Scanning Tunneling Microscopy Study of Vibronic Coupling due to Charged Impurity in Monolayer WS2 on Graphite

Electron-phonon coupling of localized atomic systems in 2D crystals act as a decoherence conduit which substantially affect transport properties of the crystal. Here, we report a novel impurity on chemical vapor deposition (CVD)-grown monolayer tungsten disulfide (WS2) on graphite. From scanning tunneling microscopy (STM) images, we have found that these impurities are negatively charged, exhibiting upward band bending with respect to the neighboring WS2 band. The defect exhibits a two-fold lobal structure observed at small positive sample bias, indicating the existence of unoccupied mid-gap states. Utilizing scanning tunneling spectroscopy (STS) to probe the impurity electronic structure, we discover that there are a series of nearly equidistant oscillatory resonant peaks in the band gap at both positive and negative bias. We observe as many as 8-10 of these peaks in the dI/dV spectrum suggesting that they do not originate from a single particle electron state. We therefore attribute them to coupling of the impurity to vibronic modes with the surrounding lattice. We hypothesize that the chemical species of this impurity to be a hydrogen atom attached at a vacant sulfur lattice site originating from the CVD growth process. By introducing excess hydrogen to deterministically dope the sulfur vacancies, we intend to uncover the origin of these impurities.