Abstract
Colloidal crystals have gained increasing importance due to their fascinating ability to mold the flow of light and sound (heat). The characteristics of these ordered assemblies of particles are strongly determined by the respective building blocks, which require complete understanding of their physical properties. In this study the mechanical properties of stretched polystyrene colloids (spheroids) are addressed. The non-invasive technique of Brillouin light scattering captures the vibrational spectra at hypersonic (GHz) frequencies. Resolved eigenmodes are considered fingerprints of the particles' shape, size and composing materials. A single particle model is used to simulate the experimental data by calculation of all active modes and subsequent evaluation of their contribution to the spectrum. Compared to spheres (high symmetry) more modes contribute to the spectra that limit the resolution at very high frequencies, due to the lifted mode degeneracy. Knowing the nature of the principal modes of spheroids is a precondition to understand the phononic dispersion in the respective colloidal crystals, in particular those responsible for anticrossing interactions with the effective medium acoustic phonon.
Highlights
A vast library of colloidal building blocks has emerged from which novel materials may be created by directed self-assembly.[1,2,3]
In order to check the accuracy, the simulation procedure was applied to the spherical particles before and a er embedding them in the poly(vinyl alcohol) (PVA)
While the shi of frequencies due to size decrease or a mixed PVA/PS surface layer can be attributed to the etching process, the observed broadening does not have such a clear attribution
Summary
A vast library of colloidal building blocks has emerged from which novel materials may be created by directed self-assembly.[1,2,3] An open challenge to use directed selfassembly techniques in the nanomanufacture of complex materials with a designed functionality is the ability to (i) identify desired ordered structures to achieve such a functionality and (ii) develop efficient routes to manipulate the interparticle interaction energies to accomplish the assembly. To accomplish both goals, an understanding of the particle's physical properties is warranted. To the best of our knowledge, this is the rst work that provides theoretical representation for vibrational spectra of spheroidal nanoparticles studied by Brillouin spectroscopy
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