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

6 Introductionconfining the EM energy below the diffraction limit and for low-loss, high-speed EM signaltransmission, a property that can be exploited in hybrid plasmonic/electronic devices[58, 85, 86].In general, the collective properties of ordered ensembles of NPs stem from a superpositionof single-NP, intrinsic geometrical factors (shape, size, orientation with respect tothe exciting field), and ensemble properties (interparticle spacing, symmetry etc.) [75–84].Changing either of the two classes of factors leads to a corresponding modification of thecollective response of the systems, thereby offering the intriguing possibility of tailoringthe functionality of NP ensembles according to the specific scientific/technological requirements,like field enhancement for nonlinear spectroscopies [87], EM signal transmission[59], sensing [55] etc.Clearly, inordertomakethemostofthecollectivepropertiesofensemblesofplasmonicnanoparticles, a strategy for their assembly must be found, which allows to flexibly modifystructural parameters of the system like mean particle-particle spacing, or the ensemblegeometry. Focusing on bottom-up approaches, based on self-assembly or self-organizedmethods, several fabrication strategies can be found. In the case of particle depositiononto a 2-dimensional support, in the purest bottom-up approach, the particles depositedon a flat surface will tend to assume a close-packed geometry [14, 88–91]. This leavesa relatively restricted number of ways to modify their arrangement: for example theparticle-particle spacing can be modified by varying the length of the surfactant molecules[14, 23, 91], or the array symmetry can be varied changing the shape of the particles[92–96]. Another possibility is instead to employ nanopatterned templates [97, 98] assupports, so that the nanoparticles arrangement can be guided in a controllable manner.Nanopatterned templates can be realized, for example, by creating a chemical contrastpattern between regions with different affinity to the nanoparticles [99–102], by printingprocesses [7, 9, 22, 103], or by inducing periodical modifications of the surface morphology[104–112]; thecombinedapplicationofnanolithography, forthepatteringofthesubstrates,and self-assembling, for the deposition of nanoparticles, is also often considered [113–116].In this thesis, we report the fabrication of self-organized 2-dimensional (2D) arrays ofAu nanoparticles with tunable particle shape and mutual arrangement, realized employingnanostructured LiF(110) substrates as patterned templates. Insulating ionic crystals, likeNaCl, LiF,CaF 2 orMgO,havealreadybeenproposedsincemorethanadecadeassuitabletemplates for nanopatterned assembly [105, 117–121]. In fact, faceting of the (110)- or(111)-like surfaces into ridge-and-valley or pyramidal structures can be spontaneouslyinduced by means of homoepitaxial deposition or mild annealing. The same pattern canthen be easily replicated by depositing a thin layer of the material of interest, allowing,for example, to realize arrays of magnetic nanowires and nanodots [105, 122, 123], or ofgold nanowires for second harmonic generation [124]. Here we apply this technique to theinvestigation of the collective optical response of a 2D arrays of gold nanodots.Our 2D arrays of gold nanoparticles were fabricated by deposition of Au onto theself-organized nanoscale ridge-and-valley morphological patterns that spontaneously formupon homoepitaxial deposition at the LiF(110) surface. Grazing-incidence evaporation ofAu onto the LiF nanopatterns leads to the formation of Au nanowires, that evolve towardsregular arrays of disconnected NPs, aligned with the LiF ridges, following a temperatureinduceddewetting. Depending on the substrate fabrication and the Au deposition parameters,the NP shape (coherently-aligned ellipsoids with tunable aspect ratio and size) andthe array characteristics (interparticle spacing, array symmetry) could be independentlycontrolled, allowing to correspondingly tune the system’s plasmonic response.In this work, we demonstrate the potential of the method by focusing on two particu-