Air‐Stable Monolayer Cu₂Se Exhibits a Purely Thermal Structural Phase Transition

Abstract

Materials possessing structural phase transformations exhibit a rich set of physical and chemical properties that can be used for a variety of applications. In 2D materials, structural transformations have so far been induced by strain, lasers, electron injection, electron/ion beams, thermal loss of stoichiometry, and chemical treatments or by a combination of such approaches and annealing. However, stoichiometry‐preserving, purely thermal, reversible phase transitions, which are fundamental in physics and can be easily induced, have not been observed. Here, the fabrication of monolayer Cu2Se, a new 2D material is reported, demonstrating the existence of a purely thermal structural phase transition. Scanning tunneling microscopy, scanning transmission electron microscopy, and density functional theory (DFT) identify two structural phases at 78 and 300 K. DFT calculations trace the phase‐transition mechanism via the existence/absence of imaginary (unstable) phonon modes at low and high temperatures. In situ, variable‐temperature low‐energy electron diffraction patterns demonstrate that the phase transition occurs across the whole sample at ≈147 K. Angle‐resolved photoemission spectra and 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. This work opens up possibilities for studying such phase transitions in 2D materials.

Novel two-dimensional transition metal chalcogenides created by epitaxial growth

“…This structure is called λ phase and the atomic model is shown in Figure 7(h). The cross-sectional STEM experiments confirmed the proposed atomic model [92]. Further variabletemperature LEED patterns demonstrate that the phase transition occurs at about 147 K [92].…”

“…The cross-sectional STEM experiments confirmed the proposed atomic model [92]. Further variabletemperature LEED patterns demonstrate that the phase transition occurs at about 147 K [92]. DFT calculations reveal that the λ phase exhibits imaginary-frequency phonons at 0 K, indicating it is unstable at low temperature.…”

“…DFT calculations also show an indirect bandgap of 1.23 eV in ζ-Cu 2 Se and a direct bandgap of 1.30 eV in λ-Cu 2 Se. Furthermore, this 2D material is stable in air [92]. These properties indicate its potential applications in electronics.…”

“…In 2D materials, structural transitions can be induced by atom substitution or chemical doping [87,88], electrostatic doping [89], electron donation from intercalation [90], or thermal loss of stoichiometry [91]. Recently, Qian et al [92] fabricated a new 2D material monolayer Cu 2 Se and discovered stoichiometry-preserving, purely thermal, reversible phase transitions in this material. They fabricated the monolayer Cu 2 Se on bilayer graphene on 6H-SiC(0001) by co-deposition of Se and Cu in an MBE chamber.…”

Two-dimensional semiconductor materials with high stability and electron mobility in group-11 chalcogenide compounds: MNX (M = Cu, Ag, Au; N = Cu, Ag, Au; X = S, Se, Te; M ≠ N)

“…Rauf et al successfully synthesized 2D β-Cu 2 S flakes with a thickness of 1.8 nm, and proved the existence of solid-liquid phase hybridization. 17 Qian et al prepared six-atom-thick, hexagonal-symmetric, air-stabilized Cu 2 Se 18 and Cu 2 Te 19 monomolecular films on double-layer graphene. It was theoretically predicted that Cu 2 S 20 and Au 2 S 21 monolayers with a thickness of three atoms had high carrier mobility and high oxidation resistance.…”