Search

Effect of ambient gases present in air on photoluminescence (PL) and electrical conductivity of films prepared using ZnO nanoparticles (NPs) have been investigated. It is observed that NPs of size below 20 nm kept inside a chamber exhibit complete reduction in their visible PL when oxygen partial pressure of the surrounding gases is decreased by evacuation. However, the visible PL from ZnO NPs is insensitive to other major gases present in the ambient air. The rate of change of PL intensity with pressure is inversely proportional to the ambient air pressure and increases when particle size decreases due to the enhanced surface to volume ratio. On the other hand, an assembly of ZnO NPs behaves as a complete insulator in the presence of dry air and its major components like N2, O2, and CO2. Electrical conduction having resistivity ∼102-103 Ωm is observed in the presence of humid air. The depletion layer formed at the NP surface after acquiring donor electrons of ZnO by the adsorbed oxygen has been found to control the visible PL and increases the contact potential barrier between the NPs which in turn enhances the resistance of the film. [ABSTRACT FROM AUTHOR]

The present article describes the size induced changes in the structural arrangement of intrinsic defects present in chemically synthesized ZnO nanoparticles of various sizes. Routine x-ray diffraction and transmission electron microscopy have been performed to determine the shapes and sizes of the nanocrystalline ZnO samples. Detailed studies using positron annihilation spectroscopy reveals the presence of zinc vacancy. Whereas analysis of photoluminescence results predict the signature of charged oxygen vacancies. The size induced changes in positron parameters as well as the photoluminescence properties, has shown contrasting or nonmonotonous trends as size varies from 4 to 85 nm. Small spherical particles below a critical size (∼23 nm) receive more positive surface charge due to the higher occupancy of the doubly charge oxygen vacancy as compared to the bigger nanostructures where singly charged oxygen vacancy predominates. This electronic alteration has been seen to trigger yet another interesting phenomenon, described as positron confinement inside nanoparticles. Finally, based on all the results, a model of the structural arrangement of the intrinsic defects in the present samples has been reconciled. [ABSTRACT FROM AUTHOR]

In this paper, we report the optical properties of magnesium (Mg)-substituted zinc oxide (ZnO) ( Zn1 - x Mg_x O) nanoparticles (diameter ≤ 20 nm). The ZnO and alloyed nanoparticles were synthesized by chemical route and have Wurzite structure. The maximum Mg substitution reported here is ≈7.5%. This leads to a blue shift of the absorption spectra by 0.5 eV as well as a blue shift of the near-band edge photo luminescence. Our analysis shows that small change in average size of the nanoparticles, as described in this paper, does not have much effect on the observed blue shift of the absorption spectra due to alloying. At the maximum concentration of Mg incorporated into ZnO, the material does not show phase segregation and retains the excitonic nature of the fundamental-absorption edge. [ABSTRACT FROM PUBLISHER]

We report the synthesis and optical properties of compact and aligned ZnO nanorod arrays (dia, ~ 50-200 nm) grown on a glass substrate with varying seed particle density. The suspension of ZnO nanoparticles (size, ~ 15 nm) of various concentrations are used as seed layer for the growth of nanorod arrays via self-assembly of ZnO from solution. We studied the effect of various growth parameters (such as seeding density, microstructure of the seed layer) as well as the growth time on the growth and alignment of the nanorods. We find that the growth, areal density and alignment of the nanorods depend on the density of seed particles which can be controlled. It is observed that there is a critical density of the seed particles at which nanorod arrays show maximum preferred orientation along [002] direction. The minimum and maximum radius of the aligned nanorods synthesized by this method lie in the range 50-220 nm which depend on the seeding density and time of growth. These nanorods have a bandgap of 3·3 eV as in the case of bulk crystals and show emission in the UV region of the spectrum (~ 400 nm) due to excitonic recombination and defect related emission in the visible region. [ABSTRACT FROM AUTHOR]

We report that the visible photoluminescence from ZnO nanoparticles can be controlled by changing the ionic or polar nature of the medium in which the nanoparticles are dispersed. We find that the presence of sufficient amount of electrolyte can even quench the emission. An explanation is proposed based on the surface charge of the ZnO nanoparticles, which controls the band bending in the depletion layer at the surface. The band bending in turn, decides the predominant nature of the visible emission. The explanation is validated by establishing a direct correlation between the visible emission and the zeta potential. [ABSTRACT FROM AUTHOR]