# Negative Out-of-Plane Electromechanical Response in Nonpiezoelectric van der Waals Layered Materials Encapsulated by Monolayer Boron Nitride

## TL;DR

Researchers discovered a way to change the electrical response of a material by stacking it with another layer, which could lead to new electronic devices.

## Contribution

A new method to manipulate the electromechanical response of nonpiezoelectric materials using heterostructures.

## Key findings

- WSe2 nanoflakes show a negative piezoresponse when capped with h-BN.
- Charge transfer between WSe2 and h-BN creates a built-in electric field.
- The negative piezoresponse is due to the electric field and weak interlayer forces.

## Abstract

The negative out-of-plane piezoelectric response is a
rare phenomenon
in nonpiezoelectric van der Waals (vdW) materials. In this study,
we present a straightforward method to manipulate the sign of the
out-of-plane piezoelectric responses of vdW centrosymmetric materials
by creating vdW heterostructures. Piezoresponse force microscopy (PFM)
results indicate that the out-of-plane piezoresponse of WSe2 nanoflakes changes from positive to negative when capped with a
monolayer of hexagonal boron nitride (h-BN). Based on the Kelvin probe
force microscopy (KPFM) results and first-principles calculations,
the charge transfer is examined to occur between WSe2 and
h-BN, resulting in a built-in electric field (EF). The observed negative
piezoresponse in the WSe2/BN heterostructures can be attributed
to the built-in EF and weak interlayer vdW forces. Our work enhances
the understanding of the electromechanical performances of vdW layered
heterostructures and may provide a new method for designing novel
vdW material-based devices utilizing electromechanical properties.