Manganese-doped nickel oxide (Ni1-xMnxO) nanoparticulate samples with x in the range 0 (undoped sample) to 0.35 were synthesized by sol-gel method involving chemical reactions between the solutions of nickel nitrate hexahydrate and manganese acetate tetrahydrate. The nanocrystallites obtained after annealing of the precipitates for different durations were characterized by X-ray diffraction and high resolution transmission electron microscopy. The samples showed high degree of purity with no secondary phase up to 35 at.% (x = 0.35) of Mn-doping. At the initial doping concentrations, the crystallite sizes increased due to vacancy type defects being recombined with some of the doped Mn2+ ions. However, substitution-induced strain soon overtook the crystallite dynamics and the sizes rapidly started reducing again as an indirect consequence of the necessity to accommodate majority of the doped cations on the surfaces of the nanocrystallites. There was conspicuous changes in the lattice parameter too which could again be attributed to the strain and charge effects. The average sizes of the crystallites were obtained in the range 5.5 nm to 13.1 nm for the different samples. UV-Vis absorption studies indicated the formation of excitonic states in NiO on Mn-doping. The band gap energy (Eg) derived from the optical absorption spectra showed a continuous increase with increase of Mn-doping of the samples. Positron lifetime and Doppler broadening spectroscopic studies were carried out on those samples to characterize the vacancy type defects and defect clusters/complexes. There were also indications to suggest positron annihilation at the crystallite surfaces owing to their sizes of nanometer order. Positron lifetimes decreased upon increase of Mn-doping. The coincidence Doppler broadened ratio curves indicated definite shifts of the prominent oxygen-electron-annihilation peak and the variation of the lineshape parameter S also indicated clearly the effects of Mn-doping.

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Ultrafine nickel oxide nanoparticles were prepared by sol-gel method using a solution of nickel nitrate hexahydrate and ammonium hydroxide and subsequently annealed in air at different temperatures in the range 200-275 C for different hours to vary the particle sizes. They were characterised for phase, purity, structure and sizes by X-ray diffraction and high-resolution transmission electron microscopy. They were found polycrystalline in nature and possessed face-centred cubic (NaCl-type) structure with lattice parameter varying with annealing temperature. The ultrafine structure clearly revealed the formation of hexagons with average diameter about 5 nm. Ultraviolet-visible absorption spectroscopy was carried out to study the optical properties and for the estimation of the bandgap. The nanoparticles exhibited weak and strong quantum confinement in the size ranges of 10.47-8.47 and 6.57-5 nm. The samples were then investigated through positron annihilation spectroscopy to characterise and closely monitor the evolution of vacancy-type defects and defect clusters during particle growth in the samples. The positron lifetime drastically increased at very low particle sizes, supporting the confinement effects demonstrated by optical absorption studies. The results from coincidence Doppler broadening measurements were consistent and indicated changes also in the electron momentum distribution during the occurrence of these finite size effects in the nanoparticles.

Calcium-ion substitution to different concentrations in the normally insulating but wide band gap semiconductor magnesium oxide nanocrystallites was successfully achieved through a sol-gel route and the synthesized samples were characterized by X-ray diffraction and transmission electron microscopy. The substitution of Mg2+ by Ca2+ gave rise to distinct variation in the nanocrystallite sizes and lattice constants. The band gap energies (Eg = 5.04 to 5.50 eV) obtained from ultra-violet and visible absorption spectroscopy hinted to the formation of Positronium Hydrogen (Ps-H) complex within nanocrystallites. Excitonic energy levels reduced the band gap after doping. In the samples of larger nanocrystallites, strong evidences to the presence of very large vacancy clusters were observed. The coincidence Doppler broadening measurements also indirectly supported this interpretation. The lineshape and wings parameters also supported the type of defects as evidenced from lifetime measurements. The results appeared significant from the context of choosing doping as an efficient way of molding the properties of semiconductor oxide nanocrystals as concomitant lattice parameter variations and the evolutions of vacancy type defects can significantly alter the expected physical changes towards other directions.

a b s t r a c tNano-crystalline ZnO has been studied with perturbed angular correlation using 111m Cd, implanted at ISOLDE/CERN and X-ray diffraction using Rietveld analysis. The data show a gradual increase in the crystal size and stress for a sample annealed at 600°C, and reaching nearly properties of standard ZnO with tempering at 1000°C. The perturbed angular correlation data show a broad frequency distribution at low annealing temperatures and small particle sizes, whereas at high annealing temperature and larger crystal sizes, results similar to bulk ZnO have been obtained. The ZnO nano-crystalline samples were initially prepared through a wet chemical route, have been examined by Fourier Transform Infrared Spectroscopy (FT-IR) and chemical purity has been confirmed with Energy Dispersive X-ray (EDAX) analysis as well as Transmission Electron Microscopy (TEM).