L. Zommer

Molecular hydrogen interactions with discontinuous and continuous thin gold films

Abstract The influence of thin gold film surface nanostructure on H2 chemisorption on unsintered gold films at 78 K was studied. Three classes of thin Au films of different thicknesses were chosen for these experiments, as follows: (i) very thin, purplish colour, transparent film formed by isolated Au islands, (ii) violet–green colour film formed by Au islands joined together and creating a kind of golden net, (iii) non-transparent, continuous Au film of golden colour. It has been found that H2 chemisorption occurs only in the case when the thin Au films are deposited at low temperatures (78 K) and unsintered. It has been suggested that this chemisorption occurs on surface Au atoms of low coordination number as a result of formation of AuH2 complexes, similarly to the compounds arising due to H2 interaction with isolated Au atoms.

Surface composition of the CoPd alloys studied by electron spectroscopies

Palladium-based alloys and bimetallic systems are of importance in heterogeneous catalysis. Recently, attention was devoted to studies of the catalytic properties of the Co-Pd binary system. An important issue to consider is the surface composition of this alloy. The aim of the present work was to determine the surface composition of the polycrystalline CoPd alloys in a wide range of constituent concentrations (Co30Pd70 at.%, Co50Pd50 at.% and Co70Pd30 at.%) at temperatures up to 900°C. Quantitative AES and XPS analyses were used for this purpose. To determine the surface composition, the values of the inelastic mean free path (IMFP) for the studied alloys are necessary. These were derived from the elastic peak intensity. The reported values of the IMFP for Pd are in agreement with the data available in the literature. Reasonable consistency of the IMFP values for CoPd alloys and Co was also observed. The surface compositions of the CoPd alloys at room temperature determined by AES and XPS are in good agreement. It has been found that Pd segregates to surfaces of the Co30Pd70 and Co50Pd50 alloys at temperatures above 300 °C.

Determination of the inelastic mean free path of electrons in GaAs and InP after surface cleaning by ion bombardment using elastic peak electron spectroscopy

Abstract The inelastic mean free path (IMFP) of electrons is an important material parameter needed for quantitative AES, EELS and non-destructive depth profiling. The distinction between the terms for IMFP and the attenuation length (AL) has been established by ASTM standards. A practical experimental method for determining values of the IMFP is elastic peak electron spectroscopy (EPES). In this method, experimentally determined ratios of elastically backscattered electrons from test surfaces and from a Ni reference standard are compared with the values evaluated theoretically. The present paper reports systematic measurements of the IMFP by EPES for GaAs and InP. They are carried out in two laboratories using two different electron spectrometers: a CMA in Budapest and DCMA in Warsaw. Prior to measurements, the samples were amorphized by high-energy Ar + ions (100–400 keV), and the surface composition was determined by quantitative XPS. Argon cleaning produces enrichment of samples in the surface layer in Ga (80%) and In (70%), respectively. The experiments refer to a such modified sample surface that was considered in Monte Carlo calculations. The experimental data were analyzed using calibration curves from Monte Carlo calculations which account for multiple elastic scattering events. This approach has been used previously for elemental solids and is now extended to amorphized binary compounds. The experimental values of IMFP obtained in both laboratories exhibited a reasonable agreement with the available literature data in the 0.1–3.0 keV energy range. With respect to the information depth of EPES, the experimental results refer to the bulk composition within a reasonable extent.

A modified adsorption microcalorimeter with computerized collection and reduction of the data

Abstract A description of a calorimetric system for adsorption of gases on thin metal films is presented. The films can be prepared under vacuum of a few nanopascals. The calorimeter operates over pressure intervals from a few nanopascals to about one hundred millipascals and over temperature intervals from 280 to 370 K. The static gain of the system at 298 K is 1.75 X 10-4 W/V and its cut-off frequency is about 2 rad/s. The deconvolution procedure presented allows rate constants of an adsorption process to be determined up to 1 s-1 with a reasonable accuracy. The calorimeter and software procedures were tested with an iron-hydrogen system. Data concerning differential heats of hydrogen adsorption on a thin iron film as a function of surface coverage at 298 K are compared with smoothed literature data.

Determination of the electron inelastic mean free path in polyacetylene by elastic peak electron spectroscopy using different spectrometers

Abstract The inelastic mean free path (IMFP) values for organic materials are very limited. Numerous data have been published mainly for elemental solids, binary alloys and semiconductors. Generally, the IMFP values for different energies can be determined from the theoretical models involving the optical data, from predictive formulas, and from the experimental method called the elastic peak electron spectroscopy (EPES). In the present work the IMFPs in N–(CH)x polyacetylene (unstretched), synthesised according to Naarmann and Theophilou, and this polyacetylene doped with Pd were determined. The IMFPs energy dependence for the above samples has been obtained from the EPES in the primary electron energy range of 200–5000 eV. The experimental data have been recorded using three different spectrometers and compared with the theoretical data available.

Relative Surface Area of the EQCM Quartz Crystals Vibrators

The relative surface area, R sa , i.e., the ratio of the surface area in a three-dimensional space to that in a two-dimensional space, of quartz crystal vibrators, used for an electrochemical quartz crystal microbalance (EQCM), was determined by atomic force microscopy in air, by coulometry in solution, and derived from the Brunauer-Emmett-Teller isotherms for xenon adsorption. The resulting R sa values were mutually consistent and were higher the better was the resolution of imaging.

Determination of the surface area of annealed continuous gold film and pyrex glass in situ by the BET adsorption isotherm and ex situ by Atomic Force Microscopy in air

Abstract The relative surface area, R sa , of annealed continuous thin gold film, thermally evaporated onto a Pyrex glass substrate at 78 K in an ultrahigh-vacuum (UHV) environment, and Pyrex glass has been determined in situ by the BET (Brunauer–Emmett–Teller) isotherm method, as well as ex situ by Atomic Force Microscopy (AFM) in air. It has been measured by the BET isotherm method that R sa for clean, continuous and annealed gold film, investigated under UHV conditions, is equal to 6.3, whereas after its exposure to air, at about 100 Pa, R sa decreased from 6.3 to 5.7. Further exposure to air, at atmospheric pressure, reduced R sa from 5.7 to 2.2, whereas the exposure of contaminated Au film to the atomic hydrogen flux caused the increase of R sa from 2.2 to 4.2. On the other hand, R sa calculated directly from the AFM image of gold film in air is equal to 1.0. In contrast to thin gold film, the Pyrex glass surface area, measured under UHV conditions by the BET adsorption isotherm and in air by the AFM technique, is equal to its geometric area, i.e., R sa =1.0.

Monte Carlo calculation of backscattering factor for Ni–C multilayer system

Auger electron spectroscopy (AES) depth profiles using the NiMVV (61 eV), CKLL (272 eV) and NiLMM (848 eV) lines were recorded for a 3 × (Ni(40 nm)/C(28 nm))/Si substrate sample. It was found that the Auger intensities corresponding to pure regions of the depth profile changed with depth. The behaviour of the change was different for the different layers and different Auger lines. The changes can be attributed to the change in the backscattering factor (BF) as the thickness of the sample changes due to the sputter removal. Zommer and Jablonski developed a Monte Carlo (MC) algorithm to calculate the BF for systems with a buried layer. This algorithm was applied to the present case; the intensities of the monitored Auger lines were calculated for the sample with decreasing thickness similarly to the Auger depth profiling. The agreement between the measured and calculated AES depth profiles is excellent. The MC calculation verifies that several layers contribute to the BF and thus the expressions developed for overlayer/substrate systems cannot be used.

Heat of adsorption of hydrogen on evaporated iron layers

Abstract The differential heats of adsorption of hydrogen on iron films prepared under UHV conditions have been measured calorimetrically at 298, 323 and 363 K. The initial heat of adsorption was 97±6 kJ/mol at 298 K, 100±4 kJ/mol at 323 K and 97±7 kJ/mol at 363 K. The maximal degree of coverage of the deposited iron surfaces with hydrogen was 0.85±05 at 298 K, 0.64±0.05 at 323 K and 0.61±0.06 at 363 K. The minimal degree of coverage to which hydrogen could be isothermally desorbed and readsorbed was 0.55±0.04 at 298 K, 0.33±0.05 and 0.30±0.06 at 363 K. The coverage range can be divided into three distinct regions. The extent of the first region, where the heat of adsorption is constant and equal to the initial heat of adsorption, is increasing with temperature and at 363 K is equal to about 70% of the whole iron surface allowed for hydrogen adsorption, about 40% of that amount can be isothermally desorbed and readsorbed. The extent of the second region, where the heat of adsorption was changing with coverage from 85 to 65 kJ/mol, is decreasing with temperature and at 363 K is vanishingly small. At the third region the heat of adsorption is continuously dropping to zero, the rate of heat evolution measured calorimetrically is much higher than the corresponding rate calculated from the sticking probability data, which can be assumed as caused by an adsorption-induced iron film reconstruction.

Simulation of the backscattered electron intensity of multi layer structure for the explanation of secondary electron contrast

The intensities of the secondary electrons (SE) and of the backscattered electrons (BSE) at energy 100 eV have been measured on a Ni/C/Ni/C/Ni/C/(Si substrate) multilayer structure by exciting it with primary electrons of 5, 2.5 and 1.25 keV energies. It has been found that both intensities similarly vary while thinning the specimen. The difference as small as 4 nm in the underlying layer thicknesses resulted in visible intensity change. Utilizing this intensity change, the thickness difference of neighboring regions could be revealed from the SE image. No simple phenomenological model was found to interpret the change of intensity, thus the intensity of the BSE electrons has been calculated by means of a newly developed Monte Carlo simulation. This code also considers the secondary electron generation and transport through the solid. The calculated and measured intensities agree well supporting the validity of the model.