by Professor Donald Bethell, University of Liverpool
Over the past 50 years the need for ever faster computers has driven down the size of electronic components and devices by an order of magnitude roughly every decade. During the same period there has been a spiralling need for the delivery of applicable electronic and optical effects and benefits. This is prompting intense worldwide competition between laboratories to discover new materials, particularly ones capable of fabrication into nanometre-scale structures.
A group of scientists at Liverpool University, led by David Schiffrin and Don Bethell of the Department of Chemistry, have recently described a new class of materials having the potential for a wide range of such applications.
These new materials are based on uniformly sized gold particles a few millionths of a millimetre across. In this size range, the gold begins to lose the characteristics associated with the bulk metal and begins to behave in remarkable ways as a result of what are termed quantum size effects.
In the Liverpool process, the nanometre-sized gold particles are generated chemically by the reaction of a gold salt and sodium borohydride in the presence of organic molecules bearing sulfur-containing thiol groups in a two-phase liquid system. The thiols self-assemble on the surface of the metallic gold as it is produced, restricting the growth of the metal particles and forming a sheath of organic molecules around them. This procedure gives highly stable material that can be readily handled and processed either in the solid state or in solution in common organic solvents.
Sinaple chemical transformations can be carried out on the surface of the sheath. Even in the absence of thiols, solutions of gold particles of only slightly larger size can be prepared, but the metal is irreversibly precipitated as the solvent is evaporated.
Simple adaptations of the basic method of thiol selfassembly, particularly using organic molecules bearing thiol groups at opposite ends, allows the construction of organized arrays of the gold particles into one-, two- and three-dimensional structures.
The techniques of electron microscopy and scanning tunnelling microscopy have been used to demonstrate the structural details of the materials so formed, including the dimensions of the gold particles.
The electrical conductivity of these materials is in the range typical of semiconductors and, as for them, it increases as the temperature rises. It also depends sensitively on the nature of the thiol spacers, especially their length and the presence within the spacer molecule of groupings capable of readily accepting or giving up an electron.
The pattern of behaviour arises because the conduction of electrons from one gold particle to the next takes place by a so-called activated hopping mechanism. From a knowledge of the size of the gold particles and the structure of the intervening spacers, the conductivity behaviour can be predicted, opening the way to the production of materials of predetermined conductivity.
Moreover, if the spacer accepts or gives up an electron to an external probe, electron transfer between the adjacent gold particles will be affected. This combination constitutes a miniature electronic device built on a nanometre scale, that is to say, some ten to one hundred times smaller than present-day electronic components.
The same basic procedures for self-assembly can be used to lay down on specially treated glass or metal surfaces monolayers or multilayers of the materials. These layers can be used to control the reflectance of the surface by application of an appropriate electrical potential across the layer.
Again, the nature of the organic spacer units separating the gold particles changes the response; in particular, the wavelength dependence of the reflectance can be modified. Furthermore, when visible light falls on such layers, in a suitable arrangement, electricity is generated.
Gold electrodes treated alternately with layers of an organic compound having thiol groupings at each end and layers of nanometre-sized gold particles allow control of the rates of electron transfer in electrochemical reactions.
When the electrode is simply coated with the dithiol, the electrode is inactive when a potential is applied; an electron-accepting system in contact with the electrode is unaffected.
Attachment to the surface thiol groups of a single layer of gold nanoparticles makes the electrode active again. This behaviour continues as the alternating layers are built up. Although this behaviour is not fully understood, it emphasises the remarkable nature of these materials.
Recent work is aimed at extending these results to electrocatalysed reactions of industrial importance, such as those occurring in fuel cells.
Many practical applications of these novel materials can be envisaged, ranging from sub-microelectronic circuitry to so-called 'smart' windows.
By choice of the organic spacers and metal particle size, the properties of the material can be matched to the application.
Although investigations have so far concentrated on materials containing gold particles, an exciting aspect of the discoveries of the Liverpool group is the possibility of extending the elegant and simple synthetic strategy to many different metals besides gold and to nanometre-sized semiconductor particles. It is expected that this will lead to a multitude of new practical applications.
For more information contact:
Department of Chemistry
University of Liverpool
P.O. Box 147,
Liverpool, United Kingdom, L69 3BX
Professor Donald Bethell
tel: +44 151 794 3508, fax: +44 151 794 3508
Professor David J. Schiffrin
tel: +44 151 794 3574, fax: +44 151