Direct imaging of band profile in single layer MoS₂ on graphite: Quasiparticle energy gap, metallic edge states, and edge band bending

Using scanning tunneling microscopy and spectroscopy, we probe the electronic structures of single layer MoS₂ on graphite. The apparent quasiparticle energy gap of single layer MoS₂ is measured to be 2.15 ± 0.06 eV at 77 K, albeit a higher second conduction band threshold at 0.2 eV above the apparent conduction band minimum is also observed. Combining it with photoluminescence studies, we deduce an exciton binding energy of 0.22 ± 0.1 eV (or 0.42 eV if the second threshold is use), a value that is lower than current theoretical predictions. Consistent with theoretical predictions, we directly observe metallic edge states of single layer MoS ₂. In the bulk region of MoS₂, the Fermi level is located at 1.8 eV above the valence band maximum, possibly due to the formation of a graphite/MoS₂ heterojunction. At the edge, however, we observe an upward band bending of 0.6 eV within a short depletion length of about 5 nm, analogous to the phenomena of Fermi level pinning of a 3D semiconductor by metallic surface states.