Thinner Is Not Always Better: Nonmonotonic Flexoelectric Scaling in Suspended MoS2 Semiconductor Membranes.

Abstract

Flexoelectricity is generally expected to strengthen as material dimensions shrink, because strain gradients are amplified at small length scales. Here, we reveal a nonmonotonic thickness dependence of the flexoelectric response in suspended semiconducting MoS2 membranes. Combining atomic force microscopy, piezoelectric force microscopy, scanning microwave impedance microscopy, and nonlinear finite element modeling, we identify a maximum flexoelectric response at ∼42 nm. Carrier redistribution increasingly screens the flexoelectric field with increasing thickness, reaching ∼16% at 80 nm, but it does not determine the optimal thickness. Instead, the optimum thickness is governed primarily by the coupled balance among bending deformation, strain-gradient elasticity, dielectric response, and flexoelectric polarization. Its independence from loading force and suspended radius further indicates a material-parameter-governed characteristic length. These findings uncover an anomalous size effect in layered semiconductors and provide design insight for high-performance 2D material-based electromechanical devices.