Colloidal quantum dots, prepared via the hot-injection synthesis, are established as promising materials for technological applications due to their cheap synthesis and tunable optical properties. The synthesis uses organic molecules to control their size and to make them dispersible in organic solvents.  However, the electronic barrier imposed by the ligand shell is problematic for applications relying on charge transport, such as photodetectors. A possible solution to this problem is the use of nanocrystal superlattices where individual nanocrystals are epitaxially connected. In this way, charge carriers can freely move through the structure while at the same time preserving the quantum confinement. Although these structures represent a next step towards the implementation of quantum dots in a variety of applications, their precise formation is currently not very well understood.

Here, we report on a study aimed at gaining insight into how 2D superlattices of lead chalcogenide nanocrystals form by focusing on the reactions occurring at the nanocrystal surface during nanocrystal attachment. The work started from the assumption that the formation of 2D superlattices requires (a) the removal of the originally present organic ligands and (b) the presence of stoichiometric and complementary crystal facets. We show that this can be accomplished in two different ways. A first involves exchanging, for example, the original oleate ligands by organic moieties that render the nanocrystal surface ligand free and stoichiometric, such as S^2- or Se^2-. As confirmed by TEM and UV-Vis analysis, this induced oriented attachment with a concomitant red shift of the first exciton transition – reflecting the growth of the nanocrystals by inclusion of the chalcogenides. A second is based on the stripping of lead oleate complexes, thereby exposing the underlying PbX crystal facet. This is accomplished by exposing PbX nanocrystals to Lewis bases such as amines which coordinate with the electrophilic lead ions. By this approach, the rate of ligand removal can be adjusted by changing the basicity of the amines or by enhancing (or reducing) steric hindrance. In this way, large area mono- or multilayer formation can be decoupled from the actual nanocrystal attachement, an approach that may lead to the formation of large area 2D PbX superlattices with controlled geometry.