- Email: [email protected]
Large-area synthesis of high-quality and uniform monolayer graphene without unexpected bilayer regions
Journal of Alloys and Compounds 615 (2014) 415–418
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Letter
Large-area synthesis of high-quality and uniform monolayer graphene without unexpected bilayer regions Jingbo Liu, Pingjian Li ⇑, Yuanfu Chen ⇑, Zegao Wang, Jiarui He, Hongjun Tian, Fei Qi, Binjie Zheng, Jinhao Zhou, Wei Lin, Wanli Zhang State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, PR China
a r t i c l e
i n f o
Article history: Received 19 March 2014 Received in revised form 11 June 2014 Accepted 1 July 2014 Available online 9 July 2014 Keywords: Semiconductors Vapor deposition Catalysis Diffusion Electronic properties
a b s t r a c t Chemical vapor deposition is a promising method to synthesize the large area monolayer graphene. However, unexpected bilayer regions are easily formed on the monolayer graphene, which will dramatically degrade the quality and uniformity of graphene. In this work, the large-area, high-quality and uniform monolayer graphene has been synthesized on the Cu foil. The studies reveal that the density of bilayer graphene regions decreases with increasing the growth time; when the growth time increases to 120 min, the formation of bilayer regions is effectively prevented. The corresponding growth mechanism was discussed. Further, the electrical studies reveal that by preventing the formation of bilayer regions, the mobility of graphene not only obviously increases, but also has a narrow distribution, indicating that the as-synthesized monolayer graphene has high quality and uniformity. We expect that this work is beneficial for not only potential applications, but also the fundamental understanding of the growth mechanism for graphene on Cu surface. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Graphene, a one-atom-thick planar sheet of sp2-hybridized carbon, has drawn much attention due to the outstanding properties and wide potential applications [1–3]. For practical applications, it is significant to achieve the large-area synthesis of monolayer graphene with high quality and uniformity. Recently, chemical vapor deposition (CVD) is considered as one of the most promising methods to synthesize large-area monolayer graphene film [4]. However, unexpected multilayer or bilayer regions are usually observed on the CVD monolayer graphene, which obviously influence the quality and uniformity of graphene [5,6]. Until now, there are many efforts to investigate the growth mechanism of CVD graphene films grown on Cu foils [6–8]. Liu et al. [6] demonstrated that the multilayer or bilayer regions were formed on the imperfection sites of Cu substrates with high chemical activation energy, and low pressure annealing in H2 (before graphene growth) could make the Cu surface smoother ⇑ Corresponding authors. Tel.: +86 028 83202291 (P. Li), +86 028 83202710 (Y. Chen). E-mail addresses: [email protected] (J. Liu), [email protected] (P. Li), [email protected] (Y. Chen), [email protected] (Z. Wang), [email protected] (J. He), [email protected] (H. Tian), [email protected] (F. Qi), [email protected] (B. Zheng), [email protected] (J. Zhou), [email protected] (W. Lin), [email protected] (W. Zhang). http://dx.doi.org/10.1016/j.jallcom.2014.07.003 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
and thereby prevent the formation of multilayer regions; however, some bilayer regions were still formed on the monolayer graphene. Herein, we report an effective way to synthesize the high-quality and uniform CVD monolayer graphene without bilayer regions. Optical microscope (OM), scanning electron microscope (SEM), Raman spectroscopy and electrical measurements were used to investigate the growth mechanism and evaluate the quality and uniformity of graphene. 2. Experimental 2.1. CVD synthesis of graphene Firstly, the Cu foil was dipped into the dilute HCl/H2O (1:10) solution for 3 min, and then washed by deionization (DI) water several times to remove the residual acid solution, and then was dried by nitrogen gas. Secondly, the Cu foil was loaded into the silica tube of the CVD system with a vacuum background of 7 104 Pa, and then the growth chamber was heated to 1000 °C and held for 20 min with 30 sccm pure H2 (99.999%), and then the 15 sccm of CH4 was introduced into the tube for graphene growth at 1000 °C for 20–180 min with the pressure of 340 Pa. Finally, cooled the system to room temperature with a cooling rate of 50 °C/min in CH4/H2 ambience. The as-synthesized graphene samples were denoted as G-X, where X represents the growth time (min). 2.2. Transfer process of graphene films Firstly, the surface of graphene grown on the Cu foil was spin-coated with poly(methyl methacrylate) solution (PMMA, A4) at 3000 rpm for 30 s, and then the Cu foil was etched away in the iron trichloride solution (FeCl3, 1 mol/L) for
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Fig. 1. SEM images of G-20 grown on the Cu foil (a) without and (b) with dilute HCl pretreatment. (c) OM image of G-20 transferred onto the SiO2/Si substrate. (d) Raman spectra of the red and black circle regions in the (c). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. (a) Schematic diagram of graphene grown on the Cu foil. (b–d) OM images of G-20, G-80, G-120 and G-180 transferred onto the SiO2/Si substrates.
12 h. Secondly, after the PMMA/graphene film was rinsed repeatedly using the DI water (10 times), it was transferred to the H2O/HCl/H2O2 (20:1:1) solution for 15 min for removing the residual metal particles, and then was transferred to the H2O/NH4OH/H2O2 (20:1:1) solution for 15 min for removing the insoluble organic contaminants. Note that the PMMA/graphene film was rinsed using the DI water to remove the residual solution after each cleaning step. Thirdly, the PMMA/ graphene was transferred onto the target substrate (SiO2/Si or PET substrates), and then was baked at 150 °C for 10 min after natural drying, and then the PMMA was removed using acetone [9].
2.3. OM, SEM, Raman and electrical measurements The surface morphology and structure of graphene were studied by using OM (Olympus BX51M), SEM (JSM-6490LV) and Raman spectrometer (Renishaw, 514 nm) at room temperature.
The graphene field-effect transistors (FETs) were fabricated by using standard photolithography after transferring the graphene onto the 270 nm SiO2/Si substrate. The Ni (30 nm) film was deposited as source and drain electrodes; the 270 nm SiO2 and n++ silicon layers acted as the gate dielectric and back-gate electrode, respectively. The electrical measurements were carried out with an Agilent 4155B semiconductor parameter analyzer in air at room temperature.
3. Results and discussions Before graphene growth, it is necessary to use dilute HCl to remove the native Cu-oxide layer on the Cu foil, otherwise the Cu-oxide nanoparticles will be formed on the graphene, as shown in Fig. 1a. Fig. 1b shows the SEM image of G-20 grown on the Cu foil with dilute HCl pretreatment. Although the graphene surface
J. Liu et al. / Journal of Alloys and Compounds 615 (2014) 415–418
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Fig. 3. (a) Optical photograph of G-120 (7 7 cm2) transferred onto the PET substrate indicating its high uniformity. (b) Typical Raman spectrum of G-120. (c) IG/I2D and (d) ID/IG Raman mapping of G-120 film (50 50 lm2).
Fig. 4. (a) Transfer characteristics curves of G-20 and G-120 FETs. (b) Hole mobilities of G-20 and G-120 FETs, and the values are mean and standard deviation (error bars) for 10 different samples.
is clean, two visibly contrasting regions are observed, indicating that G-20 has poor uniformity. In order to further investigate the uniformity of G-20, it was transferred onto the SiO2/Si substrate. Fig. 1c and d shows the OM image and corresponding Raman spectra of G-20, respectively. For the Raman spectra of the black circle region in Fig. 1c, the G to 2D peak (IG/I2D) ratio is 0.42 and the full-width at half-height maximum (fwhm) of the 2D peak is 36.4 cm1, indicating monolayer graphene [10]. For the Raman spectra of the red circle region in Fig. 1c, the IG/I2D is 0.91 and the fwhm of the 2D peak is 49.7 cm1, indicating bilayer graphene [10]. The above studies reveal that the poor uniformity of G-20 is mainly originated from the formation of unexpected bilayer regions. It is known that uniform monolayer graphene can be synthesized on the Cu substrate with ideal flat surface, having low solubility of carbon [6]. However, compared with flat regions, the imperfection sites (e.g. steps, grain boundaries) of Cu foil have higher activation energy which promotes the catalytic decomposition of CH4, and thus it results in the formation of bilayer regions,
as shown in Fig. 2a. It means that it is critical to make the surface of Cu foil flat to synthesize the monolayer graphene with high uniformity. In order to improve the uniformity of G-20, we have investigated the effect of growth time on the uniformity of graphene. Fig. 2b–d shows the OM images of G-20, G-80, G-120 and G-180 transferred onto the SiO2/Si substrates. With increasing the growth time, the density of bilayer regions decreases; when the growth time reaches 120 min, no bilayer regions are formed. It can be explained by the schematic diagram as shown in Fig. 2a. At the 1000 °C of growth temperature, the Cu and carbon atoms are sublimated, at the same time, CH4 molecules are decomposed on the fresh surface of Cu foil, which is the dynamic equilibrium process. Thus, when the growth time increases, the surface of Cu foil becomes smoother, and thereby improving the uniformity of graphene. Note that annealing in H2 before graphene growth cannot effectively prevent the formation of bilayer regions, because the surface of Cu foil will be etched by H2 during this progress.
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However, this H2 etching effect can be suppressed after introducing CH4, because H2 prefers to promote the catalytic decomposition of CH4 molecules on the Cu foil during the graphene growth process [11,12]. In addition, some narrow wrinkles with hundreds of nanometers wide are observed no matter whether bilayer graphene regions are formed, as shown in Fig. 2b–d. It may be due to the different thermal expansion coefficients of graphene and Cu foil [9], and will be further investigated in the future work. By increasing the growth time, the large-area monolayer graphene (7 7 cm2) with high uniformity (G-120) is synthesized, as shown in Figs. 2d and 3a. The typical Raman spectrum shown in Fig. 3b reveals that the G-120 is monolayer and has high quality (low D peak). For further investigating the uniformity of G-120, Raman mapping at the 50 50 lm2 scale was obtained, as shown in Fig. 3c and d. From Fig. 3c, the IG/I2D ratio is 0.34–0.56, suggesting uniform monolayer graphene [8,13]. From Fig. 3d, the ID/IG ratio is 0–0.15, suggesting few defects in the G-120 film [14]. In order to further evaluate the quality and uniformity of G-120, the graphene FETs were fabricated. Fig. 4a shows the transfer characteristics (Ids–Vg) curves of G-20 and G-120 FETs at a fixed drainsource voltage (Vds) of 1 V. The hole mobility (l) can be calculated using l ¼ g m L=WC g V ds , where g m ¼ dIds =dV g V ¼constant is obtained
and without bilayer regions. The studies reveal that G-20 and G-120 have the mobilities of 933:7 278:2 cm2 V1 s1 and 2534:3 97:6 cm2 V1 s1 , respectively. The result indicates that not only the quality, but also the uniformity of monolayer graphene are obviously improved by preventing the formation of bilayer regions. Acknowledgements The research was supported by the National Natural Science Foundation of China (Grant Nos. 51202022, 51372033 and 61378028), the Program for New Century Excellent Talents in University (Grant No. NCET-10-0291), the 111 Project (Grant No. B13042), the Specialized Research Fund for the Doctoral Program of Higher Education (Gran No. 20120185120011), Sichuan Youth Science and Technology Innovation Research Team Funding (Grant No. 2011JTD0006), the International Science and Technology Cooperation Program of China (Gran No. 2012DFA51430), and the Sino-German Cooperation PPP Program of China. We thank Prof. Ping Xu of Harbin Institute of Technology for his technical help for Raman characterization.
ds
from Fig. 4a; L and W are the length (40 lm) and width (4 lm) of graphene channel, respectively; Cg is the back-gate capacitance with the value of 12.7 nF/cm2 [15]. As a result, the hole mobilities of G-20 and G-120 are 933:7 278:2 cm2 V1 s1 and 2534:3 97:6 cm2 V1 s1 , respectively, as shown in Fig. 4b; the values are mean and standard deviation (error bars) for 10 different samples. Thus, we can conclude that the quality and uniformity of monolayer graphene are obviously improved by preventing the formation of bilayer regions. 4. Conclusions We have synthesized the large-area monolayer graphene with high quality and uniformity on Cu foil. OM, SEM, Raman spectra were used to investigate the effects of growth time on the quality and uniformity of graphene. The results reveal that when the growth time increases, the density of bilayer graphene regions decreases. It is attributed that the Cu surface becomes smoother with the sublimation of Cu atoms during the growth process. As a result, when the growth time reaches 120 min, the monolayer graphene can be synthesized without unexpected bilayer regions. Further, we have studied the electrical properties of graphene with
References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666–669. [2] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183–191. [3] Z. Wang, Y. Chen, P. Li, X. Hao, J. Liu, R. Huang, Y. Li, ACS Nano 5 (2011) 7149– 7154. [4] X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc, Science 324 (2009) 1312–1314. [5] W. Ge, B. Lu, W. Li, J. Lu, Z. Ye, Vacuum 97 (2013) 9–14. [6] Q. Li, H. Chou, J. Zhong, J. Liu, A. Dolocan, J. Zhang, Y. Zhou, R.S. Ruoff, S. Chen, W. Cai, Nano Lett. 13 (2013) 486–490. [7] W. Liu, H. Li, C. Xu, Y. Khatami, K. Banerjee, Carbon 49 (2011) 4122–4130. [8] Z. Sun, A.O. Raji, Y. Zhu, C. Xiang, Z. Yan, C. Kittrell, E. Samuel, J.M. Tour, ACS Nano 6 (2012) 9790–9796. [9] X. Liang, B.A. Sperling, I. Calizo, G. Cheng, C.A. Hacker, Q. Zhang, Y. Obeng, K. Yan, H. Peng, Q. Li, ACS Nano 5 (2011) 9144–9153. [10] L.M. Malard, M.A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, Phys. Rep. 473 (2009) 51–87. [11] I. Vlassiouk, M. Regmi, P. Fulvio, S. Dai, P. Datskos, G. Eres, S. Smirnov, ACS Nano 5 (2011) 6069–6076. [12] L. Liu, H. Zhou, R. Cheng, W.J. Yu, Y. Liu, Y. Chen, J. Shaw, X. Zhong, Y. Huang, X. Duan, ACS Nano 6 (2012) 8241–8249. [13] Z. Yan, Z. Peng, Z. Sun, J. Yao, Y. Zhu, Z. Liu, P.M. Ajayan, J.M. Tour, ACS Nano 5 (2011) 8187–8192. [14] Z. Peng, Z. Yan, Z. Sun, J.M. Tour, ACS Nano 5 (2011) 8241–8247. [15] S. Kim, J. Nah, I. Jo, D. Shahrjerdi, L. Colombo, Z. Yao, E. Tutuc, S.K. Banerjee, Appl. Phys. Lett. 94 (2009) 621071–6210714.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Letter
Large-area synthesis of high-quality and uniform monolayer graphene without unexpected bilayer regions Jingbo Liu, Pingjian Li ⇑, Yuanfu Chen ⇑, Zegao Wang, Jiarui He, Hongjun Tian, Fei Qi, Binjie Zheng, Jinhao Zhou, Wei Lin, Wanli Zhang State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, PR China
a r t i c l e
i n f o
Article history: Received 19 March 2014 Received in revised form 11 June 2014 Accepted 1 July 2014 Available online 9 July 2014 Keywords: Semiconductors Vapor deposition Catalysis Diffusion Electronic properties
a b s t r a c t Chemical vapor deposition is a promising method to synthesize the large area monolayer graphene. However, unexpected bilayer regions are easily formed on the monolayer graphene, which will dramatically degrade the quality and uniformity of graphene. In this work, the large-area, high-quality and uniform monolayer graphene has been synthesized on the Cu foil. The studies reveal that the density of bilayer graphene regions decreases with increasing the growth time; when the growth time increases to 120 min, the formation of bilayer regions is effectively prevented. The corresponding growth mechanism was discussed. Further, the electrical studies reveal that by preventing the formation of bilayer regions, the mobility of graphene not only obviously increases, but also has a narrow distribution, indicating that the as-synthesized monolayer graphene has high quality and uniformity. We expect that this work is beneficial for not only potential applications, but also the fundamental understanding of the growth mechanism for graphene on Cu surface. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Graphene, a one-atom-thick planar sheet of sp2-hybridized carbon, has drawn much attention due to the outstanding properties and wide potential applications [1–3]. For practical applications, it is significant to achieve the large-area synthesis of monolayer graphene with high quality and uniformity. Recently, chemical vapor deposition (CVD) is considered as one of the most promising methods to synthesize large-area monolayer graphene film [4]. However, unexpected multilayer or bilayer regions are usually observed on the CVD monolayer graphene, which obviously influence the quality and uniformity of graphene [5,6]. Until now, there are many efforts to investigate the growth mechanism of CVD graphene films grown on Cu foils [6–8]. Liu et al. [6] demonstrated that the multilayer or bilayer regions were formed on the imperfection sites of Cu substrates with high chemical activation energy, and low pressure annealing in H2 (before graphene growth) could make the Cu surface smoother ⇑ Corresponding authors. Tel.: +86 028 83202291 (P. Li), +86 028 83202710 (Y. Chen). E-mail addresses: [email protected] (J. Liu), [email protected] (P. Li), [email protected] (Y. Chen), [email protected] (Z. Wang), [email protected] (J. He), [email protected] (H. Tian), [email protected] (F. Qi), [email protected] (B. Zheng), [email protected] (J. Zhou), [email protected] (W. Lin), [email protected] (W. Zhang). http://dx.doi.org/10.1016/j.jallcom.2014.07.003 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
and thereby prevent the formation of multilayer regions; however, some bilayer regions were still formed on the monolayer graphene. Herein, we report an effective way to synthesize the high-quality and uniform CVD monolayer graphene without bilayer regions. Optical microscope (OM), scanning electron microscope (SEM), Raman spectroscopy and electrical measurements were used to investigate the growth mechanism and evaluate the quality and uniformity of graphene. 2. Experimental 2.1. CVD synthesis of graphene Firstly, the Cu foil was dipped into the dilute HCl/H2O (1:10) solution for 3 min, and then washed by deionization (DI) water several times to remove the residual acid solution, and then was dried by nitrogen gas. Secondly, the Cu foil was loaded into the silica tube of the CVD system with a vacuum background of 7 104 Pa, and then the growth chamber was heated to 1000 °C and held for 20 min with 30 sccm pure H2 (99.999%), and then the 15 sccm of CH4 was introduced into the tube for graphene growth at 1000 °C for 20–180 min with the pressure of 340 Pa. Finally, cooled the system to room temperature with a cooling rate of 50 °C/min in CH4/H2 ambience. The as-synthesized graphene samples were denoted as G-X, where X represents the growth time (min). 2.2. Transfer process of graphene films Firstly, the surface of graphene grown on the Cu foil was spin-coated with poly(methyl methacrylate) solution (PMMA, A4) at 3000 rpm for 30 s, and then the Cu foil was etched away in the iron trichloride solution (FeCl3, 1 mol/L) for
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J. Liu et al. / Journal of Alloys and Compounds 615 (2014) 415–418
Fig. 1. SEM images of G-20 grown on the Cu foil (a) without and (b) with dilute HCl pretreatment. (c) OM image of G-20 transferred onto the SiO2/Si substrate. (d) Raman spectra of the red and black circle regions in the (c). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. (a) Schematic diagram of graphene grown on the Cu foil. (b–d) OM images of G-20, G-80, G-120 and G-180 transferred onto the SiO2/Si substrates.
12 h. Secondly, after the PMMA/graphene film was rinsed repeatedly using the DI water (10 times), it was transferred to the H2O/HCl/H2O2 (20:1:1) solution for 15 min for removing the residual metal particles, and then was transferred to the H2O/NH4OH/H2O2 (20:1:1) solution for 15 min for removing the insoluble organic contaminants. Note that the PMMA/graphene film was rinsed using the DI water to remove the residual solution after each cleaning step. Thirdly, the PMMA/ graphene was transferred onto the target substrate (SiO2/Si or PET substrates), and then was baked at 150 °C for 10 min after natural drying, and then the PMMA was removed using acetone [9].
2.3. OM, SEM, Raman and electrical measurements The surface morphology and structure of graphene were studied by using OM (Olympus BX51M), SEM (JSM-6490LV) and Raman spectrometer (Renishaw, 514 nm) at room temperature.
The graphene field-effect transistors (FETs) were fabricated by using standard photolithography after transferring the graphene onto the 270 nm SiO2/Si substrate. The Ni (30 nm) film was deposited as source and drain electrodes; the 270 nm SiO2 and n++ silicon layers acted as the gate dielectric and back-gate electrode, respectively. The electrical measurements were carried out with an Agilent 4155B semiconductor parameter analyzer in air at room temperature.
3. Results and discussions Before graphene growth, it is necessary to use dilute HCl to remove the native Cu-oxide layer on the Cu foil, otherwise the Cu-oxide nanoparticles will be formed on the graphene, as shown in Fig. 1a. Fig. 1b shows the SEM image of G-20 grown on the Cu foil with dilute HCl pretreatment. Although the graphene surface
J. Liu et al. / Journal of Alloys and Compounds 615 (2014) 415–418
417
Fig. 3. (a) Optical photograph of G-120 (7 7 cm2) transferred onto the PET substrate indicating its high uniformity. (b) Typical Raman spectrum of G-120. (c) IG/I2D and (d) ID/IG Raman mapping of G-120 film (50 50 lm2).
Fig. 4. (a) Transfer characteristics curves of G-20 and G-120 FETs. (b) Hole mobilities of G-20 and G-120 FETs, and the values are mean and standard deviation (error bars) for 10 different samples.
is clean, two visibly contrasting regions are observed, indicating that G-20 has poor uniformity. In order to further investigate the uniformity of G-20, it was transferred onto the SiO2/Si substrate. Fig. 1c and d shows the OM image and corresponding Raman spectra of G-20, respectively. For the Raman spectra of the black circle region in Fig. 1c, the G to 2D peak (IG/I2D) ratio is 0.42 and the full-width at half-height maximum (fwhm) of the 2D peak is 36.4 cm1, indicating monolayer graphene [10]. For the Raman spectra of the red circle region in Fig. 1c, the IG/I2D is 0.91 and the fwhm of the 2D peak is 49.7 cm1, indicating bilayer graphene [10]. The above studies reveal that the poor uniformity of G-20 is mainly originated from the formation of unexpected bilayer regions. It is known that uniform monolayer graphene can be synthesized on the Cu substrate with ideal flat surface, having low solubility of carbon [6]. However, compared with flat regions, the imperfection sites (e.g. steps, grain boundaries) of Cu foil have higher activation energy which promotes the catalytic decomposition of CH4, and thus it results in the formation of bilayer regions,
as shown in Fig. 2a. It means that it is critical to make the surface of Cu foil flat to synthesize the monolayer graphene with high uniformity. In order to improve the uniformity of G-20, we have investigated the effect of growth time on the uniformity of graphene. Fig. 2b–d shows the OM images of G-20, G-80, G-120 and G-180 transferred onto the SiO2/Si substrates. With increasing the growth time, the density of bilayer regions decreases; when the growth time reaches 120 min, no bilayer regions are formed. It can be explained by the schematic diagram as shown in Fig. 2a. At the 1000 °C of growth temperature, the Cu and carbon atoms are sublimated, at the same time, CH4 molecules are decomposed on the fresh surface of Cu foil, which is the dynamic equilibrium process. Thus, when the growth time increases, the surface of Cu foil becomes smoother, and thereby improving the uniformity of graphene. Note that annealing in H2 before graphene growth cannot effectively prevent the formation of bilayer regions, because the surface of Cu foil will be etched by H2 during this progress.
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J. Liu et al. / Journal of Alloys and Compounds 615 (2014) 415–418
However, this H2 etching effect can be suppressed after introducing CH4, because H2 prefers to promote the catalytic decomposition of CH4 molecules on the Cu foil during the graphene growth process [11,12]. In addition, some narrow wrinkles with hundreds of nanometers wide are observed no matter whether bilayer graphene regions are formed, as shown in Fig. 2b–d. It may be due to the different thermal expansion coefficients of graphene and Cu foil [9], and will be further investigated in the future work. By increasing the growth time, the large-area monolayer graphene (7 7 cm2) with high uniformity (G-120) is synthesized, as shown in Figs. 2d and 3a. The typical Raman spectrum shown in Fig. 3b reveals that the G-120 is monolayer and has high quality (low D peak). For further investigating the uniformity of G-120, Raman mapping at the 50 50 lm2 scale was obtained, as shown in Fig. 3c and d. From Fig. 3c, the IG/I2D ratio is 0.34–0.56, suggesting uniform monolayer graphene [8,13]. From Fig. 3d, the ID/IG ratio is 0–0.15, suggesting few defects in the G-120 film [14]. In order to further evaluate the quality and uniformity of G-120, the graphene FETs were fabricated. Fig. 4a shows the transfer characteristics (Ids–Vg) curves of G-20 and G-120 FETs at a fixed drainsource voltage (Vds) of 1 V. The hole mobility (l) can be calculated using l ¼ g m L=WC g V ds , where g m ¼ dIds =dV g V ¼constant is obtained
and without bilayer regions. The studies reveal that G-20 and G-120 have the mobilities of 933:7 278:2 cm2 V1 s1 and 2534:3 97:6 cm2 V1 s1 , respectively. The result indicates that not only the quality, but also the uniformity of monolayer graphene are obviously improved by preventing the formation of bilayer regions. Acknowledgements The research was supported by the National Natural Science Foundation of China (Grant Nos. 51202022, 51372033 and 61378028), the Program for New Century Excellent Talents in University (Grant No. NCET-10-0291), the 111 Project (Grant No. B13042), the Specialized Research Fund for the Doctoral Program of Higher Education (Gran No. 20120185120011), Sichuan Youth Science and Technology Innovation Research Team Funding (Grant No. 2011JTD0006), the International Science and Technology Cooperation Program of China (Gran No. 2012DFA51430), and the Sino-German Cooperation PPP Program of China. We thank Prof. Ping Xu of Harbin Institute of Technology for his technical help for Raman characterization.
ds
from Fig. 4a; L and W are the length (40 lm) and width (4 lm) of graphene channel, respectively; Cg is the back-gate capacitance with the value of 12.7 nF/cm2 [15]. As a result, the hole mobilities of G-20 and G-120 are 933:7 278:2 cm2 V1 s1 and 2534:3 97:6 cm2 V1 s1 , respectively, as shown in Fig. 4b; the values are mean and standard deviation (error bars) for 10 different samples. Thus, we can conclude that the quality and uniformity of monolayer graphene are obviously improved by preventing the formation of bilayer regions. 4. Conclusions We have synthesized the large-area monolayer graphene with high quality and uniformity on Cu foil. OM, SEM, Raman spectra were used to investigate the effects of growth time on the quality and uniformity of graphene. The results reveal that when the growth time increases, the density of bilayer graphene regions decreases. It is attributed that the Cu surface becomes smoother with the sublimation of Cu atoms during the growth process. As a result, when the growth time reaches 120 min, the monolayer graphene can be synthesized without unexpected bilayer regions. Further, we have studied the electrical properties of graphene with
References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666–669. [2] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183–191. [3] Z. Wang, Y. Chen, P. Li, X. Hao, J. Liu, R. Huang, Y. Li, ACS Nano 5 (2011) 7149– 7154. [4] X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc, Science 324 (2009) 1312–1314. [5] W. Ge, B. Lu, W. Li, J. Lu, Z. Ye, Vacuum 97 (2013) 9–14. [6] Q. Li, H. Chou, J. Zhong, J. Liu, A. Dolocan, J. Zhang, Y. Zhou, R.S. Ruoff, S. Chen, W. Cai, Nano Lett. 13 (2013) 486–490. [7] W. Liu, H. Li, C. Xu, Y. Khatami, K. Banerjee, Carbon 49 (2011) 4122–4130. [8] Z. Sun, A.O. Raji, Y. Zhu, C. Xiang, Z. Yan, C. Kittrell, E. Samuel, J.M. Tour, ACS Nano 6 (2012) 9790–9796. [9] X. Liang, B.A. Sperling, I. Calizo, G. Cheng, C.A. Hacker, Q. Zhang, Y. Obeng, K. Yan, H. Peng, Q. Li, ACS Nano 5 (2011) 9144–9153. [10] L.M. Malard, M.A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, Phys. Rep. 473 (2009) 51–87. [11] I. Vlassiouk, M. Regmi, P. Fulvio, S. Dai, P. Datskos, G. Eres, S. Smirnov, ACS Nano 5 (2011) 6069–6076. [12] L. Liu, H. Zhou, R. Cheng, W.J. Yu, Y. Liu, Y. Chen, J. Shaw, X. Zhong, Y. Huang, X. Duan, ACS Nano 6 (2012) 8241–8249. [13] Z. Yan, Z. Peng, Z. Sun, J. Yao, Y. Zhu, Z. Liu, P.M. Ajayan, J.M. Tour, ACS Nano 5 (2011) 8187–8192. [14] Z. Peng, Z. Yan, Z. Sun, J.M. Tour, ACS Nano 5 (2011) 8241–8247. [15] S. Kim, J. Nah, I. Jo, D. Shahrjerdi, L. Colombo, Z. Yao, E. Tutuc, S.K. Banerjee, Appl. Phys. Lett. 94 (2009) 621071–6210714.