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Interests in two-dimensional transition-metal dichalcogenides have promptedsome recent efforts to grow ultrathin layers of these materials epitaxiallyusing molecular-beam epitaxy. However, growths of monolayer and bilayer WSe2,an important member of the transition-metal dichalcogenides family, by themolecular-beam epitaxy method remain uncharted probably because of thedifficulty in generating tungsten fluxes from the elemental source. In thiswork, we present a scanning tunneling microscopy and spectroscopy study ofmolecular-beam epitaxy-grown WSe2 monolayer and bilayer, showing atomicallyflat epifilm with no domain boundary defect. This contrasts epitaxial MoSe2films grown by the same method, where a dense network of the domain boudariesdefects is present. The scanning tunneling spectroscopy measurements ofmonolayer and bilayer WSe2 domains of the same sample reveal not only thebandgap narrowing upon increasing the film thickness from monolayer to bilayer,but also a band-bending effect across the boundary between monolayer andbilayer domains. This band-bending appears to be dictated by the edge states atsteps of the bilayer islands. Finally, comparison is made between the scanningtunneling spectroscopy-measured electronic bandgaps with the exciton emissionenergies measured by photoluminescence, and the exciton binding energies inmonolayer and bilayer WSe2/MoSe2 are thus estimated.

Molecular-beam epitaxy of monolayer and bilayer WSe2: A scanning tunneling microscopy/spectroscopy study and deduction of exciton binding energy