Indirect Band Gap in Scrolled MoS₂ Monolayers

MoS₂ nanoscrolls that have inner core radii of ∼250 nm are generated from MoS₂ monolayers, and the optical and transport band gaps of the nanoscrolls are investigated. Photoluminescence spectroscopy reveals that a MoS₂ monolayer, originally a direct gap semiconductor (∼1.85 eV (optical)), changes into an indirect gap semiconductor (∼1.6 eV) upon scrolling. The size of the indirect gap for the MoS₂ nanoscroll is larger than that of a MoS₂ bilayer (∼1.54 eV), implying a weaker interlayer interaction between concentric layers of the MoS₂ nanoscroll compared to Bernal-stacked MoS₂ few-layers. Transport measurements on MoS₂ nanoscrolls incorporated into ambipolar ionic-liquid-gated transistors yielded a band gap of ∼1.9 eV. The difference between the transport and optical gaps indicates an exciton binding energy of 0.3 eV for the MoS₂ nanoscrolls. The rolling up of 2D atomic layers into nanoscrolls introduces a new type of quasi-1D nanostructure and provides another way to modify the band gap of 2D materials.