V. V. Seminko

Spectroscopic Properties of Nanoceria Allowing Visualization of Its Antioxidant Action

Two distinct luminescence centers were revealed in ceria nanocrystals: first one – Ce3+ ions with 5d-4f luminescence at 390 nm, and second one – Ce4+–O2− complexes showing charge transfer (CT) luminescence at 630 nm. Intensity of Ce3+ luminescence depends directly on the concentration of oxygen vacancies in nanoceria and can be varied by means of change of both heat treatment atmosphere from oxidizing to reducing and the size of nanocrystal. Ce3+ luminescence can be used for visualization of the processes of interaction between ceria nanoparticles and reactive oxygen species using relative intensity of Ce3+ band as a measure of Ce4+/Ce3+ ratio during oxidation reaction.

Quantum splitting in praseodymium-doped lanthanum aluminum dimetaborate crystals at X-ray excitation

ABSTRACT Mixed lanthanum-praseodymium aluminum dimetaborate and aluminum orthoborate crystals were grown. The processes of excitation energy relaxation for the crystals were compared. For praseodymium-doped lanthanum aluminum dimetaborate crystals, the effect of quantum splitting was predicted and observed experimentally: under X-ray excitation emission of two photons at 1S0 → 1D2 and 1D2 → 3H4 transitions was shown. The possibility of quantum splitting for aluminum dimetaborates can be explained by the relatively high cation-ligand distances for this lattice.

Processes of excitation energy transport in EuPO4 and EuP3O9 nanocrystals

Processes of excitation energy transport were investigated for EuPO4 and EuP3O9 nanocrystals, in which Eu3+ ions form three- and one-dimensional subsystems, respectively. It was determined that for EuPO4 nanocrystals 5D0 luminescence quenching manifested itself even at low temperatures due to effective phonon-assisted energy migration. At the same time, for EuP3O9 nanocrystals no influence of energy migration on the luminescence quenching was observed in the whole temperature range from 10 to 293 K that can be ascribed to modification of phonon spectra for EuP3O9 nanocrystals as compared to their bulk counterparts.

Development of Nanocomposite Alpha-Detectors Based on Silica Matrices and Organic Scintillators

New materials based on highly-porous SiO2 matrices with incorporated molecules of organic scintillators (POPOP, PPO) were fabricated. The composite scintillators are characterized by decay times in the nanosecond range, a high transparency in the visible region (about 70 %) and an intensive photoluminescence. At excitation by 5.46 MeV alpha particles the light yield of SiO2 (POPOP, PPO) was equal to 4,400–5,100 photon/MeV, the resolution was 27–32 %.

Energy migration processes in phosphate nanocrystals: Size and dimensionality dependence

Peculiarities of electronic excitation energy migration in phosphate nanocrystals with three-dimensional (EuPO4) and one-dimensional (EuP3O9) arrangement of regular ions under variation of concentration of both energy traps (Nd3+ ions) and scattering centres (La3+ ions) are discussed in the paper. Processes of energy migration in both EuPO4 and EuP3O9 nanocrystals are phonon-assisted ones due to absence of resonance between energy levels of adjacent Eu3+ ions. The distance of energy migration for both EuPO4 and EuP3O9 nanocrystals was found to be equal to tens of nanometres, however, for EuPO4 nanocrystals energy migration leads to stronger quenching of Eu3+ luminescence. For 10 nm EuPO4 nanocrystals sufficient hampering of energy migration is observed due to the depletion of the low-energy phonon spectrum as a result of the phonon confinement effect.

Low-temperature spectroscopy of optical centers in cerium-yttrium (Ce1-xYxO2-x/2) and cerium-zirconium (Ce1-xZrxO2) oxides

The results of studying the impact of isovalent (Zr4+) and non-isovalent (Y3+) impurity ions on the formation of oxygen vacancies in the lattice of cerium dioxide nanocrystals. It is shown that the oxygen vacancy content increases significantly with the introduction of both the Y3+ and Zr4+ ions, and that the oxygen vacancies play a part in the formation of two types of optical centers: Ce3+-V··-Ce3+ and F+-centers. The ratio of these optical centers can be manipulated by varying the concentration of impurity ions and the atmosphere of high-temperature nanocrystal processing.

Influence of Zr-doping on the luminescence properties of ceria nanocrystals

In the paper the influence of partial substitution of cerium ions by zirconium ones on the processes of the formation of oxygen vacancies in mixed ceria-zirconia (CeO2-ZrO2) nanocrystals was investigated using conventional spectroscopic techniques. It was shown that the intensity of Ce3+ luminescence band which depends on the concentration of oxygen vacancies in ceria nanocrystal increases strongly at incorporation of Zr4+ ions with concentrations up to 20 at.% confirming the supposition that replacing of the part of Ce4+ ions by Zr4+ ones provokes formation of additional oxygen vacancies in ceria structure.

Temperature-dependent segregation of Pr{sup 3+} impurity ions in Y{sub 2}SiO{sub 5}:Pr{sup 3+} and YPO{sub 4}:Pr{sup 3+} nanocrystals

Stationary and time-resolved spectroscopic methods are used to show that the impurity ions in Y2SiO5:Pr3+ and YPO4:Pr3+ nanocrystals are distributed nonuniformly. This nonuniform distribution is found to be caused by the temperature-dependent segregation of Pr3+ ions near the surface of a nanocrystal. The motion of the activator ions from the bulk of a nanocrystal to the near-surface layer is traced when the activator concentration and the heat-treatment parameters are varied over wide ranges, and the main parameters of this effect (impurity redistribution intensity and time, diffusion coefficient) are estimated.