Electric-field-induced optical hysteresis in single-layer WSe₂

We demonstrate that the exciton energy of a monolayer of tungsten diselenide on an SiO₂/Si substrate can be tuned by an applied in-plane electric field for two samples with different dielectric capping materials. The exciton energy can be either red- or blue-shifted by up to 20 meV based on the polarity of the applied electric field. We argue that a piezoelectric effect creates a large internal electric field, which is either partially aligned or partially antialigned with the external electric field. Additionally, optical hysteresis is observed on cycling of the external electric field due to trapped charges.