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 BiOx 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 BiOx-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 BiOx 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.