## 2 Preparation of Thin-Film Materials  
The extensive applications of thin films facilitate the development of film preparation techniques. In general, typical thin-film growth can be divided into both physical and chemical processes [12; 14; 28; 30; 44; 45]. The physical process consists of the physical vapor deposition (PVD) process, which involves evaporation from a source and sputtering from a target, and then condensing on the substrate. It includes vacuum evaporation, pulsed laser deposition (PLD), molecular beam epitaxy (MBE), and sputtering. The chemical processes are composed of the chemical vapor deposition (CVD) process and the chemical solution deposition process; a discharge of ions, decomposition of the compounds, and the reaction of gases or liquids with the substrate surface. Due to the limited quantities used on the surface and/or thin-film layers, thin-film materials and devices are available for minimization of toxic materials. In addition, thin-film processing can also improve energy consumption in production, which is highly regarded as an environmentally friendly material technology in the upcoming century. A large number of methods are employed to prepare inorganic thin-film. For example, the main methods for growing oxide thin-films include oxidation [46], sputtering [47], evaporation [48; 49], CVD [47], ALD [50], PLD [51; 52], and sol-gel techniques [53; 54]. These methods all have their own advantages and disadvantages. For example, PVD is a physical process, which is an eco-friendly technology in contrast to CVD. PVD technology is clean and dry, with no hazardous materials involved, and does not generate chemical waste or water pollution. Thin-film grown by the PVD has high density and few impurities as well as good controllability and repeatability of thickness. While all PVD processes are performed under vacuum. CVD technology is a chemical process, which has a higher deposition rate than PVD. The composition of the film is precisely controllable, and the ratio range is large. For the sol-gel technique, it is a method for producing solid materials from small molecules, which is a cheap and low-temperature technique that allows the fine control of the product's chemical composition. Its mechanical properties and structural stability are enhanced. However, owing to the shrinkage of a wet gel upon drying, it often leads to fracture resulting from the generation of large capillary stresses, which makes it difficult for the attainment of large monolithic pieces. For 2D thin-film, a typical preparation method is a mechanical exfoliation, which was first used to prepare single-layer graphene in 2004 [18]. Since then, CVD [55], PLD [56], MBE [57], and pick-and-lift vdW techniques [58] were also used for preparing high-quality 2D thin films. Mechanical exfoliation is the main methodology to prepare the single-layer graphene. It is simple in operation and high in sample quality. However, it is low in efficiency and results in high cost, making it unsuitable for mass production in the industry. Therefore, some novel preparation methods were explored and pursued by the researchers, where the environmental-friendly production of graphene attracted tremendous attention and interest under the e-waste pollution and energy shortage. [PERSON] et al. reported a green methodology of synthesis of graphene oxide (GO) via aqueous extract of grapes, which was successfully utilized for the reduction of GO in order to prepare the graphene [59]. This is a promising methodology for the large-scale production owing to the environmentally benign, economical, and facile synthesis approach. Similarly, there are a multitude of preparation methods for organic thin-films, in which hydroxyl condensation reaction is one of the ways to fabricate organic thin films. As shown in the research [60], a copolymer was fabricated using this method.