Chapter 2. Graphene
2.3 Graphene Preparation
2.3.2 Synthesized Graphene with Chemical Vapor Deposition
Chemical vapor deposition studies have been used for more than 60 years to decompose hydrocarbon sources at high temperatures to synthesize thin graphitic layers on transition metal surfaces [104]. The chemical vapor deposition method of graphene synthesis was first reported in 2008 using Ni [105] and in 2009 using Cu [10]. Since then, chemical vapor deposition has become the most promising approach because it can synthesize graphene at low cost and large area than any other strategies for synthesizing graphene. The CVD synthesis that use metal to form graphene decompose hydrocarbons precursors and form carbon radicals on the metal surface when gas species flow into the reactor at high temperature zone. Single-, or few-layer graphene are synthesized. During the synthesis process, the metal substrate not only serves as a catalyst to lower the energy barrier of reaction, but also determines the mechanism of graphene generation. Ultimately, the quality of graphene is determined by the metal substrate, so the kind of metal in CVD synthesis is very important.
Table 1 Carbon solubility and growth mechanism using various metal for CVD graphene [7].
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When synthesizing graphene using polycrystalline Ni film, the gas is mixed with Ar, H2, and CH4. The ramping process raises the temperature from 900 °C to 1000 °C in an Ar/H2 atmosphere.
This process has the effect of cleaning the surface and increasing the grain size of polycrystalline Ni.
When the temperature stabilizes at high temperature, flow H2 and CH4 together. Then the carbon
precursor is decomposed by H2, and carbon atoms dissolve on the Ni film surface. The Ni catalyst has relatively higher carbon solubility (Table 1). The carbon solubility is temperature-dependent. The Ni catalyst has high solubility at high temperatures and low solubility at low temperatures. At high temperatures, high carbon solubility causes many carbons to dissolve on the surface of Ni. When the temperature is lowered, the carbon solubility of Ni is lowered, and the excess carbon is released to the Ni surface. Then it forms a graphene. In general, when Ni is used, cooling process is performed in an Ar atmosphere to prevent the decomposition of additional carbon precursors. Figure 2.7 is an illustration of the mechanism of synthesis of graphene using single crystal Ni (1,1,1). Single crystal Ni has a lattice constant similar to that of graphene with hexagonal lattices [106], so the Ni surface is
(a)
(b)
(c)
(d)
Figure 2.7 Graphene synthesis mechanism using (a) single crystal Ni (1,1,1) and (b) poly-crystalline Ni. (C) show optical images of graphene synthesized using Ni (1,1,1) and (d) poly-crystalline Ni [19].
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well matched to the lattice of graphene and is used as a good graphene synthesis catalyst. Most of metal films except Copper and Germanium have been reported as segregation mechanism to synthesize graphene in table 1. That is, at high temperatures, carbon is dissolve in metal and graphene form as temperature decreases. Hence, the cooling rate directly affects the quality and thickness of the
graphene. Also, the microstructure of the metal film changes the graphene formed. Single crystal Ni (1,1,1) is favorable for single or few layer graphene synthesis. However, in the case of poly- crystalline Ni, the surface is not exactly flat. Therefore, multilayer graphene is formed, or grain boundaries of graphene are formed in the grain boundary of Ni. The synthesis time of the carbon precursor and the gas ratio to H2 also affect the quality of the graphene. The graphene synthesized using Ni can be transferred to various substrates and large-scale synthesis of wafer scale.
The method of synthesizing graphene using a catalyst substrate of copper is one of the easily accessible synthesis methods by providing large area and high-quality graphene. Particularly for coppers with the lowest carbon solubility, it is relatively advantageous to synthesize a single layer graphene. Since the synthesis of monolayer graphene was reported in 2009 in low vacuum conditions, Cu has become a typical material for single layer graphene synthesis [10]. Recently, research on the synthesis of single layer graphene over 30 inches has also been reported [107]. Like the synthesis of graphene using Ni, the gases of CH4 and H2 are used for graphene synthesis. Ramp up to 1000 °C temperature in H2 atmosphere. And it maintains 1000 °C for about 20 ~ 30 minutes (Figure 2.8) to removes impurities from the copper surface and make the surface uniform. To synthesize graphene, flow CH4 with H2 gas in the hot temperature zone. After finishing growth process, the H2 and CH4
flow rates are kept constant in the cooling process. In the case of the copper, the carbon solubility is low. When the carbon precursor is decomposed, graphene is formed immediately. In the case of Ni substrates, the cooling process is very important because the graphene is synthesized in this process However, graphene is synthesized in the growth process at 1000 °C for Cu substrates. So, it is
Figure 2.8 CVD synthesis parameter showing temperature and gas precursor composition [10].
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important to keep CH4 and H2 flowing constantly instead of releasing Ar during cooling. When graphene is synthesized using polycrystalline Cu, a large area graphene with a grain boundary is synthesized. The grain boundaries will degrade electrical properties when fabricating electrical devices. The challenge of synthesizing large-area high-quality graphene is to synthesize a single layer single crystal graphene. To produce a single crystal graphene, the nucleation density must be lowered through various conditions such as growth temperature, flow rate, pressure, gas ratio, and pre-cleaning of Cu surface. Recently, single crystal graphene of hexagonal, rectangular and flower shapes have been reported by controlling these factors.
Optical microscopy, SEM and Raman spectroscopy provided information on the number of layers of graphene. Figure 2.9 (a) shows that single layer graphene was over 95% through a SEM image with a double layer. The optical contrast of double layer was different from the single layer graphene, and the grain boundaries are clear. Due to atomic thickness of single layer graphene, it is
difficult to obtain information about layer with the visible ray, but the information about layers can be distinguished even when graphene is on a 300 nm SiO2/Si wafer [11]. The reason is that the range of the wavelength reflected by the number of graphene layers slightly changes. It gives information about single- and double-layer using optical microscopy (Figure 2.9 (b)). Comparing I2D/ID ratios through Raman spectroscopy analysis gives information of absolute layers. The I2D/ID ratio is greater than 2 for a single layer and closer to 1 for a double layer. The I2D/ID ratio is less than 1 about the over 3 layers. Raman analysis gives accurate information about single layer or double layer information.
(Figure 2.9 (c))
The synthesis of graphene by the CVD technique is one of the widely used approaches to synthesize high-quality and large-area graphene. To make an electrical device using CVD-growth Figure 2.9 Analysis of single layer CVD graphene [10]. (a) SEM image of graphene on SiO2/Si water. (b) optical microscope image of the same area as in (a). (c) Raman spectra of single-, double- and triple-layer graphene.
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graphene, it is essential to transfer the graphene on the Cu catalyst substrate to the target substrate.
The transfer technique is used for preparing graphene. For example, mechanically exfoliated technique transfer single layer graphene from graphite [72], the graphene synthesized using CVD technique is transferred to a target substrate using polymer-assisted method [108-110], and large area graphene can be transferred using roll-to-roll method [107]. Research has also been reported on synthesizing graphene directly on the substrate to avoid the transfer process. A typical method of transferring graphene synthesized by the CVD technique is using a polymer. Representative polymers include Polydimethylsiloxane (PDMS) or PMMA. The PMMA polymer have a high tolerance to Cu etchant such as FeCl3, HCl, HNO3, Fe(NO3)3 and CuCl2. The PMMA polymer provide safely transferring atomic thickness graphene and is easily removed in acetone. Hence, the PMMA is usually used for graphene transfer. However, visually indistinguishable small PMMA residues remain on the surface of the graphene. Because the PMMA residues directly affect the properties of graphene, studies have been reported on more thorough removal of PMMA. The Ruoff group reported that PMMA residues were cleanly removed [111]. The method was that graphene was transferred to the target substrate and then the new PMMA solution was coated again on the PMMA film and was removed using acetone after 30 minutes. They suggested that a new coating PMMA solution would help to remove the transferred PMMA better. Two years later, researchers have reported that PMMA was effectively removed through modified Radio Corporation of America (RCA) cleaning.
Conventional RCA cleaning was performed using i) (SC-1) cleaning of 5: 1: 1 H2O/H2O2 /NH4OH solution to remove insoluble organic contaminants, ii) remove the thin silicon dioxide layer using a 50:
1 H2O/HF solution. iii) (SC-3) removing heavy metal contaminants using a 5: 1: 1 H2O/H2O2/HCl solution. Since the PMMA film melts in strong acid, SC-1 and SC-3 processes, which is suitable for the graphene transfer process, were applied at a ratio of 20: 1:1. The cleaning procedure was to coat the PMMA film on the graphene, remove the Cu film, clean the PMMA / graphene film using the SC- 3 cleaning process, and then clean the SC-1 solution after DI rinsing. Since then, the process was the same as before. Through this modified RCA cleaning, clean graphene was prepared on a wafer scale.
In 2013, Prof. In the Ruoff group, p-doped graphene was changed to pristine properties using the formamide [112]. The NH2 functional groups of formamide bound to PMMA residues and reduced the effects of PMMA. Studies have shown in 2013 that removal of PMMA using acetic acid can further reduce PMMA residues, rather than removing PMMA with acetone [113]. The PMMA residue is generally observed in CVD graphene, and many studies have been reported to reduce the influence of PMMA residue.
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