Elsevier

Thin Solid Films

Volume 516, Issue 10, 31 March 2008, Pages 2962-2965
Thin Solid Films

Surface alloying in the Sn/Ni(111) system studied by synchrotron radiation photoelectron valence band spectroscopy and ab-initio density of states calculations

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Abstract

Photoelectron spectroscopy using synchrotron radiation and ab-initio electronic structure calculations were used in order to describe the fine structure of the valence band in the Sn/Ni(111) system. The characteristic contributions of each metal in the valence band photoemission spectra obtained with a photon energy of 80 eV and their changes upon the formation of the (√3 × √3)R30° Sn/Ni(111) surface alloy were also born out in the calculated density-of-states curves in fair agreement with the experiments. The Sn–Ni interaction leads to a considerable broadening of the valence band width at the bimetallic surfaces.

Introduction

The controlled growth of metals on metals has drawn attention over the years, partly motivated by the broad range of technological applications of bimetallic systems in materials science, catalysis and microelectronics [1], [2]. Besides the commonly observed ordered overlayers, bimetallic systems may form true surface substitutional alloys, which often lack any direct bulk analogue [3]. Many surface studies have been devoted to Sn-based bimetallic systems and, recently, Sn addition to some transition metals was found to improve their performance in heterogeneous catalysis [4]. The Sn/Ni(111) system has been studied extensively in the past [5], [6], [7] as well as recently [8] using a number of surface sensitive techniques, such as Low-Energy Electron Diffraction (LEED), Synchrotron Radiation Photoelectron Spectroscopy (SRPES), ultra-violet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy and temperature-programmed desorption, and the conditions for the formation of the (√3 × √3)R30° surface alloy have been well documented.

Theoretical ab-initio studies addressing Sn/Ni surface alloys so far focused on geometrical effects. Due to the fact that the covalent radius of Sn is about 20% larger than that of Ni, the Sn atoms protrude from the first lattice plane. For the (2 × 2)-Sn(Ni(001) surface alloy, the calculated rippling is found to be 0.36 Å [9], somewhat smaller than the experimental value of 0.44 ±0.05 Å [7], and for the (√3 × √3)R30° Sn/Ni(111) surface alloy, the theoretical result is 0.45 Å [10], compared to 0.44 Å experimentally [6]. As far as the electronic structure is concerned, according to density functional theory calculations the presence of Sn atoms in the (2 × 2)-Sn /Ni(001) surface alloy structure strongly reduces the density of states at the Fermi energy and almost completely quenches the magnetic moment of the Ni atoms in the topmost surface layer [9].

In this work, a comparison is presented between valence band SRPES data and density-of-states (DOS) curves derived from first principles calculations for corresponding Sn/Ni systems. The comparison is made in the following three cases: clean Ni(111), ∼ 1.2 ML of as-deposited Sn overlayer on Ni(111) and the perfect (√3 × √3)R30° Sn/Ni(111) surface alloy.

Section snippets

Experimental system and calculations

An ultra-high-vacuum chamber (base pressure 5 × 10 8 Pa) was used for the experiments. The chamber, at the Material Science Beam line of the Elettra synchrotron radiation facility, was equipped with a hemispherical electron energy analyser (Specs HA-150) for high resolution SRPES, as described elsewhere [11]. The analyser axis and the direction of the synchrotron light beam were on the same horizontal plane at an angle of 60°, whereas the sample surface was located in the vertical plane and could

Results and discussion

Fig. 1 shows SRPE spectra obtained with 80 eV photon energy for the three studied cases. The bottom one is that of a clean Ni(111) single crystal, the middle one is from 1.2 ML Sn/Ni(111) and the upper one was obtained from the perfect (√3 × √3)R30° surface alloy. The main contributions from the d-electrons of the Ni crystal are seen in the photoemission spectrum of clean Ni at 0.3 eV, 0.6 eV, 1.3 eV, 2.0 eV and 3.8 eV binding energy, whereas the feature at about 6.2 eV is a plasmon satellite

Conclusions

The salient features in the valence band fine structure of Ni(111) and their changes upon near monolayer Sn deposition at RT and subsequent annealing in order to obtain the characteristic (√3 × √3)R30° surface alloy could be reproduced by ab-initio DOS calculations in fair agreement with the experiment. At the bimetallic surfaces, the spectra cannot be interpreted as a simple superposition of Ni-related and Sn-related features. The results thus indicate a strong coupling between the Sn and the Ni

Acknowledgements

The synchrotron radiation experiments at Elettra were partially supported by EU funding. Additional support by a Greek-Czech Bilateral Project (KONTAKT 2003–2005) is gratefully acknowledged. One of the authors (N.I.P.) acknowledges partial support from grant PENED-03/968 (Greek Ministry of Development, GSRT).

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