低能量光子激发下纳米ZnO的超快时间分辨光谱特性

Time Resolved Photoluminescence of ZnO Nanoparticles under Low Photon Energy Excitation

室温下获得了均匀沉积法制备的纳米ZnO颗粒在低能量2.33eV光子(532nm)的准连续皮秒脉冲激发下的频谱范围从550～1000nm的时间分辨光谱和时间积分光谱。随着样品粒径的增加,发光带谱带峰值出现规律性的红移。通过高斯拟合对光谱结构解迭发现,这种规律性的红移是由于低能端区域的高斯组分的相对比例增加所致。时间分辨光谱中超快发射的荧光衰减寿命(皮秒量级)也出现随着样品粒径的增大而相应变长。源于材料尺寸、纳米颗粒大的比表面积所引起的表面能级可以较好的解释此范围的超快发射特性。

Time-resolved photohtminescenee （TRPL）and time-integrated photoluminescenee （TIPL）measurements were performed at ZnO nanopartieles with grain size from 17 nm to 110 nrn prepared by homogenous precipitation method （HPM）. With a quasi-continuous pieoseeonds laser exeitation 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 1 000 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 whieh 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 inerease 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 dependenee of the deeay time on the grain size of nanoparticles can be suitably attributed to the surface related mechanism. Combined analysis of the lifetime dependenee on particle size validates the model of surface states recombination in nanostructures due to large surface-to-volume ratio.