Defect-induced photogating effect and its modulation in ultrathin free-standing Bi2O2Se nanosheets with a visible-to-near-infrared photoresponse

Ultrathin Bi2O2Se nanostructures possess ubiquitous structural and optical properties suitable for future optoelectronics. Herein, we have studied the unique structural and optoelectronic functionalities of ultrathin Bi2O2Se nanocrystals, synthesized via a scalable chemical reaction process with high reproducibility, and their modulation through vacuum annealing. The as-prepared nanosheets (NS) with monolayer-to-few-layer thickness possess different kinds of defects (e.g., Bi-antisites at Se vacancies, Se vacancies, interstitial O, O antisites at Se vacancies) due to their free-standing nature with Se-terminated charged surfaces. Our study reveals that vacuum annealing improves the crystal quality through defect passivation, basically through inter-layer self-assembly via electrostatic interactions. Interestingly, we observed self-powered negative persistent photoconductivity (PPC) in the as-grown defect-containing Bi2O2Se NS and positive photoconductivity (PC) in annealed Bi2O2Se NS containing fewer defects. The mechanistic origin of such a conversion from negative PPC to positive PC is discussed in detail, where the energy levels created by the defects serve as photo-gates. In addition, photoresponsivity (vis-NIR) measurements show a photo-gating effect created by the defects that play a key role in modulating the wavelength-dependent photoconductivity. The peak responsivity of the device is shown to be 0.4 A W-1 at 808 nm. This work highlights the possible origin of the crystallinity improvement of non-van der Waals Bi2O2Se nanosheets through vacuum annealing and their intriguing photoresponse properties in the visible-to-near-infrared range, which are of fundamental importance and practical application in cutting-edge optoelectronics.

Automated summary

In this study, ultrathin Bi2O2Se nanostructures were synthesized through a scalable chemical reaction process and their structural and optoelectronic functionalities were modulated through vacuum annealing. It was found that vacuum annealing improved the crystal quality and led to a conversion from negative persistent photoconductivity to positive photoconductivity. The defects in the nanocrystals also played a key role in modulating the wavelength-dependent photoconductivity. The research highlights the importance of vacuum annealing in improving the crystallinity and the practical applications of Bi2O2Se nanostructures in optoelectronics.