The self-assembly of nanoparticles (NPs) is evident all around us. Their assembled structures are complex, often astonishing, and comparable to that seen in biological self-organized systems. Yet our understanding of NP assembly processes is nascent, and their studies bring many surprises. By combining experiment and simulation, we have discovered and explained the assembly of NPs into mesoscale structures of a variety of shapes, including chains, sheets, capsules, supraparticles, stars, twisted ribbons, superlattices, etc. By comparing and contrasting the various observed structures and the processes under which they form, we are able to extract fundamental mechanisms of assembly. We will discuss two general classes of assembly: “terminal” structures that grow to a self-limiting size, and "extended" structures that grow continuously along specific directions. The distinction between these two cases will be made based on the balance of attractive and repulsive interactions between NPs using phase diagrams. Biomimetic, chiral, bionic and epitaxial assemblies will be presented, with applications from electronic devices to catalysis.
With more than two-thirds of utilized energy being lost as waste heat, there is a compelling motivation for high performance thermoelectric materials that can directly convert heat to electrical energy. However, over the decades practical realization of thermoelectric materials has been limited by hitherto low figure of merit, ZT, which governs the Carnot efficiency. This talk will describe our long-standing efforts to advance the ZT to record levels starting from exploratory synthesis and evolving into the nanostructuring and panoscopic paradigm, which has ushered in a new era of investigation for thermoelectrics. Like in any other energy conversion technology involving materials thermoelectrics research is a challenging exercise in taming "contra-indicated" properties. Critical properties such as high electrical conductivity, thermoelectric power, low thermal conductivity, and mechanical strength do not tend to favor coexistence in a single material. I will describe how these can be achieved in certain systems leading to records in ZT. Endotaxial nanostructures and mesoscale engineering in thermoelectrics enable effective phonon scattering with negligible electron scattering. By combining all relevant length scales hierarchically we can achieve large enhancements in thermoelectric performance. The field however continues to produce surprises.