# Recent Advances in Flexible Resistive Random Access Memory
## 3 Material System of Flexible Resistive Memory
### Flexible Dielectric Materials
#### 3.1.3 Flexible Organic-Inorganic Composite Materials  
Considering the excellent flexibility of organic materials and the electrical reliability of inorganic materials, using organic-inorganic composite materials seems to be a good choice. The resistance transition of inorganic dielectric materials is usually stable, but the flexibility is poor. Organic materials have good flexibility but poor stability of resistance transition. Therefore, organic-inorganic composite materials can be selected as storage media, in order to combine the stability of the storage performance of inorganic materials with the flexibility of organic materials. HKUST-1 (Cu\({}_{3}\)(BTC)\({}_{2}\), BTC = benzo-1,3,5-tricarbox acid), a composite material based on a metal-organic framework, was used by [PERSON] [70]. This kind of material has the characteristics of high porosity, three-dimensional highly ordered structure, stable physical properties, easy design, easy film and so on. It is expected to realize flexible and stable resistance devices. In their study, they found that Au/HKUST-1/Au/PET devices can maintain uniform resistance over a temperature range of \(\pm\)70 \({}^{\circ}\)C. Moreover, it has a minimum bending radius of 3.2 mm and an excellent durability of more than 10\({}^{7}\) cycles (Figure 8). Metal-organic frameworks, which are formed by the association of metal cations or clusters of cations (\"nodes\") with soft organic bridging ligands (\"linkers\"), are a fascinating class of flexible crystalline composite materials offering a potential strategy for the construction of flexible RRAM.  
Moreover, poly (4-vinylphenol): graphene oxide (PVP:GO) and ultrathin HfO\({}_{\text{x}}\) were composited by [PERSON] [71]. Under the scanning of AC and DC, the endurance can be over 1400 and 800 cycles, respectively. The same device configuration realized over a flexible PET substrate exhibited an ON/OFF ratio of \(>\)10\({}^{3}\) even after undergoing large mechanical strain (corresponding to a 5 mm bending radius). According to this report, the incorporation of GO into the PVP solution resulted in achieving better control over conductive filament growth and, therefore, improved repeatability and reliability.  
Figure 7: (**a**) Device structure, (**b**) molecular structures and (**c**) transmittance of a polyelectrolyte-based device; AFM topographic images of (**d**) bare ITO, (**e**) PEI/ITO and (**f**) PAA/PEI/ITO surfaces; current voltage characteristics (**g**); endurance performance (**h**) and retention characteristics (**i**) of polymer synaptic device [68].  
At present, there are other organic-inorganic composite materials (black phosphorous quantum dots: PMMA [72], graphene quantum dots: PVP [73], boron nitride: polyvinyl alcohol [74], copper phthalocyanine: PMMA [75], (CH3 NH3)2 PbI2(SCN)2 [76], MnO2/activated carbon fibers [77], MoS2/zeolitic imidazolate frameworks [78], Al/CH3 NH3 PbI3 [79], Fe/reduced graphene oxide [80], hybrid organic-inorganic perovskite [81], etc.) have excellent flexibility in organic materials and overcome the shortcomings of poor fatigue resistance in organic materials but also have good storage performance and stability in inorganic materials. However, their fabrication process is complex, and the resistance transition mechanism is not very clear.