The discovery of graphene, a single layer of covalently bonded carbon atoms, has at-tracted intense interest. Initial studies using mechanically exfoliated graphene unveiled its remarkable electronic, mechanical and thermal properties. There has been a growing need and rapid development in large-area deposition of graphene film and its applica-tions. Chemical vapor deposition on copper has emerged as one of the most promising methods in obtaining large-scale graphene films with quality comparable to exfoliated graphene. In this chapter, we review the synthesis and characterizations of graphene grown on copper foil substrates by atmospheric pressure chemical vapor deposition. We also discuss potential applications of such large-scale synthetic graphene.
Recently, chemical vapor deposition (CVD) on copper has been becoming a main method for preparing large-area and high- quality monolayer graphene. In this paper, we first briefly introduce the preliminary understanding of the microstructure and growth behavior of graphene on copper, and then focus on the recent progress on the quality improvement, number of layers control and transfer-free growth of graphene. In the end, we attempt to analyze the possible development of CVD growth of graphene in future, including the controlled growth of large-size single-crystal graphene and bilayer graphene with different stacking orders.
As for the synthesis of diamond it has been well established that the methyl radical is the growth precursor for CVD-diamond growth. In standard hydrogen-rich gas compositions it is produced by the abstraction of one hydrogen atom from the methane molecule via its reaction with atomic hydrogen: CH4 + H → CH3 + H2. In a hydrogen-deficient atmosphere—as used for UNCD growth—it is produced by electron collisional impact. This contribution leads to a non-vanishing growth rate even in hydrogen-free gas mixtures (Gruen ). Thus, it follows that—at constant gas composition, gas pressure, microwavepower, and substrate temperature—the plasma chemistry is fixed and the growth rate of diamond grains should be constant. There is evidence that the nucleation process for diamond differs from these reactions (Buck ). Since in deposition of nanocrystalline diamond films the strongest visible emission in the optical spectrum comes from the C2 line the C2 dimer was viewed as main growth and nucleation species (Gruen ; Gruen et al. ). Recently it was resolved for the first time by separate examination of the nucleation and the growth of diamond that both are consecutive but yet independent growth steps. It was shown that by controlling the C2/Hα ratio it is possible to adjust the nucleation density and the growth rate of the deposited films (Woehrl and Buck ). These experiments lead to the idea, that the C2/Hα ratio can also be used to adjust the deposition process between a preferential graphene and a preferential nanodiamond growth on copper substrates.
The PE-CVD process uses hydrogen and methane as reaction gases as is typically done in thermal CVD process for the deposition of Graphene as well as diamond. The chemical fundamentals of the synthesis of Graphene are based on a physisorption process of hydrogen and a consecutive chemisorption process of methane (Zhang et al. ). Physically adsorbed H2 molecules decompose due to the high temperature on the copper surface. Consecutively chemisorption of methane molecules results in carbon species bonded to the top layered copper atoms, which become successively dehydrogenized and eventually building the graphene’s honeycomb pattern. Due to the catalytic influence of copper and the minor solubility of carbon in copper, this leads to the growth of monolayer graphene. The graphene growth process is self-limiting and therefore basically stops after the growth of a monolayer (Li et al. ).
Summarizing the results of this paragraph, the quality of the synthesized graphene does not significantly depend on the C2/Hα-ratio measured in the plasma. The growth of nanodiamonds (nucleation density and diamond cluster size) which occurs in all the experiments, however, can be strongly influenced by the C2/Hα-ratio. This finding suggests that the C2/Hα-ratio is a process parameter that allows it to influence the growth of the different carbon allotropes independently. Moreover, the results confirm that the chemical processes that lead to nucleation and growth of graphene and diamond on copper are significantly different and that specific process parameters can be used to synthesize hybrid structures.
The growth of graphene is investigated on copper by atmospheric pressure chemical vapor deposition in a system free of pumping equipment. In such a system, it is evidenced that it is mandatory to flow hydrogen all along the growth process (especially the growth and cooling steps) to prevent oxidation and etching of graphene by residual oxidizing impurities inevitably present in the atmosphere of the reactor. In that case, micrometer-sized hexagon-shaped graphene domains of high structural quality are achieved .
There have been some reports of plasma-based methods to decrease the process temperature including the use of microwave plasma CVD to synthesize graphene on nickel foil (Kim et al. ), surface wave plasma CVD to synthesize graphene at temperatures in the range of 300–400 °C on large area conductive electrodes (Kim et al. ; Kalita et al. ) and the plasma-assisted deposition of graphene on copper foils at temperatures down to 600 °C (Chan et al. ).
Trinsoutrot, Pierre and Rabot, Caroline and Vergnes, Hugues and Delamoreanu, Alexandru and Zenasni, Aziz and Caussat, Brigitte Role of the gas phase in graphene formation by chemical vapor deposition on copper. (2013) Chemical Vapor Deposition, vol. 19. ISSN 0948-1907
One of the most frequently used methods to synthesize larger areas of high quality graphene is the use of CVD, in particular thermal CVD, which allows to deposit rather large areas with graphene of high quality (Reina et al. ). During the low-pressure CVD process, reactive gas species (mostly H2 and CH4) are fed into a hot-wall reactor at temperatures of around 1000 °C to initialize chemical reactions. Most of these CVD processes use copper substrates and take advantage of the catalytic influences of the copper on the dissociation of hydrogen (Li et al. ; Vlassiouk et al. ). However, the high activation temperature for the catalytic reaction of methane causes significant evaporation of the substrate material even at temperatures far below the melting point of copper (Li et al. ) resulting in defects in the growing graphene monolayer. In combination with the slow increase of temperature in a typical thermal CVD reactor leading to a long process time, the process will be affected by the evaporation of a notable amount of copper.
Among the methods known for the synthesis of high quality graphene, chemical vapor deposition (CVD) on copper is considered as one of the most promising methods because it enables large-area single-layer films [9–12] .
The simultaneous growth of both nanodiamonds and graphene on copper samples is described for the first time. A PE-CVD process is used to synthesize graphene layers and nanodiamond clusters from a hydrogen/methane gas mixture as it is typically done successfully in thermal CVD processes for graphene synthesis. However, the standard thermal CVD process is not without problems since the deposition of graphene is affected by the evaporation of a notable amount of copper caused by the slow temperature increase typical for thermal CVD resulting in a long process time. In sharp contrast, the synthesis of graphene by PE-CVD can circumvent this problem by substantially shortening the process time at holding out the prospect of a lower substrate temperature. The reduced thermal load and the possibility to industrially scale-up the PE-CVD process makes it a very attractive alternative to the thermal CVD process with respect to the graphene production in the future. Nanodiamonds are synthesized in PE-CVD reactors for a long time because these processes offer a high degree of control over the film’s nanostructure and simultaneously providing a significant high deposition rate. To model the co-deposition process, the three relevant macroscopic parameters (pressure, gas mixture and microwave power) are correlated with three relevant process properties (plasma ball size, substrate temperature and C2/Hα-ratio) and the influence on the quality of the deposited carbon allotropes is investigated. For the evaluation of the graphene as well as the nanodiamond quality, Raman spectroscopy used whereas the plasma properties are measured by optical methods. It is found that the diamond nucleation can be influenced by the C2/Hα-ratio in the plasma, while the graphene quality remains mostly unchanged by this parameter. Moreover it is derived from the experimental data that the direct plasma contact with the copper surface is beneficial for the nucleation of the diamond while the growth and quality of the graphene benefits from a larger distance to the plasma. Therefore, this work presents a basis for a method to tailor the deposition of graphene–diamond hybrid films using a MW PE-CVD process or to suppress the diamond deposition entirely if desired.
The synthesis of graphene by Chemical Vapor Deposition (CVD) process is one of the most promising way for device applications since it allows producing large area films on suitable substrates. In this work 2 were grown on copper foil substrates by CVD using hydrogen/methane or hydrogen/argon/ethanol mixtures as gas precursors. The growth processes were performed at 1000 °C, both at atmospheric and low pressures. A system for the fast cooling of the sample was implemented, allowing for a rapid decrease of the sample temperature below 600 °C in few seconds.2/Si substrates after Cu dissolution