Structure, properties and self-assembly of gold and silver nanoparticles.
Non-aqueous dispersions of colloidal gold and silver clusters have been prepared
by the reduction of HAuCl4 and AgN03 solutions with NaBH4 in the presence of
protecting agents such as quaternary ammonium salts and alkanethiol. The resulting
clusters were then investigated both in solution, and in thin film form when deposited
on TEM grids. Thiol-derivatised particles form dark brown residues upon removal of
the solvent which can easily be re-dispersed in low dielectric solvents such as toluene or
pentane. Such exceptional stability has led to the name cluster compounds.
The growing interest in nanophase systems has been motivated both by scientific
interest in their unique opto-electronic properties which show a marked deviation from
bulk behaviour, but also from their potential applications in the fields of opto-electronic
devices. Such applications will inevitably require long range order and control of such
nanoparticle assemblies and it is with this in mind that the present work is based.
A drop of such a colloidal solution when allowed to evaporate onto a carbon
coated grid results in the formation of regular self-assembled structures which were then
examined using transmission electron microscopy (TEM). Monodisperse gold and silver
particles show a tendency to self-assemble into highly ordered pseudo-hexagonal closepacked
rafts, in which the interparticle separation can be varied according to the size of
the stabilising ligand attached to the gold or silver surface. It is also shown that gold
particles can, in addition to conventional f.c.c., h.c.p. stacking in 2D structures, form
unusual non-closed packed ring and line like structures.
Ordered superlattices composed of bimodal particle size distributions of either
gold or silver, or gold-silver binary mixtures, have also been observed. Such structures
represent the first example of superlattice ordering on the nanometre scale which, up to
now, has only been observed in micrometre scale colloidal crystallites and in atomic
scale intermetallic alloys. Moreover, it is shown how the underlying principles for
intermetallic alloy formation can readily be extended to explain the formation
nanoparticle superstructures in terms of well known phase rules, thus drawing a
comparison between the two ordered states.