Directly Probing Stacking-Engineered Defect State Delocalization in Marginally Twisted Bilayer WS₂

Defect states in bilayer transition metal dichalcogenides (TMDs) represent a promising platform for optoelectronic devices, where stacking-dependent defect state delocalization influences the carrier capturing probability and device performance. However, top-layer defects dominate the electronic measurement and obscure the observation of delocalized defect states, whereas only bottom-layer defects allow direct observation of defect state delocalization at the surface. Here, we employ scanning tunneling microscopy/spectroscopy and image charge analysis to distinguish bottom-layer charged defects across distinct stacking configurations in lattice-reconstructed twisted bilayer WS₂. Our results reveal that defects aligned with W atoms exhibit weaker charge screening and stronger interlayer orbital coupling, resulting in enhanced carrier accumulation and out-of-plane state propagation. Defect state spectroscopy demonstrates that W-aligned defects show pronounced delocalization and the largest carrier capture cross-section, which is unfavorable for optoelectronic and electronic devices. Our findings elucidate the atomic-scale mechanisms governing defect state delocalization in bilayer structures, providing fundamental insights for defect engineering in TMD-based quantum devices.