Se Exhibits a Purely Thermal Structural Phase Transition*

Materials possessing structural phase transformations exhibit a rich set of physical and chemical properties that can be used for various applications. However, stoichiometry-preserving, purely thermal, reversible phase transitions have not been observed. Here, we report a purely thermal structural phase transition in a new 2D material, monolayer Cu₂Se by using scanning tunneling microscopy, scanning transmission electron microscopy, and density functional theory (DFT) calculations. DFT calculations trace the phase-transition mechanism via the existence/absence of imaginary (unstable) phonon modes at low and high temperatures. In addition, DFT calculations show that a degeneracy at the Γ point of the energy bands of the high-temperature phase is lifted in the low-temperature phase, confirmed by the angle resolved photoemission spectra. The variable-temperature low-energy electron diffraction patterns indicate that the phase transition occurs across the whole sample at ≈147 K. This work provides a new platform for future investigations of such phase transitions in 2D materials. [Adv. Mater. 32, 1908314, (2020)]