Strain-Induced Indirect to Direct Bandgap Transition in Multilayer WSe₂

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.

Single photon emission from deep-level defects in monolayerWSe2

“…The PC of X 0 peak reflects a blue-shift rate of band edge of the direct K-K interband transition. [18][19][20][21] Here, the PC of X 0 at low temperature is nearly a half of the value for 2D-X 0 at room temperature reported elsewhere. 22 The discrepancy in the PC values obtained at different temperatures is at moment not understood.…”

Strain engineering in semiconducting two-dimensional crystals

“…In Refs. [88,98] electrode to apply a uniform bias voltage to the piezoelectric material. The changes in the electronic band structure, induced by the straining with the piezoelectric substrate, were also monitored by the shift of the A excitonic feature in the photoluminescence spectra.…”

Influence of the substrate material on the optical properties of tungsten diselenide monolayers

“…Reflection contrast [34][35][36], transient absorption [35], time-integrated photoluminescence (PL) [37,38] and time-resolved photoluminescence (TRPL) [39,40] experiments, have been performed to study the emission properties of WSe 2 . In prior studInfluence of the substrate material on the optical properties of tungsten diselenide monolayers 2 S Lippert et al ies on WSe 2 , layers were deposited on SiO 2 /Si substrates [41,42], sapphire [43,44], graphene [45], and fused silica (quartz) [34,46] or sandwiched between layers of hBN [47].…”