Correlations among size, defects, and photoluminescence in ZnO nanoparticles

Abstract

We studied the correlations among size, defects, and photoluminescence emissions in ZnO nanoparticles of sizes ranging from 25 to 73 nm. The impurities and defects were characterized by Fourier-transform infrared spectroscopy and Raman spectroscopy. Particles of larger size revealed fewer surface impurities and enhanced E 2 mode of hexagonal ZnO crystals, while the oxygen vacancy centers did not vary significantly with particle size. A simultaneous increase of excitonic luminescence and defect luminescence intensities with the increase of particle size is shown, indicating both emissions are subjected to nonradiative quenching by near surface defects. The study on the size-dependent green luminescence in our samples suggests that the emission might be a bulk property instead of having a surface origin in nanostructured ZnO. Two different radiative recombination processes are involved in the excitonic emission of ZnO. While the slow decay component ͑370 ps͒ did not depend on particle size, the fast component varied from 56 to 96 ps. We attribute the slow component to free exciton recombination, while the fast component is attributed to near surface exciton recombination.

Synthesis, structural and optical properties of pure ZnO and Co doped ZnO nanoparticles prepared by the co-precipitation method

“…The bands are observed at 881 and 880 cm -1 due to the C-H outof plane bending of the polyethylene glycol group for the pure ZnO, and Co doped ZnO NPs samples. The most intense broad absorption band at $ 438 cm -1 is attributed to the stretching of vibration of ZnO [34]. The Zn-O stretching bands are observed at 438 and 427 cm -1 for the respective pure ZnO and Co doped ZnO NPs samples.…”

Phonon dynamics of Zn(Mg,Cd)O alloy nanostructures and their phase segregation

“…21 Higher wave number peaks correspond to vibration modes of various impurities such as hydroxyl, carboxylate, and alkane in the materials. 22 In this work, within the displayed frequency range, phase segregated ZnMgO and ZnCdO alloy nanostructures show two impurity bands near 600-1000 cm −1 due to C-OH bond. The impurity peaks are present in almost all the previous reports of ZnO nanostructures grown by chemical as well as physical deposition method.…”

“…The impurity peaks are present in almost all the previous reports of ZnO nanostructures grown by chemical as well as physical deposition method. 22,23 The sources of these impurities are the surface adsorbed organic precursors employed during the synthesis process. The effect of these impurities is prominent for smaller nanoparticles due to high surface to volume ratio and the impurity peaks gradually vanish for bigger particles.…”

Surface electronic properties of ZnO nanoparticles

“…Recent theoretical calculations and existing models of methanol synthesis using ZnO nanocatalysts suggest that oxygen vacancies on the ZnO surface could act as active sites for CO and CO 2 chemisorption during methanol synthesis [2,3]. In addition, surface defects and impurities have been reported to quench the excitonic luminescence of ZnO [4,5] since they can act as trapping centres to compete with the near-band-edge (NBE) emission. These studies indicate that key properties of ZnO nanostructures may depend critically on their surfaces.…”