Inside Powders: A Theoretical Model of Interfaces between MgO Nanocrystallites

The electron- and hole-trapping and optical properties of a wide variety of interfaces between MgO nanocrystallites are investigated for the first time using a quantum-mechanical embedded-cluster method and time-dependent density functional theory. We conclude that delocalized holes can be transiently trapped at a large number of places within a powder. However, it is more energetically favorable for holes to trap on low-coordinated anions on the nanocrystallite surface, forming O- species. Electrons are trapped at few interfaces but are readily trapped by surface kink and corner sites. Contrary to common perception, our calculations of optical absorption spectra indicate that a variety of features buried within a powder can be exited with photon energies less than 5 eV, usually used to selectively excite low-coordinated surface sites

From Insulator to Electride: A Theoretical Model of Nanoporous Oxide 12CaO·7Al₂O₃

Recently, a novel inorganic electride stable at room temperatures has been obtained by reducing a complex nanoporous oxide 12CaO·7Al2O3 (C12A7) in a Ca atmosphere (Matsuishi, S.; Toda, Y.; Miyakawa, M.; Hayashi, K.; Kamiya, T.; Hirano, M.; Tanaka, I.; Hosono, H. Science 2003, 301, 626). In this system, up to 2.3 × 1021/cm3 electrons can be accommodated in a three-dimensional network of cages formed by a positively charged oxide framework. We demonstrate theoretically that at all concentrations, ne, the electrons are neither associated with specific atoms nor fully delocalized. At low ne, the electrons are isolated from each other and resemble the color centers in insulating materials. They are well localized in some of the lattice cages and yield strong inhomogeneous lattice distortions that provide polaron-type cage-to-cage electron hopping. As ne increases, the electrons form a denser electron gas and become more evenly spread over all available lattice cages. At sufficiently high ne, the system becomes metallic but still retains partially localized character of the conducting electrons. We describe the nature of the electronic states at the Fermi level and predict the changes in the optical and magnetic properties of this system as a function of ne

Enhanced N₂ Dissociation on Ru-Loaded Inorganic Electride

Electrides, i.e. salts in which electrons serve as anions, are promising materials for lowering activation energies of chemical reactions. <i>Ab initio</i> simulations are used to investigate the effect of the electron anions in a prototype mayenite-based electride (C12A7:<i>e</i>^<i>–</i>) on the mechanism of N₂ dissociation. It is found that both atomic and molecular nitrogen species chemisorb on the electride surface and become negatively charged due to the electron transfer from the substrate. However, charging alone is not sufficient to promote dissociation of N₂ molecules. In the presence of Ru, N₂ adsorbs with the formation of a <i>cis</i>-Ru₂N₂ complex and the N–N bond weakens due to both the electron transfer from the substrate and interaction with Ru. This complex transforms into a more stable <i>trans</i>-Ru₂N₂ configuration, in which the N₂ molecule is dissociated, with the calculated barrier of 116 kJ mol^–1 and the overall energy gain of 72 kJ mol^–1. In contrast, in the case of the stoichiometric mayentie, the <i>cis</i>-Ru₂N₂ is ∼34 kJ mol^–1 more stable than the <i>trans</i>-Ru₂N₂, while the <i>cis</i>–<i>trans</i> transition has a barrier of 192 kJ mol^–1. Splitting of N₂ is promoted by a combination of the strong electron donating power of C12A7:<i>e</i>^<i>–</i>, ability of Ru to capture N₂, polarization of Ru clusters, and electrostatic interaction of negatively charged N species with the surface cations

Optical Absorption and Band Gap Reduction in (Fe_1–<i>x</i>Cr_<i>x</i>)₂O₃ Solid Solutions: A First-Principles Study

We provide a detailed theoretical analysis of the character of optical transitions and band gap reduction in (Fe1–xCrx)2O3 solid solutions using extensive periodic model and embedded cluster calculations. Time-dependent density functional theory is used to calculate and assign optical absorption bands for x = 0.0, 0.5, and 1.0 and photon energies up to 5 eV. Consistent with recent experimental data, a band gap reduction of as much as 0.7 eV with respect to that of pure α-Fe2O3 is found. This result is attributed predominantly to two effects: (i) the higher valence band edge for x ≈ 0.5, as compared to those in pure α-Fe2O3 and α-Cr2O3, and (ii) the onset of Cr → Fe d–d excitations in the solid solutions. Broadening of the valence band due to hybridization of O 2p with Fe and Cr 3d states also contributes to band gap reduction

Optical Properties of Nanocrystal Interfaces in Compressed MgO Nanopowders

The optical properties and charge trapping phenomena observed on oxide nanocrystal ensembles can be strongly influenced by the presence of nanocrystal interfaces. MgO powders represent a convenient system to study these effects due to the well-defined shape and controllable size distributions of MgO nanocrystals. The spectroscopic properties of nanocrystal interfaces are investigated by monitoring the dependence of absorption characteristics on the concentration of the interfaces in the nanopowders. The presence of interfaces is found to affect the absorption spectra of nanopowders more significantly than changing the size of the constituent nanocrystals and, thus, leading to the variation of the relative abundance of light-absorbing surface structures. We find a strong absorption band in the 4.0−5.5 eV energy range, which was previously attributed to surface features of individual nanocrystals, such as corners and edges. These findings are supported by complementary first-principles calculations. The possibility to directly address such interfaces by tuning the energy of excitation may provide new means for functionalization and chemical activation of nanostructures and can help improve performance and reliability for many nanopowder applications

Effect of Protons on the Optical Properties of Oxide Nanostructures

Site-specific functionalization of oxide nanostructures gives rise to novel optical and chemical surface properties. In addition, it can provide deeper insights into the electronic surface structure of the associated materials. We applied chemisorption of molecular hydrogen, induced by ultraviolet (UV) light, followed by vacuum annealing to MgO nanocubes to selectively decorate three-coordinated oxygen ions (oxygen corner sites, for simplicity) with protons. Fully dehydroxylated nanocubes exhibit 3.2 ± 0.1 eV photoluminescence induced by 4.6 eV light, where both emission and absorption are associated with three-coordinated oxygen sites. We find that partially hydroxylated nanocubes show an additional photoluminescence feature at 2.9 ± 0.1 eV. Interestingly, the excitation spectra of the 2.9 and 3.2 eV emission bands, associated with protonated and nonprotonated oxygen corner sites, respectively, nearly coincide and show well-pronounced maxima at 4.6 eV in spite of a significant difference in their local atomic and electronic structures. These observations are explained with the help of ab initio calculations, which reveal that (i) the absorption band at 4.6 eV involves four-coordinated O and Mg ions in the immediate vicinity of the corner sites and (ii) protonation of the three-coordinated oxygen ions eliminates the optical transitions associated with them and strongly red-shifts other optical transitions associated with neighboring atoms. These results demonstrate that the optical absorption bands assigned to topological surface defects are not simply determined by the ions of lowest coordination number but involve contributions due to the neighboring atoms of higher coordination. Thus, we suggest that the absorption band at 4.6 eV should not be regarded as merely a signature of the three-coordinated O2- ions but ought to be assigned to corners as multiatomic topological features. Our results also suggest that optical absorption signatures of protonated and nonprotonated sites of oxide surfaces can be remarkably similar

Probing Oxidation-Driven Amorphized Surfaces in a Ta(110) Film for Superconducting Qubit

Recent advances in superconducting qubit technology have led to significant progress in quantum computing, but the challenge of achieving a long coherence time remains. Despite the excellent lifetime performance that tantalum (Ta) based qubits have demonstrated to date, the majority of superconducting qubit systems, including Ta-based qubits, are generally believed to have uncontrolled surface oxidation as the primary source of the two-level system loss in two-dimensional transmon qubits. Therefore, atomic-scale insight into the surface oxidation process is needed to make progress toward a practical quantum processor. In this study, the surface oxidation mechanism of native Ta films and its potential impact on the lifetime of superconducting qubits were investigated using advanced scanning transmission electron microscopy (STEM) techniques combined with density functional theory calculations. The results suggest an atomistic model of the oxidized Ta(110) surface, showing that oxygen atoms tend to penetrate the Ta surface and accumulate between the two outermost Ta atomic planes; oxygen accumulation at the level exceeding a 1:1 O/Ta ratio drives disordering and, eventually, the formation of an amorphous Ta2O5 phase. In addition, we discuss how the formation of a noninsulating ordered TaO1−δ (δ < 0.1) suboxide layer could further contribute to the losses of superconducting qubits. Subsurface oxidation leads to charge redistribution and electric polarization, potentially causing quasiparticle loss and decreased current-carrying capacity, thus affecting superconducting qubit coherence. The findings enhance the comprehension of the realistic factors that might influence the performance of superconducting qubits, thus providing valuable guidance for the development of future quantum computing hardware

Nanoporous Crystal 12CaO·7Al₂O₃: A Playground for Studies of Ultraviolet Optical Absorption of Negative Ions

A novel nanoporous material 12CaO·7Al2O3 (C12A7) offers a possibility of incorporating large concentrations (>1021 cm-3) of a wide range of extraframework anions inside its nanopores. We have investigated, both experimentally and theoretically, optical absorption associated with several types of such anions, including F-, OH-, O-, O2-, O2-, and O22-, and assigned their optical absorption bands. It is demonstrated that the chemical identity and concentration of extraframework anions can be controlled by an appropriate treatment of “as grown” C12A7. We also show that the position of the adsorption edge is, in turn, determined by the chemical identity of the extraframework species and can be varied in the range of ∼4−6 eV. We suggest that C12A7 is a unique host material, which can be used as a playground for studying negatively charged species that are unstable in other environments