# Directly Synthesized Graphene-Based Photonics and Optoelectronics Devices
## 2 Growth Mechanism of Graphene
### Interfacial Growth  
A considerable number of issues, such as wrinkles, cracks, contamination, low efficiency in the growth process, and complex transferring procedures, are undeniable in metal-catalyst-based conventional CVD-grown graphene [66; 67]. Therefore, transfer-free graphene synthesis methods attracted significant attention among researchers. Interfacial growth process enables the growth of a patterned graphene layer between the substrate and catalyst layers; it is a method conceived to avoid the transferring process by optimizing critical growth factors that affect the synthesis mechanism [68; 69; 70]. A study confirmed that synthesis of graphene can be obtained at the interface of the substrate and Ni film inside a generic CVD furnace [71]. Figure 2a-d provide a pictorial presentation of the synthesis procedure. There was no external carbon source fed to the furnace, and no transfer process was required after the synthesis. In this process, an electron beam (e-beam) evaporator was used for Ni deposition, where the substrates were heated at 1000 \({}^{\circ}\)C and then cooled to room temperature to sustain nucleation and growth of the graphene crystals. Large grain boundaries were obtained after applying high-temperature heat treatment, which can be controlled by optimizing the thermal conditions. According to the study, no external carbon sources were requiredas high carbon density near the grain boundary (GB) and the high-speed diffusion paths for the carbon atoms were confirmed by the duplicated weak patterns of Ni-GB on the graphene layer [72]. The Raman spectrum of the as-grown graphene presented in Figure 2e illustrates that a few layers of graphene were grown at the interface that corresponded well with the TEM analysis. In another study [73], high carbon density and diffused carbon atoms were obtained from the Ni grain boundaries in the resultant graphene, which advances the process without any external carbon source [71; 73]. The prepared graphene was studied by Raman spectroscopy; the graphene grown at 710 \({}^{\circ}\)C was found to be more crystalline than that grown at 700 \({}^{\circ}\)C.  
Another study revealed a diffusion-assisted synthesis (DAS) method, in which graphene was synthesized on SiO\({}_{2}\)/Si substrate, plastics, or glass [74]. A Ni film was employed as a catalyst, and solid carbon was used as a carbon source to form a graphene layer under an electron evaporator within a wide range of working temperatures (from 25 \({}^{\circ}\)C on PMMA and glass to 400 \({}^{\circ}\)C on SiO\({}_{2}\)/Si (100)). In this work, thermally oxidized Si (100), poly(methyl methacrylate) (PMMA)-covered SiO\({}_{2}\), and commercially available glass were used as substrates. For the catalyst material, a thin Ni film (\(\sim\)100 nm) was deposited on the substrates, and then annealed at 1000 \({}^{\circ}\)C in an H\({}_{2}\) atmosphere to yield a strong 111-texture and micrometer-sized grains with smooth surfaces. Ni is used for possessing high carbon solubility, and Ni (111) is nearly lattice-matched with the basal planes of graphite; these two important factors provide equal epitaxial growth of graphene [64; 75]. The source was solid C, which was supplied from a paste made of graphite powder dispersed in ethanol. The molybdenum holding stage was used for applying a homogeneous pressure (\(<\)1 MPa) to the C-Ni/substrate. This ensured efficient contact at the C-Ni interface and stimulated the C-C bond breaking on the catalytic Ni (111) surface [76; 77]. Subsequently, the sample was sealed in a quartz tube, which was filled with inert argon gas and heated for a short time between 25 and 260 \({}^{\circ}\)C. The C-Ni film was treated using an FeCl\({}_{3}\) solution to obtain the resultant graphene layer. From the Raman spectra, it was confirmed that a few layers of synthesized graphene were found on SiO\({}_{2}\) and the as-synthesized graphene films were wrinkle-free and smooth over large areas.