## 4 Synthesis of Graphene and Its Derivatives
In this method, as presented in Figure 9, hydrogen gas is mixed with methane during the growth to eliminate the undesired impurities of carbon. The formed graphene monolayer is transferred to another substrate. Due to the tight bond between the carbon and copper substrate, the graphene films are covered with a protective polymethylmethacrylate (PMMA) using a technique called spin coating, then the copper substrate is etched away with FeCl\({}_{3}\). To proceed, the graphene on a polymer support is transferred onto the Si/SiO\({}_{2}\) substrate and PMMA is polished with acetone. In addition, CVD graphene films commonly acquire defects during formation due to the subsequent merging of multiple-crystalline domains and the formation of grain boundaries, which hinder the mechanical and electronic properties of graphene. Thus, several approaches occurred including the addition of surface oxygen to graphene and the predefined orientation on a single-crystalline Ge substrate [101; 125]. Earlier, chemical vapor compositions (CVD) were introduced to produce a 3D graphene-cobalt oxide electrode for glucose detection. The group used a sample hydrothermal process in which the cobalt oxide nanowire grows up on a 3D graphene foam prepared by CVD [126]. In comparison with the exfoliation methods, chemical vapor deposition can synthesize larger graphene films with high mobility up to \(10^{5}\) cm\({}^{2}\) V\({}^{-1}\) S\({}^{-1}\); however, the structural defects or corruptions that originated during the etching and transfer process possess limiting factors for high-performance graphene-based electronic and optoelectronic sensors [99; 101]. In addition to the previous top-down methods, 2D nano-graphene materials can be synthesized by wet chemistry methods, which are advantageous in low cost, scalable high yield, and uncontaminating. This method is used to prepare MoS\({}_{2}\) nanoplates, which are disorganized graphene-like sheets with about 30 nm thickness, by the introduction of Mo(O)\({}_{6}\) and S as a precursor in the one-step hydrothermal reaction as well as used to synthesize flower such as MoS\({}_{2}\) nanostructures by the usage of Mo  
Figure 9: Graphene synthesis via chemical vapor deposition (CVD): (**a**) Chemical vapor deposition reactor. (**b**) Growth mechanism of graphene on the copper substrate via deposition. (**c**) Schematic illustration of the transfer process of the graphene sheet; spin coating with a polymethylmethacrylate (PMMA) polymer, etching copper with FeCl\({}_{3}\), graphene on a PMMA support is transferred onto the Si/SiO\({}_{2}\) target substrate, and PMMA is removed by acetone. nanorodes as a precursor in the two-step hydrothermal method. However, it is still challengeable to control the size and thickness of 2D nanomaterials by chemical synthesis methods [127]. In recent years, the functionalization of graphene has gained significant attention due to the sensitivity of graphene to a reproducible, effective, and homogenous functionalization in comparison with other carbon materials; thus, enhancing its sensing capabilities for different electroanalytical applications [100; 128]. Numerous modification techniques have been utilized to incorporate biorecognition molecules in graphene that are capable of finely detecting the target biomolecules and improving the dispersion of graphene. Two major functionalization methods are the peer-reviewed covalent and noncovalent functionalization [110]. Covalent functionalization is formed on sp\({}^{2}\)-hybridized carbon atoms of the \(\pi\)-conjugated crystal which alter the electronic characterization and physical structure. The grafting of molecules can occur either at the edges and/or on the basal plane by two methods of heteroatom doping and by a reaction with the unsaturated double bond of graphene [128; 129]. Graphene is very stable thermally and chemically, thus, the covalent functionalization method needs high energy procedures and is established by converting sp\({}^{2}\)\(\pi\)-conjugation into sp\({}^{3}\).