  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.
