Construction Of One-dimensional ZnO Heterojunctions And Research On Their Optoelectronic Properties

Photodetectors is crucial for potential application in many fields,such as military,astronomy,industrial production,medicine,environmental monitoring,etc.Many one-dimensional nanomaterials have excellent photoelectric properties.ZnO nanowires,that have various properties such as semiconducting,piezoelectric and pyroelectric properties,are one of the important members.Persistent photoconductive effect in the photodetectors based on ZnO nanowires results in depression of response/recovery time heavily owing to the inevitable defects of oxygen vacancies during the preparation of ZnO nanowires.Pyroelectric potential produced at the moment of illumination can quickly regulate the separation and transportation of charge carriers,which can suppress the persistent photoconductive effect and shorten the response/recovery time to a certain extent.Although ZnO has high photothermal conversion efficiency for long-wavelength infrared light,the pyroelectric potential generated in ZnO is small for UV/visible light whose photon heat effect is weak.Especially,for UV/visible light with low power density,the pyroelectric potential/current becomes much weak,and thus the signal-to-noise ratio of the device becomes small.Moreover,the response/recovery time becomes long.To address the above problems,we propose to enhance the photothermal conversion capability of ZnO nanowire array-based materials for UV/visible light using localized surface plasmonic resonance（LSPR）.And hence,enhancing the rate of temperature change at the moment of illumination to inspire/enhance the pyroelectric effect.Finally,responsivity is increased,and response/recovery time are shortened.So,to detect low-intensity UV/visible fast and sensitively are achieved.The major achievements of this thesis are as follows:First,we designed a self-powered photodetector based on a Schottky junction（Ag electrode-ZnO Schottky junction）constructed with ZnO NWs decorated with AuNPs.The photodetectors can detect 325 nm light with a power density as low as 68 nW/cm² by exploiting localized surface plasmonic resonance（LSPR）inspired pyro-phototronic effect.Under illumination of 325 nm light with power density of 68 nW/cm²,the responsivity of the photodetector increases sharply from 0 mA/W of the original device to 0.485 mA/W of the ZnO/Auphotodetector.Similarly,the detectivity increases from 0 Jones to 2.75×10¹¹ Jones.The detectivity improves from 9.3×10⁹ Jones of the ZnO photodetector to3.16×10¹¹ Jones of the ZnO/Auphotodetector under the illumination of 170nW/cm² 325 nm light,increasing by more than 3298%.The response time of the ZnO/Auphotodetector also reaches 12 ms.Secondly,we demonstrate a self-powered photodetector,which is integrated with ZnO/CuO core-shell nanowires and Aunanoparticles,to detect UV to Vis light.Here,a practical strategy is used to improve the performance of ZnO-based photodetectors,by coupling pyroelectric effect of ZnO with LSPR of Aunanoparticles subtly.Like the Schottky-contacted photodetectors,the self-powered photodetectors with p-n junction also achieve fast and sensitive detection of UV when power density is 68 nW/cm² and the performance is even better.The responsivity and detectivity for 325 nm light of 68 nW/cm² are 1.4×10^-4 A/W and3.3×10¹¹ Jones for devices with Auloading of 230 nmol,while the devices without AuNPs cannot detect even more large light power density at 360 nW/cm².The rise time and fall time are shortened from 329.6/403.7 ms of the ZnO/CuO device to 6.3/6.9 ms of the ZnO/CuO/Auphotodetector.The LSPR induced by AuNPs is demonstrated by FDTD simulations to explain the enhancement of pyroelectric effect in ZnO.Finally,in a self-powered photodetector based on ZnO/CuO core-shell nanowires decorated with AuNPs,the couple of piezo-phototronic effect,pyro-phototronic effect and LSPR effect can be realized.The influences of LSPR,pyroelectric and external pressure on the performance of devices have been thoroughly investigated respectively.ZnO/CuO/Audevices display the most attractive performance under pressure of 73.7 N.The maximums of responsivity and detectivity are obtained as 0.81 mA/W and 3.3×10¹³ Jones respectively under pressure of 73.7 N when detecting weak ultraviolet radiation（140 nW/cm²）.Responsivity and detectivity are dramatically enhanced by 17 and 12 times compared to CuO/ZnO devices.Moreover,rise time and fall time reduce from114/75 ms of a ZnO/CuO device to 18/12 ms of a ZnO/CuO/Audevice under pressure of 64.7 N.These results demonstrate LSPR,pyro-phototronic and piezo-phototronic coupled effect make greater improvement than the individual effect in performance of photodetectors.This work can be probably a cornerstone of designing high-performance photodetectors using other nanomaterial systems as well.