MoS_2/graphene (MoS_2/Gr) vertical heterostructures have exhibited great application potentials in high speed electronic and optoelectronic devices, as well as efficient electrocatalysts in hydrogen evolution reaction (HER). The electronic property at the edge of monolayer MoS_2 is an essential parameter for addressing such applications. Herein, it is reported that, for monolayer MoS_2 synthesized on Gr/Au foils by chemical vapor deposition, a dramatic decrease of the bandgap from ≈2.20 to ≈0.30 eV occurs at the domain edge within a lateral distance of ≈6 nm, as evidenced by scanning tunneling microscopy/spectroscopy observations. The edges of monolayer MoS_2 on Gr/Au foils can thus be regarded as narrow-gap quantum wires considering of their reduced bandgaps. More intriguingly, it is found that this bandgap decrease at the domain edge is closely related to the rather high HER performance for MoS_2/Gr/Au foils comparing with that of MoS_2/Au foils. Briefly, this work should propel the band structure investigations for MoS_2 /Gr stacks and their applications in energy related fields.
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