Space‐Confined CVD‐Grown Bi ₃ O 2.5 Se ₂ Enabling Broadband Edge Detector With Sub‐Bandgap Response

Edges are fundamental structural elements in visual perception. It is imperative to develop detectors capable of resolving edges across an extended spectral range, including sub‐bandgap wavelength. Recently, a new phase in the Bi‐O‐Se system, Bi 3 O 2.5 Se 2 , was discovered. However, the formation of Bi 3 O 2.5 Se 2 is extremely sensitive to the local oxygen partial pressure, which has impeded its application. Here, we developed a reliable space‐confined CVD method to synthesize high‐quality Bi 3 O 2.5 Se 2 nanosheets. The thickness of Bi 3 O 2.5 Se 2 nanosheets can be continuously modulated from 4.4 to 43 nm, with the lateral domain size reaching up to 425 µm. We integrate these nanosheets in a back‐illuminated Schottky device to minimize metal shadowing and concentrate absorption near the junction. The detector exhibits zero‐bias broadband response from 520 to 1550 nm with a high signal‐to‐noise ratio, including an infrared response beyond the 1.14 eV bandgap that is consistent with internal hot‐carrier‐assisted collection. The device reaches a peak responsivity of 0.3 A·W −1 and a specific detectivity D * of 2.89 × 10 11 cm·Hz 1/2 W −1 (at 940 nm), with a microsecond‐level response time (107 µs). Finally, we demonstrate edge‐enhanced imaging of patterned objects with zero‐bias operation, highlighting its promise for compact in‐sensor analog preprocessing.