A.I. Popov

Radiation-induced point defects in simple oxides

We present a survey of recent theoretical studies of radiation-induced point defects in simple oxides with emphasis on highly ionic MgO, partly-covalent corundum (Al2O3) and ferroelectric KNbO3. The atomic and electronic structure of the electronic (F a and F centers) and hole centers, as well as interstitial atoms therein are discussed in light of the available experimental data. Results for defect diAusion and photo-stimulated F a fi F center conversion are also ana

LaPO4:Ce,Tb and YVO4:Eu nanophosphors: Luminescence studies in the vacuum ultraviolet spectral range

Comparative analysis of the luminescent properties of nanocrystalline LaPO4:Ce,Tb and YVO4:Eu luminescent materials with macrocrystalline analogues, commercially produced by Philips, has been performed under excitation by pulsed vacuum ultraviolet (VUV) synchrotron radiation, ranging from 3.7-40 eV. Special attention was paid to VUV spectral range, which is not reachable with commonly used lamp and laser sources. Our results clearly show distinct difference in the excitation spectra for nano- and macrocrystalline samples, especially at energies, when the spatial separation of electron-hole pairs is comparable with sizes of nanoparticles. Differences in the region of multiplication of the electronic excitations are also demonstrated and discussed.

Structural and electronic properties of single-walled AlN nanotubes of different chiralities and sizes

Four models of single-walled AlN nanotubes (NTs), which possess (i) two different chiralities (armchair or zigzag type) and (ii) two different uniform diameters for both types of NTs (1 or 6 nm) have been constructed, in order to analyse the dependence of their properties on both morphology and thickness. Periodic one-dimensional (1D) DFT calculations performed on these models have allowed us to analyse how the chirality and curvature of the NT change its properties as compared to both AlN bulk with either wurtzite or zinc-blende structures and their densely packed surfaces. We have found that the larger the diameter of the AlN NT, the smaller the width of its bandgap, the strengths of its bonds and the charge separations in them .T his confirms the recent experimental finding of the possibility to adjust electronic properties in ultimate nanoscale optoelectronic devices produced from AlN and other group III nitrides. (Some figures in this article are in colour only in the electronic version)

Vibrational properties of LaPO4 nanoparticles in mid- and far-infrared domain

Nanopowders of LaPO4 have been grown by sedimentation-micellar method. As-prepared LaPO4 nanoparticles with the average grain size of about 8 nm have a single-phase hydrated hexagonal structure. After thermal annealing at 600 and 800 °C, the average size of nanoparticles increases up to ∼35 and ∼50 nm, respectively, and the structure transforms into single-phase monoclinic. IR spectra of LaPO4 nanoparticles of different size were investigated in the wide range of wavenumbers from 130 to 5000 cm−1 in the 20–300 K temperature region. Differences between IR spectra of the bulk material and nanoparticles as well as the temperature behavior of the vibrational properties are discussed.

Semi-empirical simulations of the electron centers in MgO crystal

Abstract Semi-empirical quantum chemical simulations of 125-atom clusters have been undertaken to obtain the self-consistent atomic and electronic structure of the two basic electron defects in MgO crystals — F+ and F centers (one and two electrons trapped by an O vacancy). The calculated absorption and luminescence energies agree well with the experimental data, the excited states of both defects are found to be essentially delocalized over nearest-neighbour cations. The mechanism of the F+ → F photoconversion is discussed.

Electronic excitations in ZnWO4 and ZnxNi1−xWO4 (x = 0.1 − 0.9) using VUV synchrotron radiation

The photoluminescence spectra and luminescence excitation spectra of pure microcrystalline and nano-sized ZnWO4 as well as the ZnxNi1−xWO4 solid solutions were studied using vacuum ultraviolet (VUV) synchrotron radiation. The samples were also characterized by x-ray powder diffraction. We found that: (i) the shape of the photoluminescence band at 2.5 eV, being due to radiative electron transitions within the [WO6]6− anions, becomes modulated by the optical absorption of Ni2+ ions in the ZnxNi1−xWO4 solid solutions; and (ii) no significant change in the excitation spectra of Zn0.9Ni0.1WO4 is observed compared to pure ZnWO4. At the same time, a shift of the excitonic bands to smaller energies and a set of peaks, attributed to the one-electron transitions from the top of the valence band to quasi-localized states, were observed in the excitation spectrum of nano-sized ZnWO4.

Tracks induced in TeO2 by heavy ions at low velocities

On the basis of its thermal properties, TeO2 crystal was selected as an insulator with low threshold electronic stopping power for track formation Set. The crystals were irradiated by S, Zn, Mo, Kr, Te and Pb ions and the optical absorption and track formation were studied. Comparison is made with the published results on LiNbO3 ,Y 3Fe5O12 and SiO2 quartz. Good quantitative agreement is found with the predictions of the thermal spike model of Szenes with respect to Set and the variation of the track size with the electronic stopping power Se. It is shown that TeO2 has a high eAciency g at low ion velocities, which is a characteristic feature of the damage cross-section velocity eAect. ” 2000 Elsevier Science B.V. All rights reserved.

Quantum chemical simulations of the optical properties and diffusion of electron centres in mgo crystals

Semiempirical quantum chemical simulations have been undertaken to obtain the self-consistent atomic and electronic structure of the two basic electron defects in MgO crystals: F+ and F centres (one and two electrons trapped by an 0 vacancy, V,). The calculated absorption and luminescence energies agree well with the experimental data; the excited states of both defects are found to be essentially delocalised over nearest-neighbour cations. The activation energy for diffusion is found to increase monotonically in a series V, --f F+ --f F centre (2.50 eV, 2.72 eV and 3.13 eV, respectively).

Influence of F centres on structural and electronic properties of AlN single-walled nanotubes

We analyse the influence of uncharged N vacancies (neutral F centres), created either under conditions of AlN nanotube growth or by its soft irradiation, on the atomic and electronic structure. Periodic one-dimensional (1D) density functional theory (DFT) calculations on models of defective single-walled nanotubes (SW NTs) allow us to analyse how NT chirality and concentration of F centres change their properties compared to the corresponding defect-free nanotubes. We have simulated reconstruction around periodically repeated F centres on 1 nm AlN SW NTs with armchair- and zigzag-type chiralities. To achieve the limit of an isolated vacancy for both chiralities, we have considered different inter-defect distances repeated along the axes of these nanotubes. For dF‐F 20 ˚ A, the interaction between defects is found to be negligible, since energy dispersion does not exceed 0.02 eV. We also analyse the influence of F centres on the energy cost required to wrap up AlN graphitic nanosheets (NSs) of both chiralities into the corresponding 1 nm thick SW NTs. The electronic properties of defective NS and NTs of both chiralities have been compared with those for defective AlN bulk three-dimensional (3D) structures of wurtzite and zinc-blend. The presence of N vacancies in various aluminium nitride structures (including SW NTs) results in the appearance of defect energy levels in the band gap with the prevailing contribution from 3s and 3p atomic orbitals of the nearest Al atoms. We have found that the larger the concentration of F centres is, the smaller the maximal energy gap between defect levels is, i.e. an increase 7 Author to whom any correspondence should be addressed.

Characterization of aluminium nitride nanostructures by XANES and FTIR spectroscopies with synchrotron radiation

We investigated different AlN nano-systems using spectroscopic methods. Experiments were performed at the Synchrotron Radiation Facility of the Laboratori Nazionali di Frascati using both XANES (x-ray absorption near edge spectroscopy) and FTIR (Fourier transform infrared spectroscopy) techniques in order to investigate materials with both interesting tribological and electronic properties. Comparisons have been performed between measurements by standard x-ray diffraction (XRD) and x-ray absorption (XRS) at the K-edge of Al, a spectroscopy method sensitive to the local order and correlated to the local and empty density of states of this wide band-gap semiconductor. Preliminary XAS simulations at the Al K edge are also presented. Correlations between XRD and XAS have been drawn, since x-ray absorption reveals structural information complementary to that addressed by x-ray diffraction. Moreover, a comparison has been performed by infrared (IR) absorption both in the mid- and in the far-IR ranges between different AlN forms: namely, powders, nanoparticles and nanotubes. Data clearly show changes connected with the electronic properties and the optical phonon modes of AlN nano-systems.