Showing papers in "Applied Surface Science in 1997"

Abstract: Modification of the surface structure of solid materials by laser radiation involves a complex chain of processes. The first step is the deposition of a certain amount of optical energy in the material. The character of the material excitation is strongly dependent on the laser pulse duration. With the use of ultrashort laser pulses non-equilibrium energy distributions with large excess population in the excited states can be produced. The distinct physical processes which come into play in laser-solid interaction on the ultrafast time scale open new routes of modifying the structure and the morphology of materials and offer interesting perspectives in laser materials processing.

Abstract: Laser ablation of solid targets by Ti:sapphire laser radiation is studied. The targets are irradiated by 150 fs–5 ns laser pulses with a fluence in the range of several J/cm2. Measurements on the ablation depth yield an effective penetration depth higher than the optical penetration depth. Advantages and potentials of sub-picosecond laser radiation for the ablation and for the microstructuring of metals are demonstrated.

Abstract: We report a method for the quantitative analysis of the Fe(2p) and O(1s) core level XPS spectra which allows the determination of the Fe2+/Fe3+ ratio within a thin oxide film. The method involves the determination of the intensity of specific shake-up features within the Fe(2p) spectrum. This analytical method is demonstrated by comparing the Fe(2p) spectra of electrochemically modified Fe2+-rich and Fe3+-rich electrodes in a combined UHV-XPS-electrochemical system. Exposure of clean polycrystalline Fe surfaces to low pressures of O2 gas for different time intervals has also been carried out. Quantitative XPS analyses of the oxide films produced by O2 exposures reveal that the oxide films are predominantly trilayers of FeO, Fe3O4 and FeOOH phases, the latter due to ambient background contamination. The analyses demonstrate that a multiplicity of Fe2+ and/or Fe3+ chemical environments and binding energies is present in such films, and this must be explicitly accounted for during quantitative analyses.

Abstract: ZnO films have been successfully grown by atomic layer deposition (ALD) using diethylzinc (DEZn) and H 2 O as reactant gases. The processing window of self-limiting growth was observed for the substrate temperature range from 105 to 165°C. The films had a good uniformity in thickness due to the self-limiting feature of ALD. A high electron mobility of 30 cm 2 /V·s was obtained for undoped ZnO films with the thickness of only 220 nm. Furthermore, a resistivity of 7.5×10 −4 Ω cm was achieved for the B-doped ZnO film at a B 2 H 6 flow rate of 0.42 μ mol/min. For further improvement of electrical properties, the effect of UV light irradiation during the ALD growth was examined and it was found that the resistivity of the films grown with UV irradiation (photo-ALD) was one order of magnitude less than that grown without UV irradiation.

Abstract: XPS measurements have been performed on a series of Mg II salts (NO 3 − , CO 3 2− , SO 4 2− , Cl − , CH 3 COO − ), hydroxides and oxides, these latter compounds are both commercial and prepared in laboratory The binding energy (BE) of the anionic partner in Mg II salts compares well in any case with literature data for the same anions in other alkaline and alkaline earth compounds The BE of Mg II appears to be affected by the electroattractive nature of the parent anion The Mg II spectra, in the case of MgO samples, are not influenced either by the nature of the oxide precursor salt or by the temperature of the oxide preparation (873 K, 1073 K, 1253 K) The BE of Mg 2p ranges between 499 eV and 502 eV Oxygen spectra are regular and show the presence of the surface chemisorbed -OH component even at heating temperatures of 1473 K The choice, as internal reference, of the hydrocarbon contaminant carbon peak is discussed, specifically, in the case of MgO samples with reference to specific lattice and surface properties of the oxide itself

Abstract: Laser processing of sapphire using a Ti:sapphire laser at 790 and 395 nm and pulse widths varying between 0.2 and 5 ps is reported. A clear improvement in quality is demonstrated for multi-shot processing with sub-ps laser pulses. For fluences between 3 and 12 J/cm 2 two ablation phases were observed, in agreement with previous work from Tam et al. using 30 ps, 266 nm laser pulses [A.C. Tam, J.L. Brand, D.C. Cheng, W. Zapka, Appl. Phys. Lett. 55 (20) (1994) 2045]. During the `gentle ablation' phase periodic wavelike structures, i.e. ripples, were observed on the Al 2 O 3 surface, perpendicular to the laser polarisation and with a spacing almost equalling the laser wavelength, indicating metallic-like behaviour. The ripple modulation depth was in the order of a few tens of nm. For fluences between 1 and 2.5 J/cm 2 , below the single-shot surface damage threshold and at a pulse width above 200 fs, microstructures could be produced at the rear side of a 1 mm thick sapphire substrate without affecting the front surface.

Abstract: The chemical and electronic structures of ultrathin SiO2 thermally grown on Si(100) and Si(111) have been investigated by using Fourier-transform infrared attenuated total reflection (FT-IR-ATR) and X-ray or ultraviolet excited photoelectron spectroscopy (XPS/UPS), respectively. A red-shift of the p-polarized LO phonon peak observed for oxides thinner than 2 nm indicates that compressively strained SiOSi bonds exist near the SiO 2 Si interface. The extent of the structural strain induced in the interface region is found to be smaller for SiO2 grown at 1000°C on Si(100) than 1000°C SiO2 on Si(111) or 800°C SiO2 on Si(100). It is also found that, from the onset of the energy loss signal for O1s photoelectrons, the bandgap of the oxides thicker than ∼2.3 nm is 8.95 ± 0.05 eV irrespective of the oxide thickness. For oxides thinner than ∼2.3 nm, a remarkable increase in the 5–9 eV energy loss signal for O1s photoelectrons is observed. This could be attributed to not only the contribution of suboxides at the interface but also the built-in stress in the interface region which causes the band edge tailing.

Abstract: Fire retardancy of polymer may be due to the formation of protective surface coating, i.e. a swollen char layer (intumescence). The intumescent coatings resulting from thermal treatments of the association of ammonium polyphosphate (APP) and pentaerythritol (PER) with or without zeolite 4A (used as synergistic agent) in a polyethylenic terpolymer (LRAM3.5) are studied by X-ray photoelectron spectroscopy. The O 1s , P 2p , C 1s and N 1s spectra are discussed. The formation of pyridinic and pyrrolic nitrogen and quaternary nitrogen in polyaromatic groups in the intumescent structures is shown. The oxidised nitrogen (nitro groups and pyridine N-oxide) are only observed up to 350°C and then are evolved as NO x species. It is then proposed that the pyridinic function, observed only in the case of the system with zeolite at all temperatures, participates in the improvement of the fire proofing properties of the materials.

Abstract: Ta2O5 thin films have been deposited in atomic layer epitaxy process from Ta(OC2H5)5 and H2O. A quartz crystalline mass-sensor was exploited to detect the adsorption processes at the gas–solid interface during the film growth. It is suggested that Ta(OC2H5)5 reacts with surface hydroxyls producing intermediate surface species (-O)nTa(OC2H5)5−n where n varies with the reactor temperature. During the subsequent water pulse these species react further converting the surface back to the hydroxyl-terminated one. The uncontrolled deposition due to the temperature-induced decomposition of tantalum ethoxide with the activation energy of 100±6 kJ/mol contributes to the film growth above 275°C. The value of the diffusion coefficient D=0.0075 m2/s for gas-phase Ta(OC2H5)5 has been calculated at 250°C. Estimated sticking coefficient of Ta(OC2H5)5 is about one order of magnitude higher than that of H2O and nearly one order of magnitude lower than that of TaCl5.

Abstract: A method for accessing information in addition to the topography primarily provided by tapping mode atomic force microscopy (TM-AFM) is presented. The phase difference between the driving signal of the piezo oscillating the cantilever in the TM-AFM and the resulting motion of the tip is exploited in order to access additional information about the tip sample interaction. Adhesive forces between the tip and the surface can be mapped simultaneously to the topographic data with the same lateral resolution. We found evidence that the phase signal is closely related to the nature of the water layer present on surfaces exposed to the ambient atmosphere by examining aerosol particles on polyester foil and the corrosion of potash-lime-silica glass with medieval composition. However, also the topography as well as operating parameters such as load force and scan speed contribute to the phase signal making quantitative comparisons difficult.

Abstract: The direct silanation of nanosized superparamagnetic particles (γ-Fe2O3) using 3-aminopropyl triethoxy silane is described. The silanized films are characterized using X-ray photoelectron spectroscopy, diffuse-reflectance Fourier transform infrared spectroscopy and electrokinetics. The silanation is conducted in both organic (toluene) and water solutions to examine the solvent effect on the molecular orientation and packing density of the silanized films. Depending on the solvent, about 74 to 83% of amine groups are found to be un-protonated and remain reactive on the particles. In acidic environment, the films silanized in toluene are more stable than that in water, but both are unstable in basic environment.

Abstract: Titanium oxide species prepared in the Y-zeolite cavities via an ion-exchange method and those of the Ti-silicalite catalyst prepared hydrothermally exhibit high photocatalytic reactivity for the direct decomposition of NO into N2, O2 and N2O at 275 K with a high selectivity for the formation of N2. The in situ photoluminescence and XAFS investigations indicate that these titanium oxide species are highly dispersed and exist in a tetrahedral coordination in the zeolite cavities and its framework. The charge transfer excited state of these titanium oxide species plays a significant role in the direct decomposition of NO with a high selectivity for the formation of N2, while the catalysts involving the aggregated octahedrally coordinated titanium oxide species and the bulk powdered TiO2 catalyst mainly produce N2O.

Abstract: We (Asahi Chemical Industy Co., Ltd.) have developed a technology for a highly selective partial hydrogenation of benzene to cyclohexene and succeeded in the commercialization of a new production process for producing cyclohexanol from benzene through cyclohexene. The process has been considered to be difficult for a long time in the industry. Several innovative technologies made the partial hydrogenation reaction possible. A catalyst that consists of specific metallic ruthenium particles exhibited excellent high selectivity of cyclohexene. The catalyst was obtained by reducing a ruthenium compound which contains a zinc compound. The use of a zinc of strong acid as a co-catalyst exhibited a remarkable effect to enhance selectivity. Dispersing agents of metal oxides were found to extend the life of the metallic ruthenium catalyst and some dispersing agents had the effect of enhancing the selectivity in the partial hydrogenation reaction. One of the remarkable features of the reaction was the reaction field. This comprises four phases: vapor (hydrogen), oil, aqueous and solid (ruthenium catalyst). The catalyst was used in the aqueous phase, and the reactants (benzene and hydrogen) were dissolved in the aqueous phase where the reaction proceeded. Therefore the products and reactants transfer between four phases through dissolution, diffusion and extraction, rendering quick transfer was a very important factor in enhancing reaction selectivity. The catalyst system and the reaction field described above made the selectivity for cyclohexene very high, and a yield of 60% for cyclohexene was obtained.

Abstract: The adsorption behavior of inorganic oxyanions on soil minerals is an important factor in the transport of subsurface environmental pollutants. We have studied the adsorption of chromate (CrO 4 2− ) from aqueous solution on the surface of the mineral goethite ( α -FeOOH) as a function of pH and adsorbate concentration in 0.05 M NaNO 3 solution. Results obtained from the dried surface by X-ray photoelectron spectroscopy (XPS) are in good agreement with data from spectrophotometric analysis of chromate remaining in the supernatant liquid. Chromate adsorption increases with decreasing pH of the solution and eventually reaches a maximum at pH 6.5. The chromium XPS signal indicates that initially a small amount of chromium adsorbs in the +3 oxidation state via a redox reaction, but that the large majority of chromium remains in the +6 oxidation state.

Abstract: The reasons for the improvements gained by using intermediate zinc pulses in atomic layer epitaxy growth of TiN and NbN films were examined by a comprehensive characterization and comparison of films prepared from TiCl4 or NbCl5 and NH3 with and without zinc. The characterization techniques used comprise time-of-flight elastic recoil detection analysis, secondary ion mass spectrometry, Rutherford backscattering spectrometry, nuclear resonance broadening, proton backscattering spectrometry, deuteron induced reactions, proton induced X-ray emission, atomic force microscopy, scanning electron microscopy, X-ray diffraction, and Hall effect and reflectance measurements. The effect of zinc was found to be manifold: both compositional and structural changes were observed. In the case of TiN the major improvement gained by using zinc was significantly decreased oxygen contamination whereas a marked increase of grain size was the dominant effect observed with NbN. A clear correlation between the compositional and structural changes and the improvements of the electrical properties was established.

Abstract: In this paper we discuss the potential advantages of quantum wells (QWs) for enhancing solar cell efficiency. We present recent experimental results which show that the open-circuit voltage (Voc) of the quantum well solar cell (QWSC) is enhanced over that of comparable conventional cells formed from the well material, by more than the change in the absorption edge. We also report on theoretical and experimental studies which seek to determine the quasi-Fermi level separation in quantum wells inside a p-i-n system in order to understand the voltage behaviour of QWSCs and to be able to estimate the efficiency enhancements which may be achieved in the radiative limit. We discuss QWSCs in the InPInGaAs lattice matched system and present results which show that QWSCs in this deep well system have a better variation of efficiency with temperature than conventional cells made from either the well or barrier material. This is important for applications involving concentrated sunlight. We also consider the advantages of quantum well cells in the area of thermophotovoltaics (TPV).

Abstract: Luminescent Y2O3:Eu3+ thin films were deposited on sapphire, polycrystalline Al2O3 and indium tin oxide coated glass or sapphire substrates by two different techniques: metallorganic chemical vapor deposition (MOCVD) and laser ablation. Microcrystalline Y2O3:Eu films were grown in a MOCVD chamber by decomposing and reacting yttrium and europium organometallic precursors in an oxygen atmosphere at low pressures (1–10 mTorr) and low substrate temperatures (500–700°C). The as-deposited films showed the characteristic red fluorescence spectrum of Y2O3:Eu with the main peak centered about 611 nm wavelength. The as-deposited films averaged 1.0 μm in particle size and 2.0 μm in thickness. Post-deposition annealing treatments in the temperature range 900–1200°C enhanced the luminescent intensity of the films. The as-deposited laser ablated oxide films were amorphous and required annealing at temperatures higher than 800°C to observe luminescence, which occurred in conjunction with crystallization. The as-deposited films averaged 500 nm in thickness and after post-annealing at 1000°C were composed of 15–200 nm grains.

Abstract: A novel amorphous Ni–Co–W–B alloy powder, which showed higher catalytic activity than Ni–Co–B for the hydrogenation of benzene, was prepared by chemical reduction and characterized by TEM and XRD. The surface composition and the interactions between the components on the surface were studied by XPS. The measurement detected the presence of some oxides and hydroxides, such as NiO, Ni(OH) 2 , Co(OH) 2 , B 2 O 3 and WO 3 associated with their elemental states of Ni, Co, B and W, respectively. The presence of a few number of tungsten atoms leads to more metallic nickel on the surface.

Abstract: The adsorption of carbon dioxide in silicalite and NaZSM-5 zeolite has been studied using new Monte Carlo software In this program, sodium cations and framework are movable during the simulation The calculated adsorption isotherms are in good agreement with the experimental results The energy distribution of carbon dioxide over silicalite and NaZSM-5 shows that the increase of the adsorption energy for NaZSM-5 is mainly due the electric field generated by sodium cations

Abstract: Single-crystal surfaces have long served us well as model catalysts; however, a new type of model catalyst has been prepared using electron beam lithography. Ordered arrays of platinum nano-particles in the 2.5–50 nm size range are deposited on oxide substrates (silica, alumina, and titania) of 1 cm 2 surface area, and are used in catalyzed surface reactions at high pressures (atmospheres). Their preparation, cleaning, and reactivity is discussed. Scanning tunneling microscopy (STM) and vibrational spectroscopy by sum frequency generation (SFG) can be utilized to monitor the substrate and adsorbate structure, respectively, over a fourteen order of magnitude pressure range (10 −10 –10 4 Torr). As a consequence, we can monitor the surface structure and reaction intermediates during high pressure catalytic reactions. An STM that operates at both high pressure (atmosphere) and high temperature has been constructed and utilized to monitor platinum (111) and (110) surface structure during chemisorption of H 2 , O 2 and CO, and during catalytic reactions of olefin hydrogenation and hydrogenolysis. Changes of surface structure upon chemisorption and during reactions have been monitored. Catalysis by the platinum tip was also detected in the presence of H 2 or O 2 at high pressures and 300 K, leading to hydrogenation or oxidation of carbonaceous deposits with nanometer spatial resolution. Vibrational spectroscopy using SFG has been used to monitor pressure dependent changes in the chemisorption of CO and NO over Pt(111). Bonding — which is similar to that in Pt m (CO) n (where n m > 1 ) clusters and for an incommensurate CO overlayer — is observed above 100 Torr. Reaction intermediates that form during ethylene, propylene, and isobutene hydrogenation, as well as CO oxidation, at atmospheric pressures and 300 K were monitored by SFG. The dominant reacting species that hydrogenate are the weakly π-bonded olefins, while the strongly chemisorbed alkylidyne and di-σ bonded species are spectators during the reaction. From quantitative measurement of coverages, the absolute turnover rates can be determined.

Abstract: Catalytic NOx removal with H2 in the presence of an excess of oxygen was studied. A Pt/zeolite catalyst has a high NOx conversion efficiency, but has the disadvantages of being strongly poisoned by CO and HCs such as C3H6 and the formation of N2O as a byproduct from NOx. By reducing the oxidation efficiency of Pt, we have developed an improved catalyst, Pt-Mo-NaSiO2, which has a higher temperature range of NOx conversion and a lower N2O byproduct formation, than conventional Pt catalysts. This idea was supported by IR and XPS data of the catalyst.

Abstract: Polycrystalline ZnS films were grown from ZnCl2 and H2S on glass and mica using the atomic layer epitaxy (ALE) technique. Morphological and crystalline changes during the ALE growth of ZnS were studied by AFM and XRD. AFM measurements revealed that substantial agglomeration took place in the beginning of the growth. On glass the nucleation density of ZnS was higher than on mica and consequently the films on glass remained smoother than those on mica. XRD measurements revealed that orientation of the films was stronger on mica than on glass.

Abstract: Silicon-nitride films have been successfully deposited in a layer-by-layer manner by alternating exposures to dichloro-silane and hydrazine. The saturated deposition rate was about 2.3 A/cycle, very near to 1 monolayer/cycle, in a temperature range between 525°C and 650°C. The layer-by-layer manner deposition was confirmed to start from a very early stage of its deposition cycle. Deposited film characteristics including electron tunnelling are also presented.

Abstract: NiTi Shape Memory Alloys (SMA) are potential biomaterial candidates. However, due to its possible corrosion in physiological solution, dissolution of toxic Ni might be happening, rendering this material nonbiocompatible. We have used excimer laser surface treatment to improve corrosion resistance of NiTi SMA plates. Potentiodynamic tests in physiological Hank's solution show that the laser treatment performed in air improved all corrosion parameters. The surface is homogenized and a Scanning Electron Microscopy (SEM) observation indicates a decrease of corrosion pit size and numbers. Laser treatment improvement resistance is explained by a combination of the homogenization of the surface by melting, the hardening due to N incorporation and the thickening of the oxide layer.

Abstract: The use of X-ray photoelectron spectroscopy (XPS) as a technique for non-destructive depth profiling of technical samples is often hindered by their roughness. In this paper, we elaborate on earlier work where we reported on the apparent existence of a ‘magic’ angle for the determination of the thickness of uniform overlayers on rough substrates. Simple calculations for fully three-dimensional model rough surfaces strongly suggest that the thickness of an overlayer on a rough substrate can be determined accurately without taking the roughness into account in the analysis of the XPS intensities, the neglect of roughness effects leading to an average error less than 10% in the determined thickness.

Abstract: Due to its distinctive self-limiting growth process, Atomic Layer Epitaxy (ALE) technique exhibits a unique combination of advantageous features. In this paper, the benefits and potentials of ALE in growth of amorphous and polycrystalline films will be discussed using some selected processes as examples. These processes include low temperature deposition of high quality nanolaminate dielectrics, controlled deposition of optical multilayers, growth of low resistivity transition metal nitrides using elemental zinc as a reducing agent, and preparation of ZnS1−xSex solid solutions via surface anion substitution reactions. The major drawback of ALE, its slowness, and the possibility of overcoming this problem via large-batch processing will also be discussed.

Abstract: The surface activated bonding method has been applied to bond the III–V compound semiconductor wafers and Si wafer directly at room temperature in ultra high vacuum. The procedure is as follows: the surfaces to be bonded are sputter-cleaned and activated by Ar fast atom beam irradiation and brought into contact under slight pressure. The GaAs and Si wafers bonded very well, without any macro defect being detected along the bonded interface. An amorphous intermediate layer of about 3 nm thick in most parts of the interface boundary direct bonding without any interlayer between the GaAs and Si wafers existed in a partially bonded interface by high resolution TEM.

Abstract: We have measured Ru 3d, 4s, 4p and O 1s high-resolution core-level X-ray photoelectron spectra, along with Ru 3d and O 1s scanned-angle X-ray photoelectron diffraction angular distributions, for RuO2(110). The surfaces were prepared by oxygen-plasma-assisted molecular beam epitaxial growth of RuO2 on TiO2(110). XPS spectral interpretation and the nature of the XPD scans strongly suggest that the complex line shapes are due to final-state screening effects, rather than the presence of Ru in oxidation states other than +4.

Abstract: LaCoO 3 thin films have been deposited in a flow-type atomic layer epitaxy (ALE) reactor from La(thd) 3 and Co(thd) 2 , (thd = 2,2,6,6-tetramethyl-3,5-heptadione) precursors using ozone as oxygen source. Films were grown on both soda lime and Corning glass substrates in the temperature range of 200-400°C. Profilometry, X-ray diffraction, Rutherford backscattering spectroscopy and X-ray photoelectron spectroscopy measurements were used to determine the thickness, crystallinity and stoichiometry of the films. The films grown below reactor temperatures of 400°C were X-ray amorphous, but became crystalline LaCoO 3 when annealed at 600°C in air. Besides LaCoO 3 , the binary oxides (La 2 O 3 and Co 3 O 4 ) were also grown as thin films.

Abstract: The surface structure of TiO2 thin film photocatalyst was successfully controlled by the sol-gel method with a polymer doped dip-coating process. The thin films prepared were either transparent or translucent depending on the molecular weight of the polymer doped. The surface of the transparent thin films looked plain consisting of uniformly aggregated nanometer size TiO2 particles, while the translucent thin film was formed with cubic-like crystalline TiO2 at a micrometer level. The specific surface area of the transparent thin film was almost the same as that of the translucent one. All the thin film photocatalysts had an anatase structure with similar crystallinity. Both the transparent and translucent films showed catalytic activity for NOx elimination from air. The activity was almost equal to the commercial photocatalyst P25.