Functional Metamaterials Based on Mesoscale Gold Sponges, Particulate Aggregates, and Their Composites with Dielectric M
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Functional Metamaterials Based on Mesoscale Gold Sponges, Particulate Aggregates, and Their Composites with Dielectric Materials Michael Cortie, Abbas Maaroof, and Geoff B. Smith Institute for Nanoscale Technology, University of Technology Sydney, Sydney, 2007, Australia
ABSTRACT The optical properties of some nanoscale composites may deviate from that expected from a simple law of mixture of their individual components. In these cases the resulting structure can be considered to be a type of ‘metamaterial’. Here we explore some of the possibilities for nanoscale composite structures comprised of gold and VO2 – the latter being a functional material that undergoes a reversible insulator to metallic phase transition at 68ºC. Two microstructures are examined: aggregates of gold nanoparticles surrounded by VO2 as the continuous phase, and its geometric inverse, mesoporous gold sponge with discontinuous VO2 inclusions. A composite, right-angled parallelepiped measuring 40x100x100 nm is taken as representative of the mixture, and calculations of the optical properties performed using the discrete dipole approximation code of Draine and Flatau. The VO2 matrix strongly attenuates the dipole-dipole plasmon resonance of the gold structure, and thermochromic switching of the remaining plasmon resonance occurs.
INTRODUCTION Mesoporous gold sponges backfilled with a dielectric filler, and aggregates of gold particles in a dielectric matrix (figure 1) are two of the several microstructures conceptually possible in Au/VO2 nanocomposites. Here we explore how the visible optical properties of these prototypical structures is influenced by the morphology and volume fraction of the gold phase, and by the physical properties of the VO2. The Au sphere aggregates are less percolated than the mesoporous sponges, and this important difference alone would be expected to have a large effect. In addition to this, VO2 has the property that it transforms reversibly at 67ºC between a low temperature semi-conductor phase (VO2-M1, band gap ~0.6 eV)) to a metallic phase (VO2R) [1, 2], with an attendant change in electrical conductivity and dielectric properties. Since any plasmon resonances on the gold component of the composite are sensitive to both the presence of percolation and to the dielectric properties of the adjacent VO2, it follows that these morphological and structural considerations should also produce changes in the optical properties of the composite. It is these changes that we explore here.
(a) (b) Figure 1. Examples of the two types of microstructure considered here. In each case a block of material measuring 40 x 100 x 100 nm is shown. The gold dipoles invoked in the calculation are shown as little spheres, but those of the VO2 are not individually depicted and the VO2 is shown instead as a continuous, semi-transparent grey material. (a) Aggregates of spherical Au nanoparticles making up a volume fraction of 0.10 in a matrix of VO2. (b) Mesoporous Au sponge making up a volume fraction of 0.24 in a matrix that ha
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