Temperature-driven reversible structural transformation and conductivity switching in ultrathin Cu₉S₅ crystals

Two-dimensional (2D) materials with reversible phase transformation are appealing for their rich physics and potential applications in information storage. However, up to now, reversible phase transitions in 2D materials that can be driven by facile nondestructive methods, such as temperature, are still rare. Here, we introduce ultrathin Cu₉S₅ crystals grown by chemical vapor deposition (CVD) as an exemplary case. For the first time, their basic electrical properties were investigated based on Hall measurements, showing a record high hole carrier density of ~ 10²² cm⁻³ among 2D semiconductors. Besides, an unusual and repeatable conductivity switching behavior at ~ 250 K were readily observed in a wide thickness range of CVD-grown Cu₉S₅ (down to 2 unit-cells). Confirmed by in-situ selected area electron diffraction, this unusual behavior can be ascribed to the reversible structural phase transition between the room-temperature hexagonal β phase and low-temperature β’ phase with a superstructure. Our work provides new insights to understand the physical properties of ultrathin Cu₉S₅ crystals, and brings new blood to the 2D materials family with reversible phase transitions.