Large‐Scale Ultrathin 2D Wide‐Bandgap BiOBr Nanoflakes for Gate‐Controlled Deep‐Ultraviolet Phototransistors

Abstract Ternary two‐dimensional (2D) semiconductors with controllable wide bandgap, high ultraviolet (UV) absorption coefficient, and critical tuning freedom degree of stoichiometry variation have a great application prospect for UV detection. However, as‐reported ternary 2D semiconductors often possess a bandgap below 3.0 eV, which must be further enlarged to achieve comprehensively improved UV, especially deep‐UV (DUV), detection capacity. Herein, sub‐one‐unit‐cell 2D monolayer BiOBr nanoflakes (≈0.57 nm) with a large size of 70 µm are synthesized for high‐performance DUV detection due to the large bandgap of 3.69 eV. Phototransistors based on the 2D ultrathin BiOBr nanoflakes deliver remarkable DUV detection performance including ultrahigh photoresponsivity (R_λ, 12739.13 A W⁻¹), ultrahigh external quantum efficiency (EQE, 6.46 × 10⁶%), and excellent detectivity (D*, 8.37 × 10¹² Jones) at 245 nm with a gate voltage (V_g) of 35 V attributed to the photogating effects. The ultrafast response (τ_rise = 102 µs) can be achieved by utilizing photoconduction effects at V_g of −40 V. The combination of photocurrent generation mechanisms for BiOBr‐based phototransistors controlled by V_g can pave a way for designing novel 2D optoelectronic materials to achieve optimal device performance.