D. A. Zatsepin

Sn-loss effect in a Sn-implanted a-SiO2 host-matrix after thermal annealing: A combined XPS, PL, and DFT study

Abstract Amorphous a-SiO2 host-matrices were implanted with Sn-ions with and without posterior thermal tempering at 900xa0°C for 1xa0h in ambient air. X-ray photoelectron spectroscopy analysis (XPS core-levels, XPS valence band mapping), photoluminescence (PL) probing, and density functional calculations (DFT) were employed to enable a detailed electronic structure characterization of these samples. It was experimentally established that the process of Sn-embedding into the a-SiO2 host occurs following two dissimilar trends: the Sn4+xa0→xa0Si4+ substitution in a-SiO2:Sn (without tempering), and Sn-metal clustering as interstitials in a-SiO2:Sn (900xa0°C, 1xa0h). Both trends were modeled using calculated formation energies and partial densities of states (PDOS) as well as valence band (VB) simulations, which yielded evidence that substitutional defect generation occurs with the help of ion-implantation stimulated translocation of the host-atoms from their stoichiometric positions to the interstitial void. Experimental and theoretical data obtained coincide in terms of the reported Sn-loss effect in a-SiO2:Sn (900xa0°C, 1xa0h) due to thermally-induced electronic host-structure re-arraignment, which manifests as backward host-atoms translocation into stoichiometric positions and the posterior formation of Sn-metal clusters.

Luminescence of modified nonbridging oxygen hole centers in silica and alkali silicate glasses

This paper reports on the results of the investigation of the cathodoluminescence spectra of silica and alkali silicate glasses upon excitation with a pulsed electron beam (energy, 180 keV; current density, 700 A/cm2; pulse duration, 2 ns). The luminescence band observed in the energy range 2.4–2.6 eV is assigned to modified structural defects of the ≡Si-O·/Me+ type. These defects are revealed under high-density electronic excitation and, unlike the known L centers in alkali silicate glasses, are interpreted as a variety of nonbridging oxygen hole centers (defects of the dangling bond type) subjected to a disturbing action of the nearest neighbor alkali metal cations. The cathodoluminescence of similar centers is observed in neutron-irradiated silica glasses with lithium impurities; alkali silicate glasses with Li, Na, and K cations; and glasses in the two-alkali Na-K systems. It is established that the energy of the radiative transition of a modified nonbridging oxygen hole center, namely, ≡Si-O·/Me+, depends on the alkali cation type.

Electronic structure, charge transfer, and intrinsic luminescence of gadolinium oxide nanoparticles: Experiment and theory

Abstract The cubic (c) and monoclinic (m) polymorphs of Gd2O3 were studied using the combined analysis of several materials science techniques – X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) spectroscopy. Density functional theory (DFT) based calculations for the samples under study were performed as well. The cubic phase of gadolinium oxide (c-Gd2O3) synthesized using a precipitation method exhibits spheroidal-like nanoclusters with well-defined edges assembled from primary nanoparticles with an average size of 50u202fnm, whereas the monoclinic phase of gadolinium oxide (m-Gd2O3) deposited using explosive pyrolysis has a denser structure compared with natural gadolinia. This phase also has a structure composed of three-dimensional complex agglomerates without clear-edged boundaries that are ∼21u202fnm in size plus a cubic phase admixture of only 2u202fat.% composed of primary edge-boundary nanoparticles ∼15u202fnm in size. These atomic features appear in the electronic structure as different defects ([Gd…O OH] and [Gd…O O]) and have dissimilar contributions to the charge-transfer processes among the appropriate electronic states with ambiguous contributions in the Gd 5р – O 2s core-like levels in the valence band structures. The origin of [Gd…O OH] defects found by XPS was well-supported by PL analysis. The electronic and atomic structures of the synthesized gadolinias calculated using DFT were compared and discussed on the basis of the well-known joint OKT–van der Laan model, and good agreement was established.

Pulsed cathodoluminescence of two-alkali sodium potassium silicate glasses

Alkali silicate glasses of variable composition 22xNa2O · 22(1−x)K2O · 3CaO · 75SiO2 with equimolecular replacement of sodium ions by potassium ions are investigated using pulsed cathodoluminescence. It is revealed that localized electronic states interact with vibrations of two types, namely, polarization vibrations of the silicon-oxygen network with the frequency v0 = 820 cm−1 and bending vibrations of the modifier sublattice. At low concentrations of one of the alkali components (x < 0.1), bending vibrations are observed at two frequencies. These frequencies coincide with those of the corresponding vibrations in one-alkali systems containing Na (530 cm−1) and K (520 cm−1). At higher concentrations (x in the range ∼0.14–0.86), there occur bending vibrations of the cationic sublattice with a frequency of 420 cm−1. This can be interpreted as a luminescence analog of two-alkali (mixed-alkali) effect.