Effect of thermal annealing on the structure of ZnSe/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocomposite films

The ZnSe/Al₂O₃ nanocomposite films synthesized by laser evaporation followed by heat treatment are studied. X-ray diffraction and electron-microscopic investigations of the as-deposited films demonstrate the presence of ZnSe crystallites in an Al₂O₃ amorphous matrix. Annealing changes the structures of ZnSe and Al₂O₃, increases the ZnSe crystallite size, and causes the appearance of the ZnSeO₄ phase. The presence of aluminum oxide layers decreases the phase transformation temperature of zinc selenide.     

Effect of passivating Al<Subscript>2</Subscript>O<Subscript>3</Subscript> thin films on MnO<Subscript>2</Subscript>/carbon nanotube composite lithium-ion battery anodes

MnO₂/carbon nanotube composite electrodes for Li-ion battery application were directly coated with ultrathin thicknesses of aluminum oxide film by atomic layer deposition (ALD). The non-reactive Al₂O₃ layer not only provides a stable film to protect the manganese oxide and carbon nanotubes from undesirable reaction with the electrolyte but also restrains the volume change strain of manganese oxide during cycling. The first cycle Coulombic efficiency of coated samples was increased to different extents depending on the coating thickness. In the following cycles, the coated electrodes denote high specific capacity, good capacity retention ability, and perfect rate charge/discharge performance.     

Investigation of luminescence spectra of the Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> and Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>:Ce single-crystal films

The luminescence spectra of single-crystal films and bulk crystals of yttrium-aluminum garnet Y₃Al₅O₁₂ and Ce³⁺-activated Y₃Al₅O₁₂ were investigated. It was shown that the room-temperature luminescence intensity of the Ce³⁺-free single-crystal Y₃Al₅O₁₂ film was considerably lower than that of the bulk crystals, while the luminescence intensity of the Ce³⁺ ions in the Y₃Al₅O₁₂:Ce films was considerably higher than that one for the corresponding bulk crystal.     

Self-propagating high-temperature synthesis and magnetic features of Fe-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> granulated films

The classical solid-phase reaction between Fe₂O₃ and Al layers in thin films is initiated. It is shown that, in the reaction products, Fe granulated films are formed in the Al₂O₃ nonconducting matrix. Analysis of the reaction equation demonstrates that the volume fraction of iron in the granulated films is less than the percolation threshold. This determines the magnetic properties of iron clusters in a superparamagnetic state. It is assumed that the nanocrystalline microstructure exists in thin films after solid-phase reactions proceeding under conditions of self-propagating high-temperature synthesis.     

The transport and thermal properties of glassy LiPO<Subscript>3</Subscript>/crystalline Al<Subscript>2</Subscript>O<Subscript>3</Subscript> (ZrO<Subscript>2</Subscript>) composite electrolytes

Glassy LiPO₃/crystalline Al₂O₃ and glassy LiPO₃/crystalline ZrO₂ (0–12.5 vol.% of oxide fillers) composite solid electrolytes have been prepared by glass matrix softening. Their thermal and transport properties have been investigated by differential scanning calorimetry (DSC) and impedance spectroscopy methods. The addition of ZrO₂ leads to a decrease in the crystallization temperature of LiPO₃ glass. It was found that the conductivity behavior depends on the nature of the dispersed addition. In the case of the Al₂O₃ addition, the increase in the electrical conductivity is observed. The ionic conductivity of the LiPO₃/10% Al₂O₃ composite reaches 5.8 × 10^?8 S/cm at room temperature. In contrast, the conductivity in the LiPO₃/ZrO₂ composite system decreases.     

X-ray spectral analysis of the interface of a thin Al<Subscript>2</Subscript>O<Subscript>3</Subscript> film prepared on silicon by atomic layer deposition

The extent and phase chemical composition of the interface forming under atomic layer deposition (ALD) of a 6-nm-thick Al₂O₃ film on the surface of crystalline silicon (c-Si) has been studied by depthresolved, ultrasoft x-ray emission spectroscopy. ALD is shown to produce a layer of mixed Al₂O₃ and SiO₂ oxides about 6–8 nm thick, in which silicon dioxide is present even on the sample surface and its concentration increases as one approaches the interface with the substrate. It is assumed that such a complex structure of the layer is the result of interdiffusion of oxygen into the layer and of silicon from the substrate to the surface over grain boundaries of polycrystalline Al₂O₃, followed by silicon oxidation. Neither the formation of clusters of metallic aluminum near the boundary with c-Si nor aluminum diffusion into the substrate was revealed. It was established that ALD-deposited Al₂O₃ layers with a thickness up to 60 nm have similar structure.     

Gas sensor based on nanosize In<Subscript>2</Subscript>O<Subscript>3</Subscript>:Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> film

Nanosize films of In₂O₃:Ga₂O₃ (96:4 weight %) have been deposited on a glassceramic substrate by the method of rf magnetron sputtering. The surfaces of fabricated films were studied with use of a scanning electron microscope; sizes of grains were determined and the thicknesses of films were measured. In order to prepare a gas-sensitive structure, a thin catalytic palladium layer and ohmic comb contacts were deposited on the In₂O₃:Ga₂O₃ film surface by the method of ion-plasma sputtering. The sensitivity of sensors based on the glassceramic/In₂O₃:Ga₂O₃ (96:4 weight %)/Pd structure to different concentrations of propane and butane gas mixture, as well as to methane was investigated at temperatures of working substance from 250 to 300°C.     

On electrochemistry of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-coated LiCoO<Subscript>2</Subscript> composite cathode with improved cycle stability

LiCoO₂-based cathode does still have a powerful competition in high-end mobile electronics due to its relatively high true density (about 5.2 g/cm³). When the operation potential range is extended, the improvement in its cycle stability has attracted more attention. The extension of its operation potential can be realized by partial replacement of Co by Ni and Mn or by surface modification. However, Ni and Mn replacing partial Co results in decreased true density; for example, the true density of LiNi_0.5Mn_0.3Co_0.2O₂ is about 4.6 g/cm³. In this case, the increase in its practical energy density is impossible. As a result, the surface modification technology becomes very important to extend its operation potential range. In this article, an Al₂O₃-coated LiCoO₂ cathode was synthesized. X-ray diffraction test did not show any impurity. Scanning electron spectroscopy measurements showed that the basic microstructure of pristine LiCoO₂ grain is sustained after coating Al₂O₃. The surface characteristic of pure and Al₂O₃-coated LiCoO₂ was also analyzed using an X-ray photoelectron spectroscopy (XPS) technique. Unusual XPS peaks of O 1s, Al 2p, and Co 2p binding energy were found and may be caused by the possible H existence in crystal structure. The electrochemical behavior was systematically investigated, and the cathode was cycled at different charge cutoff voltages (4.30～4.60 V). The charge-discharge and cyclic voltammetry measurements showed an obviously improved cyclic performance after coating Al₂O₃. The electrocatalytic activity is not clearly changed before and after coating Al₂O₃. From our systematical investigation, it could be concluded that the Al₂O₃-coated LiCoO₂ cathode is suitable for practical application in the potential range of 3.70～4.50 V vs. Li/Li⁺.     

Simultaneous excitation of cavity resonance and surface plasmon resonance in Ag/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Ag layer structure

Simultaneous excitation of cavity resonance (CR) and surface plasmon resonance (SPR) was observed in the angular spectrum by substituting Ag/Al₂O₃/Ag layers for the metal film in a Kretschmann structure. Two reflective valleys, elicited respectively by CR and SPR, appeared at different positions in the angular spectrum. The former is the sum of enhanced transmission of CR and absorption of the metal, expressed in the reflection spectrum and extremely insensitive to the changes of the surface environment (refractive index). The latter behavior is like that when two metal films are stuck together: it has almost the same resonance depth and width, and is extremely sensitive to the changes of the surface environment. Moreover, two SPR peaks could be excited simultaneously at one angle but with different wavelengths in the frequency spectrum, which is not seen in traditional Kretschmann structures.     

EPR study of ordered Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-based aerogel

Samples of oriented aerogel based on aluminum oxide are studied by the electron paramagnetic resonance (EPR) technique under steady-state and pulse conditions. At least two types of paramagnetic centers interacting with Al nuclei on the surface of aerogel are revealed. Their spectra are characterized by the presence or absence of superhyperfine structure in the EPR spectra, respectively. The X-ray irradiation at room temperature gives rise to the formation of additional long-lived paramagnetic centers of the second kind. Their characteristic decay times for the “fast” and “slow” processes are determined. The interaction of induced paramagnetic centers with protons located on the surface of aerogel is revealed.     

Properties of an Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Si interface

The phase chemical composition of an Al₂O₃/Si interface formed upon molecular deposition of a 100-nm-thick Al₂O₃ layer on the Si(100) (c-Si) surface is investigated by depth-resolved ultrasoft x-ray emission spectroscopy. Analysis is performed using Al and Si L_{2, 3} emission bands. It is found that the thickness of the interface separating the c-Si substrate and the Al₂O₃ layer is approximately equal to 60 nm and the interface has a complex structure. The upper layer of the interface contains Al₂O₃ molecules and Al atoms, whose coordination is characteristic of metallic aluminum (most likely, these atoms form sufficiently large-sized Al clusters). The shape of the Si bands indicates that the interface layer (no more than 10-nm thick) adjacent to the substrate involves Si atoms in an unusual chemical state. This state is not typical of amorphous Si, c-Si, SiO₂, or SiO_x (it is assumed that these Si atoms form small-sized Si clusters). It is revealed that SiO₂ is contained in the vicinity of the substrate. The properties of thicker coatings are similar to those of the 100-nm-thick Al₂O₃ layer and differ significantly from the properties of the interfaces of Al₂O₃ thin layers.     

Thermal hysteresis of a simulated Al<Subscript>2</Subscript>O<Subscript>3</Subscript> system

Thermal hysteresis in a simulated Al₂O₃ system has been investigated using a Molecular Dynamics (MD) method. Simulations were done in the basic cube under periodic boundary conditions containing 3000 ions with Born-Mayer type pair potentials. The system was cooled down from 7000 K to 0 K and heated up from 0 K to 7000 K by the same cooling/heating rate of 1.7178×10¹⁴ K/s. The temperature dependence of the system density upon cooling and heating shows thermal hysteresis. The differences between structure and dynamics of the models obtained by cooling (MOBC) and heating (MOBH) at three different temperatures of 2100 K, 3500 K and 5600 K have been detected. Calculations show that the differences in the dynamics of the systems are more pronounced than those in the structure. Furthermore, dynamical heterogeneities in MOBC and MOBH at the temperature of 2100 K have been studied through a non-Gaussian parameter and comparison of partial radial distribution functions (PRDFs) for the 10% most mobile or immobile particles with their corresponding mean ones. Cluster size distributions of the 10% most mobile or immobile particles in MOBC and MOBH at the temperature of 2100 K have been obtained. Calculations show that differences in dynamical heterogeneities are pronounced.     

Electronic excitations and defects in nanostructural Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

A time-resolved cathodo-and photoluminescence study of nanostructural modifications of Al₂O₃ (powders and ceramics) excited by heavy-current electron beams, as well as by pulsed synchrotron radiation, is reported. It was found that Al₂O₃ nanopowders probed before and after Fe⁺ ion irradiation have the same phase composition (the γ-phase/δ-phase ratio is equal to 1), an average grain size equal to ～17 nm, and practically the same set of broad cathodoluminescence (CL) bands peaking at 2.4, 3.2, and 3.8 eV. It was established that Al₂O₃ nanopowders exhibit fast photoluminescence (PL) (a band at 3.2 eV), whose decay kinetics is described by two exponential stages (τ₁ = 0.5 ns, τ₂ = 5.5 ns). Three bands, at 5.24, 6.13, and 7.44 eV, were isolated in the excitation spectrum of the fast PL. Two alternate models of PL centers were considered, according to which the 3.2-eV luminescence either originates from radiative relaxation of the P^? centers (anion-cation vacancy pairs) or is due to the formation of surface analogs of the F⁺ center (F _S ⁺ -type centers). In addition to the fast luminescence, nano-Al₂O₃ was found to produce slow luminescence in the form of a broad band peaking at 3.5 eV. The excitation spectrum of the 3.5-eV luminescence obtained at T = 13 K exhibits two doublet bands with maxima at 7.8 and 8.3 eV. An analysis of the luminescent properties of nanostructural and single-crystal Al₂O₃ suggests that the slow luminescence of nanopowders at 3.5 eV is due to radiative annihilation of excitons localized near structural defects.     

Al<Subscript>2</Subscript>O<Subscript>3</Subscript> plasma production during pulsed laser deposition

A Nd:YAG laser operating in second harmonic (532 nm), 3 ns pulse duration, 150 mJ pulse energy, and 10 Hz repetition rate, is employed to irradiate Al₂O₃ target placed in high vacuum. The produced plasma is investigated by an ion collector used in time-of-flight configuration and by a mass quadrupole spectrometer, in order to determine the equivalent plasma temperature and the atomic and molecular composition. Pulsed laser deposition technique has been used to produce thin films on different substrates placed close to the target. Different surface analyses, such as energy dispersive X-ray fluorescence (EDXRF), X-ray photoelectron spectroscopy (XPS) and surface profilometry are employed to characterize the produced films. Measurements of ablation yield, plasma equivalent temperature, acceleration voltage and characterization of grown thin films are presented and discussed.     

Investigation of electrical and optical properties of ZnO thin films grown with O<Subscript>2</Subscript>/O<Subscript>3</Subscript> gas mixture

The electrical and optical properties of ZnO thin films grown with an O₂/O₃ gas mixture are compared with samples grown with pure oxygen gas. The ZnO films were grown on sapphire(0001) by pulsed laser deposition. The residual background carrier concentration is reduced by using an O₂/O₃ gas mixture as compared to pure molecular oxygen. In particular, a one order of magnitude reduction in residual background carrier density (6.15×10¹⁶ cm^-3) is achieved by using an O₂/O₃ gas mixture. The lower donor defect density is attributed to the generation of acceptor defects compensating for the residual donor defects. Photoluminescence results show that the deep level emission increased and the band edge emission decreased for the ZnO films grown with ozone, as compared to the samples grown with pure oxygen gas. PACS 73.61.Ga; 78.55Et; 81.05 Dz; 81.15.Fg     

Magnetic order in thin films of the heavy fermion superconductor UNi<Subscript>2</Subscript>Al<Subscript>3</Subscript>

Resonant magnetic X-ray scattering was employed to investigate the magnetic state of epitaxial a^* oriented thin films of the heavy fermion superconductor UNi₂Al₃. The observed incommensurate propagation vector as well as the Ne l temperature correspond to those of bulk samples. The 1200 film shows magnetic order with a correlation length >800 ? parallel to the growth axis. Out of the three possible magnetic domains the one with the moment direction perpendicular to the film surface is not realized.     

The study of electronic conduction in amorphous thin films of Al-In<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al structure deposited by thermal evaporation

A discussion of electronic conduction in amorphous thin films of Al-In₂O₃-Al structure is presented. Particular attention is given to the question of film thickness, substrate temperature during deposition and post-deposition annealing, since these conditions are known to have a profound effect on the structure and electrical properties of the films. The effects of temperature on the V-I characteristics and effects of frequency on conductivity and capacitance of the Al-In₂O₃-Al structure are also reported. Activation energies for conduction processes are estimated and the results are discussed in terms of the hopping model. The conduction at higher temperature is seemingly a contact-limited, i.e. Schottky type process, so a transition from hopping to free-band conduction takes place. The capacitance decreases with the rise of frequency and the lowering of temperature. The values of dielectric constants are estimated and the results are discussed in terms of Schottky type of conduction. The increase in conductivity with the increase in temperature during measurements of electrical properties, film thickness, substrate temperature and post deposition annealing is reported and results are discussed in terms of current theory.     

Synthesis and magnetic properties of FeNi<Subscript>3</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> core-shell nanocomposites

In this study, FeNi₃/Al₂O₃ core-shell nanocomposites, where individual FeNi₃ nanoparticles were coated with a thin layer of alumina, were fabricated by a modified sol-gel method. Several physical characterizations were performed on the samples of FeNi₃/Al₂O₃ nanocomposites with different thickness of Al₂O₃ shell. The encapsulation of FeNi₃ nanoparticles with alumina stops FeNi₃ agglomeration during heat treatment, and prevents interaction among the closely spaced magnetic FeNi₃ nanoparticles. The Al₂O₃ insulating shell improves the soft magnetic properties of FeNi₃. The study of the complex permeability of the samples shows that the real part μ’ of the permeability of the sample with Al molar content of 20% (Al/(Fe+Ni)) is as high as 12, and independent of frequency up to at least 1 GHz. The tunneling magnetoresistance arising from the presence of the Al₂O₃ shell have also been studied.     

Metastable superconductivity of the Al/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> interface

Superconductivity in the interface region between metallic Al and its oxide, Al₂O₃, has been detected at about 45 K in the measured dynamic magnetic susceptibility.     

Effect of annealing temperature on the structural reorganization of Eu<Superscript>3+</Superscript> optical centers in Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Eu<Subscript>2</Subscript>O<Subscript>3</Subscript>-BiOF gel films

The dependence of the structural reorganization of Eu³⁺ optical centers in Al₂O₃-Eu₂O₃-BiOF films on the annealing temperature has been investigated. It is shown by the methods of crystal field theory and computer simulation that the increase in the annealing temperature from 700 to 1100 °C leads to removal of bismuth from Eu-O-Bi complex centers with the C _3V symmetry in the Al₂O₃ structure and the change in symmetry from D ₃ to O _h for a large fraction of EuAlO₃ centers.