Abstract
Phonon engineering of solids enables the creation of materials with tailored heat-transfer properties, controlled elastic and acoustic vibration propagation, and custom phonon–electron and phonon–photon interactions. These can be leveraged for energy transport, harvesting, or isolation applications and in the creation of novel phonon-based devices, including photoacoustic systems and phonon-communication networks. Here we introduce nanocrystal superlattices as a platform for phonon engineering. Using a combination of inelastic neutron scattering and modeling, we characterize superlattice-phonons in assemblies of colloidal nanocrystals and demonstrate that they can be systematically engineered by tailoring the constituent nanocrystals, their surfaces, and the topology of superlattice. This highlights that phonon engineering can be effectively carried out within nanocrystal-based devices to enhance functionality, and that solution processed nanocrystal assemblies hold promise not only as engineered electronic and optical materials, but also as functional metamaterials with phonon energy and length scales that are unreachable by traditional architectures.
Highlights
Phonon engineering of solids enables the creation of materials with tailored heat-transfer properties, controlled elastic and acoustic vibration propagation, and custom phonon–electron and phonon–photon interactions
To gain an understanding of the expected phononic structure and energy scale for a NC superlattice, we model it as a three-dimensional mass-spring system, where the NCs are the masses, m, and the surface terminating ligands are the springs providing a force constant, k, between neighboring NCs
In a masps-sffipffiffiffirffiffiiffinffi g mqodffiffieffiffilffiffi,ffiffitffiffihffiffieffiffiffiffieffiffinffiffiergy of the phonon modes will scale with k=m 1⁄4 nðrÞklig=m, where n(r) is the number of ligands providing an interaction between neighboring nanocrystals of radius r, and klig is the effective spring constant of a single ligand
Summary
Phonon engineering of solids enables the creation of materials with tailored heat-transfer properties, controlled elastic and acoustic vibration propagation, and custom phonon–electron and phonon–photon interactions. The inter-NC spacing and packing of the NCs into these superlattices can be tuned by the size and shape of the NCs and choice of ligand[1,14], with structures ranging from primary crystal structures (e.g., cubic[3,15], BCC14, FCC16, and hexagonal2) to complex binary systems (e.g., NaCl, MgZn2...)[5,17] This multi-parameter tunability can potentially be exploited to control the collective vibrational structure of the NC superlattice, which would enable the design of new materials via phonon engineering. We highlight that the design of long-range phonons in NC superlattices can be used to achieve materials with novel low temperature properties, and that the vibrational structure in NC superlattices enables them to be used as functional 2D and 3D acoustic metamaterials spanning energy and length scales not achievable using standard methods
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