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Rare earth nickelates including LaNiO3 are promising catalysts for water electrolysis to produce oxygen gas. Recent studies report that Fe substitution for Ni can significantly enhance the oxygen evolution reaction (OER) activity of LaNiO3. However, the role of Fe in increasing the activity remains ambiguous, with potential origins that are both structural and electronic in nature. On the basis of a series of epitaxial LaNi1-xFexO3 thin films synthesized by molecular beam epitaxy, we report that Fe substitution tunes the Ni oxidation state in LaNi1-xFexO3 and a volcano-like OER trend is observed, with x = 0.375 being the most active. Spectroscopy and ab initio modeling reveal that high-valent Fe3+δ cationic species strongly increase the transition-metal (TM) 3d bandwidth via Ni-O-Fe bridges and enhance TM 3d-O 2p hybridization, boosting the OER activity. These studies deepen our understanding of structural and electronic contributions that give rise to enhanced OER activity in perovskite oxides.

Two-dimensional materials exhibit properties promising for novel applications. Topologically protected states at their edges can be harnessed for use in quantum devices. We use ab initio simulations to examine properties of edges in 1T'-WTe2 monolayers, known to exhibit topological order, and their interactions with Cu atoms. Comparison of (010)-oriented edges that have the same composition but different terminations shows that, as the number of Cu atoms increases, their thermodynamically preferred arrangement depends on the details of the edge structure. Cu atoms aggregate into a cluster at the most stable edge; while the cluster is nonmagnetic, it spin-polarizes the W atoms along the edge, which removes the topological protection. At the metastable edge, Cu atoms form a chain incorporated into the WTe2 lattice; the topological state is preserved in spite of the dramatic edge restructuring. This suggests that exploiting interactions of metal species with metastable edge terminations can provide a path toward noninvasive interfaces.

We report the formation of neutral nitrogen molecules in the cages of [Ca12Al14O32]2+ (C12A7) framework compensated by extra-framework anions. NH3 treatment of C12A7 electride (C12A7:e-) at 800 °C leads to the formation of N2 and NH2- species in the C12A7 cages. N2 and NHx species in the cages are identified using the Raman spectroscopy of 14NH3 and 15NH3-treated C12A7:e-. The concentration of H and N in the C12A7 cages after NH3 treatment is ∼1021 cm-3. We propose a two-step mechanism, supported by density functional theory (DFT) modeling, of N2 incorporation into the C12A7 cages, i.e., incorporation of NH2- formed from decomposition of NH3 at C12A7:e- surface followed by the NH2- species reacting to form N2 molecules. Encapsulation of neutral molecules, as opposed to negatively charged species reported in C12A7 previously, offers new opportunities for trapping and storing gaseous substances in nanoporous materials.

Efficient ammonia synthesis and decomposition processes under mild conditions are important to meet the expanding demand in major applications of ammonia as the energy carrier and to provide feedst...

We investigate the structure, thermodynamic stability, and electronic properties of (001), (110), and (111) surfaces of mayenite (12CaO·7Al2O3 or C12A7) in its oxygen-deficient phase known as mayenite-derived electride Ca12Al14O32 using simulations based on the density functional theory. Seven types of surface terminations were identified, depending on the orientation of the lattice nanosize cages with respect to the surface plane. We find that the surface termination has a strong effect on the distribution of electron anions in the near-surface region, as demonstrated by the range of the ionic charges and binding energies of adsorbed Au atoms and dimers. Notably, in all cases, Au species acquire a negative charge and preferentially adsorb as isolated ions rather than aggregate into clusters. These results show that C12A7 electride can provide a simple, yet versatile substrate to achieve high adsorption capacity and steric selectivity for single atom catalysts.

The epitaxial growth of multifunctional oxides on semiconductors has opened a pathway to introduce new functionalities to semiconductor device technologies. In particular, the integration of gate materials that enable non-volatile or hysteretic functionality in field-effect transistors could lead to device technologies that consume less power, or allow for novel modalities in computing. Here we present electrical characterization of ultrathin single crystalline SrZrxTi1-xO3 (x = 0.7) films epitaxially grown on a high mobility semiconductor, Ge. Epitaxial films of SrZrxTi1-xO3 exhibit relaxor behavior, characterized by a hysteretic polarization that can modulate the surface potential of Ge. We find that gate layers as thin as 5 nm corresponding to an equivalent-oxide-thickness of just 1.0 nm exhibit a ~ 2 V hysteretic window in the capacitance-voltage characteristics. The development of hysteretic metal-oxide-semiconductor capacitors with nanoscale gate thicknesses opens new vistas for nanoelectronic devices.

We investigate the formation mechanisms of vacancy-ordered phase and collective mass transport in epitaxial SrCrO3−δ films using ab initio simulations within the density functional theory formalism. We show that as the concentration of oxygen vacancies (VO) increases, they form 1D chains that feature Cr-centered tetrahedra. Aggregation of these 1D VO chains results in the formation of (111)-oriented oxygen-deficient planes and an extended vacancy-ordered phase observed in recent experiments. We discuss atomic-scale mechanisms enabling the quasi-2D VO aggregates to expand along and translate across (111) planes. The corresponding lowest activation energy pathways necessarily involve rotation of Cr-centered tetrahedra, which emerges as a universal feature of fast ionic conduction in complex oxides. These findings explain reversible oxidation and reduction in SrCrO3−δ at low temperatures and provide insights into transient behavior necessary to harness ionic conductive oxides for high-performance and low-tem...

Emergent phenomena at complex oxide interfaces could provide the basis for a wide variety of next-generation devices, including photovoltaics and spintronics. To date, detailed characterization and computational modeling of interfacial defects, cation intermixing, and film stoichiometry have helped to explain many of the novel behaviors observed at a single heterojunction. Unfortunately, many of the techniques employed to characterize a single heterojunction are less effective for a superlattice made up of a repeating series of interfaces that induce collective interfacial phenomena throughout a film. These repeating interfaces present an untapped opportunity to introduce an additional degree of complexity, such as confined electric fields, that cannot be realized in a single heterojunction. In this work, we explore the properties of SrTiO3–LaCrO3 superlattices to understand the role of defects, including variations in cation stoichiometry of individual layers of the superlattice, intermixing across inter...

Double perovskites of the form R2BB′O6 (where R is a rare earth cation and B and B′ are chemically distinct transition metal cations with half-filled and empty eg orbitals, respectively) are of significant interest for their magnetoelectric properties. La2MnNiO6 is particularly attractive because of its large expected ferromagnetic moment per formula unit (5 μB f.u.–1) and its semiconducting character. If the ideal structure nucleates, superexchange coupling can take place via the B—O—B′ bonds that form, and the moment per formula unit can attain its maximum theoretical value. However, we show that even in the case of layer-by-layer deposition via molecular beam epitaxy, the system can follow multiple reaction pathways that lead to deviations from the double perovskite structure. In particular, we observe a spatially extended phase in which B-site cation disorder occurs, resulting in Mn—O—Mn and Ni—O—Ni antiferromagnetic domains, as well as the formation of quasi-epitaxial, antiferromagnetic NiO nanoscale...

We show that despite very similar crystallographic properties and work function values in bulk Fe and Cr, epitaxial films of these metals on Nb:SrTiO3(001) exhibit completely different heterojunction electronic properties. The Cr/SrTiO3 interface is ohmic, whereas Fe/SrTiO3 forms a Schottky barrier with a barrier height of 0.50 eV. This difference arises because of variations in interface chemistry. In contrast to Cr [Chambers, S. A. , Adv. Mater. 2013, 25, 4001.], in-diffused Fe exhibits a +2 oxidation state and occupies Ti sites in the perovskite lattice, resulting in negligible charge transfer to Ti, upward band bending, and Schottky barrier formation. The differences between Cr and Fe are understood by performing first-principles calculations of the energetics of defect formation, which corroborate experimental results.

Magnesium is of great interest as a replacement for lithium in next-generation ion-transfer batteries but Mg-metal anodes currently face critical challenges related to the formation of passivating layers during Mg-plating/stripping and anode-electrolyte-cathode incompatibilities. Alternative anode materials have the potential to greatly extend the spectrum of suitable electrolyte chemistries but must be systematically tailored for effective Mg(2+) storage. Using analytical (scanning) transmission electron microscopy ((S)TEM) and ab initio modeling, we have investigated Mg(2+) insertion and extraction mechanisms and transformation processes in β-SnSb nanoparticles (NPs), a promising Mg-alloying anode material. During the first several charge-discharge cycles (conditioning), the β-SnSb particles irreversibly transform into a porous network of pure-Sn and Sb-rich subparticles, as Mg ions replace Sn atoms in the SnSb lattice. After electrochemical conditioning, small Sn particles/grains (33 ± 20 nm) exhibit highly reversible Mg-storage, while the Sb-rich domains suffer substantial Mg trapping and contribute little to the system performance. This result strongly indicates that pure Sn can act as a high-capacity Mg-insertion anode as theoretically predicted, but that its performance is strongly size-dependent, and stable nanoscale Sn morphologies (40 nm) are needed for superior, reversible Mg-storage and fast system kinetics.

We show that by co-doping SrTiO3 (STO) epitaxial thin films with equal amounts of La and Cr, it is possible to produce films with an optical band gap ∼0.9 eV lower than that of undoped STO. Sr1–xLaxTi1–xCrxO3 thin films were deposited by molecular beam epitaxy and characterized using X-ray photoelectron spectroscopy and X-ray absorption near-edge spectroscopy to show that the Cr dopants are almost exclusively in the Cr3+ oxidation state. Extended X-ray absorption fine structure measurements and theoretical modeling suggest that it is thermodynamically preferred for La and Cr dopants to occupy nearest-neighbor A- and B-sites in the lattice. Transport measurements show that the material exhibits variable-range hopping conductivity with high resistivity. These results create new opportunities for the use of doped STO films in photovoltaic and photocatalytic applications.

Electrides, i.e. salts in which electrons serve as anions, are promising materials for lowering activation energies of chemical reactions. Ab initio simulations are used to investigate the effect of the electron anions in a prototype mayenite-based electride (C12A7:e(-)) 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 cis-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 trans-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 cis-Ru2N2 is ~34 kJ mol(-1) more stable than the trans-Ru2N2, while the cis-trans 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:e(-), ability of Ru to capture N2, polarization of Ru clusters, and electrostatic interaction of negatively charged N species with the surface cations.

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.

We investigate desorption of Br atoms from the α-CsBr(110) and β-CsBr(100) surfaces induced by 6.4 and 7.9 eV ultraviolet laser irradiation. The mechanisms of Br-atom desorption were modeled using density functional theory (DFT) calculations. Together the experimental data and theoretical predictions demonstrate that the sub-bandgap irradiation at 6.4 eV predominantly excites the CsBr surface, leading to desorption of neutral Br atoms with a hyperthermal kinetic energy distribution. Excitation above the bandgap at 7.9 eV leads to desorption of Br atoms with both thermal and hyperthermal energies. Our theoretical modeling suggests that desorption of Br atoms with thermal velocities originates from the decay of subsurface excitons, which produces interstitial Br atoms that subsequently diffuse to the CsBr surface. Hyperthermal desorption can be explained by the surface-exciton-based desorption model. The computed maximum kinetic energy of desorbed Br atoms agrees well with the experimental observables.

An atomistic model of the SiO2/BaTiO3 interface was constructed using ab initio molecular dynamics. Analysis of its structure and electronic properties reveals that (i) the band gap at the stoichiometric SiO2/BaTiO3 interface is significantly smaller than those of the bulk BaTiO3 and SiO2, and (ii) the interface contains ∼5.5 nm(-2) oxygen vacancies (V(2+)) in the outermost TiO2 plane of the BaTiO3 and ∼11 nm(-2) Si-O-Ti bonds resulting from breaking Si-O-Si and Ti-O-Ti bonds and subsequent rearrangement of the atoms. This structure gives rise to the interface polar region with positive and negative charges localized in the BaTiO3 and SiO2 parts of the interface, respectively. We propose that high dielectric response, observed experimentally in the SiO2-coated nanoparticles of BaTiO3, is due to the electron gas formed in oxygen-deficient BaTiO3 and localized in the vicinity of the polar interface.

We report qualitatively new highly hyperthermal (HHT) oxygen atom emission from nanostructured CaO excited by 6.4 eV nanosecond laser pulses. The kinetic energy distribution of emitted O-atoms peaks at 0.7 eV, which is over 4 times greater than previously observed. Excitation of MgO and CaO nanostructures with UV laser pulses is known to result in thermal and hyperthermal emission of oxygen atoms when photons with energies above and below the band gap, respectively, are used. The highly energetic atomic desorption we observe, following bulk excitation, challenges the conventional view that bulk excitation can only induce thermal desorption. Using density functional theory and an embedded cluster method, we propose a mechanism for this HHT feature based on the interaction of surface holes with bulk excitons. These experimental and theoretical results suggest that specific atomic desorption mechanisms in wide-bandgap materials can be controlled by selective electronic excitation of not only the surface but also the bulk of these materials.

We present a method and a computer code for accurate calculation of electrostatic potential in an arbitrary crystalline lattice modeled using a finite system. The method is based on complementing a lattice unit cell with a set of point charges in order to annihilate simultaneously all components of any number of the lowest multipole moments. The positions and the values of the complementary charges are determined analytically. The electrostatic potential produced by each modified cell is short range, and the corresponding lattice series converges absolutely, which makes it convenient to use in embedded cluster calculations of solids, surfaces, and low-dimensional structures. The method is illustrated by application to the rutile TiO2 and α-quartz SiO2 lattices and to those of several complex minerals.

We have studied the intra- and intermolecular hydrogen transfer in a crystalline 1,1-diamino-2,2-dinitroethylene (DADNE) and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) by means of an embedded cluster method and density functional theory (DFT). We found that, even though both of these materials have similar amino- and nitro- functional groups and layered crystalline structures, there are important differences in the mechanisms of hydrogen transfer. In particular, our calculations suggest that the proton migration from an amino-group to a nitro-group of the same molecule is a feasible process in TATB but not in DADNE. At the same time, we have found that no intermolecular hydrogen transfer occurs in either molecular crystal. These results imply that the activation of the decomposition reactions proceeds via different paths in these two materials.

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.

A Monte Carlo method has been developed to investigate the dynamic configurations of nanometer-sized An nanocrystallites (NCs) supported on the MgO(100) surface. We have found significant concentrations of An atoms that transiently occupy adatom positions on Au(111) facets. Their concentration increases from 10(-4) per NC at 250 K to 10(-2) per NC at 550 K. A complex roughening transition involving the creation of steps on Au(111) facets is observed close to 500 K. The appreciable numbers of various local atom configurations that may transiently appear on NC at finite temperature may be important for many applications using NCs.

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 x 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.

Ultraviolet excitation of high surface area MgO films using 4.7 eV femtosecond pulses results in neutral Mg-atom desorption with hyperthermal kinetic energies in the range 0.1-0.4 eV. The Mg-atom hyperthermal energies and power dependences are similar to those previously observed using nanosecond pulsed excitation. Femtosecond two-pulse correlation measurements reveal the existence of different dynamical paths for Mg-atom desorption. One mechanism displays a sub-150 fs time scale and involves the simultaneous or near-simultaneous transition of two electrons to a 3-coordinated Mg(2+) site. Other paths display picosecond time scales that we associate with dynamics following electron trapping at 3-coordinated Mg(2+) surface sites.

The dynamics of photoinduced formation of electron and hole centers at the surface of MgO powder has been investigated in situ using EPR spectroscopy. Monochromatic excitation of the sample with 282 nm photons leads to creation of well-separated electron and hole centers at the surface. Polychromatic (200-900 nm) irradiation leads to a very different time evolution of the EPR signal of the photoinduced centers. We argue that this difference is due to secondary reactions stimulated by the low energy part of the radiation spectrum. It can be explained by the excitation of the localized holes to the valence band and localized electrons into the conduction band and their consequent trapping at defect states and powder particle interfaces. The results demonstrate the existence of various types of EPR active and inactive electron and hole traps at the surface of MgO powder and provide preliminary data on their energy distribution.

We have calculated the ionization energies, electron affinities, optical excitation energies, and relaxed electron and hole states at corners, kinks, and steps of the MgO (001) surface. The calculations are performed using an embedded cluster model and density functional theory and take into account the long-range surface polarization. The extent of localization of electronic states associated with specific structural defects at the surface is studied by the participation function method. The positions of energy levels of the surface sites with respect to the top of the surface valence band and the vacuum level are determined. The results demonstrate the existence of deep and shallow electron traps at steps, corners, and kinks of the MgO (001) surface, and establish direct correlation between common surface features and their spectroscopic and other electronic properties.