Showing papers on "Overlayer published in 1980"
TL;DR: In this paper, a short-range repulsive interaction model was proposed to explain the mutual configuration of the adsorbed molecules, which is supported by the observation that the isosteric heat of adsorption (Ead=38.5 kcal/mole) is constant up to a coverage of Θ?0.5.
Abstract: Adsorption of CO on a Pd(100) surface was studied in detail mainly by LEED, UPS, work function and thermal desorption measurements. Analysis of the ordered c(2√2×√2) R 45° structure occurring at Θ=0.5 revealed that in this phase each CO molecule is bridge bonded to 2 Pd atoms with Pd–C distances of 1.93±0.07 A and a C–O bond length of 1.15±0.1 A, the molecular axis being oriented normal to the surface. The mutual configuration of the adsorbed molecules is explained in terms of a short‐range repulsive interaction model, which is supported by the observation that the isosteric heat of adsorption (Ead=38.5 kcal/mole) is constant up to a coverage of Θ?0.45. The photoelectron spectra exhibit two maxima at 7.9 (5σ+1π level) and 10.8 eV (4σ level) below the Fermi level which are in agreement with the observations with other Pd planes. This also holds for an electronic excitation associated with an energy of 13.5 eV as observed by electron energy loss spectroscopy. The variation of the sticking coefficient with coverage is described in terms of a second‐order precursor state model with repulsive interactions. The pre‐exponential factor for desorption (3×1016 sec−1) varies only little with coverage. The dipole moment of the adsorbate remains constant up to Θ?0.35 (0.17 D) where the overlayer starts to order, and exhibits an appreciable higher value near Θ=0.5. The maximum work function increase is Δφmax=0.93 V at Θ=0.5. The differential entropy of the adsorbed layer around 450 K was derived from the experimental adsorption isotherms. Up to Θ=0.35 the data fall well between the limits of two theoretical models for localised and delocalised adsorption of noninteracting particles. Deviations at higher coverages reflect again the onset of ordering. The energetic and work function data are in some variance to results reported earlier which is ascribed to the fact that these quantities may be sensitively influenced by spurious amounts of carbon impurities on the surface.
286 citations
TL;DR: In this paper, the results of a detailed investigation of the chemisorption and reactivity of acetylene and C2H2 on the Rh(111) single crystal surface were reported.
Abstract: We report the results of a detailed investigation of the chemisorption and reactivity of acetylene (C2H2) and ethylene (C2H4) on the Rh(111) single crystal surface. Below 270 K ELS measurements indicate that acetylene chemisorbs on Rh(111) with its C–C bond oriented parallel to the surface forming an approximately sp2 hybridized species. LEED investigations show that both C2H2 and C2H4 form metastable (2×2) surface structures on Rh(111) below 270 K. An irreversible order–order transformation occurs between 270 and 300 K to a stable c(4×2) hydrocarbon overlayer. The stable species formed from both molecules are identical. Hydrogen addition to chemisorbed acetylene is necessary to complete this conversion. The geometry of the adsorbed ethylene species does not change during this transformation although the overlayer structure does. This stable hydrocarbon species is identical to the hydrocarbon species formed from the chemisorption of either C2H4 or C2H4 and hydrogen on Pt(111) above 300 K. The addition of ...
139 citations
TL;DR: In this article, a SiO2 cap, either intentionally deposited or produced by laser crystallization in the presence of oxygen, was found to be necessary for Si graphoepitaxy.
Abstract: Silicon graphoepitaxy has been achieved using a strip‐heater oven and a sample configuration consisting of a relief grating in a SiO2 substrate, a deposited amorphous silicon film, and a deposited SiO2 overlayer or ’’cap.’’ The resulting films are free of cracks and superior in crystallographic orientation and surface smoothness to graphoepitaxial films produced by laser crystallization. Enhancement‐mode, n‐channel, insulated polysilicon gate field‐effect transistors were fabricated and gave surface mobilities of 400 cm2/V sec at a p doping of 1016 cm−3. A SiO2 cap, either intentionally deposited or produced by laser crystallization in the presence of oxygen, was found to be necessary for Si graphoepitaxy. We attribute this effect to shear stresses produced by the SiO2 cap during crystallization.
108 citations
TL;DR: In this paper, the energy and angle variation of large-angle, inelastic, electron scattering produced by vibrations in a c (2 x 2) overlayer of CO on Ni(100) are presented.
Abstract: Calculations of the energy and angle variation of large-angle, inelastic, electron scattering produced by vibrations in a c (2 x 2) overlayer of CO on Ni(100) are presented. The results show pronounced structure related to the bonding site and symmetry associated with the adsorbed CO molecule. Limitations of the elementary form of the dipole-scattering formula applicable to the regime of small-angle scattering are also discussed.
93 citations
TL;DR: The LaB6 (110) and (111) clean surfaces have been studied by angle-resolved XPS, ISS and LEED as discussed by the authors, and the work functions of the (100, (110), (110, and) surfaces were measured from the width of UPS spectra and are ~2.3, 2.5 and 3.3 eV, respectively.
84 citations
TL;DR: In this article, a multilayer growth of AgCl is investigated by dosing with AgCl(g) and it is found that the desorption spectra evolve in an unusual way and the observed energy of small adsorbed crystallites.
68 citations
TL;DR: In this paper, the authors studied the oxidation of the Rh(111) surface using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and thermal desorption mass spectrograph (TDS).
64 citations
TL;DR: In this article, the interaction of 3H/sub 2/CO at 1 atm and 573 to 673/sup 0/K with unsupported Ru, Fe, and RuFe alloy powders was investigated using x-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), and differential chemical kinetics.
61 citations
TL;DR: In this article, the final state oxidation products and their distribution in thin native oxides (30-40 A) on GaAs have been studied using x-ray photoelectron spectroscopy in conjunction with chemical depth profiling.
Abstract: The final‐state oxidation products and their distribution in thin native oxides (30–40 A) on GaAs have been studied using x‐ray photoelectron spectroscopy in conjunction with chemical depth profiling. Extended room‐temperature‐oxidation conditions have been chosen to allow the native oxide to attain its equilibrium composition and structure. The work emphasizes the use of chemical depth‐profiling methods which make it possible to examine the variation in chemical reactivity of the oxide structure. A minimum of two distinct regions of Ga2O3 with differing chemical reactivity is observed. Arsenic oxide species show one distinct phase at the outer oxide surface. Intensity analysis indicates an oxidized Ga to oxidized As composition ratio of 2:1 at the outer surface, which changes to approximately 10:1 at the oxide/GaAs interface. Chemical shift data indicate the presence of As2O3 in the oxide together with an elemental As overlayer at the interface. A change in relative charge transfer between oxygen and both arsenic and gallium–oxide species is observed in the region of the interface. The presence of mixed As/Ga oxides and different gallium oxide coordination environments is also discussed.
53 citations
TL;DR: In this article, a simple positron diffusion model, including surface and vacancy trapping, is fitted to the positronium data and an estimate of the binding energy of a positron in this trap is made.
Abstract: Well-characterized Al(111) and Al(100) samples were studied with monoenergetic positrons before and after exposure to oxygen. Both positronium-formation and positron-emission curves were obtained for various incident positron energies at sample temperatures ranging from 160-900 K. The orthopositronium decay signal provides a unique signature that the positron has emerged from the surface region of a clean metal. In the clean Al crystals part of the positronium formed near the surface is found to be associated with a temperature-activated process described as the thermally activated detrapping of a positron from a surface state. A simple positron diffusion model, including surface and vacancy trapping, is fitted to the positronium data and an estimate of the binding energy of the positron in this trap is made. The positron diffusion constant is found to have a negative temperature dependence before the onset of positron trapping at thermally generated monovacancies (g500 K), in reasonable agreement with theoretical predictions. The depth of the positron surface state is reduced or positronium is formed in the chemisorbed layer as oxygen is adsorbed on both Al sample surfaces, thus increasing the positronium fraction and decreasing the positron emission. At higher oxygen exposures [g500 L (1 L = ${10}^{\ensuremath{-}6}$ torr sec)] positron or positronium traps are generated in the overlayer and the positronium fraction is reduced. The amorphous-to-crystalline surface transition of ${\mathrm{Al}}_{x}{\mathrm{O}}_{y}$ on Al is observed between 650 and 800 K by the change in the positronium fraction and is interpreted as the removal of trapping centers in the metal-oxide overlayer. At the higher temperatures and incident energies vacancy trapping is observed by the decrease in the positron diffusion length in both the clean and the underlying Al of the oxygen-exposed samples. Similar vacancy formation enthalpies for Al are extracted in both the clean and oxygen-covered samples by a simple model and are in good agreement with those measured by other experimental methods. This technique provides a new experimental means for the study of interfaces and thin films and the vacancy-type defects associated with them.
52 citations
TL;DR: Ion-bombarded and annealed Ge surfaces, maintained in ultrahigh vacuum and characterized by standard LEED and Auger surface analysis techniques, are examined by spectroscopic ellipsometry over the energy range 1.5 − 6.0 eV as discussed by the authors.
Abstract: Sputter‐removal rates of overlayer and interfacial species on silicon are analyzed to determine sputtering yields for the species involved. Sputtering yields up to two orders of magnitude lower than those measured for silicon are found, and the results are interpreted in terms of a cascade mixing process which continually reburies much of the overlayer material beyond the escape depth of the sputtered atoms.
TL;DR: In this article, the interaction of activated nitrogen (AN) with Cu(210) and Ni(210), and of O 2 with Ni (210) have been studied using LEED, RHEED and AES.
TL;DR: In this paper, the adsorption of 12CO on Ir films evaporated under ultrahigh vacuum (UHV) conditions was studied using infrared reflection-absorption spectroscopy (IRAS).
TL;DR: In this paper, the C-O stretching vibrations were studied as a function of composition of the overlayer and the data were successfully interpreted by a dipole-dipole coupling theory.
Patent•
10 Jul 1980
TL;DR: In this article, an intermediate layer of woven fiber glass fabric that includes strands of bulked yarn in a relatively loose weave is embedded in the coating system between the underlayer and overlayer.
Abstract: A coating system of high tensile strength and resilience for use as a coating for roofs, swimming pools or the like having an underlayer of coating material, an intermediate layer of woven fiber glass fabric that includes strands of bulked yarn in a relatively loose weave, and an overlayer of coating material, with the fiber glass fabric being embedded in the coating system between the underlayer and overlayer. Alternatively the underlayer may be of asphalt material and the overlayer may be a first overlayer bonded to the underlayer with the intermediate layer embedded therebetween and a second overlayer bonded to the first overlayer. The coating material contains water, a high solids thermoplastic acrylic emulsion, calcium carbonate, titanium dioxide, sodium salt of polymeric carboxylic acid, a wetting, emulsifying and stabilizing agent and defoamers, with zinc oxide being used if desired for mildew resistance.
TL;DR: In this article, the authors used a time-of-flight (TOF) system which collects both neutrals and ions to reveal the scattered yield from second layer as well as first layer atoms.
TL;DR: In this article, electron energy-loss spectroscopy was used on vacuum-cleaved ZnSe(110) surfaces and on Ar+ bombarded-annealed surfaces of ZnTe, ZnO, CdSe, and CdTe.
Abstract: Oxidation properties of ZnSe, ZnTe, CdSe, and CdTe surfaces were studied. The following combinations of experimental techniques were used: electron‐energy‐loss spectroscopy on vacuum‐cleaved ZnSe(110) surfaces and on Ar+ bombarded‐annealed surfaces of ZnSe, ZnTe, CdSe, and CdTe. Photoemission spectroscopy also was used on a vacuum cleaved ZnTe(110) surface. The oxygen uptake is very slow with exposures to molecular oxygen. Thus oxide overlayers were prepared by an electron‐beam irradiation technique which greatly enhanced the oxygen uptakes. Changes in loss spectrum detected on oxidation of CdTe can be explained by formation of TeO2. For the case of ZnTe, the shifted Te 4d loss due to TeO2 appears at the oxygen coverage, ϑ, of about 0.5 of a monolayer, and losses characterizing formation of ZnO are seen at ϑ≳1. The oxide overlayer on ZnTe is composed of TeO2 and ZnO and, in addition, an unidentified complex oxide for the heavily oxidized surface. On further oxygen exposures and beam irridation of this oxi...
TL;DR: In this article, a model of the VBr 2 system was developed which fits all the thermal, structural and kinetic data, and a fractional order kinetics were exhibited which reflected the increasing Madelung energy of the growing VBr 2 crystal.
TL;DR: In this paper, a phase diagram based on the theory of a two-dimensional Ising ferromagnet in zero magnetic field is constructed for the condensation of a lattice gas system with net attractive interactions between the particles.
TL;DR: The influence of imperfections of single crystal surfaces on two-dimensional phase transitions is studied by LEED and AES in this paper, where the first adsorbed layer of xenon undergoes a 2D gas-incommensurable solid first order transition.
TL;DR: In this article, the surface electronic states and oxidation properties of CdTe (110, 111, and (100) surfaces and CdSe {0001} surfaces have been studied with low-energy electron-loss spectroscopy, Auger-electron spectrographs, and low energy electron diffraction, and the surface losses due to the transitions from the Cd-$4d$ core level locate the empty Cd-derived surface states at 2.5 [for the (110) and (111) plane] or 3.7 eV in Cd
Abstract: Surface electronic states and oxidation properties of CdTe (110), {111}, and (100) surfaces and CdSe {0001} surfaces have been studied with low-energy electron-loss spectroscopy, Auger-electron spectroscopy, and low-energy-electron diffraction. The surface losses due to the transitions from the Cd-$4d$ core level locate the empty Cd-derived surface states at 2.5 [for the (110) and (111) plane] or 3.0 eV [for the (100)] in CdTe and 2.7 eV in CdSe, with respect to the valence-band maximum. The losses appearing at 61 and 66.2 eV in CdSe are identified as due to the transitions from the Se-$3d$ core level to the Se-derived surface states lying at about 8 and 13 eV above the valence-band maximum. No corresponding Te-derived empty surface states, i.e., transitions from the Te-$4d$ level to Te-derived surface states, have been detected for CdTe. Oxygen uptake on the ordered surface is very slow when the surface is exposed to molecular oxygen; the initial sticking coefficients are ${10}^{\ensuremath{-}13}$ for CdTe (100), less than ${10}^{\ensuremath{-}14}$ for CdTe (110), and ${10}^{\ensuremath{-}14}$ for CdSe (0001). Oxidation is stimulated drastically by electron-beam irradiation on the surface exposed to molecular oxygen and bulklike oxide overlayers are formed. Upon oxidation of CdTe surfaces, the shifted Te-$4d$ loss due to Te${\mathrm{O}}_{2}$ occurs at rather early stages of oxidation and grows in magnitude with oxygen uptake, and the loss spectrum from the heavily oxidized surface resembles that of Te${\mathrm{O}}_{2}$, while formation of neither a CdO nor a Cd element is evident. These findings indicate that the oxygen adsorbs preferentially on surface Te atoms by breaking the back bonds at the initial stages of oxidation, and then oxidation proceeds into the bulk, forming the oxide overlayer composed entirely of Te${\mathrm{O}}_{2}$. In the case of the CdSe (0001) surface, oxygen adsorbs only on the surface Se atoms without breaking the back bonds at the initial stages of oxidation, and then it seems to adsorb with a breaking of the back bonds, and both selenium oxide and cadmium oxide are formed, although part of the selenium oxide sublimes away from the surface, leaving the oxide overlayer rich in CdO.
TL;DR: In this article, the presence of a magnetic overlayer on FeTi made up of iron clusters and the absence of magnetic moments on the iron contained in the bulk was found to originate from this segregated iron layer on the surface of FeTi.
Abstract: Mossbauer spectroscopy of 57 Fe conversion electrons was used to study FeTi subjected to 51 hydrogen absorption-desorption cycles. This technique probes both the surface and part of the bulk, and it can also distinguish between magnetic and non-magnetic iron in the lattice. Our measurements suggest the presence of a magnetic overlayer on FeTi made up of iron clusters and the absence of magnetic moments on the iron contained in the bulk. This results shows that the magnetic moment observed in bulk magnetization studies in fact originates from this segregated iron layer on the surface of FeTi.
TL;DR: In this paper, the effect of the angle of incidence of 600 eV Ar+ ions on the yields and mechanisms of particle ejection from a clean Cu(001) and a c (2×2) overlayer of oxygen on Cu(1) has been examined.
Abstract: The effect of the angle of incidence of 600 eV Ar+ ions on the yields and mechanisms of particle ejection from a clean Cu(001) and a c (2×2) overlayer of oxygen on Cu(001) has been examined. The total yield of particles ejected as a function of polar angle is in qualitative agreement with experiment. The azimuthal dependence of the yields for both the clean Cu(001) and Cu(001) with a c (2×2) overlayer has been predicted. Mechanisms of particle ejection which are specific to a particular angle of incidence have been identified. Of particular interest is a shearing mechanism which contributes to the ejection process at an angle of ϑ=45° in the <100≳ directions. This shearing is responsible for the ejection of a large number of dimers which were originally next nearest neighbors on the surface. By energy selecting the dimers which eject, one can preferentially select the original sites of the two components. The angular distribution of the ejected particles, both substrate and adsorbate, due to normal incidence ion bombardment has previously been found to reflect the original surface site symmetry. The angular patterns of the ejected atoms are still dominated by the site sensitive effects, but the intensity is shifted due to the primary ion’s off‐normal angle of incidence.
TL;DR: In this paper, a full dynamical LEED calculation and analysis have been performed on the initial stages of oxidation (150 L O2 exposure) of fresh deposited Al(111) surfaces grown epitaxially on mica substrates.
TL;DR: In this paper, an accurate determination of the relative uptakes of CO in the formation of three successive overlayer structures on Cu(111) has been achieved by sequential adsorption of 12 CO and 13 CO followed by isotopic analysis of the desorbing gas.
TL;DR: In this article, the interfacial electronic states associated with Al and Ga overlayers on cleaved GaAs (110) surfaces are studied by ultraviolet photoemission spectroscopy (UPS) and low energy electron diffraction (LEED).
Abstract: The interfacial electronic states associated with Al and Ga overlayers on cleaved GaAs (110) surfaces are studied by ultraviolet photoemission spectroscopy (UPS) and low energy electron diffraction (LEED). Deposition of Al or Ga can produce valence‐band spectra nearly devoid of new structure. Sb produces quite significant changes in the surface valence band and is shown as a contrasting example. Pseudopotential calculations from the literature for Al‐ and Ga‐ordered overlayers are found to be in somewhat better agreement with the data than the corresponding tight‐binding calculations, particularly in the adatom states which lie above the GaAs valence‐band maximum (VBM), although neither calculation gives a good description of the observed overlayer states. It is shown that Al and Ga most likely form two‐dimensional clusters or rafts on the surface at submonolayer coverage which are not in registry with the GaAs surface lattice. A comparison is made with results by other workers for Al, Ga, and In on GaAs ...
TL;DR: In this paper, the authors studied the effect of CO adsorption on Ni(110) ridges by infrared ellipsometric spectroscopy and showed that at low coverages, CO enters short bridge sites on the Ni (110) ridge up to one half coverage corresponding to a c(2 × 2) structure.
IBM1
TL;DR: The interaction of Nb(110) and nb(750) with oxygen has been studied using LEED, AES, SIMS and ELS as mentioned in this paper, and it has been shown that the topmost layer of the oxide consists of a highly irregular two-dimensional network of nbO 6 octahedra linked together in a way to yield the stoichiometry of the Nb 2 O 5.
TL;DR: In this paper, the electronic structure of a model Si (111) surface with a Pd overlayer is investigated and the self-consistent pseudopotential method in the mixed basis set (Gaussian orbitals plus plane waves) representation is used to describe the system.
Abstract: The electronic structure of a model Si (111) surface with a Pd overlayer is investigated. The self-consistent pseudopotential method in the mixed basis set (Gaussian orbitals plus plane waves) representation is used to describe the system. Two different adsorption sites of Pd (the top site and the triangular site) studied here give different dispersions for the Pd d bands and different hybridization between Si and Pd atoms. The Pd d peak centers around -1.8 and -2.2 eV below the Si valence-band maximum in each case. Partially filled Pd-Si hybridized states exist in the gap. The Schottky barrier height is 1.1 eV for the top site and 0.95 eV for the triangular site, respectively. Various localized states are examined, and the behavior of the Schottky barrier is discussed. Comparisons with experiment are made when possible.