Thermal Properties of Organic-Inorganic Materials Superstructured at the Nanoscale
The thermal properties of nanocrystal arrays and large unit cell molecular crystals are studied using experimental and computational techniques. The major objective is to understand the mechanisms of thermal transport through three-dimensional organic-inorganic superstructured materials that are built from superatoms. The frequency domain thermoreflectance technique is applied to measure thermal conductivity in thin films and nanoliter sized single crystals. Molecular dynamics simulations, lattice dynamics calculations, and density functional theory calculations are employed to interpret the measurements and to explore experimentally-inaccessible nanoscale phenomena. A superatom is a cluster of atoms that behaves as a stable or metastable unit with emergent properties distinct from its elemental atoms. Superatoms can self-assemble into three-dimensional hierarchical materials with each superatom occupying a lattice site to form a periodic solid (i.e., a superlattice). By varying geometry and composition, the resulting solid can have tunable electrical and optical properties. This work presents the first investigation of the thermal properties of solids built from two classes of superatoms: (i) monodispersed nanocrystals that form a nanocrystal array (NCA) and (ii) inorganic-organic superatoms of precise stoichiometric composition that form a molecular crystal (LUMC). The thermal conductivity of NCAs was measured to be between 0.1 to 0.3 W/m-K. Experiments revealed that energy transport is mediated by the density and chemistry of the organic/inorganic interfaces as well as the volume fractions of nanocrystal cores and surface ligands. The NCA thermal conductivity trends upward then plateaus with increasing temperature suggesting elastic scattering events dominate transport at the organic-inorganic interfaces. The onset temperature of the plateau is dependent on the overlap of the vibrational states in the core and ligand. Atomistic computational analysis of the thermal transport explored experimentally inaccessible trends that provided new insights for controlling heat flow in NCAs. A decreasing interfacial thermal conductance trend for the organic-inorganic interface with increasing nanocrystal diameter was discovered. This trend can be related to the interfacial thermal conductance of a self-assembled monolayer (SAM) interface through a geometrical scaling law. Changing the atomic mass of the nanocrystal core to vary its vibrational states resulted in a non-monotonic trend in both the thermal conductivity and interfacial thermal conductance. Peaks in both properties occur at the same small atomic mass and are related to the overlap and coupling of the organic and inorganic vibrational states. Preliminary measurements of LUMCs indicate that the thermal conductivity is between 0.2 and 0.4 W/mK at the temperature of 300 K, comparable to that of an amorphous polymer. A slight increase in thermal conductivity is observed for the binary-species LUMCs that contain fullerene derivatives over their corresponding mono-species LUMCs composed of inorganic core superatoms. This increase may be attributed to the stronger ionic intermolecular bonds in the binary LUMCs. The presence of a larger chalcogenide element in the inorganic core superatom decreases the thermal conductivity of the LUMC. This decrease is consistent with lower frequency vibrational modes that have a lower group velocity in the crystal. The temperature dependent thermal conductivity of a mono-species CoSe LUMC has a crystalline-like behavior, unlike most low thermal conductivity materials. Nanoscale superstructured organic-inorganic materials self-assemble from solutions and can be scalable to replace single crystal semiconductors for many technologies. Arrays of ligand-stabilized colloidal nanocrystals with size-tunable electronic structure are promising alternatives to single-crystal semiconductors in electronic, opto-electronic, and energy-related applications. The low thermal conductivity in the NCA presents a challenge for thermal management but a boon for thermoelectric waste heat scavenging. The class of large unit cell molecular crystals investigated here have low thermal conductivity and a moderate electrical conductivity, making them novel candidates for thermoelectricity.