Oleg Konovalov2 Jaco Geuchies2 1 Carlo van Overbeek1 Wiel Evers3 4 Bart Goris5 Annick de Backer5 Freddy Rabouw1 Anjan Gantapara6 Andrei Petukhov7 8 Jan Hilhorst2 Joep Peters1 Marjolein Dijkstra6 Laurens Siebbeles3 Sandra van Aert5 Sara Bals5 Daniel Vanmaekelbergh1

2, Synchrotron Radiation Facility (ESRF), Grenoble, , France
1, Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, Utrecht University, Utrecht, 3584, Netherlands
3, Optoelectronic Materials Section, Department of Chemical Engineering, Delft University of Technology, Delft, , Netherlands
4, Kavli Institute of Nanoscience, Delft University of Technology, Delft, , Netherlands
5, Electron Microscopy for Materials Science (EMAT), University of Antwerp, Antwerpen, , Belgium
6, Soft Condensed Matter, Debye Institute for Nanomaterials Science, Utrecht, , Netherlands
7, Physical and Colloidal Chemistry, Debye Institute for Nanomaterials Science, Utrecht University, Utrecht, , Netherlands
8, Laboratory of Physical Chemistry, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, Eindhoven, , Netherlands

Oriented attachment of PbSe nanocubes can result in the formation of two-dimensional (2D) superstructures with long-range nanoscale and atomic order1,2. This questions the applicability of classic models in which the superlattice grows by first forming a nucleus, followed by sequential irreversible attachment of nanocrystals3,4, as one misaligned attachment would disrupt the 2D order beyond repair. Here, we demonstrate the formation mechanism of 2D PbSe superstructures with square geometry by using in situ grazing-incidence X-ray scattering (small angle and wide angle), ex situ electron microscopy, and Monte Carlo simulations. We observed nanocrystal adsorption at the liquid/gas interface, followed by the formation of a hexagonal nanocrystal monolayer. The hexagonal geometry transforms gradually through a pseudo-hexagonal phase into a phase with square order, driven by attractive interactions between the {100} planes perpendicular to the liquid substrate, which maximize facetto- facet overlap. The nanocrystals then attach atomically via a necking process, resulting in 2D square superlattices.