Stabilizing the B-site oxidation state in ABO3 perovskite nanoparticles

Impact of Sc3+-Modified Local Site Symmetries on Er3+ Ion Upconversion Luminescence in Y2O3 Nanoparticles

Rare earth (RE) doped yttria sesquioxide has been widely used as host materials for upconversion (UC) phosphors due to their high refractive index, wide band gap, and high melting point. Meanwhile, while fluoride matrices with low phonon cutoff energies exhibit stronger UC emissions, RE-doped oxides exhibit better thermal stability and higher thermal sensitivity when applied as optical temperature sensors. In this work, Sc3+ is substituted in RE-doped Y2O3 lattices to generate smaller cation sites, enhancing the crystal field and modifying the allowed optical transitions. Er3+ is used as a photoluminescent probe to study the effect of site position and symmetry on the UC performance. In comparison with the traditional hydrothermal method, Sc3+ is successfully incorporated into the Y2O3 lattice via the co-precipitation/molten salt method without segregating observed. The Judd-Ofelt analysis was applied to determine the local symmetry and efficiency changes. Sc was found to be able to improve the luminescence performances of Er in Y2-x Sc x O3 (YScO) hosts by adjusting the local symmetry level around the luminescent sites. The local symmetry level was reduced with less than 30 mol % of Sc doping concentration based on the changes in Ω2 values. Meanwhile, the YScO oxide was found to significantly improve the luminescence intensity and red-to-green ratio at a lower Yb3+ concentration (5 mol %) instead of a higher concentration (20 mol %) commonly used. This was attributed to an increased energy transfer between the closer Yb3+-Er3+ pairs. Overall, this work allows the spatial occupancy of luminescence centers in the metal oxide host materials to optimize the UC luminescence performance and develop a high-efficiency oxide material for high-temperature applications such as optical thermometry.

Modifying Metastable Sr1-xBO3-δ (B = Nb, Ta, and Mo) Perovskites for Electrode Materials

The presence of surface/deep defects in 4d- and 5d-perovskite oxide (ABO3, B = Nb, Ta, Mo, etc.) nanoparticles (NPs), originating from multivalent B-site cations, contributes to suppressing their metallic properties. These defect states can be removed using a H2/Ar thermal treatment, enabling the recovery of their electronic properties (i.e., low electrical resistivity, high carrier concentration, etc.) as expected from their electronic structure. Therefore, to engineer the electronic properties of these metastable perovskites, an oxygen-controlled crystallization approach coupled with a subsequent H2/Ar treatment was utilized. A comprehensive study of the effect of the post-treatment time on the electronic properties of these perovskite NPs was performed using a combination of scattering, spectroscopic, and computational techniques. These measurements revealed that a metallic-like state is stabilized in these oxygen-reduced NPs due to the suppression of deep rather than surface defects. Ultimately, this synthetic approach can be employed to synthesize ABO3 perovskite NPs with tunable electronic properties for application into electrochemical devices.

Effect of Oxide Ion Distribution on a Uranium Structure in Highly U-Doped RE2Hf2O7 (RE = La and Gd) Nanoparticles

Rare-earth based A₂B₂O₇ compounds have been considered as potential host materials for nuclear waste due to their exceptional chemical, physical, capability of accommodating high concentration of actinides at both A- and B-sites, negligible leaching, tendency to form antisite defects, and radiation stabilities. In this work, La₂Hf₂O₇ (LHO) and Gd₂Hf₂O₇ (GHO) nanoparticles (NPs) were chosen as the RE-based hafnates to study the structural changes and the formation of different U molecular structures upon doping (or alloying) at high concentration (up to 30 mol %) using a combined coprecipitation and molten-salt synthesis. These compounds form similar crystal structures, i.e., ordered pyrochlore (LHO) and disordered fluorite (GHO), but are expected to show different phase transformations at high U doping concentration. X-ray diffraction (XRD) and Rietveld refinement results show that the LHO:U NPs have high structural stability, whereas the GHO:U NPs exhibit a highly disordered structure at high U concentration. Alternatively, the vibrational spectra show an increasingly random oxygen distribution with U doping, driving the LHO:U NPs to the disordered fluorite phase. X-ray spectroscopy indicates that U is stabilized as different U⁶⁺ species in both LHO and GHO hosts, resulting in the formation of oxygen vacancies stemming from the U local coordination and different phase transformation. Interestingly, the disordered fluorite phase has been reported to have increased radiation tolerance, suggesting multiple benefits associated with the LHO host. These results demonstrate the importance of the structural and chemical effect of actinide dopants on similar host matrices which are important for the development of RE-based hafnates for nuclear waste hosts, sensors, thermal barrier coatings, and scintillator applications.

Simulated field-modulated x-ray absorption in titania

In this paper, we present a method to compute the x-ray absorption near-edge structure (XANES) spectra of solid-state transition metal oxides using real-time time-dependent density functional theory, including spin-orbit coupling effects. This was performed on bulk-mimicking anatase titania (TiO₂) clusters, which allows for the use of hybrid functionals and atom-centered all electron basis sets. Furthermore, this method was employed to calculate the shifts in the XANES spectra of the Ti L-edge in the presence of applied electric fields to understand how external fields can modify the electronic structure, and how this can be probed using x-ray absorption spectroscopy. Specifically, the onset of t_2g peaks in the Ti L-edge was observed to red shift and the e_g peaks were observed to blue shift with increasing fields, attributed to changes in the hybridization of the conduction band (3d) orbitals.

Adsorption of Polarized Molecules for Interfacial Band Engineering of Doped TiO2 Thin Films

Owing to their chemical and mechanical stability, metal-oxides have emerged as potential alternatives for conventional pure-metal and organic molecule-based solid-state electronic devices. Traditionally, band engineering of these metal-oxides has been performed to improve the efficiency of solar cells and transistors. However, recent advancements in the field of oxide-based electronic devices demand reversible band structure engineering for applications in next-generation adaptive electronics and memory devices. Therefore, this work aims to reversibly engineer the surface band structure of doped metal-oxides using stable organic ligands with weak dipoles. Para-substituted benzoic acid (BZA) ligands with positive and negative dipole moments were adsorbed in situ on the surface of TiO₂:Ni²⁺ thin film to modify the interfacial dipole moment, and the valence band structure was probed using surface-sensitive ultraviolet photoelectron spectroscopy (UPS). UPS, paired with density functional theory (DFT) simulations, demonstrate the ability to selectively tune interfacial electronic/chemical landscapes with ligand-dependent dipole moment. The unique ability to reversibly tune the band bending at the organic-inorganic interface of doped metal-oxide semiconductors using molecular dipoles is expected to play a key role in the development of metal-oxide-based adaptive electronics that outperform the conventional polymer-based and Si-based devices.