Yang, Zi-LiangZi-LiangYangLin, Yu-ChiehYu-ChiehLinChaudhary, MayurMayurChaudharyLin, Li-ShengLi-ShengLinHuang, Chih-YangChih-YangHuangLin, You-JieYou-JieLinJYH PIN CHOUChen, Li-ChyongLi-ChyongChenChen, Kuei-HsienKuei-HsienChenChueh, Yu-LunYu-LunChuehYA-PING CHIU2026-01-052026-01-052025-12-23https://scholars.lib.ntu.edu.tw/handle/123456789/735024https://www.scopus.com/record/display.uri?eid=2-s2.0-105025728356&origin=resultslistJanus 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.en2D materialsdefectelectronic deviceelectronic structureJanusscanning tunneling microscopySTMjournal article10.1021/acsnano.5c144462-s2.0-105025728356