Assembling nanocrystal superstructures

At the nanoscale, electronic materials can develop size-dependent properties due to quantum-confinement effects. This is the case for semiconductor nanocrystals, or quantum dots (QDs). In these materials, the confinement of charge carriers leads to a size-dependent electronic bandgap; consequently, the light absorption and emission properties of QDs are tunable over a wide spectral range by simply changing their size. Furthermore, QDs are synthesized as a colloidal dispersion, making their use compatible with inexpensive and efficient roll-to-roll printing technologies. The unique photophysics of QDs, together with their ease of fabrication and processability, make them promising candidates as building blocks for novel optoelectronic devices, such as solar cells, photodetectors, light emitting diodes, and lasers. However, to be used as active layer in a device, QDs must be assembled to form macroscopic solids.
In this thesis, we propose novel approaches to drive the assembly of QDs into programmable artificial solids with morphology-defined properties. By exploiting an emulsion-templated approach, we direct the assembly of QDs into spherical crystals of nanocrystals, or supercrystals. These mesoscopic objects feature enhanced light-matter interaction, increasing the potential of QDs for applications. Furthermore, we show that QDs crystallize similarly to hard spheres, when taking into account the role of surface ligands. To explore further the role of these surface-bound molecules, we use colloidal nanoplatelets as a model system and show that surface repassivation can result in either structural damage or effective reconstruction depending on experimental conditions. Finally, we demonstrate the use of a novel colloidal interaction, the critical Casimir effect, to drive the growth of QD superstructures of morphologies varying from 3D fractals to 2D epitaxial monolayers.