Nanocrystal superlattices as phonon-engineered solids and acoustic metamaterials

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.

Ligand Dynamics in Nanocrystal Solids Studied with Quasi-Elastic Neutron Scattering

“…The NC surface and its ligands thus have a significant effect on how NCs are processed (e.g., their solvation, stability in solution, and assembly from colloidal form into solids) , and on their functionality (e.g., their catalytic, electronic, optical, phononic, and thermal performance). − For example, for NCs used in catalysis, the surface ligands can impact the catalyst energy efficiency and selectivity. − In the case of (opto)­electronics, the type of ligand as well as its orientation determines the surface electrical dipole of individual NCs. This tunes the energy position of the electronic states of individual NCs as well as their assemblies and thus optical absorption and emission. , Furthermore, the electronic transport properties of NC solids depend on the electronic wave function confinement of individual NCs as well as the NC packing in the NC solid (facet-to-facet alignment and spacing), all of which are determined by the ligands. , Mechanical, phononic, and thermal conductivity of solids assembled from NCs are governed by the number of ligand interconnections, their orientation, and the elasticity. − These examples illustrate the importance of ligands in optimizing the performance of NCs and NC thin films for different applications.…”

“…Information on the crystal structure is contained in elastic scattering, for which there is no energy transfer, S ( Q , E = 0) . Inelastic neutron scattering (INS) spectra have been used to obtain information on a number of vibrational processes in NCs, ranging from interparticle vibrations to soft surface modes, and phonons in the nanocrystal core. , Scattering centered around the elastic line at a low energy transfer of micro- to millielectronvolts is referred to as quasi-elastic neutron scattering (QENS). To date, sparsely used for studying ligands on NCs, it probes the individual, diffusive motion of atoms (in this case hydrogen) and can thus be used to develop and validate models for ligand dynamics.…”

Extracellular Vesicles in CNS Developmental Disorders

“…In the specific case of neurons, exosomes are released from post-synaptic soma or dendrites [ 24 ], and they mediate several processes, such as the maintenance of homeostasis by triggering synapse pruning by microglial cells [ 30 ], or the outflow of molecular information to neighboring cells, mediated by miRNAs. These miRNAs may induce gene expression in recipient cells in an activity-dependent manner [ 31 ]; as previously described, miR-124 internalized by astrocytes is capable of regulating the glutamate transporter (GLT1) levels, as well as glutamate uptake in the brain [ 23 ].…”

“…In a more recent breakthrough, Men et al have demonstrated that the miRNA profile of secreted exosomes is different from the one observed in live neuronal cells [ 24 ]. By generating a cell-type-specific ILVs/exosome reporter (CD63-GFPf/f) in mice, the authors observed that an undescribed neuron-specific miRNA, miR-124-3p, was internalized into astrocytes, also upregulating the glutamate transporter GLT1 [ 24 ].…”

Interconnection of organic–inorganic hybrid nano-building blocks towards thermally robust mesoporous structures

“…Organic-molecules-adsorbed cubic nanoparticles have been assembled to form nanoparticle bilayers and show tandem catalytic properties. 13 Superlattices of molecule-adsorbed nanoparticles have been investigated as optical, 14 phonon-engineering, 15 and lithium-ion storage materials, 16 owing to the nano scale periodicity.…”