Direct imaging of the band profile in single layer $Mo{S}_2$ on graphite: quasiparticle energy gap, metallic edge states and edge band bending

Abstract

Using Scanning Tunneling Microscopy and Spectroscopy, we probe the electronic structures of single layer $Mo{S}_2$ on graphite. We show that the quasiparticle energy gap of single layer $Mo{S}_2$ is 2.15 $\pm$ 0.07 eV at 77 K. Combining with temperature dependent photoluminescence studies, we deduce an exciton binding energy of 0.22 $\pm$ 0.1 eV, a value that is much lower than current theoretical predictions. Consistent with theoretical predictions we directly observed metallic edge states of single layer $Mo{S}_2$. In the bulk region of $Mo{S}_2$, the Fermi level is located at 1.8 eV above the valence band maximum, possibly due to the formation of a graphite/$Mo{S}_2$ 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.

17 pages, 4 figures