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Rapid growth of single-layer graphene on the insulating substrates by thermal CVD
Applied Surface Science 346 (2015) 41–45
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Rapid growth of single-layer graphene on the insulating substrates by thermal CVD C.Y. Chen a,b,1 , D. Dai b,1 , G.X. Chen b , J.H. Yu b , K. Nishimura b,c , C.-T. Lin b , N. Jiang b,∗ , Z.L. Zhan a,∗ a
Faculty of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, PR China Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, PR China c Advanced Nano-processing Engineering Lab, Mechanical Systems Engineering, Kogakuin University, Japan b
a r t i c l e
i n f o
Article history: Received 25 November 2014 Received in revised form 29 March 2015 Accepted 29 March 2015 Available online 5 April 2015 Keywords: Single-layer graphene Rapid growth Insulating substrates Thermal CVD
a b s t r a c t The advance of CVD technique to directly grow graphene on the insulating substrates is particularly significant for further device fabrication. As graphene is catalytically grown on metal foils, the degradation of the sample properties is unavoidable during transfer of graphene on the dielectric layer. Moreover, shortening the treatment time as possible, while achieving single-layer growth of graphene, is worthy to be investigated for promoting the efficiency of mass production. Here we performed a rapid heating/cooling process to grow graphene films directly on the insulating substrates by thermal CVD. The treating time consumed is ≈25% compared to conventional CVD procedure. In addition, we found that high-quality, single-layer graphene can be formed on quartz, but on SiO2 /Si substrate only few-layer graphene can be obtained. The pronounced substrate effect is attributed to the different dewetting behavior of Ni films on the both substrates at 950 ◦ C. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Graphene is a one-atom-thick layer of sp2 -bonded carbon atoms packed into a two-dimensional honeycomb crystal lattice [1]. Due to its outstanding performance such as ultrafast carrier mobility [2], excellent mechanical strength [3], and high transparency in visible light region [4], graphene is regarded being a promising multifunctional material for advanced electronic [5,6]/optoelectronic devices [7,8] and sensing applications [9]. In recent years, highquality graphene has been obtained by chemical vapor deposition (CVD) using metal foils as catalytic substrates, such as Cu [10–12], Ni [13–15], Pt [16,17], and Ru [18,19], etc. So far 100-meter long graphene sheet can be made by roll-to-roll CVD production [20,21]. Based on catalytic CVD, the graphene film was grown on the conducting substrates, thus it’s necessary to perform a transfer process to detach graphene from metal foils, and then set onto the insulating substrates for further device fabrication [22,23]. However, the transfer process would cause additional contamination, raise
∗ Corresponding authors. E-mail addresses: [email protected] (N. Jiang), zl [email protected] (Z.L. Zhan). 1 These authors contributed equally to this study. http://dx.doi.org/10.1016/j.apsusc.2015.03.204 0169-4332/© 2015 Elsevier B.V. All rights reserved.
the surface roughness, and eventually degrade the performance of graphene-based devices [24,25]. Therefore, the development of a CVD technique to directly grow graphene on the insulating substrates, especially on silicon-based substrates, attracts great attentions. Some works have been done to grow graphene on the surface of catalyst layer-deposited silicon wafer [7,26,27], but it’s not feasible for direct use in device fabrication due to the fact that the underneath metal layer still dominates the device properties. To create an approach to direct growth of graphene films on the insulating substrates, Su et al. indicated that graphene thin layers may be formed on SiO2 /Si substrates using thermal CVD with 300 nm Cu-deposited silicon as starting substrate [28]. They found that during CVD the dissociated carbon atoms not only migrate on copper surfaces, but also diffuse to the interface between Cu and underlying insulator through the boundary between copper grains. This results in the formation of graphene films on both sides of Cu layer. In Su’s work, the growth temperature (900 ◦ C) is lower than conventional one (950–1000 ◦ C) to avoid the evaporation and dewetting of Cu films [29], but this leads the limitation of graphene quality. Moreover, Kato et al. introduced a transfer-free method for growing carrier-density-controlled graphene on SiO2 /Si substrates by rapid-heating plasma CVD [30]. However, the generation of a uniform plasma region is difficult to be controlled for depositing large-area graphene films. Expect using hydrocarbon gases as
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C.Y. Chen et al. / Applied Surface Science 346 (2015) 41–45
Fig. 1. (a) The schematic illustration of graphene formation on insulators by CVD. (b) The temperature profile of rapid and normal growth processes. (c) Photos of the samples on quartz prepared by rapid and normal growth, respectively.
precursors, Peng et al. obtained bilayer graphene on SiO2 /Si by the diffusion of decomposed carbon through nickel layer with a sandwich structure (polymer film/400 nm Ni/300 nm SiO2 /Si) [31]. As a result, the direct formation technique is a simple, straightforward way to prepare graphene films with smooth surface and less contamination [32–37], which is especially important for the fabrication of high-performance graphene-based devices. So far it still needs more efforts to investigate the controllability of graphene growth based on this approach. In this study, we conducted a rapid heating/cooling process by thermal CVD to grow graphene films using Ni-deposited SiO2 /Si and quartz as substrates, respectively. The rapid thermal treatment was performed in a sliding tube furnace. After removal of nickel, singleor few-layer graphene films can be seen on the both substrates. Using this process, not only the treating time can be greatly shortened, we also found that the substrate effect is more pronounced for the quality of graphene than that grown by conventional CVD process. 2. Experimental 300 nm-thick nickel films were deposited on 300 nm-thick SiO2 /Si substrates or quartz by mini-type ion sputtering apparatus (Quorum 150 T ES, UK). To compare with the rapid growth investigated in this study, another CVD process commonly utilized in the literatures (named normal growth) was performed as well [38,39]. The difference between two methods is that the heating rate (10 ◦ C/min) of the samples in normal growth is much slower than ours (95 ◦ C/min). In rapid thermal process, the Ni-deposited substrates were set in a sliding tube furnace (BTF-1200C-II-SL, AnHui BEQ Equipment Tech., China), but out of the heating zone before growth. First, the furnace was preheated to 950 ◦ C with H2 flow of 20 sccm. While the temperature reaches 950 ◦ C, the furnace was moved to align the substrate at the center of the heating zone, resulting in rapid temperature increase of the substrates. Meanwhile, the gas mixture of H2 /CH4 (20/5 sccm) at total pressure of 0.38 Torr was introduced into the system to trigger the growth of graphene. After 5–20 min growth, the samples were moved out of the heating zone for rapid cooling down (300 ◦ C/min) with flowing 20 sccm H2 and 150 sccm Ar. The samples were determined by Raman spectroscopy (Renishaw plc, Wotton-under-Edge) employing a laser wavelength of 532 nm and transmission electron microscopy (TEM, JEOL JEM-2100).
3. Results and discussions A schematic illustration shown in Fig. 1a presents the concept of graphene growth mechanism on both side of catalytic nickel layer by thermal CVD. At 950 ◦ C, the methane gas decomposed to form carbon atoms, which dissolved into and diffused through Ni layer due to high solubility of carbon in nickel. Note that during cooling step, the carbon atoms would be precipitated and assembled into graphene films not only on the top of nickel, but also at the interface between Ni and the substrate [31]. The graphene on the insulators can be obtained after removal of nickel by immersing the samples in etching solution (10 g CuSO4 and 50 ml HCl in 50 ml DI water) for half an hour. To confirm that the graphene was formed at the interface, plasma treatment was performed to etch the graphene on Ni surface, and we still found graphene on the insulating substrates after Ni etching. Differing from the conventional CVD process (normal growth) [40,41], the treatment period of our method (rapid growth) is much shorter, as shown in Fig. 1b. The sample quality and layer number, as a function of the growth time, will be investigated and discussed later. Fig. 1c exhibits the samples prepared by two kinds of processes on quartz substrates after removal of nickel. Fig. 2 shows the typical Raman spectra of graphene grown by rapid thermal CVD using (300 nm) Ni-deposited SiO2 /Si and quartz as the substrates, respectively. The samples prepared by normal growth method were also determined for comparison. Note that the thickness of Ni layer has been fine-tuned: the graphene quality is worse as the catalytic layer is too thin (200 nm), and no graphene can be seen on the substrate surface after Ni etching when thick layer (400 nm) was used. The spectra in Fig. 2a and c come from the graphene films on the Ni surface after CVD, and those in Fig. 2b and d are the graphene formed on the surface of the insulating substrates after etching of nickel layer. All the spectra exhibit the characteristic peaks of graphene from single- to few-layers: Gband (1579–1588 cm−1 ) is attributed to the stretching-vibration mode of sp2 sites, and 2D-band located at 2688–2699 cm−1 is originated from a double-resonance process [42–44]. Very weak D-band (1345–1355 cm−1 ) indicates fewer defects in graphene films. Moreover, Ni et al. reported that larger I2D /IG ratio represents higher degree of sp2 -hybridized carbon-carbon bonding in graphene, thus I2D /IG ratio and the full width at half maximum (FWHM) of the 2D-band can be an indicator for determining the graphene layer number [45,46]. Our results indicate that in rapid growth, the graphene films formed originally at the interface exhibit higher
C.Y. Chen et al. / Applied Surface Science 346 (2015) 41–45
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Fig. 2. The typical Raman spectra of graphene films grown on SiO2 /Si substrates (a) before and (b) after Ni etching, as well as those samples grown on quartz (c) before and (d) after Ni etching.
Fig. 3. The estimated (a) I2D /IG ratios and (b) 2D-band FWHM of graphene films as a function of the growth time at 950 ◦ C. The results prepared by normal growth method are also shown for comparison.
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Fig. 4. The cross-sectional TEM images of (a) the sample after CVD growth and few-layer graphene observed on (b) the surface of quartz and (c) the top of nickel.
I2D /IG ratio than that on the surface of nickel, regardless of on SiO2 /Si or quartz substrate. And, the I2D /IG ratio of the samples increases with the increase of the growth time. In addition, the samples grown on quartz show a better quality than that on SiO2 /Si substrates. Note that under the same conditions the sample prepared by normal growth is usually composed of few-layer graphene. The changes of I2D /IG ratios and 2D-band FWHM of graphene films as a function of the growth time have been summarized in Fig. 3, respectively. In Fig. 3b, I2D /IG ratio of the samples after 5min rapid growth on the top of Ni-deposited quartz (before etching) is 0.75, which is increased to 0.96 when the time reaches 20 min, showing the characteristics of few-layer graphene. Meanwhile, the I2D /IG of the sample formed at the interface (after etching) increases from 1.02 to 1.71, and the FWHM decreases from 43.1 to 40.2 cm−1 (Fig. 3d) [31], demonstrating the formation of single-layer graphene on quartz by rapid growth method. On the contrary, after Ni etching, the I2D /IG of the samples adhered on SiO2 /Si slightly increases from 1.34 to 1.41, as the growth time increases from 5 to 20 min (Fig. 3a). The formation of thinner graphene at the interface is attributed to the different precipitation rate of carbon atoms above and underneath Ni layer during cooling. In the cooling step, the heat may be efficiently extracted by the substrates to freeze the carbon precipitation at the interface between Ni and the substrate, whereas few-layer graphene is grown on the Ni surface due to the poor convection effect under low pressure (0.90 Torr). As shown in Fig. 3a and b, the I2D /IG ratios of the samples increases with the increase of the growth time, suggesting that the appropriate concentration of the dissolved carbon atoms in nickel would help to form uniform graphene films with large domain size. Fig. 4a is the cross-sectional TEM image of the sample obtained using quartz as the substrate, exhibiting a multilayer structure (graphene/Ni/graphene/quartz). The graphene films composed of 3–6 layers attached on quartz surface and the top of nickel can be seen in Fig. 4b and c, respectively. Note that we do not obtain distinct images of single-layer graphene at the cross-section of the insulating substrates, possibly due to the destruction of oneatom-thick layer during sample preparation for TEM observation. In addition, we notice that the growth temperature is a key factor affecting the graphene quality. As the growth temperature rises to 1000 ◦ C, nickel nanoparticles would be formed over the insulating substrates during CVD, resulting in a discontinuous graphene structure. This can be attributed to the dewetting and partial evaporation of nickel film at high temperature and under low pressure [31]. Moreover, even reacting at 950 ◦ C, the dewetting phenomena of the samples on SiO2 /Si substrate is still stronger than on quartz. As shown in Fig. 3a and b, the I2D /IG of the samples directly formed on quartz increases with increasing growth time, in contrast to those on SiO2 /Si substrate, which exhibit lower I2D /IG ratio. The competition between Ni evaporation/reconstruction and graphene growth may influence on the resulting quality of graphene. We conclude that during CVD growth, the rapid thermal process is an efficient approach to suppress the dewetting and evaporation of Ni film in
contrast with the normal growth, achieving the better quality of graphene films with shorter growth time. 4. Conclusions In summary, we developed a rapid thermal CVD process to grow graphene films both on Ni surface and at the interface between Ni layer and insulating substrate. After removal of nickel, single-layer and few-layer graphene directly adhered on quartz and SiO2 /Si can be obtained, respectively. The treating time consumed is ≈25% compared to conventional CVD procedure. The formation of thinner graphene at the interface is attributed to the faster precipitation rate of carbon atoms underneath than above Ni layer during cooling. In addition, only few-layer graphene can be found using conventional CVD both on quartz and silicon substrates, because of the dewetting behavior of Ni films during the long treatment time. The contamination-free single-layer graphene may stimulate further development in device applications, such as highly-sensitive sensors/detectors and transparent electrodes. Acknowledgements This work was supported by the National Science Foundation of China (Grant No. 51303034). 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 (2004) 666–669. [2] T. Hesjedal, Continuous roll-to-roll growth of graphene films by chemical vapor deposition, Appl. Phys. Lett. 98 (2011) 133106. [3] C. Lee, X. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene, Science 321 (2008) 385–388. [4] R. Nair, P. Blake, A. Grigorenko, K. Novoselov, T. Booth, T. Stauber, N. Peres, A. Geim, Fine structure constant defines visual transparency of graphene, Science 320 (2008) 1308. [5] Y.M. Lin, C. Dimitrakopoulos, K.A. Jenkins, D.B. Farmer, H.Y. Chiu, A. Grill, P. Avouris, 100-GHz transistors from wafer-scale epitaxial graphene, Science 327 (2010) 662. [6] D.-H. Kim, J.-H. Ahn, W.M. Choi, H.-S. Kim, T.-H. Kim, J. Song, Y.Y. Huang, Z. Liu, C. Lu, J.A. Rogers, Stretchable and foldable silicon integrated circuits, Science 320 (2008) 507–511. [7] K.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, J.M. Kim, K.S. Kim, J.H. Ahn, P. Kim, J.Y. Choi, B.H. Hong, Large-scale pattern growth of graphene films for stretchable transparent electrodes, Nature 457 (2009) 706–710. [8] H.C. Ko, M.P. Stoykovich, J. Song, V. Malyarchuk, W.M. Choi, C.-J. Yu, J.B. Geddes Iii, J. Xiao, S. Wang, Y. Huang, A hemispherical electronic eye camera based on compressible silicon optoelectronics, Nature 454 (2008) 748–753. [9] X. Dong, Y. Shi, W. Huang, P. Chen, L.J. Li, Electrical detection of DNA hybridization with single-base specificity using transistors based on CVDgrown graphene sheets, Adv. Mater. 22 (2010) 1649–1653. [10] X. Li, C.W. Magnuson, A. Venugopal, J. An, J.W. Suk, B. Han, M. Borysiak, W. Cai, A. Velamakanni, Y. Zhu, Graphene films with large domain size by a two-step chemical vapor deposition process, Nano Lett. 10 (2010) 4328–4334. [11] X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc, Large-area synthesis of high-quality and uniform graphene films on copper foils, Science 324 (2009) 1312–1314.
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Rapid growth of single-layer graphene on the insulating substrates by thermal CVD C.Y. Chen a,b,1 , D. Dai b,1 , G.X. Chen b , J.H. Yu b , K. Nishimura b,c , C.-T. Lin b , N. Jiang b,∗ , Z.L. Zhan a,∗ a
Faculty of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, PR China Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, PR China c Advanced Nano-processing Engineering Lab, Mechanical Systems Engineering, Kogakuin University, Japan b
a r t i c l e
i n f o
Article history: Received 25 November 2014 Received in revised form 29 March 2015 Accepted 29 March 2015 Available online 5 April 2015 Keywords: Single-layer graphene Rapid growth Insulating substrates Thermal CVD
a b s t r a c t The advance of CVD technique to directly grow graphene on the insulating substrates is particularly significant for further device fabrication. As graphene is catalytically grown on metal foils, the degradation of the sample properties is unavoidable during transfer of graphene on the dielectric layer. Moreover, shortening the treatment time as possible, while achieving single-layer growth of graphene, is worthy to be investigated for promoting the efficiency of mass production. Here we performed a rapid heating/cooling process to grow graphene films directly on the insulating substrates by thermal CVD. The treating time consumed is ≈25% compared to conventional CVD procedure. In addition, we found that high-quality, single-layer graphene can be formed on quartz, but on SiO2 /Si substrate only few-layer graphene can be obtained. The pronounced substrate effect is attributed to the different dewetting behavior of Ni films on the both substrates at 950 ◦ C. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Graphene is a one-atom-thick layer of sp2 -bonded carbon atoms packed into a two-dimensional honeycomb crystal lattice [1]. Due to its outstanding performance such as ultrafast carrier mobility [2], excellent mechanical strength [3], and high transparency in visible light region [4], graphene is regarded being a promising multifunctional material for advanced electronic [5,6]/optoelectronic devices [7,8] and sensing applications [9]. In recent years, highquality graphene has been obtained by chemical vapor deposition (CVD) using metal foils as catalytic substrates, such as Cu [10–12], Ni [13–15], Pt [16,17], and Ru [18,19], etc. So far 100-meter long graphene sheet can be made by roll-to-roll CVD production [20,21]. Based on catalytic CVD, the graphene film was grown on the conducting substrates, thus it’s necessary to perform a transfer process to detach graphene from metal foils, and then set onto the insulating substrates for further device fabrication [22,23]. However, the transfer process would cause additional contamination, raise
∗ Corresponding authors. E-mail addresses: [email protected] (N. Jiang), zl [email protected] (Z.L. Zhan). 1 These authors contributed equally to this study. http://dx.doi.org/10.1016/j.apsusc.2015.03.204 0169-4332/© 2015 Elsevier B.V. All rights reserved.
the surface roughness, and eventually degrade the performance of graphene-based devices [24,25]. Therefore, the development of a CVD technique to directly grow graphene on the insulating substrates, especially on silicon-based substrates, attracts great attentions. Some works have been done to grow graphene on the surface of catalyst layer-deposited silicon wafer [7,26,27], but it’s not feasible for direct use in device fabrication due to the fact that the underneath metal layer still dominates the device properties. To create an approach to direct growth of graphene films on the insulating substrates, Su et al. indicated that graphene thin layers may be formed on SiO2 /Si substrates using thermal CVD with 300 nm Cu-deposited silicon as starting substrate [28]. They found that during CVD the dissociated carbon atoms not only migrate on copper surfaces, but also diffuse to the interface between Cu and underlying insulator through the boundary between copper grains. This results in the formation of graphene films on both sides of Cu layer. In Su’s work, the growth temperature (900 ◦ C) is lower than conventional one (950–1000 ◦ C) to avoid the evaporation and dewetting of Cu films [29], but this leads the limitation of graphene quality. Moreover, Kato et al. introduced a transfer-free method for growing carrier-density-controlled graphene on SiO2 /Si substrates by rapid-heating plasma CVD [30]. However, the generation of a uniform plasma region is difficult to be controlled for depositing large-area graphene films. Expect using hydrocarbon gases as
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Fig. 1. (a) The schematic illustration of graphene formation on insulators by CVD. (b) The temperature profile of rapid and normal growth processes. (c) Photos of the samples on quartz prepared by rapid and normal growth, respectively.
precursors, Peng et al. obtained bilayer graphene on SiO2 /Si by the diffusion of decomposed carbon through nickel layer with a sandwich structure (polymer film/400 nm Ni/300 nm SiO2 /Si) [31]. As a result, the direct formation technique is a simple, straightforward way to prepare graphene films with smooth surface and less contamination [32–37], which is especially important for the fabrication of high-performance graphene-based devices. So far it still needs more efforts to investigate the controllability of graphene growth based on this approach. In this study, we conducted a rapid heating/cooling process by thermal CVD to grow graphene films using Ni-deposited SiO2 /Si and quartz as substrates, respectively. The rapid thermal treatment was performed in a sliding tube furnace. After removal of nickel, singleor few-layer graphene films can be seen on the both substrates. Using this process, not only the treating time can be greatly shortened, we also found that the substrate effect is more pronounced for the quality of graphene than that grown by conventional CVD process. 2. Experimental 300 nm-thick nickel films were deposited on 300 nm-thick SiO2 /Si substrates or quartz by mini-type ion sputtering apparatus (Quorum 150 T ES, UK). To compare with the rapid growth investigated in this study, another CVD process commonly utilized in the literatures (named normal growth) was performed as well [38,39]. The difference between two methods is that the heating rate (10 ◦ C/min) of the samples in normal growth is much slower than ours (95 ◦ C/min). In rapid thermal process, the Ni-deposited substrates were set in a sliding tube furnace (BTF-1200C-II-SL, AnHui BEQ Equipment Tech., China), but out of the heating zone before growth. First, the furnace was preheated to 950 ◦ C with H2 flow of 20 sccm. While the temperature reaches 950 ◦ C, the furnace was moved to align the substrate at the center of the heating zone, resulting in rapid temperature increase of the substrates. Meanwhile, the gas mixture of H2 /CH4 (20/5 sccm) at total pressure of 0.38 Torr was introduced into the system to trigger the growth of graphene. After 5–20 min growth, the samples were moved out of the heating zone for rapid cooling down (300 ◦ C/min) with flowing 20 sccm H2 and 150 sccm Ar. The samples were determined by Raman spectroscopy (Renishaw plc, Wotton-under-Edge) employing a laser wavelength of 532 nm and transmission electron microscopy (TEM, JEOL JEM-2100).
3. Results and discussions A schematic illustration shown in Fig. 1a presents the concept of graphene growth mechanism on both side of catalytic nickel layer by thermal CVD. At 950 ◦ C, the methane gas decomposed to form carbon atoms, which dissolved into and diffused through Ni layer due to high solubility of carbon in nickel. Note that during cooling step, the carbon atoms would be precipitated and assembled into graphene films not only on the top of nickel, but also at the interface between Ni and the substrate [31]. The graphene on the insulators can be obtained after removal of nickel by immersing the samples in etching solution (10 g CuSO4 and 50 ml HCl in 50 ml DI water) for half an hour. To confirm that the graphene was formed at the interface, plasma treatment was performed to etch the graphene on Ni surface, and we still found graphene on the insulating substrates after Ni etching. Differing from the conventional CVD process (normal growth) [40,41], the treatment period of our method (rapid growth) is much shorter, as shown in Fig. 1b. The sample quality and layer number, as a function of the growth time, will be investigated and discussed later. Fig. 1c exhibits the samples prepared by two kinds of processes on quartz substrates after removal of nickel. Fig. 2 shows the typical Raman spectra of graphene grown by rapid thermal CVD using (300 nm) Ni-deposited SiO2 /Si and quartz as the substrates, respectively. The samples prepared by normal growth method were also determined for comparison. Note that the thickness of Ni layer has been fine-tuned: the graphene quality is worse as the catalytic layer is too thin (200 nm), and no graphene can be seen on the substrate surface after Ni etching when thick layer (400 nm) was used. The spectra in Fig. 2a and c come from the graphene films on the Ni surface after CVD, and those in Fig. 2b and d are the graphene formed on the surface of the insulating substrates after etching of nickel layer. All the spectra exhibit the characteristic peaks of graphene from single- to few-layers: Gband (1579–1588 cm−1 ) is attributed to the stretching-vibration mode of sp2 sites, and 2D-band located at 2688–2699 cm−1 is originated from a double-resonance process [42–44]. Very weak D-band (1345–1355 cm−1 ) indicates fewer defects in graphene films. Moreover, Ni et al. reported that larger I2D /IG ratio represents higher degree of sp2 -hybridized carbon-carbon bonding in graphene, thus I2D /IG ratio and the full width at half maximum (FWHM) of the 2D-band can be an indicator for determining the graphene layer number [45,46]. Our results indicate that in rapid growth, the graphene films formed originally at the interface exhibit higher
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Fig. 2. The typical Raman spectra of graphene films grown on SiO2 /Si substrates (a) before and (b) after Ni etching, as well as those samples grown on quartz (c) before and (d) after Ni etching.
Fig. 3. The estimated (a) I2D /IG ratios and (b) 2D-band FWHM of graphene films as a function of the growth time at 950 ◦ C. The results prepared by normal growth method are also shown for comparison.
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Fig. 4. The cross-sectional TEM images of (a) the sample after CVD growth and few-layer graphene observed on (b) the surface of quartz and (c) the top of nickel.
I2D /IG ratio than that on the surface of nickel, regardless of on SiO2 /Si or quartz substrate. And, the I2D /IG ratio of the samples increases with the increase of the growth time. In addition, the samples grown on quartz show a better quality than that on SiO2 /Si substrates. Note that under the same conditions the sample prepared by normal growth is usually composed of few-layer graphene. The changes of I2D /IG ratios and 2D-band FWHM of graphene films as a function of the growth time have been summarized in Fig. 3, respectively. In Fig. 3b, I2D /IG ratio of the samples after 5min rapid growth on the top of Ni-deposited quartz (before etching) is 0.75, which is increased to 0.96 when the time reaches 20 min, showing the characteristics of few-layer graphene. Meanwhile, the I2D /IG of the sample formed at the interface (after etching) increases from 1.02 to 1.71, and the FWHM decreases from 43.1 to 40.2 cm−1 (Fig. 3d) [31], demonstrating the formation of single-layer graphene on quartz by rapid growth method. On the contrary, after Ni etching, the I2D /IG of the samples adhered on SiO2 /Si slightly increases from 1.34 to 1.41, as the growth time increases from 5 to 20 min (Fig. 3a). The formation of thinner graphene at the interface is attributed to the different precipitation rate of carbon atoms above and underneath Ni layer during cooling. In the cooling step, the heat may be efficiently extracted by the substrates to freeze the carbon precipitation at the interface between Ni and the substrate, whereas few-layer graphene is grown on the Ni surface due to the poor convection effect under low pressure (0.90 Torr). As shown in Fig. 3a and b, the I2D /IG ratios of the samples increases with the increase of the growth time, suggesting that the appropriate concentration of the dissolved carbon atoms in nickel would help to form uniform graphene films with large domain size. Fig. 4a is the cross-sectional TEM image of the sample obtained using quartz as the substrate, exhibiting a multilayer structure (graphene/Ni/graphene/quartz). The graphene films composed of 3–6 layers attached on quartz surface and the top of nickel can be seen in Fig. 4b and c, respectively. Note that we do not obtain distinct images of single-layer graphene at the cross-section of the insulating substrates, possibly due to the destruction of oneatom-thick layer during sample preparation for TEM observation. In addition, we notice that the growth temperature is a key factor affecting the graphene quality. As the growth temperature rises to 1000 ◦ C, nickel nanoparticles would be formed over the insulating substrates during CVD, resulting in a discontinuous graphene structure. This can be attributed to the dewetting and partial evaporation of nickel film at high temperature and under low pressure [31]. Moreover, even reacting at 950 ◦ C, the dewetting phenomena of the samples on SiO2 /Si substrate is still stronger than on quartz. As shown in Fig. 3a and b, the I2D /IG of the samples directly formed on quartz increases with increasing growth time, in contrast to those on SiO2 /Si substrate, which exhibit lower I2D /IG ratio. The competition between Ni evaporation/reconstruction and graphene growth may influence on the resulting quality of graphene. We conclude that during CVD growth, the rapid thermal process is an efficient approach to suppress the dewetting and evaporation of Ni film in
contrast with the normal growth, achieving the better quality of graphene films with shorter growth time. 4. Conclusions In summary, we developed a rapid thermal CVD process to grow graphene films both on Ni surface and at the interface between Ni layer and insulating substrate. After removal of nickel, single-layer and few-layer graphene directly adhered on quartz and SiO2 /Si can be obtained, respectively. The treating time consumed is ≈25% compared to conventional CVD procedure. The formation of thinner graphene at the interface is attributed to the faster precipitation rate of carbon atoms underneath than above Ni layer during cooling. In addition, only few-layer graphene can be found using conventional CVD both on quartz and silicon substrates, because of the dewetting behavior of Ni films during the long treatment time. The contamination-free single-layer graphene may stimulate further development in device applications, such as highly-sensitive sensors/detectors and transparent electrodes. Acknowledgements This work was supported by the National Science Foundation of China (Grant No. 51303034). 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 (2004) 666–669. [2] T. Hesjedal, Continuous roll-to-roll growth of graphene films by chemical vapor deposition, Appl. Phys. Lett. 98 (2011) 133106. [3] C. Lee, X. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene, Science 321 (2008) 385–388. [4] R. Nair, P. Blake, A. Grigorenko, K. Novoselov, T. Booth, T. Stauber, N. Peres, A. Geim, Fine structure constant defines visual transparency of graphene, Science 320 (2008) 1308. [5] Y.M. Lin, C. Dimitrakopoulos, K.A. Jenkins, D.B. Farmer, H.Y. Chiu, A. Grill, P. Avouris, 100-GHz transistors from wafer-scale epitaxial graphene, Science 327 (2010) 662. [6] D.-H. Kim, J.-H. Ahn, W.M. Choi, H.-S. Kim, T.-H. Kim, J. Song, Y.Y. Huang, Z. Liu, C. Lu, J.A. Rogers, Stretchable and foldable silicon integrated circuits, Science 320 (2008) 507–511. [7] K.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, J.M. Kim, K.S. Kim, J.H. Ahn, P. Kim, J.Y. Choi, B.H. Hong, Large-scale pattern growth of graphene films for stretchable transparent electrodes, Nature 457 (2009) 706–710. [8] H.C. Ko, M.P. Stoykovich, J. Song, V. Malyarchuk, W.M. Choi, C.-J. Yu, J.B. Geddes Iii, J. Xiao, S. Wang, Y. Huang, A hemispherical electronic eye camera based on compressible silicon optoelectronics, Nature 454 (2008) 748–753. [9] X. Dong, Y. Shi, W. Huang, P. Chen, L.J. Li, Electrical detection of DNA hybridization with single-base specificity using transistors based on CVDgrown graphene sheets, Adv. Mater. 22 (2010) 1649–1653. [10] X. Li, C.W. Magnuson, A. Venugopal, J. An, J.W. Suk, B. Han, M. Borysiak, W. Cai, A. Velamakanni, Y. Zhu, Graphene films with large domain size by a two-step chemical vapor deposition process, Nano Lett. 10 (2010) 4328–4334. [11] X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc, Large-area synthesis of high-quality and uniform graphene films on copper foils, Science 324 (2009) 1312–1314.
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