Evidence of the direct-to-indirect band gap transition in strained two-dimensional WS2 , MoS2 , and WSe2

report a strain-induced direct-to-indirect band gap transition in mechanically deformed WS2 monolayers(MLs). The necessary amount of strain is attained by proton irradiation of bulk WS2 and the ensuing formationof 1-ML-thick, H2-filled domes. The electronic properties of the curved MLs are mapped by spatially andtime-resolved microphotoluminescence, revealing the mechanical stress conditions that trigger the variation ofthe band gap character. This general phenomenon, also observed in MoS2 and WSe2, further increases ourunderstanding of the electronic structure of transition metal dichalcogenide MLs and holds a great relevance fortheir optoelectronic applications.