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

We report the two-dimensional self-assembly at the liquid-airinterface of spherical gold nanoparticles (NPs) with diameters between2 and 14 nm. By exploring the self-assembly conditions, such as thedispersing solvent and the coating ligand (thiols with different lengths,oleylamine, polystyrene), we identify suitable conditions for long-rangeclose-packed monolayers obtention. We show that though NPs with diametersbelow 3 nm yield glassy films or fuse during self-assembly dependingon the ligand length, NPs with larger sizes dispersed in toluene yieldwell-ordered monolayers over distances that can span tens of micrometers.Adding a free ligand in solution before the self-assembly triggerslong-range ordering into close-packed structures of otherwise amorphousfilms. The equilibrium distance between the NPs within the monolayersis compared to predictions by packing models, and the OTM displaysqualitative agreement. We also observed a CaCu5 phase infew-layers assemblies which results from the size segregation of amonomodal population of NPs into two populations of different meansizes occupying the two different sites of this complex lattice. Insome instances, Moire patterns consisting of two close-packedhexagonal monolayers superimposed with a twist angle are evidenced.By comparing the experimental structures with numerically simulatedpatterns, we show that a twist angle of 30(circle) yields a quasicrystallineorder with 12-fold rotational symmetry. Our work provides insightsinto fundamental processes behind the self-assembly of colloidal nanocrystalsinto ordered mono- and few-layers as well as more complex assembliessuch as quasicrystalline or Frank-Kasper phases. These structuresare of great significance for bottom-up fabrication of functionaldevices that take advantage of (collective) plasmonic properties orsurface enhanced Raman scattering.