Synthesis of Ultra-Thin Superionic Cu2Se and New Aspects of the Low-Temperature Crystal Configurations

Superionic conductors offer unique advantages for novel technological devices in various fields, such as energy storage and neuromorphic computing. Above 414 K, Cu2Se turns into a well-known superionic conductor via a phase transition, and it is demonstrated to exhibit peculiar electrical and thermoelectric properties in bulk. Here, we report a large-area synthesis of ultra-thin single crystalline Cu2Se using the chemical vapor deposition method. We demonstrate that Cu2Se crystals exhibit optically and electrically controllable robust phase reconfiguration below 414 K. Moreover, our results show that the mobility of the liquid-like Cu ion vacancies in Cu2Se causes macroscopic fluctuations in the Cu ordering. Consequently, phase variations are not dictated by the diffusive motion of the ions but by the local energy minima formed due to the interplay between the extrinsic and the intrinsic material parameters. As a result, long-range ordering of the crystal below 414 K is optically observable at a micrometer scale. Our results show that Cu2Se could find applications beyond thermoelectric such as smart optical coatings, optoelectronic switching, and ionic transistors.