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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:343682 |
| Date | January 2000 |
| Creators | Fink, John |
| Publisher | University of Liverpool |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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