# Recent Advances in Flexible Resistive Random Access Memory
## 3 Material System of Flexible Resistive Memory
### Flexible Dielectric Materials
#### 3.1.1 Flexible Inorganic Materials  
In recent years, a large number of flexible inorganic materials with resistance transformation effects have been applied to the dielectric layer of flexible RRAM, including binary metal oxides (TiO\({}_{2}\)[21], NiO\({}_{\text{x}}\)[22; 23], ZnO\({}_{\text{x}}\)[24], AlO\({}_{\text{x}}\)[13; 25; 26; 27], WO\({}_{\text{x}}\)[15; 28; 29], HfO\({}_{\text{x}}\)[17; 27; 30; 31], MoO\({}_{\text{x}}\)[32; 33], etc. ), multinated oxides (HfTiO\({}_{\text{4}}\)[34], ZnSnO\({}_{\text{x}}\)[35], etc. ), solid electrolyte (Ag\({}_{2}\)Se [36], Ge\({}_{2}\)Sb\({}_{2}\)Te\({}_{5}\)[37], MoS\({}_{2}\)[38], etc.) and other flexible inorganic materials (C\({}_{3}\)N\({}_{4}\)[39], BN [8], ZrN [40], Si\({}_{3}\)N\({}_{4}\)[41], SiO\({}_{\text{x}}\)[42], amorphous silicon [43], graphene oxide [44; 45; 46; 47], etc.). In flexible inorganic medium materials, binary metal oxides with simple chemical composition and excellent resistance are the most common. Binary metal oxides are favored by researchers because of their simple structure, controllable composition, simple preparation, stable chemical properties, high ON/OFF ratio and compatibility with CMOS technology. The resistance transformation effect of most binary metal oxides is based on the valence state change caused by oxygen ion migration under an electric field or the conductive channel generated by oxygen vacancy accumulation. Few show the resistance transformation effect by forming metal conductive filaments, such as Ti-doped ZnO dielectric [48] and Al-doped HfO\({}_{\text{x}}\) dielectric [18]. At present, there is a flexible RRAM based on binary metal oxides with excellent performance. The characteristics of the binary metal oxides are shown in Table 1 below:  
As shown in Table 1 above, the resistive memory based on binary metal oxides has an excellent performance in retention time, flexibility and other aspects. The ON/OFF ratios of NiO\({}_{\text{x}}\), ZnO\({}_{\text{x}}\) and AlO\({}_{\text{x}}\) are high, owing to the high interface barrier with the electrode. LRS and HRS are easy to distinguish in the working process, resulting in small errors, but the flexibility of these is relatively low and is not conducive to the application of flexible devices. The material of HfO\({}_{\text{x}}\) has an outstanding advantage in flexibility, with the lowest bending radius and the highest number of bending cycles. Although the ON/OFF ratio is relatively low, it is within the acceptable range. In general, HfO\({}_{\text{x}}\) is the most widely used as flexible inorganic medium because of the best mechanical properties. Hf\({}^{2+}\) has large mobility and solid solution rate, which makes it form conductive filaments more easily, so it can withstand greater bending strain. However, the more detailed resistance transition mechanism of these binary metal materials is not clear, so it could still be a hot spot in this field to research the resistance transition mechanism for guiding the selection principle of flexible dielectric materials. The multinated oxides used in flexible RRAM are usually transitioning metal oxides, and these oxides have good performance. For example, a two-layer TiO\({}_{2}\):HfO\({}_{2}\) film for flexible storage media was reported by [PERSON] [34]. The storage medium exhibits superb \(10^{3}\) direct-current (DC) switching cycles, outstanding \(10^{7}\) pulse endurance and high-thermal stability (\(10^{5}\) s at 125 \({}^{\circ}\)C), and its cross-array memory even has the potential to be used in biomimetic neuromorphic systems (Figure 3). Flexible RRAM based on multinetal oxides is mostly used in the field of synaptic bionics, which requires high durability. The