## 3 Thin-Film Materials and Their Applications
### Organic Thin-Films
#### 3.2.1 Small Molecule and Polymer Thin-Films  
Due to the various types of organic materials, different organic materials vary from one another, resulting in different usage. Nowadays, \"green\" materials and devices are garnering increasing attention worldwide. Society is well aware of the degrading state on the earth's environment due to pollution caused by waste electronics. The requirements of \"green\" electronics and energy are becoming stronger. Next, we will expand our review from the preparation methods and applications of these thin films. [PERSON] et al. introduced a new method that allows thin-film composite (TFC) membrane in reverse osmosis (RO) and forward osmosis (FO) to be used for addressing the environmental issues related to oily wastewater [176]. Such economic and environmental-friendly treatment processes for oily wastewater treatment can offer a source of freshwater for reuse and meet the discharge standard, further minimizing serious environmental pollution. Similarly, [PERSON] et al. presented TFC and/or TFC polymer hollow fiber (HF) membranes for water treatment, pervaporation dehydration, and gas/vapor separation [177]. In this paper, different types of membranes such as ultrafiltration, reverse osmosis, nanofiltration, and microfiltration are prepared for softening, disinfection, organic removal, and desalination of water and wastewater. In addition, TFC HF membranes prepared via the dip-coating technique were used for sensing and separation of CO\({}_{2}\) gas, which can relieve environmental concerns, for instance, climate change, and global warming. [PERSON] et al. proposed a poly(3,4-ethylenedioxy-thiophene): Poly(styrene sulfonate) (PEDOT: PSS) sensors using photolithography on fully biodegradable and flexible silk, which has proven to have benign cytocompatibility and biodegradability [62]. These sensors can be completely degradable under enzymatic action. Furthermore, they can maintain excellent performances under complex mechanical deformations, showcasing the potential in biosensors. [PERSON] et al. put forward a new material, Alginate [178]. As a natural linear polysaccharide derived from brown sea algae using the simple freeze-drying method, the calcium alginate film has porous structures which will be beneficial for triboelectric power generation. This calcium alginate film originates from the ocean initially and can be finally degraded in the ocean, which is thought to be an environmental-friendly ecosystem. [PERSON] et al. raised a degradable and flexible nanocomposite film fabricated with poly lactic acid (PLA), nano ZIF-8@GO, and degradable flame-retardant resorcinol diphenyl phosphate (RDP) [179]. These environmentally friendly biodegradable polymers are ideal candidates for solving today's electronic waste pollution. Therefore, further works develop the degradable and transient devices for \"green\" microelectronic applications. In 2016, [PERSON] et al. fabricated a biodegradable synaptic transistor on graphene-coated PET substrate gated by a chitosan-based biopolymer electrolyte film [180]. Chitosan is a naturally abundant, non-toxic, and degradable polymer [181]. The proposed transistor could be dissolved in deionized water, indicating the potential biodegradable and \"green\" synaptic transistor applications. In 2018, a water degradable biopolymer-based indium-tin-oxide (ITO) synaptic transistor was proposed by [PERSON] et al., where starch-based biopolymer electrolyte was used as the gate dielectric [178]. The proposed biopolymer-based oxide synaptic transistor exhibits high stabilities in synaptic response. After mimicking both short-term and long-term synaptic plasticity, it shows the potential applications in \"green\" neuromorphic platforms. In 2019, our group used natural chicken album thin-film prepared by spin coating as the gate dielectric to fabricated electric-double-layer (EDL) TFTs [182].