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 Cu2 Se, 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.

中文翻译：

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空气稳定的单层Cu2 Se具有纯的热结构相变。

具有结构相变的材料具有丰富的物理和化学特性，可用于多种应用。迄今为止，在2D材料中，结构变形是由应变，激光，电子注入，电子/离子束，化学计量热损失和化学处理引起的，或者由此类方法和退火的组合引起的。然而，尚未观察到化学计量保持的，纯热的，可逆的相变，其在物理学中是基本的并且易于诱导。在这里，报道了一种新型2D材料单层Cu2 Se的制造，这表明存在纯热结构相变。扫描隧道显微镜，扫描透射电子显微镜，密度泛函理论（DFT）确定了在78 K和300 K时的两个结构相。DFT计算通过在低温和高温下是否存在虚（不稳定）声子模式来追踪相变机理。原位，可变温度低能电子衍射图谱表明，整个样品在约147 K处发生了相变。角度分辨光发射光谱和DFT计算表明，高能带的Γ点处发生简并温度阶段在低温阶段被提升。这项工作为研究2D材料中的这种相变打开了可能性。可变温度低能电子衍射图谱表明，整个样品在约147 K处发生了相变。角度分辨光发射光谱和DFT计算表明，高温相能带的Γ点处存在简并性在低温阶段被提升。这项工作为研究2D材料中的这种相变打开了可能性。可变温度低能电子衍射图谱表明，整个样品在约147 K处发生了相变。角度分辨光发射光谱和DFT计算表明，高温相能带的Γ点处存在简并性在低温阶段被提升。这项工作为研究2D材料中的这种相变打开了可能性。