2007 NSTI Nanotechnology Conference and Trade Show - Nanotech 2007 - 10th Annual

Hypersonic Phononic Colloidal Crystals

G. Fytas
University of Crete, GR

Keywords:
phononic properties, colloidal crystals, self-assembly, vertical lifting deposition

Abstract:
The phononic properties of fabricated closed packed fcc colloidal crystals formed by self-assembly during vertical lifting deposition of spherical submicrometer particles e.g. polystyrene (diameter d=256nm and 307nm, polymethylmethacrylate (d=330nm)) with subsequent fluid infiltration (e.g. silicon oil and low molar mass polydimethylsiloxane) were investigated by high resolution Brillouin light scattering spectroscopy in the GHz frequency range. The dispersion relation between the frequency and the wave vector (k|| parallel to the fcc (111) plane) of the thermally excited acoustic waves has revealed two phononic band gaps: (i) a Bragg ñgap occurring at the boundary of the first Brillouin zone and (ii) a hybridization-gap resulting from the interaction of particle eigenmodes with the acoustic mode of the effective medium. Crystallinity is a prerequisite for the appearance only of the Bragg-gap. Depending on the particle size and the speed of sound in the infiltrated fluid, the frequency and the width of the Bragg-gap can be tuned. The elastic parameters of the particles can be extracted from their vibration eigenfrequencies in the multipole resonance spectrum of the corresponding dry opal, whereas the particle size polydispersity is reflected in the line shape of the lowest-frequency mode. Since hypersonic crystals can simultaneously exhibit phononic and photonic band gaps in the visible spectral region, the technological applications could range from tunable filters and heat management to acoustic-optical devices. The phononic properties for different volume fraction are under investigation for colloidal crystals of core/shell particles in polymeric matrices prepared by shear flow under compression.

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