Visualization of anomalous electromechanical coupling at atomic scale

Piezoelectric materials enable the direct interconversion of mechanical and electrical energy and are therefore widely used in stress sensors, wearable devices, and information-storage technologies. Controllably modulating piezoelectric coefficients could significantly enhance piezoelectric responses and thereby improve device performance. Sliding ferroelectrics in two-dimensional van der Waals materials offer opportunities to regulate piezoelectricity through stacking manipulation. Here, using atomic-scale in situ biasing high-angle annular dark-field scanning transmission electron microscopy, we directly visualized electric-field-induced interlayer sliding and intralayer atomic displacements in γ-InSe under different bias voltages and correlated the structural evolution with a field-dependent, nonmonotonic polarity reversal in the transverse electromechanical response, revealing anomalous electromechanical coupling in γ-InSe. Together with theoretical calculations, these observations attribute the anomalous piezoelectric behavior to competition between clamped-ion and internal-strain contributions induced by concurrent changes in interlayer stacking configuration and intralayer atomic displacement. These findings provide insights into the ferroelectric mechanism of γ-InSe and suggest routes for tuning piezoelectric coefficients in sliding ferroelectrics, thereby informing the design of next-generation piezoelectric and electromechanical devices.