# Directly Synthesized Graphene-Based Photonics and Optoelectronics Devices
## 2 Growth Mechanism of Graphene
### Synthesis of Graphene by the ACS Method  
Despite striving persistently to grow 3D graphene, which is in demand for practical electronic, optoelectronic, and optical devices, the inadequacy of synthetic methods and the practice of conventional transfer approaches have limited its realization. Accordingly, a metal-free, etching-free, transfer-free, direct synthesis method of functional graphene that contours 3D-structured surfaces of nonlinear optical devices has been proposed in [109]. In this method, first, the catalytic effect of \(\gamma\)-Al\({}_{2}\)O\({}_{3}\), i.e., the ceramic material employed as the growth catalyst, was verified by synthesizing graphene on it. Inside the CVD furnace, dehydrogenation of CH\({}_{4}\) occurs on the surface of \(\gamma\)-Al\({}_{2}\)O\({}_{3}\), resulting in the generation of C atoms at lower activation energies. As the density of C atoms increases, the concentration gradient of C atoms between the catalyst and the target surfaces evolves gradually. The increasing concentration gradient leads to the development of a driving force for C atoms to flow through air towards the neighboring surface via surface diffusion and undergo graphitization. Accordingly, the graphene clusters on the neighboring surface resemble the \"sprayed\" C atoms from the catalyst. This mechanism has been defined as the ACS method and is graphically presented in Figure 6a.  
Figure 5: (**a**) SEM image of the conformal graphene/SiNHs structure; (**b**) Raman spectra of graphene on SiNHs (red) and Si (black); (**a,b**) Reproduced with permission from [108]. Copyright American Chemical Society, 2019.  
Figure 6: (**a**) Schematic representation of graphene growth on the targeted surface by spatial diffusion; (**b**) Schematic of the setup for synthesizing graphene around the 3D structured fiber by the ACS process; (**c**) Raman intensity ratio of \(\mathrm{I_{D}/I_{G}}\) (red curve) and \(\mathrm{I_{D}/I_{G}}\) (blue curve) acquired from the fiber perimeter. (**a–c**) Reproduced with permission from [109]. Copyright WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2020.  
To fabricate a practical 3D structured optical device coated with conformal 3D graphene by the ACS method, a V-shaped groove pre-deposited with a \(\gamma\)-Al\({}_{2}\)O\({}_{3}\) thin film and covered with a SiO\({}_{2}\) substrate from the top was employed, as shown in Figure 6b. The grown graphene around the fiber was further analyzed by evaluating both \(I_{\text{D}}/I_{\text{G}}\) and \(I_{\text{2D}}/I_{\text{G}}\) peak ratios, as presented in Figure 6c. The region nearest to the fiber from the \(\gamma\)-Al\({}_{2}\)O\({}_{3}\) surface during synthesis (from \(-90^{\circ}\) to \(+90^{\circ}\) in Figure 6b) was coated with higher-quality graphene compared to other regions, which indicates the impact of a longer distance between \(\gamma\)-Al\({}_{2}\)O\({}_{3}\) and the number of O\({}_{2}\) atoms on the growth of graphene. This method requires no metal catalyst, has less complexity, and yet produces high quality graphene growth. Owing to the ability to control semiconducting properties by tuning the growth condition, this process has a higher potential for further functional systems than the conventional techniques.