Morphology and plasmonic properties of self-organized arrays of ...

104 Conclusionsparticular cases of 2D arrays. The first featured a 2D array of in-plane spherical goldparticles, having different interparticle spacings along and 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 properties of the two classes of samples were described by means of asimple but powerful effective medium model, including interparticle dipolar interactions,morphological disorder in the NPs shapes, and substrate effects. Combining experimentswithmodelcalculations, weshowedthedependenceoftheplasmonicresponseonthearraydimensionality, and we could separately estimate the effects of the single contributions(intrinsic and collective) on the LSP characteristics. In this way, we found that theplasmonic response of the NPs in the arrays with rectangular symmetry is relativelyweakly affected by the interparticle EM dipolar coupling, via a partial cancellation effectof the dipolar fields of neighbouring NPs. Instead, the single-particle optical anisotropy ofelongated NPs is further increased by the action of the EM interactions, even for squarearrays.Toconclude, wepresentedamethodtofabricate2Darraysofmetallicnanoparticlesentirelybasedonself-organizationprocesses. Theproceduredeveloped herecouldbeappliedfor the cheap and easy fabrication of flexibly-tunable plasmonic supports for applicationsin which the high-degree of 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 of 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 of 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 of the LSPresonances can be tuned from the visible to the UV range, and by realizing arrays ofmetals alloys NPs it could be even possible to fine-tune the optical range of the device.The same methodology can also be applied to systems with different functionalitiesthan plasmonic. For example nanopatterned NaCl(110) substrates have been employedfor the fabrication of ferromagnetic iron nanowires [105, 122, 221], iron nanodots andundulating films [105], and arrays of cobalt nanoparticles [123].Finally, we showed how the 2D arrays of metallic nanoparticles can be employed asbase for the realization of composite devices. For example, an optically active device couldbe obtained by depositing a monolayer of magnetic nanoparticles, where the plasmonicresponse of the metallic NPs are tuned by the application of an external magnetic field.