Photophysics of Atomically Thin Freestanding β-Ga ₂ O ₃ Nanosheets: Implications for Luminescent Thermometry

Atomically thin non-van der Waals oxides are difficult to access because strong 3D bonding resists exfoliation. Here we report an amine-assisted liquid-phase exfoliation that produces freestanding β-Ga 2 O 3 nanosheets only 2.1–2.4 nm thick with lateral sizes up to ∼10 μm, while preserving the monoclinic β-phase and a bulk-like bandgap of 4.79 ± 0.10 eV. Temperature-dependent luminescence reveals emergent 2D photophysics: self-trapped-hole emission dominates at low temperature, whereas a 2.4 eV green band exhibits negative thermal quenching from 80 to 200 K, consistent with self-trapped-hole migration to near-surface gallium-vacancy centers and highlighting opportunities for luminescent thermometry and optical sensing.