Thermo-stimulated surface segregation in the ordering alloy Pt 80 Co 20 (1 1 1): Experiment and modeling M.A. Vasylyev a, * , V.A. Tinkov a , A.G. Blaschuk a , J. Luyten b , C. Creemers b a Institute of Metal Physics, Nat. Acad. Sci. Ukraine, Akad Vernadsky Blvd. 36, 03680 Kiev-142, Ukraine b Chemical Engineering Department, W. de Croylaan 46, B-3001 Leuven, Belgium Received 17 July 2005; accepted 18 December 2005 Available online 3 April 2006 Abstract In this paper, a method of Ionization Spectroscopy (IS) is proposed for the non-destructive layer-by-layer analysis of the elemental composition of a solid surface. Using ionization energy loss spectra, a layer-by-layer concentration profile of the Pt 80 Co 20 (1 1 1) alloy surface is obtained for different annealing temperatures. For the disordered Pt 80 Co 20 (1 1 1) at room temperature, the first atomic layer consists of pure Pt with damped oscillations in the deeper layers. Heating the sample reduces the oscillations. However, at a temperature of 823 K, a sandwich-like structure of the type Pt/Co/Pt was found in the first three atomic layers. For the ordered state the first atomic layer also consists of pure Pt with bulk concentration in other layers. LEED analysis shows a p(2  2) superstructure for the surface of the ordered Pt 80 Co 20 (1 1 1) alloy. The segregation behavior in this alloy is further studied by Monte Carlo (MC) simulations combined with the Constant Bond Energy (CBE) model. The results of the MC simulations agree well with the experiments at the higher temperatures, both for the surface composition and the concentration depth profile. At lower temperatures, some discrepancies exist between the MC results and the measured concentration profile. # 2006 Elsevier B.V. All rights reserved. PACS: 79.20.H; 64.75; 68.35.B; 05.70.; 07.05.Tp Keywords: Layer-by-layer non-destructive analysis; Ionization Spectroscopy; Surface segregation; Surface structure; (Dis)ordered alloy; Pt 80 Co 20 (1 1 1); Monte Carlo modeling 1. Introduction The structural and quantitative properties of alloy surfaces have been the subject of numerous studies in view of developing a better understanding of the alloys catalysis. In particular, binary alloys and intermetallic compounds of platinum have quite interesting physico-chemical properties and these materials are widely used in nanotechnology and catalysis [1,2]. A lot of work was devoted to the study of segregation in the Pt–Co alloys [1–8]. Basically a competition is expected between two mechanisms that have a significant effect on the segregation: lowering of the surface energy by segregation and the tendency for chemical ordering in the bulk and also in the surface region (with possibly, the formation of a reconstructed surface). Pt–Co is known to have a strong chemical ordering enthalpy, which is the origin of the ordered L1 2 and L1 0 phases of Pt 3 Co and PtCo, respectively. Depending on the bulk composition, temperature and surface orientation, the interplay of these driving forced leads to different degrees of surface enrichment [1–5]. Furthermore, in order to minimize the total energy, Pt–Co alloys may also undergo a (2  1) missing-row reconstruction which is the thermodynamically stable config- uration of the clean Pt(1 1 0) surface [7]. In recent years several non-destructive methods have been used for surface analysis. Surface segregation in metallic alloys can be studied with Auger Electron Spectroscopy and X-ray Photoelectron Spectroscopy, but the possibilities for obtaining a layer-by-layer composition depth profile with these methods are not well established [9]. Low Energy Ion Scattering is extremely sensitive to the surface composition, but it cannot give direct information on the composition in deeper atomic layers. Furthermore, the study of alloys of components with comparable atomic masses is difficult. Low Energy Electron Diffraction (LEED) is capable of accurately measuring the composition of the three outermost layers of substitutionally www.elsevier.com/locate/apsusc Applied Surface Science 253 (2006) 1081–1089 * Corresponding author. Tel.: +380 44 424 25 20. E-mail address: vasil@imp.kiev.ua (M.A. Vasylyev). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.12.172