Nanoscale Thermal Transport Phenomena in Superstructured Semiconductors

The underlying transport phenomena that dictate properties like thermal conductivity have been extensively investigated in small unit cell semiconductors and metals. However, more complex structures have received less attention. Here, we investigate heat transfer in superstructures, or materials with nanometer-scale periodicities that are longer than those seen in atomic crystals. We use a combination of theoretical, experimental, and modeling techniques to elucidate thermal transport mechanisms in superatomic structures, colloidal nanocrystals, and two-dimensional perovskites. Most notably, we use frequency-domain thermoreflectance to measure thermal conductivity in single crystal and thin film samples. Our results in certain superatom and perovskite materials are indicative of ordered-to-disordered phase transitions and coherent phonon transport. Our work highlights the tunability of all three groups of materials and emphasizes multifunctionalities that may be exploited in the design of next-generation photovoltaic, optoelectronic, and thermoelectric devices. Additionally, we shed light on phonon transport behaviors that result in ultralow thermal conductivities on the order of the lowest ever measured in fully-dense solids. Our results may be helpful towards a deeper understanding of thermal transport in superstructures as well as towards the development of novel technologies that are able to either functionalize or manage the heat transfer characteristics we observe.