Piezoelectricity of Atomically Thin WSe2 via Laterally Excited Scanning Probe Microscopy

TL;DR

This paper introduces a lateral excitation scanning probe microscopy technique to directly measure the piezoelectric response of atomically thin WSe2, revealing layer-dependent electromechanical properties with high spatial resolution.

Contribution

The study develops a novel lateral excitation SPM method enabling direct, high-resolution piezoelectric measurements of 2D WSe2 on a substrate, overcoming limitations of conventional techniques.

Findings

Monolayer and trilayer WSe2 exhibit strong piezoelectric response.

Bilayer WSe2 shows negligible piezoelectric response.

Estimated piezoelectric coefficient is 5.2 pm/V.

Abstract

Lattices of odd-layered two-dimensional (2D) transition metal dichalcogenides (TMDs) such as WSe2 are non-centrosymmetric, and thus could possess linear piezoelectricity that is attractive for applications such as nanoelectromechanical systems (NEMS) and nanogenerators. Measuring the electromechanical coupling of 2D TMDs, however, is rather challenging, since its D3h point group symmetry makes conventional piezoresponse force microscopy (PFM) inapplicable. Here we develop a lateral excitation scanning probe microscopy (SPM) technique that enables mapping of the piezoelectric response of atomically thin WSe2 directly on a substrate with high spatial resolution. Planar electrodes are used to excite piezoelectric vibrations while imposing anisotropic in-plane mechanical constraint to the WSe2, resulting in an out-of-plane deformation due to Poisson's effect that can be measured by an SPM probe. Using this technique, we show that WSe2 monolayer and trilayers exhibit strong electromechanical response linear to the applied excitation biases that is distinct from the substrate, while WSe2 bilayer show negligible electromechanical response as expected. The effective piezoelectric coefficient is estimated to be 5.2 pm/V from the measurement, consistent with theoretical predictions. This method can be conveniently applied to a wide range of 2D materials with similar symmetry.