M. Krawczyk

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 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.

Experimental determination of the inelastic mean free path of electrons in GaSb and InSb

Abstract The inelastic mean free path (IMFP) of electrons is a fundamental material parameter for quantitative surface- and thin-film analysis by AES and XPS. Experimental determination of IMFP is based on the elastic peak electron spectroscopy (EPES) The intensity of the elastic peak recorded for the sample is compared with that of the Ni reference. The IMFP is evaluated from the Monte Carlo (MC) calculations of the elastic backscattering probability. The MC algorithm is based on elastic scattering cross-sections from the NIST 64 database and IMFP values of Ni. Experiments have been carried out in three laboratories working with different types of electron spectrometers and energy ranges: HSA, E =0.2–5 keV; CMA, E =0.2–2 keV, and RFA, E =0.2–1.5 keV. GaSb(100) and InSb(100) samples have been cleaned and their surface layer amorphized by an Ar + ion bombardment at E ion =2 keV. The surface composition after cleaning was checked in situ by XPS. No metallic Ga, In or Sb phases were evidenced by plasmon losses on the surface after E ion =2 keV Ar + ion treatment. The MC calculations were based on the real surface composition. Thus, the IMFP values experimentally obtained for the ion bombarded samples can be considered as the volume parameters for E >0.5 keV. A reasonable agreement was found with the calculated IMFP data of NIST and with other theoretically determined values of the IMFP.

Decomposition of diborane on Pd(111): thermal and chemical behaviour of surface boron

Abstract Recent Auger electron spectroscopy (AES)–low energy electron diffraction (LEED)–thermal desorption spectroscopy (TDS) results on interactions of diborane (B 2 H 6 ) with Pd(111) surface and oxygen with a surface of Pd(111)–B solid solution are reported. The physicochemical behaviour of surfaces covered with the post-interaction layer is described. AES results following exposure of Pd(111) to B 2 H 6 at 300 K indicate the formation of a B overlayer on the surface due to complete decomposition of the B 2 H 6 molecules. The B surface concentration increases with rise of the B 2 H 6 exposure, exhibiting a maximum constant value for exposures greater than 16 langmuirs (L). As found from TDS results, hydrogen is the only desorbing product (360 K T des 2 H 6 exposures studied. Exposure of a clean Pd(111)–B solid solution surface to O 2 at T >600 K leads to a strong chemical interaction between dissolved boron and oxygen, evidenced by the formation of the B x O y overlayer on the surface. The kinetics of the B surface segregation process is analysed. The rate of this process, which is initially controlled by the bulk diffusion, diminishes at T >800 K due to the competitive dissolution of B into the Pd(111) bulk. The B x O y overlayer demonstrates a high level of thermal stability and a distinct chemical inertness in respect to hydrogen.

Corrected electron inelastic mean free paths (IMFPs) for selected wide band semiconductors

Elastic peak electron spectroscopy (EPES) has been widely used to determine the electron inelastic mean free paths (IMFPs) in solids. In this work, we investigated quantitatively the influence of surface excitations on electron IMFPs determined by EPES. We used IMFPs obtained from the early EPES measurements of the electron elastic backscattering probability from GaN and Cd0.88Mn0.12 Te wideband-gap semiconductors, and the Ni standard in the energy range 200–2000 eV. The total surface-excitation parameter (SEP) was evaluated using Chen and Werner approaches, and was applied for correcting the EPES IMFPs. These corrected values were then compared with those predicted by the TPP-2M formula. We found that implementation of the surface-excitation correction improved agreement between the resulting IMFPs for selected wide band semiconductors and the TPP-2M values at low-energy (E > 500 eV) electrons. The extent to which the IMFPs measured by EPES differ from the corresponding bulk values (on account of surface excitations) was found to depend on the semiconductor material with finite surface. Our results also clearly demonstrated the importance of accounting for surface excitations for accuracy of the IMFPs measured for GaN.