Magnesium Oxide Films Research Articles

Characterisation of magnesium oxide and its interface with α-Mg in Mg–Al-based alloys

Magnesium oxide (MgO) films and particles have been collected by pressurised filtration of a Mg-8.6wt%Al-0.67wt%Zn (AZ91D) alloy melt. The morphology of the oxides and their interfaces with the α-Mg phase were investigated by high-resolution transmission electron microscopy. It was found that the oxide films consisted of large numbers of sub-micrometre-sized MgO particles, and that melt shearing can effectively break up the oxide films and disperse the oxide particles. For the first time, orientation relationships (ORs) of OR I: [1 1 1]MgO∼2° from (0 0 0 1)α-Mg and (0 1 1)MgO //[2 1 1 0)α-Mg; and OR II: (1 1 1)MgO//(1 1 0 1)α-Mg and [0 1 1]MgO//[1 2 1 1]α-Mg, were observed between the MgO particles and the α-Mg matrix. The calculated Bramfitt planar disregistries were 5.5% and 2.5% for the two ORs, respectively, indicating good lattice matching between MgO and α-Mg at the interface. With the evidence of grain refinement effect observed in the sheared AZ91D magnesium alloy, the possibility of MgO particles to act as potent nucleants for heterogeneous nucleation of α-Mg grains is discussed in terms of the crystallographic criterion.

Enhanced Secondary Electron Emission Yield of MgO Thin Films by Vapor Chopping Technique for Plasma Display Panels

Department of Physics, Shivaji University, Kolhapur 416004, IndiaWe report the enhanced secondary electron emission yield and reduced firing voltage of ac-PDPsby using vapour chopped MgO thin film as protective layer. MgO thin films have been prepared bythermal oxidation (in air) of vacuum evaporated magnesium films at 573, 623 and 673 K. SEE yieldshowed variation with oxidation temperature. The use of vapour chopping technique approach wasaimed to improve the SEE yield. The reduced surface roughness and improved SEE yield by useof vapour chopping technique supports the better performance and greater lifetime of the panelassisting to reduce the operational costs.

Plasmon Response of a Quantum-Confined Electron Gas Probed by Core-Level Photoemission

We demonstrate the existence of quantized "bulk" plasmons in ultrathin magnesium films on Si(111) by analyzing plasmon-loss satellites in core-level photoemission spectra, recorded as a function of the film thickness d. Remarkably, the plasmon energy is shown to vary as 1/d2 all the way down to three atomic layers. The loss spectra are dominated by the n=1 and n=2 normal modes, consistent with the excitation of plasmons involving quantized electronic subbands. With decreasing film thickness, spectral weight is gradually transferred from the plasmon modes to the low-energy single-particle excitations. These results represent striking manifestations of the role of quantum confinement on plasmon resonances in precisely controlled nanostructures.

Effects of magnesium film thickness and annealing temperature on formation of Mg 2Si films on silicon (1 1 1) substrate deposited by magnetron sputtering

Magnesium films of various thicknesses were first deposited on silicon (1 1 1) substrates by magnetron sputtering method and then annealed in annealing furnace filled with argon gas. The effects of the magnesium film thickness and the annealing temperature on the formation of Mg 2Si films were investigated by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The Mg 2Si thin films thus obtained were found to be polycrystalline and the Mg 2Si (2 2 0) orientation is preferred regardless of the magnesium film thickness and annealing temperature. XRD results indicate that high quality magnesium silicide films are produced if the magnesium/silicon samples are annealed at 400 °C for 5 h. Otherwise, the synthesized films annealed at annealing temperatures lower than 350 °C or higher than 450 °C contain magnesium crystallites or magnesium oxide. SEM images have revealed that microstructure grains in the polycrystalline films are about 1–5 μm in dimensions, and the texture of the Mg 2Si films becomes denser and more homogeneous as the thickness of the magnesium film increases.

In situ infrared thermographic screening of compositional spread Mg–Ti thin-film libraries

An infrared imaging set-up and our own data treatment methodology have been developed to investigate hydrogen storage properties of combinatorial metallic thin-film libraries in situ. The technique has been tested by analysing the kinetics of hydrogen sorption and desorption in pure metallic magnesium films and in compositional-spread Mg–Ti libraries. Thermography was used to screen the surface emissivity of the libraries, since hydrogenated and dehydrogenated phases present very different values of this property. The proposed data treatment allows for direct visualisation of onset reaction temperatures ( T hydrogenation and T dehydrogenation) in function of composition which, in the present case, clearly reveals the catalytic influence of titanium on the kinetic properties of magnesium film dehydrogenation.

Sapphire surface polariton splitting due to resonance with magnesium oxide film phonon

Surface polariton spectra of magnesium oxide films (of thickness ranging from 10 to 300 nm) prepared by chemical vapor deposition technique on sapphire substrates have been studied. The splitting of the dispersion curve of the sapphire surface polariton appears near 700 cm−1 due to the resonance interaction of the sapphire substrate surface polariton with the film optical phonon. This splitting is proportional to the square root of the film thickness. Optical constants of ultrathin magnesium oxide films have been obtained from surface polariton spectra measurements. For the thinnest film (10 nm) they appear to be close to the bulk crystal values.

Biaxial texture development in the ion beam assisted deposition of magnesium oxide

Low-energy ion-beam irradiation (<1 keV) during the concurrent deposition of cubic oxide materials results in the growth of crystallographically textured thin films. A model system, magnesium oxide (MgO), has been successfully used as a biaxially textured template film for the heteroepitaxial deposition of many materials with texture dependent properties like high temperature superconductors, tunable microwave materials, and ferroelectrics. Here, we present data on the initial nucleation of biaxial crystallographic texture in this model system using an in situ quartz crystal microbalance (QCM) substrate combined with in situ reflected high-energy electron diffraction (RHEED). Correlation of mass uptake with the RHEED images of the growing surface shows that the development of crystallographic biaxial texture in this material system occurs suddenly as the initially polycrystalline MgO films reaches a critical film thickness of 2 nm. This texture continues to improve during subsequent growth. A simple model shows that the effect is not due simply to coverage effects. We use a combination of in situ RHEED and ex situ transmission electron microscopy to further elucidate the mechanism of this sudden texture formation. We present a physical model to describe this behavior and explain the role of ion-to-atom arrival ratio and underlying nucleation surface on texture development.

Influence of Sputtering Gas on Morphological and Optical Properties of Magnesium Films

The influence of sputtering gas (He & Ar) on the structural properties of Mg thin films has been investigated. The optical property (reflectance) that results from the growth of films at varying substrate temperatures ( T sub) was also studied. The deposited films were characterized by using X-ray diffraction (XRD), field emission scaning electron microscopy (FE-SEM), atomic force microscopy (AFM) and UV-Vis-NIR spectrophotometer. The smaller crystallite size and lower deposition rate were observed in the presence of Helium atmosphere compared to Argon. Morphology of the films shows 2D hexagonal geometry of grains in the deposition temperature range ( T sub≈50–150°C) in both the sputtering gases. The surface roughness of the polycrystalline films were found to increase with increase in the deposition temperature of both ambient gases. Optical reflectance of Mg films was measured in near infrared region and larger reflectance was observed from Mg films sputtered in He atmosphere compared to that in argon.

PREPARATION AND CHARACTERIZATION OF ELEC-TROLESS Ni COATING ON THE SURFACE OF MgO WITH POROUS STRUCTURE

Generally,the surface pretreatments such as sensitization and activation are necessary for depositing a nickel metal layer on oxide surface by electroless plating,however,for an oxide surface with porous structure it is possible that the pretreatment is not necessary.In this paper,only by use of the surface activity of porous structure,an electroless plated nickel layer can be prepared on the surface of microarc-oxidation-fabricated porous magnesium oxide film in a conventional electroless nickel plating solution,consisted of nickel sulfate as main salt and sodium hypophosphite as reducing agent.Furthermore,the phase,microstructure,electrical conductivity and corrosion resistance of the obtained nickel layer were characterized.The results indicate that the 5μm-thick nickel layer is composed of fine and homogeneously distributed nickel particles,at the same time the microstructure of nickel layer is dense.Nickel layer spreads into the micropores on the surface of porous magnesium oxide film,so that an interleaving interface is formed at nickel/oxide interface.XRD results reveal that the nickel layer contains crystalline Ni and amorphous Ni-P.Four-point probe measurement indicates that the nickel layer exhibits well electrical conductivity.Meanwhile,polarization curve reveals that corrosion potential elevates notably due to the presence of nickel layer.During electroless nickel plating the nickel ions in solution were reduced and deposited in the micropores of porous magnesium oxide film under the action of reducing agent ions,so as to generate tiny primary nickel particles, subsequently,these primary nickel particles continuously grew and spread,and finally formed an entire nickel layer on oxide surface.

Hydrogen separation under high-flux, low-energy molecular ion implantation at elevated temperature

Storage of hydrogen in magnesium films was investigated using plasma immersion ion implantation (PIII). Mg films have been magnetron sputter deposited on fused quartz substrates and subsequently irradiated by molecular ions extracted from plasmas generated in hydrogen containing molecular gases: either water vapor, CH 4 or a mixture of water vapor plus CH 4. Irradiations were performed at a substrate temperature of 450 K with pulsed 0.5–1 kV bias voltage. The depth profiles of C, O and H atoms have been analyzed using Nuclear Reaction Analysis and Secondary Ion Mass Spectrometry techniques. It is shown that hydrogen can be separated from hydrogen containing molecular ions and stored as MgH 2 compound in Mg films avoiding hydrogen gas purification process. This is discussed on the basis of H transport mechanisms in plasma activated Mg films.

Adsorption of nitrogen oxide molecules to the surface of nanosized nickel clusters formed on the (111) surface of a magnesium oxide film

The properties of the systems formed on deposition of Ni atoms on the (111) surface of a MgO film of thickness equal to six monomolecular layers grown on a Mo(110) crystal face and the adsorption of NO nitrogen oxide molecules to the system surface have been studied by methods of electron spectroscopy (AES, XPES, LEED, LEIBSS) and reflective infrared absorption spectroscopy. On deposition of Ni atoms on the surface of MgO at a substrate temperature of 600 K, three-dimensional islands of Ni are formed. The subsequent adsorption of NO results in molecule dissociation even at 110 K. The efficiency of this process depends on the morphology of the Ni layer.

Hydrogenation mechanism in lanthanum-activated magnesium films

With density-functional theory, the dissociative chemisorptions and diffusion processes of hydrogen on both pure and La-doped Mg(0001) surfaces are studied. Calculation results show that the energy barrier obtained for hydrogen dissociation on the La-doped Mg(0001) surface is smaller due to back-donated bonding between molecular H2 and doped La atom. The obtained diffusion barriers (0.8–0.22 eV) imply a fast motion of atomic H on La-doped Mg(0001) surface.

Hydrogenation of Mg thin films in CH 4 + Ar plasmas

Magnesium films were magnetron sputter deposited on AISI 316L stainless steel and hydrogenated at 450 K by the plasma immersion ion implantation technique using CH 4 + Ar gases under a pressure of 10 Pa. The goal was to obtain hydrogen separation and formation of hydrides in Mg film thus avoiding a hydrogen gas purification process. The depth profiles of C and H atoms across the Mg film were analyzed by the Nuclear Reaction Analysis and secondary ion mass spectrometry. After 0.5 h of plasma treatment with a 1 kV bias voltage, C atoms remained in the near surface region while H atoms were homogeneously distributed across the entire film thickness. The results are explained on the basis of C and H ion implantation and atomic transport kinetics along the grain boundaries of a nanocrystalline Mg film.

ChemInform Abstract: Nanoassembled Model Catalysts

The present review outlines, on the basis of specific examples taken from our laboratory, the most important steps in (i) preparing supported nanoassembled model catalysts and (ii) investigating their size-dependent catalytic properties. We describe the cluster generation and present evidence for softlanding of the clusters onto solid surfaces. Subsequently, the growth and the characterization of the cluster support material, thin magnesium oxide films, are discussed, including the role of surface defects. Then the thermal stability of Cu clusters and the individual electronic structure of small Ag and Cu clusters on MgO are addressed. Finally, two examples of cluster size-dependent heterogeneous catalytic reactions are presented, (i) the cyclotrimerization of acetylene on nanoassembled Pd catalysts, and (ii) the CO oxidation on nanoassembled Au catalysts. We show experimentally and together with first-principle calculations that the interaction of such small metal clusters with the oxide surface strongly changes their catalytic properties. In contrast to large particles an additional electron in localized valence states of small clusters represents a major change. It is this charge transfer which turns inactive gold and palladium clusters into active model catalysts opening new perspectives to tune catalytic processes on the nanoscale.

Improvement in adhesion and decrease in stress of MgO thin films due to vapour chopping

We report the enhancement of adhesion and reduction of stress of MgO thin films by using vapour chopping technique. Vapour chopped (VC) and nonchopped (NC) magnesium oxide thin films (thickness ∼ 600 nm) have been prepared by thermal oxidation (in air) of vacuum evaporated magnesium films. The effect of oxidation temperature of 573, 623 and 673 K and duration 90 and 180 min on the adhesion and intrinsic stress of MgO thin films due to vapour chopping were studied. Adhesion and stress was investigated by direct pull off method and interferometric method respectively. Adhesion was in the range 27–54.8 × 10 4 N/m 2 for vapour chopped thin films, as compared to 20.1–41.9 × 10 4 N/m 2 of nonchopped MgO thin films. Intrinsic stress was found to decrease from 70.07 to 22.86 × 10 7 N/m 2 for nonchopped and to 45.63 to 11.83 × 10 7 N/m 2 for vapour chopped MgO thin films.

28.4: Characterization of Negative Electron Affinity for MgO Film by Ultraviolet Photoelectron Spectroscopy

AbstractMany attempts to investigate electronic band structures of various metal oxides such as magnesium oxide (MgO) and calcium oxide (CaO) have been made for a long time, but exact electronic band structures of metal oxides have not been achieved successfully so far due to surface charging during measurement analysis except for ultrathin film. In this study, ultraviolet photoelectron spectroscopy (UPS) is introduced and electronic band structure for MgO film has been obtained by using a unique way of heating the film, which suppresses the surface charging of MgO film through improvement of electrical conductivity during measurement. Negative electron affinity has been characterized for MgO film in the measured UPS spectrum. The characteristics of negative electron affinity can be understood as originating from image‐potential states on MgO surface.

Thermionic vacuum arc (TVA) technique for magnesium thin film deposition

In this study, magnesium thin films were deposited on glass substrate by the Thermionic Vacuum Arc (TVA) technique for the first time. We present a different technique for deposition of high-quality magnesium thin films. By means of this technique, the production of films is achieved by condensing the plasma of anode material generated using Thermionic Vacuum Arc (TVA) under high vacuum conditions onto the surface to be coated. The crystal orientation and morphology of the deposited films were investigated by using XRD, EDX, SEM and AFM. The aim of this study is to search the use of TVA technique to coat magnesium thin films and to determine some of the physical properties of the films generated. Furthermore, this study will contribute to the scientific studies which search the thin films of magnesium or the compounds containing magnesium. In future, this study will be preliminary work to entirely produce magnesium diboride (MgB 2) superconductor thin film with the TVA technique.

Critical examination of growth rate for magnesium oxide (MgO) thin films deposited by molecular beam epitaxy with a molecular oxygen flux

The authors report a study of molecular beam deposition of MgO films on amorphous SiO2 and (0001) GaN surfaces over a large range of temperatures (25–400 °C) and molecular oxygen growth pressures (10−7–10−4 Torr). This study provides insight into the growth behavior of an oxide with volatile metal constituents. Unlike other materials containing volatile constituents (e.g., GaAs, PbTiO3), all components of MgO become volatile at normal epitaxial growth temperatures (≥250 °C). Consequently, defining which species is the adsorption controller becomes ambiguous. Different growth regimes are delineated by the critical substrate temperature for Mg re-evaporation and the Mg:O flux ratio. These regimes have impact on phase purity, quartz crystal microbalance calibration, and film microstructure. The universal decay in deposition rate above growth 10−5 Torr O2 is also considered. By introducing a third flux of inert argon gas, rate reduction is attributed to increased molecular scattering and not oxidation of the metal source.

Synthesis and electron emission properties of MgO nanowires

Magnesium oxide film has high secondary electron emission yield and is found as applications in flat panel display widely. In this work, MgO nanowire electron emission properties, including both secondary and field emissions, are reported. MgO nanowire thin films have been synthesized on stainless steel substrate by thermal vapor deposition. The turn-on field of 9 MV/m for the field emission is measured with the MgO nanowire thin film, and the maximum secondary electron emission (SEE) yield of 8.3 is obtained. In addition, the good stability of SEE yield is also observed under long-time electron bombardment.

Adhesion and stress of magnesium oxide thin films: Effect of thickness, oxidation temperature and duration

Nanoscale magnesium oxide thin films have been deposited on glass substrate by thermal oxidation (in air) of vacuum evaporated magnesium films. X-ray diffraction (XRD) showed orientation along (2 0 0) and (2 2 0) directions. The mechanical properties of the MgO thin films were found to be the function of thickness (300, 450 and 600 nm), oxidation temperature (573, 623 and 673 K) and oxidation duration (90 and 180 min). As oxidation temperature and oxidation duration increases, adhesion and intrinsic stress were found to increase. Intrinsic stress decreased whereas adhesion increased due to increase in thin film thickness. The value of intrinsic stress was in range 28.902–73.212 (×10 7 N/m 2) and that of adhesion was 12.1–27.4 (×10 4 N/m 2) for the thin film of thickness 300 nm.