Low-temperature synthesis of graphene on Cu using plasma-assisted thermal chemical vapor deposition
© Chan et al.; licensee Springer. 2013
Received: 10 May 2013
Accepted: 5 June 2013
Published: 12 June 2013
Plasma-assisted thermal chemical vapor deposition (CVD) was carried out to synthesize high-quality graphene film at a low temperature of 600°C. Monolayer graphene films were thus synthesized on Cu foil using various ratios of hydrogen and methane in a gaseous mixture. The in situ plasma emission spectrum was measured to elucidate the mechanism of graphene growth in a plasma-assisted thermal CVD system. According to this process, a distance must be maintained between the plasma initial stage and the deposition stage to allow the plasma to diffuse to the substrate. Raman spectra revealed that a higher hydrogen concentration promoted the synthesis of a high-quality graphene film. The results demonstrate that plasma-assisted thermal CVD is a low-cost and effective way to synthesis high-quality graphene films at low temperature for graphene-based applications.
Graphene, a sp2-hybridized carbon film with unique properties, has attracted substantial interest in recent years, and it is a candidate for several applications. The carriers in graphene are transported in the π-orbitals that are perpendicular to the surface so the optical transparency of a single layer of graphene can be as high as approximately 97%, and it can exhibit excellent electronic properties with reported mobilities of between 3,000 and 27,000 cm2/V·s [1–3]. Various methods for synthesizing graphene have been developed. One of them is the mechanical exfoliation from highly oriented pyrolytic graphite, but it has low throughput and produces graphene with a limited area [4–7]. Chemical exfoliation is a promising method; it has high throughput and produces graphene flakes from bulk graphite . Sulfuric acid is a common oxidizing agent that reacts strongly with the surface of aromatic carbon compounds to form graphene oxide flakes that are subsequently reduced to graphene [9, 10]. This method forms various defects that degrade the electronic properties of the formed graphene. Another method is the thermal decomposition from SiC substrate. In this case, a Si atom on a SiC surface is exposed to a temperature of 1,050°C to 1,100°C [11, 12]. The epitaxial graphene on SiC has high quality, but the use of an expensive SiC substrate is not practical. Recently, synthesis of uniform and large-scale graphene films by chemical vapor deposition (CVD) on transition metals has been demonstrated [13–19]. This method is operated at a high temperature of 1,000°C, and it depends on the source of hydrocarbon gas, limiting its range of applications. Therefore, a low-temperature process for synthesizing graphene is required for graphene applications. Hence, the plasma CVD system is effective for synthesizing a high-quality graphene film by deposition at low temperature. Kim et al. used microwave plasma CVD to synthesize graphene films on nickel foil at a low temperature of 750°C , and surface wave plasma CVD has been used to synthesize graphene conductive electrodes on a large scale at low temperatures in the range of 300°C to 400°C [21, 22]. However, these approaches require expensive equipment, produce multilayer graphene with low transparency, and form many defects that suffer from ion bombardment. In this work, plasma-assisted thermal CVD was utilized to grow a monolayer of graphene at low temperature. Unlike the aforementioned plasma-based CVD methods, plasma-assisted thermal CVD is low-cost and forms a monolayer of graphene with few defects on Cu foil without the ion bombardment effect. Additionally, the plasma emission spectra of the plasma-assisted thermal CVD system were obtained to elucidate the mechanism of graphene growth.
Graphene films were grown on a 25-μm-thick copper foil (99.8%, Alfa Aesar, item no.13382, Ward Hill, MA, USA) using the proposed plasma-assisted thermal CVD system by a method similar to one described elsewhere . Prior to growth, the copper foil was electropolished with 100 mL of phosphoric acid and 50 mL of deionized (DI) water in a homemade electrochemical bath, and a voltage of 3 V was applied for 30 s. Thereafter, the copper foil was rinsed in DI water with sonication before being dried in a nitrogen atmosphere for 5 min. The copper foil was then mounted in the CVD chamber, and the furnace was heated to 1,035°C in 40 min with constant flow of 20 sccm H2 plasma. After the temperature had reached 1,035°C, the sample was annealed for 30 min, as presented in Figure 1b. Graphene was grown at a lower temperature of 600°C. Methane (CH4) gas, flowing at 1 sccm, was the carbon source; it was mixed with various flows of H2 and fed into the tube for 5 min to form a monolayer of graphene. Subsequently, the sample was rapidly cooled by removing it from the hot zone of the thermal furnace. The synthesized graphene films were transferred onto the SiO2 (300 nm)/Si substrates by etching away the copper foil in an iron chloride (FeCl3) solution. Prior to wet etching, a 200-nm-thick thin film of PMMA (poly-methyl methacrylate) was spin-coated on the top of graphene/copper foil and then baking it at 130°C for 1 min. The PMMA/graphene thin films were washed with dilute hydrochloric acid solution to remove the metal ions and then rinsed in DI water. PMMA/graphene films were placed on the SiO2 (300 nm)/Si substrate, and the PMMA was then dissolved in an acetone bath over 24 h.
Results and discussion
This study elucidates the effect of hydrogen on graphene grown on Cu by plasma-assisted thermal CVD at a low temperature of 600°C. The mechanism of growth of graphene by plasma-assisted thermal CVD was clarified by obtaining plasma emission spectra at various H2 flow rates. When the H2 flow rate increased, the Raman spectra of the samples have I2d/Ig ratios that increase from 0.98 to 2.29 and the FWHMs of the 2D band that decrease from 39 to 35, both indicate that the graphene film is high quality. Plasma-assisted thermal CVD is a more effective method for depositing high-quality graphene films on metal substrates.
The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under contract no. NSC 102-ET-E-008-002-ET.
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