Using Scanning Tunneling Microscopy and Spectroscopy, we probe the electronic structures of single layer ${small MoS_2}$ on graphite. We show that the quasiparticle energy gap of single layer ${small MoS_2}$ is 2.15 $pm$ 0.07 eV at 77 K. Combining wi
th 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 ${small MoS_2}$. In the bulk region of ${small MoS_2}$, the Fermi level is located at 1.8 eV above the valence band maximum, possibly due to the formation of a graphite/${small MoS_2}$ 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.
