A3 rods have high crystallinity. The PL spectrum also has two
weak emission peaks at 474 nm and 518 nm. The UV emission
peak which is also known as near band emission (NBE) could be
attributed to the recombination of free excitons through an
exciton–exciton collision process [28]. The blue transition at
474 nm is caused due to transition from the level of ionized
oxygen vacancy to the valence band. The green emission, called
deep-level emission occurring by the recombination of the photo-
generated holes with singly ionized oxygen vacancies in ZnO, is
centered at 518 nm [29]. As the ratio of the intensity of the NBE
peaks and green emission peaks are high, our ZnO rods synthe-
sized by a microwave assisted simple chemical precipitation
technique have good optical properties [30] and so have less
concentration of oxygen vacancies. Besides, the increased inten-
sity of the deep-level emission in A3 compared to A2 reveals that
the increased concentration of OH ions enhances the surface
defects A3 rod greatly [31].