104 Conclusionsparticular cases <strong>of</strong> 2D <strong>arrays</strong>. The first featured a 2D array <strong>of</strong> in-plane spherical goldparticles, having different interparticle spacings along <strong>and</strong> across the ripples, providingan optical anisotropy mainly due to the anisotropic EM coupling between the particles.The second array accommodated elongated gold nanoparticles arranged on a square grid,providing an optical anisotropy mainly due to the anisotropic single-particle polarizability.The optical <strong>properties</strong> <strong>of</strong> the two classes <strong>of</strong> samples were described by means <strong>of</strong> asimple but powerful effective medium model, including interparticle dipolar interactions,morphological disorder in the NPs shapes, <strong>and</strong> substrate effects. Combining experimentswithmodelcalculations, weshowedthedependence<strong>of</strong>the<strong>plasmonic</strong>responseonthearraydimensionality, <strong>and</strong> we could separately estimate the effects <strong>of</strong> the single contributions(intrinsic <strong>and</strong> collective) on the LSP characteristics. In this way, we found that the<strong>plasmonic</strong> response <strong>of</strong> the NPs in the <strong>arrays</strong> with rectangular symmetry is relativelyweakly affected by the interparticle EM dipolar coupling, via a partial cancellation effect<strong>of</strong> the dipolar fields <strong>of</strong> neighbouring NPs. Instead, the single-particle optical anisotropy <strong>of</strong>elongated NPs is further increased by the action <strong>of</strong> the EM interactions, even for square<strong>arrays</strong>.Toconclude, wepresentedamethodt<strong>of</strong>abricate2D<strong>arrays</strong><strong>of</strong>metallicnanoparticlesentirelybasedon<strong>self</strong>-organizationprocesses. Theproceduredeveloped herecouldbeappliedfor the cheap <strong>and</strong> easy fabrication <strong>of</strong> flexibly-tunable <strong>plasmonic</strong> supports for applicationsin which the high-degree <strong>of</strong> order achievable by lithographic methods [58, 68, 69, 71–74, 77, 86] is not strictly required, like substrates for LSP-enhanced optical spectroscopiesor LSP-based sensing.The method has been applied to the case <strong>of</strong> gold NPs, but it is very general, suggestingfacile extension to other materials than gold, for which similar results are expected. Inthis respect, it can represent an advantageous solution over, for example, the chemicalsynthesis <strong>of</strong> nanoparticles, which instead requires specific procedures for each material.The possibility to employ different materials gives the opportunity to address variousspectral regions; in fact, by using metals like Cu, Au, Ag or Al, the position <strong>of</strong> the LSPresonances can be tuned from the visible to the UV range, <strong>and</strong> by realizing <strong>arrays</strong> <strong>of</strong>metals alloys NPs it could be even possible to fine-tune the optical range <strong>of</strong> the device.The same methodology can also be applied to systems with different functionalitiesthan <strong>plasmonic</strong>. For example nanopatterned NaCl(110) substrates have been employedfor the fabrication <strong>of</strong> ferromagnetic iron nanowires [105, 122, 221], iron nanodots <strong>and</strong>undulating films [105], <strong>and</strong> <strong>arrays</strong> <strong>of</strong> cobalt nanoparticles [123].Finally, we showed how the 2D <strong>arrays</strong> <strong>of</strong> metallic nanoparticles can be employed asbase for the realization <strong>of</strong> composite devices. For example, an optically active device couldbe obtained by depositing a monolayer <strong>of</strong> magnetic nanoparticles, where the <strong>plasmonic</strong>response <strong>of</strong> the metallic NPs are tuned by the application <strong>of</strong> an external magnetic field.
AcknowledgementsFirst<strong>of</strong>all, IwouldliketothankDr.FrancescoBisio<strong>and</strong>Pr<strong>of</strong>.LorenzoMatteraformakingthis work possible <strong>and</strong> for their precious guidance. A big thank also to Pr<strong>of</strong>. MaurizioCanepa for his useful advices <strong>and</strong> discussions. A thank to Dr. Riccardo Moroni for hisassistance in the experiments, <strong>and</strong> to Ennio Vigo for the technical support during therealization <strong>of</strong> the preparation chamber. A grateful thank also to Dr. Mirko Prato for hisprecious help on ellipsometry.I gratefully acknowledge the group <strong>of</strong> biophysics, in particular Pr<strong>of</strong>. Ornella Cavalleri,for allowing me to use the AFM <strong>and</strong> DLS instrumentations <strong>and</strong> access the chemistrylaboratory. A particular thank also to Dr. Am<strong>and</strong>a Penco for her kind assistance in theAFM measurements.I deeply thank the group <strong>of</strong> Maria Del Puerto Morales at CSIC/ICCM <strong>of</strong> Madrid forthe collaboration in the synthesis <strong>of</strong> the magnetic nanoparticles.Finally, I would like to thank Laura Caprile, Michael Caminale, Dr. Elena Gatta <strong>and</strong>Chiara Toccafondi for making my thesis a pleasant experience.105