Evidence of the direct-to-indirect band gap transition in strained two-dimensional WS_{2}, MoS_{2}, and WSe_{2}

Abstract

We report a strain-induced direct-to-indirect band gap transition in mechanically deformed WS_{2} monolayers (MLs). The necessary amount of strain is attained by proton irradiation of bulk WS_{2} and the ensuing formation of 1-ML-thick, H_{2}-filled domes. The electronic properties of the curved MLs are mapped by spatially and time-resolved microphotoluminescence, revealing the mechanical stress conditions that trigger the variation of the band gap character. This general phenomenon, also observed in MoS_{2} and WSe_{2}, further increases our understanding of the electronic structure of transition metal dichalcogenide MLs and holds a great relevance for their optoelectronic applications.