Paul A. Salvador

In situ characterization of strontium surface segregation in epitaxial La0.7Sr0.3MnO3 thin films as a function of oxygen partial pressure

Using in situ synchrotron measurements of total reflection x-ray fluorescence, we find evidence of strontium surface segregation in (001)-oriented La0.7Sr0.3MnO3 thin films over a wide range of temperatures (25–900 °C) and oxygen partial pressures (pO2=0.15–150 Torr). The strontium surface concentration is observed to increase with decreasing pO2, suggesting that the surface oxygen vacancy concentration plays a significant role in controlling the degree of segregation. Interestingly, the enthalpy of segregation becomes less exothermic with increasing pO2, varying from −9.5 to −2.0 kJ/mol. In contrast, the La0.7Sr0.3MnO3 film thickness and epitaxial strain state have little impact on segregation behavior.

Spatially selective visible light photocatalytic activity of TiO2/BiFeO3 heterostructures

Heterostructures of thin titania films on BiFeO3 substrates were grown by pulsed laser deposition. The heterostructures, when excited by visible light with energies between 2.53 and 2.70 eV, photochemically reduce aqueous silver cations from solution in patterns that mimic the structure of the ferroelectric domains in the substrate. Under the same conditions, titania by itself reduces insignificant amounts of silver. The observations indicate that electrons generated in the substrate are influenced by dipolar fields in the ferroelectric domains and transported through the titania film to reduce silver on the surface.

Mobility of oxygen vacancy in SrTiO3 and its implications for oxygen-migration-based resistance switching

Capacitance−voltage characteristics of high quality Pt Schottky diodes fabricated on oxygen-vacancy-doped SrTiO3 single crystals were used to obtain the oxygen vacancy profiles within one microns of the Pt interface. Computer simulations based on solving the drift-diffusion equations for electrons and ionized vacancies were performed to understand the experimentally observed oxygen vacancy profile’s time-evolution at room temperature and 0 V applied bias. Building upon this understanding, the diode’s room temperature profile evolution under −35 V applied bias was analyzed to yield a vacancy mobility value of 1.5 × 10−13 cm2/V·s at an electric field of 500 kV/cm. This mobility is 8 orders of magnitude too low to produce nanosecond resistance switching in thin film devices. The applicability of the results to oxygen-migration-based resistance switching is discussed relative to recent observations and modeling.

Electron tunneling characteristics on La0.7Sr0.3MnO3 thin-film surfaces at high temperature

We report on the electron tunneling characteristics on La0.7Sr0.3MnO3 (LSM) thin-film surfaces up to 580 °C in 10−3 mbar oxygen pressure, using scanning tunneling microscopy/spectroscopy (STM/STS). A thresholdlike drop in the tunneling current was observed at positive bias in STS, which is interpreted as a unique indicator for the activation polarization in cation-oxygen bonding on LSM cathodes. Sr-enrichment was found on the surface at high temperature using Auger electron spectroscopy, and was accompanied by a decrease in tunneling conductance in STS. This suggests that Sr-terminated surfaces are less active for electron transfer in oxygen reduction compared to Mn-terminated surfaces on LSM.

Growth and magnetoresistive properties of (LaMnO3)m(SrMnO3)n superlattices

Artificial (LaMnO3)m(SrMnO3)n superlattices, n/(m+n)=0.26, were grown with pulsed-laser deposition, and the superlattice period Λ was varied. Their structural characteristics were investigated using x-ray diffraction and electron microscopy. When Λ<ap (perovskite lattice parameter), bulk-like properties of La0.74Sr0.26MnO3 are obtained: TMI (metal-to-insulator transition temperature) ≈Tc (Curie temperature) ≈345 K and −MR (magnetoresistance) ≈100%. Increasing Λ (to 21 ap) leads to a decrease in the Tc, the low-temperature magnetization, the magnetoresistance, and the TMI—eventually becoming insulating at low temperatures. These effects can be explained by Mn3+/Mn4+ separation arising from the artificially induced La/Sr order.

Visible light photochemical activity of heterostructured PbTiO3–TiO2 core–shell particles

Heterostructured powders composed of microcrystalline PbTiO3 cores coated with nanostructured TiO2 shells were prepared by a sol–gel method. When exposed to visible light (λ > 420 nm), the heterostructured powder degrades methylene blue at a rate 4.8 times greater than PbTiO3, TiO2, or mechanical mixtures of the phases. The rate at which the heterostructured powder degrades methylene blue depends on the processing temperature; samples annealed at 500 °C are the most reactive, even though they do not have the highest surface area. Enhanced degradation rates were not observed for Pb-doped TiO2, indicating that it is the core–shell architecture and not contamination by lead that increases the reactivity. The improved reactivity of microcrystalline-PbTiO3/nanostructured-TiO2 heterostructures in visible light is attributed to visible light absorption in the PbTiO3 core, charge separation at the buried interface between the ferroelectric PbTiO3 and the dielectric TiO2, and dye degradation on the nanostructured TiO2 shell.

Effect of Crystal and Domain Orientation on the Visible-Light Photochemical Reduction of Ag on BiFeO3

The reduction of silver from an aqueous solution on BiFeO₃ surfaces, activated by visible light, was investigated as a function of crystal and ferroelectric domain orientation. When excited by light with energy between 2.53 and 2.70 eV, BiFeO₃ photochemically reduces silver cations from solution in patterns corresponding to the underlying ferroelectric domain structure. Silver is preferentially reduced on domains with a positive polarization directed toward the surface. The amount of reduced silver depends on whether the component of the domain polarization normal to the surface is positive or negative, but is relatively insensitive to the crystal orientation. It is concluded that the ferroelectric polarization decreases electron drift to the surface in domains with a negative polarization, causing spatially selective photochemical behavior, and that the direction of the polarization is more important than the amplitude.

Visible-Light Photochemical Activity of Heterostructured Core–Shell Materials Composed of Selected Ternary Titanates and Ferrites Coated by TiO2

Heterostructured photocatalysts comprised of microcrystalline (mc-) cores and nanostructured (ns-) shells were prepared by the sol-gel method. The ability of titania-coated ATiO3 (A = Fe, Pb) and AFeO3 (A = Bi, La, Y) catalysts to degrade methylene blue in visible light (λ > 420 nm) was compared. The catalysts with the titanate cores had enhanced photocatalytic activities for methylene blue degradation compared to their components alone, whereas the catalysts with ferrite cores did not. The temperature at which the ns-titania shell is crystallized influences the photocatalytic dye degradation. mc-FeTiO3/ns-TiO2 annealed at 500 °C shows the highest reaction rate. Fe-doped TiO2, which absorbs visible light, did not show enhanced photocatalytic activity for methylene blue degradation. This result indicates that iron contamination is not a decisive factor in the reduced reactivity of the titania coated ferrite catalysts. The higher reactivity of materials with the titanate cores suggests that photogenerated charge carriers are more easily transported across the titanate-titanate interface than the ferrite-titanate interface and this provides guidance for materials selection in composite catalyst design.

The Effect of Chromium Oxyhydroxide on Solid Oxide Fuel Cells

Hexavalent chromium species like the oxyhydroxide, CrO{sub 2}(OH){sub 2}, or hexoxide, CrO{sub 3}, are electrochemically reduced to Cr{sub 2}O{sub 3} in solid oxide fuel cells and adversely affect the cell operating potentials. Using a narrowly focused beam from the Advanced Photon Source, such chromium oxide deposits were unequivocally identified in the active region of the cathode by X-ray diffraction, suggesting that the triple phase boundaries were partially blocked. Under fuel cell operating conditions, the reaction has an equilibrium potential of about 0.9 V and the rate of chromium oxide deposition is therefore dependent on the operating potential of the cell. It becomes diffusion limited after several hours of steady operation. At low operating potentials, lanthanum manganite cathodes begin to be reduced to MnO, which reacts with the chromium oxide to form the MnCr{sub 2}O{sub 4} spinel.

Surface engineering along the close-packed direction of SrTiO3

We have investigated the effects of wet etching with a 3 : 1 mixture of HCl : HNO3 and of annealing at 8508C on the surface morphology of [1 1 1]-oriented SrTiO3 single crystals. Atomic force microscopy is used to demonstrate that the surface morphology is a strong function of both etching and annealing time. All surfaces have step heights equal to integral or half-integral multiples of the (1 1 1) interplanar spacing. However, step bunching, non-regular step heights, granularity, inhomogeneous surface morphology, and etch pits are observed on many surfaces. A combination of etching and annealing leads to surfaces that are free of these irregularities and are characterized by well-developed step