# Recent Advances in Flexible Resistive Random Access Memory
## 3 Material System of Flexible Resistive Memory
### Flexible Electrode Materials
#### 3.2.3 Other Flexible Electrode Materials  
Other flexible electrode materials include silver nanowire (AgNW) [91; 105; 109], ZnS [110; 111], poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) [112; 109; 113], Si [13; 43; 113] and so on. According to the report of [PERSON] [105], the AgNW electrode exhibits a light transmittance of more than 85% at a 550 nm light source, as well as excellent flexibility and stability (after bending \(10^{4}\) times at a 4 mm radius, the resistivity changes only 0.2%). ITO is widely used due to its low resistivity and high light transmittance. However, ITO has many problems such as high raw material price, high manufacturing cost and high technological requirements (it easily deteriorates when exposed to plasma in the process of treatment, resulting in characteristic changes). Therefore, AgNW becomes an excellent alternative material for ITO. The AgNW's grid layer exhibits high optical transparency, excellent flexibility and conductivity, and has the advantages of mass production by the solution process, which is suitable for transparent electrode. Using ZnS as a flexible electrode, it was proved that multishell ZnS can provide quantum dot sites according to the report by [PERSON] [111]. When the device is in HRS, all traps in the quantum dots are fully filled, enhancing the charge storage capability of the device and making the formation of conductive filaments in the device more rapid and stable. Meanwhile, the ZnS electrode also has the characteristics of transparency and flexibility, and its transmittance in the visible light band is more than 70%. After bending at a 10 mm radius 100 times, there was no significant change in conductivity. PEDOT:PSS is one of the common polymers with excellent stretchability [109]. This polymer has the characteristics of a simple fabrication process at room temperature, low cost, good flexibility and strong compatibility with flexible substrates, but it is not resistant to high temperature, which is a common shortcoming of organic materials. It is often used together with AgNW as an electrode of the device. As shown in Figure 11a, AgNPs were placed randomly with each other. Due to having overlapping parts of AgNWs, the contact resistance between AgNW layers was very high, resulting in an increase in the resistivity of AgNW layers. The stacking of PEDOT:PSS layers on the AgNW layer can improve the contact between them (Figure 11b). The PEDOT:PSS layer was deposited on AgNW film by spin coating, which not only helped to improve the conductivity of AgNW but also improved the surface uniformity of AgNW electrode and reduced its roughness (Figure 11c,d). Meanwhile, the maximum transmittance of the PEDOT:PSS/AgNW composite layer is 89% (the highest transmittance for electrode). As for Si electrodes, p-doped and n-doped monocrystalline silicon are generally used as the top and bottom electrodes, respectively. Such electrodes can be used to prepare flexible memory with self-rectification resistance transformation characteristics. The self-rectifying effect is due to the different doping types in the silicon disk that forms a self-sufficiency diode within each junction. In addition, the effect of the flexible electrodes on the mechanical properties of the device was studied by [PERSON] [19]. The devices of ITO/HfO\({}_{\text{x}}\)/ITO and ITO/HfO\({}_{\text{x}}\)/Pt were prepared separately. The result shows that the minimum bending radius of the device is 3 mm when the top and bottom electrodes are ITO electrodes, while the minimum bending radius of the device can be reduced to 2 mm when the top electrode is ITO and the bottom electrode is Pt. The mechanical properties of the device can be improved by using Pt as the electrode, mainly because there is a great difference between the crack formed by Pt fracture and the crack formed by ITO fracture when the curvature exceeds the critical value. The ITO electrode will form a crack through the whole film, which makes it lose the ability to conduct current.