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Chromium iodide monolayers, which have different magnetic properties in comparison to the bulk chromium iodide, have been shown to form skyrmionic states in applied electromagnetic fields or in Janus-layer devices. In this work, we demonstrate that spin-canted solutions can be induced into monolayer chromium iodide by select substitution of iodide atoms with isovalent impurities. Several concentrations and spatial configurations of halide substitutional defects are selected to probe the coupling between the local defect-induced geometric distortions and orientation of chromium magnetic moments. This work provides atomic-level insight into how atomically precise chemical doping can be used to create and control complex magnetic patterns in chromium iodide layers and lays out the foundation for investigating the field- and geometric-dependent magnetic properties in similar two-dimensional materials.

Internal electric fields that underpin functioning of multi-component materials systems and devices are coupled to structural and compositional inhomogeneities associated with interfaces in these systems. Hard-x-ray photoelectron spectroscopy is a valuable source of information on band-edge profiles, governed by the distribution of internal fields, deep inside semiconductor thin films and heterojunctions. However, extracting this information requires robust and physically meaningful decomposition of spectra into contributions from individual atomic planes. We present an approach that utilizes the physical requirements of a monotonic dependence of the built-in electrostatic potential on depth and continuity of the potential function and its derivatives. These constraints enable efficient extraction of band-edge profiles and allow one to capture details of the electronic structure, including determination of the signs and magnitudes of the band bending as well as the valence band offsets. The utility of this approach to generate quantitative insight into the electronic structure of complex materials is illustrated for epitaxial $$\hbox {SrTiO}_3$$SrTiO3 on intrinsic Si(001).

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 demonstrate that the interfacial dipole associated with bonding across the $\mathrm{SrTi}{\mathrm{O}}_{3}/\mathrm{Si}$ heterojunction can be tuned through space charge, thereby enabling the band alignment to be altered via doping. Oxygen impurities in Si act as donors that create space charge by transferring electrons across the interface into $\mathrm{SrTi}{\mathrm{O}}_{3}$. The space charge induces an electric field that modifies the interfacial dipole, thereby tuning the band alignment from type II to III. The transferred charge, accompanying built-in electric fields, and change in band alignment are manifested in electrical transport and hard x-ray photoelectron spectroscopy measurements. Ab initio models reveal the interplay between polarization and band offsets. We find that band offsets can be tuned by modulating the density of space charge across the interface. Modulating the interface dipole to enable electrostatic altering of band alignment opens additional pathways to realize functional behavior in semiconducting hybrid heterojunctions.

Since the advent of the first computers, chemists have been at the forefront of using computers to understand and solve complex chemical problems. As the hardware and software have evolved, so have the theoretical and computational chemistry methods and algorithms. Parallel computers clearly changed the common computing paradigm in the late 1970s and 80s, and the field has again seen a paradigm shift with the advent of graphical processing units. This review explores the challenges and some of the solutions in transforming software from the terascale to the petascale and now to the upcoming exascale computers. While discussing the field in general, NWChem and its redesign, NWChemEx, will be highlighted as one of the early codesign projects to take advantage of massively parallel computers and emerging software standards to enable large scientific challenges to be tackled.

Extreme shear deformation is used for several material processing methods and is unavoidable in many engineering applications in which two surfaces are in relative motion against each other while in physical contact. The mechanistic understanding of the microstructural evolution of multi-phase metallic alloys under extreme shear deformation is still in its infancy. Here, we highlight the influence of shear deformation on the microstructural hierarchy and mechanical properties of a binary as-cast Al-4 at.% Si alloy. Shear-deformation-induced grain refinement, multiscale fragmentation of the eutectic Si-lamellae, and metastable solute saturated phases with distinctive defect structures led to a two-fold increase in the flow stresses determined by micropillar compression testing. These results highlight that shear deformation can achieve non-equilibrium microstructures with enhanced mechanical properties in Al–Si alloys. The experimental and computational insights obtained here are especially crucial for developing predictive models for microstructural evolution of metals under extreme shear deformation. Extreme deformation of alloys is important for processing and applications. Here, extreme shear deformation of an Al–Si alloy induces grain refinement, multi-scale fragmentation of lamellae and generation of various defects, enhancing the mechanical properties of micropillars.

Understanding and controlling heat transfer across interfaces has become an important issue for the performance of micro- and nanoscale electronics, as well as achieving a high figure of merit for thermoelectrics. Intrinsic and extrinsic defects can have a significant impact on thermal transport in bulk materials and across interfaces, but the mechanism is not well understood. In this work, nonequilibrium molecular dynamics simulations are used to determine the impact of interfacial atomic vacancies on thermal transport across a Cu/Si junction. In contrast to the reduction in thermal transport typically seen with bulk defects, we find that by introducing atomic vacancies at a concentration of 6.3% near the interface in either or both materials, the interfacial thermal conductance can be increased by up to 76%. By controlling the initial positions of the vacancies and keeping track of their movements and population, we find that interfacial thermal transport is dependent on temperature, vacancy concentration, and distribution, and a positive correlation between the conductance and point defect activities (extent of vacancy migration, rate of Frenkel defect creation and annihilation) is observed. Further calculations based on the phonon density of states and normal mode decomposition reveal that the increase in interfacial thermal conductance originates primarily from high-frequency phonons, supported by enhanced inelastic phonon transport which contributes to more than 60% of the increase. Our findings suggest a practical way to manipulate inelastic phonon conversion through the presence of defects, which provides an alternative perspective on improving thermal transport between materials with a large lattice mismatch.

Two-dimensional stacking fault defects embedded in a bulk crystal can provide a homogeneous trapping potential for carriers and excitons. Here we utilize state-of-the-art structural imaging coupled with density-functional and effective-mass theory to build a microscopic model of the stacking-fault exciton. The diamagnetic shift and exciton dipole moment at different magnetic fields are calculated and compared with the experimental photoluminescence of excitons bound to a single stacking fault in GaAs. The model is used to further provide insight into the properties of excitons bound to the double-well potential formed by stacking fault pairs. This microscopic exciton model can be used as an input into models which include exciton-exciton interactions to determine the excitonic phases accessible in this system.

We report charge transfer and built-in electric fields across the epitaxial SrNb_{x}Ti_{1-x}O_{3-δ}/Si(001) interface. Electrical transport measurements indicate the formation of a hole gas in the Si and the presence of built-in fields. Hard x-ray photoelectron measurements reveal pronounced asymmetries in core-level spectra that arise from these built-in fields. Theoretical analysis of core-level spectra enables built-in fields and the resulting band bending to be spatially mapped across the heterojunction. The demonstration of tunable charge transfer, built-in fields, and the spatial mapping of the latter, lays the groundwork for the development of electrically coupled, functional heterojunctions.

We have investigated the electronic and optical properties of epitaxial $\mathrm{L}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{r}}_{x}\mathrm{Fe}{\mathrm{O}}_{3}$ for $0\ensuremath{\le}x\ensuremath{\le}1$ prepared by molecular-beam epitaxy. Core-level and valence-band x-ray photoemission features monotonically shift to lower binding energy with increasing $x$, indicating downward movement of the Fermi level toward the valence-band maximum. Combining valence-band photoemission and O $K$-edge x-ray absorption data, we map the evolution of the occupied and unoccupied bands and observe a narrowing of the gap, along with a transfer of state density from just below to just above the Fermi level as a result of hole doping. In-plane transport measurements confirm that the material becomes a $p$-type semiconductor at lower doping levels and exhibits a conversion from semiconducting to metallic behavior at $x=1$. Low-energy optical transitions revealed by spectroscopic ellipsometry are explained based on insight from theoretical densities of states and first-principles calculations of optical absorption spectra.

Metastable orientation relationships (ORs) between Cu and Cr are stabilized via epitaxial thin film deposition, with the initial layer of Cu(001) or Cr(001) grown epitaxially on MgO(001). The Bain OR is observed by x-ray diffraction and scanning/transmission electron microscopy for Cu(001) / Cr(001). In contrast, three Cr/Cu ORs are found for Cr deposition on Cu(001): the Pitsch OR, and two previously unreported ORs related to the Bain and Pitsch ORs, respectively. Ab initio calculations predict the energetics of these metastable ORs, and reveal that the deformation resistance of Cr makes the Bain OR energetically unfavorable when Cr films are deposited on Cu(001). These results show that kinetic constraints imposed by controlling the substrate surface and deposition conditions can lead to metastable interfacial structures.

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...

Creating new functionality in materials containing transition metals is predicated on the ability to control the associated charge states. For a given transition metal, there is an upper limit on valence that is not exceeded under normal conditions. Here, it is demonstrated that this limit of 3+ for Ni and Fe can be exceeded via synthesis of (SrNiO3 )m /(LaFeO3 )n superlattices by tuning n and m. The Goldschmidt tolerance constraints are lifted, and SrNi4+ O3 with holes on adjacent O anions is stabilized as a perovskite at the single-unit-cell level (m = 1). Holding m = 1, spectroscopy reveals that the n = 1 superlattice contains Ni3+ and Fe4+ , whereas Ni4+ and Fe3+ are observed in the n = 5 superlattice. It is revealed that the B-site cation valences can be tuned by controlling the magnitude of the FeO6 octahedral rotations, which, in turn, determine the energy balance between Ni3+ /Fe4+ and Ni4+ /Fe3+ , thus controlling emergent electrical properties such as the band alignment and resulting hole confinement. This approach can be extended to other systems for synthesizing novel, metastable layered structures with new functionalities.

Photoemission features associated with states deep in the band gap of n−SrTiO₃ (001) are found to be ubiquitous in bulk crystals and epitaxial films. These features are present even when there is little signal near the Fermi level. Analysis reveals that these states are deep-level traps associated with defects. The commonly investigated defects—O vacancies, Sr vacancies, and aliovalent impurity cations on the Ti sites—cannot account for these features. Rather, ab initio modeling points to these states resulting from interstitial oxygen and its interaction with donor electrons.

Identification of kinetic and thermodynamic factors that control crystal nucleation and growth represents a central challenge in materials synthesis. Here we report that apparently defect-free growth of ${\mathrm{La}}_{2}{\mathrm{MnNiO}}_{6}$ (LMNO) thin films supported on ${\mathrm{SrTiO}}_{3}$ (STO) proceeds up to 1--5 nm, after which it is disrupted by precipitation of NiO phases. Local geometric phase analysis and ensemble-averaged x-ray reciprocal space mapping show no change in the film strain away from the interface, indicating that mechanisms other than strain relaxation induce the formation of the NiO phases. Ab initio simulations suggest that the electrostatic potential build-up associated with the polarity mismatch at the film-substrate interface promotes the formation of oxygen vacancies with increasing thickness. In turn, oxygen deficiency promotes the formation of Ni-rich regions, which points to the built-in potential as an additional factor that contributes to the NiO precipitation mechanisms. These results suggest that the precipitate-free region could be extended further by either incorporating dopants that suppress the built-in potential or by increasing the oxygen fugacity in order to suppress the formation of oxygen vacancies.

Significance Electrides are ionic materials in which electrons act as anions. Solvated electrons in ammonia are one of the early known examples of such anions in liquids. Electron anions in crystalline materials have previously been shown to dramatically alter their electronic properties, for example, to turn an insulator into a superconductor with minimal changes in the structure and stoichiometry. We show that electron anions modify the disordered glass network by creating highly mobile weak links, thus fundamentally altering the thermodynamic and electronic properties of the network and enabling a new design paradigm for amorphous materials.

A series of mixed cation silicate glasses of the composition A2O–2MO–4SiO2, with A = Li, Na, K and M = Ca, Sr, Ba has been investigated by means of molecular dynamics simulations in order to understand the effect of the nature of the cations on the mobility of the alkaline-earth ions within the glass network. The size of the alkaline-earth cation was found to affect the inter-atomic distances, the coordination number distributions and the bond angle distributions, whereas the medium-range order was almost unaffected by the type of the cation. All the alkaline-earth cations contribute to lower vibrational frequencies but it is observed that there is a shift to smaller frequencies and the vibrational density of states distribution gets narrower as the size of the alkaline-earth increases. The results from our modeling for the ionic diffusion of the alkaline-earth cations are in a qualitative agreement with the experimental observations in that there is a distinct correlation between the activation energy for diffusion of alkaline earth-ions and the cation radius ratio. An asymmetrical linear behavior in the diffusion activation energy with increasing size difference is observed. The results can be described on the basis of a theoretical model that relates the diffusion activation energy to the electrostatic interactions of the cations with the oxygens and the elastic deformation of the silicate network.

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.

A multi-scale simulation study of Ni/InAs nano-scale contact aimed for the sub-14 nm technology is carried out to understand material and transport properties at a metal-semiconductor interface. The deposited Ni metal contact on an 11 nm thick InAs channel forms an 8.5 nm thick InAs leaving a 2.5 nm thick InAs channel on a p-type doped (1 × 1016 cm−3) AlAs0.47Sb0.53 buffer. The density functional theory (DFT) calculations reveal a band gap narrowing in the InAs at the metal-semiconductor interface. The one-dimensional (1D) self-consistent Poisson-Schrodinger transport simulations using real-space material parameters extracted from the DFT calculations at the metal-semiconductor interface, exhibiting band gap narrowing, give a specific sheet resistance of Rsh = 90.9 Ω/sq which is in a good agreement with an experimental value of 97 Ω/sq.

Thin-film synthesis methods that have developed over the past decades have unlocked emergent interface properties ranging from conductivity to ferroelectricity. However, our attempts to exercise precise control over interfaces are constrained by a limited understanding of growth pathways and kinetics. Here we demonstrate that shuttered molecular beam epitaxy induces rearrangements of atomic planes at a polar/nonpolar junction of ${\mathrm{LaFeO}}_{3}$ (LFO)/$n\text{\ensuremath{-}}{\mathrm{SrTiO}}_{3}$ (STO) depending on the substrate termination. Surface characterization confirms that substrates with two different $({\mathrm{TiO}}_{2}$ and SrO) terminations were prepared prior to LFO deposition; however, local electron-energy-loss spectroscopy measurements of the final heterojunctions show a predominantly LaO/${\mathrm{TiO}}_{2}$ interfacial junction in both cases. Ab initio simulations suggest that the interfaces can be stabilized by trapping extra oxygen (in LaO/${\mathrm{TiO}}_{2})$ and forming oxygen vacancies (in ${\mathrm{FeO}}_{2}$/SrO), which points to different growth kinetics in each case and may explain the apparent disappearance of the ${\mathrm{FeO}}_{2}$/SrO interface. We conclude that judicious control of deposition time scales can be used to modify growth pathways, opening new avenues to control the structure and properties of interfacial systems.

A novel pathway of increasing the surface density of catalytically active oxygen radical sites on a MoVTeNb oxide (M1 phase) catalyst during alkane oxidative dehydrogenation is reported. The novel sites form when a fraction of Te4+ is reduced and emitted from the M1 crystals under catalytic operating conditions, without compromising structural integrity of the catalyst framework. Density functional theory calculations show this Te reduction induces multiple inter-related electron transfers, and the associated cooperative effects lead to the formation of O- radicals. The in situ observations identify complex dynamic changes in the catalyst on an atomistic level, highlighting a new way to tailor structure and dynamics for highly active catalysts.

Epitaxial thin films of $\mathrm{C}{\mathrm{r}}_{2\ensuremath{-}x}\mathrm{T}{\mathrm{i}}_{x}{\mathrm{O}}_{3}$ were deposited by oxygen-plasma-assisted molecular beam epitaxy for $0.04\ensuremath{\le}x\ensuremath{\le}0.26$. Ti valence is verified by both x-ray photoelectron spectroscopy (XPS) and Ti $K$-edge x-ray absorption near-edge spectroscopy (XANES) to be $\mathrm{T}{\mathrm{i}}^{4+}$. Substitution of Ti for Cr in the corundum lattice is verified by fitting the Ti $K$-edge extended x-ray absorption fine structure (EXAFS) data. Room temperature electrical transport measurements confirm the highly resistive nature of Ti-doped $\mathrm{C}{\mathrm{r}}_{2}{\mathrm{O}}_{3}$, despite the presence of aliovalent $\mathrm{T}{\mathrm{i}}^{4+}$. For comparison, the resistivity of very pure, undoped $\mathrm{C}{\mathrm{r}}_{2}{\mathrm{O}}_{3}$ was measured to be four orders of magnitude higher than for Ti-doped $\mathrm{C}{\mathrm{r}}_{2}{\mathrm{O}}_{3}$. Analysis of the XPS and EXAFS data reveal the presence of Cr vacancies at intermediate and high Ti concentrations. This conclusion is corroborated by the results of density functional modeling. At low Ti concentrations, a strong increase of the XPS $\mathrm{Ti}\phantom{\rule{0.16em}{0ex}}2p$ core level peak width is observed as the Ti concentration decreases. In this limit, the formation of Cr vacancies becomes less favorable due to the increased distance between Ti dopants, and compensation by O interstitials contributes to broadening of the $\mathrm{Ti}\phantom{\rule{0.16em}{0ex}}2p$ XPS peak. The differences in electronic structure which render $\mathrm{T}{\mathrm{i}}^{4+}$-doped $\mathrm{C}{\mathrm{r}}_{2}{\mathrm{O}}_{3}$ resistive due to the formation of compensating defects, but $\mathrm{T}{\mathrm{i}}^{4+}$-doped $\mathrm{F}{\mathrm{e}}_{2}{\mathrm{O}}_{3}$ conductive, are discussed. The defect compensation model developed here provides insight into previous, conflicting reports of $n$-type versus $p$-type conductivity in Ti-doped $\mathrm{C}{\mathrm{r}}_{2}{\mathrm{O}}_{3}$ at high temperature, and will inform future studies to exploit the wide variety of electronic and magnetic properties of corundum structure oxides.

Through systematic control of the Nd concentration, we show that the carrier density of the two-dimensional electron gas (2DEG) in ${\mathrm{SrTiO}}_{3}/{\mathrm{NdTiO}}_{3}/{\mathrm{SrTiO}}_{3}(001)$ can be modulated over a wide range. We also demonstrate that the ${\mathrm{NdTiO}}_{3}$ in heterojunctions without a ${\mathrm{SrTiO}}_{3}$ cap is degraded by oxygen absorption from air, resulting in the immobilization of donor electrons that could otherwise contribute to the 2DEG. This system is, thus, an ideal model to understand and control the insulator-to-metal transition in a 2DEG based on both environmental conditions and film-growth processing parameters.

Oxygen vacancies in LaAlO3 (LAO) play an important role in the formation of the two-dimensional electron gas observed at the LaAlO3/SrTiO3 interface and affect the performance of MOSFETs using LAO as a gate dielectric. However, their spectroscopic properties are still poorly understood, which hampers their experimental identification. Here we predict the absorption spectra and ESR parameters of oxygen vacancies in LAO using periodic and embedded cluster methods and density functional theory (DFT). The structure, charge distribution, and spectroscopic properties of the neutral and charged (1+ and 2+) oxygen vacancies in cubic and rhombohedral LaAlO3 are investigated. The highest intensity optical transitions [calculated using time-dependent DFT (TDDFT)], from the oxygen vacancy states to the conduction-band states have onsets at 3.5 and 4.2 eV for the neutral vacancy and 3.6 eV for the 1+ charged vacancy in rhombohedral LAO and 3.3 and 4.0 eV for the neutral vacancy and 3.4 eV for the 1+ charged vacancy in cubic LAO, respectively. Also reported are the isotropic g value (2.004026) and hyperfine coupling constants of the 1+ charged oxygen vacancy, which are compared to the experimental data obtained using electron spin resonance (ESR) spectroscopy, and accurately predict both the position and the width (3 mT) of its ESR signature. These results may further facilitate the experimental identification of oxygen vacancies in LAO and help to establish their role at the LAO/STO interfaces and in nanodevices using LAO.

Epitaxial interfaces and superlattices comprised of polar and non-polar perovskite oxides have generated considerable interest because they possess a range of desirable properties for functional devices. In this work, emergent polarization in superlattices of SrTiO$_3$ (STO) and LaCrO$_3$ (LCO) is demonstrated. By controlling the interfaces between polar LCO and non-polar STO, polarization is induced throughout the STO layers of the superlattice. Using x-ray absorption near-edge spectroscopy and aberration-corrected scanning transmission electron microscopy displacements of the Ti cations off-center within TiO6 octahedra along the superlattice growth direction are measured. This distortion gives rise to built-in potential gradients within the STO and LCO layers, as measured by in situ x-ray photoelectron spectroscopy. Density functional theory models explain the mechanisms underlying this behavior, revealing the existence of both an intrinsic polar distortion and a built-in electric field, which are due to alternately positively and negatively charged interfaces in the superlattice. This study paves the way for controllable polarization for carrier separation in multilayer materials and highlights the crucial role that interface structure plays in governing such behavior., 19 pages, 3 figures, supplement: 13 pages, 7 figures, Advanced Materials Interfaces, Early View, 2016

Surface structures of stoichiometric and oxygen-deficient complex subnanoporous oxide 12CaO·7Al 2 O 3 (C12A7) are generated by simulating lattice rupture under the influence of an external strain. Extra-framework anions are found to serve as buffers, maintaining stability of the lattice cages in both elastic and inelastic stretching regimes. Modification of the local atomic structure in the near-surface region reduces the band gap in stoichiometric insulating C12A7. On the contrary, the band gap appears in the oxygen-deficient form of C12A7, which is metallic in the bulk. This is due to formation of the surface electron traps, which differ both in the type of the local atomic structure and stability of the electronic states. The implications of this electronic structure for the surface chemical and electron emission properties are discussed.

Author(s): Kaspar, TC; Sushko, PV; Spurgeon, SR; Bowden, ME; Keavney, DJ; Comes, RB; Saremi, S; Martin, L; Chambers, SA | Abstract: The behavior of polar LaMnO3 (LMO) thin films deposited epitaxially on nonpolar SrTiO3(001) (STO) is dictated by both the LMO/STO band alignment and the chemistry of the Mn cation. Using in situ X-ray photoelectron spectroscopy, the valence band offset (VBO) of LMO/STO heterojunctions is directly measured as a function of thickness, and found that the VBO is 2.5 eV for thicker (≥3 u.c.) films. No evidence of a built-in electric field in LMO films of any thickness is found. Measurements of the Mn valence by Mn L-edge X-ray absorption spectroscopy and by spatially resolved electron energy loss spectra in scanning transmission electron microscopy images reveal that Mn2+ is present at the LMO surface, but not at the LMO/STO interface. These results are corroborated by density functional theory simulations that confirm a VBO of ≈2.5 eV for both ideal and intermixed interfaces. A model is proposed for the behavior of polar/nonpolar LMO/STO heterojunctions in which the polar catastrophe is alleviated by the formation of oxygen vacancies at the LMO surface.

Nonuniform and highly localized Li dendrites are known to cause deleterious and, in many cases, catastrophic effects on the performance of rechargeable Li batteries. However, the mechanisms of cathode failures upon contact with Li metal are far from clear. In this study, using in situ transmission electron microscopy, the interaction of Li metal with well-defined, epitaxial thin films of LiCoO2 , the most widely used cathode material, is directly visualized at an atomic scale. It is shown that a spontaneous and prompt chemical reaction is triggered once Li contact is made, leading to expansion and pulverization of LiCoO2 and ending with the final reaction products of Li2 O and Co metal. A topotactic phase transition is identified close to the reaction front, resulting in the formation of CoO as a metastable intermediate. Dynamic structural and chemical imaging, in combination with ab initio simulations, reveal that a high density of grain and antiphase boundaries is formed at the reaction front, which are critical for enabling the short-range topotactic reactions and long-range Li propagation. The fundamental insights are of general importance in mitigating Li dendrites related issues and guiding the design principle for more robust energy materials.

The question of stability against diffusional mixing at the prototypical LaAlO3/SrTiO3(001) interface is explored using a multi-faceted experimental and theoretical approach. We combine analytical methods with a range of sensitivities to elemental concentrations and spatial separations to investigate interfaces grown using on-axis pulsed laser deposition. We also employ computational modeling based on the density function theory as well as classical force fields to explore the energetic stability of a wide variety of intermixed atomic configurations relative to the idealized, atomically abrupt model. Statistical analysis of the calculated energies for the various configurations is used to elucidate the relative thermodynamic stability of intermixed and abrupt configurations. We find that on both experimental and theoretical fronts, the tendency toward intermixing is very strong. We have also measured and calculated key electronic properties such as the presence of electric fields and the value of the valence band discontinuity at the interface. We find no measurable electric field in either the LaAlO3 or SrTiO3, and that the valence band offset is near zero, partitioning the band discontinuity almost entirely to the conduction band edge. Moreover, we find that it is not possible to account for these electronic properties theoretically without including extensive intermixing in our physical model of the interface. The atomic configurations which give the greatest electrostatic stability are those that eliminate the interface dipole by intermixing, calling into question the conventional explanation for conductivity at this interface - electronic reconstruction. Rather, evidence is presented for La indiffusion and doping of the SrTiO3 below the interface as being the cause of the observed conductivity.

First principles density functional theory has been used to calculate the 2-D band structure of Si slabs with different thicknesses. From the calculated 2-D band structure, electron longitudinal and transverse effective masses have been extracted as a function of the slab thickness. These thickness-dependent electron effective masses have then been used to simulate I D-VG characteristics of scaled, sub-10 nm double-gate (DG) MOSFETs and to compare them with the results obtained using bulk masses. The channel thickness dependence of the Si band structure starts to affect noticeably DG MOSFET performance at channel lengths below 10 nm, lowering the on-current by approximately 10, for transistors with a body thickness of 2.6 nm, and by 20, for transistors with a body thickness of 1.3 nm. On the other hand, the subthreshold swing is improved by 10, in the 6-nm-gate length DG MOSFET and by 15, in the 4-nm-gate length device. Finally, the impact of thickness-dependent effective masses has been related to the behavior of the transmission coefficients.

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.

Novel approaches to efficient ammonia synthesis at an ambient pressure are actively sought out so as to reduce the cost of ammonia production and to allow for compact production facilities. It is accepted that the key is the development of a high-performance catalyst that significantly enhances dissociation of the nitrogen–nitrogen triple bond, which is generally considered a rate-determining step. Here we examine kinetics of nitrogen and hydrogen isotope exchange and hydrogen adsorption/desorption reactions for a recently discovered efficient catalyst for ammonia synthesis—ruthenium-loaded 12CaO·7Al2O3 electride (Ru/C12A7:e−)—and find that the rate controlling step of ammonia synthesis over Ru/C12A7:e− is not dissociation of the nitrogen–nitrogen triple bond but the subsequent formation of N–Hn species. A mechanism of ammonia synthesis involving reversible storage and release of hydrogen atoms on the Ru/C12A7:e− surface is proposed on the basis of observed hydrogen absorption/desorption kinetics., Development of catalysts that enhance dissociation of the nitrogen–nitrogen triple bond will reduce costs of ammonia production. Here, the authors study ammonia synthesis over a ruthenium loaded electride catalyst and show that the rate-determining step is shifted to nitrogen–hydrogen bond formation.

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.

We used an ab initio embedded cluster method to study and compare three charged states of the Si-Si dimer configurations of oxygen vacancies in @a-quartz and amorphous silica. The Si-Si bond in the neutral vacancy remains largely the same in both crystalline and amorphous SiO"2. In @a-quartz the positively charged dimer E' centre exists only as a meta-stable configuration, whereas in amorphous silica stable dimer configuration can be formed at favorable precursor sites. Our results demonstrate that negatively charged dimer oxygen vacancies can be formed in @a-quartz.