Nanoscale Behavior and Manipulation of the Phase Transition in Single-Crystal Cu2Se

Phase transition is a fundamental physical phenomenon that has been widely studied both theoretically and experimentally. According to the Landau theory, the coexistence of high- and low-temperature phases is thermodynamically impossible during a second-order phase transition in a bulk single crystal. Here, the coexistence of two (alpha and beta) phases in wedge-shaped nanosized single-crystal Cu2Se over a large temperature range are demonstrated. By considering the surface free-energy difference between the two phases and the shape effect, a thermodynamic model is established, which explicitly explains their coexistence. Intriguingly, it is found that with a precise control of the heating temperature, the phase boundary can be manipulated at atomic level. These discoveries extend the understanding of phase transitions to the nanoscale and shed light on rational manipulation of phase transitions in nanomaterials.