# Directly Synthesized Graphene-Based Photonics and Optoelectronics Devices
## 2 Growth Mechanism of Graphene  
### Conventional Methods of Graphene Synthesis  
Since the discovery of graphene, extensive research has been conducted regarding the synthesis of graphene. Amongst all the conventional methods of graphene production, CVD on catalytic metal substrates seems to be one of the better prospective strategies. There are several available reports on the synthesis of graphene with Ni and Cu catalysts [40; 42; 43; 50; 51], as well as various other metal catalysts, such as Co [52], Ru [53], Ir [54], Pt [55], Re [56], and Pd [57] that have been employed for CVD-based synthesis of graphene. In the case of the Ni-catalyst-based synthesis, polycrystalline Ni films are annealed at 900-1000 \({}^{\circ}\)C under an Ar/H\({}_{2}\) atmosphere to increase the grain boundaries, facilitating the growth of graphene with larger grain size [43; 58]. The Ni films were then exposed to hydrocarbons to initiate the growth process. The hydrocarbon gas is decomposed into C atoms, which dissolve in bulk Ni. Because Ni possesses high carbon solubility [59], the dissolved C atoms continue to form Ni-C solid bonds until the C atoms reach a super saturation state in the Ni film. Consequently, monolayer graphene forms by C segregation, which can be observed on the surface [60]. The outcome of CVD-based synthesis of graphene using Ni catalysts is highly dependent on the temperature used. The higher the temperature, the more the carbon solubility Ni possesses. When the temperature is cooled down, more C atoms start diffusing from the solid Ni-C solution and start precipitating out to the Ni surface, thereby forming a graphene film of a certain thickness [61; 62; 63]. In addition to temperature, the microstructure of Ni plays a vital role in graphene formation [43; 50; 51]. The grain boundaries in polycrystalline Ni substrates lead to the formation of multilayer graphene, whereas the Ni (111) substrate renders a smooth surface, which results in graphene uniformity. In an experiment to produce graphene [43], the synthesis was performed over Ni films. The Ni films were deposited on a SiO\({}_{2}\)/Si substrate by e-beam evaporation, as shown in the optical image in Figure 1a and then was annealed at an elevated temperature before the synthesis process in CVD. After exposure of the polycrystalline Ni film to highly diluted CH\({}_{4}\) flow under ambient pressure at 900\(-\)1000 \({}^{\circ}\)C, an atomically thin (1-10 layers) graphene film was grown on the Ni surface. Formation of single- and multi-layered films bridge together yielding a continuous film of graphene crystal. Figure 1b presents the optical image of the transferred resultant graphene, demonstrating the preservation of the geometry of the transferred film. Furthermore, the grain size at micrometer scale from the transferred graphene film calculated by Raman spectroscopy (as presented in Figure 1c) shows the advantages of annealing the Ni film in the synthesis process. Figure 1: Metal catalyst assisted growth of graphene: (**a**) Micrograph of a patterned Ni film on SiO\({}_{2}\)/Si; (**b**) Micrograph of the grown graphene transferred to targeted SiO\({}_{2}\)/Si substrate; (**c**) Raman spectroscopy of the transferred graphene film; (**d**) Schematic presentation of graphene growth mechanism on Ni (111); (**e**) Schematic diagrams of graphene growth mechanism on polycrystalline Ni surface; (**f**) The I\({}_{G}\)/I\({}_{G}\) intensity ratio versus the FWHM of G\({}^{\prime}\) peak of graphene grown on Ni (111) and polycrystalline Ni; (**g**) SEM image of the transferred graphene; (**h**) Micrograph of the transferred graphene; (**i**) Raman spectra taken from different points of the transferred graphene film. (**a–c**) Reproduced with permission from [43]. Copyright American Chemical Society, 2009; (**d–f**) Reproduced with permission from [58]. Copyright American Chemical Society, 2010; (**g–i**) Reproduced with permission from [42]. Copyright Science, 2009.