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.

Identifying air-stable two-dimensional (2D) ferromagnetism with high Curie temperature (T_c) is highly desirable for its potential applications in next-generation spintronics. However, most of the work reported so far mainly focuses on promoting one specific key factor of 2D ferromagnetism (T_c or air stability), rather than comprehensive promotion of both of them. Herein, ultrathin Cr_1–xTe crystals grown by chemical vapor deposition (CVD) show thickness-dependent T_c up to 285 K. The out-of-plane ferromagnetic order is well preserved down to atomically thin limit (2.0 nm), as evidenced by anomalous Hall effect observed in non-encapsulated samples. Besides, the CVD-grown Cr_1−xTe nanosheets present excellent ambient stability, with no apparent change in surface roughness or electrical transport properties after exposure to air for months. Our work provides an alternative platform for investigation of intrinsic 2D ferromagnetism and development of innovative spintronic devices.