Electronic interaction at the TiO 2 ±Al 2 O 3 interface as observed by X-ray absorption spectroscopy M. S anchez-Agudo a , L. Soriano a, * , C. Quir os a , J. Avila b,c , J.M. Sanz a a Departamento de F  õsica Aplicada, Instituto de Ciencia de Materiales Nicol  as Cabrera, Universidad Aut onoma de Madrid, Cantoblanco E-28049 Madrid, Spain b Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, E-28049 Madrid, Spain c LURE, Universit e Paris-Sud, B^ atiment 209d, F-91405 Orsay, France Abstract The electronic structure of the TiO 2 ±Al 2 O 3 interface has been studied using X-ray absorption spectroscopy. Special attention has been paid to the early stages of growth, i.e. the sub-monolayer regime h < 1. The Ti 2p spectra for coverages below 1 ML show signi®cant changes with respect to those for large coverages and bulk TiO 2 indicating the presence of interfacial states. The spectra have been compared with atomic multiplet calculations reported in the lit- erature. From this comparison it is concluded that strong electronic interactions occur at the interface, as deduced from the signi®cant lowering of the crystal ®eld of the Ti atoms at the interface 1.0 eV) as compared with bulk TiO 2 1.8 eV). It is suggested that the important covalent character of the bonding of the Al 2 O 3 substrate is the responsible of this crystal ®eld lowering. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Near edge extended X-ray absorption ®ne structure NEXAFS); Interface states; Aluminum oxide; Titanium oxide; Coatings; Insulating ®lms 1. Introduction The study of oxide±oxide interfaces is a research topic since the last few years because most of thin- ®lms applications are aected by interface phe- nomena such as wetting, adhesion, reactivity, etc. The purpose of this paper is to investigate the electronic structure of the TiO 2 ±Al 2 O 3 interface by means of X-ray absorption spectroscopy XAS). XAS has already proved its potential in the anal- ysis of interfaces, being able to detect the presence of interfacial states [1,2] due to the high sensitivity to the local symmetry, i.e. chemical state and co- ordination, of the initial state involved in the metal 2p absorption spectra [3]. In fact, strong crystal- ®eld eects at the TiO 2 ±SiO 2 interface, as revealed by unique Ti 2p spectra, have been recently pub- lished [2]. In this work, we present a detailed analysis of the Ti 2p spectra as a function of the TiO 2 deposition on Al 2 O 3 , paying special attention to the sub-monolayer regime, i.e. h < 1. The re- sults clearly show the existence of strong electronic interactions at the TiO 2 ±Al 2 O 3 interface which is consistent with the formation of Ti±O±Al cross- linking bonds. The importance of metal oxides in technological applications has led to the development of physical methods to produce thin ®lms of oxides supported Surface Science 482±485 2001) 470±475 www.elsevier.nl/locate/susc * Corresponding author. Tel.: +34-913-974-192; fax: +34- 913-973-969. E-mail address: l.soriano@uam.es L. Soriano). 0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0039-602800)01048-7