## 3 Results and Discussion
To further confirm that the synthesized material is Bi\({}_{2}\)O\({}_{2}\)Se, it was also characterized by energy-dispersive X-ray photoelectron spectroscopy (XPS). As described in Figure 1f-h, the peaks of Bi 4f\({}_{7/2}\) and Bi 4f\({}_{5/2}\), which are located at 158.8 eV and 164.2 eV, respectively. By performing Lorentzian fitting on the broad peak near 54 eV using XPS software (Avantage V5.9931), it is possible to resolve the peaks at 53.2 eV and 54.2 eV, which correspond to the Se 3d\({}_{5/2}\) and 3d\({}_{3/2}\) peaks, respectively. The O 1s spectrum contains two peaks that are located at 532.1 eV and 529.8 eV; these two peaks correspond to adsorbed oxygen and lattice oxygen [37]. All bonding energies are consistent with the elements in Bi\({}_{2}\)O\({}_{2}\)Se. Subsequently, a Au electrode with a thickness of 50 nm was fabricated via electron beam evaporation deposition for establishing an electrical connection with Bi\({}_{2}\)O\({}_{2}\)Se (Figure S3a). As shown in Figure 2a, the photocurrent of the Bi\({}_{2}\)O\({}_{2}\)Se photodetector  
Figure 1: Synthesis and characterization of 2D Bi\({}_{2}\)O\({}_{2}\)Se nanosheets. (**a**) Schematic diagram of the structure of Bi\({}_{2}\)O\({}_{2}\)Se. (**b**) Schematic illustration of the CVD growth of Bi\({}_{2}\)O\({}_{2}\)Se. (**c**) Optical microscopy image of the Bi\({}_{2}\)O\({}_{2}\)Se sample. (**d**) AFM image. (**e**) Raman spectrum of the Bi\({}_{2}\)O\({}_{2}\)Se sample. (**f**–**h**) XPS spectra of the few-layer Bi\({}_{2}\)O\({}_{2}\)Se on mica. increases with the enhancement of light intensity at 550 nm. Due to the bandgap of about 0.8 eV in few-layer Bi\({}_{2}\)O\({}_{2}\)Se, the device's dark current reaches 10\({}^{-6}\) A. As shown in Figure 2b, Bi\({}_{2}\)O\({}_{2}\)Se exhibits excellent photoresponse curves in the 405-1000 nm range, while almost no photocurrent is detected beyond 1000 nm. Figure 2c shows the responsivity and detectivity of the Bi\({}_{2}\)O\({}_{2}\)Se photodetector from 405 nm to 1000 nm. The experiment found that the maximum responsivity and detectivity of the few-layer Bi\({}_{2}\)O\({}_{2}\)Se photodetector are both at 600 nm, with a responsivity reaching 0.604 A/W and a detectivity reaching 7.61 \(\times\) 10\({}^{9}\) Jones. Thus, the detection range of the few-layer Bi\({}_{2}\)O\({}_{2}\)Se photodetector prepared in the experiment is 405-1000 nm, mainly demonstrating its excellent detection capability in the visible light region. In the near-infrared region, however, the photodetection capability of Bi\({}_{2}\)O\({}_{2}\)Se is very weak, mainly due to the intrinsic indirect bandgap of Bi\({}_{2}\)O\({}_{2}\)Se [42,43], which limits its application in the field of photodetection. As shown in Figure 2d, the \(\tau_{\text{rise}}/\tau_{\text{decay}}\) values of the Bi\({}_{2}\)O\({}_{2}\)Se photodetector are 0.63/1.27 s (V\({}_{\text{ds}}\) = 1 V). However, by taking advantage of the high mobility of Bi\({}_{2}\)O\({}_{2}\)Se, an efficient broadband photodetector can be constructed by integrating it with QDs with high extinction coefficients [30]. As shown in Figure 3a, we further prepared photosensitive PbS QDs with a characteristic diameter of approximately 4.13 nm on the pre-fabricated Bi\({}_{2}\)O\({}_{2}\)Se transistor. These QDs possess a narrow bandgap of 1.1 electron volts, enabling spectral detection up to 1300 nm. In Figure 3b, the absorption of the quantum dots exhibits a first exciton absorption peak at approximately 1100 nm. The results are consistent with the predicted bandgap. The experiment involved spin-coating PbS QDs onto Bi\({}_{2}\)O\({}_{2}\)Se nanosheets, as illustrated in Figure 3c. Subsequently, the sample was annealed at 90 \({}^{\circ}\)C under Ar gas conditions for 15 min to induce solvent removal and atomic restructuring of the quantum dot surfaces, releasing surface partial stress defects, thereby enhancing the photodetection performance  
Figure 2: Optoelectronic detection performance of the bare Bi\({}_{2}\)O\({}_{2}\)Se device.