## 3 Thin-Film Materials and Their Applications
### Complex Metal Oxides
#### 3.1.3 Two-Dimensional (2D) Thin-Film Materials  
In the past decade, owing to the unique electronic, mechanical and optical properties [23; 118; 119; 120; 121; 122], researchers had an extensive study for 2D materials for various applications. Since the discovery of graphene [18], the field of 2D thin-film materials and research pertaining to the discovery, characterization, and application of novel 2D materials have grown exponentially [123]. The prominent properties of graphene, such as the emergence of a Dirac cone, lead to higher conductivity in the monolayer, and further pushing people to search other promising 2D materials. Hexagonal boron nitride (h-BN), a structural analog of graphene, was one of the several 2D materials which have been researched next. This special 2D material was predicted to have an obvious bandgap opening and relatively high carrier mobility. Figure 3 shows the structures of three representative 2D materials [124]. The top view and the side view of graphene, boron nitride, and silica (2D silicon dioxide film) are well demonstrated. MoS\({}^{2}\), with a wide bandgap [125], displays an indirect to direct bandgap transition between monolayer forms and the bulk [126]. While other members of the 2D transition metal dichalcogenide (TMDC) of materials principally have a semiconducting nature, and band gaps are larger than their bulk counterparts. Recently, transition metal carbides, carbonitrides, and nitrides (referred to as MXenes) are among the latest additions to the 2D world [127; 128; 129; 130; 131; 132; 133]. Also, III-V semiconductors [134], II-VI semiconductors [135; 136], and many other synthetic varied 2D materials [137; 138; 139; 140; 141] have also been predicted. In this section, we mainly discuss three 2D materials, that is, Graphene, TMDC, and MXene, and summarize their structural characteristics, the status of their development, and some applications. Graphene, a monolayer carbon hexagonal sheet, is one of the most common materials in both academia and industry. In the past decades, thousands of researchers have studied and reported the fascinating electronic properties of graphene, in both experimental and theoretical aspects. In this section, we introduce graphene and discuss the development history and the discovery of its unique properties. Due to its various properties, this 2D thin-film can be implemented in many different areas such as graphene-based electronics, energy storage, and composite materials. Especially, environmental-friendly applications are often pursued by individuals. Since graphene was isolated via mechanical exfoliation in the famous \"scotch tape\" method for the first time in 2004 [18], [PERSON] and [PERSON] received the Nobel Prize in Physics in 2010 for their pioneering research on the preparation of single-layer graphene. Henceforth, graphene in the process  
Figure 3: Comparison of graphene, boron nitride monolayer, and silica bilayer. Top view of **(a)** graphene, **(b)** boron nitride monolayer, and **(c)** silica bilayer. Side view of **(d)** graphene, **(e)** boron nitride monolayer, and **(f)** silica bilayer. Reproduced with permission from [124], Copyright American Chemical Society (ACS), 2016. It is worth noting that further permissions related to the material excerpted should be directed to the ACS. of being commercialized. There are many potential applications in almost every field from electronics to the environment all thanks to its fascinating electrical and chemical properties. Most commonly known, solar cells play a key role in solving the energy crisis. Graphene has been proven that it could be utilized in solar cells. Extensive attention has been paid to a variety of solar cells such as crystalline silicon solar cells, amorphous silicon film solar cells, dye-sensitized solar cells, and organic dye solar cells.