Fabrication Of Bi₂O₂Se Nanosheet And Its Surface Modification For Photodetection

Recently,Bi₂O₂Se nanosheet,as an emerging layered material with ultrahighmobility(value of～29000 cm²V^-1s^-1at 1.9 K),a narrow bandgap（0.8 e V）and excellent air stability,which are promising to support the further development of high-speed and low-power optoelectronic devices in the future,has attracted a lot of attention.However,as there are a variety of vacancy defects in Bi₂O₂Se manifested as donor doping,Bi₂O₂Se nanosheet generally shows a high carrier concentration(10¹⁹-10²⁰cm^-3),which causes excessive dark current problems in optoelectronic devices.And previous studies have shown that the use of surface charge transfer doping is a suitable doping method for ultrathin two-dimensional semiconductors.In this paper,Bi₂O₂Se nanosheets are synthesized by chemical vapor deposition（CVD）,and Bi₂O₂Se nanosheet photoconductive detectors are constructed.Based on the principle of charge transfer doping,we explore the modification by oxygen plasma treatment or MoO₃ thin films by vacuum deposition and study their influence on the electrical properties and photodetection performance of Bi₂O₂Se.The main research contents are as follows:1.We synthesize Bi₂O₂Se nanosheets by chemical vapor deposition（CVD） method,and characterize their electrical properties and photodetection performance.The analysis shows that lateral size of as-synthesized Bi₂O₂Se nanosheets reaches 110μm and thickness is mainly between 10-30 nm,while their conductivity is mainly 10-10⁵ S/m.Photoconductive detectors based on Bi₂O₂Se nanosheets are constructed by laser direct writing/electron beam exposure.The tests exhibit that the best responsivity is about 9.7×10³ A/W at a wavelength of 680 nm,the shortest response time is about7.4 ms and the highest detectivity is 8×10¹⁰ Jones;and based on the dependence between photocurrent and optical power I_ph∝P^0.25,it is concluded that photodetection mechanism comes from the photogating effect caused by a large number of native defects in the material.2.We use O₂ plasma treatment and MoO₃ thin films by vacuum deposition to modify Bi₂O₂Se nanosheets with thickness varying from 10-30 nm,in order to improve their photodetection performance.The results have shown that with the increase of the oxygen plasma processing time,the dark current of devices gradually decreases,and the photogating effect in the device is enhanced,which increases the response switching ratio of the detector and improves the detectivity to 1.4×10¹¹ Jones.However,the optimal time for oxygen plasma treatment varies due to the difference among Bi₂O₂Se nanosheets,and the surface oxygen absorption generated during the treatment causes the poor stability.In order to achieve stable surface modification,the research continues to explore the deposition of～13 nm MoO₃ film on the surface of Bi₂O₂Se nanosheets by vacuum thermal evaporation,leading to the surface acceptor-type charge transfer doping through its higher work function（4.4 e V）.Although the results show that the introduction of the surface MoO₃ film can effectively reduce the dark current of the device,it still faces the problem of the decrease in the responsivity caused by the modification,which needs further research and optimization.