## 3 Thin-Film Materials and Their Applications
### Complex Metal Oxides
#### 3.1.3 Two-Dimensional (2D) Thin-Film Materials
There are many properties that prompt graphene to the area of a single atomic layer surface which can reach up to 2630 m\({}^{2}\)/g, the semiconductor intrinsic mobility can attain 200,000 cm\({}^{2}\)/(V-s), thermal conductivity is about 5000 W/(m\(\cdot\)k), and the transmittance is 97.7% with excellent transparency. Thus, graphene is the ideal candidate to substitute previous expensive ITO or fluorine-doped tin oxide (FLO), which is used in solar cells for injection or charge collection. More importantly, the graphene processes unique biocompatibility and biodegradability, which has stimulated the enormous interest of researchers. Biocompatibility can be said as a type of environmental-friendly property somehow, which means that the materials have the ability to interact with cells, tissues, or a living body without causing harmful effects [43]. Due to its special effects on stem cell lineage specifications, graphene has attracted attention as a platform for cell culture to promote the cardiomyocyte differentiation process of mesenchymal stem cells [142]. Furthermore, further studies show that graphene prepared by the CVD could operate as the favoring growth of neuronal cell substrates with no cytotoxic effects. This property of graphene is not only eco-friendly but also plays a significant role in promoting development in the biomedical field. There is no need to overstate the environmental implications of biodegradability, and it has been proven that some nanostructures like graphene oxide are capable of catalyzing the degradation at the cellular level and in vivo [143; 144; 145; 146]. Transition Metal Dichalcogenide (TMDC)  
Due to the layered structure of TMDC similar to graphene, as shown in Figure 4, much research has been done about it. In 2009, [PERSON] et al. performed a first principle study in which the MoS\({}_{2}\) thin films were acquired in the experiments by means of chemical bath deposition [147]. In further studies, various feasible methods have been used for the deposition of MoS\({}_{2}\) thin films, such as the spin coating, microwave synthesis, CVD, metal-organic chemical vapor deposition, sputtering technique, laser evaporation, and pulsed electrodeposition [149; 150; 151; 152; 153; 154]. The adjacent layer of this crystal structure may slide due to the weak Van der Waals forces between the layers of S-Mo-S [155]. For investigating the electronic properties of the graphene-like structure TMDC, [PERSON] et al. came to the conclusion that the lattice parameters and stabilities of the MX\({}_{2}\) (M = Mo, Nb, W, Ta; X = S, Se, Te) sheets depend on the chalcogen atoms [148]. Meanwhile, the electronic properties are primarily determined by the chalcogen. The variation of optical absorbance with wavelength and the variations with inverse temperature have been well demonstrated. Owing to the unique electronic properties of the graphene-like structure TMDC, many applications are already realized, based on it. Figure 4: The side and top view of the MoS\({}_{2}\) sheet. Reproduced with permission from [148], Copyright Elsevier, 2011. In 2012, [PERSON] introduced a model for the surface potential and drain current for monolayer TMD field-effect transistor (FET) [156], considering the 2D density-of-states of the atomic layer thick TMDC and its influence on the quantum capacitance. The work developed an analytical drain current expression that is applicable for both subthreshold and above threshold regions of operation. In 2014, [PERSON] et al. also performed an analytical current-voltage model formulated specifically for 2D TMDC semiconductor-based FETs [157]. In this work, various non-ideal secondary effects have been taken into consideration, such as interface traps, mobility degradation, and inefficient doping. In 2015, [PERSON] et al. considering dielectric interface trap states presented a surface potential-based low-field drain current compact model for 2D TMDC FET [158].