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

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 InSe synthesized chemical vapor deposition, a recently discovered 2D ferroelectric material. We observed that InSe 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 phase that was distinct from previously reported phases in 2D InSe. The kinetics of the reversible thermally driven β-to- 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 phase. Scanning tunneling spectroscopy measurements show that the indirect bandgap of monolayer InSe 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 InSe with thickness down to the monolayer limit and the associated electronic properties will provide insights to tune the functionalities of 2D InSe and other emerging 2D ferroelectric materials and shed light on their numerous potential applications (, nonvolatile memory devices).