(716c) Anisotropic Hypersonic Phonon Propagation in Films of Aligned Ellipsoids

The assembly of colloidal building blocks into ordered structures using directing electric fields is a rapid, highly scalable, and potentially low-cost route for manufacturing functional nanomaterials and devices.  The characteristics of these ordered assemblies of particles are inherently determined by the size, shape and chemistry of the respective building blocks and the final ordered arrangement.  In this talk, we describe the assembly of colloidal polystyrene ellipsoids of three aspect ratios into densely packed, orientationally ordered thin films using AC electric fields and evaluate the phononic properties of the films by Brillouin light scattering.  The measurements indicate anisotropic elastic mechanical properties and the emergence of a unidirectional hypersonic band gap in the fabricated materials.  The frequency of the gap and the effective longitudinal sound velocities (c_L) can be tuned by the ellipsoid aspect ratio. We hypothesize that the band gap originates from the primary eigenmode peak, the m-splitted (s,1,2) mode, in the vibrational spectrum of the individual particle resonating with the effective medium. These results reveal the potential for powerful control of the hypersonic phononic band diagram by combining anisotropic particles and self-assembly.