Reversible Switching of Non-Volatile Bistable Defect Charge States in Monolayer MoS₂

In-depth understanding of charged objects in atomic-scale is crucial for both fundamental science and unprecedented functional applications such as ultrasmall charge-storage memory devices. Here, we demonstrate a non-volatile reversible switching of hydrogenic defect states in monolayer MoS₂, employing scanning tunneling microscopy and spectroscopy (STM/S). Bistable defect charge states are identified: negatively charged (-) and neutral (0), where they can be switched reversibly via STM tip manipulation. The negatively charged state is characterized by the presence of the upward band bending and resulting depletion region in vicinity of the defects. The Coulomb potential of the negative charged perturbation is renormalized via dielectric screening of the host two-dimensional semiconducting MoS₂, which admits additional localized states near the valence band side. Thus, by controlling the defect charge, we “switch on” or “off” the in-gap states. We show that the observed in-gap states are the physical manifestation of the two-dimensional Rydberg states.