Reversible Thermally Driven Phase Transformation in Ultrathin Ferroelectric In₂Se₃ Layers

Phase transformation in emerging two dimensional (2D) ferroelectric materials is crucial for their applications, such as in nonvolatile memory devices. We combined STM, STEM, Raman spectroscopy and first-principles calculation to investigate ultrathin layered In₂Se₃ synthesized by chemical vapor deposition (CVD). At room temperature, we observed that ultrathin In₂Se₃, ranging from ~20 layers to monolayer, stabilized at the β phase with a superlattice. Strikingly, at around 210 K the β phase converted to a novel and more stable β^′ phase, which has never been revealed in 2D In₂Se₃ with atomic resolution. The thermally driven kinetics of the reversible β-to-β^′ phase transformation was studied with temperature dependent STEM and Raman spectroscopy, which corroborated with the expected minimum-energy pathways for the transformation obtained from our DFT calculation. The calculated energy difference between the β phase and the β^′ phase is 0.033 eV/In₂Se₃, with an energy barrier of 0.006 eV/In₂Se₃. The DFT calculation further suggested in-plane ferroelectricity in the β^′ phase, and in contrast, both in-plane and out-of-plane ferroelectricity in the β phase.