Atomic-Scale Spectroscopy of Gated Monolayer MoS₂

Abstract

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