Effect of surface condition on the kinetics of scandium-deuteride formation

We studied the effects of surface oxidation on the kinetics of deuterium (D₂) absorption by Sc. A Sieverts apparatus was used to measure the D₂ absorption kinetics and X-ray photoelectron spectroscopy (XPS) was used to determine the surface oxidation state during heat treatments in ultra-high vacuum. In the as-received condition, the bulk Sc sample was covered by an Sc₂O₃ film. However, the Sc₂O₃ phase decomposed dramatically at 573 K due to the fast diffusion of oxygen from the surface to the bulk. The initial surface of Sc was cleaned of most oxygen by heating up to 873 K whereas the obtained fresh surface at temperatures over 873 K was contaminated by oxygen again when cooled down to room temperature. Due to fresh surfaces available for D₂ absorption, the sample at temperatures over 873 K started to absorb D₂ as soon as it was introduced. The absorption rate at these temperatures appears to be governed by D diffusion in the Sc bulk. However, the sample deuterated at room temperature presented an incubation time before the absorption was observed. And this stage was followed by a surface controlled process that changed gradually to a stage related to deuterium diffusion into the bulk.     

Comparative microscopic and spectroscopic analysis of temperature-dependent growth of WO3 and W0.95Ti0.05O3 thin films

We present a comparative microscopic and spectroscopic study of the morphology and composition of WO₃ and W_0.95Ti_0.05O₃ thin films, grown by radio-frequency magnetron reactive sputtering at substrate temperatures varied from room temperature to 500 °C, using atomic force microscopy (AFM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). With increasing growth temperature, the AFM results show increase in the average crystallite size and in the surface roughness for both undoped and doped samples. The AFM data, along with the Raman results, clearly indicate that for the given set of experimental conditions, higher growth temperatures are required to obtain crystalline Ti-doped WO₃ films than for WO₃ films. Also, the Raman results suggest a potential phase transformation from a monoclinic WO₃ structure to an orthorhombic, but more probably a tetragonal, configuration in the W_0.95Ti_0.05O₃ thin films. This remark is based on the observed shifting, with Ti doping, to lower frequencies of the Raman peaks corresponding to W–O–W stretching modes of WO₃ at 806 and 711 cm⁻¹ to 793 and 690 cm⁻¹, respectively. XPS data indicate that the doped material has a reduced WO_3−x stoichiometry at the surface, with the presence of W⁶⁺ and W⁵⁺ oxidation states; this observation could also be related to the existence of a different structural phase of this material, corroborating with the Raman measurements.     

Pressure dependence of the superconducting transition temperature in alkali tungsten bronzes

The superconducting transition temperatures have been measured for the tetragonal I phase of Na_0.23WO₃, the hexagonal phase of K_0.33 WO₃, and the hexagonal phase of Rb_0.33WO₃ for pressures up to 20,000 bar. The slope of the curves is + 1.7 × 10^–5 K/bar for the sodium bronze, –5.8 × 10^–5 K /bar for the potassium bronze, and –1.5 × 10^–5 K/bar for the rubidium bronze. A sharp change in slope of the transition temperature vs. pressure curve for the potassium bronze at 4000 bar may indicate a phase change.Prepared for the U.S. Energy Research and Development Administration under Contract No. W-7405-eng-82.     

Annealing effect on the characteristics of La0.67Sr0.33MnO3 polycrystalline thin films produced by the sol–gel dip-coating process

The effect of annealing temperature on selected characteristics of polycrystalline La_0.67Sr_0.33MnO₃ films, which have been produced on quartz substrates, was investigated. X-Ray powder diffraction patterns showed that the phase formation started at 873 K and all the films had perovskite structure. By increasing the annealing temperature, the lattice parameters were decreased. Scanning electron microscope indicated that the film thicknesses were approximately 3 μm and the average grain size of the samples varied between 30–100, 50–110, 70–120, and 100–150 nm for films annealed at 873, 973, 1,073, and 1,173 K, respectively. All the films showed a paramagnetic–ferromagnetic (T_C) and metal–insulator (T_IM) phase transition. The T_C indicated a small variation [from 131 K (S4) to 124 K (S1)] as a function of annealing temperature, whereas the T_IM went down from 212 K (S4) to 110 K (S1), a strong decrease of 102 K. A colossal magneto resistance with magneto resistance ratios of 130, 139, 156, and 163% were observed near T_C and at 6 T magnetic field.     

Microstructural, mechanical and magnetic properties of shape memory alloy Ni55Mn23Ga22 thin films deposited by radio-frequency magnetron sputtering

The near-stoichiometric Ni₂MnGa ferromagnetic alloys are one of the smart materials, that are of a great interest when they are deposited as a thin film by r.f. sputtering. These thin films of shape memory alloys are prospective materials for micro and nanosystem applications. However, the properties of the shape memory polycrystalline thin films depend strongly on their structure and internal stress, which develop during the sputtering process as well as during the post-deposition annealing treatment. In this study, about 1 μm Ni₅₅Mn₂₃Ga₂₂ thin films were deposited in the range 0,45 to 1,2 Pa of Ar pressure and P = 40 to 120 W. Their composition, crystallographic structure, internal stress and stress gradient, indentation modulus, hardness, deflection induced by magnetic field and magnetic properties were systematically studied as a function of the temperature of the silicon substrate ranging from 298 to 873 K and the vacuum annealing treatment at 873 K for 21,6 ks and 36 ks. A silicon wafer having a native amorphous thin SiO_x buffer layer was used as a substrate. This substrate influences the microstructure of the films and blocks the diffusion process during the heat treatment.The crystal structure of the martensitic phase in each film was changed systematically from bct or 10 M or 14 M. In addition, the evolution of the mechanical properties such as mean stress, stress gradient, roughness, hardness and indentation modulus with the temperature (of substrate or of heat treatment) were measured and correlated to crystal structure and morphology changes.Moreover, it has been shown that it is necessary to associate a high temperature (873 K) annealing during a long time (21 ks and 36 ks) to obtain good ferromagnetic properties. Thus, for the well annealed films (36 ks at 873 K) the magnetostrain is about - 170 ppm for a magnetic field of 1 MA m^- 1 applied along the beams.As a conclusion, the response of free-standing magnetic shape memory films to a magnetic field of 0,2 MA m^- 1 depends strongly on the martensitic structure, internal mechanical stress (mean and gradient) and magnetic properties. The free-standing annealed film at 873 K for 36 ks points out a considerable magnetic actuation associated with bct or 10 M or 14 M martensitic structures.     

Calcium phosphates formation on CaTiO<Subscript>3</Subscript> coated titanium

In this study, performance of calcium phosphate formation of CaTiO₃ coating film on Ti in Hanks’ balanced saline solution (HBSS) was investigated. CaTiO₃ thin films with a thickness of 50 nm were deposited on Ti using radiofrequency (RF) magnetron sputtering. The temperature of Ti substrate was adjusted to room temperature (RT) and 873 K. Thereafter, the specimens deposited at RT were annealed at 873 K in air for 7.2 ks. The films were characterized by grazing incident angle X-ray diffractometry (GI-XRD) and X-ray photoelectron spectroscopy (XPS). After immersion in HBSS for 60 d, on CaTiO₃ coated Ti, the formation of hydroxyapatite (HAP) was observed. Furthermore, HAP layer formed was thicker on the specimen on which CaTiO₃ film was deposited at RT and annealed than that prepared at 873 K. The major difference between both specimens was the chemical properties of the outermost surface. In summary, CaTiO₃ thin film deposited at RT and followed by annealing at 873 K for 7.2 ks in air enhances calcium phosphate formation ability on Ti.     

Influence of Fe dopant concentration and annealing temperature on the structural and optical properties of ZnO thin films deposited by sol–gel method

Nanostructured Fe doped ZnO thin films were deposited onto glass substrates by sol–gel spin coating method. Influence of Fe doping concentration and annealing temperature on the structural, compositional, morphological and optical properties were investigated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), UV–Vis spectroscopy and photoluminescence (PL) measurements. XRD analysis showed that all the films prepared in this work possessed a hexagonal wurtzite structure and were preferentially oriented along the c-axis. Pure ZnO thin films possessed extensive strain, whereas Fe doped films possessed compressive strain. In the doped films, least value of stress and strain was observed in the 0.5 at.% Fe doped thin film, annealed at 873 K. Average crystallite size was not significantly affected by Fe doping, but it increased from 15.57 to 17.79 nm with increase in annealing temperature from 673 to 873 K. Fe ions are present in +3 oxidation state as revealed by XPS analysis of the 0.5 at.% Fe doped film. Surface morphology is greatly affected by changes in Fe doping concentration and annealing temperature which is evident in the SEM images. The increase in optical band gap from 3.21 to 3.25 eV, with increase in dopant concentration was attributed to Moss–Burstein shift. But increase in annealing temperature from 673 to 873 K caused a decrease in band gap from 3.22 to 3.20 eV. PL spectra showed emissions due to excitonic combinations in the UV region and defect related emissions in the visible region in all the investigated films.     

Oxidation study of polycrystalline InN film using in situ X-ray scattering and X-ray photoemission spectroscopy

Oxidation process of polycrystalline InN films were investigated using in situ X-ray diffraction (XRD) and X-ray photoemission spectroscopy (XPS). The films were grown by dc sputter on sapphire (0001) substrates and were oxidized in air at elevated temperatures. The XRD data showed that the structure of the films changed to the bixbyite In₂O₃ (a = 10.11 Å) above 450 °C. Chemical configurations of the sample surfaces were investigated using high-resolution XPS. For the non-intentionally oxidized InN film, XPS analysis on the In 3d peak and the N 1s main peak at 396.4 eV suggests that indium and nitrogen are bound dominantly in the form of InN. An additional peak observed at 397.4 eV in the N 1s photoelectrons and the O 1s peaks indicate that the InN film surface is partly oxidized to have InO_xN_y configuration. After oxidation of the InN film at elevated temperature, the O 1s spectrum is dominated by In₂O₃ peak, which indicates that the structure is stable chemically with In₂O₃ configuration at least within the XPS probing depth of a few nm.     

Effect of vacuum annealing on the phase composition of In/SnO/Si, In/SnO2/Si, and Sn/In2O3/Si heterostructures

The chemical interaction between indium and thin SnO and SnO₂ films and between tin and thin In₂O₃ films during vacuum annealing was studied. The metallic films were deposited onto single-crystal silicon substrates by magnetron sputtering, the SnO and SnO₂ films were produced by heat-treating the Sn film in flowing oxygen at 673 and 873 K, respectively, and the In₂O₃ film was produced by heat-treating the In film at 573 K. The results indicate that annealing of the In/SnO/Si and In/SnO₂/Si heterostructures in vacuum (residual pressure of 0.33 × 10^?2 Pa) at 773 K gives rise to the reduction of Sn and oxidation of In, whereas annealing of Sn/In₂O₃/Si causes partial tin substitution for indium in the cubic indium oxide lattice.     

Thin film deposition and characterization of pure and iron-doped electron-beam evaporated tungsten oxide for gas sensors

Pure tungsten oxide (WO₃) and iron-doped (10 at.%) tungsten oxide (WO₃:Fe) nanostructured thin films were prepared using a dual crucible Electron Beam Evaporation (EBE) technique. The films were deposited at room temperature under high vacuum onto glass as well as alumina substrates and post-heat treated at 300 °C for 1 h. Using Raman spectroscopy the as-deposited WO₃ and WO₃:Fe films were found to be amorphous, however their crystallinity increased after annealing. The estimated surface roughness of the films was similar (of the order of 3 nm) to that determined using Atomic Force Microscopy (AFM). As observed by AFM, the WO₃:Fe film appeared to have a more compact surface as compared to the more porous WO₃ film. X-ray photoelectron spectroscopy analysis showed that the elemental stoichiometry of the tungsten oxide films was consistent with WO₃. A slight difference in optical band gap energies was found between the as-deposited WO₃ (3.22 eV) and WO₃:Fe (3.12 eV) films. The differences in the band gap energies of the annealed films were significantly higher, having values of 3.12 eV and 2.61 eV for the WO₃ and WO₃:Fe films respectively. The heat treated films were investigated for gas sensing applications using noise spectroscopy. It was found that doping of Fe to WO₃ produced gas selectivity but a reduced gas sensitivity as compared to the WO₃ sensor.     

Synthesis and characterization of WO<Subscript>3</Subscript> and WS<Subscript>2</Subscript> hexagonal phase nanostructures and catalytic test in sulfur remotion

The aim of the present study was the synthesis and characterization of WO₃ and WS₂ nanostructures in hexagonal phases and the evaluation of the latter as catalyst in the dibenzothiophene hydrodesulfurization reaction. 2H-WS₂ nanostructures were obtained from a precursor WO₃ nanostructure by a two-step hydrothermal/gas phase reaction under well-controlled conditions. All nanostructures were characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, and the specific surface area of the materials was measured using the BET method. The catalytic activity and selectivity measurements of the resulting unsupported WS₂ nanocatalysts are also presented. Catalytic activity was found to be highest for the 2H-WS₂ from the WO₃ nanostructure sulfided at 773 K (rate constant of 3 × 10⁻⁷ mol/g s).     

Dependence of target-substrate distance on crystallographic and optical properties of WO3 films prepared by reactive radio frequency magnetron sputtering

WO₃ films were deposited on glass substrates using radio frequency magnetron sputtering in a mixed gas of 80%Ar-20%O₂. From the X-ray diffraction patterns, the WO₃ films deposited at source to substrate distance (D_T-S) of 40 mm were polycrystalline, crystallizing in the monoclinic crystal structure, with highly preferred c-axis orientation perpendicular to the film plane. On the other hand, amorphous films were observed in the WO₃ films deposited at 70 mm. The crystallite sizes of the WO₃ films deposited at D_T-S of 40 mm were larger for films deposited at lower working gas pressures (P_W). The optical absorption edge of the films shifted to shorter wavelength region with increase in P_W irrespective of the D_T-S. The oxygen deficient films are obtained when the films are deposited at D_T-S of 40 mm and P_W of 0.3 Pa and stoichiometric films are formed at higher P_W. The photocatalytic oxidation of methanol proceeds via CO and formaldehyde as the intermediate species on WO₃/TiO₂ bilayer films and CO₂ is identified as the final product.     

Influence of the annealing temperature on violet emission of ZnO films obtained by oxidation of Zn film on quartz glass

The photoluminescence (PL) emission properties of ZnO films obtained on quartz glass substrate by the oxidation of Zn films with the oxygen pressure of 50Pa at temperature of 773 K~973 K were studied. The strong single violet emission centering on 424 nm (or 2.90 eV) without any accompanying deep-level emission and UV emission was observed in the PL spectra of the ZnO films at room temperature. The intensity of violet emission increased with increasing annealing temperature in the range of 773 K~873 K and decreased with increasing annealing temperature in the range of 873 K~973 K. These violet emission bands are attributed to the electron transition from interstitial zinc (Zn_i) level (2.91 eV) to the valence band.     

Investigation of RuO2-SnO2 thin film formation by thermogravimetry-mass spectrometry and infrared emission spectroscopy

The thermal evolution process of RuO₂-SnO₂/Ti mixed oxide thin films of varying noble metal content has been investigated by thermogravimetry-mass spectrometry and Fourier transform infrared emission spectroscopy. The gel-like films prepared from aqueous solutions of the precursor salts Sn(OH)₂(CH₃COO)_2−xCl_x and Ru(OH)Cl₃ on titanium metal support were heated in an atmosphere containing 20% O₂ and 80% Ar up to 873 K.Chlorine evolution takes place in a single stage process between 573 and 773 K, while the decomposition of organic species like various types of carbonyls, carbonates and carboxylates occurs in two main stages between 423 and 773 K. The combustion temperature of the organic species formed decreases with the increase of the ruthenium content, indicating the catalytic effect of the noble metal. The secondary processes can significantly influence the morphology and electrochemical properties of the films when used as anodes in electrochemical processes.     

Growth, structure and friction behavior of titanium doped tungsten disulphide (Ti-WS2) nanocomposite thin films

This study describes the synthesis, structure and friction behavior of titanium doped tungsten disulphide (Ti-WS₂) nanocomposite solid lubricant thin films grown by cosputtering at room and 300 °C in situ substrate temperatures. The films were studied by focused ion beam (FIB) prepared cross-sectional scanning and transmission electron microscopies and X-ray diffraction (XRD) to determine the thin film structure and crystallinity as a function of varying titanium atomic percent and sputtering power. XRD confirmed that the pure WS₂ thin films grown at room temperature (RT) and 300 °C were crystalline with hexagonal texture. Basal planes with c-axis orientated parallel to the substrate surface [(100) and (101) texture] were predominantly observed in all thin films. Co-sputtering at RT with any amount of Ti induced a dramatic change in the microstructure, i.e., Ti prevented the formation of crystalline WS₂, making it amorphous with well-dispersed nanocrystalline (1-3 nm) precipitates. For RT friction tests, longer thin film lifetimes were exhibited when the thin films were doped with low amounts of Ti (∼ 5-14 at.%) in comparison to pure WS₂ but there was no change in friction coefficient (∼ 0.1). For high temperature (500 °C) friction tests, slightly higher friction coefficients (0.2) but longer lifetimes were observed for the low at.% Ti doped thin films. Mechanisms of solid lubrication were studied by FIB prepared cross-sectional specimens and Raman spectroscopy wear maps inside the wear tracks to determine the sub-surface deformation behavior and formation of tribochemical products, respectively. It was determined that WS₂ oxidized to form relatively low shear strength WO₃ during wear (tribo-oxidation) and heating at 500 °C (thermal oxidation) as determined by Raman spectroscopy in the wear track and transfer film (third body) on the counterface.     

Preparation of La0.8Sr0.2Cr0.97V0.03O3 − δ films for solid oxide fuel cell application

La_0.8Sr_0.2Cr_0.97V_0.03O_3 − δ (LSC) is commonly studied as a ceramic interconnect material as well as a coating material for metallic interconnects for solid oxide fuel cell applications. However, it is difficult to sinter this type of material to high density. In order to overcome this problem and to study the material in form of a thin film we have used Pulsed Laser Deposition to obtain a dense, uniform film with the right stochiometry. Investigation of preparation-parameter dependence of the LSC films deposited on a stainless steel substrate during pulsed-laser deposition was carried out. The LSC films were deposited with KrF excimer laser (248 nm) on a stainless steel substrate at different oxygen pressure and substrate temperatures. The substrate temperature (873-1073 K) and the oxygen background pressure (5-20 Pa) were varied in order to obtain optimal growth conditions. The surface morphology and structural information of the films were obtained using scanning electron microscope (SEM) and X-ray diffraction, respectively. Under the optimal preparation-parameter conditions: substrate temperature of 1023 K and an oxygen pressure of 10 Pa the structure of the film agreed with the target structure and the SEM micrographs show that the surfaces are homogeneous, smooth, crack-free and dense.     

Nanostructured tungsten and tungsten trioxide films prepared by glancing angle deposition

Nanostructured tungsten (W) and tungsten trioxide (WO₃) films were prepared by glancing angle deposition using pulsed direct current magnetron sputtering at room temperature with continuous substrate rotation. The chemical compositions of the nanostructured films were characterized by X-ray photoelectron spectroscopy, and the film structures and morphologies were investigated using X-ray diffraction and high resolution scanning electron microscopy. Both as-deposited and air annealed tungsten trioxide films exhibit nanostructured morphologies with an extremely high surface area, which may potentially increase the sensitivity of chemiresistive WO₃ gas sensors. Metallic W nanorods formed by sputtering in a pure Ar plasma at room temperature crystallized into a predominantly simple cubic β-phase with <100> texture although evidence was found for other random grain orientations near the film/substrate interface. Subsequent annealing at 500 °C in air transformed the nanorods into polycrystalline triclinic/monoclinic WO₃ structure and the nanorod morphology was retained. Substoichiometric WO₃ films grown in an Ar/O₂ plasma at room temperature had an amorphous structure and also exhibited nanorod morphology. Post-deposition annealing at 500 °C in air induced crystallization to a polycrystalline triclinic/monoclinic WO₃ phase and also caused a morphological change from nanorods into a nanoporous network.     

Tribological behavior of radio-frequency sputtering WS2 thin films with vacuum annealing

Thin films of tungsten disulfide (WS₂) were deposited on 3Cr13 martensitic stain less steel substrate by radio-frequency (RF) sputtering. The as-deposited films were annealed at 473, 673 and 873 K respectively for 2 h in 5 × 10^− 4 Pa vacuum. Composition of the films was inspected by energy dispersive spectroscopy. Surface morphology and structure properties were studied by scanning electron microscopy and X-ray diffraction techniques. Tribological behavior was also examined using tribometer. At 473 K, the films exhibited low crystallization structure and no significant improvement in the tribological performance. At 673 K, the tribological performance was improved and a transition from non-crystalline to hexagonal structure took in place. When the annealing temperature rose up to 873 K, the films cracked and fell off from the substrate. The results suggested that with suitable technical parameters vacuum annealing could promote crystallization and improve tribological performance of RF sputtering WS₂ films.     

Epitaxial growth and relaxation of γ-Al2O3 on silicon

0.5-10 nm-thick single crystal γ-Al₂O₃ films was epitaxially grown, at high temperature, on Si(001) and Si(111) substrates using electron-beam evaporation techniques. Reflection High Energy Electron Diffraction studies showed that the Al₂O₃ films grow pseudomorphically on Si (100) up to thickness of 2 nm. For higher thicknesses, a cubic to hexagonal surface phase transition occurs. Epitaxial growth and relaxation were also observed for Si(111). The film surfaces are smooth and the oxide-Si interfaces are atomically abrupt without interfacial layers.     

Developments in hot-filament metal oxide deposition (HFMOD)

Hot-filament metal oxide deposition (HFMOD) is a variant of conventional hot-filament chemical vapor deposition (HFCVD) recently developed in our laboratory and successfully used to obtain high-quality, uniform films of MO_x, WO_x and VO_x. The method employs the controlled oxidation of a filament of a transition metal heated to 1000 °C or more in a rarefied oxygen atmosphere (typically, of about 1 Pa). Metal oxide vapor formed on the surface of the filament is transported a few centimetres to deposit on a suitable substrate. Key system parameters include the choice of filament material and diameter, the applied current and the partial pressures of oxygen in the chamber. Relatively high film deposition rates, such as 31 nm min^− 1 for MoO_x, are obtained. The film stoichiometry depends on the exact deposition conditions. MoO_x films, for example, present a mixture of MoO₂ and MoO₃ phases, as revealed by XPS. As determined by Li⁺ intercalation using an electrochemical cell, these films also show a colouration efficiency of 19.5 cm² C^− 1 at a wavelength of 700 nm. MO_x and WO_x films are promising in applications involving electrochromism and characteristics of their colouring/bleaching cycles are presented. The chemical composition and structure of VO_x films examined using IRRAS (infrared reflection-absorption spectroscopy), RBS (Rutherford backscattering spectrometry) and XPS (X-ray photoelectron spectrometry) are also presented.