Anisotropic phononic bandgaps in colloidal crystals of dumbbell-shaped nanoparticles

Nanoparticles are excellent building blocks for creating crystalline structures. Using an electric-filed-directed self-assembly technique, we fabricate colloidal crystals of dumbbell-shaped nanoparticles. The base-centered monoclinic lattice structure exhibits phononic bandgaps due to the periodicity of the elastic properties. The phonon dispersion recorded by Brillouin light spectroscopy (BLS) reveals direction-dependent Bragg gaps resulting from the anisotropic lattice structure. In addition, a hybridization bandgap robust to disorder is also observed in the disordered films of three different dumbbell-shape nanoparticles. We present theoretical calculations of the BLS-measured particle vibrational eigenmode spectra and the band structure of the phononic crystal. The calculation results elucidate the origin of the hybridization bandgap and its relation to the particle vibrational eigenmodes.