Atomically Resolved Defects Modulate Electronic Structure in Plasma-Assisted 2D Janus MoSSe Monolayers

Janus transition metal dichalcogenides, such as MoSSe, are potential materials for advanced electronics, yet their real-world device performance often fails to meet theoretical expectations. The origin of this discrepancy, rooted in atomic-scale imperfections, has remained critically unexplored. Here, using scanning tunneling microscopy and spectroscopy, this work provides atomic-scale insights into the complex electronic structures of monolayer Janus MoSSe, revealing distinct defect species that govern device performance. The residual sulfur dopants are found to introduce a broad band (≈0.5 eV) of shallow in-gap states near the valence band with spatially inhomogeneous distribution. Moreover, this work unveils two distinct native charge defects with spatially electronic influence extending ≈2.5 nm: conductive charge traps that reduce the local effective bandgap by more than half and insulating scattering centers that impede carrier transport. This microscopic understanding of defect-induced electronic modifications explains how atomic-scale imperfections influence macroscopic device limitations, providing fundamental design criteria for the engineering of Janus devices.