Enhanced photoresponse of annealed ZnO nanoparticle film near its bandgap

The ultraviolet (UV) photoconduction characteristics of the ZnO nanoparticles (NPs) deposited on glass substrates have been studied as deposited and after induction of oxygen vacancy through thermal annealing in a vacuum. The morphology, structural, and optical properties of the samples have been characterized using X-ray diffraction (XRD) patterns, field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), UV-Vis absorption, and photoluminescence spectroscopies. The results confirmed the presence of oxygen vacancies on the surface of ZnO nanostructures after annealing. The photoconductivity behavior of the annealed ZnO NP film was investigated using a two-point probe method. The results showed significant photosensitivity under UV irradiance near its bandgap. The wavelength-dependent and time-resolved photoconductivity measurements revealed a fast response to UV illumination (rise and decay durations of 4 s and 18 s, respectively) for the annealed film optimized at 380 nm, which is the wavelength corresponding to its bandgap. The improved photoresponse characteristics of the film could be attributed to the increased surface oxygen vacancy of the annealed ZnO NP film, which helps the chemisorption of oxygen molecules from ambient air. The photoconductivity properties of the ZnO NP film investigated in this study indicate their great potential for usage in photonic devices such as photodetectors.