Ionic and electronic transport in Ag₂S nanocrystal-GeS₂ matrix composites with size-controlled Ag₂S nanocrystals.

The Ag-Ge-S system is among the best-performing active materials for a new class of high-performance electronic memory known as programmable metallization cells.[1,2] Memory switching in a programmable metallization cell relies on the electrochemical formation and dissolution of an electronically conducting filament within a solid electrolyte. In the Ag-Ge-S system, this electrochemical process is facilitated by the transport of both Ag+ ions and electrons. Consequently, it is important to understand how the ionic conductivity and electronic conductivity relate to the structure of the Ag-Ge-S system. However, this understanding is hampered by the ill-defined morphology of Ag-Ge-S, which consists of an amorphous Ge-rich matrix with embedded Ag-rich nanoinclusions of random size, shape, and interparticle spacing.[1,2] We circumvent these morphological ambiguities by controlling these structural variables with our recently developed nanocomposite formation technique.[3] We create Ag2S nanocrystal–GeS2 matrix composites and demonstrate that their ionic and electronic properties can be systematically controlled by varying the diameter of the Ag2S nanocrystals. We also observe an ionic conductivity enhancement relative to pure Ag2S and (GeS2)0.5(Ag2S)0.5 glass. Additionally, the thermal phase transition of Ag2S into its superionic phase exhibits differences in thermal hysteresis and transition temperature when comparing the composites to pure Ag2S. The combined structural control and chemical composition flexibility of our nanocomposite formation technique will allow the careful study of structure-property relationships in many important nanocomposite systems. The flexibility of the nanocomposite fabrication is enabled by a modular formation process in which nanocrystals and chalcogenidometallate (ChaM) clusters are independently synthesized. After assembling the nanocrystals into a thin film, the ChaM clusters are intercalated into the interstitial space between nanocrystals. The ChaM clusters are then converted into an inorganic matrix