Blooming flexoelectricity in 2D materials revealed via second-harmonic generation

Abstract

Flexoelectricity—the coupling between strain gradients and polarization—offers a powerful route to engineer electromechanical responses, yet its straightforward detection at the nanoscale remains challenging. Here, we establish second-harmonic generation (SHG) as a noninvasive optical probe for the nanoscale strain gradient in two-dimensional (2D) materials. Using MoS2 as a model D3h system, we show that the bending-induced strain gradient εxx,z breaks mirror symmetry and transforms the canonical six-lobed SHG polar pattern into a “blooming core.” SHG imaging resolves strain gradients on the order of 0.1 μm−1, with intensity scaling positively with the gradient—opposite to the monotonic decay under uniform strain. This signature is universal across MoSe2, WS2, WSe2, and hexagonal boron nitride (h-BN), and recurs in diverse geometries, including wrinkles, bubbles, and microholes. Quantitative analysis further reveals a linear scaling between the square root of SHG intensity and the strain gradient. Our results establish a versatile optical platform for nanoscale flexoelectric studies in 2D materials.