(558b) Thermal Transport in Nanocrystal Arrays and Self Assembled Monolayers

The thermal conductivity of nanocrystal arrays (NCAs) is found to be tunable with chemistry and the nanocrystal diameter. Nanocrystals (inorganic crystalline cores 2-10 nm in size encapsulated in organic monolayers) self-assemble from colloidal suspensions into close-packed 3D NCA films. These films exhibit tunable electronic and optical properties that can meet various demands in energy conversion and optoelectronic applications. When coupled with scalable solution-based fabrication processes, these properties make NCAs a cheap and versatile replacement for conventional single crystal semi-conductors and nanostructured materials.

Though electronic transport in NCAs has been studied extensively, little is known, about the nature of thermal transport in these complicated materials. We show that the thermal conductivity of NCAs shows independence from the bulk core thermal conductivity, yet is tunable from 0.1-0.3 W/m-K by increasing the nanocrystal diameter. Effective medium approximations (EMA) modified by incorporating a fintite thermal conductance at the core ligand interface correctly captures the measured diameter-dependent magnitudes and trends. Changing the chemistry of the nanocrystal (i.e., different core and ligand species) enables further control for manipulating the NCA thermal conductivity. To learn more about thermal transport at this interface we have directly studied the thermal conductance of a series of self assembled monolayer (SAM) junctions. By considering various metal-SAM-metal junctions we find that the interface conductance is influenced by both the SAM-metal bond and the overlap of their vibrational states. Molecular dynamics simulations of NCAs and SAMs help us to sort out these effects and how they more generally apply at the organic-inorganic interface.