A Van der Waals Material Exhibiting Room Temperature Broken Inversion Symmetry with Ferroelectricity.

Since the initial synthesis of van der Waals 2D indium selenide is first documented in 1957, five distinct polymorphs and their corresponding polytypes are identified. In this study, a unique phase of indium selenide is reported via Scanning Transmission Electron Microscopy (STEM) analysis in the synthesized large-area films - which is named the β p $\ubeta^{\text{p}}$ phase. The quintuple layers of the β p $\ubeta^{\text{p}}$ phase, characterized by a unique zigzag atomic configuration with unequal indium-selenium bond lengths from the middle selenium atom, are distinct from any other previously reported phase of indium selenide. Cross-sectional STEM analysis has revealed that the β p $\ubeta^{\text{p}}$ layers exhibit intralayer shifting. It is found that indium selenide films with β p $\ubeta^{\text{p}}$ layers display electric-field-induced switchable polarization characteristic of ferroelectric materials, suggesting the breaking of the inversion symmetry. Experimental observations of nonlinear optical phenomena - Second Harmonic Generation (SHG) responses further support this conclusion. This study reports a β p $\ubeta^{\text{p}}$ phase of indium selenide showing ferroelectricity over large areas at room temperature in a low-dimensional limit.