Two-step colloidal synthesis of micron-scale Bi2O2Se nanosheets and their electrostatic assembly for thin-film photodetectors with fast response

Recently discovered bismuth oxychalcogenide (Bi₂O₂Se) has aroused great interest due to its ultrahigh carrier mobility, tunable band gap and good environmental stability, making it a promising candidate for high-performance electronics and optoelectronics. Their synthesis by colloidal approaches represents a cost-effective alternative to well-established chemical vapor deposition methods, and the resulting electronic-grade inks are important for large-area printed or wearable electronics. However, it is still challenging to control the colloidal growth of Bi₂O₂Se nanosheets in solution in addition to their assembly into high-performance thin films. Here, we report a two-step colloidal synthesis of Bi₂O₂Se nanosheets by separating the seeding and growth steps, thereby achieving controllable production of nanosheets with a lateral size of 1.4 μm and a thickness of 10 nm at optimized reaction conditions. These Bi₂O₂Se nanosheets are electrostatically assembled into large-area thin films, from which a photodetector is fabricated with a responsivity of 6.1 A/W and a short response time of 368 μs under the 520-nm laser illumination. The device exhibits fast response to modulations as high as 100 kHz, along with a −3 dB bandwidth of 1 kHz. This work provides an important understanding of the controlled colloidal synthesis of Bi₂O₂Se nanosheets, and demonstrates their potential applications in fast photodetectors.