# Directly Synthesized Graphene-Based Photonics and Optoelectronics Devices
## 2 Growth Mechanism of Graphene
### Conventional Methods of Graphene Synthesis
Another study reported that [58] multilayer graphene domain formation is influenced by the concentration of Ni boundaries based on the precipitation of carbon atoms [64]. Single-crystalline Ni (111) film favors the growth of mono/bilayer graphene with higher uniformity, which hinders the growth of multilayer graphene. In contrast, polycrystalline Ni facilitates the growth of multilayer graphene. The researchers synthesized graphene on both single-crystalline (in Figure 1d) and polycrystalline Ni (in Figure 1e) surfaces. The growth temperature was elevated up to 900 \({}^{\circ}\)C under H\({}_{2}\) ambience. After the CVD growth process, micro-Raman spectroscopy verified the uniform formation of graphene layers on both of the Ni surfaces; formation of monolayer/bilayer graphene accounted for 91.4% of the area for single-crystalline Ni contrasting to 72.8% for that of polycrystalline Ni (as portrayed in Figure 1f) under comparable growth condition. The Raman peaks of Ni (111) and polycrystalline Ni are assigned to the G and G' (D band was negligible) bands of the graphene layers, respectively, confirming that the graphene produced on both surfaces have low defects [58]. The CVD process on polycrystalline Cu foils was performed by [PERSON] et al. in a similar setup like that used for Ni [42]. However, unlike Ni, Cu has no partially filled d-orbital that can interact with C atoms; therefore, Cu possesses ultralow carbon solubility. Furthermore, the graphene formation on Cu is not a case of segregation and precipitation but a surface reaction process. This means that the graphene growth process is controlled by surface-absorption growth in the case of Cu, where the carbon decomposes from the hydrocarbon on the Cu surface. From the beginning of nucleation, the formation of graphene domains continues progressively until the decomposition of the precursor hydrocarbon becomes super-saturated. Therefore, the growth mechanism is a self-limiting process, generally resulting in high surface coverage by a monolayer. This mechanism has been verified with isotope variant precursor molecule-based growth and Raman characterization [65]. Because graphene and Cu have different thermal expansion coefficients, graphene \"wrinkles\" can be seen in Figure 1g (SEM image) and Figure 1h (optical image). The \"wrinkles\" can also go across Cu grain boundaries, indicating that multilayer graphene is not formed in the grain boundaries and the transferred graphene film is continuous. Further speculation with Raman spectra can be made based on Figure 1i, in which the peaks assigned as D, G, and 2D bands carry information, such as defect density and film thickness. In addition, the D map proves the uniformity of the as-grown graphene, except for the regions where wrinkles are visible that are close to the multilayer regions. The G and 2D maps clearly show the existence of multilayer graphene in the flakes. By analyzing all the SEM characterizations, the amount of monolayer graphene was found to be 95%, less than 1% of monolayer/trilayer graphene and 3-4% of bilayer graphene [42].