Polarity-Programmable Bismuth Oxide Overlayers on Bi(111)/MoS₂ Heterostructures via Oxidation and Annealing

Band-structure engineering in bismuth (Bi) oxides is frequently hampered by their amorphous or poorly ordered nature, which obscures the relationship among stoichiometry, band dispersion, and transport polarity. We find that native BiO_x overlayers on epitaxial Bi(111)/MoS₂ undergo a reversible n → p → n polarity control of the heterostructure system under controlled air exposure and annealing. As-grown BiO_x-rich surfaces are n-type and exhibit a photoluminescence peak at ∼2.1-2.2 eV. Mild air annealing (100 °C, 1 h) of native BiO_x overlayers on epitaxial Bi(111)/MoS₂ yields a p-type surface with a phase-mixed Bi₂O₃ (α+β) overlayer, characterized by a highly dispersive Bi-O valence band, and in-situ annealing at 300 °C reduces the thickness of the oxide layer and restores the n-type band alignment governed by the Bi(111)/MoS₂ stack. First-principles calculations for hexagonal Bi₂O₃ monolayers and Bi₂O₃/Bi(111) heterostructures reveal a transition from a wide-gap, O-2p-dominated oxide to a narrow, Bi-dominated direct gap at Γ, which supports the observed p → n band structure evolution. These findings provide a fundamental mechanism for tuning the polarity and band alignment of Bi-oxide-based interfaces on 2D semiconductors.