Formation and electron diffraction studies of ordered 2-D and 3-D superlattices of amine-stabilized gold nanocrystals

Two- and three-dimensional superlattices formed from a family of amine-stabilized gold nanoparticles were investigated by transmission electron microscopy and selected area diffraction studies. The samples were prepared by ligand-exchange reactions between a phosphine-stabilized 1.5 nm precursor and pentadecylamine and exhibit metal core diameters ranging from 1.8 to similar to8 nm. Several of the observed specimens have surprisingly narrow core size dispersity and, as a consequence, form highly organized two- and three-dimensional superlattices. TEM imaging and electron diffraction studies are used to determine both the packing arrangement and the type and degree of order present in these superlattices. Smaller (d(CORE) similar to 1.8 nm) nanoparticles form three-dimensional fee superlattices, a surprising finding in light of the fact that the 1.4 nm precursors have not previously been observed to form such highly ordered superlattices. Narrow dispersity samples of larger nanoparticles (d(CORE) > 5 nm) form organized superlattices in both two and three dimensions. All of these samples exhibited at least translational ordering of the metal cores. In one class of nanoparticle (d(CORE) similar to 8 nm) electron diffraction studies provide evidence that the atomic lattices within neighboring nanoparticles are oriented in the same fashion (orientational ordering). The high degree of order found with these superlattices suggests it may be possible, with sufficiently monodisperse samples, to obtain single crystals of this family of nanoparticles.