Vancho Kocevski

Controlling and predicting the morphology of lanthanide sesquioxides in thin film form is vital to their use in current applications. In the present study, single and codeposited Sm2O3, Er2O3, and Lu2O3 thin films were grown on yttria-stabilized zirconia (8%) substrates by radio frequency magnetron sputtering at room temperature and 500 °C. The effect of two different substrate temperatures and altering the oxide cation on the structural and morphological properties of the films was analyzed. The thin films were characterized by profilometry, scanning electron microscopy, transmission electron microscopy, and x-ray diffraction. The single-component Lu2O3 and Sm2O3 films obtained were of the cubic phase, and the Er2O3 was a mix of cubic and monoclinic phases. It was observed for both the Er2O3 and Lu2O3 films that increasing the substrate temperature to 500 °C resulted in larger grained polycrystalline films. In contrast, large grained polycrystalline films were obtained at both room temperature and 500 °C for Sm2O3 and uneven granularity increased as temperature increased. Codeposition of Lu2O3 and Sm2O3, and Lu2O3 and Er2O3 resulted in a cubic bixbyite phase (the C phase of the lanthanide sesquioxide) solid solution. It was observed that the structure and morphology of the films can be controlled by manipulating deposition parameters. Both substrate temperature and altering the oxide cation contributed to changes in crystallinity and grain structure, which can modify the chemical and physical properties of the films for their applications.

Metastable phase formation of Ln₂O₃ phases was explained and successfully predicted using calculated metastable phase diagrams. Irradiation experiments of Lu₂O₃ show formation of 3 phases, confirming the prediction, showing a unique behavior.

A series of Ca-containing lanthanide thiophosphates has been obtained and their structural evolution from 3D for LnPS₄ and Cs_0.3(Ln_0.7Ca_0.3)PS₄ to 2D in Cs_0.5(Ln_0.5Ca_0.5)PS₄ was shown as a function of Ca content.

Single crystals of CsGa₇O₁₁, RbGa₇O₁₁, and RbGa₄In₅O₁₄ were grown from alkali halide melts and their structures were characterized by single crystal and powder X‐ray diffraction. CsGa₇O₁₁ and RbGa₇O₁₁ adopt the same structure type, reminiscent of the hollandite structure type, as it contains nearly rectangular channels made up of two dimers of edge‐sharing GaO₆ octahedra, and two corner‐sharing octahedron/tetrahedron pairs. The structure of RbGa₄In₅O₁₄ is more complex and is comprised of indium octahedra, gallium trigonal bipyramids, and gallium tetrahedra, and contains similar sized tunnels as CsGa₇O₁₁ and RbGa₇O₁₁. CsGa₇O₁₁ and RbGa₄In₅O₁₄ were further characterized by TGA, ion exchange experiments, and DFT studies revealing that both structures are thermodynamically stable up to 850°C; however, CsGa₇O₁₁ decomposes to GaO(OH) xH₂O when heated in warm aqueous solutions. CsGa₇O₁₁ undergoes ion exchange in both an aqueous solution of RbCl and a RbNO₃ melt, as predicted by DFT studies, where the ion exchange is more extensive in the RbNO₃ melt.

Triuranium disilicide (U₃Si₂) fuel with silicon carbide (SiC) composite cladding is being considered as an advanced concept/accident tolerant fuel for light water reactors thus, understanding their chemical compatibility under operational and accident conditions is paramount. Here we provide a comprehensive view of the interaction between U₃Si₂ and SiC by utilizing density functional theory calculations supported by diffusion couple experiments. From the calculated reaction energies, we demonstrate that triuranium pentasilicide (U₃Si₅), uranium carbide (UC), U₂₀Si₁₆C₃, and uranium silicide (USi) phases can form at the interface. A detailed study of U₃Si₂ and SiC defect formation energies of the equilibrated materials yielding the interfacial phases U₂₀Si₁₆C₃, U₃Si₅ and UC reveal a thermodynamic driving force for generating defects in both fuel and cladding. The absence of either the U₃Si₂ or SiC phase, however, causes the defect formation energies in the other phase to be positive, removing the driving force for additional interfacial reactions. The diffusion couple experiments confirm the conclusion with demonstrated restricted formation of U₃Si₅, UC, and U₂₀Si₁₆C₃/USi phases at the interface. The resulting lack of continuous interaction between the U₃Si₂ and SiC, reflects the diminishing driving force for defect formation, demonstrating the substantial stability of this fuel-cladding system.

Single crystals of new cesium cobalt silicates and germanates exhibiting three-dimensional, ion-exchangeable crystal structures were grown from a mixed CsCl–CsF flux, and their electronic and magnetic properties were studied using DFT calculations.

Using high-throughput first-principles calculations, a list of 92 double spinel chemistries are screened for their stability; 49 stable double spinels are identified, representing potentially new compounds with unique properties and functionality.

Single crystal growth of new germanate salt inclusion materials. [(Rb₆F)(Rb₄F)][Ge₁₄O₃₂] exhibits room temperature luminescence and [(Rb₆F)(Rb_3.1Co_0.9F_0.96)][Co_3.8Ge_10.2O₃₀F₂] demonstrates Co/Ge mixing and an unanticipated Rb/Co inclusion.

The Linde Type A (LTA) zeolite capability to exchange alkaline earth ions is analyzed using DFT calculations, considering the systems to be in water and vacuum, investigating the LTA's potential as a material for radioactive decontamination processes.

Crystals of three new uranium(v) containing oxyfluorides were grown out of an alkali fluoride flux and adopt a perovskite-type structure and are examined by SXRD, PXRD, XANES, XPS, EDS, magnetic susceptibility measurements, DFT calculations, and UV-vis spectroscopy.

Using first-principles calculations, we studied the adsorption of alkali ions in pure silica Linde Type A (LTA) zeolite. The probability of adsorbing alkali ions from solution and the driving force for ion exchange between Na+ and other alkali ions at the different adsorption sites were analyzed. From the calculated ion exchange isotherms, we show that it is possible to exchange Na+ with K+ and Rb+ in water, but that is not the case for systems in a vacuum. We also demonstrate that a solvation model should be used for the accurate representation of ion exchange in an LTA and that dispersion interactions should be introduced with care.

Despite the known temperature effects on the optical and photoluminescence properties of silicon nanocrystals (Si NCs), most of the density functional theory calculations thus far have been carried out at zero temperature, i.e., fixed atomic positions. We present a study of the effect of finite temperature on the radiative lifetimes and bandgaps of Si NCs capped with six different organic ligands, CH3, C2H5, C2H4Cl, C2H4OH, C2H4SH, and C2H4NH2. In addition, we show the differences in electronic and optical properties, as well as the wavefunctions (WFs) around the bandgap, of the capped Si NCs at zero temperature. We show that the NCs capped with alkyl and C2H4Cl ligands have larger HOMO-LUMO and optical absorption gaps compared to the C2H4NH2, C2H4OH, and C2H4SH capped NCs. We demonstrate that this big difference in both gaps comes from the increased contribution to the states at the top of the valence band from the NH2, OH, and SH groups of the C2H4NH2, C2H4OH, and C2H4SH ligands, respectively. Additionally, we assigned the rather weak dependence of the radiative lifetimes of C2H4NH2 capped NCs on the NC size to the slightly changing symmetry of the highly localized HOMO WF at the NH2 group. Furthermore, we demonstrate that the temperature effect on the radiative lifetimes and bandgaps is larger in alkyl and C2H4Cl capped Si NCs. We indicate that the decrease in radiative lifetime of the CH3 capped NCs with increasing temperature comes from the changing symmetry of the LUMO WF and the increased dipolar overlap between the HOMO and LUMO WFs. Finally, we show that there is a constant decrease in the bandgaps of the Si NCs with increasing size, with the bandgap change of CH3 capped NCs being larger compared to the bandgap change of the C2H4NH2 capped NCs.

Thermodynamically stable ground state half-Heusler structures are valence balanced irrespective of electron count or stoichiometry.

We present results of a scanning tunneling spectroscopy (STS) study of the impact of dehydrogenation on the electronic structures of hydrogen-passivated silicon nanocrystals (SiNCs) supported on the Au(111) surface. Gradual dehydrogenation is achieved by injecting high-energy electrons into individual SiNCs, which results, initially, in reduction of the electronic bandgap, and eventually produces midgap electronic states. We use theoretical calculations to show that the STS spectra of midgap states are consistent with the presence of silicon dangling bonds, which are found in different charge states. Our calculations also suggest that the observed initial reduction of the electronic bandgap is attributable to the SiNC surface reconstruction induced by conversion of surface dihydrides to monohydrides due to hydrogen desorption. Our results thus provide the first visualization of the SiNC electronic structure evolution induced by dehydrogenation and provide direct evidence for the existence of diverse dangling bond states on the SiNC surfaces.

Semiconducting nanocrystals (NCs) have become one of the leading materials in a variety of applications, mainly due to their size tunable band gap and high intensity emission. Their photoluminescence (PL) properties can be notably improved by capping the nanocrystals with a shell of another semiconductor, making core-shell structures. We focus our study on the CdS/ZnS core-shell nanocrystals that are closely related to extensively studied CdSe/CdS NCs, albeit exhibiting rather different photoluminescence properties. We employ density functional theory to investigate the changes in the electronic and optical properties of these nanocrystals with size, core/shell ratio, and interface structure between the core and the shell. We have found that both the lowest unoccupied eigenstate (LUES) and the highest occupied eigenstate (HOES) wavefunction (WF) are localized in the core of the NCs, with the distribution of the LUES WF being more sensitive to the size and the core/shell ratio. We show that the radiative lifetimes are increasing, and the Coulomb interaction energies decrease with increasing NC size. Furthermore, we investigated the electronic and optical properties of the NCs with different interfaces between the core and the shell and different core types. We find that the different interfaces and core types have rather small influence on the band gaps and the absorption indexes, as well as on the confinement of the HOES and LUES WFs. Also the radiative lifetimes are found to be only slightly influenced by the different structural models. In addition, we compare these results with the previous results for CdSe/CdS NCs, reflecting the different PL properties of these two types of NCs. We argue that the difference in their Coulomb interaction energies is one of the main reasons for their distinct PL properties.