Local Strain Induced Band Gap Modulation and Photoluminescence Enhancement of Multilayer Transition Metal Dichalcogenides

The photocarrier relaxation between direct and indirect band gaps along the high symmetry K-γ line in the Brillion zone reveals interesting electronic properties of the transition metal dichalcogenides (TMDs) multilayer films. In this study, we reported on the local strain engineering and tuning of an electronic band structure of TMDs multilayer films along the K-γ line by artificially creating one-dimensional wrinkle structures. Significant photoluminescence (PL) intensity enhancement in conjunction with continuously tuned optical energy gaps was recorded at the high strain regions. A direct optical band gap along K-K points and an indirect optical gap along γ-K points measured from the PL spectra of multilayer samples monotonically decreased as the strain increased, while the indirect band gap along -γ was unaffected owing to the same level of local strain in the range of 0%-2%. The experimental results of band gap tuning were in agreement with the density functional theory calculation results. Local strain modified the band structure in which K-conduction band valley (CBV) was aligned below the -CBV, and this explained the observed local PL enhancement that made the material indirect via the K-γ transition. The study also reported experimental evidence for the funneling of photogenerated excitons toward regions of a higher strain at the top of the wrinkle geometry.