JOURNAL OF RAMAN SPECTROSCOPY J. Raman Spectrosc. 29, 693È702 (1998) Laser-deposited Silver Island Films : An Investigation of their Structure, Optical Properties and Activity* SERS E. Vogel,1 W. Kiefer,1¤ V. Deckert2 and D. Zeisel2 1 Institut fu r Physikalische Chemie, Universitat Wu rzburg, Am Hubland, D-97074 Wu rzburg, Germany 2 Laboratorium fu r Organische Chemie, Eidgeno ssische Technische Hochschule Zu rich, Universitatsstr. 16, CH-8092 Zu rich, Switzerland Silver island Ðlms prepared by pulsed laser deposition were tested as substrates for surface enhanced raman spec- troscopy. They exhibit a position-dependent thickness and SERS activity distribution. Cresyl Violet and Crystal Violet were chosen as test molecules. The annealing of the Ðlms leads to coalescence of the initial island structures and the development of longer ellipsoidal particles, which generally improves the enhancement properties of the substrates. The dimensions of the surface structures depend on the local initial Ðlm thickness. The changes in the island shape and dimensions manifest themselves in changes in the absorption spectra. 1998 John Wiley & Sons, ( Ltd. INTRODUCTION The investigation of molecules adsorbed on metal sur- faces is of great interest in a wide range of applications. Surface-enhanced Raman spectroscopy (SERS) is regarded as a valuable tool in this respect. It is a highly sensitive method, which permits the detection and spec- troscopy of even single molecules.1 In addition, the application of speciÐc “surface selection rulesÏ allows one to draw conclusions about the adsorption geometry.2 The magnitude of the surface enhancement depends critically on the size, shape and spacing of the nanos- tructures causing the electromagnetic Ðeld enhance- ment.3 The investigation and determination of these structural parameters, which provide optimum enhance- ment of the Raman signals of adsorbed molecules, are of crucial importance for the practical applications of SERS spectroscopy and for the better understanding of the nature of the surface-enhanced scattering process. Di†erent techniques are used for nanostructuring the SERS substrates : electrochemical roughening of electrodes4 and the preparation of metal colloids5 or Ðlms6 by chemical reduction of the corresponding salts. Sputtering7 or thermal evaporation8 is used to manu- facture metal island Ðlms and cold deposited metal Ðlms.9 Recently, laser ablation of metals was used to prepare of SERS-active colloids.10 Generally, the laser ablation technique is becoming more and more common for the preparation of thin Ðlms of functional materials, e.g. high transition temperature superconduc- * Dedicated to Professor F. W. Schneider on the occasion of his 65th birthday. ¤ Correspondence to: W. Kiefer, Institut fu r Physikalische Chemie, Universitat Wu rzburg, Am Hubland, D-97074 Wu rzburg, Germany. Contract/grant sponsor : Deutsche Forschungsgemeinschaft ; Contract/grant number : Sonderforschungsbereich 347, Teilprojekt C2. Contract/grant sponsor : Fonds der Chemischen Industrie. tors, ferroelectric oxides or the laser processing of metals and alloys.11 A wide range of comprehensive studies on laser ablation have been reported, even though investigations on some fundamental aspects of the process are still in progress. An advantage of pre- paring SERS-active substrates by laser ablation over chemical reduction or redox procedures is that the surface of such colloids is free of organic or ionic con- taminants. Hence the chemical and physical e†ects of SERS can be studied under accurately controlled condi- tions.10 However, the structural characterization of col- loidal particles by means of conventional structure-sensitive methods such as transition electron microscopy (TEM) or atomic force microscopy (AFM) still requires their deposition on a solid substrate.12 In this respect, sputtered or thermally evaporated island Ðlms on transparent dielectric substrates are con- venient for comprehensive characterization with di†er- ent methods. A detailed study of the structural and optical properties of evaporated thin Ðlms of di†erent mass thickness by means of TEM and UVÈVIS spec- troscopy was performed by Singer et al.13 Van Duyne et al.14 coupled quantitative measurements of the SERS enhancement factor for Ag island Ðlms and Ag Ðlms over polymer nanospheres with AFM data and inter- preted the results in terms of the electromagnetic theory of SERS. The e†ect of annealing on the optical proper- ties, surface structure and SERS e†ectivity of thin evaporated silver Ðlms was extensively studied by Mas- kevich and co-workers by means of AFM and UVÈVIS spectroscopy. In addition, the long-range component of the Raman enhancement was investigated by the same group16 using the LangmuirÈBlodgett technique. In this study, we investigated laser-deposited island Ðlms as possible substrates for SERS. These substrates can be characterized by morphology-sensitive methods and have a continuously varying Ðlm thickness distribu- tion. The variation in Ðlm thickness allows one to inves- tigate its inÑuence on the optical and structural CCC 0377È0486/98/080693È10 $17.50 Received 5 January 1998 ( 1998 John Wiley & Sons, Ltd. Accepted 8 May 1998