No abstract

Nanocrystalline europium doped yttria was synthesized using a chemical vapor technique. The powder was characterized by x-ray diffraction, transmission electron microscopy, and ultraviolet spectroscopy. For the first time it was possible to obtain single phase Y2O3:Eu nanoparticles crystallized in the cubic structure with an average particle size of only 10 nm. The reflection, excitation, and emission spectra were studied. The nanoparticles show blue shifted absorption bands with respect to coarse grained material.

Nanocrystalline materials, which have been proposed to represent a new solid state structure, are investigated by Mössbauer spectroscopy. Nanocrystalline materials are polycrystals with a crystal size of typically 1–10 nm. These materials consist of two components of comparable volume fractions: a crystalline component and an interfacial component, formed by the atoms located either in the crystals or in the interfacial regions between them. As the atomic configurations of both components are different, two kinds of Mössbauer spectra are expected. Iron nanocrystalline material is found to exhibit a two-component Mössbauer spectrum, consisting of a crystalline component and a second one with different Mössbauer parameters. The Mössbauer parameters of the second subspectrum are consistent with the model of the interfacial component of a nanocrystalline material.

Metal nanoparticles can display a unique behavior when deposited on substrates with a significantly lower surface energy. Co nanoparticles in the 10 nm size regime burrow into clean Cu(100) and Ag(100) substrates when deposited at 600 K and also assume the substrate orientation. Deposition at room temperature fails to show either burrowing or reorientation. Crucial in understanding these results are the capillary forces and surface tension associated with a nanoparticle: they must be high enough to drive atoms away from underneath the cluster.

The luminescence of nanocrystalline cubic yttria has been measured for 300 and 80 K. A blueshift and a broadening of the absorption edge, both depending on the particle size were found. The energy of the radiation emitted due to the recombination of a bound exciton does not depend on particle size. The observations are not caused by quantum or phonon confinement. Using a quantum mechanical configurational coordinate diagram an explanation can be given. Hydrostatic pressure increasing the phonon energy of the excited state of the bound exciton, or an increase of the exciton-phonon coupling may generate the size dependent optical properties of nanocrystalline yttria.