## 3 Thin-Film Materials and Their Applications
### Complex Metal Oxides
#### 3.1.3 Two-Dimensional (2D) Thin-Film Materials
This work, by means of a compact model, successfully extracted the interface trap distribution of MoS\({}_{2}\) MOSFETs in which there were a surface potential interface trap charge self-consistent calculation procedure and a drain current expression. As a graphene-like material, the applications of TMDC are similar to what graphene has to offer. TMDC also has the degradation ability which can be regarded as environmental-friendly. [PERSON] et al. have reported a study that demonstrated that MoS\({}_{2}\) is thermodynamically and kinetically unstable to O\({}_{2}\), being degraded under ambient conditions in various oxidizing aqueous environments [159]. MXene  
Recently, a new kind of 2D material has been rapidly studied ever since the initial synthesis of Ti\({}_{3}\)C\({}_{2}\)T\({}_{x}\) (MXene). MXene is a new family of 2D graphene-like transitional metal carbides, nitrides, and carbonitrides, whose precursors are MAX phases, as shown in Figure 5[160]. Ti\({}_{3}\)C\({}_{2}\)T\({}_{x}\) was first synthesized successfully by [PERSON] et al. via selective etching of the raw Ti\({}_{3}\)AlC\({}_{2}\) (MAX phases) [127]. Till now, more than 70 species of MAX materials have been developed, which can generally be prepared by the selective etching of certain atomic layers from their corresponding MAX phases using aqueous hydrofluoric acid as the etchant at room temperature [161; 162; 163; 164]. Various unique properties account for its popularity in the research area. Firstly, [PERSON] et al. have studied that the stability of MXene is excellent via employing first-principle band structure calculations and density functional theory [165]. The lattice energy of MXenes was figured out to be negative values. As is well known, the lattice energy is a crucial parameter to judge the stability of a crystal. The more negative the energy is, the more stable of the crystal. Secondly, MXene possesses fine electronic properties according to the density functional theory. [PERSON] et al. reported the research that the dipole moment generated by the OH functional group is favorable to electron transport, and the conductivity of MXenes is comparable to that of multilayered graphene [166]. Hence, MXene has semiconductor properties which are well worth studying. Thirdly, MXenes have also been found that  
Figure 5: Fragment of the Periodic table. Elements forming MAX phases of the general composition M\({}_{n+1}\)AX\({}_{n}\) [n = 1, 2, 3...; M is a transition d-metal, A is a p-element (Si, Ge, Al, S, Sn, etc. ), X is carbon or nitrogen] [160]. Inset: An example of the MAX phases. Reproduced with permission from [160], Copyright Turpion Ltd., 2013.  
its elastic modulus and bending strength are prominently higher than that of other thin films with the same thickness, such as graphene, which is reported by [PERSON] et al. [167]. What is more, MXene can hold ions and molecules as a layered structure, which has a higher capacity volume than conventional carbon materials per unit. Therefore, lithium-ion batteries (LIBs) can be effectively improved in theory. There are tremendous applications of MXene materials. Here we specifically discuss the application of it being environmental-friendly. Various studies showed that MXenes have a high hydrogen storage capacity. Hydrogen is widely recognized as a clean energy source [168; 169]. It is found to be the most potential alternative to traditional fossil fuel which leads to air pollution and greenhouse gas emission. In 2013, [PERSON] et al. devoted to research the hydrogen storage availability of Ti\({}_{2}\)C [168]. Considering all absorbed hydrogen storage molecules and atoms, the maximum hydrogen storage capacity of Ti\({}_{2}\)C was calculated to be 8.6 wt% which meets the gravimetric storage capacity requirement set by the U.S. DOE. Seeking for reversible hydrogen storage material remains a great challenge for material scientists and chemists. Hence, MXenes is making a groundbreaking breakthrough in the development of clean energy.