[Time resolved photoluminescence of ZnO nanoparticles under low photon energy excitation]

Time-resolved photoluminescence (TRPL) and time-integrated photoluminescence (TIPL)measurements were performed at ZnO nanoparticles with grain size from 17 nm to 110 nm prepared by homogenous precipitation method (HPM). With a quasi-continuous picoseconds laser excitation at a low photon energy of 2. 33 eV (wavelength 532 nm)from a Vanguard 2000-HM532 laser, ultrafast emission with the wavelength ranging from 550 to 1000 nm was detected. It was observed that the wavelength position of all the peaks of the samples regularly redshifted with the increase in the particle sizes. The PL spectra were fitted with Gaussian curves, in which a good fitting consisting of four Gaussian peaks was obtained. The Gaussian analysis indicates that the redshift in PL is due to the increased relative magnitude of the Gaussian combination in the low energy region. The analysis of the experiment suggests the existence of surface states in the bad gap near Fermi level of ZnO. At the same time, a strong dependence of the PL lifetime on grain size was found. Upon the increase in particle size, the decay of the emission shortened much, in which the PL intensity reveals bi-exponential decay for samples with small diameters while no identified decay of emission was observed within 2 nanoseconds for large particles. The dependence of the decay time on the grain size of nanoparticles can be suitably attributed to the surface related mechanism. Combined analysis of the lifetime dependence on particle size validates the model of surface states recombination in nanostructures due to large surface-to-volume ratio.