Fig. 4 shows the PL emission spectra acquired from the end of
the representative ZnO nanowire upon excitation at 325 nm. The PL
spectra of a single nanowire indicated that the defect emission is
much stronger than the near UV emission. Therefore, the study of a
single nanowire is mainly focused on its green emission. The emissions
at 545 and 385 nm are generally attributed to the electronic
transition from lower energy levels originating from defects in the
ZnO crystal field and the direct recombination of the free excitons
near band edge, respectively [21,22].