Stanovyi O. Energy spectrum of traps in oxide and chalcogenide nanostructures

Thesis is dedicated to the study and development of models for describing the regularities in energy spectrum of traps (point defects) in bulk and microcrystals with an ideal and deformed crystal lattice and nanoparticles. The investigations were carried out by the methods of thermoluminescence, photoluminescence, Raman scattering, and also by the methods of mathematical modeling. Alkaline halide crystals have been used as a good model material for studying regularities in the energy spectrum of traps, mainly due to the simple crystalline structure and the nature of chemical bonds. It is determined that the activation energies of traps in alkaline halide crystals form the oscillatory series with the vibrational quantum. The energy of the quantum in oscillator regularity corresponds to the local vibration mode of the halide molecule (H-center). The mathematical modeling of the contours of the thermoluminescence peaks makes it possible to decompose the thermoluminescence curve on the components. Activation energy of traps is obtained as parameters of approximation. The combination of the initial rise method and the method of peaks approximation allows to expand the range of materials for which the activation energy of traps can be determined with high accuracy. The comparative investigations of the low-temperature thermoluminescence of micro-, single crystals and nanoparticles of CdSe and CdS has been made for the first time. The activation energy of most traps is slightly changing (within a few hundredths eV) from single crystal to microcrystalline specimens and further to nanoparticles. For example, the activation energies of traps corresponding to the high-temperature peaks in CdSe coincide within ± 0.02 eV, and the positions of the maxima change slightly for all samples of CdSe. Accordingly, the difference in depth of traps (energy levels of points defects) for nanoparticles was not detected in comparison with bulk crystals. The study of the thermoluminescence of nanosized powders ZrO2:Y2O3, with different percentages of Y2O3 (used to stabilize the tetragonal or cubic structure of zirconium dioxide) was performed. It was found that in ZrO2:Y2O3 crystall lattice disorder and the inhomogeneous distribution of oxygen vacancies cause the activation energies variation and broadening of the thermoluminescence peaks. The full width at half maximum of the activation energy distribution in ZrO2:Y2O3 is from 0.05 to 0.07 eV for the samples with various Y2O3 concentrations. The equation describing the contour of the broaden thermoluminescence peaks was proposed. A model of thermoluminescence of ZrO2:Y2O3 is proposed. In this model O–-centers act as traps and the centers of recombination are F+ or T-centers: the hole releases from the trap and recombines at the electronic center. Such mechanism is confirmed by the results of comparison of the temperature dependence of the photoluminescence spectra that demonstrate their different behavior before and after the accumulation of lightsum (vacant and filled traps, respectively). The traps empting and the change in the charge state of point defects (in particular, F+ and T-centers) affect on radiative and non-radiative recombination process. Consequently, the change of the intensity of photoluminescence at a constant intensity of excitation is observed.