Sn–CeO 2 thin films prepared by rf magnetron sputtering: XPS and SIMS study Karel Mas ˇek a, *, Michal Va ´ clavu ˚ a , Petr Ba ´ bor b , Vladimı ´r Matolı ´n a a Charles University, Faculty of Mathematics and Physics, Department of Surface and Plasma Science, V Holesˇovicˇka ´ch 2, 18000 Prague 8, Czech Republic b Brno University of Technology, Faculty of Mechanical Engineering, Institute of Physical Engineering, Technicka ´ 2896/2, 616 69 Brno, Czech Republic 1. Introduction The electronic structure of the CeO 2 oxide is characterized by unoccupied 4f states of Ce 4+ (4f 0 ) whilst the Ce 2 O 3 oxide has a Ce 3+ (4f 1 ) configuration. Different 4f configurations for Ce 4+ and Ce 3+ result in different core level and valence band structures. One of the important properties of ceria is its oxygen storage capacity, which can provide oxygen to the gas mixture in catalytic contexts. The key factor for this property is the reversible transformation from Ce 4+ to Ce 3+ . The cerium chemical state may be determined by a photoelec- tron spectroscopy. Using a conventional laboratory X-ray source, Ce 3d levels can be analysed [1–3]. The Ce 3d spectrum of partially reduced ceria consists of three 3d 3/2 –3d 5/2 spin–orbit–split doublets representing different 4f 0 (Ce 4+ ) configurations in the photoemission final state. Two other doublets represent 4f 1 (Ce 3+ ) states. The previous X-ray photoelectron spectroscopy (XPS) and resonance photoelectron spectroscopy (RPES) studies on epitaxial CeO 2 thin films grown on Cu(1 1 1) showed that the Sn deposition on cerium oxide causes Ce 3+ intensity to appear in the Ce 3d spectra [4,5]. The mixed oxide Sn–Ce–O has been found to be efficient CO oxidation catalyst which was more active than the individual oxides. The purpose of this study was to contribute to the elucidation of the Sn–Ce interaction mechanism in mixed oxides prepared by simultaneous magnetron sputtering of ceria and tin. There are several reports concerning the deposition of CeO 2 films using sputtering, mainly for superconducting and microwave applications, e.g., [6–9]. Secondary ion mass spectroscopy (SIMS) study of the chemical nature of ceria thin films showed the possibility to determine depth profiles by analysing different positive ion clusters [10]. We report that the Sn introduction in the CeO 2 thin film causes Ce 3+ concentration enhancement and complete Ce 4+ ! Ce 3+ conversion could be caused by a charge transfer from Sn atoms to unoccupied orbital Ce 4f 0 of cerium oxide by forming Ce 4f 1 state. 2. Experimental Non-reactive rf magnetron sputtering was used to deposit CeO 2 and Sn–CeO 2 thin films on Si(1 0 0) wafer substrates. CeO 2 sputtering was performed using a pure CeO 2 target (Kurt J. Lesker Company, 99.99% purity) at a distance of 90 mm from the Si substrate (Si wafer covered by natural oxide layer) with rf power of Applied Surface Science 255 (2009) 6656–6660 ARTICLE INFO Article history: Received 17 April 2008 Received in revised form 5 January 2009 Accepted 23 February 2009 Available online 6 March 2009 PACS: 68.47.Gh 82.80.Pv 68.49.Sf 81.15.Cd Keywords: Cerium oxide Tin–cerium mixed oxide SIMS XPS Magnetron sputtering ABSTRACT Sn addition in the CeO 2 thin film by simultaneous Sn metal and cerium oxide magnetron sputtering causes growth of Ce 3+ rich films whilst pure cerium oxide sputtering provides stoichiometric CeO 2 layers. Ce 4+ ! Ce 3+ conversion is explained by a charge transfer from Sn atoms to unoccupied orbital Ce 4f 0 of cerium oxide by forming Ce 4f 1 state. XPS and SIMS revealed a formation of a new chemical Ce(Sn) + state, which belongs to SnCeO 2 species. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +420 22191 2753. E-mail address: karel.masek@mff.cuni.cz (K. Mas ˇek). Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.02.080