Shape controllability and photoluminescence properties of ZnO nanorods grown by chemical bath deposition

Zinc oxide (ZnO) nanorods (NRs) were synthesized on glass substrates and Au seed layers by chemical bath deposition from the aqueous solution of ZnCl2 and the mixed aqueous solution of zinc acetate dihydrate (ZnAc) and hexamethylenetetramine (HMT) at a low temperature of ~ 90 °C. Vertically aligned NRs were successfully grown on the Au seed layers. For the NRs synthesized from the ZnCl2 solution of 0.17 M, when the growth time increased from 15 to 180 min, the average diameter and length increase from ~ 350 to ~ 1020 nm and from ~ 1000 to ~ 5600 nm, respectively. The increase in average diameter with the concentration of solution was observed on the NRs synthesized from the mixed solution of ZnAc and HMT. The influence of additional HMT was found on the shapes and density of the NRs. Photoluminescence (PL) spectra of the NRs synthesized from the solutions of ZnCl2 exhibited a dominant orange band (OB) emission at ~ 640 nm associated with the excess-oxygen atoms. On the other hand, the NRs synthesized from the mixed solution of ZnAc and HMT exhibited a strong near-band-edge (NBE) emission at ~ 380 nm, suggesting their high crystalline quality. For the NRs synthesized from the mixed solution of ZnAc and HMT, the OB emission is effectively excited at the photon energy corresponding to the A free exciton emission. For the NRs synthesized from the solution of ZnCl2, however, the secondary phase Zn(OH)2 formed at the surface regions of the NRs contributes to the excitation process for the OB emission. Photoacoustic (PA) measurements revealed that the intra-band-gap absorption band extending from 400 to 660 nm responsible for nonradiative transitions were suppressed in the NRs synthesized from the mixed solutions of ZnAc and HMT in comparison with those from the ZnCl2 solutions.