Bandgap Engineering of Strained Monolayer and Bilayer MoS₂

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 <i>E</i>′ 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