Showing papers on "Oxide published in 1999"

Oxide thin film

Abstract: An object of the invention is to provide an oxide thin film which exhibits a widegap or transparency and p-type conductivity although it has heretofore been very difficult to form. The oxide thin film formed on a substrate contains copper oxide and strontium oxide as a main component and exhibits p-type conductivity at a bandgap of at least 2 eV.

Block Copolymer Templating Syntheses of Mesoporous Metal Oxides with Large Ordering Lengths and Semicrystalline Framework

Abstract: A simple and general procedure has been developed for the syntheses of ordered largepore (up to 14 nm) mesoporous metal oxides, including TiO2, ZrO2 ,N b 2O5 ,T a 2O5 ,A l 2O3, SiO2, SnO2 ,W O 3, HfO2, and mixed oxides SiAlOy ,A l 2TiOy, ZrTiOy, SiTiOy, ZrW2Oy. Amphiphilic poly(alkylene oxide) block copolymers were used as structure-directing agents in nonaqueous solutions for organizing the network-forming metal oxide species. Inorganic salts, rather than alkoxides or organic metal complexes, were used as soluble and hydrolyzable precursors to the polymerized metal oxide framework. These thermally stable mesoporous oxides have robust inorganic frameworks and thick channel walls, within which high densities of nanocrystallites can be nucleated. These novel mesoporous metal oxides are believed to be formed through a mechanism that combines block copolymer self-assembly with alkylene oxide complexation of the inorganic metal species.

The electronic structure at the atomic scale of ultrathin gate oxides

Abstract: The narrowest feature on present-day integrated circuits is the gate oxide—the thin dielectric layer that forms the basis of field-effect device structures. Silicon dioxide is the dielectric of choice and, if present miniaturization trends continue, the projected oxide thickness by 2012 will be less than one nanometre, or about five silicon atoms across1. At least two of those five atoms will be at the silicon–oxide interfaces, and so will have very different electrical and optical properties from the desired bulk oxide, while constituting a significant fraction of the dielectric layer. Here we use electron-energy-loss spectroscopy in a scanning transmission electron microscope to measure the chemical composition and electronic structure, at the atomic scale, across gate oxides as thin as one nanometre. We are able to resolve the interfacial states that result from the spillover of the silicon conduction-band wavefunctions into the oxide. The spatial extent of these states places a fundamental limit of 0.7 nm (four silicon atoms across) on the thinnest usable silicon dioxide gate dielectric. And for present-day oxide growth techniques, interface roughness will raise this limit to 1.2 nm.

Wettability at high temperatures

Abstract: Chapter headings and selected sub-headings: Series Preface. Preface. Fundamental Equations of Wetting. Surface and interfacial energies in solid / liquid / vapour systems. Dynamics of Wetting by Metals and Glasses. Non-reactive wetting. Reactive wetting. Methods of Measuring Wettability Parameters. Sessile drop experiments. Surface Energies. Data for metals and alloys. Data for non-metallic compounds. Wetting Properties of Metal / Metal Systems. Effects of alloying elements. Systems that form intermetallic compounds. Wetting Properties of Metal / Oxide Systems. Non-reactive pure metal / ionocovalent oxide systems. Effect of electronic structure of the oxide. Wetting of fluorides. Wetting Properties of Metal / Non-Oxide Ceramic Systems. Metals on predominantly covalent ceramics. Wetting Properties of Metal / Carbon Systems. Non-reactive systems. Reactive systems. Wetting by Glasses and Salts. The glassy state. Wetting behaviour. Wetting When Joining. Flow into capillary gaps. Effects on mechanical properties. Appendices. List of symbols. Index

Atomic layer deposition with nitrate containing precursors

Abstract: Metal nitrate-containing precursor compounds are employed in atomic layer deposition processes to form metal-containing films, eg metal, metal oxide, and metal nitride, which films exhibit an atomically abrupt interface and an excellent uniformity

Theory of Surface States

Abstract: The properties of metal-to-semiconductor junctions and of free semiconductor surfaces are usually explained on the basis of surface states. The theory of the metal contacts is discussed critically, because strictly speaking localized surface states cannot exist in such junctions. However, it is shown that virtual or resonance surface states can exist which behave for practical purposes in the same way. They are really the tails of the metal wave functions rather than separate states. In the past, the length of this tail has often been ignored. Some estimates of its length are made and its consequences pointed out. A semiquantitative discussion is given of various recent data, including the effect of an oxide layer on barrier height, the variation of barrier height with the metal, the work function of a free surface at high doping, and the effect of a cesium layer on the work function.

Bismuth based oxide electrolytes— structure and ionic conductivity

Abstract: Bismuth oxide systems exhibit high oxide ion conductivity and have been proposed as good electrolyte materials for applications such as solid oxide fuel cells and oxygen sensors. However, due to their instability under conditions of low oxygen partial pressures there has been difficulty in developing these materials as alternative electrolyte materials compared to the state-of-the-art cubic stabilised zirconia electrolyte. Bismuth oxide and doped bismuth oxide systems exhibit a complex array of structures and properties depending upon the dopant concentration, temperature and atmosphere. In this paper we comprehensively review the structures, thermal expansion, phase transitions, electrical conductivity and stability of bismuth oxide and doped bismuth oxide systems. ©

Passivity : the key to our metals-based civilization

Abstract: Humankind has been able to develop a metals-based civilization primarily because the reactive metals (Fe, Ni, Cr, Al, Ti, Zr, . . .) exhibit extraordinary kinetic stabilities in oxidizing environments. From the time of Schonbein and Faraday (1830s), the reason for this stability has been attributed to the existence of a thin reaction product ®lm on the metal (or alloy) surface. This ®lm effectively isolates the metal from the corrosive environment. However, attempts to elucidate the mechanisms of the formation of passive oxide ®lms, which generally comprise bilayer structures consisting of a defective oxide that grows directly into the metal and an outer, precipitated hydroxide (or oxyhydroxide on even oxide) layer, have yielded only a rudimentary understanding of the chemistry and physics of the growth and breakdown processes. In this paper, selected aspects of passivity and passivity breakdown are reviewed, with emphasis on the physical models that have been proposed to account for the experimental observations. One such model, the Point Defect Model, is shown to account for most, if not all, experimental observations, and to provide a robust basis for predicting the occurrence of passivity breakdown in any given system. By combining the Point Defect Model with deterministic models for pit growth and crack growth, it is now possible to predict the evolution of localized corrosion damage in a wide range of systems.

Interfacial Electrochemistry: Theory: Experiment, and Applications

Abstract: Theory and modelling: theoretical modelling of the solid-liquid interface - chemically specific simulation methods theory of the metal-electrolyte interface structure of water at the water-metal interface - molecular dynamics computer simulations Monte Carlo methods for equilibrium and nonequilibrium problems in interfacial electrochemistry electronic structure calculations of polyatomic oxyanions adsorbed on metal surfaces. electrochemical surface science: electro-chemical processes involving H adsorbed at metal electrode surfaces reconstruction of gold surfaces in situ surface X-ray scattering studies of electrosorption atomic-scale aspects of anodic dissolution of metals - studies by in situ scanning funnelling microscopy Auger spectroscopy of electrode surfaces. Nonmetallic and semiconductor electrodes: electrochemical properties of carbon surfaces bulk and surface states of reactive oxide films - an extended semiconductor model with Ti, Ni, and Fe as examples solid-state voltammetry conducting polymer films as electrodes photoelectrochemistry of nanostructured semiconductors - the case of anatase TiO2. Electrocatalysis: interfacial electrochemistry of conductive oxides for electrocatalysis kinetic modelling of electrocatalytic reactions - methanol oxidation on platinum electrodes properties of bimetallic electrodes - spectroscopic characterization and electrocatalysis electrocatalysis as well-defined surfaces - kinetics of oxygen reduction and hydrogen oxidation/evolution on Pr(hkl) electrodes mechanism of methanol electro-oxidation. (Part contents).

A Solution-Phase Chemical Approach to a New Crystal Structure of Cobalt.

Abstract: Kinetic control of crystal growth in the presence of a coordinating ligand is critical for the formation of a new structure of elemental cobalt (e-cobalt, the unit cell with the two different types of cobalt atoms is shown), which was discovered upon analyzing the metallic powder produced by the thermal decomposition of [Co2 (CO)8 ] in solution in the presence of trioctylphosphane oxide [TOPO, Eq. (1)].

H2 or O2 Evolution from Aqueous Solutions on Layered Oxide Photocatalysts Consisting of Bi3+ with 6s2 Configuration and d0 Transition Metal Ions

Abstract: Bi2W2O9, Bi2WO6, and Bi3TiNbO9 consisting of layered structure with perovskite slabs interleaved with Bi2O2 layers showed photocatalytic activities for H2 evolution from an aqueous methanol solution and O2 evolution from an aqueous silver nitrate solution. Bi2WO6 with the Aurivillius structure and a 2.8 eV band gap was active for the O2 evolution reaction under visible light irradiation (λ > 420 nm).

Epitaxial cubic gadolinium oxide as a dielectric for gallium arsenide passivation

Abstract: Epitaxial growth of single-crystal gadolinium oxide dielectric thin films on gallium arsenide is reported. The gadolinium oxide film has a cubic structure isomorphic to manganese oxide and is (110)-oriented in single domain on the (100) gallium arsenide surface. The gadolinium oxide film has a dielectric constant of approximately 10, with low leakage current densities of about 10(-9) to 10(-10) amperes per square centimeter at zero bias. Typical breakdown field is 4 megavolts per centimeter for an oxide film 185 angstroms thick and 10 megavolts per centimeter for an oxide 45 angstroms thick. Both accumulation and inversion layers were observed in the gadolinium oxide-gallium arsenide metal oxide semiconductor diodes, using capacitance-voltage measurements. The ability to grow thin single-crystal oxide films on gallium arsenide with a low interfacial density of states has great potential impact on the electronic industry of compound semiconductors.

Solid-State Coordination Chemistry: The Self-Assembly of Microporous Organic–Inorganic Hybrid Frameworks Constructed from Tetrapyridylporphyrin and Bimetallic Oxide Chains or Oxide Clusters

Abstract: The hydrothermal reactions of MoO(3), tetrapyridylporphyrin (tpypor), water, and the appropriate M(II) precursor yield the first examples of three-dimensional framework materials constructed from metal oxide and porphyrin subunits. The picture shows a section of [{Fe(tpypor)}(3)Fe(Mo(6)O(19))(2)] small middle dotx H(2)O with the [Fe(8)(tpypor)(6)](8+) building block of the cationic framework and the entrained {Mo(6)O(19)}(2-) cluster.

Direct observation of d-orbital holes and Cu–Cu bonding in Cu2O

Abstract: A striking feature of metal oxide chemistry is the unusual electronic and chemical behaviour of Cu(I) and Ag(I): a case in point is that detailed understanding of Cu–O bonding is essential to the theory of high-temperature copper oxide superconductors. Both cations are usually coordinated in a linear fashion to two oxygens, particularly for Cu(I). In many compounds, the Cu(I) and Ag(I) cations also adopt close-packed (and related) configurations with short metal–metal distances that are strongly suggestive of the occurrence of metal–metal bonding1,2 despite their formal nd10 configuration. Such observations have been explained3,4 by invoking the participation in bonding of electronic orbitals of higher principal quantum number—that is, (n + 1)s and (n + 1)p—accompanied by the creation of d-orbital holes on the metal ion. To test this hypothesis, we have used a recently developed method of quantitative convergent-beam electron diffraction5 combined with X-ray diffraction to map the charge-density distribution in the simple oxide Cu2O, the results of which we then compare with electronic-structure calculations. We are able to image directly the d holes on the copper atoms, and also demonstrate the existence of Cu–Cu bonding in this compound.

Structure of the polymer electrolyte poly(ethylene oxide)6:LiAsF6

Abstract: Polymer electrolytes—salts (such as LiCF3SO3) dissolved in solid, high-molar-mass polymers (for example, poly(ethylene oxide), PEO)1,2,3—hold the key to the development of all-solid-state rechargeable lithium batteries4. They also represent an unusual class of coordination compounds in the solid state5. Conductivities of up to 10−4 S cm−1 may be obtained, but higher levels are needed for applications in batteries5,6,7. To achieve such levels requires a better understanding of the conduction mechanism, and crucial to this is a knowledge of polymer-electrolyte structure. Crystalline forms of polymer electrolytes are obtained at only a few discrete compositions. The structures of 3 : 1 and 4 : 1 complexes (denoting the ratio of ether oxygens to cations) have been determined5,8,9. But the 6 : 1 complex is of greater interest as the conductivity of polymer electrolytes increases significantly on raising the polymer content from 3 : 1 to 6 : 1 (refs 10, 11). Furthermore, many highly conducting polymer-electrolyte systems form crystalline 6 : 1 complexes whereas those with lower conductivities do not. Here we report the structure of the PEO:LiAsF6 complex with a 6 : 1 composition. Determination of the structure was carried out abinitio by employing a method for flexible molecular structures, involving full profile fitting to the X-ray powder diffraction data by simulated annealing12. Whereas in the 3 : 1 complexes the polymer chains form helices, those in the 6 : 1 complex form double non-helical chains which interlock to form a cylinder. The lithium ions reside inside these cylinders and, in contrast to other complexes, are not coordinated by the anions.

Inhibition of Crystallite Growth in the Sol-Gel Synthesis of Nanocrystalline Metal Oxides

Abstract: Crystal growth upon firing of hydrous transition metal oxide gels can be effectively inhibited by replacing the surface hydroxyl group before firing with another functional group that does not condense and that can produce small, secondary-phase particles that restrict advancing of grain boundaries at elevated temperatures. Accordingly, fully crystallized SnO 2 , TiO 2 , and ZrO 2 materials with mean crystallite sizes of ∼20, 50, and 15 angstroms, respectively, were synthesized by replacing the hydroxyl group with methyl siloxyl before firing at 500°C. An ultrasensitive SnO 2 -based chemical sensor resulting from the microstructural miniaturization was demonstrated.

Sodium Hydride as a Powerful Reducing Agent for Topotactic Oxide Deintercalation: Synthesis and Characterization of the Nickel(I) Oxide LaNiO2

Abstract: The capability of sodium hydride as a reducing agent in oxide deintercalation reactions is explored. The Ni(III) perovskite LaNiO3 can be reduced topotactically to LaNiO2, isostructural with the “infinite layer” cuprates, using solid sodium hydride in a sealed evacuated tube at 190 ≤ T/°C ≤ 210, and a similar infinite-layer phase is prepared by reduction of NdNiO3. Structural characterization indicates the coexistence of incompletely reduced regions, with five-coordinate Ni centers due to the introduction of oxide anions between the NiO23- sheets, giving samples with a refined stoichiometry of LaNiO2.025(3). Neutron powder diffraction and magnetization measurements indicate that the lamellar Ni(I) phase does not show the long-range antiferromagnetic ordering characteristic of isoelectronic Cu(II) oxides. This may be due either to the influence of the interlamellar oxide defect regions or to the reduced covalent mixing of Ni 3d and O 2p levels.

Melt corrosion of oxide and oxide–carbon refractories

Abstract: Penetration and dissolution mechanisms are reviewed for predominantly single phase oxide, two phase oxide, and oxide–carbon composite refractories by liquid silicates. Theoretical models of these processes, as well as static (sessile drop, dipping, and crucible) and dynamic (rotating finger and rotary slag) experimental tests, along with their practical limitations are considered. Direct (congruent or homogeneous) attack involves dissolution of a solid into a liquid with no intermediate solid phase leading to active corrosion. Indirect (incongruent or heterogeneous) attack leads to formation of one or more new solid phases at the original solid/liquid interface. This may lead to passive corrosion. Examples of direct and indirect attack in a range of refractory–liquid systems are described highlighting the critical influence of the composition and hence viscosity of the local liquid adjacent to the solid. Penetration and corrosion can be controlled either through the local liquid composition via th...

Dynamics of oxidation of aluminum nanoclusters using variable charge molecular-dynamics simulations on parallel computers

Abstract: Oxidation of aluminum nanoclusters is investigated with a parallel molecular-dynamics approach based on dynamic charge transfer among atoms. Structural and dynamic correlations reveal that significant charge transfer gives rise to large negative pressure in the oxide which dominates the positive pressure due to steric forces. As a result, aluminum moves outward and oxygen moves towards the interior of the cluster with the aluminum diffusivity $60%$ higher than that of oxygen. A stable 40 \AA{} thick amorphous oxide is formed; this is in excellent agreement with experiments.

Titanium dioxide (TiO 2 )-based gate insulators

Abstract: Titanium dioxide has been deposited on silicon for use as a high-permittivity gate insulator in an effort to produce low-leakage films with oxide equivalent thicknesses below 2.0 nm. Excellent electrical characteristics can be achieved, but TEM and electrical measurements have shown the presence of a low-resistivity interfacial layer that we take to be SiO2. The leakage current follows several mechanisms depending on the bias voltage. Reasonably good agreement has been seen between current-voltage measurements and a 1D quantum transport model.

Generation of superoxide ions at oxide surfaces

Abstract: The superoxide radical anion O2- is both an important intermediate in heterogeneous catalytic oxidation and a useful probe for positive charges in ionic solids, such as metal oxides and zeolites. The paper illustrates the main circumstances under which stable superoxide anions are formed at surfaces: (i) direct surface–oxygen electron transfer; (ii) photoinduced electron transfer; (iii) surface intermolecular electron transfer; (iv) decomposition of hydrogen peroxide.

Fluoroalkylsilane Monolayers Formed by Chemical Vapor Surface Modification on Hydroxylated Oxide Surfaces

Abstract: Water-repellent surfaces have been prepared by exposing Si substrates with a hydroxylated surface oxide to fluoroalkyl silane (FAS) vapor. Since this chemical vapor surface modification (CVSM) is based on the chemical reaction between organosilane molecules and hydroxyl groups at the oxide surface, prior to CVSM, the substrate surface was completely hydroxylated by irradiating in air with a 172-nm ultraviolet light until the water contact angle of the surface became almost 0°. Under atmospheric pressure, the substrate was then exposed to vapor of an FAS precursor, that is, one of three types of FAS having different perfluoroalkyl chain lengths [CF3(CF2)nCH2CH2Si(OCH3)3, where n = 0, 5, or 7, referred to as FAS-3, FAS-13, and FAS-17, respectively]. The FAS molecules chemically reacted with the hydroxyl groups on the substrate surface and adsorbed onto it, forming a thin layer of less than 2 nm in thickness. The water repellency of the substrate surface increased with an increase in perfluoroalkyl chain len...