Effect of Strain and hBN Encapsulation on the Optical Transitions of Ultrathin Transition Metal Dichalcogenides

The optical spectra of transition metal dichalcogenides (TMDCs) are dominated by excitons and they experience an indirect-gap to direct-gap transition at the monolayer limit. For monolayer TMDCs, encapsulation by hexagon BN (hBN) has been shown to reduce the linewidth of direct exciton at both low and room temperatures. However, the effect of hBN encapsulation on the optical spectra of bilayer TMDCs with their indirect gap is not clear. We have therefore prepared hBN encapsulated bilayers and studied how indirect excitons are affected. Strain engineering on TMDCs has also been demonstrated to modify their optical fingerprints, including excitonic linewidths by changing the band structure and the excitonic states. We apply uniaxial tensile strain to both bare monolayer and bilayer TMDCs, as well as hBN encapsulated ones. By comparing the strain dependent modifications of the linewidth and other optical fingerprints of hBN encapsulated samples and bare samples, we are able to distinguish different contributions to the exciton linewidth and the role of hBN encapsulation.