Fabrication of Nano-Structured Gold Arrays by Guided Self-assembly for Plasmonics
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Fabrication of Nano-Structured Gold Arrays by Guided Self-assembly for Plasmonics Xiaoli V. Li1, Clelia A. Milhano2, Robin M. Cole3, Phil N. Bartlett2, Jeremy J. Baumberg3, and Cornelis H. de Groot1 1 School of Electronics and Computer Science, University of Southampton, Southampton, SO17 1BJ, United Kingdom 2 School of Chemistry, University of Southampton, Southampton, SO17 1BJ, United Kingdom 3 NanoPhotonics Centre, University of Camrbidge, Camrbidge, CB3 0HE, United Kingdom ABSTRACT Gold inverse spherical nanoscale voids have been fabricated in linear arrays for directional plasmon measurements in the visible spectral range. We show that by KOH anisotropic etching in Si, we are able to make V-grooves in which latex spheres of the order of 500 nm self-assemble with largely defect-free cubic symmetry. Both single layer and multilayer assembly in a facecentered close-packed (FCC) lattice can be achieved by varying the width of the trenches. This template is subsequently used for electrodeposition of gold to create the inverse spherical nanovoids. INTRODUCTION Monodispersed spherical colloids can self-assemble into one-dimensional to threedimensional lattices [1–3] by various types of driving forces, such as gravity [4], convection [5], spin-coating [6] or electrostatics [7]. To achieve long-range well-ordered colloidal lattices, a promising approach is guided self-assembly, in which colloidal spheres self-assemble onto a patterned substrate [8, 9]. For colloidal spheres, the main packing force is the interactions between spheres, which are often non-directional, in a sense, very similar to ionic bonding and metallic bonding. On a flat substrate, they self-assemble into face-centered or hexagonal close packed monolayer or multi-layers because this type of packing has the maximum density [10]. However, on a patterned substrate, the physical constraint, which is the volume interactions between spheres and the walls of the templates, is the main packing force. Since it is directional, the packing stucture of colloidal crystals on patterned substrates is based primarily on physical constraints, as shown in Ref. [11-15]. These high quality three dimensional colloidal sphere arrays have attracted attention in photonic applications, such as waveguide structures, optical filters and switches because of their potential as template for the fabrication of photonic crystals. In particular, metal inverse spherical nanoscale voids are useful because they possess plasmonic modes significantly different from those of metal nanoparticles. Submicron spheres are demanded for visible light plasmonic study. The dimensions of the spheres that were self-assembled in Ref. [11-15] were too large to be suitable for visible light study. In previous work, hexagonally close-packed spherical nanoscale voids have been fabricated by ourselves through self-assembled latex sphere templates by metal electrodeposition. The inverse spherical nanovoids support both propagating and localized plasmon modes [16, 17]. However, it was not possibl
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