Research On The Preparation And Properties Of Ultraviolet Optoelectronic Devices Based On Novel Two-dimensional Semiconductor Materials

Ultraviolet(UV)optoelectronic devices possess many advantages,such as high energy,short wavelength,strong penetration,small spot limit size and high-resolution imaging,which have promising and expansive application prospect in military detection,deep-space communication,biochemical detection,optoelectronic neural network computing,and so on.Two-dimensional(2D)UV optoelectronic materials have a bright application prospect in the field of UV optoelectronic devices due to their atomic-scale thickness,controllable band structure,and various low-dimensional quantum effects.However,present 2D UV materials with small bandgap,low photoelectric conversion efficiency,weak UV light absorption.In addition,there are also great shortcomings in device design and combination with computer algorithms.Therefore,the poor performance parameters and the weak combination level of software and hardware cannot meet the needs of the new-generation UV optoelectronic systems.Herein,the dissertation focuses on exploring new 2D UV optoelectronic materials,constructing and designing high-performance UV optoelectronic devices,and combining neural network algorithm,which has been carried out from three aspects:1.2D non-layered?-CuBr nanosheets,2D quasi-layered BiOBr nanosheets and 2D quasi-layered Bi₂O₂Se nanosheets are synthesized by self-limiting CVD,Cu-catalyzed CVD and soluble-substrate CVD,respectively.2.The self-driven 2D?-CuBr UV photodetector,the2D BiOBr UV phototransistor with high speed and high gain,and 2D Bi₂O₂Se photomemory with UV light writing and backgating-voltage erasure are realized by designing the electrode structure of asymmetric Schottky junction,the optical response mechanism regulated by the back gate,and the continuous photoconductivity effect of the defect mechanism,respectively.The above three kinds of UV optoelectronic devices are dominated by coexisted ultraviolet photoresponse mechanism of photoconductivity effect and photogating effect.By controlling the trap states of low-dimensional materials,the light response mechanism can be changed from an untunable light response mechanism and tunable light response mechanism to a special continuous light response mechanism,which gradually completed the research and exploration of new 2D UV optoelectronic devices.3.Through function fitting and back-propagation algorithm,the hardware implementation of neural network calculation is achieved by 2D Bi₂O₂Se photomemorys.The detailed researches are as follows:(1)2D?-CuBr nanosheets with different thickness can be synthesized by controlling the space-confined distance and growth temperature.The thinness is 0.91 nm and the maximum size is 45?m.The 2D?-CuBr nanosheets exhibit a direct bandgap of 2.92 e V,the light absorption properties of Z_1,2,and Z₃ excitons and the luminescence properties of enhanced Z_f excitons,as well as the nonlinear optical properties of two-photon fluorescence and angular-polarization second harmonic generation.The self-driven,high-performance?-CuBr UV photodetectors by constructing Schottky junction with asymmetrical electrodes,performed high photoresponsibility(R_?)of 3.17 A W^-1,external quantum efficiency(EQE)of 1126%,detectivity(D*)of 1.4×10¹¹ Jones and fast photoresponse process(?_rise=32 ms,?_decay=48 ms).(2)The 2D BiOBr nanosheets with different morphologies and thicknesses were synthesized by controlling the source temperature and flow rate.The thickness of monolayer BiOBr nanosheets was about 0.57 nm and the maximum size was over 70?m.Through theoretical calculation and light absorption spectrum,it is found that BiOBr has strong deep-UV(DUV)light absorption and wide bandgap in few layers,and the maximum bandgap of monolayer BiOBr can reach?3.69 e V.2D DUV BiOBr-based phototransistors achieved excellent UV detection performance by gate voltage regulation through the design of conversion of photoconduction effect and photogating effect.The ultrahigh R_?of 12739.13 A W^-1,EQE of 6.46×10⁶%and D*of 8.37×10¹² Jones were obtained at the gate voltage of 35 V.The ultrafast photoresponse process(?=102?s)could be realized at the gate voltage of-40 V.High-photogain and high-speed photodetectors can be achieved at the same device by gate voltage control.(3)2D Bi₂O₂Se nanosheets were synthesized by CVD method on soluble Na Cl substrates,which can realize the nondestructive transfer.Photomemorys with UV light writing and back-gate voltage erasure can be implemented by photosensitive properties and inherent defects of 2D Bi₂O₂Se nanosheets.The hardware implementation of neural network algorithm is achieved through function fitting and software programming.After20 times of epoch training by using back-propagation algorithm in MINST handwritten digit training set,the recognition accuracy of handwritten digit test can reach 95%.