Determination of the surface composition of palladium-nickel alloy film catalysts using Auger electron spectroscopy

Palladium-nickel films evaporated in UHV on room temperature substrates form alloys of good bulk homogeneity as indicated by X-ray diffraction. The average composition of the outermost 3 to 5 atom layers has been determined from the intensities of the high energy 848 eV nickel and 330 eV palladium Auger electrons. This average composition is in close agreement with the bulk composition determined by X-ray diffraction, X-ray fluorescence and atomic absorption spectrophotometry. If the nickel concentration is determined from the intensity of the low energy 61 eV nickel Auger electrons, when the analysis refers more critically to the first 1 to 3 atom layers, then a surface enrichment of palladium is indicated for all alloy compositions. From the decrease in the relative intensities of the low energy and high energy nickel Auger electrons with increasing palladium concentration it may be deduced that the enrichment of palladium in the first atom layer is higher than in the second and third layers and that a complete monolayer of palladium is formed for bulk concentrations of 65 at% or more. The experimental observations are in qualitative agreement with theoretical predictions of surface composition from bulk thermodynamic data. The palladium-nickel alloys form a range of surface compositions which can be controlled by changing the bulk composition and which are useful for studying catalytic activity as a function of composition. The alloy films are stable under electron irradiation in the AES analysis in UHV but air exposed films analysed in a residual pressure of 1.3 microPa water vapour show a decrease in palladium surface concentration on irradiation indicating a diffusion of nickel to the surface to form an overlayer of nickel oxide.     

Low-energy electron diffraction study of the surface structures of adsorbed amino acid monolayers and ordered films deposited on copper crystal surfaces

Monolayer structures and ordered multilayer films of several amino acids on single-crystal substrates were studied using low-energy electron diffraction. At monolayer coverage, ordered layers of glycine, alanine, D- and L-tryptophan were observed on both Cu(100) and Cu(111). With increasing film thickness, ordered multilayer structures of the tryptophans were also seen on both substrates. The monolayer structures are considered in terms of molecular size and packing. The optically isomeric D- and L-tryptophans yielded surface structures which were related by mirror inversion. Ordered multilayer films, apparently unlike the bulk crystals, were obtained only for tryptophan. Electron beam damage was serious for both glycine and alanine multilayer films while tryptophan films remained unaffected in agreement with previously observed stabilization by conjugated electron systems.     

Mechanism and site of attack for direct damage to DNA by low-energy electrons

We report results on the desorption of OH- induced by 0-19 eV electrons incident on self-assembled monolayer films made of single and double DNA strands of different orientations with respect to a gold substrate. Such measurements make it possible to deduce the mechanism and site of OH- formation within a biomolecule as complex as DNA. This type of damage is attributed to dissociative electron attachment to the phosphate group of DNA, when it contains the counterion H+.     

溶胶-凝胶SiO2酸性膜与碱性膜的激光损伤行为

 采用溶胶-凝胶技术分别在K9基片上镀制了光学厚度相近的单层SiO₂酸性膜和碱性膜。测试了两类薄膜的激光损伤阈值;分别采用透射式光热透镜技术、椭偏仪、原子力显微镜、扫描电镜和光学显微镜研究了两类薄膜的热吸收、孔隙率、微观表面形貌、激光辐照前薄膜的杂质和缺陷状况以及激光辐照后薄膜的损伤形貌。实验结果表明：相对于碱性膜,酸性膜有更大的热吸收和更小的孔隙率,因此其激光损伤阈值较小;两类薄膜不同的损伤形貌与薄膜的热吸收系数与微观结构有关。     

钛酸镧薄膜的制备及工艺优化

为了获得制备钛酸镧（LaTiO3）薄膜的最优工艺条件,采用电子束热蒸发技术在K9基底上制备了单层LaTiO3激光薄膜。研究了不同工艺条件对LaTiO3薄膜激光损伤特性的影响。研究结果表明,对LaTiO3薄膜激光损伤阈值（laser-induced damage threshold, LIDT）影响最大的工艺条件是沉积温度,其次是工作真空度,最后是蒸发束流。获得了制备单层LaTiO3激光薄膜的最优工艺条件:沉积温度175 ℃、工作真空度2.010-2 Pa、蒸发束流120 mA（8 keV）;证明了最优工艺下制备的LaTiO3薄膜具有良好的激光损伤特性、稳定性以及重复性,所制备LaTiO3薄膜的激光损伤阈值为16.9 J/cm2（1 064 nm,10 ns）。     

Experimental research of laser-induced damage of the monolayer ZrO2 PVD and sol–gel films

Monolayer zirconia physical vapor deposition (PVD) and sol–gel films on K9 glass substrates were prepared by electron beam evaporation and spin coating methods, respectively. The laser-induced damage threshold (LIDT) of each film was measured. Properties of the films were analyzed using Stanford photo-thermal solutions (SPTS), ellipsometry, atomic force microscopy (AFM) and optical microscopy to study the damage mechanism of films under laser irradiation. The experimental results showed that, compared with the monolayer zirconia sol–gel film, the monolayer zirconia PVD film had larger absorption and smaller porous ratio, and that it had smaller LIDT. The different damage morphologies of films were influenced by their different absorption and microstructure characteristics. The zirconia sol–gel film is more suitable for applications involving high-power lasers.     

Combined application of AES and XPS to study carbon diffusion through the selvedge of an Fe(100) surface

Diffusion of carbon from an Fe(100) surface into the bulk, following halocarbon adsorption, is modelled using experimental data from AES and XPS experi electrons (272 eV) in a dense monolayer of halogen atoms (Cl, Br) is found by a variety of experimental methods to be ～3Å, a value substantially sm inferred from the combined information provided by the AES and XPS experiments, the concentration profile of carbon perpendicular to the surface follow function. The importance of accurate data for electron escape depths and of experimental detection limits for quantitative electron spectroscopy of a s     

Metal-induced gap states at well defined alkali-halide/metal interfaces

In order to search for states specific to insulator/metal interfaces, we have studied epitaxially grown interfaces with element-selective near edge x-ray absorption fine structure. An extra peak is observed below the bulk edge onset for LiCl films on Cu and Ag substrates. The nature of chemical bonds as probed by x-ray photoemission spectroscopy and Auger electron spectroscopy remains unchanged, so we regard this as evidence for metal-induced gap states (MIGS) formed by the proximity to a metal, rather than local bonds at the interface. The dependence on the film thickness shows that the MIGS are as thin as one monolayer. An ab initio electronic structure calculation supports the existence of the MIGS that are strongly localized at the interface.     

L'adsorption du bismuth sur des films minces d'or etudiée par variation de résistance electrique

Adsorption of bismuth on gold thin films is studied by electrical resistance variations. At low coverage the increase of gold resistivity due to Bi adatoms is 2.3 ± 0.1 μΩ cm/at% and is independent of temperature and thickness of the Au layers. At 20° C and ?150°C, the shape of the resistance variation curve for increasing Bi coverage indicates that the adatoms form a first monolayer having a higher density at lower temperature. At 85°C two Bi monolayers are made successively: the first is adsorbed on the free surface of the gold film; the second grows on the other face, between the gold and the glass substrate, after migration of the Bi atoms through the grain boundaries or other defects. These results are verified by Auger electron spectroscopy. They are used to show that the initial reflectivities for conduction electrons on the two surfaces are similar in recrystallized gold films; the specularity coefficients P and Q have been evaluated using the Fuchs theory. For clean surface, the electron reflection is large specular (P ? Q ? 0.75 ± 0.05) and becomes entirely diffuse when a monolayer of bismuth is adsorbed on each face of the gold film.     

Structural and compositional mapping of a phase-separated Langmuir–Blodgett monolayer by X-ray photoelectron emission microscopy

The structure and composition of phase-separated Langmuir–Blodgett monolayer films comprised of mixtures of arachidic acid (C₁₉H₃₉COOH) and perfluorotetradecanoic acid (C₁₃F₂₇COOH) were characterized using a combination of X-ray photoelectron emission microscopy (X-PEEM), secondary electron emission microscopy (SEEM) and atomic force microscopy (AFM). X-PEEM provides high lateral spatial resolution and is directly sensitive to the elemental and chemical (functional group) composition of these ultrathin films through the chemical sensitivity of NEXAFS spectroscopy; AFM provides high-resolution imaging, both in terms of lateral and vertical (height) film topography. SEEM provides additional structural and electronic information through work function and electron scattering effects. The combination is used for chemical mapping of the phase-separated domains in the monolayer film. Our results directly confirm previous AFM measurements that suggested that the discontinuous domains are enriched in arachidic acid, whereas the surrounding continuous domain is a mixture of both arachidic acid and perfluorotetradecanoic acid.     

The application of reflection infrared and surface enhanced Raman spectroscopy to the characterization of chemisorbed organic disulfides on Au

We have found that a variety of organic molecules containing the S---S linkage strongly adsorb on freshly evaporated or electrochemically cleaned gold surfaces. Adsorption stops at monolayer coverage and the packing density of the molecules can be close to the limit set by the size of the molecules. Reflection infrared spectra show that the films are strongly oriented and in the case of long alkyl-chain-substituted disulfides the films have structures similar to those of Langmuir-Blodgett type films.

Most of the films were prepared by freshly evaporating gold onto polished single crystal silicon substrates. Immediately after the deposition the films were removed from the vacuum system (under N₂) and placed in solutions of the disulfides. After rinsing the films were examined by ellipsometry at 6328Å and 4420Å and in general, the results are consistent with monolayer structures with close packing similar to bulk crystallites. Infrared spectra were obtained using a Digilab 15B Fourier transform infrared spectrometer with a high f/number optical system and small beam spot size for glancing reflection. Spectra were obtained between 800 and 3200cm⁻¹ using a mercury cadmium telluride detector. The spectrum of a monolayer of the di-p-nitrobenzoate ester of trans-o-Dithiane-4,5-diol is shown in Figure 1. The structure of the diol is given below.

The spectrum of the monolayer of the di-ester of the C₁₆ acid (n-C₁₅H₃₁CO₂H) is given in Figure 2. Notice the alkyl chain mode band series in the monolayer spectrum which indicates crystalline packing of the chains. The infrared spectra are consistent with oriented structures which maximize packing densities. No direct evidence is yet available for defining a Au---S bond. However, a sample in which the monolayer of the p-nitrobenzoate derivative has been covered with 400Å average thickness of CaF₂ and 200Å of smooth silver has yielded an enhanced Raman spectrum (λ(excite)=4880Å 50mW power) in which the normally strong S-S stretching mode observed at 504cm⁻¹ in bulk samples is absent in the monolayer.

These studies demonstrate an application of surface vibrational spectroscopy to defining molecular structures at surfaces. Work is presently underway to quantitatively calculate molecular orientation from the infrared spectra and to more carefully determine the extent of any ordering in appropriate monolayers.     

Studies of clean and oxidized tellurium surfaces

Clean and oxidized surfaces of tellurium films have been studied using electron-excited Auger electron spectroscopy, X-ray photoelectron spectroscopy, energy loss spectroscopy, and electron-stimulated desorption. The results for clean tellurium are in general agreement with previous studies, but the oxidation studies have provided new information. Reaction between oxygen gas and tellurium was found to be an activated process requiring tellurium temperatures in excess of 60°C to produce detectable oxide for 30 min exposures to ~800 Torr of oxygen. Increasing the temperature to 200°C produced a 10.6 A layer of TeO₂. This layer was rapidly removed by electron irradiation with a cross-section of ~3 × 10^?18 cm² for electron-stimulated desorption of oxygen by 2 keV electrons; however, there was evidence for reduced cross sections for thinner TeO₂ layers.     

Distinct electronic and transport properties between 1T-HfSe2 and 1T-PtSe2

In this study, we employed the first-principles calculation to investigate the structural, electronic and transport properties of 1T-HfSe₂ and 1T-PtSe₂ transition metal dichalcogenides, and further explain why they share the same 1T (octahedral) layered structure but exhibit very different electronic and transport properties. There are two underlying concepts: the degree of interlayer bond ionicity and the number of 5d valence electrons of transition metal. The high degree of Hf-Se bond ionicity not only gives rise to the indirect energy gap of HfSe₂ bulk and thin films, but also results in the weak Se-Se vdW interlayer coupling to further restrict the electron transport only within a HfSe₂ layer. On the other hand, the modulation of metallic/semiconducting property of PtSe₂ bulk and thin films can be understood by the significant vdW interlayer coupling, which induces charge redistribution of Se atom and allows electrons to transport within a PtSe₂ layer as well as cross neighboring layers. Finally, our transport calculation for 1T-HfSe₂/1T-PtSe₂ bulks and monolayers suggests the great electron transport within Hf-Se/Pt-Se layer but suppresses/allows electron from neighboring layers. The robust two-dimensional characteristic of 1T-HfSe₂ and the metal-to-semiconductor transition of 1T-PtSe₂ may provide more knowledge for future application in nanoelectronic and optoelectronic devices.     

The growth mode of aluminium on silver (111)

The geometric and electronic structures occuring during the growth of Al on a single crystal Ag(111) surface have been studied using a combination of low energy electron diffraction (LEED), Auger electron spectroscopy (AES), energy loss spectroscopy (ELS) and work function measurements. The Auger signal versus deposition time plots, which were used to monitor the growth mode, are shown to behave in an identical fashion to that expected for layer-by-layer (Frank-van der Merwe) growth. LEED was used to determine the lateral periodicity of thin Al films and shows that Al forms, at very small coverages, 2D islands which have the same structure as the Ag(111) substrate and which grow together to form the first monolayer. At substrate temperatures of 150 K a well defined (1 × 1) structure with the same orientation as the underlying Ag(111) can be seen up to at least 12 ML. After completion of the third monolayer the ELS spectrum approached that observed for bulk aluminium. At a coverage of 3 ML the work function decreases by 0.4 eV from the clean silver value.     

Geometric structure and electronic states of copper films on a ruthenium (0001) surface

Auger electron spectroscopy (AES), combined with thermal desorption mass spectroscopy (TDS), work function (Δφ) measurements and energy-dependent angular resolved UV photoemission using synchrotron radiation were used to investigate the geometric and electronic properties of submonolayer and monolayer copper films grown by vapor deposition on a clean Ru(0001) substrate. A pronounced influence of the deposition temperature on the morphology of the Cu films was established in that lower temperatures favor an island growth mechanism (Stranski-Krastanov or Volmer-Weber type), whereas higher deposition temperatures lead to a more uniform spreading and a layer-by-layer growth (Frank-van der Merwe type). For Cu films grown under the latter conditions angular resolved photoemission reveals the existence of two-dimensional Cu bands even before the monolayer has reached completion; the experimentally determined band dispersions agree quite well with recent theoretical calculations.     

Spin Noise Spectroscopy in N-GaAs:Spin Relaxation of Localized Electrons

Spin noise spectroscopy(SNS) of electrons in n-doped bulk GaAs is studied as functions of temperature and the probe-laser energy. Experimental results show that the SNS signal comes from localized electrons in the donor band. The spin relaxation time of electrons, which is retrieved from the SNS measurement, depends on the probe light energy and temperature, and it can be ascribed to the variation of electron localization degree.     

Zum Hall-Effekt abschreckend kondensierter Metallschichten

Measurements of the annealing behaviour of the low-field Hall-coefficient are carried out on quenched films of Au, Pb, Zn and Tl, which are evaporated in vacuum at liquid helium temperatures. The Hall coefficient of quenched Au-films agrees with the free electron value. When the films are warmed up to temperatures above 260 K, the Hall-coefficient of the bulk material is observed. Quenched Pb-films have a negative Hall-coefficient, which changes its sign during annealing. At 300 K the positive bulk value is observed. When the annealed films are cooled down, the Hall-effect becomes negative again. The Hall-coefficients of Zn- and Tl-films are positive in the whole temperature range. For amorphous Tl-films a negative Hall-effect has been measured. The discussion of the results is based on the different scattering behaviour of electrons with phonons and with lattice defects.     

Photochemistry on thin metal films: probe of electron dynamics in metal-semiconductor heterosystems

The surface photochemistry of on ultrathin epitaxial Ag films on Si(100) substrates has been studied with the goal to employ it as a tool to unravel the electron dynamics in such films. An increase of the photodesorption cross section is observed--a factor of 5 for 266 nm light and 12 nm film thickness--when the film thickness is decreased, despite the fact that the optical absorbtivity decreases. The increased photodesorption cross section is interpreted to result from photon absorption in the Si substrate producing electrons at energies and parallel momenta which are not allowed in Ag. These electrons penetrate through the Ag film despite the gap in the surface projected band structure utilizing quantum resonances.     

Optical spectroscopy study of modifications induced in cerium dioxide by electron and ion irradiations

UV-visible absorption spectroscopy and Raman spectroscopy were used to study damage production in cerium dioxide epitaxial films and polycrystalline sintered samples after irradiation with electrons for three energies to span the threshold displacement energies of cerium and oxygen atoms, and 2.4-MeV Cu ions. Neither amorphization nor specific colour-centre bands were detected. Evolutions of the refractive index were derived from the interference fringes in the optical transmission spectra of epilayers after irradiation. No significant change of the refractive index occurred for the 1.0-MeV electron irradiation, whereas a maximum decrease by 28?±?8% was deduced for the 1.4-MeV and 2.5-MeV energies. These modifications are consistent with ballistic damage on the cerium sublattice for high electron energies producing Ce³⁺ ions. However, no significant change of refractive index was found for the Cu ion irradiation. This likely stems from the high rate of Frenkel pair recombination in the collision cascades induced by more energetic recoils than for the electron irradiations, combined with electronic excitations and hole capture on Ce³⁺ ions. This study reveals modifications of the electronic structure upon irradiation that could take place in other non-amorphizable oxide systems.     

物理法和化学法制备的单层ZrO2膜的激光损伤行为差异

 分别采用电子束热蒸发技术和溶胶-凝胶技术在K9基片上镀制了光学厚度相近的ZrO₂单层薄膜,测试了两类薄膜的激光损伤阈值。分别采用透射式光热透镜技术、椭偏仪、原子力显微镜和光学显微镜研究了两类薄膜的热吸收、孔隙率、微观表面形貌、激光辐照前薄膜的杂质和缺陷状况以及激光辐照后薄膜的损伤形貌。实验结果表明：两类薄膜的不同损伤形貌与薄膜的热吸收与微观结构有关， 物理法制备的ZrO₂膜结构致密紧凑，膜层的杂质和缺陷多；化学法制备的ZrO₂膜结构疏松多孔，膜层纯净杂质少，激光损伤阈值达26.9 J/cm²；因物理法制备的ZrO₂膜拥有更大的热吸收(115.10×10^-6)和更小的孔隙率(0.20)，其激光损伤阈值较小(18.8 J/cm²)，损伤主要为溅射和应力破坏，而化学法制备的ZrO₂膜的损伤主要为剥层。理论上对实验结果进行了解释。