# Directly Synthesized Graphene-Based Photonics and Optoelectronics Devices
## 2 Growth Mechanism of Graphene
### Metal-Free Growth Technique  
Direct growth techniques on metal oxide substrates are encouraging, as these substrates avoid deleterious transfers and exhibit an insulating nature. Catalytic graphene growth has been analyzed on sapphire [87; 88], quartz [89; 90], and SiO2 [90], but these are considered to be high-temperature reactions. Although using MgO [91; 92] or ZrO2 [92] substrates reduced the processing temperature (-350 \({}^{\circ}\)C), high surface diffusion barrier of C atoms, small band gaps, uneven growth of graphene [91; 92], and high leakage current density (-1 \(\times\) 10\({}^{-4}\) A cm \({}^{-2}\)) [93; 94] are considered to be limitations.  
The fabrication of a ready-to-use graphene sticker comprising a polymer spin-coat and separated by peeling off the polymer film [95] are shown schematically in Figure 4a,b. Then, graphene/\(\gamma\)-Al2O3 is scaled off from the additional layers. Atomic layer deposition (ALD) is used to deposit the initial amorphous Al2O3, which later transforms into gamma phase at elevated temperatures. The transformed \(\gamma\)-Al2O3 acted as a substrate inside the CVD furnace where dehydrogenation of CH4 occurred.  
Another study [96] introduced the generation of nanographene (nGr) on \(\gamma\)-Al2O3, where a weak adhesion was observed between the graphene and substrate; therefore,  
Figure 3: **(a)** Schematic representation of the laser induced synthesis of graphene by the in-situ method onto SMF end facet; **(b)** Schematic illustrations of in-situ synthesis of graphene onto the flat surface of a D-shaped fiber. **(a)** Reproduced with permission from [82]. Copyright WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2015. **(b)** Reproduced with permission from [83]. Copyright American Chemical Society, 2018.  
the synthesized nGr film was easily separable, and no conventional chemical [60; 97; 98] etchant was required. The sheet resistance (R\({}_{\text{s}}\)) was found to be increased in the grown nGr [90; 99; 100], which is an indication of a reduction in the grain population.  
The outcome of oxygen being present during graphene synthesis on a ceramic catalyst has been inspected in a study [101], which observed the effect of graphene grown at distinct oxygen exposures with altering positions inside the CVD furnace. This inspection confirmed that the O\({}_{2}\) influx from the system leakage acted as a growth inhibitor for graphene. Oxygen leakage in CVD systems has adverse effects, as it leads to defects in the grown graphene [99; 102]. The presence of oxygen modifies the contact resistance and sheet resistance; these were found to directly influence the device performance [103; 104; 105]. In this experiment, the highly reactive tri-coordinated Al(III) sites on \(\gamma\)-Al\({}_{2}\)O\({}_{3}\) surfaces acted as catalytic sites because they possess strong reactivity [101; 106; 107]. Leaked oxygen reacted on these Al(III) sites, as schematized in Figure 4c.  
By increasing the distance between the inlet and substrate, the oxygen and sp\({}^{3}\) portions declined from 25.76% to 10.59% and from 21.15% to 14.70%, respectively. Meanwhile, the values of \(I_{\text{G}}/I_{\text{D}}\), which is the ratio of the G-peak intensity (corresponding to the graphene crystal) to the D-peak intensity (corresponding to the defect), have been found to be inversely functional to the distance from the inlet, as shown in Figure 4d. By examining the sample grown at different positions, different values of sheet resistance (R\({}_{\text{s}}\)) were found. Within the 0-6 cm position from the inlet, the value of R\({}_{\text{s}}\) approached infinity, whereas the samples at the 12, 18, and 24 cm positions had R\({}_{\text{s}}\) values of 6.64, 1.06, and 0.64 k\(\Omega\) sq\({}^{-1}\), respectively.