Hoydoo You

Substrate effects on the structure of epitaxial PbTiO3 thin films prepared on MgO, LaAlO3, and SrTiO3 by metalorganic chemical‐vapor deposition

Epitaxial PbTiO3 films were prepared by metalorganic chemical‐vapor deposition on MgO(001)‐, SrTiO3(001)‐, and LaAlO3(001)‐oriented substrates. Four‐circle x‐ray diffraction, transmission electron microscopy, Rutherford backscattering (RBS) channeling, and optical waveguiding were performed to characterize the deposited films. Epitaxial, single‐crystal films were obtained on all three substrate materials under the same growth conditions. However, the defect structure of the films, including grain tilting, threading dislocation density, and 90° domain formation, was strongly dependent on the choice of substrate material. Films grown on MgO(001) and LaAlO3(001) (pseudocubic indices) substrates are nominally c‐axis oriented; however, the PbTiO3 grains in the film form a fourfold domain structure, with the grains tilted ∼0.6° and ∼0.7°, respectively, toward the [100] directions (cubic or pseudo‐cubic) of the substrates. In addition, these films contain a significant volume fraction of 90°‐domain (a‐axis) stru...

Critical thickness of crystallization and discontinuous change in ferroelectric behavior with thickness in ferroelectric polymer thin films

We report on the observation of the critical thickness of crystallization of ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymer thin films, which were solution spun cast on platinum coated silicon wafer. The effect occurs at about 100 nm thickness, which is significantly above any currently known spatial dimensions of the polymer, so that for films at thickness below about 100 nm, the crystallization process is strongly hindered, resulting in a low crystallinity in these films. This low crystallinity leads to a large and discontinuous change of the dielectric constant and ferroelectric polarization in the films below the critical thickness.

Unique Activity of Platinum Adislands in the CO Electrooxidation Reaction

The development of electrocatalytic materials of enhanced activity and efficiency through careful manipulation, at the atomic scale, of the catalyst surface structure has long been a goal of electrochemists. To accomplish this ambitious objective, it would be necessary both to obtain a thorough understanding of the relationship between the atomic-level surface structure and the catalytic properties and to develop techniques to synthesize and stabilize desired active sites. In this contribution, we present a combined experimental and theoretical study in which we demonstrate how this approach can be used to develop novel, platinum-based electrocatalysts for the CO electrooxidation reaction in CO(g)-saturated solution; the catalysts show activities superior to any pure-metal catalysts previously known. We use a broad spectrum of electrochemical surface science techniques to synthesize and rigorously characterize the catalysts, which are composed of adisland-covered platinum surfaces, and we show that highly undercoordinated atoms on the adislands themselves are responsible for the remarkable activity of these materials.

Applications of surface X-ray scattering to electrochemistry problems

Abstract Applications of the synchrotron X-ray scattering technique to electrochemistry problems are briefly reviewed ranging from submonolayer level phenomena, through nanometer size phenomena, to submicron size phenomena; that is, covering the full range of the ‘interphase’ at an electrode surface. The examples include, (i) incipient oxidation/reduction of platinum single crystal surfaces; (ii) submonolayer/monolayer level oxidation/reduction of ruthenium dioxide single crystal surfaces; (iii) copper passivation/depassivation; and (iv) anodic formation of porous silicon and silicon dioxide layers. The design of several X-ray/electrochemical cells is also described.

Shape-Dependent Activity of Platinum Array Catalyst

We produced millions of morphologically identical platinum catalyst nanoparticles in the form of ordered arrays epitaxially grown on (111), (100), and (110) strontium titanate substrates using electron beam lithography. The ability to design, produce, and characterize the catalyst nanoparticles allowed us to relate microscopic morphologies with macroscopic catalytic reactivities. We evaluated the activity of three different arrays containing different ratios of (111) and (100) facets for an oxygen-reduction reaction, the most important reaction for fuel cells. Increased catalytic activity of the arrays points to a possible cooperative interplay between facets with different affinities to oxygen. We suggest that the surface area of (100) facets is one of the key factors governing catalyst performance in the electrochemical reduction of oxygen molecules.

Lead adsorption at the calcite-water interface: Synchrotron x-ray standing wave and x-ray reflectivity studies

Abstract By combining synchrotron X-ray standing wave (XSW) measurements with synchrotron X-ray reflectivity measurements, we have determined: (1) the precise three-dimensional location within the calcite unit cell of submonolayer Pb ions adsorbed at the calcite (104) surface from dilute aqueous solutions, and (2) the precise one-dimensional location of these unit cells relative to the calcite surface. Our XSW measurements, using three separate calcite Bragg reflections for triangulation, show that most adsorbed Pb ions occupy Ca sites in the calcite lattice with an ordered coverage of 0.05 equivalent monolayers, while the remaining Pb ions are disordered with a coverage of 0.03 equivalent monolayers. Our X-ray reflectivity measurements show that the ordered Pb ions occur primarily (>70%) in the surface atomic layer of calcite. Atomic force microscopy (AFM) was used to characterize the topography of the calcite (104) surface under conditions similar to the X-ray experiments. The quantitative morphological information obtained by AFM was used to develop realistic models of the calcite surface. The calculated X-ray reflectivities for these model surfaces were compared with the measured X-ray reflectivities. The new combined X-ray method that we have developed can be used to determine the atomic-scale structure of other metals adsorbed at mineral-water interfaces. Such high-resolution structural determinations are essential before detailed conceptual and theoretical models can be further developed to understand and predict the behavior of dissolved metals in mineral-water systems.

In‐situ x‐ray reflectivity study of incipient oxidation of Pt(111) surface in electrolyte solutions

Electrochemical oxidation causes the lifting of Pt atoms of the surface layer, substantiating a place‐exchange mechanism. Furthermore, for a charge transfer of ≲1.7e−/Pt atom, the flat surface is recovered by reduction, while the surface is irreversibly roughened by more excessive oxidation. Roughening involves only the atoms in the top layer.

Epitaxial TiO2 and VO2 films prepared by MOCVD

Abstract Titanium and vanadium oxide systems were selected to study the growth of thin film in a metal-organic chemical vapor deposition (MOCVD) process. Epitaxial TiO 2 and VO 2 films were obtained on sapphire (112¯0), (0001), and (11¯02) but not on Si(111). Eight distinct substrate-film epitaxial relationships have been determined by X-ray diffraction studies using a four-circle diffractometer. It was found that none of the eight epitaxial systems had a good lattice match between substrate and film. But further investigation revealed that substantial similarity existed in the local atomic patterns of the substrate and the film for all these systems. Nevertheless, it should be emphasized that mismatches of the local atomic patterns for these systems are, in general, substantially larger than those observed in the epitaxial systems containing semiconductor materials such as silicon and GaAs.

Uniaxial in-plane magnetic anisotropy of Ga1−xMnxAs

The anisotropic magnetic properties of a series of epitaxial Ga1−xMnxAs films with thicknesses ranging from 0.2to6.8μm were investigated using magnetometry and x-ray diffraction. The films all show a distinct uniaxial contribution to the magnetic anisotropy along the in-plane [110] direction, whose relative importance increases with the temperature. The uniaxial anisotropy field is found to be essentially thickness independent, ruling out the possibility that this is an effect produced by a single surface or interface due to well-known surface reconstruction. Furthermore, even the very thick samples are found to be tetragonal and coherently strained to the GaAs substrate. Our results suggest that the uniaxial anisotropy arises from surface reconstruction induced preferential Mn incorporation occurring at every step of layer-by-layer growth and thus uniformly penetrates the entire sample.

ELECTROCHEMICAL AND X-RAY SCATTERING STUDY OF WELL DEFINED RUO2 SINGLE CRYSTAL SURFACES

Abstract Electrochemical and synchrotron surface-X-ray scattering measurements were performed on two low index faces, (110) and (100), of RuO 2 single crystal electrodes in a variety of solutions. The two crystal faces displayed uniquely different cyclic voltammograms and the structural changes associated with cyclic voltammograms were investigated with synchrotron surface-X-ray scattering measurements. In sulfuric acid solution, the cyclic voltammogram of the (100) surface exhibits a reduction signature near the hydrogen evolution potential. The reduction feature was found to be associated with an expansion of the top ruthenium layer approximately along the (110) direction. The same reduction feature was seen much less clearly at the (110) surface. However, the associated displacement of ruthenium atoms is very similar to that of the (100) surface, leading to the conclusion that the oxygen bonds on the surface are elongated by a chemical reaction of the oxygen atoms with hydronium molecules in solution. In NaOH solution, the cyclic voltammogram of the (110) surface indicates two clearly identifiable oxidation features near the oxygen evolution potential. We find that the oxidation features are associated with the clearly identifiable surface-structure changes and these structure models are presented. The structure models and corresponding charge-transfer amounts were consistent with the pH-dependent cyclic voltammograms and the super-nernstian behavior measured by adding phosphoric acid to the solution.