Latest studies on resistance switching of molecular thin films embedded with nanoparticles

Abstract

Resistive switching of molecular film incorporated with nanoparticles(NPs) has become a hot topic in the information storage industry, which is systematically reviewed from the aspects of electrodes, film structure, NPs, switching mechanism and mechanical properties. There are three sorts of structures i.e., layered, core-shell and complexed films, in which the film thickness affects the device charge transport and switching performance to a large extent. Usually, higher on/off ratio and lower threshold voltage can be expected for device with less-conductive active layers than that with more conductive ones. As a key factor, the interfaces of electrode/organic and molecule/NPs may largely affect the switching performance. It is shown that the type, size and distribution of NPs and molecular structure govern the interfacial behaviors, which in turn influences the switching mechanisms including filament formation/ rupture, charge trapping/ detrapping or charge transfer. For the case of filament theory, it may be ascribed to metallic, oxygen vacant or carbon-rich model. The as-embedded NPs can be classified as metal, metal oxide and/or carbon-like materials such as Au, Ag, Al, ZnO, TiO2, or graphene etc. The Au NPs show distinguishing features of little diameter, high chemical stability and large work function. On the other hand, the metal oxide NPs may form deep interfacial barrier with the target molecules and thus improve the switching characteristics. Small molecular-weight organics are also studied as embedding materials complexed with polymers as to strengthen the switching properties, and charge transfer is believed to be responsible for such an enhancement. Except for concentration and diameter of the NPs, their distribution in the active layer critically influences the memory behavior. The NPs can be made onto the molecular layer in-situ by vacuum thermal evaporation of different metals or sputtering deposition of various metal oxides. In such cases, the thickness of the deposition layer is a key parameter to obtain good switching performance. Although great progress has been made for static devices in small-scale, it is crucial to develop roll-to-roll manufacturing, precise NPs' distribution and dynamic mechanical properties in order to fabricate large-scale, low-cost and flexible memory devices. It still needs hard work on understanding the switching mechanism and engineering the interfacial properties of molecule/electrode and molecule/NPs, especially under bending conditions. New techniques should be developed to fabricate organic memory films embedded with NPs so as to avoid the problems of pinhole, effects of solvent and dust normally existing in traditional spin-coating films.