4 CONTENTS5.2.2 Optical anisotropy <strong>of</strong> <strong>self</strong>-<strong>organized</strong> Au NPs <strong>arrays</strong> . . . . . . . . . 856 Composite media based on Au/LiF <strong>arrays</strong> 97Conclusions 103Acknowledgements 105
IntroductionIn a world in which the information <strong>and</strong> communication technology has gradually assumeda pivotal role for scientific, technological <strong>and</strong> social purposes, there is a constantly growingdem<strong>and</strong> for smaller <strong>and</strong> faster devices able <strong>of</strong> manipulating information, typically in theform <strong>of</strong> electronic, magnetic or optical signals. The request <strong>of</strong> smaller <strong>and</strong> faster deviceshas inevitably led to a constant process <strong>of</strong> miniaturization <strong>of</strong> their elementary components,thereby bringing along a huge load <strong>of</strong> scientific <strong>and</strong> technological challenges thatresearchers <strong>and</strong> engineers have to face. Nowadays, for example, state-<strong>of</strong>-the-art technologyrequires excellent control <strong>of</strong> structures having typical lateral dimension <strong>of</strong> the order<strong>of</strong> approximately ten nanometers, not too far <strong>of</strong>f molecular dimensions.While conventional lithographic methods [1–6] always represent the st<strong>and</strong>ard choicefor the fabrication <strong>of</strong> technological nanostructures, the challenges associated with theconstant size reduction have promoted intense research aimed at exploiting alternativestructures for the fabrication <strong>of</strong> nanosized systems [7–20].Among the various strategies, in the past years we have witnessed a growing interest tothe world <strong>of</strong> colloid <strong>and</strong> cluster science, leading to the consolidation <strong>of</strong> nanoparticles (NPs)as convenient structural elements for the construction <strong>of</strong> functional interfaces [10, 14, 21–28]. In fact, nanoparticles can be fashioned from many different materials, presentinga wide diversity <strong>of</strong> electronic, optical, catalytic <strong>and</strong> magnetic <strong>properties</strong>, which in manycases originate from the reduced dimensionality <strong>of</strong> the systems <strong>and</strong> thus are not found inthe bulk counterparts [29–34].While the <strong>properties</strong> <strong>of</strong> individual NPs can be exploited in a variety <strong>of</strong> ways [8, 21, 23,26,35–47], otherinterestingfunctionalitiesarisewhentheNPsareassembledinsuchawaythat each one “feels” the presence <strong>of</strong> neighbouring particles via some mutual interaction.Typical examples <strong>of</strong> systems exhibiting such collective functionality are assemblies <strong>of</strong>magnetic particles [14, 27, 28, 34, 48–50], interacting with one another through theirmagnetic dipolar field or, most relevant for this thesis, assemblies <strong>of</strong> metallic particlescharacterized by peculiar optical functionalities [24, 25, 38, 51–60].Undertheinfluence<strong>of</strong>theelectromagnetic(EM)field,metallicnanoparticlescaninfactexhibit strong resonant absorptions, absent in bulk counterparts, referred to as localizedsurface plasmon resonances (LSPRs) [54, 61–65]. The main features <strong>of</strong> LSPRs (frequency,width <strong>and</strong> intensity) are strongly sensitive to intrinsic geometrical factors, like the NPshape <strong>and</strong> size, <strong>and</strong> external variables, like the NP dielectric environment [54, 62, 66, 67].The near proximity <strong>of</strong> the NPs to another metallic material, in the form <strong>of</strong> a surface or<strong>of</strong> other neighbouring NPs, causes dramatic variations <strong>of</strong> the LSPR characteristics drivenby the near-field EM coupling between the NP <strong>and</strong> its metallic surroundings [62, 68–84]. Beside modifying the LSPR, EM coupling leads to interesting <strong>properties</strong> in terms <strong>of</strong>localization, enhancement <strong>and</strong> guiding <strong>of</strong> the EM field on subwavelength scales [59]. Forexample, 1-dimensional (1D) chains <strong>of</strong> NPs are extremely appealing for their capability <strong>of</strong>5
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