  We 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 formation of
one-ML-thick, H2-filled domes. The electronic properties of the curved MLs are
mapped by spatially- and time-resolved micro-photoluminescence revealing the
mechanical stress conditions that trigger the variation of the band gap
character. This general phenomenon, also observed in MoS2 and WSe2, further
increases our understanding of the electronic structure of transition metal
dichalcogenide MLs and holds a great relevance for their optoelectronic
applications.
