Delivered by Ingenta to: Ecole Polytechnique IP : 129.104.38.5 Thu, 26 Jul 2012 07:17:07 ARTICLE Copyright © 2012 by American Scientific Publishers All rights reserved. Printed in the United States of America Science of Advanced Materials Vol. 4, pp. 708–718, 2012 (www.aspbs.com/sam) Formation and Evolution of Nanoscale Metal Structures on ITO Surface by Nanosecond Laser Irradiations of Thin Au and Ag Films F. Ruffino 1, 2, * , E. Carria 1, 2 , S. Kimiagar 1, 3 , I. Crupi 2 , F. Simone 1, 2 , and M. G. Grimaldi 1, 2 1 Dipartimento di Fisica ed Astronomia-Università di Catania, via S. Sofia 64, 95123 Catania, Italy 2 MATIS IMM-CNR, via S. Sofia 64, 95123 Catania, Italy 3 Islamic Azad University, Central Tehran Branch, Tehran, Iran ABSTRACT The effect of nanosecond laser irradiations on 5 nm thick sputter-deposited Au and Ag films on Indium-Tin- Oxide surface is investigated by atomic force microscopy (AFM) and scanning electron microscopy (SEM). After 500, 750, and 1000 mJ/cm 2 fluence irradiations, the breakup of the Au and Ag films into nanoscale islands is observed as a consequence of fast melting and solidification processes. The mean nanoparticles size and surface density are quantified, as a function of the laser fluence, by the AFM and SEM analyses. In particular, the comparison between the Au and Ag islands reveals the formation of larger islands in the case of Ag for each fixed fluence. The mechanism of the nanoscale islands formation is discussed, both for Au and Ag, in terms of the starting film thickness fluctuations (influencing the local threshold for melting), dewetting phenomenon and the Rayleigh criterion. KEYWORDS: ITO, Gold, Nanosecond Laser Irradiation, Nanoclusters, Nanostructuring, Atomic Force Microscopy. 1. INTRODUCTION The generation of metallic nanostructures on surfaces is a thrust in materials science from scientific viewpoint and technological applications in areas such as catalysis, pho- tonics, single electron and quantum devices, plasmonics, solar cells, etc. 1 2 In particular, the interest in assembling Au and Ag nanoclusters (NCs) onto conductive transpar- ent substrates, as indium-tin-oxide (In 2 O 3 :SnO 2 , ITO), has increased 3–9 as the resulting interfaces show interesting optical properties exhibiting structure-dependent transmis- sion/absorption spectra due to the occurrence of localized surface plasmon resonance effects (LSPR) 3–8 that could be successfully used in plasmonic solar cell to increase the energy conversion efficiency. 10–12 Au and Ag NCs exhibit extraordinary optical resonances: when excited by an electromagnetic radiation, they exhibit LSPR due to the collective oscillations of their conduction electrons. The resonant excitation of LSPR leads to selective pho- ton absorption and enhancement of local electromagnetic fields near the NCs by orders of magnitude. The possibility ∗ Author to whom correspondence should be addressed. Email: francesco.ruffino@ct.infn.it Received: 24 September 2011 Accepted: 27 January 2012 of controllably tuning the LSPR wavelength through the visible to near infrared region makes Au and Ag NCs very important and promising for a wide range of appli- cations in chemical and biological sensors, 13–15 optical filters, optical tweezers, ultrafast optical switching, and surface enhanced Raman spectroscopy. 16–22 On the other hand, ITO films are widely used in optical devices due to the high electrical conductivity and high transparency at visible light wavelengths. 23–25 ITO thin film is a highly degenerate n-type semiconductor which has a low electri- cal resistivity (usually ∼2–4 × 10 -4 cm). The low resis- tivity of ITO films is due to a high carrier concentration because the Fermi level is located above the conduction level. The degeneracy is caused by both oxygen vacancies and substitutional tin dopants created during film deposi- tion. The carrier concentration of high conductivity ITO films is in the range of 10 20 –10 21 cm -3 . Furthermore, ITO is a wide band gap semiconductor (3.5–4.3 eV), which shows high transmission in the visible and near-IR regions of the electromagnetic spectrum. So, ITO films for appli- cation in transparent electrodes for display devices, trans- parent coating for solar energy heat mirrors and window films in n–p heterojunction solar cells, are, today, routinely used. A key technological point concerns the generation and control of the structural properties (shape, size) of metallic 708 Sci. Adv. Mater. 2012, Vol. 4, No. 7 1947-2935/2012/4/708/011 doi:10.1166/sam.2012.1342