RF-sputtering of gold on silica surfaces: Evolution from clusters to continuous films L. Armelao a , D. Barreca a, * , G. Bottaro a , G. Bruno b , A. Gasparotto c , M. Losurdo b , E. Tondello c a ISTM-CNR and INSTM, Department of Chemistry, University of Padova, via Marzolo, 1-35131 Padova, Italy b IMIP-CNR and INSTM, via Orabona, 4-70126 Bari, Italy c Department of Chemistry and INSTM, University of Padova, via Marzolo, 1-35131 Padova, Italy Available online 2 August 2005 Abstract Au/SiO 2 nanosystems were prepared by RF-sputtering of gold from Ar plasmas on amorphous silica substrates at temperatures as low as 60 -C. The interrelation between nanosystem properties and synthesis conditions was investigated in detail, with particular attention to the nucleation and coalescence processes of Au nanoparticles on the SiO 2 surface. To this regard, special emphasis was given to the interplay between the system morphology and the resulting optical properties, as probed by both in-situ and ex-situ characterization techniques. In this context, Spectroscopic Ellipsometry (SE) enabled to gain important information on the energy dispersion of the complex dielectric function and, subsequently, on the evolution from dispersed clusters to continuous films. Such variations were tailored by proper combinations of the bias potential (V bias ) and deposition time, indicating the possibility of a fine modulation of the optical response. D 2005 Elsevier B.V. All rights reserved. Keywords: Au/SiO 2 nanosystems; Spectroscopic Ellipsometry; Clusters; Continuous films 1. Introduction Metal nanoparticles on oxide substrates have gained a markedly increasing consideration with regard to both scientific and technological purposes [1]. In particular, gold-silica nanosystems are among the most studied, thanks to their extensive applications in heterogeneous catalysis [2,3] and optics (non-linear devices, fiber optics chemical sensors) [1,4–6]. As a matter of fact, supported Au nanoparticles display size- and shape-dependent properties, which can further be tailored by varying their distribution on the substrate and interparticle spacing [4]. In particular, significant variations in the Au/SiO 2 optical response are induced by modifications of the system morphology from cluster-like systems, where gold nanoparticles are dispersed on the silica surface, to island-like structures, where Au aggregates are partially interconnected between each other, and ultimately, to continuous films [5,7]. The control of the Au particle distribution and concentration is therefore a key- step in order to develop nanosystems endowed with well- tailored optical properties. To this regard, in-situ and ex-situ optical methods for real-time growth monitoring and feedback-control of the deposition process have attracted a great deal of attention due to their non-destructive and non-invasive nature [8]. In particular, Spectroscopic Ellipsometry (SE) is a powerful and versatile technique for the determination of the energy dispersion of the complex dielectric function, ( (x)= ( 1 (x)+ i( 2 (x)=(n + ik ) 2 , and of the refractive index, n , and the extinction coefficient, k , of Au/SiO 2 nanosystems with high accuracy, even in the spectral region of strong absorption. The use of a suitable modeling enables to obtain valuable information on the interrelations between structure and optical properties even for low-size nanosystems, whose detection and analysis is a hard task by means of conven- tional structural techniques alone [9]. Despite different SE studies on both Au island-like systems and continuous films on Si and SiO 2 have appeared in the literature [1,8,10], most 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.06.007 * Corresponding author. Tel.: +39 49 8275170; fax: +39 49 8275161. E-mail address: davide@chin.unipd.it (D. Barreca). Materials Science and Engineering C 25 (2005) 599 – 603 www.elsevier.com/locate/msec