Superstructure assembly 2D by oriented attachment of PbX nanocrystals

Abstract

The current interest in materials with a nanoscopic 2D honeycomb geometry is due to their unique optoelectronic properties. In previous research in our group, we have shown that oriented attachment of colloidal lead selenide (PbSe) nanocrystals can result in semiconducting materials that possess such a honeycomb geometry and that have a long-range nanoscale and atomic order. This questions the applicability of classic models in which the superstructure grows by first forming a nucleus, followed by sequential irreversible attachment of nanocrystals, as one misaligned irreversible attachment would disrupt the 2D order beyond repair. In the present thesis, we investigate the synthesis and formation mechanism of these materials We found that reproducibly synthesizing these materials requires extremely precise chemical and thermodynamic tuning of the reaction vessel. Particularly the presence of oxidizing agents, a few ppm of oxygen in the glovebox atmosphere is enough, can profoundly disrupt the formation of a superstructure. Nevertheless, we succeeded in increasing the number of building block options for the oriented attachment procedure from only PbSe to all lead chalcogenide (PbX, X = S, Se or Te) nanocrystals. All these different PbX nanocrystals can form the same 2D honeycomb structure and other 2D or 1D nanoperiodic structures, such as square, linear and zigzag structures. The synthesis of these PbX superstructures may be combined with cation exchange under preservation of the nanogeometry. This way, materials might be made that possess both the exotic optoelectronic properties of the geometry and many different material properties, e.g. the high spin-orbit coupling of HgX compounds or the optical bandgap of CdX compounds. Furthermore, we investigated the formation mechanism of 2D PbSe superstructures with a square geometry, the most reproducible superstructure to synthesize. This was done with in situ grazing-incidence X-ray scattering, ex situ electron microscopy, and Monte Carlo simulations. We observed a remarkable series of intermediate stages in the formation of these superstructures: from nanocrystal interfacial adsorption, to the formation of a hexagonal phase that transforms into a square phase, finally followed by crystal attachment. Form further scattering studies and trends in the superstructure synthesis outcomes, a tentative formation mechanism of the honeycomb materials is also presented.