Phonon-engineered solids constructed from nanocrystals

Abstract

Here, we present the concept of colloidal nanocrystal-based solids as solution-processed, phonon-engineered materials. In particular, we discuss superlattice phonons, the vibrational modes associated with the motion of the nanocrystals away from their equilibrium positions in the nanocrystal solid (or superlattice). We calculate the characteristic energies and density of states of these superlattice phonons by modeling the nanocrystal solids as three-dimensional mass-spring networks. In this model, the nanocrystals correspond to the masses and their surface terminating organic or inorganic moieties (i.e., ligands) act as springs. We parameterize this model by determining nanocrystals masses based on crystal size and material density and by using density functional theory to determine the spring constant associated with ligand-mediated bonding between the nanocrystals. We show that by varying the type of nanocrystals, their ligands, and the topology of the nanocrystal superlattice, it is possible to systematically tune the density of states of the superlattice phonons in the energy range of 0.01 meV–10 meV. We then highlight how the construction of binary nanocrystals superlattices can be used, for example, to introduce phononic bandgaps at specific energies. Finally, we show that even with disorder stemming from finite nanocrystal size distributions and variations in the bonding between the nanocrystals and the ligands, distinct superlattice phonons modes will still be present.