Atomic-Scale Observation of Reversible Thermally Driven Phase Transformation in 2D In₂Se₃

Abstract

Phase transformation in emerging two-dimensional (2D) materials is crucial for understanding and controlling the interplay between structure and electronic properties. In this work, we investigate 2D In₂Se₃ synthesized <i>via</i> chemical vapor deposition, a recently discovered 2D ferroelectric material. We observed that In₂Se₃ layers with thickness ranging from a single layer to ∼20 layers stabilized at the β phase with a superstructure at room temperature. At around 180 K, the β phase converted to a more stable <i>β'</i> phase that was distinct from previously reported phases in 2D In₂Se₃. The kinetics of the reversible thermally driven β-to-<i>β'</i> phase transformation was investigated by temperature-dependent transmission electron microscopy and Raman spectroscopy, corroborated with the expected minimum-energy pathways obtained from our first-principles calculations. Furthermore, density functional theory calculations reveal in-plane ferroelectricity in the <i>β'</i> phase. Scanning tunneling spectroscopy measurements show that the indirect bandgap of monolayer <i>β'</i> In₂Se₃ is 2.50 eV, which is larger than that of the multilayer form with a measured value of 2.05 eV. Our results on the reversible thermally driven phase transformation in 2D In₂Se₃ with thickness down to the monolayer limit and the associated electronic properties will provide insights to tune the functionalities of 2D In₂Se₃ and other emerging 2D ferroelectric materials and shed light on their numerous potential applications (<i>e.g.</i>, nonvolatile memory devices).