A van der Waals material exhibiting room temperature broken inversion symmetry with ferroelectricity

Since the initial synthesis of van der Waals two-dimensional indium selenide was first documented in 1957, five distinct polymorphs and their corresponding polytypes have been identified. In this study, we report a unique phase of indium selenide via Scanning Transmission Electron Microscopy (STEM) analysis in the synthesized large-area films -- which we have named the $\beta^\text{p}$ phase. The quintuple layers of the $\beta^\text{p}$ 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 $\beta^\text{p}$ layers exhibit intralayer shifting. We found that indium selenide films with $\beta^\text{p}$ 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 $\beta^\text{p}$ phase of indium selenide showing ferroelectricity over large areas at room temperature in a low-dimensional limit.