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

## TL;DR

Researchers discovered a new form of indium selenide that shows ferroelectric properties at room temperature, which could be useful for electronic devices.

## Contribution

The discovery of a new βp phase of indium selenide with room-temperature ferroelectricity in large-area films.

## Key findings

- The βp phase of indium selenide exhibits electric-field-induced switchable polarization.
- The material shows broken inversion symmetry and ferroelectric behavior at room temperature.
- Second Harmonic Generation (SHG) confirms the nonlinear optical properties of the βp phase.

## 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.

This study reports a βp phase of indium selenide showing ferroelectricity over large areas at room temperature in a low‐dimensional limit. 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.