Eur. Phys. J. AP 9, 105–114 (2000) T HE EUROPEAN P HYSICAL JOURNAL APPLIED PHYSICS c  EDP Sciences 2000 Optical study of Ag-TiO 2 nanocermet thin films prepared by R.F. co-sputtering A. Dakka 1, 2 , J. Lafait 2, a , M. Abd-Lefdil 1 , C. Sella 2 , and M. Maaza 3 1 Laboratoire de Physique des Mat´ eriaux, Universit´ e Mohammed V, Facult´ e des Sciences, avenue Ibn Batouta, B.P. 1014 Rabat, Morocco 2 Laboratoire d’Optique des Solides b , Universit´ e Pierre et Marie Curie, Case 80, 4 place Jussieu, 75252 Paris Cedex 05, France 3 Physics Department, University of the Witwatersrand, Wits 2050, Johannesburg, South Africa Received: 8 March 1999 / Revised: 17 May 1999 / Accepted: 6 December 1999 Abstract. The optical properties of Ag-TiO2 nanocermet thin films are studied with the aim of optical filtering applications. Beyond the classical properties of cermets with noble metal inclusions predicted by the effective medium theories, the optical properties of Ag-TiO2 nanocermets deposited by R.F. co- sputtering are governed by their columnar morphology and the under-stoichiometry of the TiO2 matrix. A careful experimental analysis of the different parameters and effects involved in the optical response of these nanocermets is performed both on TiO2 and Ag-TiO2: film thickness, silver volume fraction, thermal treatments, oxidation. The influence of these parameters on the surface plasmon resonance and the infrared transmission of the nanocermet thin films is optimized. PACS. 78.20.-e Optical properties of bulk materials and thin films – 81.05.Ys Nanophase materials – 81.05.Mh Cermets, ceramic and refractory composites – 81.40.Tv Optical and dielectric properties (related to treatment conditions) 1 Introduction Nanocermet thin films are used for applications in various fields such as optoelectronics, magnetic storage, energy conversion, optical filters, etc. The question we address in this paper concerns the realization of optical filters in the middle of the visible spectrum, at wavelengths larger than 500 nm. The concept of such a filter is based on a characteristic optical property of heterogeneous media composed of noble metal grains, of size much smaller than the wavelength, dispersed in a transparent dielectric ma- trix. These media exhibit a strong absorption band, not present in the bulk, due to the surface plasmon resonance in the free electron metal inclusions. Such a resonance is quite insensitive to the angle of incidence of the light on the material, in contrast to interference filters (although interference filters can be much more efficient). The Maxwell-Garnett model [1], developed in the framework of the quasi-static approximation predicts the position of the plasmon resonance, λ r , as a function of the dielectric function of the metal, ε i , and of the matrix, ε m , and the metal volume fraction, p. Silver, because of the occurrence of its interband transitions below 320 nm, is the quasi-free electron metal best suited for realizing this a e-mail: lafait@ccr.jussieu.fr b UMR 7601 resonance in the visible spectrum. In Figure 1, we rep- resent the resonance wavelength versus the silver vol- ume fraction in cermets having different matrix refractive indices, as obtained from Maxwell-Garnett calculations. The realization of an efficient absorption filter requires a high transmission outside the absorbing region. For this only low metal fractions, less than 10–15%, are accept- able. Within this range, resonance wavelengths larger than 500 nm can be achieved only with matrix indices larger than 2.0. Very few optical materials fulfil this condition, except TiO 2 . A large number of experimental results are available for the plasmon resonance in nanocermets with matrix refractive indices less than 2.0: SiO 2 [2, 3], MgF 2 [4, 5], MgO [6,7], A1 2 O 3 [8–10], Cr 2 O 3 [11], PbI 2 [12], etc. The lack of results concerning TiO 2 matrices can be at- tributed to the difficulty in obtaining stoichiometric ma- trices with this material. For this reason we present in detail the preparation conditions as well as the morpho- logical, structural and optical properties of both TiO 2 and Ag-TiO 2 thin films prepared by R.F. sputtering. Many other techniques have been used to prepare TiO 2 films such as evaporation [13], ion beam technique [14], chemical vapor deposition [15] and reactive d.c. mag- netron sputtering [16,17]. We have chosen R.F. diode sputtering because it is a common industrial technique