Zhongyi Luo

Ternary two-dimensional (2D) Nb2SiTe4 has recently garnered significant interest due to its exceptional stability under ambient conditions, a property rarely observed in conventional 2D semiconductors. Despite this advantage, the microscopic mechanism underpinning its environmental robustness remains unresolved. In typical 2D chalcogenides, instability is closely associated with electronically active surface anions, which facilitate the formation of anion vacancies and promote surface oxidation. In this work, we employ first-principles calculations to elucidate the electronic structure and intrinsic defect physics of Nb2SiTe4. Our results demonstrate that Te atoms in Nb2SiTe4 are electronically inert, contributing negligibly to the valence-band maximum as a consequence of weak Te-5p/Nb-4d hybridization. This inert electronic character gives rise to both the high formation energy and the delocalized defect state near the conduction band minimum of the Te vacancy. Removal of a Te atom enables direct Nb–Nb orbital interaction within the face-sharing trigonal prisms, generating an unoccupied bonding state that destabilizes the local coordination environment and renders the formation of Te vacancy energetically unfavorable. Moreover, pristine Nb2SiTe4 exhibits only weak adsorption of the O2 molecule, attributed to the limited electronic perturbation between Te and O2. Even in the presence of a Te vacancy, O2 adsorption induces minimal and highly localized charge transfer, indicating a strongly suppressed oxidation propensity relative to conventional 2D chalcogenides. Our findings reveal the intrinsic origin of the high anion-vacancy formation energy and outstanding oxidation resistance of Nb2SiTe4. This work provides fundamental insight into its exceptional environmental stability and underscores its suitability for reliable and durable 2D electronic applications.

Silver-bismuth iodide (ABI) ternary semiconductors, such as AgBi2I7, AgBiI4, Ag2BiI5, and Ag3BiI6, have emerged as promising lead-free light absorbers for photovoltaic applications due to their favorable optoelectronic properties. Despite recent advances that have improved power conversion efficiencies from ∼1% to over 5%, ABI-based solar cells still show substantial open-circuit voltage (VOC) losses of up to ∼1 V, which significantly hinder the device performance. These losses have been experimentally attributed to the non-radiative recombination originating from intrinsic defects, however, theoretical understanding of these defect mechanisms remains limited. Here, using density functional theory calculations, we systematically investigate the defect properties of AgBiI4. We identify the dominant intrinsic defects as acceptor-like Ag vacancies (VAg) and AgBi antisites, as well as donor-like Ag interstitials (Agi) and BiAg antisites. Among these defects, VAg and AgBi are shallow defects, while Agi and BiAg create deep trap states. Our calculations reveal that I-rich synthesis conditions with a carefully balanced Ag/Bi ratio are essential to suppressing the formation of deep defects and mitigating non-radiative recombinations. These insights provide theoretical guidance for defect modulation in ABI compounds and highlight AgBiI4 as a model system for understanding defect physics in ABI photovoltaic materials.