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 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 are compared to predictions by the OPM and OCM geometrical models. We also observed a CaCu5 phase in few-layers assemblies which results from the size segregation of a single monodisperse 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 a 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 advantage of (collective) plasmonics properties or surface enhanced raman scattering.