Distinct absolute bandgap deformation potentials of direct and indirect gaps in bilayer WSe2

Abstract

Bilayer (BL) transition metal dichalcogenides exhibit rich optoelectronic phenomena, including strong strain-induced bandgap tunability and creation of strain-induced spectrally isolated quantum-light emitters. However, two important governing parameters of strain tunability—absolute bandgap deformation potentials (DPs) and mode Grüneisen parameters—remain largely unexplored. Here, we focus on the Kc and Qc conduction-band valleys and the Kv valence-band valley of BL WSe2, and create localized biaxial strain using dielectric nanoparticle stressors. By combining spatially resolved Raman spectroscopy at room temperature and photoluminescence spectroscopy at low (4–130 K) and room temperatures with a theoretical model, we extract DPs of −6.04 eV for the Qc–Kv indirect bandgap and −8.45 eV for the Kc–Kv direct bandgap. These results indicate that approximately 0.9% biaxial tensile strain is sufficient to drive an indirect-to-direct bandgap transition in BL WSe2, in agreement with previous reports. We also measure a Grüneisen parameter of 1.55 for the E2g mode. Notably, a localized strain of only ∼0.4% renders BL WSe2 optically as bright as an unstrained monolayer. This work paves the way for advances in flexible electronics, sensors, optoelectronics, and quantum-photonic devices through precise strain engineering of BL WSe2.