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Effect of copper surface morphology on grain size uniformity of graphene grown by chemical vapor deposition
Current Applied Physics 19 (2019) 1414–1420
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Effect of copper surface morphology on grain size uniformity of graphene grown by chemical vapor deposition
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Jaewoon Kang, Chang-Ju Lee, Jaeeuk Kim, Honghwi Park, Changhee Lim, Junyeong Lee, Muhan Choi, Hongsik Park∗ School of Electronics Engineering, Kyungpook National University, Daegu, 41566, South Korea
ARTICLE INFO
ABSTRACT
Keywords: Graphene grain boundaries Electropolishing process Small-grain regions Global uniformity
The graphene grain boundaries (GGBs) of polycrystalline graphene grown by chemical vapor deposition (CVD) typically constitute a major reason of deterioration of the electrical properties of graphene-based devices. To reduce the density of GGB by increasing the grain size, CVD growth conditions with a reduced CH4 flow rate have been widely applied and, recently, electropolishing of copper (Cu) foil substrates to flatten the surface has been undertaken prior to graphene growth. In this study, we show that polycrystalline graphene layer grown on typical Cu foil features two heterogeneous regions with different average grain sizes: small-grain regions (SGRs) and large-grain regions (LGRs). Statistical analysis of the grains of the graphene layers grown under different process conditions showed that SGRs (which form on Cu striations) limit the average grain size, the ability to control the grain size through adjustment of growth conditions, and global grain-size uniformity. Analysis showed that the surface-flattening process significantly improves grain-size uniformity, and monolayer coverage, as well as the average grain size. These results suggest that a process for flattening the surfaces of Cu substrates is critical to controlling the quality and uniformity of CVD-grown graphene layers for practical device applications.
1. Introduction Since the development of chemical vapor deposition (CVD) for graphene growth on copper (Cu) foils, it has been the most widely used growth method for device applications because it enables large-scale graphene growth and transfer to different substrates [1–4]. However, since the CVD-grown polycrystalline graphene layers have grain boundaries, which are the most significant factor degrading the carrier mobility [5,6], reducing the density of the grain boundary is very important to enhance the electrical properties of polycrystalline graphenebased devices. To suppress the effect of graphene grain boundaries (GGBs), researchers have attempted to increase the average grain size by controlling the H2-to-CH4 gas ratio and the partial pressure of CH4 by supplying Ar [7–9]. Increasing the ratio of H2 to CH4 flow rate reduces the concentration of active carbon species on the Cu surface as well as the graphene nuclei and enables larger-size graphene grains during graphene growth. Recently, an electropolishing process for reducing the surface roughness of Cu foil substrates was reported to increase the grain size of CVD-grown graphene [9–11]. It is known that the electropolishing process can lower the nucleation density of graphene by removing the rolling features and crystal defects of Cu foil,
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resulting in increase in the overall graphene grain sizes. In addition to the average grain size of CVD-grown graphene, uniformity of the grain size is also an important factor that affects the applicability of graphene-based devices. In this work, we systematically investigated how a surface flattening process affects the nucleation density, the average grain size and the grain size uniformity of CVD-grown graphene. We statistically investigated the grain size and its distribution of polycrystalline graphene grown on a conventional (unpolished) Cu foil substrate and its uniformity and compared the results with those obtained for graphene grown on an electropolished Cu substrate. We also studied the effect of the surface flattening process on the controllability of graphene grain sizes by the growth condition adjustment. 2. Experimental 2.1. Graphene growth For graphene growth, the Cu foil (25 μm thickness, 99.8% purity, Alfa Aesar) was loaded into the CVD furnace and heated up to 1050 °C with a gas flow of 570 cm3/min (sccm) of Ar and 100 sccm of H2. After
Corresponding author. E-mail address: [email protected] (H. Park).
https://doi.org/10.1016/j.cap.2019.09.005 Received 17 August 2019; Accepted 10 September 2019 Available online 11 September 2019 1567-1739/ © 2019 Korean Physical Society. Published by Elsevier B.V. All rights reserved.
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reaching 1050 °C, the Cu foil was annealed for 1 h under the same gas flow conditions. A monolayer graphene was then grown at 1050 °C for 1 h under a pressure of 2 Torr and a gas flow of Ar (570 sccm), H2 (100 sccm), and CH4 (2 sccm), which represent typical growth conditions for graphene with an average grain size of several micrometers [5,8].
mechanism of graphene. Since the average grain size of graphene is determined by the nucleation density of the initial growth stage [7,15–17], we think that the presence of SGR/LGR originates from different nucleation densities over the Cu surface. P. Braeuninger–Weimer et al. reported that carbon impurities in the Cu bulk are concentrated along striations on the Cu surface during the Cu annealing step before the growth step [18]. The striation is a rolling feature on the Cu surface produced during foil manufacturing. H. Kim et al. also reported that the carbon concentration required for the nucleation of graphene (i.e. the concentration for supersaturation condition) at the rolling features (striations) is lower than that at flat regions because carbon-adatom species are less mobile on striations [16]. Thus, the probability of nucleation is relatively high at striations [16]. Based on these reported results, we attributed the formation of the SGRs to the relatively high nucleation density along the striations on the Cu foil substrate. This explanation on the formation of SGRs and LGRs is described in the schematics of the nucleation and growth of graphene in Fig. 1(c). Since the nucleation density and graphene grain sizes are affected by the surface roughness, we investigated the surface morphology of the Cu substrate used for the graphene growth using CLSM. The surface morphology of the Cu substrate changed because of a hightemperature annealing process during the graphene growth [19–21]. To observe the surface morphology of the Cu foil in the growth stage, we annealed the Cu foil under the same conditions as those used for graphene growth before CLSM imaging. As shown in Fig. 1(d), there were striation features on the Cu foil surface even after the annealing process and the distance between adjacent striations was several tens of micrometers. In the study of graphene CVD growth, one of the most important research topics has been the control of the average grain size of graphene gained by the adjusting growth conditions [5,12,22]. In addition, the uniformity of grain sizes is as important as the average grain size of graphene. The coexistence of SGR/LGR in a graphene layer implies that the global uniformity might be worse although the grain sizes in each region were relatively uniform. To investigate the effect of the coexistence of SGRs/LGRs on the average grain size and uniformity of grain sizes, we statistically analyzed the distributions of grain sizes. Fig. 1(e) shows the average grain sizes and grain size uniformity of SGRs, LGRs, and the overall region in the graphene layer. The average grain sizes of LGR and SGR were 9.26 ± 2.55 μm and 5.29 ± 1.76 μm, respectively. The average grain size of the overall region including both SGRs and LGRs was 6.81 ± 2.85 μm. This result implies that it is difficult to consider this graphene layer as a single uniform material and that the SGRs considerably decrease the average grain size of the overall graphene layer. The large difference between the average grain size of SGRs and LGRs adversely affects the uniformity of grain sizes of the overall graphene layer. Although the standard deviation is a measure representing data uniformity, it tends to increase or decrease proportionally as the mean increases or decreases [23]. Thus, the coefficient of variation (CV), which is the ratio of the standard deviation to the mean, is a more suitable measure for comparison and evaluation of the uniformity of multiple data sets with different average values [24,25]. Since the CV is a standard deviation normalized by an average, this value is smaller in a region with more uniform grain sizes. In this study, we evaluated the uniformity of different graphene regions with the reciprocal of CV, denoted as the “uniformity factor”:
2.2. UV/ozone treatment for GGB visualization To statistically evaluate the grain size and its uniformity, observation of the shape of graphene grains is critical. Direct observation of the GGBs is very difficult because the width of GGB regions is typically in the sub-nanometer scale. To visualize the GGBs, we applied an ultraviolet (UV) treatment to the as-grown polycrystalline graphene [12]. The graphene on the Cu foil was placed in a UV/ozone chamber (PSDPro, Novascan Technologies) and irradiated with UV light for 30 min under ambient conditions to selectively oxidize the Cu regions under defective GGBs. Since the volume of the oxidized Cu region beneath the GGBs increased, this treatment enabled direct visualization of the GGBs. Scanning electron microscopy (SEM, S-4800, Hitachi) images of UV/ ozone-treated graphene layers were further used for the statistical analysis of graphene grains to evaluate the average grain size and uniformity of the grown graphene layer (For details, see Fig. S1 in supplementary data.). 2.3. Characterization of surface roughness of Cu substrates To evaluate the surface roughness of the Cu substrates, atomic force microscopy (AFM) images of the Cu surface were taken by a tappingmode operation using a scanning probe microscope (NX20, Park Systems). Confocal laser-scanning microscopy (CLSM, LSM 700, Carl Zeiss) images were used for large-area evaluation of the Cu surface. 2.4. Electropolishing process Electropolishing of Cu foil was performed in an 85% phosphoric acid solution at a constant voltage of 1.2 V for 20 min. Two unpolished Cu foils were used as the anode and cathode electrodes. The size of the Cu foils was 2 × 6 cm2 and the distance between the electrodes was 5 cm. The Cu foil for the anode was electropolished and used as the substrate for graphene growth. The voltage and electropolishing time were optimized by monitoring the surface morphology of the polished Cu foils. An excess voltage or excess polishing time created surface defects [13,14]. 3. Results and discussion We synthesized graphene layers on a Cu foil using typical growth conditions, performed a surface treatment on the grown graphene layers for visualization of GGBs, and statistically analyzed the average grain size and its uniformity. Fig. 1(a) shows an SEM image of the graphene layer for which the grain boundaries were visualized by UV/ ozone treatment. In this image, the GGBs of the UV/ozone-treated graphene layer are highlighted in yellow to make them more visible. The original image of the UV/ozone-treated graphene layer is shown in supplementary data (Fig. S2). The average grain size, evaluated using more than 140 grains, was 6.81 μm with a standard deviation of 2.85 μm. Fig. 1(b) shows the distribution of the grain sizes. The peaks of the histogram indicate that there are two distinguishable regions that differ in their average grain sizes. One is denoted as the “small-grain region” (SGR), while the other is termed the “large-grain region” (LGR). These two regions can also be observed in the SEM image [Fig. 1(a)]. The approximate boundaries of the SGR and LGR are indicated by the red dashed lines. Although there has been no report on the coexistence of SGR/LGR in a continuous graphene layer grown on a Cu foil, the causes of the presence of both regions with different average grain sizes can be explained based on several reports discussing the growth
U
µ 1 = CV
(1)
where µ is the average grain size and is the standard deviation of the grain size. This result shows that the uniformity factor for LGRs is larger than that for SGRs in the case of the graphene layer used in this study. The uniformity factor for the global grain region is lower than those for the SGR and LGR, which clearly indicates that the coexistence of SGR/ LGR degrades the global uniformity of the graphene layer as expected. This result implies that we can increase the average grain size and improve the grain size uniformity of CVD-grown graphene using a 1415
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Fig. 1. (a) SEM image of the CVD-grown graphene. GGBs are highlighted in yellow and approximate boundaries of the SGR and LGR are indicated by red dashed lines. (b) Distribution of graphene grain sizes measured from SEM images of graphene grains. Two Gaussian peaks indicate the existence of heterogeneous grain regions. (c) Schematic illustration of graphene growth mechanism on conventional Cu foil. (d) CLSM image of the annealed Cu-foil surface. (e) Average grain size (µ ) and uniformity factor (U) of graphene grains in SGRs, LGRs, and overall region. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
process to suppress the SGR formation during the graphene growth. Because the formation of SGRs is associated with the surface morphology of Cu substrates, a process for surface flattening could be effective to suppress the formation of SGRs. In recent years, a Cu foil electropolishing process has been applied to graphene growth to increase the average grain size of CVD-grown graphene. It was reported that electropolishing could remove surface defects and make the Cu foil surface flat [10,11,16,19]. We applied Cu foil electropolishing to the graphene growth process as shown in Fig. 2(a) and investigated the effect of the surface flattening process on SGR formation and grain size uniformity. The difference in the surface morphology of unpolished and electropolished Cu foils was observed using CLSM. Since the Cu foils were annealed just before the graphene growth step, we annealed the Cu foils at 1050 °C for 1 h before CLSM imaging. Fig. 2(b) and (c) show the CLSM images and AFM topographic images of annealed unpolished and electropolished Cu foil surfaces. The images show that the unpolished Cu foil had striations originating from Cu foil rolling features even after annealing and the height of striations was considerably reduced by the electropolishing process. The rootmean-square (RMS) roughness of the electropolished Cu foil surface was 21.2 nm, while that of the unpolished Cu foil surface was 69.1 nm. Fig. 2(d) shows the height profiles of the two Cu foil surfaces, corresponding to the red lines in the AFM images in Fig. 2(c). The
electropolishing process effectively flattened the Cu foil surface although the striations were not completely removed with the process conditions guaranteeing a low-defect surface condition. To investigate the effect of the electropolishing process on the formation of SGRs, we synthesized graphene on the unpolished and electropolished Cu foils and analyzed the grain sizes of the grown graphene layers. The graphene layers were grown at 1050 °C for 1 h under a working pressure of 2 Torr and a gas flow of Ar (570 sccm), H2 (100 sccm), and CH4 (2 sccm). We visualized the GGBs of the grown graphene layers using UV/ozone treatment and measured the sizes of graphene grains grown on the electropolished Cu foil from more than 70 grain images. Representative SEM images for the graphene layers grown on the unpolished and electropolished Cu foils are shown in Fig. 3(a) and (b). In Fig. 3(a), an SGR is observed as seen in Fig. 1(a). However, it is notable that heterogeneous regions that are distinguishable with grain sizes are not observed for the graphene on the electropolished Cu foil [Fig. 3(b)]. Fig. 3(c) and (d) show the distribution of measured grain sizes for the graphene layers grown on the unpolished and electropolished Cu foils, respectively. As expected from the SEM images, there were no two distinguishable peaks in the histogram for the graphene on the electropolished Cu substrate differing from those for the graphene on the unpolished Cu foil. This result implies that the formation of SGRs could be suppressed by the electropolishing process. For further 1416
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Fig. 2. (a) Schematic of the process for electropolishing Cu foils. (b) Confocal laser-scanning microscopy images of the unpolished (top) and electropolished (bottom) Cu-foil surfaces. (c) AFM topographic images of the unpolished (top) and electropolished (bottom) Cu foils. (d) Height profiles of the two Cu-foil surfaces corresponding to the red lines in the AFM images. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
observation, we transferred the graphene layers on Cu foils onto oxidized silicon substrates. It has been reported that an excess growth time causes the formation of bilayer graphene during graphene growth [25]. Thus, if there were SGRs and LGRs in a graphene layer, bilayers would be formed in SGRs by the excess growth time because a relatively shorter time is required for grain islands to merge with adjacent grains and form a continuous polycrystalline single-layer graphene [26,27]. Fig. 3(e) and (f) show optical microscopy images of the transferred graphene layers. In Fig. 3(e), it is observed that graphene bilayers were formed and concentrated in long and narrow regions. The distance between the bilayer graphene regions was comparable with the pitch of striation. In contrast, bilayers were rarely found in the graphene grown
on the electropolished Cu foil [Fig. 3(f)]. This result is indirect evidence for the fact that the grain sizes of the graphene on the electropolished Cu foil were larger than those of the graphene on the unpolished Cu foil and the electropolishing process effectively suppressed the formation of SGRs along Cu striations. Because the average grain size is determined by the nucleation density, investigation of the nucleation density of graphene and distribution of graphene grains is important to understand the effect of Cu striations on the nucleation of graphene. We synthesized graphene for a short time and finished the growth step at the initial stage of graphene nucleation. Except the growth time, the growth conditions were not changed. We synthesized two sets of graphene layers. The first set was
Fig. 3. SEM images of graphene grains on (a) an unpolished and (b) electropolished Cu foil. Distribution of graphene grain sizes for graphene grown on (c) the unpolished and (d) electropolished Cu foil. Optical images of graphene layers transferred on oxidized silicon substrates: graphene grown on (e) the unpolished and (f) electropolished Cu foil. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 1417
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Fig. 4. SEM images of graphene grains grown for 1 min on (a) an unpolished and (b) electropolished Cu foil. SEM images of the graphene grains grown for 5 min on (c) an unpolished and (d) electropolished Cu foil. Distribution of the angles (θ) of the lines connecting each grain with its nearest grains for graphene grains grown on (e) the unpolished and (f) electropolished Cu foil.
grown for 1 min on an unpolished Cu foil and electropolished Cu foil simultaneously, and the second set was grown for 5 min. Fig. 4(a) and (b) show SEM images of the first set of graphene layers grown for 1 min. The graphene grains grown on the unpolished Cu substrate were arranged in the form of parallel lines. The distances between the lines were 10–30 μm which was comparable with the pitch of Cu striation [Fig. 4(a)]. In contrast, the graphene grains grown on the electropolished Cu substrate were randomly distributed as shown in Fig. 4(b). This result indicates that the nucleation of graphene was concentrated in the region of Cu foil striations. When we increased the growth time to 5 min, the graphene grains grown on the unpolished Cu substrate increased in size and maintained the parallel distribution that is similar to that of the grains grown for the initial 1 min as shown in Fig. 4(c). The grain boundaries are formed when these grains are further enlarged and meet each other. A region with a higher density of nucleation and grain islands would be a region with a smaller average grain size after the formation of a continuous graphene layer [15,28]. Thus, these results indicate that graphene grown on an unpolished Cu foil had small-grain regions in the region with the smaller density of nucleation and the graphene grown on an electropolished Cu foil had relatively large grain sizes. To quantitatively show the effect of the
surface morphology of Cu substrates on the formation of graphene nucleation and grains, we investigate the patterns of the arranged small grains by measuring the angles (θ) of the lines connecting each grain with its nearest grains as shown in Fig. 4(c) (dashed line). Fig. 4(e) and (f) show the histogram plots for the distribution of θ for the graphene grains grown on the unpolished and electropolished Cu foils, respectively. These plots clearly indicates that the graphene grains grown on the unpolished Cu substrate were concentrated along a specific direction [Fig. 4(e)] while the grains on the electropolished Cu substrate were randomly distributed [Fig. 4(f)]. This result implies that the electropolishing process could effectively eliminate the formation of the high-density nucleation in specific regions corresponding to Cu-foil striations. We showed that the average grain size of the graphene layer grown on the electropolished Cu foil was larger than that of the graphene layer grown on the unpolished Cu foil. In literature, it has been reported that the nucleation density of graphene grown on Cu foils could be controlled by the ratio of H2:CH4 flow rate. An increase in the ratio of H2:CH4 resulted in a reduced density of graphene nucleation and an increased average grain size due to the decrease in the fraction of the carbon source (CH4) in the supplied gas [7–9]. Since the density of the 1418
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Fig. 5. (a) Average grain sizes of graphene grown on unpolished (black squares) and electropolished (red circles) Cu foils under different growth conditions with the H2:CH4 ratio of 10, 50, and 200. (b) Uniformity of the graphene grain sizes evaluated by the uniformity factors for the graphene layers grown on unpolished and electropolished Cu foils with different growth conditions. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
graphene nucleation was affected by the morphology of the Cu-foil surface, the controllability of the average grain size by the growth condition adjustment also could be affected by the Cu-foil surface morphology. To investigate this effect, we synthesized graphene layers on unpolished and electropolished Cu foils under different growth conditions. Fig. 5(a) shows the average grain sizes of the graphene layers grown on the unpolished and electropolished Cu foils under three different growth conditions (H2:CH4 ratios of 10, 50, and 200). As expected, the average grain size increased as H2:CH4 ratios was increased. It can be noted that the average grain size of the graphene layer on the electropolished Cu foil was more effectively controlled by the H2:CH4 ratio than that of graphene layer on the unpolished Cu foil. When we increased the H2:CH4 ratio from 10 to 200, the average grain size of graphene on the electropolished Cu foil increased from 4.9 to 26.7 μm while the average grain size of graphene on the unpolished Cu foil increased only from 4.47 to 7.26 μm. This result indicates that the electropolishing process significantly enhanced the effectiveness of the control over the average grain size by the ratio of H2:CH4 flow rate. Fig. 5(b) shows the uniformity of the graphene grain sizes evaluated by the uniformity factors for the graphene layers grown on unpolished and electropolished Cu foils with different growth conditions. The uniformity factor of the grain sizes decreased as the average grain size increased by the increase in the H2:CH4 ratio because the coefficient of variation (CV) typically increases as the average value of a distribution increases [23]. Nevertheless, the grain-size uniformity factors of the graphene layers on the electropolished Cu substrates were higher than those of the graphene layers on the unpolished Cu foil. These results show that the coexistence of SGRs/LGRs in a graphene layer decreases the average grain size of CVD-grown graphene, deteriorates the global uniformity of grain sizes, and hinders the control of the grain size by adjustment of growth conditions. Since the small-grain regions (SGRs) of graphene were grown on the Cu-foil striations, the electropolishing process could effectively suppress the SGR formation and enhance the quality of the CVD-grown graphene by increasing the average grain size, monolayer coverage, and the uniformity of grain sizes.
Conflicts of interest There are no conflicts to declare. Acknowledgements This work was supported in part by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2017R1A4A1015565), and the Bio & Medical Technology Development Program of the NRF funded by the Ministry of Science & ICT (2017M3A9G8083382). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cap.2019.09.005. References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (5696) (2004) 666–669 https://doi.org/10.1126/science.1102896. [2] X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc, S.K. Banerjee, L. Colombo, R.S. Ruoff, Large-area synthesis of high-quality and uniform graphene films on copper foils, Science 324 (5932) (2009) 1131–1314 https://doi.org/10.1126/science.1171245. [3] S. Bae, H. Kim, Y. Lee, X. Xu, J.-S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H.R. Kim, Y.I. Song, Y.-J. Kim, K.S. Kim, B. Özyilmaz, J.-H. Ahn, B.H. Hong, S. Iijima, Roll-toRoll production of 30-inch graphene films for transparent electrodes, Nat. Nanotechnol. 5 (2010) 574–578 https://doi.org/10.1038/nnano.2010.132. [4] K.S. Novoselov, V.I. Fal′ko, L. Colombo, P.R. Gellert, M.G. Schwab, K. Kim, A roadmap for graphene, Nature 490 (2012) 192–200 https://doi.org/10.1038/ nature11458. [5] A.W. Tsen, L. Brown, M.P. Levendorf, F. Ghahari, P.Y. Huang, R.W. Havener, C.S. Ruiz-Vargas, D.A. Muller, P. Kim, J. Park, Tailoring electrical transport across grain boundaries in polycrystalline graphene, Science 336 (6085) (2012) 1143–1146 https://doi.org/10.1126/science.1218948. [6] Q.K. Yu, L.A. Jauregui, W. Wu, R. Colby, J.F. Tian, Z.H. Su, H.L. Cao, Z.H. Liu, D. Pandey, D.G. Wei, T.F. Chung, P. Peng, N.P. Guisinger, E.A. Stach, J.M. Bao, S.S. Pei, Y.P. Chen, Control and characterization of individual grains and grain boundaries in graphene grown by chemical vapour deposition, Nat. Mater. 10 (2011) 443–449 https://doi.org/10.1038/NMAT3010. [7] X. Li, C.W. Magnuson, A. Venugopal, J. An, J.W. Suk, B. Han, M. Borysiak, W. Cai, A. Velamakanni, Y. Zhu, L. Fu, E.M. Vogel, E. Voelkl, L. Colombo, R.S. Ruoff, Graphene films with large domain size by a two-step chemical vapor deposition process, Nano Lett. 10 (2010) 4328–4334 https://doi.org/10.1021/nl101629g. [8] B. Vlassiouk, S. Smirnov, M. Regmi, S.P. Surwade, N. Srivastava, R. Feenstra, G. Eres, C. Parish, N. Lavrik, P. Datskos, S. Dai, P. Fulvio, Graphene nucleation density on copper: fundamental role of background pressure, J. Phys. Chem. C 117 (2013) 18919–18926 https://doi.org/10.1021/jp4047648. [9] Z. Yan, J. Lin, Z. Peng, Z. Sun, Y. Zhu, L. Li, C. Xiang, E. Loïc Samuel, C. Kittrell, J.M. Tour, Toward the synthesis of wafer-scale single-crystal graphene on copper foils, ACS Nano 6 (10) (2012) 9110–9117 https://doi.org/10.1021/nn303352k. [10] B. Zhang, W.H. Lee, R. Piner, I. Kholmanov, Y. Wu, H. Li, H. Ji, Rodney S. Ruoff, Low-temperature chemical vapor deposition growth of graphene from toluene on electropolished copper foils, ACS Nano 6 (3) (2012) 2471–2476 https://doi.org/10. 1021/nn204827h. [11] X. Wu, G. Zhong, L. D'Arsiè, H. Sugime, S. Esconjauregui, A.W. Robertson, J. Robertson, Growth of continuous monolayer graphene with millimeter-sized domains using industrially safe conditions, Sci. Rep. 6 (2016) 21152 https://doi.
4. Conclusions We observed that polycrystalline graphene layer grown on a typical Cu foil features two heterogeneous regions with different average grain sizes: small-grain regions (SGRs) and large-grain regions (LGRs). Based statistical analysis on the grain sizes, we investigated the effect of the surface morphology of Cu-foil substrates on the average grain size and the uniformity of grain sizes of graphene layers grown on an unpolished and electropolished Cu-foil. This work suggests that the suppression of the SGR formation through a surface-flattening process enhance the quality of the CVD-grown graphene by increasing the average grain size, monolayer coverage, and the uniformity of grain sizes. 1419
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Effect of copper surface morphology on grain size uniformity of graphene grown by chemical vapor deposition
T
Jaewoon Kang, Chang-Ju Lee, Jaeeuk Kim, Honghwi Park, Changhee Lim, Junyeong Lee, Muhan Choi, Hongsik Park∗ School of Electronics Engineering, Kyungpook National University, Daegu, 41566, South Korea
ARTICLE INFO
ABSTRACT
Keywords: Graphene grain boundaries Electropolishing process Small-grain regions Global uniformity
The graphene grain boundaries (GGBs) of polycrystalline graphene grown by chemical vapor deposition (CVD) typically constitute a major reason of deterioration of the electrical properties of graphene-based devices. To reduce the density of GGB by increasing the grain size, CVD growth conditions with a reduced CH4 flow rate have been widely applied and, recently, electropolishing of copper (Cu) foil substrates to flatten the surface has been undertaken prior to graphene growth. In this study, we show that polycrystalline graphene layer grown on typical Cu foil features two heterogeneous regions with different average grain sizes: small-grain regions (SGRs) and large-grain regions (LGRs). Statistical analysis of the grains of the graphene layers grown under different process conditions showed that SGRs (which form on Cu striations) limit the average grain size, the ability to control the grain size through adjustment of growth conditions, and global grain-size uniformity. Analysis showed that the surface-flattening process significantly improves grain-size uniformity, and monolayer coverage, as well as the average grain size. These results suggest that a process for flattening the surfaces of Cu substrates is critical to controlling the quality and uniformity of CVD-grown graphene layers for practical device applications.
1. Introduction Since the development of chemical vapor deposition (CVD) for graphene growth on copper (Cu) foils, it has been the most widely used growth method for device applications because it enables large-scale graphene growth and transfer to different substrates [1–4]. However, since the CVD-grown polycrystalline graphene layers have grain boundaries, which are the most significant factor degrading the carrier mobility [5,6], reducing the density of the grain boundary is very important to enhance the electrical properties of polycrystalline graphenebased devices. To suppress the effect of graphene grain boundaries (GGBs), researchers have attempted to increase the average grain size by controlling the H2-to-CH4 gas ratio and the partial pressure of CH4 by supplying Ar [7–9]. Increasing the ratio of H2 to CH4 flow rate reduces the concentration of active carbon species on the Cu surface as well as the graphene nuclei and enables larger-size graphene grains during graphene growth. Recently, an electropolishing process for reducing the surface roughness of Cu foil substrates was reported to increase the grain size of CVD-grown graphene [9–11]. It is known that the electropolishing process can lower the nucleation density of graphene by removing the rolling features and crystal defects of Cu foil,
∗
resulting in increase in the overall graphene grain sizes. In addition to the average grain size of CVD-grown graphene, uniformity of the grain size is also an important factor that affects the applicability of graphene-based devices. In this work, we systematically investigated how a surface flattening process affects the nucleation density, the average grain size and the grain size uniformity of CVD-grown graphene. We statistically investigated the grain size and its distribution of polycrystalline graphene grown on a conventional (unpolished) Cu foil substrate and its uniformity and compared the results with those obtained for graphene grown on an electropolished Cu substrate. We also studied the effect of the surface flattening process on the controllability of graphene grain sizes by the growth condition adjustment. 2. Experimental 2.1. Graphene growth For graphene growth, the Cu foil (25 μm thickness, 99.8% purity, Alfa Aesar) was loaded into the CVD furnace and heated up to 1050 °C with a gas flow of 570 cm3/min (sccm) of Ar and 100 sccm of H2. After
Corresponding author. E-mail address: [email protected] (H. Park).
https://doi.org/10.1016/j.cap.2019.09.005 Received 17 August 2019; Accepted 10 September 2019 Available online 11 September 2019 1567-1739/ © 2019 Korean Physical Society. Published by Elsevier B.V. All rights reserved.
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reaching 1050 °C, the Cu foil was annealed for 1 h under the same gas flow conditions. A monolayer graphene was then grown at 1050 °C for 1 h under a pressure of 2 Torr and a gas flow of Ar (570 sccm), H2 (100 sccm), and CH4 (2 sccm), which represent typical growth conditions for graphene with an average grain size of several micrometers [5,8].
mechanism of graphene. Since the average grain size of graphene is determined by the nucleation density of the initial growth stage [7,15–17], we think that the presence of SGR/LGR originates from different nucleation densities over the Cu surface. P. Braeuninger–Weimer et al. reported that carbon impurities in the Cu bulk are concentrated along striations on the Cu surface during the Cu annealing step before the growth step [18]. The striation is a rolling feature on the Cu surface produced during foil manufacturing. H. Kim et al. also reported that the carbon concentration required for the nucleation of graphene (i.e. the concentration for supersaturation condition) at the rolling features (striations) is lower than that at flat regions because carbon-adatom species are less mobile on striations [16]. Thus, the probability of nucleation is relatively high at striations [16]. Based on these reported results, we attributed the formation of the SGRs to the relatively high nucleation density along the striations on the Cu foil substrate. This explanation on the formation of SGRs and LGRs is described in the schematics of the nucleation and growth of graphene in Fig. 1(c). Since the nucleation density and graphene grain sizes are affected by the surface roughness, we investigated the surface morphology of the Cu substrate used for the graphene growth using CLSM. The surface morphology of the Cu substrate changed because of a hightemperature annealing process during the graphene growth [19–21]. To observe the surface morphology of the Cu foil in the growth stage, we annealed the Cu foil under the same conditions as those used for graphene growth before CLSM imaging. As shown in Fig. 1(d), there were striation features on the Cu foil surface even after the annealing process and the distance between adjacent striations was several tens of micrometers. In the study of graphene CVD growth, one of the most important research topics has been the control of the average grain size of graphene gained by the adjusting growth conditions [5,12,22]. In addition, the uniformity of grain sizes is as important as the average grain size of graphene. The coexistence of SGR/LGR in a graphene layer implies that the global uniformity might be worse although the grain sizes in each region were relatively uniform. To investigate the effect of the coexistence of SGRs/LGRs on the average grain size and uniformity of grain sizes, we statistically analyzed the distributions of grain sizes. Fig. 1(e) shows the average grain sizes and grain size uniformity of SGRs, LGRs, and the overall region in the graphene layer. The average grain sizes of LGR and SGR were 9.26 ± 2.55 μm and 5.29 ± 1.76 μm, respectively. The average grain size of the overall region including both SGRs and LGRs was 6.81 ± 2.85 μm. This result implies that it is difficult to consider this graphene layer as a single uniform material and that the SGRs considerably decrease the average grain size of the overall graphene layer. The large difference between the average grain size of SGRs and LGRs adversely affects the uniformity of grain sizes of the overall graphene layer. Although the standard deviation is a measure representing data uniformity, it tends to increase or decrease proportionally as the mean increases or decreases [23]. Thus, the coefficient of variation (CV), which is the ratio of the standard deviation to the mean, is a more suitable measure for comparison and evaluation of the uniformity of multiple data sets with different average values [24,25]. Since the CV is a standard deviation normalized by an average, this value is smaller in a region with more uniform grain sizes. In this study, we evaluated the uniformity of different graphene regions with the reciprocal of CV, denoted as the “uniformity factor”:
2.2. UV/ozone treatment for GGB visualization To statistically evaluate the grain size and its uniformity, observation of the shape of graphene grains is critical. Direct observation of the GGBs is very difficult because the width of GGB regions is typically in the sub-nanometer scale. To visualize the GGBs, we applied an ultraviolet (UV) treatment to the as-grown polycrystalline graphene [12]. The graphene on the Cu foil was placed in a UV/ozone chamber (PSDPro, Novascan Technologies) and irradiated with UV light for 30 min under ambient conditions to selectively oxidize the Cu regions under defective GGBs. Since the volume of the oxidized Cu region beneath the GGBs increased, this treatment enabled direct visualization of the GGBs. Scanning electron microscopy (SEM, S-4800, Hitachi) images of UV/ ozone-treated graphene layers were further used for the statistical analysis of graphene grains to evaluate the average grain size and uniformity of the grown graphene layer (For details, see Fig. S1 in supplementary data.). 2.3. Characterization of surface roughness of Cu substrates To evaluate the surface roughness of the Cu substrates, atomic force microscopy (AFM) images of the Cu surface were taken by a tappingmode operation using a scanning probe microscope (NX20, Park Systems). Confocal laser-scanning microscopy (CLSM, LSM 700, Carl Zeiss) images were used for large-area evaluation of the Cu surface. 2.4. Electropolishing process Electropolishing of Cu foil was performed in an 85% phosphoric acid solution at a constant voltage of 1.2 V for 20 min. Two unpolished Cu foils were used as the anode and cathode electrodes. The size of the Cu foils was 2 × 6 cm2 and the distance between the electrodes was 5 cm. The Cu foil for the anode was electropolished and used as the substrate for graphene growth. The voltage and electropolishing time were optimized by monitoring the surface morphology of the polished Cu foils. An excess voltage or excess polishing time created surface defects [13,14]. 3. Results and discussion We synthesized graphene layers on a Cu foil using typical growth conditions, performed a surface treatment on the grown graphene layers for visualization of GGBs, and statistically analyzed the average grain size and its uniformity. Fig. 1(a) shows an SEM image of the graphene layer for which the grain boundaries were visualized by UV/ ozone treatment. In this image, the GGBs of the UV/ozone-treated graphene layer are highlighted in yellow to make them more visible. The original image of the UV/ozone-treated graphene layer is shown in supplementary data (Fig. S2). The average grain size, evaluated using more than 140 grains, was 6.81 μm with a standard deviation of 2.85 μm. Fig. 1(b) shows the distribution of the grain sizes. The peaks of the histogram indicate that there are two distinguishable regions that differ in their average grain sizes. One is denoted as the “small-grain region” (SGR), while the other is termed the “large-grain region” (LGR). These two regions can also be observed in the SEM image [Fig. 1(a)]. The approximate boundaries of the SGR and LGR are indicated by the red dashed lines. Although there has been no report on the coexistence of SGR/LGR in a continuous graphene layer grown on a Cu foil, the causes of the presence of both regions with different average grain sizes can be explained based on several reports discussing the growth
U
µ 1 = CV
(1)
where µ is the average grain size and is the standard deviation of the grain size. This result shows that the uniformity factor for LGRs is larger than that for SGRs in the case of the graphene layer used in this study. The uniformity factor for the global grain region is lower than those for the SGR and LGR, which clearly indicates that the coexistence of SGR/ LGR degrades the global uniformity of the graphene layer as expected. This result implies that we can increase the average grain size and improve the grain size uniformity of CVD-grown graphene using a 1415
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Fig. 1. (a) SEM image of the CVD-grown graphene. GGBs are highlighted in yellow and approximate boundaries of the SGR and LGR are indicated by red dashed lines. (b) Distribution of graphene grain sizes measured from SEM images of graphene grains. Two Gaussian peaks indicate the existence of heterogeneous grain regions. (c) Schematic illustration of graphene growth mechanism on conventional Cu foil. (d) CLSM image of the annealed Cu-foil surface. (e) Average grain size (µ ) and uniformity factor (U) of graphene grains in SGRs, LGRs, and overall region. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
process to suppress the SGR formation during the graphene growth. Because the formation of SGRs is associated with the surface morphology of Cu substrates, a process for surface flattening could be effective to suppress the formation of SGRs. In recent years, a Cu foil electropolishing process has been applied to graphene growth to increase the average grain size of CVD-grown graphene. It was reported that electropolishing could remove surface defects and make the Cu foil surface flat [10,11,16,19]. We applied Cu foil electropolishing to the graphene growth process as shown in Fig. 2(a) and investigated the effect of the surface flattening process on SGR formation and grain size uniformity. The difference in the surface morphology of unpolished and electropolished Cu foils was observed using CLSM. Since the Cu foils were annealed just before the graphene growth step, we annealed the Cu foils at 1050 °C for 1 h before CLSM imaging. Fig. 2(b) and (c) show the CLSM images and AFM topographic images of annealed unpolished and electropolished Cu foil surfaces. The images show that the unpolished Cu foil had striations originating from Cu foil rolling features even after annealing and the height of striations was considerably reduced by the electropolishing process. The rootmean-square (RMS) roughness of the electropolished Cu foil surface was 21.2 nm, while that of the unpolished Cu foil surface was 69.1 nm. Fig. 2(d) shows the height profiles of the two Cu foil surfaces, corresponding to the red lines in the AFM images in Fig. 2(c). The
electropolishing process effectively flattened the Cu foil surface although the striations were not completely removed with the process conditions guaranteeing a low-defect surface condition. To investigate the effect of the electropolishing process on the formation of SGRs, we synthesized graphene on the unpolished and electropolished Cu foils and analyzed the grain sizes of the grown graphene layers. The graphene layers were grown at 1050 °C for 1 h under a working pressure of 2 Torr and a gas flow of Ar (570 sccm), H2 (100 sccm), and CH4 (2 sccm). We visualized the GGBs of the grown graphene layers using UV/ozone treatment and measured the sizes of graphene grains grown on the electropolished Cu foil from more than 70 grain images. Representative SEM images for the graphene layers grown on the unpolished and electropolished Cu foils are shown in Fig. 3(a) and (b). In Fig. 3(a), an SGR is observed as seen in Fig. 1(a). However, it is notable that heterogeneous regions that are distinguishable with grain sizes are not observed for the graphene on the electropolished Cu foil [Fig. 3(b)]. Fig. 3(c) and (d) show the distribution of measured grain sizes for the graphene layers grown on the unpolished and electropolished Cu foils, respectively. As expected from the SEM images, there were no two distinguishable peaks in the histogram for the graphene on the electropolished Cu substrate differing from those for the graphene on the unpolished Cu foil. This result implies that the formation of SGRs could be suppressed by the electropolishing process. For further 1416
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Fig. 2. (a) Schematic of the process for electropolishing Cu foils. (b) Confocal laser-scanning microscopy images of the unpolished (top) and electropolished (bottom) Cu-foil surfaces. (c) AFM topographic images of the unpolished (top) and electropolished (bottom) Cu foils. (d) Height profiles of the two Cu-foil surfaces corresponding to the red lines in the AFM images. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
observation, we transferred the graphene layers on Cu foils onto oxidized silicon substrates. It has been reported that an excess growth time causes the formation of bilayer graphene during graphene growth [25]. Thus, if there were SGRs and LGRs in a graphene layer, bilayers would be formed in SGRs by the excess growth time because a relatively shorter time is required for grain islands to merge with adjacent grains and form a continuous polycrystalline single-layer graphene [26,27]. Fig. 3(e) and (f) show optical microscopy images of the transferred graphene layers. In Fig. 3(e), it is observed that graphene bilayers were formed and concentrated in long and narrow regions. The distance between the bilayer graphene regions was comparable with the pitch of striation. In contrast, bilayers were rarely found in the graphene grown
on the electropolished Cu foil [Fig. 3(f)]. This result is indirect evidence for the fact that the grain sizes of the graphene on the electropolished Cu foil were larger than those of the graphene on the unpolished Cu foil and the electropolishing process effectively suppressed the formation of SGRs along Cu striations. Because the average grain size is determined by the nucleation density, investigation of the nucleation density of graphene and distribution of graphene grains is important to understand the effect of Cu striations on the nucleation of graphene. We synthesized graphene for a short time and finished the growth step at the initial stage of graphene nucleation. Except the growth time, the growth conditions were not changed. We synthesized two sets of graphene layers. The first set was
Fig. 3. SEM images of graphene grains on (a) an unpolished and (b) electropolished Cu foil. Distribution of graphene grain sizes for graphene grown on (c) the unpolished and (d) electropolished Cu foil. Optical images of graphene layers transferred on oxidized silicon substrates: graphene grown on (e) the unpolished and (f) electropolished Cu foil. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 1417
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Fig. 4. SEM images of graphene grains grown for 1 min on (a) an unpolished and (b) electropolished Cu foil. SEM images of the graphene grains grown for 5 min on (c) an unpolished and (d) electropolished Cu foil. Distribution of the angles (θ) of the lines connecting each grain with its nearest grains for graphene grains grown on (e) the unpolished and (f) electropolished Cu foil.
grown for 1 min on an unpolished Cu foil and electropolished Cu foil simultaneously, and the second set was grown for 5 min. Fig. 4(a) and (b) show SEM images of the first set of graphene layers grown for 1 min. The graphene grains grown on the unpolished Cu substrate were arranged in the form of parallel lines. The distances between the lines were 10–30 μm which was comparable with the pitch of Cu striation [Fig. 4(a)]. In contrast, the graphene grains grown on the electropolished Cu substrate were randomly distributed as shown in Fig. 4(b). This result indicates that the nucleation of graphene was concentrated in the region of Cu foil striations. When we increased the growth time to 5 min, the graphene grains grown on the unpolished Cu substrate increased in size and maintained the parallel distribution that is similar to that of the grains grown for the initial 1 min as shown in Fig. 4(c). The grain boundaries are formed when these grains are further enlarged and meet each other. A region with a higher density of nucleation and grain islands would be a region with a smaller average grain size after the formation of a continuous graphene layer [15,28]. Thus, these results indicate that graphene grown on an unpolished Cu foil had small-grain regions in the region with the smaller density of nucleation and the graphene grown on an electropolished Cu foil had relatively large grain sizes. To quantitatively show the effect of the
surface morphology of Cu substrates on the formation of graphene nucleation and grains, we investigate the patterns of the arranged small grains by measuring the angles (θ) of the lines connecting each grain with its nearest grains as shown in Fig. 4(c) (dashed line). Fig. 4(e) and (f) show the histogram plots for the distribution of θ for the graphene grains grown on the unpolished and electropolished Cu foils, respectively. These plots clearly indicates that the graphene grains grown on the unpolished Cu substrate were concentrated along a specific direction [Fig. 4(e)] while the grains on the electropolished Cu substrate were randomly distributed [Fig. 4(f)]. This result implies that the electropolishing process could effectively eliminate the formation of the high-density nucleation in specific regions corresponding to Cu-foil striations. We showed that the average grain size of the graphene layer grown on the electropolished Cu foil was larger than that of the graphene layer grown on the unpolished Cu foil. In literature, it has been reported that the nucleation density of graphene grown on Cu foils could be controlled by the ratio of H2:CH4 flow rate. An increase in the ratio of H2:CH4 resulted in a reduced density of graphene nucleation and an increased average grain size due to the decrease in the fraction of the carbon source (CH4) in the supplied gas [7–9]. Since the density of the 1418
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Fig. 5. (a) Average grain sizes of graphene grown on unpolished (black squares) and electropolished (red circles) Cu foils under different growth conditions with the H2:CH4 ratio of 10, 50, and 200. (b) Uniformity of the graphene grain sizes evaluated by the uniformity factors for the graphene layers grown on unpolished and electropolished Cu foils with different growth conditions. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
graphene nucleation was affected by the morphology of the Cu-foil surface, the controllability of the average grain size by the growth condition adjustment also could be affected by the Cu-foil surface morphology. To investigate this effect, we synthesized graphene layers on unpolished and electropolished Cu foils under different growth conditions. Fig. 5(a) shows the average grain sizes of the graphene layers grown on the unpolished and electropolished Cu foils under three different growth conditions (H2:CH4 ratios of 10, 50, and 200). As expected, the average grain size increased as H2:CH4 ratios was increased. It can be noted that the average grain size of the graphene layer on the electropolished Cu foil was more effectively controlled by the H2:CH4 ratio than that of graphene layer on the unpolished Cu foil. When we increased the H2:CH4 ratio from 10 to 200, the average grain size of graphene on the electropolished Cu foil increased from 4.9 to 26.7 μm while the average grain size of graphene on the unpolished Cu foil increased only from 4.47 to 7.26 μm. This result indicates that the electropolishing process significantly enhanced the effectiveness of the control over the average grain size by the ratio of H2:CH4 flow rate. Fig. 5(b) shows the uniformity of the graphene grain sizes evaluated by the uniformity factors for the graphene layers grown on unpolished and electropolished Cu foils with different growth conditions. The uniformity factor of the grain sizes decreased as the average grain size increased by the increase in the H2:CH4 ratio because the coefficient of variation (CV) typically increases as the average value of a distribution increases [23]. Nevertheless, the grain-size uniformity factors of the graphene layers on the electropolished Cu substrates were higher than those of the graphene layers on the unpolished Cu foil. These results show that the coexistence of SGRs/LGRs in a graphene layer decreases the average grain size of CVD-grown graphene, deteriorates the global uniformity of grain sizes, and hinders the control of the grain size by adjustment of growth conditions. Since the small-grain regions (SGRs) of graphene were grown on the Cu-foil striations, the electropolishing process could effectively suppress the SGR formation and enhance the quality of the CVD-grown graphene by increasing the average grain size, monolayer coverage, and the uniformity of grain sizes.
Conflicts of interest There are no conflicts to declare. Acknowledgements This work was supported in part by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2017R1A4A1015565), and the Bio & Medical Technology Development Program of the NRF funded by the Ministry of Science & ICT (2017M3A9G8083382). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cap.2019.09.005. References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (5696) (2004) 666–669 https://doi.org/10.1126/science.1102896. [2] X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc, S.K. Banerjee, L. Colombo, R.S. Ruoff, Large-area synthesis of high-quality and uniform graphene films on copper foils, Science 324 (5932) (2009) 1131–1314 https://doi.org/10.1126/science.1171245. [3] S. Bae, H. Kim, Y. Lee, X. Xu, J.-S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H.R. Kim, Y.I. Song, Y.-J. Kim, K.S. Kim, B. Özyilmaz, J.-H. Ahn, B.H. Hong, S. Iijima, Roll-toRoll production of 30-inch graphene films for transparent electrodes, Nat. Nanotechnol. 5 (2010) 574–578 https://doi.org/10.1038/nnano.2010.132. [4] K.S. Novoselov, V.I. Fal′ko, L. Colombo, P.R. Gellert, M.G. Schwab, K. Kim, A roadmap for graphene, Nature 490 (2012) 192–200 https://doi.org/10.1038/ nature11458. [5] A.W. Tsen, L. Brown, M.P. Levendorf, F. Ghahari, P.Y. Huang, R.W. Havener, C.S. Ruiz-Vargas, D.A. Muller, P. Kim, J. Park, Tailoring electrical transport across grain boundaries in polycrystalline graphene, Science 336 (6085) (2012) 1143–1146 https://doi.org/10.1126/science.1218948. [6] Q.K. Yu, L.A. Jauregui, W. Wu, R. Colby, J.F. Tian, Z.H. Su, H.L. Cao, Z.H. Liu, D. Pandey, D.G. Wei, T.F. Chung, P. Peng, N.P. Guisinger, E.A. Stach, J.M. Bao, S.S. Pei, Y.P. Chen, Control and characterization of individual grains and grain boundaries in graphene grown by chemical vapour deposition, Nat. Mater. 10 (2011) 443–449 https://doi.org/10.1038/NMAT3010. [7] X. Li, C.W. Magnuson, A. Venugopal, J. An, J.W. Suk, B. Han, M. Borysiak, W. Cai, A. Velamakanni, Y. Zhu, L. Fu, E.M. Vogel, E. Voelkl, L. Colombo, R.S. Ruoff, Graphene films with large domain size by a two-step chemical vapor deposition process, Nano Lett. 10 (2010) 4328–4334 https://doi.org/10.1021/nl101629g. [8] B. Vlassiouk, S. Smirnov, M. Regmi, S.P. Surwade, N. Srivastava, R. Feenstra, G. Eres, C. Parish, N. Lavrik, P. Datskos, S. Dai, P. Fulvio, Graphene nucleation density on copper: fundamental role of background pressure, J. Phys. Chem. C 117 (2013) 18919–18926 https://doi.org/10.1021/jp4047648. [9] Z. Yan, J. Lin, Z. Peng, Z. Sun, Y. Zhu, L. Li, C. Xiang, E. Loïc Samuel, C. Kittrell, J.M. Tour, Toward the synthesis of wafer-scale single-crystal graphene on copper foils, ACS Nano 6 (10) (2012) 9110–9117 https://doi.org/10.1021/nn303352k. [10] B. Zhang, W.H. Lee, R. Piner, I. Kholmanov, Y. Wu, H. Li, H. Ji, Rodney S. Ruoff, Low-temperature chemical vapor deposition growth of graphene from toluene on electropolished copper foils, ACS Nano 6 (3) (2012) 2471–2476 https://doi.org/10. 1021/nn204827h. [11] X. Wu, G. Zhong, L. D'Arsiè, H. Sugime, S. Esconjauregui, A.W. Robertson, J. Robertson, Growth of continuous monolayer graphene with millimeter-sized domains using industrially safe conditions, Sci. Rep. 6 (2016) 21152 https://doi.
4. Conclusions We observed that polycrystalline graphene layer grown on a typical Cu foil features two heterogeneous regions with different average grain sizes: small-grain regions (SGRs) and large-grain regions (LGRs). Based statistical analysis on the grain sizes, we investigated the effect of the surface morphology of Cu-foil substrates on the average grain size and the uniformity of grain sizes of graphene layers grown on an unpolished and electropolished Cu-foil. This work suggests that the suppression of the SGR formation through a surface-flattening process enhance the quality of the CVD-grown graphene by increasing the average grain size, monolayer coverage, and the uniformity of grain sizes. 1419
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