Search

Zinc oxide (ZnO) samples in the form of hexagonal-based bipyramids and particles of nanometer dimensions were synthesized through solvothermal route and characterized by x-ray diffraction and transmission electron microscopy. Positron annihilation experiments were performed to study the structural defects such as vacancies and surfaces in these nanosystems. From coincidence Doppler broadening measurements, the positron trapping sites were identified as Zn vacancies or Zn–O–Zn trivacancy clusters. The positron lifetimes, their relative intensities, and the Doppler broadened lineshape parameter S all showed characteristic changes across the nanobipyramid size corresponding to the thermal diffusion length of positrons. In large nanobipyramids, vacancies within the crystallites also trapped positrons and the effects of agglomeration of such vacancies due to increased temperatures of synthesis were reflected in the variation of the annihilation parameters with their base diameters. The sizes of the nanoparticles used were all in the limit of thermal diffusion length of positrons and the annihilation characteristics were in accordance with the decreasing contribution from surfaces with increasing particle size. [ABSTRACT FROM AUTHOR]

Nanostructures of ZnS, both particles and rods, were synthesized through solvothermal processes and characterized by x-ray diffraction and high resolution transmission electron microscopy. Positron lifetime and Doppler broadening measurements were made to study the features related to the defect nanostructures present in the samples. The nanocrystalline grain surfaces and interfaces, which trapped significant fractions of positrons, gradually disappeared during grain growth, as indicated by the decreasing fraction of orthopositronium atoms. The crystal vacancies present within the grains also trapped positrons. These vacancies further agglomerated into clusters during the thermal treatment given to effect grain growth. The positron lifetime was remarkably large at extremely small grain sizes (∼1.5 nm) and this was attributed to the occurrence of quantum confinement effects, as verified through optical absorption measurements. Positron lifetimes in ZnS nanorods increased with increasing content of cubic phase in the samples and this observation is assigned to the annihilation of positrons in sites with increased cubic unit cell volume. The Doppler broadened spectra also indicated qualitative changes consistent with these observations. [ABSTRACT FROM AUTHOR]

Phase pure rutile SnO2 nanoparticles and nanorods were prepared by solvothermal technique. The diameters of the nanoparticles were in the range of 5–10 nm while the length and width of the nanorods were within 100–200 nm and ∼50 nm, respectively. The crystal structure, morphology, and sizes of the SnO2 nanocrystals were determined by x-ray diffraction and transmission electron microscopy. Optical properties of the products were explored by Fourier transform infrared spectroscopy, optical absorption, photoluminescence, and Raman studies. Size dependent blueshift of the optical band gap of the nanocrystals was observed due to quantum confinement. The photoluminescence spectra showed broad UV emission. The influence of the particle size and the morphology of the SnO2 nanocrystals on the vibration band in the Raman scattering was studied. [ABSTRACT FROM AUTHOR]