Building up: The chemical structure of lead chalcogenide nanocrystals and superlattices

Building new materials is one of the most important things a chemist does. During my research, we made crystals in 1, 2 and 3 dimensions, the superstructures also have a precise periodic nanogeometry. But instead of atoms as building blocks, we used pre-synthesized nanocrystals of PbX (X = S, Se, Te). In a conventional self-assembled nanocrystal solid, the long organic capping molecules form substantial electronic barriers between the nanocrystals, thus impeding electron transport. This is not a problem for applications of nanocrystals as phosphors, but long capping molecules will limit electronic and opto-electronic applications, such as electroluminescent devices, photo-detectors and solar cells. However, there are methods of self-assembly whereby the facets form atomic crystalline necks between the nanocrystals; this is called self-assembly followed by oriented attachment. During my doctoral research, I studied the nanocrystal building blocks in more detail and showed how we can use these building blocks to make larger superstructures with a periodic nanogeometry. First, we determined the size-dependence of the energy of the lowest optical transition for PbTe nanocrystals and established a formula to determine their concentration. Moreover, the binding coordination and the structure of the ligands on the surface of the nanocrystal was determined for the entire PbX nanocrystal family. In another study, we determined the precise shape of the PbSe nanocrystal core and showed that the shape of the crystalline core depends on the density of the ligands on the surface. We also showed how we can protect this type of nanocrystal from oxygen. We then also applied this protection to structures made from nanocrystals. To makes those structures, we developed a new ultra-slow evaporation method which resulted in much larger domains, a more selective synthesis of honeycomb structures and the possibility to create a new three-dimensional structure. In addition, we studied how PbSe nanocrystals assembled in a hexagonal nanocrystal bi-layer become more and more atomically aligned, and how such a hexagonal bilayer transforms gradually into a buckled silicene superstructure.