Atomic-Scale Spectroscopy of Gated Monolayer MoS₂

The electronic properties of semiconducting monolayer transition-metal dichalcogenides can be tuned by electrostatic gate potentials. Here we report gate-tunable imaging and spectroscopy of monolayer MoS₂ by atomic-resolution scanning tunneling microscopy/spectroscopy (STM/STS). Our measurements are performed on large-area samples grown by metal-organic chemical vapor deposition (MOCVD) techniques on a silicon oxide substrate. Topographic measurements of defect density indicate a sample quality comparable to single-crystal MoS₂. From gate voltage dependent spectroscopic measurements, we determine that in-gap states exist in or near the MoS₂ film at a density of 1.3 × 10¹² eV^-1 cm^-2. By combining the single-particle band gap measured by STS with optical measurements, we estimate an exciton binding energy of 230 meV on this substrate, in qualitative agreement with numerical simulation. Grain boundaries are observed in these polycrystalline samples, which are seen to not have strong electronic signatures in STM imaging.