Valentyna V. Nosenko

Mechanisms of radiation-induced defects formation in m-ZrO2 powders were investigated using DFT+U calculations. In unirradiated crystal, oxygen vacancies exist in doubly charged state (VO2+) in threefold coordinated lattice sites together with two remote electron polarons. As a consequence, additional configurational entropy leads to considerable increase in vacancy and polaron concentrations. After crystal irradiation, the Frenkel pairs VO2+ - Oi2− and electron-hole polaron pairs are formed. Interaction of such quasiparticles with Frenkel pairs leads to formation of the long-lived metastable centers observed in EPR experiment. Various charge states of oxygen vacancies were represented as complexes of doubly charged vacancy and one, two or three electron polarons. The most stable is complex VO+ with one trapped polaron, while concentration of another paramagnetic state of vacancy VO− is negligibly small at room temperature. Concentration of equilibrium electron polarons (Zr3+ centers) is sufficiently decreased after irradiation because such polarons are captured by nonequilibrium VO2+ vacancies of Frenkel pairs. O− centers were identified as the interstitial oxygen which traps the hole polaron. Electron-hole polaron pairs in irradiated m-ZrO2 itself are short-lived defects and cannot be observed in experiment. However, interaction of polarons with native defects and impurities is important for formation of long-lived defects. In particular, we elucidate the nature of radiation-induced activation of phosphorus impurity. We show that in the ZrO2 doped with phosphorus the self compensation occurs. After irradiation, polaron is captured by PZr+ that converts this defect into paramagnetic state PZr0.

Defect transformations in tetragonal and monoclinic ZrO2 stimulated by high-energy electron irradiation were studied by electron paramagnetic resonance (EPR) method. The undoped and Y-doped ZrO2 powders prepared by co-precipitation approach and annealed at different temperatures as well as their commercial counterparts were used. Both ZrO2 powders and commercial ZrO2 consisted of two phases - monoclinic (m-ZrO2) and tetragonal (t-ZrO2), while Y-doped ZrO2 was of tetragonal phase. In the EPR spectra of ZrO2, the signal caused by Zr3+ centers located in m-ZrO2 and t-ZrO2 phases with parameters g⊥=1.9755, g||=1.9635 and g⊥=1.9755, g||=1.9590, correspondingly, was identified. The concentration of Zr3+ centers is much larger in t-ZrO2, which implies that tetragonal phase is more favorable for the existence of Zr3+ centers. Electron irradiation of ZrO2 decreases the amount of Zr3+ centers, and leads to formation of two singly ionized oxygen vacancies, VO+, in both ZrO2 phases and hole trapped at oxygen atom, O-, in monoclinic phase. The parameters of corresponding EPR centers are: g⊥=2.0029, g||=2.0006 (VO+ in m-ZrO2), g⊥=2.004, g||=2.002 (VO+ in t-ZrO2), and gx=2.0125, gy=2.0113, gz=2.0046 (O- in m-ZrO2). The EPR signal with the parameters g⊥=2.0024 and g||=2.0110 observed in irradiated Y-doped ZrO2 was assigned to O- in tetragonal phase. In the EPR spectra of irradiated commercial ZrO2 powders an additional EPR signal with the parameters g||= 2.000, g⊥=2.001, A||=1265×10-4 cm-1, A⊥=1075×10-4 cm-1 was revealed and ascribed to PZr4+ defects in the Zr sublattice of the m-ZrO2 phase. From the analysis of DFT simulations the radiation-induced defect reactions in ZrO2 were proposed. The EPR data of intrinsic and radiation-induced centers in ZrO2 were systematized and updated.

In this work, we studied Cu-doped and (Cu,Y)-codoped ZrO₂ nanopowders produced through a coprecipitation approach to identify the nature of Cu-related bulk and surface paramagnetic centers. We conducted EPR, NMR, and Raman scattering studies on Cu- and (Cu,Y)-doped ZrO₂ powders calcined at different temperatures. At low calcination temperatures (400 °C) and low Cu loading (0.1–1.0 mol.% of CuO), the EPR signal was found to be attributed to surface-related Cu-H₂O complexes. For powders with higher Cu content (up to 8.0 mol.% of CuO), the superparamagnetic signal associated with the formation of copper clusters was observed. At higher calcination temperatures, the destruction of Cu-related surface complexes promotes the incorporation of Cu²⁺ ions into the bulk of ZrO₂ nanocrystals at Zr positions. Co-doping ZrO₂ with Cu and Y was observed to facilitate the incorporation of Cu²⁺ ions into cation sites at lower calcination temperatures when compared with Cu-doped ZrO₂.

In this study, we report a significant enhancement in the performance of GaNAs-based single nanowire lasers through optimization of growth conditions, leading to a lower lasing threshold and higher operation temperatures. Our analysis reveals that these improvements in the laser performance can be attributed to a decrease in the density of localized states within the material. Furthermore, we demonstrate that owing to their excellent nonlinear optical properties, these nanowires support self-frequency conversion of the stimulated emission through second harmonic generation (SHG) and sum-frequency generation (SFG), providing coherent light emission in the cyan-green range. Mode-specific differences in the self-conversion efficiency are revealed and explained by differences in the light extraction efficiency of the converted light caused by the electric field distribution of the fundamental modes. Our work, therefore, facilitates the design and development of multiwavelength coherent light generation and higher-temperature operation of GaNAs nanowire lasers, which will be useful in the fields of optical communications, sensing, and nanophotonics.

GaAsBi nanowires represent a novel and promising material platform for future nano-photonics. However, the growth of high-quality GaAsBi nanowires and GaAsBi alloy is still a challenge due to a large miscibility gap between GaAs and GaBi. In this work we investigate effects of Bi incorporation on lattice dynamics and carrier recombination processes in GaAs/GaAsBi core/shell nanowires grown by molecular-beam epitaxy. By employing photoluminescence (PL), PL excitation, and Raman scattering spectroscopies complemented by scanning electron microscopy, we show that increasing Bi-beam equivalent pressure (BEP) during the growth does not necessarily result in a higher alloy composition but largely affects the carrier localization in GaAsBi. Specifically, it is found that under high BEP, bismuth tends either to be expelled from a nanowire shell towards its surface or to form larger clusters within the GaAsBi shell. Due to these two processes the bandgap of the Bi-containing shell remains practically independent of the Bi BEP, while the emission spectra of the NWs experience a significant red shift under increased Bi supply as a result of the localization effect.