Influence of preparation conditions on the depth-dependent composition of AuPd nanoparticles grown on planar oxide surfaces

The high resolution depth profiling capabilities of medium energy ion scattering are employed to determine the depth dependent composition of Au/Pd nanoparticles grown on thin silica films on Si(100) as functions of Au/Pd composition, total metal loading and annealing temperature. We show that, despite the fact that Au is deposited prior to Pd, the surface of the particles is generally enriched in Au compared to the bulk composition. The extent of this Au enrichment decreases with annealing temperature. In addition, we examine the influence of the adsorption of acetic acid on the surface composition of Au/Pd particles grown on thin alumina films on NiAl(110). We find that acetic acid causes limited segregation of Pd to the bimetallic surface of relatively Au-rich particles.     

Energy spectra of 6–32 keV neutral and ionized Ar and He scattered from Au targets; ionized fractions as functions of energy

The neutralization of ions is an important aspect of low energy ion scattering for surface analysis. Electrostatic energy analyzers (ESA) have been used almost exclusively in such work, and information on charge neutralization efficiencies is needed for quantitative interpretation of ESA data. In the past, the occurrence in low energy ion spectra of surface peaks and low backgrounds due to scattering from inside the solid has been attributed to preferential neutralization of ions which penetrate beyond the surface. In the work to be described, a time-of-flight technique was used to measure energy spectra of both neutral and ionized Ar and He scattered at 90° from a polycrystalline gold target. Incident energies of 6–32 keV were used. The energy spectra of neutral Ar scattered from polycrystalline gold exhibit sharp surface peaks, and double scattering shoulders, over this entire energy range. For He there is a gradual downward slope toward lower energy rather than a sharp surface peak. The behavior in both cases is attributed to large scattering cross-sections which cause a loss of beam particles during penetration. A calculation using a $\text{1}\text{r}{}^2$ potential illustrates this effect as a function of energy for helium. In the present experiments we find that the ion fraction of scattered argon does indeed depend on depth of penetration. This is in contrast to the behavior of He and H at higher energies, e.g. 100 keV, in which cases the charge state depends on emergent velocity but not on depth of penetration. The characteristic shapes of ion scattering spectra in this energy range appear to result from both neutralization and beam attenuation inside the target.     

Depth profiling by ion-beam spectrometry

A procedure is developed for determining the stoichiometry of a sample as function of depth from ion-beam analysis energy spectra. The approach is in principle equally applicable to back scattering experiments, experiments involving nuclear reactions with known cross sections and experiments combining these techniques. The procedure is especially suitable for routine computer evaluation of energy spectra. It is more straightforward and/or involves less rigid assumptions and approximations than alternative approaches. If all elemental signals are measured, an accurate beam dose is not needed. The limitations are basically those inherent to ion beam analysis in general.     

Spectroscopy of charged particles elastically scattered by plane-parallel solid layers

The energy spectra of electrons elastically scattered by plane-parallel solid layers are presented. The solid surface is analyzed by a method based on the identification of similar spectra and is called electron Rutherford scattering in analogy with the well-known ion spectroscopy method. The effect of multiple scattering processes on peak intensities in the energy spectra of elastically scattered particles is analyzed. The applicability range of the strong single scattering approximation for the interpretation of the energy spectra of elastically scattered electrons is established.     

Adsorption of (S)-glutamic acid induces segregation of Ni at bimetallic Au/Ni surfaces

The adsorption of the catalytically important chiral modifier, (S)-glutamic acid, was investigated on bimetallic Ni/Au surfaces created by annealing thin Ni films on Au{1 1 1}. Reflection absorption infrared spectroscopy revealed that adsorption of (S)-glutamic acid at room temperature resulted in the immediate formation of pyroglutamate species on Ni-rich surfaces and the non-zwitterionic glutamic acid species on Au-rich bimetallic surfaces. For Au-rich bimetallic surfaces, TPD revealed that pyroglutamate is also formed on annealing. Medium energy ion scattering studies showed that significant adsorbate-induced Ni segregation was observed on Au-rich bimetallic surfaces – the driving force for segregation is likely to be the formation of nickel pyroglutamate. The implications of our findings for enantioselective catalysis are discussed.     

Energy spectra and charge fractions for keV hydrogen and helium backscattered from silicon

Energy spectra and charge fractions for hydrogen and helium backscattered from silicon targets are reported. The primary energy of the incident particle varies from 5 to 15 keV. The backscattered energy distributions are measured down to 500 eV and the results are compared to a Monte-Carlo computer simulation. Good agreement is found between the theoretical model and the experimental data. Charge fractions are measured by differentiating between scattered ions and neutrals. For hydrogen, neutralization occurs primarily at the surface for the backscattered particles and no depth effects are found. Helium shows a large peak in the ion yield for surface scattering with a much reduced ion yield for particles scattered from within the solid.     

Energy exchange in structure scattering: a molecular dynamics study for Cs from Pt(1 1 1)

We have employed a classical molecular dynamics simulation to investigate the energy transfer of a heavy projectile ion to a surface, i.e. Cs⁺ impacting onto Pt(1 1 1), for incidence energies between 25 and 100 eV and an incidence angle of 45°. The in-plane scattering results show a continuous increase of the final energy with increasing scattering angle. All scattering intensities have a main supraspecular peak and scattering into subspecular angles increases with increasing incidence energy. The large projectile/target mass ratio causes a high energy loss and a strong angular dependence of the final energy distribution. The trends of the energy transfer and its angular dependence can be understood in terms of a binary collision model, augmented with double collisions and an the image charge correction. Backscattering at high incidence energies leads to a distribution of very low final energies, indicating the onset of surface sputtering. Peaks in the energy spectra arise from impact site dependent scattering and can be assigned to single, double, triple or sputtering type collisions.     

Simple technique for measurement of energy spectra of ions from laser produced plasma

Old absorption technique is revived for measuring the energy spectra of ions arising from laser generated plasma. The technique suggested relies on implantation of produced ions into a light element target situated close to the ion production site. Then the ion energy spectra are reconstructed from the depth profile of the implanted ions measured by some of the common depth profiling techniques such as RBS or SIMS. Theoretical range vs. ion energy relations are used for this purpose. In this study the principles of the method are explained and its capabilities are assessed. The feasibility of the method is demonstrated on the energy spectra of Ta ions arising from the laser generated plasma.     

Resonant charge transfer in low-energy ion scattering: Information depth in the reionization regime

Time-Of-Flight Low-energy ion scattering (TOF-LEIS) experiments were performed for He(+) ions scattered from Cu(100) and Cu(0.5)Au(0.5)(100). Probabilities for resonant neutralization and reionization in close collisions were deduced in a wide energy range. To learn about the information depth in LEIS, in a next step ion spectra were analyzed for polycrystalline Cu samples. The relative yield of backscattered projectiles, which have undergone distinct charge exchange processes, was calculated. Results indicate a strong contribution to the ion yield that origins from particles reionized in a close collision in deeper layers when experiments are performed at energies where reionization is prominent. The surface sensitivity of the ion signal at different energies is quantified. Based on these results, the total ion spectrum was quantitatively modelled by two consistent, but different approaches.     

Experimental confirmation of the EPES sampling depth paradox for overlayer/substrate systems

Elastic-peak electron spectroscopy (EPES) has been one of the main tools for obtaining the inelastic mean free path of electrons in solids. Recently it has become clear that, if this type of experiment is done using an energetic electron beam (20-40 keV) and large scattering angles, then the recoil energies of the elastic scattering event for different elements can be resolved. This recoil energy is mass dependent and this fact makes it possible to separate the elastic-peak contributions due to electrons scattered from light and heavy elements. Here we use this energy separation to determine experimentally the sampling depth for an overlayer/substrate system. The sampling depth for a (high-Z) Au overlayer on a (low-Z) C substrate is found to be about two orders of magnitude smaller than for a C overlayer on a Au substrate, whereas the inelastic mean free path of electrons in both materials differ much less. This effect is shown to be a consequence the strong Z dependence of the elastic scattering cross section. The dependence of the spectra on the electron kinetic energy and sample rotation is also dramatically different for both sample geometries.     

Preparation of large-area surface-enhanced Raman scattering active Ag and Ag/Au nanocomposite films

Ag films on tinning glass substrates were fabricated by modified silver-mirror (Tollen’s) reaction with the advantage of low-cost, simple and quick fabrication process. The obtained Ag films were served as sacrificial materials for preparation of Ag/Au nanocomposite films by immersing in a chlorauric acid (HAuCl₄) solution at room temperature. After a short time of galvanic replacement reaction, Ag/Au bimetallic nanostructures were synthesized with “concave” structures. The morphology, properties and composition of the Ag and Ag/Au nanocomposite films were analyzed by using scanning electron microscopy (SEM), UV-visible spectroscopy, X-ray diffraction (XRD), energy-dispersive X-ray spectrometry (EDS) and surface enhanced Raman scattering (SERS). SEM images displayed that the large area of Ag film and Ag/Au bimetallic nanostructures experienced structural evolution process during galvanic reaction. The UV-Vis spectra showed the absorbencies characterization of Ag film and Ag/Au nanocomposite films. SERS measurements using methylene blue as an analyte showed that SERS intensities of bimetallic films were enhanced significantly compared with that of pure Ag films. The SERS enhancement ability of Ag/Au bimetallic films was dependent on the immersion time for galvanic replacement reaction.     

低速高电荷态重离子在C_(60)薄膜中引起的势效应研究

为了研究低速高电荷态离子在C60薄膜中引起的势效应,用能量为200keV的高电荷态Xen+(n=3,10,13,15,17,20,22,23)离子辐照了C60薄膜。用原子力显微镜(AFM)和Raman散射技术分析了辐照过程中高电荷态Xen+离子所储存势能在C60薄膜中引起的效应,即势效应。AFM分析结果表明,辐照C60薄膜的表面粗糙度随辐照Xen+离子电荷态(即势能)的增加而减小,揭示了势效应的存在。而Raman分析结果表明,由于Xe离子的动能远大于其所储存的势能,因此,尽管有表面势效应的影响,但在Raman分析的深度范围内,弹性碰撞还是主导了C60薄膜的损伤过程。     

Comparative analysis of characteristic electron energy loss spectra and inelastic scattering cross-section spectra of Fe

The inelastic electron scattering cross section spectra of Fe have been calculated based on experimental spectra of characteristic reflection electron energy loss as dependences of the product of the inelastic mean free path by the differential inelastic electron scattering cross section on the electron energy loss. It has been shown that the inelastic electron scattering cross-section spectra have certain advantages over the electron energy loss spectra in the analysis of the interaction of electrons with substance. The peaks of energy loss in the spectra of characteristic electron energy loss and inelastic electron scattering cross sections have been determined from the integral and differential spectra. It has been shown that the energy of the bulk plasmon is practically independent of the energy of primary electrons in the characteristic electron energy loss spectra and monotonically increases with increasing energy of primary electrons in the inelastic electron scattering cross-section spectra. The variation in the maximum energy of the inelastic electron scattering cross-section spectra is caused by the redistribution of intensities over the peaks of losses due to various excitations. The inelastic electron scattering cross-section spectra have been analyzed using the decomposition of the spectra into peaks of the energy loss. This method has been used for the quantitative estimation of the contributions from different energy loss processes to the inelastic electron scattering cross-section spectra of Fe and for the determination of the nature of the energy loss peaks.     

New opportunities for quantitative analysis as applied to reflected electron energy loss spectroscopy of Fe/Si structures

The feasibility of determining the elemental composition, chemical state, and element distribution across the depth in a subsurface region using the computer simulation of the electron inelastic scattering cross section is demonstrated with iron layers on silicon substrates. Analysis is carried out based on the dielectric theory and on the experimental determination of the product of the electron inelastic mean free path by the inelastic scattering cross section from reflected electron energy loss spectra.     

Strain and defect densities in Si/Si1-xGex heterostructures investigated by ion scattering and X-ray diffraction

MBE-grown Si/Si_1-xGe_x heterostructures on (100) Si have been characterized by Rutherford backscattering spectroscopy (RBS), ion channeling and X-ray diffraction to investigate defect densities and tetragonal lattice distortion. Critical layer thickness and relaxation of strain by formation of misfit dislocations are strongly dependent on the growth temperature. A Si_0.67Ge_0.33 layer with a thickness of 2000 Å is found to be still fully strained at a growth temperature of 450°C, whereas the same layer grown at 550°C shows considerable strain relaxation by dislocations. To obtain better depth resolution than with conventional RBS, medium energy ion scattering (MEIS) experiments have been performed on Si/Ge superlattices with layer thicknesses of 10–40 Å. A position-sensitive toroidal electrostatic analyser was employed to detect the backscattered ions simultaneously over an angular range of 30° with an energy resolution of 1 keV FWHM for 250 keV He ions, corresponding to a depth resolution of about 10 Å.     

Multiple scattering and neutralization aspects of low energy noble gas ions incident on a Cu (110) crystal

Experiments have been done with 1 keV Ne⁺ ions bombarding a Cu (110) single crystal in a (111) plane. From the measured energy spectra of the scattered ions the minimum and maximum scattering angles for multiple scattering on the surface are determined. These minimum and maximum scattering angles were also calculated using a computer model. The parameters for an interatomic potential function between ion and metal atom are determined by comparison of the experimental and calculated scattering angles. A neutralization model for an energetic ion at an atomic surface is given. The reliability of this model is tested by comparison of the results of computer calculations and the measured peak intensity distributions as a function of the scattering angle.     

Direct numerical reconstruction of inelastic cross sections from REELS and ISS spectra

The reconstruction of inelastic scattering cross sections faces two problems: the measured signal (energy spectrum) is a multiple scattering signal; the inelastic energy loss is nonuniform over the target depth. In this paper, we present a method for numerical reconstruction of cross sections from characteristic energy loss spectra, which efficiently solves both problems within a multilayer model. It is shown that the inverse problem of cross section extraction in the three-layer model is ill-conditioned, and the method is practically inapplicable to the three-layer model. The direct numerical reconstruction method yields a strongly “noised” result and can be applied only to obtain a priori information on the inelastic cross section form for further fitting. Using a combination of two methods, inelastic scattering cross sections were reconstructed for aluminum from characteristic energy loss spectra at probe beam energies of 5 and 40 keV. It is shown that ionization in solids should be described as a local process and as a collective one using the dispersion formula similarly to the case of excitation plasmons.     

Composition dependence on dispersion of Au/Cu bimetallic nanoparticles in nylon 11 thin films

Effect of composition on the dispersion of Au_xCu_1-x bimetallic nanoparticles into nylon 11 matrix has been investigated. TEM, EDX, and XPS depth profiling were used for characterizing the changes in the composition of the bimetallic particles and in the depth distribution of the particles in the nylon 11 layer caused by heat treatment in N₂ atmosphere. The island-like bimetallic particles were found to be formed on the nylon 11 surface before heat treatment. The results of XPS depth profiling revealed that, by the heat treatment, the Au_xCu_1-x bimetallic particles with x? 0.55 were not dispersed into the nylon 11 layer while those with x≥ 0.70 were homogeneously dispersed in the films, indicating the existence of critical composition for penetration of the bimetallic particles. By comparing the composition and structure of the bimetallic particles, the cause of these finding is discussed in terms of surface free energy of the particles. Received 29 November 2000     

Anomalous transport due to drift-wave turbulence

The anomalous transport of particles and thermal energy is analyzed for the electron and ion temperature gradient driven drift-wave turbulence. The entropy production from the anomalous flows is calculated. Approximate formulas for the fluctuation spectra are briefly compared with a microwave scattering experiment in a low beta tokamak.     

Energy spectra of ions sputtered from elements by O+2: A comprehensive study

The results of a comprehensive study of the energy distributions of ions sputtered from 31 pure elements and two compounds by a 5.5 keV O⁺₂ beam are presented. The spectra were determined under three different ambient conditions: residual vacuum or low argon pressure backfill, oxygen backfill, and nitrogen backfill. The energy spectra of polyatomic ions are narrower and peaked at lower energies than the atomic ion spectra and decrease in width and average energy with greater ion complexity. The statistical model of polyatomic ion formation does not correctly describe the shape of the energy distributions. The atomic ion energy spectra reveal that more than one ionization mechanism is responsible for the formation of ions from the various elements and that the dominant mechanism is frequently different in the presence of a reactive gas than in a vacuum. Periodic trends of the parameters describing the energy distributions are dependent on the electronic structure of the ions. The average energies of the distributions correlate with the sublimation energies of the pure elements, but the most probable energies do not. Apparent surface binding energies calculated from the energy distributions are presented.