BRIGE: Thermal Transport in Single-Domain Three-Dimensional Colloidal Nanocrystal Superlattices

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Abstract#1227979Wang, RobertColloidal nanocrystals are inorganic nanoparticles with organic ligand molecules bonded to their surface. These nanocrystals can self-assemble into periodic arrays due to the van der Waals interactions between their ligand molecules. In analogy to the atomic lattice of a crystal, these nanocrystal assemblies are termed nanocrystal superlattices. These superlattices are best known for their optical and electronic transport properties, however their thermal transport properties remain unexplored. I hypothesize that these superlattices should have an extremely low thermal conductivity because of: (i) a large interfacial density, (ii) a large acoustic impedance mismatch between the inorganic nanoparticle cores and organic ligand matrix, and (iii) filtering of lattice vibrations (i.e. phonons) resulting from its microstructure. A phonon is a quantum of crystal vibrational energy and is analogous to the photon, which is a quantum of electromagnetic energy. In non-metallic solids, phonons are the dominant mode of heat conduction. Do to the exquisite periodicity of these self-assembled nanocrystals, these superlattices should behave as phononic crystals. Phononic crystals are artificially structured materials with periodic variations in acoustic impedance. This periodicity results in a phononic band gap, which forbids the propagation of phonons in a particular energy range. The phononic crystal is analogous to the well-known photonic crystal, which uses periodic variations in refractive index to create a photonic band gap. Phononic crystals can be used for phonon filters, waveguides, resonators, and superlenses.To investigate these thermal transport hypotheses, I will (1) Synthesize monodisperse nanocrystals and assemble them into single-domain three-dimensional nanocrystal superlattices, (2) Experimentally measure their thermal conductivity as a function of nanocrystal size, (3) Numerically determine their phonon band structure, and thereby identify their potential as phononic crystals, and (4) Create a phonon spectroscopy apparatus to be used in future proposals to experimentally measure the phonon band gap of these superlattices as a function of nanocrystal size.Intellectual Merit: This proposal takes significant steps forward by investigating thermal transport in nanocrystal superlattices, which are an unexplored class of materials. It also numerically explores the phononic crystal properties of these superlattices and motivates future proposals to experimentally study this topic. Due to the small ~10 nm superlattice periodicity, their phononic band gap should be in the 100 GHz ? 1 THz range, which is 100x higher than the best-reported 3-dimensional phononic crystal. Due to their frequency-dependent phonon transport properties, these materials should have drastically different heat conduction properties than conventional solids. Furthermore, this type of phonon transport engineering could lead to significant advances in thermoelectricity and electronics thermal management.Broader Impacts: To enhance diversity as well as integrate research and education, I have developed a comprehensive outreach plan that addresses instruction at all education levels. This plan includes: (1) Engaging K-12 students via interactive presentations on-site in their classrooms that focus on thermal energy and related topics. Included in this presentation will be hands-on demonstrations that integrate research topics on thermal transport and thermoelectricity. (2) Designing a laboratory module on colloidal nanocrystal synthesis and characterization for incorporation into curriculum at the community college level. The integration of research tools such as transmission electron microscopy is included. (3) Recruiting individuals from underrepresented groups into my research and mentoring them toward their respective career goals. My plan for these initiatives includes metrics to gauge their effectiveness. For example, the K-12 initiative will be assessed using concept knowledge questions from the AAAS Science Assessment database. The questions in this database meet rigorous psychometric standards and have a large data set with which to compare the answers of the service recipients.