Controlling proton volatility in SiO₂-capped TiO₂thin films for neuromorphic functionality

Volatile memories are one of the essential analog devices used in recent electronics, such as neuromorphic circuits and reservoir computing. However, since the application of volatile memories is relatively recent, basic research on the volatility function in the materials is still lacking. In this study, we focused on electrochemically doped protons inside the TiO2 thin film as one of the simplest platforms for volatile functions and aimed to clarify the design principle for volatile memories. It was found that the proton volatility from the TiO2 surface can be controlled by the slow proton diffusion inside a few-nm SiO2 cap layer, where TiO2 and SiO2 can be viewed as a proton container and a sealing cap, respectively. It was further shown that both volatile and nonvolatile protons exist in TiO2 thin films, and hence, suppressing the influence of the latter would be essential for exploiting functions from volatile protons. These results indicate that simple material stacks with nanoscale thickness can control volatile protons in solid-state devices, providing a basic design principle for volatile memories.