed Ea of ~4 eV. Since the domain size is mainly related to (iv) while the density relates to (iii), it might be a coincidence that we observed the similar values for both the processes. However, both have the dissociation process of C-H bonds to attach a

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domain or a very small nucleus, which we think the most important in the graphene growth. In our previous work of single-walled carbon nanotubes, we reported Cu metal has much lower catalytic activity than Fe and Co toward the nanotube growth.43 Also, Cu required the highest CH4 concentration to grow nanotubes among Fe, Co, Ni, and Cu.43 These previous results strongly suggest that the catalytic activity for the dissociation of C-H bonds of CH4 or CHx is weak for Cu metal. Kim et al. estimated the activation energy of 3 eV from the nucleation density for the temperature range of 900-1000 ºC in low pressure CVD.26 They also claimed that the growth limiting factor is the attachment of carbon at the growing front of a nucleus. Their claim agrees with our conclusion. Vlassiouk et al. reported the activation energies for the domain size (5 eV) and density (9 eV) by ambient pressure CVD in the temperature range of 900-1080 ºC.35 Their activation energies are higher than the present data, and this indicates that our ambient pressure CVD on the Cu(111) film is good to reduce the activation energy for high productivity. For the graphene domains grown at 1070 ºC, the slightly rotated structure was confirmed by the low-energy electron microscopy (LEEM) measurements. Figure 4 shows a brightfield LEEM image and the selected-area electron diffraction patterns. In the merged graphene domains which looks to have the same angle (Figure 4a), the orientation was slightly different with misorientation angle of 3º, as determined by the LEED patterns taken at two positions, A and B. From the comparison with the LEED pattern from the bare Cu lattice (Figure 1d,e), the domain A has the orientation consistent with the Cu lattice, while the domain B has 3º-rotation against the Cu. This is consistent with the previous LEED data (see Figure 2d), and it suggests that a careful analysis is necessary for the determining the domain orientation even for the faceted hexagonal domains. 4. Band structure of CVD graphene. The band structures of CVD graphene grown at different temperature were measured by angle-resolved photoelectron spectroscopy (ARPES). The band structures measured along the Γ-K direction are depicted in Figure 5. The graphene grown at 1000 ºC showed a clear but relatively thick linear band dispersion reaching to the Fermi level, representing the graphene’s band structure. This is a stark contrast with CVD-grown polycrystalline graphene which does not show clear ARPES due to multiple rotation of each domain. The relatively thick band of the graphene grown at 1000 ºC can be originated from a number of small graphene domains in the measured area which have almost the same hexagon orientation. On the other hand, the sample grown at 1070 ºC has a limited number of graphene domains but they are slightly rotated, as we discussed above. Therefore, the several weak band dispersions are seen in Figure 5b. These results indicate that the domain structure and orientation of graphene strongly influence its band structure, and it is important to control the domain structure for obtaining the single-crystalline electronic structure.32 As already reported, the clear orbital mixing of graphene π-band and Cu 3d-band was not seen in Figure 5a,b, indicating very weak electronic coupling between graphene and the Cu film.23 We think that such very weak interaction causes the


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temperature-dependent variation in the orientation of graphene hexagonal lattice, as discussed above (see Figure 2e,f). The interface structure between two merged graphene domains is a very important issue, because the seamless connection of the merged domains without defects can potentially result in a large sheet of single-crystalline graphene. In particular, in our 1000 ºC sample, the orientation of graphene hexagonal lattice is highly controlled and there can be a chance to connect without formation of defects. There are different discussion on the boundary formation at interface of hexagonal domains merged with the same angle.44-46 This is out of the scope of this paper, and more studies are necessary to reach solid conclusion for this sensitive issue. 5. Graphene growth on epitaxial Cu(100) films. It is interesting to compare the graphene growth on the epitaxial Cu(100) with the Cu(111), since the domain orientation of graphene is strongly dependent on the Cu lattice.20,37 We measured the XRD and LEED for the as-sputtered Cu film on a spinel(100) substrate and that after the CVD at 1000 and 1070 ºC. The as-sputtered Cu film (black line in Figure 6a) showed an intense fcc(200) peak, confirming the formation of epitaxial Cu(100) film on the spinel(100). However, after the CVD at 1000 ºC, the fcc(111) peak appeared together with the original fcc(200) peak (blue in Figure 6a). Further increase of the CVD temperature to 1070 ºC completely suppressed the fcc(200) peak and made the fcc(111) peak stronger (red line). These XRD patterns demonstrate that the as-sputtered Cu(100) film drastically changed its structure to Cu(111) during the CVD process. This suggests that the Cu(111) is more energetically stable than Cu(100) and that the Cu lattice dynamically reorganizes during the CVD process. It is surprising that such phase change occurs even for the thin, 500 nm-thick Cu(100) film directly supported on the spinel(100) substrate. Reflecting the structural change of the Cu film, the LEED pattern drastically changed when the growth temperature was increased from 1000 ºC to 1070 ºC, as seen in Figures 6b and 6c. At 1000 ºC, clear four diffraction spots originated from the Cu(100) surface were obtained, which are marked by pink circles in Figure 6b. We observed that the graphene has two main orientations rotated by 30º, being in consistent with our previous work.37,47 This can be accounted for by the mismatch of the lattice symmetry between graphene (six-fold symmetry) and Cu(100) (four-fold). In both orientations, a C-C bond of graphene is aligned parallel to one of Cu-Cu bonds, but due to the symmetry mismatch the two energetically equivalent orientations are observed on the Cu(100) surface.38 In the present case, both graphene domains are slightly rotated from the original Cu(100) lattice as seen in Figure 6b. This may be related to the distorted Cu(100) lattice as seen in the XRD pattern which shows both Cu(100) and Cu(111) peaks. On the other hand, when the CVD temperature was elevated to 1070 ºC, blur six diffraction spots were observed as marked by yellow circles in Figure 6c. In addition, broad and weak diffractions (orange) were also seen. Instead, the square spots originating in the Cu(100) disappeared. The appearance of six diffraction patterns is in agreement with the XRD profile of the Cu film which indicates the structural conversion from Cu(100) to Cu(111) during the CVD at 1070 ºC. The XRD profile (Figure 6a) also signifies that not only the surface of the Cu but also the bulk of the film is transformed to fcc(111). The blur six spots (marked by the yellow circles) as

well as the broad and weak diffraction patterns (orange) indicate that the converted Cu(111) film on spinel(100) is less ordered than that directly deposited on spinel(111) substrates so that the orientation of graphene is less controlled. It is interesting how graphene grows while reconstructing Cu lattice. We note that uniform single-layer graphene was obtained on Cu/spinel(100) after the CVD at 1070 ºC. In addition, the shape of graphene domains is mainly related to the growth condition and not to the underlying Cu lattice, as the hexagonal shapes are widely observed on Cu foils21,32,35,37,39, and Cu(111), (110), (100) films.20,23 Therefore, it can be considered that the graphene growth process on the reconstructing Cu film is similar to that on melting Cu, where hexagonal graphene domains float on the melting Cu surface.24,25

■ CONCLUSIONS Dynamics of single-layer graphene growth on Cu catalyst is presented by using heteroepitaxial Cu(111) and Cu(100) films. On the Cu(111) catalyst, a new orientation of graphene is revealed which is rotated by 3.4º against the Cu(111) lattice when the CVD temperature is higher than 1040 ºC. On the other hand, the growth at the relatively lower temperatures, 900-1030 ºC, gave the graphene whose orientation is well matched with that of Cu(111) lattice. Further increase of the growth temperature above 1070 ºC induced different misoriented graphene formation. These results are explained in terms of the larger domain size of graphene and thermal fluctuation of the Cu lattice at elevated temperatures. Furthermore, drastic change of the Cu lattice was seen for the Cu film deposited on spinel(100) substrates, which shows the phase transformation from Cu(100) to Cu(111) during the CVD process at 1070 ºC. The temperature-induced change of domain size and orientation of graphene is found to influences the band structure of graphene from the ARPES measurements. Our results shed light on the growth mechanism of CVD graphene on Cu surface and the influence on the band structure, which are useful for further development of high-performance graphene-based electronic devices.

■ EXPERIMENTAL SECTION Sample preparation: Spinel substrates, MgAl2O4(111) and MgAl2O4(100), purchased from CRYSTAL GmbH, Germany were used as supports for the epitaxial Cu film deposition. A 500 nm-thick Cu was deposited by radio frequency (RF) magnetron sputtering at temperature of 220 ºC in Ar atmosphere. Single-layer graphene was grown by ambient pressure CVD with CH4, H2, and Ar gases at 1000-1080 ºC.23 Firstly, the Cu/spinel substrate was placed inside a quartz boat equipped in a tubular furnace. The system including the quartz tube was pumped by a rotary pump to remove residual gas. Before introducing CH4 the Cu/spinel was annealed at 1000 ºC for 40 min in the flow of H2/Ar gas (H2 2.3%) to clean and reduce the Cu surface. Then, the temperature was changed to the target temperature in the same H2/Ar flow (900-1080 ºC). After reaching the target temperature, diluted CH4 gas was added with a concentration of 10 ppm for all the experiments except for the CVD at 900-1000 ºC (40 ppm CH4 was used due to low reactivity of CH4 feedstock at the low temperature). The growth time set 20 min for evaluating the domain size and density, while longer growth time (60-90 min) was used to fully cover the Cu surface. Finally, the reac-



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tion was quenched by rapidly taking out the substrate from the heating zone, followed by pumping to set vacuum. For the transfer from a Cu film, the graphene surface was covered with a PMMA film by spin coating. After removing the Cu film by immersing in aqueous solution of ammonium persulfate ((NH4)2S2O8), the PMMA/graphene stack was transferred onto a SiO2/Si substrate. Finally, the PMMA film was removed by dipping the substrate into acetone solution. Characterization: Scanning electron microscope (SEM, HITACHI S-4800) and atomic force microscope (AFM, Bruker Nanoscope V) were used to image the surfaces of Cu and graphene. Raman spectra were measured for a transferred graphene sheet with a Nanofinder30 (Tokyo Instruments) using a 532 nm excitation. XRD and EBSD were by RIGAKU RINT TR-III and TSL Solutions, OIM, respectively. Crystal orientations of as-grown graphene films were characterized by LEED and LEEM equipment. LEED patterns of as-grown graphene were recorded in a UHV chamber of 8 × 10−9 Pa using BDL600IR (OCI, Canada). To measure the LEED from both graphene and Cu surface, the Cu surface was fully covered with uniform graphene by extending the growth time to 60-90 min. Otherwise, the surface oxidation of a Cu film occurs, making difficult to obtain LEED patterns. LEEM images and selected-area diffraction were measured with Elmitec LEEM III. ARPES was measured at Kyushu Synchrotron Light Research Center (Beam line 10). An oval-shaped beam with a size of several tens of microns by several hundred microns was used to measure the ARPES with further reduction by a slit for the detector.

■ ASSOCIATED CONTENT Supporting Information. Additional EBSD data, LEED patterns at different growth temperatures, simulation of a LEED pattern, and theoretical study of the structural geometry of graphene and Cu(111) lattice. This material is available free of charge via the Internet at http://pubs.acs.org.”

■ AUTHOR INFORMATION ■ Corresponding Author *E-mail: [email protected]

■ ACKNOWLEDGMENTS This work is supported by PRESTO-JST and KAKENHI (Grant numbers 15H03530 and 15K13304 from JSPS). The authors acknowledge Dr. P. Solís Fernández, Ms. H. Endo, Mr. K. Kawahara, and Mr. H. Kinoshita of Kyushu Univ. for experimental help. The ARPES measurements were performed at the BL10 of the SAGA Light Source (Proposal No. 0911127Pi).

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Figure 1 XRD profile (a) and EBSD image (b) of the Cu thin film after the CVD growth of graphene at 1070 ºC. Inset (a) is the schematic of the as-graphene grown on the epitaxial Cu(111) film. (c) Raman spectra of the transferred graphene on SiO2/Si measured at three different points. Inset shows a photograph of the transferred graphene. AFM images of the surfaces of the as-sputtered Cu (d) and that annealed at 1000 ºC (e) and 1075 ºC (f) in H2 flow. Inset of (f) is a corresponding FFT image, indicating high symmetry of the Cu(111) surface.


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Figure 2 (a-d) LEED patterns from graphene/Cu(111) samples grown at the different temperatures. (e) Magnified LED pattern of (a) and the corresponding atomic structures of graphene and Cu(111) lattice with 0º rotation. (f) Magnified LEED pattern of (c) and the atomic model with 3.4º rotation. Black and red lattices show graphene and Cu, respectively.



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Figure 3 SEM images of graphene domains grown on epitaxial Cu(111) films. (a) 1010, (b) 1020, (c) 1030, (d) 1040, (e) 1050, (f) 1060, (g) 1070, and (h) 1080 ºC. Arrhenius plots of domain size (i) and density (j) of graphene domains. In all the syntheses, the concentrations of CH4 (10 ppm) and H2 (2.3%) as well as the growth time (20 min) was fixed.


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Figure 4 (a) Bright-field LEEM image of merged graphene domains grown on Cu(111) at 1070 ºC. (b,c) LEED patterns measured at the areas, A and B, marked in (a). (d,e) BF-LEEM and the corresponding LEED pattern of the bare Cu surface. Yellow, pink, and light blue dotted lines indicate the diffraction from Cu, area A, and area B, respectively. The graphene domain at the area B is rotated from the underlying Cu lattice by ~-3º.



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Figure 5 ARPES data of the as-grown graphene synthesized at 1000 C º(a) and 1070 º C (b). The spectra are measured along the Γ-K direction with an photon energy of 95 eV. The different d-band intensity comes from measurement condition and

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Figure 6 XRD profiles of the Cu films as-sputtered (black line) on spinel(100) substrates and that after CVD at 1000 ºC (blue line) and 1070 ºC (red line). LEED patterns of graphene grown on Cu(100)/Spinel(100) substrates at 1000 ºC (b) and 1070 ºC (c).

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Table of contents artwork

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Growth Dynamics of Single-Layer Graphene on Epitaxial Cu Surfaces

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