Strain-Induced Indirect to Direct Bandgap Transition in Multilayer WSe₂

TLDR

Transition metal dichalcogenides such as MoS₂ and WSe₂ are direct bandgap in monolayer form but become indirect in multilayers, limiting light‑harvesting devices, though WSe₂’s small direct‑indirect gap difference makes strain‑induced bandgap crossover feasible. We show that applying up to 2 % uniaxial tensile strain to 2–4‑layer WSe₂ boosts its photoluminescence by up to 35×, evidencing an indirect‑to‑direct bandgap transition confirmed by DFT and highlighting strain engineering as a route to enhance few‑layer WSe₂ optoelectronic performance.

Abstract

Transition metal dichalcogenides, such as MoS2 and WSe2, have recently gained tremendous interest for electronic and optoelectronic applications. MoS2 and WSe2 monolayers are direct bandgap and show bright photoluminescence (PL), whereas multilayers exhibit much weaker PL due to their indirect optical bandgap. This presents an obstacle for a number of device applications involving light harvesting or detection where thicker films with direct optical bandgap are desired. Here, we experimentally demonstrate a drastic enhancement in PL intensity for multilayer WSe2 (2–4 layers) under uniaxial tensile strain of up to 2%. Specifically, the PL intensity of bilayer WSe2 is amplified by ∼35× , making it comparable to that of an unstrained WSe2 monolayer. This drastic PL enhancement is attributed to an indirect to direct bandgap transition for strained bilayer WSe2, as confirmed by density functional theory (DFT) calculations. Notably, in contrast to MoS2 multilayers, the energy difference between the direct and indirect bandgaps of WSe2 multilayers is small, thus allowing for bandgap crossover at experimentally feasible strain values. Our results present an important advance toward controlling the band structure and optoelectronic properties of few-layer WSe2 via strain engineering, with important implications for practical device applications.