Gold Nanoparticle Superlattices: Conditions for Long-Range Order, Moiré Patterns, and Binary Phase from a Single Population

We report the two-dimensional self-assembly at the liquid–air interface of spherical gold nanoparticles (NPs) with diameters between 2 and 14 nm. By exploring the self-assembly conditions, such as the dispersing solvent and the coating ligand (thiols with different lengths, oleylamine, polystyrene), we identify suitable conditions for long-range close-packed monolayers obtention. We show that though NPs with diameters below 3 nm yield glassy films or fuse during self-assembly depending on the ligand length, NPs with larger sizes dispersed in toluene yield well-ordered monolayers over distances that can span tens of micrometers. Adding a free ligand in solution before the self-assembly triggers long-range ordering into close-packed structures of otherwise amorphous films. The equilibrium distance between the NPs within the monolayers is compared to predictions by packing models, and the OTM displays qualitative agreement. We also observed a CaCu 5 phase in few-layers assemblies which results from the size segregation of a monomodal population of NPs into two populations of different mean sizes occupying the two different sites of this complex lattice. In some instances, Moiré patterns consisting of two close-packed hexagonal monolayers superimposed with a twist angle are evidenced. By comparing the experimental structures with numerically simulated patterns, we show that a twist angle of 30° yields a quasicrystalline order with 12-fold rotational symmetry. Our work provides insights into fundamental processes behind the self-assembly of colloidal nanocrystals into ordered mono- and few-layers as well as more complex assemblies such as quasicrystalline or Frank–Kasper phases. These structures are of great significance for bottom-up fabrication of functional devices that take advantage of (collective) plasmonic properties or surface enhanced Raman scattering.