## 2 Memristor: Models, Material, and Structure Design
### Memristor Material and Structure Design
Common perovskites include MAPbI\({}_{3}\), (C\({}_{4}\)H\({}_{9}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\), LaFeO\({}_{3}\), MAPbBr\({}_{3}\), CH\({}_{3}\)NH\({}_{3}\)PbClXI\({}_{3}\)[23], etc. These materials are ideal for solar cells, photodetectors, and light-emitting diodes due to their superior optoelectronic properties. They are also promising for designing next-generation neuromorphic memristors due to their mixed ion and electron conductivity, switchable majority carrier concentration, and slow photocurrent decay. However, perovskite materials lack environmental stability, which has been addressed by using lead-free double perovskite Cs\({}_{2}\) AgBiBr\({}_{6}\) for environmentally robust memristors (TO/Cs\({}_{2}\)AgBiBr\({}_{6}\)/Au structure) [26]. These memristors can last 10 s in alcohol burner flames and withstand temperatures of 180 \({}^{\circ}\)C or 60 \({}^{\circ}\)C \(\gamma\)-ray irradiation. Perovskite memristors, despite their extensive research, face limitations such as compatibility issues with CMOS processes, which restrict their practical applications. The development of portable and wearable devices has led to the need for flexible memristors for data storage in electronic systems. Organic materials like organic small molecules, Ru-complexes, and polymers are promising candidates due to their low cost, solution processability, flexibility, and large-scale preparation. The authors of [27] prepared an Ag/2 DP\({}_{\text{BTA+PDA}}\)/ITO memristor that exhibited good switching performance but had low endurance. The authors of [28] reported a new, flexible, and robust diffusion memristor based on a copolymer of trifluorochloroethylene and vinylidene fluoride (FK-800) Ag/FK-800/Pt, with an endurance of over 10\({}^{6}\) cycles. However, their manufacturing yield remained low. In [29], a record yield of 90% for polymer memristors is achieved with miniaturization and low power consumption. The authors of [30] fabricated a solution-processed flexible organic memristor array with self-selectivity, achieving high recognition accuracy. [PERSON] et al. prepared Cu/poly (1,3,5-trivinyl- 1,3,5-trimethylcyclottrisiloxane) (pV\({}_{3}\)D\({}_{3}\))/Al\({}_{2}\)O\({}_{3}\)/Pt memristors, improving reliability. Two-dimensional materials have gained attention due to their atomic-scale thickness, excellent electrical properties, and novel characteristics like flexibility and transparency. Researchers have investigated 2D memristors with vertical and planar structures, such as flexible memristors with vertical structures based on polyvinyl alcohol-graphene oxide hybrid nanocomposites [23]. These devices have great optical and electrical properties. They are also very stable under mechanical stress and can be separated from the bulk 2D crystal because of weak van der Waals forces between layers. Additionally, 2D material-based memristors are typically prepared on silicon wafers or polymer substrates, but these are not cheap or biodegradable. Cellulose paper is a good alternative due to its biodegradability, low cost, recyclability, light weight, and mechanical flexibility. The authors of [31] prepared a paper-based flexible memristor device using a nanohybrid material with silver and copper electrodes, achieving 500 cycles and a switching ratio of \(\sim\)10\({}^{4}\). However, 2D materials face challenges such as a large area of preparation, instability in the ambient atmosphere, controllable doping, and the direct deposition of metal electrodes, which can lead to defects and large contact resistance. Researchers are continuing to explore and optimize these materials to further enhance the capabilities of memristors in various electronic devices. One area of focus is improving the stability and reliability of memristors, particularly in high-speed applications. Additionally, efforts are being made to reduce the energy consumption of these devices to make them more sustainable for future technology.