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.

Elastic strain has been an important method to regulate the electronic structures and physical properties of nanoscale semiconductors due to the promising potentials in improving the performance of their optoelectronic devices. Here, we report the investigation of bending strain effects on the optical and optoelectric properties of individual gallium nitride (GaN) nanowires (NWs). By charactering the near-band emission spectrum of individual GaN NWs at different bending strains with low temperature cathodoluminescence (CL), we reveal that the near-band emission splits into two peaks, where the low energy peak displays a linear redshift with increasing the bending strain while the high energy one shows a slight blueshift. Further localized ultraviolet (UV) photoresponse measurements illustrate that the photoresponse of the GaN NWs shows a linear increase with the bending train, and the maximum enhancement is more than two orders of magnitude. The experimental observations are well interpreted by theoretical calculations on the strain modulation on the electronic band structure of GaN combined with analysis of carrier dynamics and optical waveguide effect in the bending strain field. Our results not only shed light on the bending strain effects on the optical and optoelectric properties of semiconductors, but also hold potential to help the future design of high performance nano-optoelectric devices.