Anomalous packing in thin nanoparticle supercrystals

We report a structural study of gold nanoparticle supercrystals formed by a few layers and of micrometer size, where the long-range order has been sti mulated by a soft thermal annealing. We have clearly identified an expansio n of the first layer lying on amorphous carbon substrate and an ordered sec ond layer sitting on twofold saddle points. The third nanoparticle layer re covers the conventional close-packed stacking. This anomalous packing has b een explained by means of a simple theoretical model based on dispersional forces.