Y.G. Shen

Studies of surface composition and structure of Cu3Pt(111) by low energy alkali ion scattering

Abstract The composition and structure of the Cu 3 Pt(111) surface have been investigated by low energy alkali ion scattering. The surface composition of the top two layers was determined by Li + ion scattering using selective scattering geometries with calibration measurements on reference samples of Cu(111) and Pt(111). Under thermal equilibrium conditions, these results consistently showed a surface composition of 80%Cu20%Pt in the first layer, while the second layer was 69%Cu31%Pt. The surface structure was probed by measuring the intensity of Li + ion scattering as a function of incident angle along the main azimuthal directions. The interpretation of experimental data was based on a chain model simulation. The results showed that the Pt atoms were coplanar with the Cu atoms in the first layer. A clustering effect of Pt has also been tested using 1 keV K + ion scattering. The results revealed that the Pt atoms in the first layer were not clustered by lack of the PtPt pair double scattering peak along the two long azimuthal directions.

The role of the electronic structure in charge exchange between low energy ions and surfaces

In this study some of the fundamental aspects of neutralisation have been addressed. The validity of the three-stage approach to neutralisation which involves treating neutralisation along the approach to and departure from the surface as an Auger-like process, and the neutralisation during the collisional phase as an atomic-like process has been justified by measurements on the interaction of He+ with Pb. Neutralisation along the exit path is shown to be independent of the oscillatory niutralisation cross section associated with the collisional phase. In order to determine the role of the work function and valence band width in determining neutralisation rates for inert gas ions, a range of elements has been studied (Ni, Cu, Zr, Pd, Ag, Au, Pb). The role of the projectile has been assessed by the use of three inert gas ions (He+, Ne+, Ar+) as projectiles. This has been extended by comparison with previously reported results involving the neutralisation of Al+ of an Al surface and Si+ off a Si surface. The conclusion of this study is that there is no experimental evidence for higher neutralisation rates in situations where a resonance process is considered to be more likely than an Auger process. A linear correlation has been found between the conduction band width and vcv1. There is also some preliminary evidence for a universal behaviour for the variation of neutralisation rates with particle velocity, not previously reported.

An unusual bimetallic p4g phase induced by Pd on Cu(001)

Abstract The surface composition and structure of Pd on Cu(001) have been studied by a combination of low-energy ion scattering (LEIS) and low-energy electron diffraction (LEED). Palladium is incorporated into the surface at room temperature (≤ 50° C ). This leads to the formation of two surface alloys: an ordered c(2 × 2) structure at 0.5 ML Pd, and a p4g structure at 1.0 ML Pd. The surface composition of the top two layers was determined by 1 keV Li + ion scattering, using selective scattering geometries with calibration measurements on reference samples of Cu(001) and Pd(001). For the p4g structure, the results show a surface composition of 53at%Cu47at% Pd in the first layer, while the second layer is 58at%Cu42at%Pd. Several structural models that could be responsible for the p4g structure are proposed and investigated. The randomly mixed top CuPd layer was found to be reconstructed such that the Pd (Cu) atoms are rotated by a lateral displacement of 0.25 ± 0.10 A . The magnitude of this displacement was determined by measuring the azimuthal distributions of the PdPd double scattering peak with 2 keV K + ions. It is proposed, based on a hard-sphere model, that it is the moderate size mismatch between Cu and Pd atoms in the second c(2 × 2) mixed layer which drives the clockwise-anticlockwise rotation.

Composition and structure of Cu3Pt(001): a (1×1) Cu termination with c(2×2) underlayer ordering

Abstract The composition and structure of the (001) oriented surface of the ordered alloy Cu 3 Pt were investigated by low-energy Li + and He + ion scattering, and low-energy electron diffraction. The clean and well-annealed surface exhibited a c(2×2) LEED pattern. We present evidence that the c(2×2) structure corresponds to an ordered c(2×2) Cu–Pt second layer below a (1×1) Cu termination. Using Li + ions, from analysis of the Cu and Pt critical shadowing edges, the first interlayer spacing was determined to be contracted by about 3%. The results are discussed on the basis of surface segregation and ordering effects.

The adsorption of Al on Pd(001) and of O and CO on an interfacial Al-alloyed Pd(001) surface

Abstract The adsorption of ultrathin Al films on Pd(001) and adsorption of oxygen and carbon monoxide on an interfacial Al-alloyed Pd(001) surface have been examined via low energy He + ion scattering, low energy electron diffraction and work function change (ΔΦ) measurements. For low Al coverages at 325 K, interaction between Al and the Pd substrate is observed, resulting in the loss of near surface order by an Al coverage of 0.5 monolayer (ML). The adsorption of Al results in a work function decrease. Annealing Al films with initial coverages from 0.5 ML to larger than 1 ML at 750–950 K leads to interfacial alloying and a stable (2 × 2)p4g LEED pattern dominates the surface structure. This structure has been determined to be a clock rotated (001) Pd top layer above a mixed c(2 × 2) AlPd underlayer. Adsorption of oxygen on the (2 × 2)p4g surface at 325 K induces Al segregation and lifts the reconstruction. In contrast, the interaction of CO with the (2 × 2)p4g surface dose not lift the reconstruction. The implications of these results for relative stability of AlO bond strength are discussed.

Formation of Ni(100)Al surface alloy

Abstract Early studies of the growth of Ni 3 Al(100) on a Ni(100) surface reported that an alloy extending to some depth was formed. In a series of studies undertaken using Low and Medium Energy Ion Scattering it has been found that there is evidence for the formation of a surface alloy extending to a depth of 2 to 3 atomic layers. A stable c-(2 × 2) structure was observed which, from MEIS measurements it was determined that 0.5 ML Al was incorporated into the first three atomic layers. From LEIS it was possible to confirm that the Al did not form an overlayer and that the surface atomic layer comprised 50% Al. There was less than 5% Al observed on the second layer which is consistent with Ni 3 Al(100) termination.

Au-segregated dealloying and Pd-induced clock reconstructing of Cu(001)

The structure and growth of ultrathin Au and Pd films on Cu(001) have been studied by low-energy ion scattering (LEIS) and low-energy electron diffraction (LEED). Due to the existence of a kinetic pathway for intermixing, both Au and Pd are incorporated into the Cu(001) surface at room temperature, forming a surface alloy at a coverage of 0.5 monolayer (ML). At Au coverage near 1.2 ML, the surface layer is pure Au; this is caused by Au-induced segregation from the underlying layer to the surface. No second- and/or third-layer Au is evident. In contrast to this, for the Pd/Cu(001) case a clock reconstruction is formed after deposition of 1 ML Pd. The LEIS data, through comparisons with 3-D computer simulation, reveal a 0.25 A lateral clockwise - anticlockwise displacement of the first-layer Pd atoms. A simple model for this clock reconstruction is proposed. The driving force behind the dealloying for the Cu - Au alloy and Pd-induced reconstructing for the Cu - Pd surface is also discussed.

Neutralisation in low energy ion scattering

Abstract Although low energy ion scattering (LEIS) has become an important method of analysing the composition and structure of surfaces, the problem of understanding the neutralisation of the scattered ion remains. In recent years there have been significant advances in the theoretical and experimental study of neutralisation. In this paper we will review some current experiments aimed at evaluating the role of various electronic parameters such as work function and band width and relate those experiments to others reported in the literature. By comparing the rate parameters of various neutralisation events involving different ions, some significant comments on the role of resonant and Auger electron exchange processes can be made.

The growth of thin Cu films on an O-precovered Ru(0001) surface studied by low energy ion beams

The growth of thin Cu films on an O-precovered Ru(0001) surface has been studied by low-energy ion scattering, Auger electron spectroscopy and low-energy electron diffraction. During deposition of Cu, the majority of oxygen (about 70%) originally on the clean Ru(0001) surface was found to float out onto the top of the surface of the growing film. This displacement process could be observed up to a Cu coverage of 10 monolayers, which appeared to be independent of the deposition rate, the O precoverage and the substrate temperature. The floating O atoms in the top layer have been determined to be a disordered overlayer by measuring the azimuthal scan dependence at grazing incidence.

Oxygen structure on Ni(100) using low energy Li+, negative recoil and H+ ions

Abstract The c(2 × 2)O structure on Ni(001) has been probed by low energy Li + , negative recoil and H + ion scattering. Li + ion scattering has the ability to derive structural information directly from experimental data and from simple calculations on the basis of shadowing and blocking effects. Experimental results obtained by analyzing the incident angle dependence associated with shadowing of Ni by oxygen confirm that the O atoms are located in symmetrical fourfold hollow sites, though off-centre positions cannot totally be ruled out. The proper consideration of a relaxation of the top Ni layer proves to be essential for the accurate determination of the adsorption height of oxygen. The perpendicular height of oxygen was determined to be d τ = 0.84 ± 0.08 A and the first interlayer spacing d 12 is 1.86 ± 0.05 A , i.e. substrate layer relaxation is 0.10 A (5.7% outward relaxation). The low energy recoil process is extremely sensitive to the electronegative atoms, and the detection of the recoiling species ensures that the adsorption site is probed directly. The angular distribution of negative ions was used as a direct measure of the adsorption site. Hydrogen scattering experiments at low energy yield similar information to its medium energy (MEIS) and high energy (RBS) analogues by the appropriate choice of scattering geometries to probe the surface structure. Experimental results indicate that these two approaches are good complementary tools with LEIS for O adsorption studies. The value of the O height was independently obtained to be 0.75±0.10 and 0.8±0.2 A respectively. The good agreement between the O adsorption heights obtained from the different methods lends confidence to the O height value and the reliability of the experimental and computational procedures.