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Epitaxial growth is a powerful tool for synthesizing heterostructures and integrating multiple functionalities. However, interfacial mixing can readily occur and significantly modify the properties of layered structures, particularly for those containing energy storage materials with smaller cations. Here, we show a two-step sequence involving the growth of an epitaxial LiCoO

The functionality of magnetite, Fe3O4, for catalysis and spintronics applications is dependent on the molar ratio of Fe2+ and Fe3+ and their distribution at the surface. In turn, this depends on a poorly understood interplay between crystallographic orientation, dopants, and the reactive adsorption of atmospheric species such as water. Here, (100)-, (110)-, and (111)-oriented films of titanomagnetite, Fe(3-x)TixO4, were grown by pulsed laser deposition and their composition, valence distribution, magnetism, and interaction with water were studied by ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and X-ray magnetic circular dichroism. Although the bulk compositions match the desired stoichiometry, the surfaces were found to be enriched in Ti4+, especially the top 1 nm. The highest surface energy (110) film was the most reduced, tied to local Ti enrichment, and a corresponding decreased magnetic moment. AP-XPS showed that incorporation of x = 0.25 Ti dramatically lowered the propensity to form hydroxyl species at a given relative humidity, and also that hydroxylation is relatively invariant with orientation. In contrast, the affinity for water is similar across orientations, regardless of Ti incorporation, suggesting that relative humidity controls its uptake. The findings may help demystify the interactions that lead to specific distributions of Fe2+ and Fe3+ at magnetite surfaces, toward design of more deliberately active catalysts and magnetic devices.

We assess the validity of criteria based on size mismatch and thermodynamics in predicting the stability of the rare class of high-entropy alloys (HEAs) that form in the hexagonal close-packed crystal structure. We focus on nanocrystalline HEA particles composed predominantly of Mo, Tc, Ru, Rh, and Pd along with Ag, Cd, and Te, which are produced in uranium dioxide fuel under the extreme conditions of nuclear reactor operation. The constituent elements are fission products that aggregate under the combined effects of irradiation and elevated temperature as high as 1200 °C. We present the recent results on alloy nanoparticle formation in irradiated ceria, which was selected as a surrogate for uranium dioxide, to show that radiation-enhanced diffusion plays an important role in the process. This work sheds light on the initial stages of alloy nanoparticle formation from a uniform dispersion of individual metals. The remarkable chemical durability of such multiple principal element alloys presents a solution, namely, an alloy waste form, to the challenge of immobilizing Tc.

Metallic particles formed in oxide fuels (e.g., UO2) during neutron irradiation have an adverse impact on fuel performance. A fundamental investigation of particle precipitation is needed to predict the fuel performance and potentially improve fuel designs and operations. This study reports on the precipitation of Mo-dominant β-phase particles in polycrystalline CeO2 (surrogate for UO2) films doped with Mo, Pd, Rh, Ru, and Re (surrogate for Tc). In situ heating scanning transmission electron microscopy indicates that particle precipitation starts at ∼1073 K with a limited particle growth to ∼10 nm. While particle concentration increases with increasing temperature, particle size remains largely unchanged up to 1273 K. There is a dramatic change in the microstructure following vacuum annealing at 1373 K, probably due to phase transition of reduced cerium oxide. At the high temperature, particles grow up to 75 nm or larger with distinctive facets. The particles are predominantly composed of Mo with a body-c...

Epsilon-phase alloy precipitates have been observed with varied compositions and sizes in spent nuclear fuels, such as UO2. The presence of the inclusions, along with other oxide precipitates, gas bubbles, and irradiation-induced structural defects, can significantly degrade the physical properties of the fuel. To predict fuel performance, a fundamental study of the precipitation processes is needed. This study uses ceria (CeO2) as a surrogate for UO2. Polycrystalline CeO2 films doped with Mo, Ru, Rh, Pd, and Re (surrogate for Tc) were grown at 823 K using pulsed laser deposition, irradiated at 673 K with He+ ions, and subsequently annealed at higher temperatures. A number of methods, including transmission electron microscopy and atom probe tomography, were applied to characterize the samples. The results indicate that there is a uniform distribution of the doped metals in the as-grown CeO2 film. Pd particles of ∼3 nm in size appear near the dislocation edges after He+ ion irradiation to ∼13 dpa. Thermal...

Understanding the interaction of water with compositionally tuned metal oxides is central to exploiting their unique catalytic and magnetic properties. However, processes such as hydroxylation, wetting, and resulting changes in electronic structure at ambient conditions are challenging to probe in situ. Here, we examine the hydroxylation and wetting of Fe(3–x)TixO4 (001)-oriented epitaxial films directly using ambient pressure X-ray photoelectron spectroscopy under controlled relative humidity. Fe2+ formation promoted by Ti4+ substitution for Fe3+ increases with hydroxylation, commensurate with a decrease in the surface work function or change in the surface dipole. The incorporation of small amounts of Ti (x = 0.25) as a bulk dopant dramatically impacts hydroxylation, in part due to surface segregation, leading to coverages closer to that of TiO2 than Fe3O4. However, the Fe(3–x)TixO4 compositional series shows a similar affinity for water physisorption, which begins at notably lower relative humidity tha...

Polarization-induced weak ferromagnetism (WFM) was demonstrated a few years back in LiNbO3-type compounds, MTiO3 (M = Fe, Mn, Ni). Although the coexistence of ferroelectric polarization and ferromagnetism has been demonstrated in this rare multiferroic family before, first in bulk FeTiO3, then in thin-film NiTiO3, the coupling of the two order parameters has not been confirmed. Here, we report the stabilization of polar, ferromagnetic NiTiO3 by oxide epitaxy on a LiNbO3 substrate utilizing tensile strain and demonstrate the theoretically predicted coupling between its polarization and ferromagnetism by X-ray magnetic circular dichroism under applied fields. The experimentally observed direction of ferroic ordering in the film is supported by simulations using the phase-field approach. Our work validates symmetry-based criteria and first-principles calculations of the coexistence of ferroelectricity and WFM in MTiO3 transition metal titanates crystallizing in the LiNbO3 structure. It also demonstrates the ...

Complex oxides and semiconductors exhibit distinct yet complementary properties owing to their respective ionic and covalent natures. By electrically coupling complex oxides to traditional semiconductors within epitaxial heterostructures, enhanced or novel functionalities beyond those of the constituent materials can potentially be realized. Essential to electrically coupling complex oxides to semiconductors is control of the physical structure of the epitaxially grown oxide, as well as the electronic structure of the interface. Here we discuss how composition of the perovskite A- and B-site cations can be manipulated to control the physical and electronic structure of semiconductor—complex oxide heterostructures. Two prototypical heterostructures, Ba1−xSrxTiO3/Ge and SrZrxTi1−xO3/Ge, will be discussed. In the case of Ba1−xSrxTiO3/Ge, we discuss how strain can be engineered through A-site composition to enable the re-orientable ferroelectric polarization of the former to be coupled to carriers in the semiconductor. In the case of SrZrxTi1−xO3/Ge we discuss how B-site composition can be exploited to control the band offset at the interface. Analogous to heterojunctions between compound semiconducting materials, control of band offsets, i.e., band-gap engineering, provides a pathway to electrically couple complex oxides to semiconductors to realize a host of functionalities.

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...

Pacific Northwest National Laboratory (PNNL), in collaboration with government agencies and industries, is actively engaged in the development, testing, and characterization of high efficiency, low cost modular solid oxide fuel cell power generation systems for stationary, automotive and military applications. Advanced SOFC systems are being developed which will offer ease of operation on a variety of gaseous liquid hydrocarbon and coal-derived fuels as well as "zero emissions" capability. SOFC R&D activities at PNNL continue in the areas of cell component materials, electrochemistry, cell design and modeling, high temperature corrosion, and fuel processing. Specific activities include development of optimized materials and cost effective fabrication techniques for high power density anode-supported cells operating at temperatures below 800 degrees C, characterization of processes responsible for high electrical performance and long term performance degradation, optimization and cell and stack designs using computational engineering models, and hydrocarbon fuel processing using micro technology.

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 LiCoO

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.

Complex oxides and semiconductors exhibit distinct yet complementary properties owing to their respective ionic and covalent natures. By electrically coupling oxides to semiconductors within epitaxial heterostructures, enhanced or novel functionalities beyond those of the constituent materials can potentially be realized. Key to electrically coupling oxides to semiconductors is controlling the physical and electronic structure of semiconductor – crystalline oxide heterostructures. Here we discuss how composition of the oxide can be manipulated to control physical and electronic structure in Ba1-xSrxTiO3/ Ge and SrZrxTi1-xO3/Ge heterostructures. In the case of the former we discuss how strain can be engineered through composition to enable the re-orientable ferroelectric polarization to be coupled to carriers in the semiconductor. In the case of the latter we discuss how composition can be exploited to control the band offset at the semiconductor - oxide interface. The ability to control the band offset, i.e. band-gap engineering, provides a pathway to electrically couple crystalline oxides to semiconductors to realize a host of functionalities.

Hysteretic behavior was studied in a series of Fe thin films, grown by molecular beam epitaxy, having different grain sizes and grown on different substrates. Major and minor loops and first order reversal curves (FORCs) were collected to investigate magnetization mechanisms and domain behavior under different magnetic histories. The minor loop coefficient and major loop coercivity increase with decreasing grain size due to higher defect concentration resisting domain wall movement. First order reversal curves allowed estimation of the contribution of irreversible and reversible susceptibilities and switching field distribution. The differences in shape of the major loops and first order reversal curves are described using a classical Preisach model with distributions of hysterons of different switching fields, providing a powerful visualization tool to help understand the magnetization switching behavior of Fe films as manifested in various experimental magnetization measurements.

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.

Polarization-induced weak ferromagnetism has been predicted a few years back in compounds MTiO 3 (M = Fe, Mn, Ni) (Fennie, 2008). We set out to stabilize this metastable, distorted perovskite structure by growing NiTiO 3 epitaxially on sapphire Al 2 O 3 (001) substrate, and to control the polar and magnetic properties via strain. Epitaxial Ni 1 − x Ti 1 − y O 3 films of different Ni/Ti ratios and thicknesses were deposited on Al 2 O 3 substrates by pulsed laser deposition at different temperatures, and characterized using several techniques. The effect of film thickness, deposition temperature, and film stoichiometry on lattice strain, film structure, and physical properties was investigated. Our structural data from x-ray diffraction, electron microscopy, and x-ray absorption spectroscopy shows that substrate-induced strain has a marked effect on the structure and crystalline quality of the films. Physical property measurements reveal a dependence of the Neel transition and lattice polarization on strain, and highlight our ability to control the ferroic properties in NiTiO 3 thin films by film stoichiometry and thickness.

High quantum efficiency enhancement is found for hybrid metal-insulator photocathodes consisting of thin films of CsI deposited on Cu(1 0 0), Ag(1 0 0), Au(1 1 1) and Au films irradiated by 266 nm laser pulses. Low work functions (near or below 2 eV) are observed following ultraviolet laser activation. Work functions are reduced by roughly 3 eV from that of clean metal surfaces. We discuss various mechanisms of quantum efficiency enhancement for alkali halide/metal photocathode systems and conclude that the large change in work function, due to Cs accumulation of Cs metal at the metal–alkali halide interface, is the dominant mechanism for quantum efficiency enhancement.

Ultrathin films of barium oxide were grown on Ag(001) and Ag(111) using the evaporation of Ba metal in an O2 atmosphere by molecular beam epitaxy. Ultraviolet photoemission spectroscopy reveals that films consisting of predominantly BaO or BaO2 result in Ag(001) work function reductions of 1.74 eV and 0.64 eV, respectively. On the Ag(001) surface, Ba oxide growth is initiated by two-dimensional nucleation of epitaxial BaO, followed by a transition to three-dimensional dual-phase nucleation of epitaxial BaO and BaO2. Three-dimensional islands of primarily BaO2(111) nucleate epitaxially on the Ag(111) substrate leaving large patches of Ag uncovered. We find no indication of chemical reaction or charge transfer between the films and the Ag substrates. These data suggest that the origin of the observed work function reduction is largely due to a combination of BaO surface relaxation and an electrostatic compressive effect.

Epitaxial rutile-structured single-crystal Mn x Ti 1 − x O 2 − δ films were synthesized on rutile- (110) and -(001) substrates using pulsed laser deposition. The films were characterized by reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and aberration-corrected transmission electron microscopy (ACTEM). Under the present conditions, 400 °C and P O 2 = 20 mTorr, single crystal epitaxial thin films were grown for x = 0.13, where x is the nominal average mole fraction of Mn. In fact, arbitrarily thick films could be grown with near invariant Mn/Ti concentration profiles from the substrate/film interface to the film surface. In contrast, at x = 0.25, Mn became enriched towards the surface and a secondary nano-scale phase formed which appeared to maintain the basic rutile structure but with enhanced z-contrast in the tunnels, or interstitial sites. Ab initio thermodynamic calculations provided quantitative estimates for the destabilizing effect of expanding the β-MnO 2 lattice parameters to those of TiO 2 -rutile, the stabilizing effect of diluting Mn with increasing Ti concentration, and competing reaction pathways for surface oxide formation.

We have measured high-resolution core-level and valence-band x-ray photoemission spectra for single-crystal $\mathrm{T}{\mathrm{i}}_{2}{\mathrm{O}}_{3}$ cleaved anoxically. The Ti(III) spectra for this lattice are considerably more complex than those measured for Ti(IV)-based oxides due to the presence of a single unpaired electron in the conduction band. This open-shell electron configuration leads to ligand-field split and frequently unresolved multiplets. The Ti $2p$ and $3p$ spectra have been calculated using relativistic Dirac-Hartree-Fock (DHF) theory with the sudden approximation for the intensities. Agreement between theory and experiment is excellent for the $3p$ spectrum, and very good for the $2p$ spectrum, the primary deficiency being a pair of features not captured by theory for the latter. The spectral line shapes are driven by final-state effects associated with angular momentum coupling of the unpaired valence electron with the core hole, one- and two-electron ligand-to-metal charge-transfer (shake) processes accompanying core photoionization, and core-hole screening by conduction-band electrons. The first two of these are accurately predicted by DHF theory with a small embedded cluster containing a single Ti cation and six oxygen ligands. The third effect is not predicted using this cluster in which screening of the core hole from electrons associated with more distant atoms is not possible.

The magnetic susceptibility of synthesized magnetite (Fe3O4) microspheres was found to decline after the growth of a metal–organic framework (MOF) shell on the magnetite core. Detailed structural analysis of the core–shell particles using scanning electron microscopy, transmission electron microscopy, atom probe tomography, and57Fe–Mossbauer spectroscopy suggests that the distribution of MOF precursors inside the magnetic core resulted in the oxidation of the iron oxide core.

Polarization-induced weak ferromagnetism (WFM) was demonstrated a few years back in LiNbO

Mixed Mn/Ti oxides present attractive physicochemical properties such as their ability to accommodate Li for application in Li-ion batteries. In this work, atomic parameters for Mn were developed to extend an existing shell model of the Li–Ti–O system and allow simulations of pure and lithiated Mn and mixed Mn/Ti oxide polymorphs. The shell model yielded good agreement with experimentally derived structures (i.e., lattice parameters and interatomic distances) and represented an improvement over existing potential models. The shell model was employed in molecular dynamics (MD) simulations of Li diffusion in the 1 × 1 c-direction channels of LixMn1–yTiyO2 with the rutile structure, where 0 ≤ x ≤ 0.25 and 0 ≤ y ≤ 1. In the infinite dilution limit, the arrangement of Mn and Ti ions in the lattice was found to have a significant effect on the activation energy for Li diffusion in the c channels due to the destabilization of half of the interstitial octahedral sites. Anomalous diffusion was demonstrated for Li ...

Following the Drude model for dielectric constant, e(E) = (E) + i(E), the plasmonic resonance energy Eres in a semiconductor depends on four parameters: () (or ) , effective mass m*, optical mobility µopt, and optical carrier concentration nopt. By solving the Drude equation at e(Eres) = 0, we obtain a relationship between µopt and nopt at constant Eres [or wavelength res(µm) = 1.2395/Eres(eV)]. A family of µopt vs nopt curves covering a range of res values (including the limiting wavelength res = ) constitute a plasmonic resonance phase diagram (PRPD) for a semiconductor defined by only e and m*. The PRPD is a convenient instrument that allows an immediate prediction of res from Hall-effect measurements of µH and nH. Furthermore, if the µH/nH point falls outside the family of µopt vs nopt curves, it shows that no resonance at all is possible. We apply the PRPD analysis to a series of ten ZnO samples grown by pulsed laser deposition at 200 C in an ambient of 33%H2:67%Ar and annealed in 25-C steps for 10 min in air at various temperatures from 400 C to 600 C. The unannealed sample had a resistivity  = 1.67 x 10-4 -cm, nmore » = 1.39 x 1021 cm-3, and µ = 26.8 cm2/V-s, and the 600-C sample,  = 1.52 -cm, n = 5.03 x 1017 cm-3, and µ = 8.2 cm2/V-s. Each of the ten µH/nH points was plotted on the PRPD. For the samples that were annealed at 550 C or lower, the µH/nH points yielded predicted values of res that ranged from 1.07 to 2.80 µm, respectively; however, the 575-C and 600-C samples were predicted to have no resonances at all. Reflectance curves for the eight samples annealed up to 550 C de-creased slowly from 6 eV down to about 1 eV, and then increased rapidly at an energy evidently close to Eres. In contrast, there was no such increase for the 575-C and 600-C samples, and thus presumably, no resonance. Satisfactory agreement is found between the reflectance and Hall-effect-predicted values of res.« less

X-ray photoelectron spectroscopy (XPS) is, in principle, based on a particularly simple process. Electrons within a sample absorb photons of a particular energy and then emerge from the solid. The kinetic energy analysis of electrons emitted from the surface yields information on the electronic states of atoms in the surface region. XPS is prominent in its popularity, versatility, and utility compared with many other techniques. The flexibility and efficacy of XPS will be highlighted through several applications. In addition, a look into spectroscopic imaging as well as future trends may add excitement to this seemingly utilitarian technique.

MTiO3 (M = Fe, Mn, Ni) compounds have received recent attention as possible candidates for multiferroic materials capable of magnetization switching by application of an electric field. In an attempt to stabilize NiTiO3 in the rhombohedral R3 structure, epitaxial Ni1 − xTi1 − yO3 films of different thickness and composition were deposited on Al2O3(0001) by pulsed laser deposition, and characterized using several techniques. Structural parameters for ilmenite-type NiTiO3 and the metastable LiNbO3-type NiTiO3 structure with the space group R3c were predicted using density functional theory calculations, and compared with the experimental results. Our structural data from X-ray diffraction and X-ray absorption spectroscopy indicate that epitaxial ilmenite-type NiTiO3 films were grown. Furthermore, lattice strain exerted by the sapphire substrate results in a distorted ilmenite structure similar to the LiNbO3-type one. While R3c NiTiO3, the desired structure based on recent theory, cannot be claimed at this point, our results demonstrate the potential of oxide heteroepitaxy to stabilize metastable multiferroic phases that may be difficult to prepare or are inaccessible in the bulk.

We show that the HF acid etch commonly used to prepare SrTiO3(001) for heteroepitaxial growth of complex oxides results in a non-negligible level of F doping within the terminal surface layer of TiO2. Using a combination of x-ray photoelectron spectroscopy and scanned angle x-ray photoelectron diffraction, we determine that on average ~ 13% of the O anions in the surface layer are replaced by F, but that F does not occupy O sites in deeper layers. Despite this perturbation to the surface, the Fermi level remains unpinned, and the surface-state density, which determines the amount of band bending, is driven by factors other than F doping. The presence of F at the STO surface is expected to result in lower electron mobilities at complex oxide heterojunctions involving STO substrates because of impurity scattering. Unintentional F doping can be substantially reduced by replacing the HF-etch step with a boil in deionized water, which in conjunction with an oxygen tube furnace anneal, leaves the surface flat and TiO2 terminated.

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.

In situ X-ray photoelectron spectroscopy (XPS) was utilized to identify the chemical state of silver in a range of silver oxide thin films obtained by codeposition of silver and atomic oxygen. A highly oxidized silver species was observed at an unexpectedly low Ag 3d5/2 binding energy (BE) of 366.8 eV with an associated broad satellite at 368.2 eV; this species was assigned as Ag(III). It was found to be highly unstable in vacuum but could be regenerated by further exposure to atomic oxygen. Both BE shifts and intensity changes of the O 1s peak were found to correlate with changes in the silver oxidation state. The theoretical XPS spectrum of high spin Ag(III) was calculated for both an isolated cation and an embedded AgO6 cluster.

The flux of atomic oxygen generated in an electron cyclotron resonance microwave plasma source was quantified by two different methods. The commonly applied approach of monitoring the frequency change of a silver-coated quartz crystal microbalance (QCM) deposition rate monitor as the silver is oxidized was found to underestimate the atomic oxygen flux by an order of magnitude compared to a more direct deposition approach. In the mixed-phase Ag/Ag 2 O deposition method, silver films were deposited in the presence of atomic oxygen such that the films were partially oxidized to Ag 2 O; X-ray photoelectron spectroscopy was utilized for quantification of the oxidized fraction. The inaccuracy of the QCM oxidation method was tentatively attributed to efficient catalytic recombination of O atoms on the silver surface.

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.

There is a growing interest in knowing the sputter rates for a wide variety of oxides because of their increasing technological importance in many different applications. To support the needs of users of the Environmental Molecular Sciences Laboratory, a national scientific user facility, as well as our research programs, the authors made a series of measurements of the sputter rates from oxide films that have been grown by oxygen plasma-assisted molecular beam epitaxy, pulsed laser deposition, atomic layer deposition, electrochemical oxidation, or sputter deposition. The sputter rates for these oxide films were determined in comparison with those from thermally grown SiO2, a common reference material for sputter rate determination. The film thicknesses and densities for most of these oxide films were measured using x-ray reflectivity. These oxide films were mounted in an x-ray photoelectron or Auger electron spectrometer for sputter rate measurements using argon ion sputtering. Although the primary objec...

We describe the synthesis and characterization of high-performance, superparamagnetic, iron oxide nanoparticle-based, heavy metal sorbents, which demonstrate excellent affinity for the separation of heavy metals in contaminated water systems (i.e., spiked Columbia River water). The magnetic nanoparticle sorbents were prepared from an easy-to-synthesize iron oxide precursor, followed by a simple, one-step ligand exchange reaction to introduce an affinity ligand to the nanoparticle surface that is specific to a heavy metal or class of heavy metal contaminants. The engineered magnetic nanoparticle sorbents have inherently high active surface areas, allowing for increased binding capacities. To demonstrate the performance of the nanoparticle sorbents, river water was spiked with specific metals and exposed to low concentrations of the functionalized nanoparticles. In almost all cases, the nanoparticles were found to be superior to commercially available sorbent materials as well as the unfunctionalized iron oxide nanoparticles.

Copper thiocyanate (CuSCN) is a candidate as a transparent solid p-type conductor for optoelectronic and photovoltaic applications, such as solar cells. We calculate the band structure, bonding characteristics, and basic native defect configurations of hexagonal β-CuSCN. β-CuSCN is predicted to be an indirect-gap semiconductor with an unusual orbital character: although the highest valence bands have the expected character of Cu 3d levels hybridized with S 3p states, the conduction band minimum (at the K point of the hexagonal Brillouin zone) has mostly cyanide antibonding character. This quasi-molecular character results in some unusual properties, including that the electron effective masses are comparable to or even larger than the hole effective masses. Calculated results match well with the valence band spectrum of thin film CuSCN, although optical absorption measurements do not conclusively confirm the predicted indirect nature of the lowest transitions. The dominant p-type character of this materia...

Antibody recognition force microscopy showed that OmcA and MtrC are expressed on the exterior surface of living Shewanella oneidensis MR-1 cells when Fe(III), including solid-phase hematite (Fe 2 O 3 ), was the terminal electron acceptor. OmcA was localized to the interface between the cell and mineral. MtrC displayed a more uniform distribution across the cell surface. Both cytochromes were associated with an extracellular polymeric substance.

Dissimilatory iron-reducing bacteria transfer electrons to solid ferric respiratory electron acceptors. Outer-membrane cytochromes expressed by these organisms are of interest in both microbial fuel cells and biofuel cells. We use optical waveguide lightmode spectroscopy (OWLS) to show that OmcA, an 85 kDa decaheme outer-membrane c-type cytochrome from Shewanella oneidensis MR-1, adsorbs to isostructural Al2O3 and Fe2O3 in similar amounts. Adsorption is ionic-strength and pH dependent (peak adsorption at pH 6.5–7.0). The thickness of the OmcA layer on Al2O3 at pH 7.0 [5.8 ± 1.1 (2σ) nm] from OWLS is similar, within error, to that observed using atomic force microscopy (4.8 ± 2 nm). The highest adsorption density observed was 334 ng cm−2 (2.4 × 1012 molecules cm−2), corresponding to a monolayer of 9.9 nm diameter spheres or submonolayer coverage by smaller molecules. Direct electrochemistry of OmcA on Fe2O3 electrodes was observed using cyclic voltammetry, with cathodic peak potentials of −380 to −320 mV versus Ag/AgCl. Variations in the cathodic peak positions are speculatively attributed to redox-linked conformation change or changes in molecular orientation. OmcA can exchange electrons with ITO electrodes at higher current densities than with Fe2O3. Overall, OmcA can bind to and exchange electrons with several oxides, and thus its utility in fuel cells is not restricted to Fe2O3.

Photoemission electron microscopy (PEEM) excited by x-ray and UV sources is used to investigate epitaxial anatase thin films embedded with rutile nanocrystals, a model system for the study of heterocatalysis on mixed-phase TiO2. Both excitation sources show distinct contrast between the two TiO2 phases, however, the contrast is reversed. Rutile nanocrystals appear darker than the anatase film in X-ray PEEM images but brighter in UV-PEEM images. Topography-induced contrast is dominant X-ray PEEM imaging, whereas work function contrast, dominates for UV-PEEM. Work function contrast results from the differences in work function and surface defect state densities between the two phases near the Fermi level. This assertion is confirmed by UPS data that shows the rutile work function to be 0.2 eV lower and a greater occupied valence band density-of-states in rutile (100) than in anatase (001). Since the boundaries between rutile nanocrystals and the anatase film are clearly resolved, these results indicate that PEEM studies of excited state dynamics and heterocatalysis are possible at chemically intriguing mixed-phase TiO2 interfaces and grain boundaries.

Shewanella oneidensis MR-1 is purported to express outer membrane cytochromes (e.g., MtrC and OmcA) that transfer electrons directly to Fe(III) in a mineral during anaerobic respiration. A prerequisite for this type of reaction would be the formation of a stable bond between a cytochrome and an iron oxide surface. Atomic force microscopy (AFM) was used to detect whether a specific bond forms between a hematite (Fe 2 O 3 ) thin film, created with oxygen plasma-assisted molecular beam epitaxy, and recombinant MtrC or OmcA molecules coupled to gold substrates. Force spectra displayed a unique force signature indicative of a specific bond between each cytochrome and the hematite surface. The strength of the OmcA-hematite bond was approximately twice that of the MtrC-hematite bond, but direct binding to hematite was twice as favorable for MtrC. Reversible folding/unfolding reactions were observed for mechanically denatured MtrC molecules bound to hematite. The force measurements for the hematite-cytochrome pairs were compared to spectra collected for an iron oxide and S. oneidensis under anaerobic conditions. There is a strong correlation between the whole-cell and pure-protein force spectra, suggesting that the unique binding attributes of each cytochrome complement one another and allow both MtrC and OmcA to play a prominent role in the transfer of electrons to Fe(III) in minerals. Finally, by comparing the magnitudes of binding force for the whole-cell versus pure-protein data, we were able to estimate that a single bacterium of S. oneidensis (2 by 0.5 μm) expresses ∼10 4 cytochromes on its outer surface.

Thermally-induced martensitic phase transformations in polycrystalline CuZnAl and thin-film NiTiCu shape memory alloys were probed using photoemission electron microscopy (PEEM). Ultra-violet photoelectron spectroscopy shows a reversible change in the apparent work function during transformation, presumably due to the contrasting surface electronic structures of the martensite and austenite phases. In situ PEEM images provide information on the spatial distribution of these phases and the evolution of the surface microstructure during transformation. PEEM offers considerable potential for improving our understanding of martensitic transformations in shape memory alloys in real time.

We have explored the systematics of TiO2 polymorph nucleation during film growth by molecular beam epitaxy on perovskite substrates. The accidental lattice match between anatase (0 0 1) and LaAlO3(0 0 1) or SrTiO3(0 0 1) typically results in anatase nucleation at the interface. However, the growth conditions dictate whether or not rutile also nucleates, and the associated morphological and structural properties of the composite film. Four symmetry equivalent epitaxial orientations of rutile on anatase are observed when rutile nucleates as discrete particles on LaAlO3(0 0 1). Such films constitute model systems for studying the anatase/rutile interface, which is of considerable current interest in photochemistry.

The thermally-induced martensitic phase transformation in a polycrystalline NiTiCu thin film shape memory alloy was probed by photoelectron emission microscopy (PEEM). In situ PEEM images reveal distinct changes in microstructure and photoemission intensity at the phase transition temperatures. In particular, images of the low temperature, martensite phase are brighter than that of the high temperature, austenite phase, due to the relatively lower work function of the martensite. Ultra-violet photoelectron spectroscopy shows that the effective work function changes by about 0.16 eV during thermal cycling. In situ PEEM images also show that the network of trenches observed on the room temperature film disappear suddenly during heating and reappear suddenly during subsequent cooling. These trenches are also characterized by atomic force microscopy at selected temperatures. We describe implications of these observations with respect to the spatial distribution of phases during thermal cycling in this thin film shape memory alloy.

Over the past five years, considerable work has been carried out in the exploration of candidate diluted oxide magnetic semiconductors with high Curie temperatures. Fueled by early experimental results and theoretical predictions, claims of ferromagnetism at and above room temperature in doped oxides have abounded. In general, neither the true nature of these materials nor the physical causes of the magnetism have been adequately determined. It is now apparent that these dilute magnetic systems are deceptively complex. We consider two well-characterizedn-type magnetically doped oxide semiconductors and explore the relationship between donor electrons and ferromagnetism.

It has recently been demonstrated that magnetically doped TiO2 and SnO2 show ferromagnetism at room temperature and Curie temperatures above room temperature. However, accurate knowledge of dopant concentrations is necessary to quantify magnetic moments in these materials. Rutherford backscattering spectrometry (RBS) is one of the powerful techniques to quantify magnetic transition-metal dopant concentrations in these materials. However, in some cases, the interference of RBS signals for different dopants and substrate elements in these materials makes analysis difficult. In this work, we demonstrate that particle induced X-ray emission (PIXE) can be successfully used to quantify the magnetic transition-element dopants in several room temperature ferromagnetic materials synthesized using three different synthesis methods: oxygen plasma-assisted molecular-beam epitaxy, ion implantation and wet chemical synthesis.

The surface of an anatase titanium dioxide (TiO2) film grown on LaAlO3(001) was observed using noncontact atomic force microscopy (NC-AFM). After cleaning with cycles of argon ion sputtering and annealing in vacuum, (1 × 4)- and (1 × 5)-reconstructed terraces appeared. In addition to the terraces, the sputter-annealed surface included many agglomerations. The presence of Tin+ () shown by x-ray photoelectron spectroscopy indicates that the agglomerations are due to TiOx () species. Limited replenishment of oxygen by the substrate and the low diffusivity of the Tin+ to the substrate are possible causes for the generation of the TiOx.

We reexamine a well-established method for determining valence band maxima (VBM) in semiconductors based on fitting photoemission valence band spectra to theoretical densities of states. In contrast to the situation for covalent semiconductors, application of this technique to SrTiO3 produces poor fits when the density of states is computed within the local density or generalized gradient approximation. The resulting VBM is too high by several tenths of an eV. However, an excellent fit, and a more physically reasonable VBM, is obtained when the density of states is computed within a recently-developed self-consistent GW approximation. Extrapolating the X-ray excited leading edge to the energy axis, and finding the energy at which the UV-excited leading edge intensity goes to zero, also yield physically reasonable VBM values that are in good mutual agreement, and in good agreement with the VBM obtained by fitting to GW theory. These numbers are useful for accurate band offset determination.