- Nano Express
- Open Access
Controlled synthesis of bilayer graphene on nickel
Nanoscale Research Lettersvolume 7, Article number: 437 (2012)
We report a uniform and low-defect synthesis of bilayer graphene on evaporated polycrystalline nickel films. We used atmospheric pressure chemical vapor deposition with ultra-fast substrate cooling after exposure to methane at 1,000°C. The optimized process parameters, i.e., growth time, annealing profile and flow rates of various gases, are reported. By using Raman spectroscopy mapping, the ratio of 2D to G peak intensities (I2D/IG) is in the range of 0.9 to 1.6 over 96% of the 200 μm × 200 μm area. Moreover, the average ratio of D to G peak intensities (ID/IG) is about 0.1.
Graphene, a monolayer of sp2-hybridized C atoms arranged in a honeycomb structure, has attracted a lot of attention due to its excellent electrical, mechanical and optical properties [1–3]. Monolayer and bilayer graphene (BLG) are semi-metals with zero band gap. The intrinsic band gap has been the key in semiconducting devices. Band gap can be induced by patterning graphene into nano-ribbons [4–10]. Another method to introduce band gap is to apply electric field in the stacking direction of BLG [11–14].
Graphene synthesis on transition metals by chemical vapor deposition (CVD) or via segregation of solid carbon sources is generally a scalable process [15–24]. Transition metals such as Ni, Cu, Pt, Ir and Pd have been used as substrates for graphene growth [25–29]. Several hydrocarbons, like methane (CH4), acetylene, ethylene, propane, etc., have been used in atmospheric and low-pressure CVD as a carbon source . Besides the CVD of the above mentioned gases, C60 and solid polymers such as poly(methyl metha-crylate), polystyrene and acrylonitrile butadiene styrene have also been decomposed to grow graphene [30–32].
Graphene synthesizes on Ni due to segregation of carbon at high temperatures. Due to the high solubility of carbon in Ni, precipitation of extra carbon occurs at the metal surfaces during cooling. Since the precipitation is a non-equilibrium process, this makes thickness control of graphene a challenge [33, 34]. The segregation of extra carbon during the cooling process can be decreased by reducing Ni film thickness. Moreover, extra carbon segregation can also be controlled by controlling the cooling rate [15, 22, 35–37]. Very high cooling rate has been reported to deposit amorphous carbon deposition, whereas low cooling rate leads to no growth . It has been reported that the carbon segregation on Ni is non-uniform at low temperatures. However, CH4 has high decomposition temperature, which helps in constant carbon coverage over the Ni surfaces . Furthermore, high melting point of Ni enables high-temperature annealing, which results in larger domains, thus making it favorable for large-area low-defect growth .
In this paper, we report a method to control the precipitation of extra carbon on Ni surface during the cooling-down process. We reduce the sample temperature from the growth temperature of 1,000°C to room temperature in a few seconds, which leads to a uniform BLG growth.
BLG was grown on a 300 nm Ni film, evaporated on SiO2 (300 nm)/Si substrate. SiO2/Si substrate was treated with acetone (10 min), methanol (10 min), deionized (DI) water rinse (10 min) and nanostrip (20 min; commercial Piranha substitute), followed by another DI water rinse (10 min). After cleaning, Ni was evaporated by using an e-beam evaporator at 1Å/s. Ni/SiO2/Si samples were gently cleaned in UV ozone for 2 min before loading in the CVD furnace. UV ozone eliminates organic contaminants from the Ni film, which is important for uniform growth. Process gases were supplied by Airgas (Denver, CO, USA) with research grade 5.0 (minimum purity 99.999%). The samples were loaded into the CVD furnace (1-inch tube diameter; Lindbergh/Blue, Thermo Scientific, Logan, UT, USA) at room temperature and heated to 700°C in 200 sccm Ar ambient. At 700°C, 65 sccm H2 was introduced in addition to Ar, and the samples were annealed for another 10 min. The temperature was ramped to 1,000°C in Ar/H2 ambient. To stabilize the growth temperature, the samples were further annealed for 10 min after reaching 1,000°C. Ar/H2 annealing sequence leads to increased grain size and decreased surface roughness [18, 22]. Finally, H2 was turned off, and BLG was synthesized by introducing CH4 into the furnace in addition to the already flowing Ar gas. A wide process parameter space was explored, which includes (a) varying the growth time (50, 60 and 120 s) under a constant CH4 flow rate (23 sccm) and (b) varying the flow rates (6, 12 and 23 sccm) under a constant growth time (120 s). After the growth, the sample temperature was reduced to room temperature within a few seconds by pulling the quartz tube out of the hot region of the furnace.
For material characterization, micro-Raman spectroscopy (Raman Nicolet Almega XR Spectrometer, Thermo Scientific) was used in the point scan and the area scan mode [3, 39, 40]. A 532 nm laser (10 mW power) was used with a 0.6 μm spot size, 15 s scan time and four scans per point. To examine the uniformity of the synthesized graphene, the ratio of 2D to G peak intensities (I2D/IG ratio) was taken over an area of 200 μm × 200 μm. In the area scan, a 2.1 μm spot size was used with 15 s scan time, four scans per point and 10 μm step size.
Results and discussions
Figure 1a,d shows the ratio of 2D to G peak intensities (I2D/IG) at two different locations for a sample that was grown under 23 sccm CH4 for 120 s. In each case, the I2D/IG ratio is in the range of 0.9 to 1.6 over 96% of the total 200 μm × 200 μm area. This suggests that the BLG is grown over a larger percentage of area on polycrystalline Ni film [21, 22, 39, 40]. The I2D and IG plots over these locations are shown in Figure 1b,e and Figure 1c,f, respectively. These plots show a uniform intensity distribution for the G and D peaks, which further implies the graphene sample uniformity.
Next, the growth time was varied to study the effect on the number of layers, uniformity and defect density of the synthesized graphene. Figure 2a shows the Raman spectra of the samples that were treated under 23 sccm of CH4 for 50, 60 and 120 s. The I2D/IG ratio is close to unity with these varying growth times. This quenching method inhibits the precipitation of extra C on the Ni surface and thus controls the number of layers and the uniformity of the graphene for various growth times. Furthermore, after turning the CH4 off, if there is some residual C inside the furnace, the ultra-fast cooling suppresses its further segregation. Moreover, the effect of the CH4 flow rate was also studied for constant growth time. Figure 2b shows the Raman spectra of samples for which the growth time was 120 s with CH4 flow rates of 6, 12 and 23 sccm. It shows that the BLG growth is consistent for a wide range of flow rates.
Another important observation is that the intensity of the D peak decreases as the growth time increases from 50 to 120 s, as shown in Figure 2a. The ratio of D to G peak intensities (ID/IG ratio) was taken over 20 locations for the samples grown under 23 sccm of CH4 for 50, 60 and 120 s. The mean and standard deviation of ID/IG ratio are plotted in the error bar graph shown in Figure 3. The average ID/IG ratio for the sample grown under 23 sccm of CH4 for 120 s is 0.1 with a standard deviation of 0.05, which suggests a low defect density of BLG for these parameters. Moreover, Figure 3 also shows that the average defect density decreases with the increasing growth time. Figure 2b also shows that the defect density of BLG is small for the samples grown under wide CH4 flow rates for 120 s.
We find that quenching the samples from the hot region of the furnace helps in reducing the non-equilibrium precipitation of extra carbon on the Ni surfaces during the cooling process, and that the main growth mechanism is diffusion of carbon on Ni surface due to the decomposed CH4. With fast cooling, the reduced sample temperature stops further segregation of carbon due to any residual carbon inside the furnace, even after CH4 flow was turned off. The thickness of the graphene is almost constant even with a wide range of CH4 flow rates (6 to 23 sccm), which shows that the segregation process is rather self-limiting. Furthermore, the growth temperature is high due to high decomposition temperature of CH4 that supports the uniform carbon diffusion over the Ni surface. This helps in growing uniform BLG with less defect density. Moreover, as the growth time is decreased, the average intensity of the D peak increases, which indicates incomplete growth. This further verifies the self-limiting equilibrium segregation of carbon on Ni surface, with reduced out-diffused carbon atoms from the C-Ni solution due to fast cooling. To verify the proposed growth mechanism, graphene was grown on 300 nm Ni film, with 23 sccm CH4 flow rated for 120 s, cooling the samples within the furnace. Due to slow cooling, the precipitation of carbon on Ni surface from the C-Ni solution is a dominant process.
Yet, another way to reduce the precipitation of extra carbon is to reduce the thickness of Ni film as less thick films would absorb less carbon and thus contribute to further decrease in out-diffused carbon. To characterize this effect, the growth was performed on 200 and 100 nm thick Ni films, with 23 sccm CH4 flow rate for 120 s. For the 200 nm Ni film, the I2D/IG ratio is close to unity, and the area uniformity is similar to the 300 nm thick films. However, growth on 100 nm Ni film results in increased surface roughness. Although the I2D/IG ratio is still around unity in this process, surface coverage is only 50%.
In conclusion, we have reported a method to synthesize bilayer graphene through CVD of CH4 on polycrystalline Ni films with an ultra-fast cooling technique. The number of graphene layers is uniform over a wide area with low defect density. The growth is consistent over a certain variation of CH4 flow rate and growth time.
Geim AK, Novoselov KS: The rise of graphene. Nat Mater 2007, 6: 183–191. 10.1038/nmat1849
Castro Neto AH, Peres NMR, Novoselov KS, Geim AK: The electronic properties of graphene. Rev Mod Phys 2009, 81: 109–162. 10.1103/RevModPhys.81.109
Raza H: Graphene Nanoelectronics: Metrology, Synthesis, Properties and Applications. Springer, Berlin; 2012.
Nakada K, Fujita M, Dresselhaus G, Dresselhaus MS: Edge state in graphene ribbons: nanometer size effect and edge shape dependence. Phys Rev B 1996, 54: 17954–17961. 10.1103/PhysRevB.54.17954
Fujita M, Wakabayashi K, Nakada K, Kusakabe K: Peculiar localized states at zig-zag edges. J Phys Soc Jpn 1996, 65: 1920–1923. 10.1143/JPSJ.65.1920
Wakabayashi K, Fujita M, Ajiki H, Sigrist M: Electronic and magnetic properties of nanographite ribbons. Phys Rev B 1999, 59: 8271–8282. 10.1103/PhysRevB.59.8271
Kawai T, Miyamoto Y, Sugino O, Koga Y: Graphitic ribbons without hydrogen-termination: electronic structures and stabilities. Phys Rev B 2000, 62: R16349-R16352. 10.1103/PhysRevB.62.R16349
Raza H, Kan EC: Armchair graphene nanoribbons: electronic structure and electric-field modulation. Phys Rev B 2008, 77: 245434.
Raza H: Zigzag graphene nanoribbons: bandgap and midgap state modulation. J Phys Condens Matter 2011, 23: 382203. 10.1088/0953-8984/23/38/382203
Raza H: Edge and passivation effects in armchair graphene nanoribbons. Phys Rev B 2011, 84: 165425.
McCann E, Abergel DSL, Fal’ko VI: Electrons in bilayer graphene. Solid State Commun 2007, 143: 110–115. 10.1016/j.ssc.2007.03.054
Castro E, Novoselov K, Morozov S, Peres N, Dos Santos J, Nilsson J, Guinea F, Geim A, Neto A: Biased bilayer graphene: semiconductor with a gap tunable by the electric field effect. Phys Rev Lett 2007, 99: 216802.
Xia F, Farmer DB, Lin Y-M, Avouris P: Graphene field-effect transistors with high on/off current ratio and large transport band gap at room temperature. Nano Lett 2010, 10: 715–718. 10.1021/nl9039636
Raza H, Kan EC: Field modulation in bilayer graphene band structure. J Phys Condens Matter 2009, 21: 102202. 10.1088/0953-8984/21/10/102202
Yu Q, Lian J, Siriponglert S, Li H, Chen YP, Pei S-S: Graphene segregated on Ni surfaces and transferred to insulators. Appl Phys Lett 2008, 93: 113103. 10.1063/1.2982585
Reina A, Jia X, Ho J, Nezich D, Son H, Bulovic V, Dresselhaus MS, Kong J: Large area, few-layer graphene films on arbitrary substrates by chemical vapor deposition. Nano Lett 2009, 9: 30–35. 10.1021/nl801827v
Kim KS, Zhao Y, Jang H, Lee SY, Kim JM, Kim KS, Ahn J-H, Kim P, Choi J-Y, Hong BH: Large-scale pattern growth of graphene films for stretchable transparent electrodes. Nature 2009, 457: 706–710. 10.1038/nature07719
Li X, Cai W, An J, Kim S, Nah J, Yang D, Piner R, Velamakanni A, Jung I, Tutuc E, Banerjee SK, Colombo L, Ruoff RS: Large-area synthesis of high-quality and uniform graphene films on copper foils. Science 2009, 324: 1312–1314. 10.1126/science.1171245
Lee Y, Bae S, Jang H, Jang S, Zhu S-E, Sim SH, Song YI, Hong BH, Ahn J-H: Wafer-scale synthesis and transfer of graphene films. Nano Lett 2010, 10: 490–493. 10.1021/nl903272n
Liu W, Li H, Xu C, Khatami Y, Banerjee K: Synthesis of high-quality monolayer and bilayer graphene on copper using chemical vapor deposition. Carbon 2011, 49: 4122–4130. 10.1016/j.carbon.2011.05.047
Peng Z, Yan Z, Sun Z, Tour JM: Direct growth of bilayer graphene on SiO2 substrates by carbon diffusion through nickel. ACS Nano 2011, 5: 8241–8247. 10.1021/nn202923y
Wan D, Lin T, Bi H, Huang F, Xie X, Chen I-W, Jiang M: Autonomously controlled homogenous growth of wafer-sized high-quality graphene via a smart Janus substrate. Adv Funct Mater 2012, 22: 1033–1039. 10.1002/adfm.201102560
Arco LGD, Zhang Y, Kumar A, Zhou C: Synthesis, transfer, and devices of single- and few-layer graphene by chemical vapor deposition. IEEE Trans Nanotechnol 2009, 8: 135–138.
Chae SJ, Gunes F, Kim KK, Kim ES, Gang HH, Kim SM, Shin H-J, Yoon S-M, Choi J-Y, Park MH, Yang CW, Pribat D, Le YH: Synthesis of large-area graphene layers on poly-nickel substrate by chemical vapor deposition: wrinkle formation. Adv Mater 2009, 21: 2328–2333. 10.1002/adma.200803016
Rümmeli MH, Rocha CG, Ortmann F, Ibrahim I, Sevincli H, Börrnert F, Kunstmann J, Bachmatiuk A, Pötschke M, Shiraishi M, Meyyappan M, Büchner V, Roche S, Cuniberti G: Graphene: piecing it together. Adv Mater 2011, 23: 4471–4490. 10.1002/adma.201101855
Park HJ, Meyer J, Roth S, Skákalová V: Growth and properties of few-layer graphene prepared by chemical vapor deposition. Carbon 2010, 48: 1088–1094. 10.1016/j.carbon.2009.11.030
Pollard AJ, Nair RR, Sabki SN, Staddon CR, Perdigao LMA, Hsu CH, Garfitt JM, Gangopadhyay S, Gleeson HF, Geim AK, Beton PH: Formation of monolayer graphene by annealing sacrificial nickel thin films. J PhysChemLett 2009, 113: 16565–16567.
Nandamuri G, Roumimov S, Solanki R: Chemical vapor deposition of graphene films. Nanotechnology 2010, 21: 145604. 10.1088/0957-4484/21/14/145604
Chen Z, Ren W, Liu B, Gao L, Pei S, Wu Z-S, Zhao J, Cheng H-M: Bulk growth of mono- to few-layer graphene on nickel particles by chemical vapor deposition from methane. Carbon 2010, 48: 3543–3550. 10.1016/j.carbon.2010.05.052
Sun Z, Yan Z, Yao J, Beitler E, Zhu Y, Tour JM: Growth of graphene from solid carbon sources. Nature 2010, 468: 549–552. 10.1038/nature09579
Shin H-J, Choi WM, Yoon S-M, Han GH, Woo YS, Kim ES, Chae SJ, Li X-S, Benayad A, Loc DD, Gunes F, Lee YH, Choi J-Y: Transfer-free growth of few-layer graphene by self-assembled monolayers. Adv Mater 2011, 23: 4392–4397. 10.1002/adma.201102526
Sabki SN, Garfitt JM, Capiod P, Beton PH: Graphene formation by decomposition of C60. J Phys Chem 2011, 115: 7472–7476. 10.1021/jp200628b
Pang S, Hernandez Y, Feng X, Müllen K: Graphene as transparent electrode material for organic electronics. Adv Mater 2011, 23: 2779–2795. 10.1002/adma.201100304
Li X, Cai W, Colombo L, Ruoff RS: Evolution of graphene growth on Ni and Cu by carbon isotope labeling. Nano Lett 2009, 9: 4268–4272. 10.1021/nl902515k
Miyata Y, Kamon K, Ohashi K, Kitaura R, Yoshimura M, Shinohara H: A simple alcohol-chemical vapor deposition synthesis of single-layer graphenes using flash cooling. Appl Phys Lett 2010, 96: 263105. 10.1063/1.3458797
Liu W, Chung C-H, Miao C-Q, Wang Y-J, Li B-Y, Ruan L-Y, Patel K, Park Y-J, Woo J, Xie Y-H: Chemical vapor deposition of large area few layer graphene on Si catalyzed with nickel films. Thin Solid Films 2010, 518: S128-S132. 10.1016/j.tsf.2009.10.070
Zhou Q, Lin L In IEEE 23rd International Conference on Micro Electro Mechanical Systems. In Synthesis of graphene using micro chemical vapor deposition. IEEE, Wanchai. Hong Kong; 2010:43–46.
Shelton JC, Patil HR, Blakely JM: Equilibrium segregation of carbon to a nickel (111) surface: a surface phase transition. Surf Sci 1974, 43: 493–520. 10.1016/0039-6028(74)90272-6
Ferrari AC, Meyer JC, Scardaci V, Casiraghi C, Lazzeri M, Mauri F, Piscanec S, Jiang D, Novoselov KS, Roth S, Geim AK: Raman spectrum of graphene and graphene layers. Phys Rev Lett 2006, 97: 187401.
Malard LM, Pimenta MA, Dresselhaus G, Dresselhaus MS: Raman spectroscopy in graphene. Phys Rep 2009, 473: 51–87. 10.1016/j.physrep.2009.02.003
We are grateful to A. Mohsin for the useful discussions, J. Baltrusaitis for helping with the Raman spectroscopy and T. Z. Raza for critically reviewing an early draft of this manuscript. We are also thankful to C. Coretsopoulos, D. Norton and J. Olesberg for their help in the Micro Fabrication Facility. Evaporation was carried out in the Micro Fabrication Facility, and the Raman spectroscopy was performed in the Central Microscopy Research Facility at the University of Iowa. This work is supported by the MPSFP program of the VPR office at the University of Iowa.
Both authors declare that they have no competing interests.
HR and AU have equal contribution to this work and the manuscript. Both authors read and approved the final manuscript.