Photoluminescence characterization of vertically aligned ZnO microrods

A detailed optical characterization of vertically aligned ZnO microrods (μRs) grown using a Ni-based catalyst was carried out by excitation-power- and temperature-dependent photoluminescence (PL) measurements. Low-temperature PL spectra of ZnO μRs are dominated by near-band-edge (NBE) emission consisted of a series of sharp lines typical for the bulk ZnO. Starting from the higher energy free exciton (FX) emission feature, the majority of them can be explained by radiative recombination of excitons bound to neutral donors (D⁰X), defect bound exciton (DBX), two-electron satellites emission, free-to-bound, i.e. free electrons to the neutral acceptors (eA⁰) transition, as well as their longitudinal-optical phonon replicas. An additional excitonic line located in between the FX and D⁰X lines, denoted as the surface excitons (SX) for ZnO μRs is observed. The intensity of the SX line is found to be smaller than that of the nanosized counterpart and has been attributed to the surface–volume ratio effects. The excitation-power-dependent results of FX line at low and high power regimes show quite close values corresponding to, respectively, p=2 and p=1 limits of the theoretical power law expression I～L^p and larger deviations for the D⁰X, SX and DBX lines. The temperature-dependent measurements confirmed the presence of eA⁰ line showing kT/2 influence to the position of eA⁰ emission line in comparison with FX. FX emissions persist up to 300 K and together with the dominant eA⁰ emission govern the line shape of the NBE emission range, while D⁰X and SX lines are quenched completely at 150 K.