Abstract
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 βp phase. The quintuple layers of the βp 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 βp layers exhibit intralayer shifting. It is found that indium selenide films with βp 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 βp phase of indium selenide showing ferroelectricity over large areas at room temperature in a low-dimensional limit.