Regulation Of Photoelectric Response And Persistent Photoconductivity In ZnO-Based Nanorod Array Films

As a traditional metal oxide semiconductor material,ZnO has shown great application potential in the area of light emitting diode,photocatalyst,photodetector and sensor.Among a variety of morphologies of ZnO nanostructures,one-dimensional(1D)structures have the potential to be directly used in device fabrication.Moreover,its unique morphologic characteristics provide an efficient platform for carrier transport,which is very advantageous for electronic and optoelectronic equipment.To ameliorate or improve the performance of device,it is necessary to study the generation and recombination process of carriers under the illumination of light.In this paper,ZnO nanorod array films prepared by hydrothermal method were selected as the research object.By modifying Ag nanoparticles(NPs)with varied content on the surface of ZnO nanorods and regulating the diameter of ZnO nanorods,the underlying intrinsic and extrinsic factors that may influence the photoelectric response and relaxation process of ZnO were systematically adjusted.Time-domained photoconductivity spectra of ZnO-based nanorod array films under UV light excitation at 365 nm were carefully characterized,and the relationship between photoelectric response and relaxation characteristics of ZnO and material microstructure was meticulously studied.Firstly,Ag NPs were modified on the surface of ZnO nanorods through dip-coating method.The effects of Ag NPs on the photoelectric response and relaxation process of ZnO nanorod array films were studied.It turned out that the dark conductance of ZnO greatly decreased with the introduction of Ag nanoparticles on the surface.The higher the Ag concentration,the smaller the dark conductance.Even if the photoconductance slightly decreased compring with pure ZnO,the responsivity of Ag/ZnO during the photoelectric response stage has been greatly improved,with the maximum value nearly 50 times of the responsivity of pure ZnO.In addition,Ag NPs had a good controllability on the persistent photoconductivity(PPC)effect of the ZnO nanorod array films during the relaxation phase.The band theory was applied to explain the mechanism of Ag NPs.Since the Fermi level of Ag is within the forbidden band of ZnO,the electrons on ZnO that were not completely consumed would be continuously transferred to Ag until a new uniform Fermi level between the metal and semiconductor was formed after switching off the excitation light.The electron transfer process attributed to the potential difference was very fast,which weakened the PPC effect of the ZnO nanorod array films.The greater the Ag concentration,the more Ag-ZnO contact were formed as the channels for the electron transfer process,at last the less obvious the PPC effect.Then,in view of the significant PPC effect in ZnO nanorod array films,further research was carried out from the perspective of the storage of photo-generated carriers.Under the premise of surface-modified Ag NPs,the diameter of ZnO nanorods was changed,and it was found that the PPC effect of ZnO nanorod array films showed a certain controllability.As the diameter of ZnO nanorods increased,the PPC effect were significantly enhanced.Moreover,the Ag NPs and diameters can work simultaneously.Finally,quantitative characterization of storage of photogenerated carriers in ZnObased nanorod array films was performed based on the cyclic photoconductance spectra under UV light excitation.The storage of carriers after turning off the excitation light can be regarded as the capture of carriers by the defects.The conditions of defects varied with ZnO nanorod array films with different diameters and Ag concentrations,which are reflected in the photoconductive spectrum as the differences in photogenerated carrier storage efficiency.Quantitative characterization of photo-generated carrier storage may provide a new idea for the quantitative characterization of defects in ZnO.