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

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.