Bandgap Engineering of Strained Monolayer and Bilayer MoS₂

Abstract

We report the influence of uniaxial tensile mechanical strain in the range 0-2.2% on the phonon spectra and bandstructures of monolayer and bilayer molybdenum disulfide (MoS2) two-dimensional crystals. First, we employ Raman spectroscopy to observe phonon softening with increased strain, breaking the degeneracy in the E' Raman mode of MoS2, and extract a Grüneisen parameter of ~1.06. Second, using photoluminescence spectroscopy we measure a decrease in the optical band gap of MoS2 that is approximately linear with strain, ~45 meV/% strain for monolayer MoS2 and ~120 meV/% strain for bilayer MoS2. Third, we observe a pronounced strain-induced decrease in the photoluminescence intensity of monolayer MoS2 that is indicative of the direct-to-indirect transition of the character of the optical band gap of this material at applied strain of ~1%. These observations constitute a demonstration of strain engineering the band structure in the emergent class of two-dimensional crystals, transition-metal dichalcogenides.

Strain and structure heterogeneity in MoS2 atomic layers grown by chemical vapour deposition

“…The corresponding difference in strain is estimated to be B0.2%. Although the DFT simulations do not account for the excitonic effects, the relative change of the calculated bandgap with respect to uniaxial strain ( À 44 meV per % strain) matches quite well with previous experiments on the PL peak position shift with strain ( À 45 meV per % strain 26 and À 36 meV per % strain 27 . Furthermore, as we mentioned, due to the fast cooling, a global tensile strain is found in MoS 2 .…”

“…Substrate-induced straining of 2D materials for band structure engineering has been effectively explored in previous studies 26,28 . However, these experiments have only been performed on exfoliated samples and a comprehensive study of the role of substrate and homogeneity on the transferred strain has not been explored.…”

“…However, these experiments have only been performed on exfoliated samples and a comprehensive study of the role of substrate and homogeneity on the transferred strain has not been explored. Most recently, the strain-induced bandgap changes in monolayer MoS 2 , using polymer substrates, has been demonstrated 26,[28][29][30] . However, because of the weak adhesion between MoS 2 and substrates, strain cannot be completely transferred to MoS 2 from the substrate, as exemplified in graphene strain-engineering studies 31,32 .…”

Pressure-induced semiconducting to metallic transition in multilayered molybdenum disulphide

“…In light of these observations, the out-of-plane vibrational mode (A 1g ) can be considered the more prominent Raman mode that is correlated with the semiconducting to metallic transition in MoS 2 , in contrast to the in-plane mode (E 2g mode) that shows a less distinct character across the transition. The IS can be understood within the context of phonon softening (PS), which describes the electron and A 1g phonon interaction where inter-valence charge transfer between the MoS 2 layers plays an important role in the structural distortion of the crystal symmetry 33 . Several materials have been previously reported to exhibit an intermediate state with soften phonon behaviour while undergoing a transition from a semiconductor to a metallic state 34,35 or when stress or strain is applied 36,37 .…”

Spin-resolved photoemission study of epitaxially grown MoSe2and WSe2thin films

“…They often exhibit versatile electronic structure controllable by thickness, surface chemical adsorption, and strain [5,6,7]. They can also be stacked into heterostructures to develop a new state in the interfaces [8] as well as through proximity effect [9].…”