Structure and electronic properties of lead-selenide nanocrystal solids

Recent advances in the controlled formation of nanocrystal superlattices have potential for creating materials with properties by design. The ability to tune nanocrystal size, shape and composition as well as symmetry of the superlattice opens routes to new materials. Calculations of such materials predict interesting electronic phenomena including topological states and Dirac cones, however experimental support is lacking. We have investigated electron localization in nanocrystal superlattices using a combination of advanced structural characterization techniques and charge transport measurements.;Recent experimental efforts to improve the electronic properties of nanocrystal solids have focused on increasing inter-dot coupling. However, this approach only leads to electronic bands if the coupling energy can overcome energetic and translational disorder. We have investigated oriented-attachment as a method to create nanocrystal superlattices with increased coupling and translational order. We show that epitaxially connected superlattices form by a coherent phase transformation that is sensitive to structural defects and ligand length. In order to measure intrinsic electronic properties we demonstrate control over electronic defects by tailoring surface chemistry and device architecture.;To probe charge transport in these structures we performed variable temperature field-effect measurements. By integrating structure analysis, surface chemistry, and transport measurements we find that carriers are localized to a few superlattice constants due to disorder. Importantly, our analysis shows that greater delocalization is possible by optimizing dot-to-dot bonding, thus providing a path forward to create quantum dot solids in which theoretically predicted properties can be realized.