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The bulk behavior of materials is often controlled by minor impurities that create nonperiodic localized defect structures due to ionic size, symmetry, and charge balance mismatches. Here, we used transmission electron microscopy (TEM) of atom-resolved dynamics to directly map the topology of Fe vacancy clusters surrounding structurally incorporated U6+ in nanohematite (α-Fe2O3). Ab initio molecular dynamic simulations provided additional independent constraints on coupled U, Fe, and vacancy mobility in the solid. A clearer understanding of how such an apparently incompatible element can be accommodated by hematite emerged. The results were readily interpretable without the need for sophisticated data reconstruction methods, model structures, or ultrathin samples, and with the proliferation of aberration-corrected TEM facilities, the approach is accessible. Given sufficient z-contrast, the ability to observe impurity-vacancy structures by means of atom hopping can be used to directly probe the association of impurities and such defects in other materials, with promising applications across a broad range of disciplines.

Ovonic Threshold Switch (OTS) devices are used as selector elements in 3D crosspoint memory arrays. The stacking of multiple decks in this architecture is a key challenge to dramatically enhance memory capacity but is currently limited by the absence of conformality of physical vapor deposition (PVD) deposited chalcogenide films. Atomic layer deposition (ALD) is necessary to enable large memory-capacity integrated 3D structures, but the development of OTS films with this technique is still confined to As-free compositions with poor performance. Here we present - for the first time - a quaternary ALD GeAsSeTe OTS device with good selectivity (≈ 104), low threshold voltage (V th ) drift ( 109 cycles) and robust stability. These results pave the way for future development of ALD chalcogenide-based selectors as a leading technology for multiple-stack integration of crosspoint memory arrays.

Oriented aggregation (OA), whereby crystals grow by successive attachment of precursor nanoparticles that are crystallographically co-aligned with the growing crystal, is a fundamental process by which many low solubility phases and minerals reach maturity in aqueous solution. Here, we investigated whether U(VI) can be incorporated into the structure of goethite that grows by OA. Wet chemical analysis and acid treatments showed that the additions of U(VI) early in the OA process hindered crystallization and increased the recalcitrant fraction of sorbed U(VI) relative to later addition of U(VI). Over time, however, the recalcitrant fraction of U(VI) decreased and converged to similar values for both early and late additions of U(VI). Electron microscopy indicated a portion of the acid resistant sorbed U(VI) was likely associated with grain boundaries between aggregating particles, not nanopores; and that these features were annealed out with increasing reaction time. Further, time elapsed imaging of U atom diffusion, that was stimulated by the electron beam, indicated that most sorbed U(VI) was not incorporated into the structure of goethite. This was confirmed and more rigorously quantified by ab initio molecular dynamics informed extended X-ray absorption fine structure spectra which indicated that only up to 5% of the recalcitrant U(VI) might be incorporated with an upper solubility limit of only U/Fe ∼0.02 atom% in goethite where U(VI) was added early in the process. We conclude that most of the recalcitrant U(VI) is simply strongly adsorbed to the surface and that it is the uranyl oxygens that both inhibit crystallization of goethite as well as incorporation of U into the structure of goethite.

The electron-catalyzed dissolution and reprecipitation of iron (oxyhydr)oxides constitute critical steps in natural geochemical iron cycling. However, the atomic-scale mechanisms of reductive dissolution and oxidative precipitation of Fe2+ remain poorly understood because they are difficult to directly experimentally probe. Using in situ synchrotron X-ray scattering and a novel electrochemical cell, we interrogate the interfacial structure between the hematite (α-Fe2O3) (1102) surface and acidic aqueous solution (5 mM Na2SO4, pH 4.0) under controlled electrochemical potential from the open circuit condition to cathodic bias as the reductive dissolution potential is approached and then exceeded. The crystalline order of the surface improves under mild reducing conditions, and the surface Fe stoichiometry changes with cathodic bias. After significant reductive dissolution occurs and cathodic bias is removed, dissolved Fe is redeposited, forming a disordered interface. Unlike at circumneutral pH, water laye...

The formation of traditional ionic materials occurs principally via joint accumulation of both anions and cations. Herein, we describe a previously unreported phenomenon by which macroscopic liquid-like thin layers with tunable self-organization properties form through accumulation of stable complex ions of one polarity on surfaces. Using a series of highly stable molecular anions we demonstrate a strong influence of the internal charge distribution of the molecular ions, which is usually shielded by counterions, on the properties of the layers. Detailed characterization reveals that the intrinsically unstable layers of anions on surfaces are stabilized by simultaneous accumulation of neutral molecules from the background environment. Different phases, self-organization mechanisms and optical properties are observed depending on the molecular properties of the deposited anions, the underlying surface and the coadsorbed neutral molecules. This demonstrates rational control of the macroscopic properties (morphology and size of the formed structures) of the newly discovered anion-based layers., Using ions of one polarity to form functional layers on surfaces is usually challenging because of counter ions which are inevitably present in the condensed phase. Here the authors demonstrate accumulation of mass-selected anions and neutral molecules from the gas phase to form a self-organizing liquid-like layer on a surface.

The characterization of trace elements in minerals using extended X-ray absorption fine structure (EXAFS) spectroscopy constitutes a first step toward understanding how impurities and contaminants interact with the host phase and the environment. However, limitations to EXAFS interpretation complicate the analysis of trace concentrations of impurities that are distributed across multiple phases in a heterogeneous system. Ab initio molecular dynamics (AIMD)-informed EXAFS analysis was employed to investigate the immobilization of trace uranium associated with nanophase iron (oxyhydr)oxides, a model system for the geochemical sequestration of radiotoxic actinides. The reductive transformation of ferrihydrite [Fe(OH)3] to nanoparticulate iron oxyhydroxide minerals in the presence of uranyl (UO2)2+(aq) resulted in the preferential incorporation of U into goethite (α-FeOOH) over lepidocrocite (γ-FeOOH), even though reaction conditions favored the formation of excess lepidocrocite. This unexpected result is sup...

The synthesis of fully epitaxial ferroelectric Hf0.5 Zr0.5 O2 (HZO) thin films through the use of a conducting pyrochlore oxide electrode that acts as a structural and chemical template is reported. Such pyrochlores, exemplified by Pb2 Ir2 O7 (PIO) and Bi2 Ru2 O7 (BRO), exhibit metallic conductivity with room-temperature resistivity of

The catalytic behavior of oxide-supported metal oxide species depends on the nature of the support and the presence of co-catalysts. We use density functional theory (DFT) to explore the relationship between the structure and chemical behavior of vanadium oxide in light of its industrial use for the selective catalytic reduction of nitric oxide with ammonia (NO-SCR). The relative stabilities of dispersed VO X monomers, dimers, and long-chain oligomers on two model oxide support surfaces with similar structure but drastically different chemical behavior, α-Al 2 O 3 (0001) and α-Fe 2 O 3 (0001), are determined. The effect of added tungsten, known to promote NO-SCR, is also investigated on the relatively inert α-Al 2 O 3 (0001) support. We find that the adsorption behavior of NH 3 , representing the first step of the NO-SCR reaction, depends strongly on the VO X local structure. Protonation of NH 3 to NH 4 + over surface hydroxyls is energetically favorable over VO X -WO X dimers and VO X oligomers, which are stabilized by the reducible α-Fe 2 O 3 (0001) support.

Memory technology continues to be at a crossroads. Established devices such as DRAM, Flash, and SRAM remain dominant, but each are facing increasingly formidable challenges in scaling, cost, and performance. Emerging memory technologies are attempting to displace these three or to find application spaces between those they traditionally support. However, no emerging memory has been able to demonstrate significant advantages over the incumbents, resulting in a proliferation of options such as phase change memory (PCM), filamentary resistive memory (ReRAM), ferroelectric memory (FERAM), magnetic memory (e.g. STT-MRAM and SOT-MRAM), and several others. Of these, the most successful has been PCM as evidenced by the commercial introduction of PCM based crosspoint memory in 2017. PCM based crosspoint memory has had limited market penetration in part due to the high manufacturing cost; the current layer-by-layer approach to PCM fabrication makes scaling difficult. In order to be cost competitive, crosspoint arrays will need to be manufactured in a style similar to that of 3D V-NAND where the active layers are deposited conformally inside deep features simultaneously forming hundreds of junctions. Chemical Vapor Deposition or Atomic Layer Deposition of these films is likely required to make this a reality. Some progress in materials for the PCM memory element has been reported; however, a remaining concern for crosspoint memory is the selector device, used in suppressing the sneak path current, consisting of a non-linear two-terminal device to properly select each memory element. In addition to high non-linearity, a selector device also requires high on-state current density JON, fast switching time, high endurance, stability, high scalability and back-end-of-line (BEOL) integration. Several selector devices have been proposed to date, e.g., mixed-ionic-electronic-conduction (MIEC), field-assisted superlinear threshold (FAST), and amorphous silicon (a-Si) selectors, but Ovonic Threshold Switch (OTS) composed of chalcogenide multinary materials clearly appears as the best candidate to cover all the mentioned requirements for high-density crosspoint applications. Currently, chalcogenide materials are deposited by using physical vapor deposition (PVD) that allows for fine tuning of the film stoichiometry, but at the same time limits its conformality and homogeneity on a large scale. To address this challenge, atomic layer deposition (ALD) is desirable; unfortunately, only ALD OTS As-free compositions with high leakage and limited endurance have been developed so far. In this report we describe the process and materials for GeSbTe PCM and a novel quaternary GeAsSeTe ALD OTS composition. We show a physical characterization of the ALD films and provide a comprehensive electrical characterization of integrated devices in DC and pulsed regimes. The OTS selector performance, in particular, shows excellent results (See Figure 1) - good selectivity (> 104), low threshold voltage (Vth) drift (< 40 mV/dec), fast switching (< 10 ns), excellent endurance (> 109 cycles) and robust stability. These results pave the way for future development of ALD chalcogenide-based selectors as a leading technology for multiple-stack integration of crosspoint memory arrays. To provide a physical understanding of the devices under study, we propose an analytical model for the subthreshold conduction mechanism, allowing for future development and optimization of the composition for 3D crosspoint memory applications. Figure 1. Electrical performance of an OTS selector over one billion cycles a) Pulsed-IV taken at each decade of the 1e9 endurance cycles. b) Sub-threshold current measured at each decade of the 1e9 endurance cycles. c) Evolution of the threshold voltage throughout the endurance test. Each point is measured three times, and all three measurements are reported d) Selector window at each decade of the endurance test. Figure 1

Radiotoxic uranium contamination in natural systems and nuclear waste containment can be sequestered by incorporation into naturally abundant iron (oxyhydr)oxides such as hematite (α-Fe

We demonstrate that the different surface terminations exhibited by α-Fe2O3 (hematite) and α-Cr2O3 (eskolaite) in superlattices (SL) of these materials, synthesized with exquisite control by molecular beam epitaxy, determine the heterojunction interface structure and result in controllable, non-commutative band offset values. Precise atomic control of the interface structure allowed us to vary the valence band offset from 0.35 eV to 0.79 eV. This controllable band alignment can be harnessed to generate a built-in potential in Fe2O3-Cr2O3 SLs. For instance, in a 2.5-period SL, a built-in potential of 0.8 eV was realized as measured by x-ray photoelectron spectroscopy of Ti dopants as probe species. The high quality of the SL structure was confirmed by atom probe tomography and scanning transmission electron microscopy. Enhanced photocurrents were measured for a thick Fe2O3 epitaxial film capped with an (Fe2O3)3-(Cr2O3)3 SL; this enhancement was attributed to efficient electron-hole separation in the SL as a result of the band alignment. The Fe-O-Cr bonds at the SL interfaces also red-shifted the onset of photoconductivity to ~1.6 eV. Exploiting the band alignment and photoabsorption properties of Fe2O3-Cr2O3 SLs has the potential to increase the efficiency of hematite-based photoelectrochemical water splitting.

The catalytic activity of oxide-supported vanadium oxide is improved by the presence of tungsten oxide for the selective catalytic reduction of nitric oxides. We propose a mechanism for V–W synergy through studies of the reduction–oxidation behavior of near-monolayer VOX and WOX species grown by atomic layer deposition on the α-Al2O3 (0001) single crystal surface. In situ X-ray standing wave measurements reveal an overlayer of W6+ species that is correlated with the substrate lattice as well as a redox-reversible shift from uncorrelated V5+ to correlated V4+. X-ray photoelectron spectroscopy and electronic structure calculations show a partial reduction of W6+ in the presence of V4+, improving the Bronsted acidity in mixed V–W catalyst systems. This mechanism of V–W synergy suggests that control of W d-states might be used as a design parameter for Bronsted acid sites in multicomponent oxide catalysts.

The interaction of water with metal oxide surfaces plays a crucial role in the catalytic and geochemical behavior of metal oxides. In a vast majority of studies, the interfacial structure is assumed to arise from a relatively static lowest energy configuration of atoms, even at room temperature. Using hematite (α-Fe2O3) as a model oxide, we show through a direct comparison of in situ synchrotron X-ray scattering with density functional theory-based molecular dynamics simulations that the structure of the (1102) termination is dynamically stabilized by picosecond water exchange. Simulations show frequent exchanges between terminal aquo groups and adsorbed water in locations and with partial residence times consistent with experimentally determined atomic sites and fractional occupancies. Frequent water exchange occurs even for an ultrathin adsorbed water film persisting on the surface under a dry atmosphere. The resulting time-averaged interfacial structure consists of a ridged lateral arrangement of adso...

The morphology of ultrathin zinc oxide films grown on Pt(111) was studied as a function of preparation and exposure conditions. The results show that submonolayer films exhibit a large variety of structures that may transform into each other depending on ambient conditions. The transformations are accompanied by substantial mass transport across the surface even at room temperature, indicating the presence and high diffusivity of migrating ZnOx species. Comparison with other metal-supported ZnO films shows that the metal substrate may play a role in such transformations. The structural diversity of ultrathin ZnO may be responsible for the continuing controversy over the role of ZnO in the catalytic performance of ZnO/metal systems.

We studied structure and reactivity of ZnO(0001) ultrathin films grown on Ag(111) and Cu(111) single crystal surfaces. Structural characterization was carried out by scanning tunneling microscopy, Auger electron spectroscopy, low-energy electron diffraction, and temperature programmed desorption. The CO oxidation behavior of the films was studied at low temperature (450 K) at near atmospheric pressures using gas chromatography. For ZnO/Cu(111), it is shown that under reaction conditions ZnO readily migrates into the Cu crystal bulk, and the reactivity is governed by a CuOx oxide film formed in the reaction ambient. In contrast, the planar structure of ZnO films on Ag(111) is maintained, similarly to the previously studied ZnO films on Pt(111). At sub-monolayer coverages, the “inverse” model catalysts are represented by two-monolayer-thick ZnO(0001) islands on Pt(111) and Ag(111) supports. While the CO oxidation rate is considerably increased on ZnO/Pt(111), which is attributed to active sites at the metal/oxide boundary, sub-monolayer ZnO films on Ag(111) did not show such an effect, and the reactivity was inhibited with increasing film coverage. The results are explained by much stronger adsorption of CO on Pt(111) as compared with Ag(111) in proximity to O species at the oxide/metal boundary. In addition, the water–gas shift and reverse water–gas shift reactions were examined on ZnO/Ag(111), which revealed no promotional effect of ZnO on the reactivity of Ag under the conditions studied. The latter finding suggests that wetting phenomena of ZnO on metals does not play a crucial role in the catalytic performance of ZnO-based real catalysts in those reactions.

X-ray studies of vanadium–tungsten mixed-monolayer-oxide catalysts grown on the rutile α-TiO2 (110) single crystal surface show redox behavior not observed for lone supported vanadium or tungsten oxides. Two cases are presented: sub-monolayer (sub-ML) vanadium oxide (vanadia) grown on ML tungsten oxide and ML vanadia grown on sub-ML tungsten oxide. The X-ray standing wave (XSW) and X-ray photoelectron spectroscopy (XPS) observations for both cases show coverage-dependent reversible redox-induced atomic-scale structural and chemical state changes. Atomic force microscopy shows that the mixed VOX/WOX overlayers have a conformal film-like structure in the as-deposited state. XSW analysis in light of XPS reveals that the V and W cations that are uncorrelated with the substrate lattice play an important role in catalytic redox reactions. Distinct differences in the redox-induced changes for these two mixed catalysts result from tuning the ratio of V to W, and relationships are drawn between the catalyst composition, structure, and chemistry. Comparison of these V–W mixed cases and the corresponding unmixed cases reveals a synergistic effect in which the reduction of W can be significantly enhanced by the addition of V.

Incorporation of economically or environmentally consequential polyvalent metals into iron (oxyhydr)oxides has applications in environmental chemistry, remediation, and materials science. A primary tool for characterizing the local coordination environment of such metals, and therefore building models to predict their behavior, is extended X-ray absorption fine structure spectroscopy (EXAFS). Accurate structural information can be lacking yet is required to constrain and inform data interpretation. In this regard, ab initio molecular dynamics (AIMD) was used to calculate the local coordination environment of minor amounts of U incorporated in the structure of goethite (α-FeOOH). U oxidation states (VI, V, and IV) and charge compensation schemes were varied. Simulated trajectories were used to calculate the U LIII-edge EXAFS function and fit experimental EXAFS data for U incorporated into goethite under reducing conditions. Calculations that closely matched the U EXAFS of the well-characterized mineral ura...

Relaxed structures and the related electronic environments of atomic monolayers and half-monolayers of tungsten with varying degrees of oxidation on the (0001) surface of hematite (α-Fe2O3) are modeled using first-principles density functional theory (DFT). This report focuses on the effect of nominally oxidizing and reducing chemical environments on surface structure and chemistry. By considering the position of W atoms relative to the substrate, calculated surface structures are compared to synchrotron X-ray standing wave (XSW) imaging results recently reported for this system. The question of W valence state, previously reported as nominally W5+ or W6+ in reducing or oxidizing surroundings, respectively, is addressed and discussed in light of X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (XAFS) results to clarify the relationship between valence state, oxygen coordination, and bond lengths.

Transparent, conducting p-La1−xSrxCrO3 epitaxial layers were deposited on Nb-doped SrTiO3(001) by oxygen-assisted molecular beam epitaxy to form structurally coherent p-n junctions. X-ray photoelectron spectroscopy reveals a type II or “staggered” band alignment, with valence and conduction band offsets of 2.0 eV and 0.9 eV, respectively. Diodes fabricated from these heterojunctions exhibit rectifying behavior, and the I-V characteristics are different from those for traditional semiconductor p-n junctions. A rather large ideality factor is ascribed to the complex nature of the interface.

Well-ordered ultrathin ZnO(0 0 0 1) films were grown on Pt(1 1 1) in a layer-by-layer mode. The reactivity of the films as a function of the film thickness and coverage was examined by the CO oxidation reaction at near-atmospheric pressures. At low temperatures (∼450 K), CO2 production is found to be much higher on the films of partial coverage than on dense ZnO(0 0 0 1) films and bare Pt(1 1 1). Under reaction conditions, monolayer islands and an entire monolayer film transform into two-monolayers-thick islands, which dominate the surface of the active catalysts. The results provide an adequate structural model for elucidating the reaction mechanism on the oxide/metal boundary at technologically relevant conditions.

Epitaxial chromium ferrite (Fe2CrO4), prepared by state-of-the-art oxygen plasma assisted molecular beam epitaxy, is shown to exhibit unusual electronic transport properties driven by the crystallographic structure and composition of the material. Replacing 1/3 of the Fe cations with Cr converts the host ferrimagnet from a metal into a semiconductor by virtue of its fixed valence (3+); Cr substitutes for Fe at B sites in the spinel lattice. By contrast, replacing 2/3 of the Fe cations with Cr results in an insulator. Three candidate conductive paths, all involving electron hopping between Fe2+ and Fe3+, are identified in Fe2CrO4. Moreover, Fe2CrO4 is shown to be photoconductive across the visible portion of the electromagnetic spectrum. As a result, this material is of potential interest for important photo-electrochemical processes such as water splitting.

The specific heat, and dc and ac magnetic susceptibility are reported for a large single crystal of PrOs(4)Sb(12) and, after grinding, its powder. The physical properties of the crystal are typical of the majority of reported PrOs(4)Sb(12) samples. The room temperature effective paramagnetic moment of the crystal was consistent with the Pr(3+) ionic configuration and full (or nearly full) occupancy of the Pr sublattice. The crystal showed two distinct anomalies in the specific heat and an overall discontinuity in C/T of approximately 1000 mJ K(-2) mol(-1). The upper transition (at T(c1)) was characteristically rounded. The anomaly at T(c2) was very sharp, consistent with a good quality for the crystal. We observed a shoulder in χ(') and two peaks in χ('') below T(c1). However, there were no signatures in χ(') of the lower temperature transition. Grinding to powder size smaller than 50 µm completely suppresses the upper superconducting transition in both the specific heat and magnetic susceptibility. It also strongly reduces ΔC/T(c) at T(c2). Stress annealing brings back some of this lost ΔC/T(c) but does not restore the upper temperature transition. Possible explanations of the existence of two superconducting specific heat anomalies for single crystals are discussed.

The electronic specific heat of ${\text{PrOs}}_{4}{\text{Sb}}_{12}$ was measured on different single crystals at temperatures down to 40 mK using a relaxation method. All investigated crystals exhibited a broad shoulder in specific heat near 0.4 K. This anomaly seems to be related to that found previously in Sb nuclear quadrupolar resonance study and possibly in lower critical-field and penetration-depth measurements. The onset of the specific-heat anomaly shows no response to magnetic fields as large as 0.2 T. These results imply a modification of superconducting properties (and normal-state properties) rather than appearance of a new superconducting phase at low temperatures. Our measurement detects low-temperature nuclear specific heat, which we suggest might be due to nuclear quadrupolar specific heat of Pr atoms, frozen in off-center positions.

The intercalation of various atomic species, such as hydrogen, to the interface between epitaxial graphene (EG) and its SiC substrate is known to significantly influence the electronic properties of the graphene overlayers. Here, we use high-resolution X-ray reflectivity to investigate the structural consequences of the hydrogen intercalation process used in the formation of quasi-free-standing (QFS) EG/SiC(0001). We confirm that the interfacial layer is converted to a layer structurally indistinguishable from that of the overlying graphene layers. This newly formed graphene layer becomes decoupled from the SiC substrate and, along with the other graphene layers within the film, is vertically displaced by ∼2.1 A. The number of total carbon layers is conserved during the process, and we observe no other structural changes such as interlayer intercalation or expansion of the graphene d-spacing. These results clarify the under-determined structure of hydrogen intercalated QFS-EG/SiC(0001) and provide a precise model to inform further fundamental and practical understanding of the system.