Role of oxygen vacancies on the green photoluminescence of microwave-assisted grown ZnO nanorods

Mechanism of commonly seen defect mediated photoluminescence (PL) from ZnO nanostructures is a topic of much discussion mainly due to its broad nature originating from several types of defects. Identification of such surface related defects requires suitable specimen having high surface to volume ratio characterized by complimentary techniques. Here, ZnO nanorods of different aspect ratio within the quantum confinement regime have been prepared using microwave-assisted synthesis method by varying time in the range of 1–4 min. The band gap as well as intensity of visible PL emission is found to decrease with the increase in size of nanorods obtained by allowing longer preparation time. Excitation wavelength dependent PL shows a strong green emission at ∼540 nm for above the band gap excitation (ABE) as well as below the band gap excitation (BBE) near to the band edge. Whereas a blue emission at ∼485 nm is seen for excitations far below the band gap. A systematic shift in emission from green to blue is observed for decreasing values of BBE, whereas almost no shift was seen for green emission with the variation of ABE. It is proposed that the green PL can be seen when ABE generated electrons in the conduction band transit rediatively to singly ionized oxygen vacancy (Vo+) and finally to reach the valence band whereas the blue emission may appear by direct excitation of electrons to a sub-band within the band gap by BBE. The presence of Vo+ in ZnO nanorods is confirmed by x-ray photoelectron spectroscopic studies and correlated with the green PL that shows increased intensity for higher concentration of Vo+ in smaller nanorods. •Highly defective ZnO nanorods of diameter below ∼20 nm and lengths in the range of ∼30–∼60 nm have been synthesised using microwave assisted growth technique.•The number of defect states in ZnO nanorods have been found to increase with decrease in aspect ratio (AR) of the nanorods.•The presence of defects has resulted in an intense green photoluminescence (PL) emission from the ZnO nanorods and the intensity of this PL emission is seen to decrease with increase in AR.•Using complementary characterization techniques it has been established that the origin of this PL is due to singly ionized oxygen vacancies in ZnO nanorods.•The present study helps to clearly identify the mechanism of green PL emission of ZnO nanorods of diameter below ∼20 nm which was not very clear earlier.