Structured green luminescence band of ZnO nanorods -- the optical emissions coming from the sample surface

The optical emissions of O-annealed ZnO nanorods (ZnO NRs) before and after surface passivation are studied at temperatures of 295 to 17 K in terms of photoluminescence (PL) spectra, PL excitation spectra, and PL quantum efficiency. Because the ZnO NR samples maintain the surface free of mechanical damages during the sample preparation, unambiguous evidence is obtained to substantiate that the structured green luminescence (GL-S) bands come from the optical emissions on the sample surfaces. We suggest that the GL-S bands are excited by the excitons diffused to the sample surface and thus substantially reduced in the PL spectra of ZnO NRs after surface passivation. PL fitting shows that many kinds of surface states may have contributions to the optical emissions in the GL-S bands, leading to the variation of GL-S bands with annealing temperature and ambient environment.