Ultraviolet Photodetection Based on ZnO Colloidal Nanoparticles Made by Top-Down Wet-Chemistry Synthesis Process

ZnO bulk material (bandgap 3.26 eV) was reduced in size via a top-down wet-chemistry synthesis process into colloidal nanoparticles that were then coated with polyvinyl-alcohol (PVA). When the nanoparticles were excited at 340 nm, strong UV emission at 377 nm was observed, and parasitic green emission (510 nm) which normally appears in ZnO nano-material was suppressed. Quantum confinement effects of the nanoparticles were observed by a 5 nm blue shift from ZnO bulk material (380 nm) in the absorption spectrum. The same characteristics held true when the particles were deposited onto substrates to form a photoconductive detector. Point contact current-voltage characteristics of the detector were measured under darkness and 340 nm UV LED. The ratio of the UV photogenerated current to dark current was as high as 10⁵. Spectral response of the detector demonstrates a sharp cut-off wavelength at 375 nm, with a UV responsivity equivalent to twice that of a commercial UV enhanced Si-based photodetector. The time response indicated the rise and fall time were 22s and 11s respectively. The results confirm that top-down wet-chemistry synthesized ZnO nanoparticles are suitable for small signal visible blind UV detection.