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II1.08 - Synthesis of 2D Spherical Periodic and Aperiodic Nanoparticle Arrays via Au-Enhanced Oxidation of Silicon 
April 22, 2014   11:30am - 11:45am

Advanced nanofabrication techniques have enabled rapid advancements in creating plasmonic nanoparticle arrays with complex optical resonance properties that are determined by their intrinsic geometry and extrinsic environment. These structures have been adopted for a wide range of applications that rely on either near-field enhancement or far-field diffractive coupling, including waveguides, sensors, solar cells, and metamaterials. The most common approach to fabricate nanoparticle arrays employs conventional top-down lithographic patterning followed by lift-off of an evaporated plasmonic metal. Although this method provides excellent control of nanoparticle size and placement, the lift-off process creates cylindrical particles with asymmetric tilted sidewalls, which can broaden the plasmonic peak width compared to an ideal isotropic spherical particle array. An alternative strategy has used localized laser induced heating of a planar evaporated Au film to create 2D spherical plasmonic particle arrays. However, the minimum spacing between adjacent particles has been limited to the micron scale, which prevents fabrication of arrays with strong interparticle coupling. This presentation will describe a new nanofabrication method that employs Au-enhanced oxidation to synthesize 2D spherical plasmonic nanoparticle arrays with well-controlled particle placement, diameter, and spacing down to the nanometer scale. The process begins by defining an array of cylindrical amorphous-Si/Au nanoparticles on a fused silica substrate using electron-beam lithography and evaporation. The amorphous-Si/Au particle array is then converted into a Au/SiO2 core-shell nanoparticle array by thermally oxidizing the entire structure in dry O2. During the thermal treatment, the Au core is transformed from a cylinder into a sphere to reduce the interfacial energy between the Au core and the SiO2 shell. In this process, the final spherical Au nanoparticle diameter and interparticle spacing is determined entirely by the starting lithographic pattern and the evaporated Au volume. To evaluate the optical properties of these structures, the angular response of a 2D periodic spherical nanoparticle array with 135 nm diameter Au particles and 360 nm spacing was compared to a conventional evaporated Au cylindrical nanoparticle array with the same geometry. The spherical nanoparticle array had a narrower resonance peak width and coincident transmission spectra for transverse electric (TE) and transverse magnetic (TM) polarizations at normal incidence. Additionally the isotropic particle geometry resulted in congruent reflectance in both the TE and TM modes at oblique angles of incidence. This approach is also being applied to fabricate 2D aperiodic nanoparticle arrays with Ammann-Beenker and Penrose tilings, which lack transitional symmetry and thus excite multiple plasmonic/photonic hybrid modes for a broader resonance response.

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