Interface‐Engineered Sb‐Doped Bi ₂ O ₂ Se via Temperature‐Mediated By‐Product Formation for Efficient Exfoliation and Enhanced Ultrafast Photonic Performance

ABSTRACT Layered materials such as Bi 2 O 2 Se hold great promise for ultrafast photonic applications, yet their strong interlayer electrostatic coupling poses a major challenge for efficient liquid‐phase exfoliation and subsequent optoelectronic integration. Here, we uncover a subtle but controllable structural transformation in Sb‐doped Bi 2 O 2 Se that occurs within a narrow 50°C synthesis window. At 630°C, the simultaneous formation of minor BiSbO 4 with the host lattice modifies the local chemical environment during crystallization. Contrary to the conventional view, BiSbO 4 functions as a structural modulator that weakens interlayer electrostatics and lowers exfoliation barriers. Ultrasonic exfoliation in ethanol yields ultrathin nanosheets with record‐large average‐lateral size (∼1.41 µm), atomic‐scale thickness (∼1.6 nm), and exceptional flatness (RMS ≈ 0.83 nm), representing the largest and smoothest Bi 2 O 2 Se‐based nanosheets to date. First‐principles calculations and zeta‐potential measurements reveal that BiSbO 4 redistributes interfacial charge and introduces shallow electronic states without generating deep traps, enhancing light–matter interactions. As a saturable absorber, the composite maintains comparable modulation‐depth but achieves an 18‐fold reduction in saturation intensity, enabling low‐threshold mode‐locking (182.4 mW) and advanced pulse regimes including 20th‐order harmonic and bound‐state solitons in fiber laser. This work introduces a by‐product‐mediated interfacial engineering strategy that offers a new paradigm for designing high‐performance ultrafast photonic materials.