An interdisciplinary team of researchers at the Georgia Institute of Technology has isolated a new series of highly stable and massive gold-cluster molecules that possess a set of "extraordinary" quantum properties.
"With these properties, the molecules are very attractive building blocks for testing one type of ultra miniaturized architecture envisioned by some for 21st-century nanoelectronics, as well as for other chemical and molecular-biological applications," said Dr. Robert L. Whetten, Professor of Physics and Chemistry at Georgia Tech.
Supported by the National Science Foundation, the U.S. Office of Naval Research, the Packard Foundation, and the Georgia Tech Foundation, the work will be reported April 16 at the 213th National Meeting of the American Chemical Society in San Francisco.
Each molecule in the new series has a compact, crystalline gold core. This pure metallic core, just one-to-two billionths of a meter (1-2 nanometers) across, is encapsulated within a shell of tightly packed hydrocarbon chains linked to the core via sulfur atoms.
The principal members of the series have core-masses of about 14,000; 22,000 and 28,000 protons, corresponding to about 75, 110 and 145 gold atoms, respectively, and are thus in the same mass range as larger protein molecules, as reported by M. M. Alvarez and colleagues in a paper published recently in Chemical Physics Letters. These differ, both in size and the higher yield with which they are obtained, from their heavier analogs described in 1996 by Whetten and colleagues in Advanced Materials.
The precise structures of the cores are as yet unknown, but theoretical and experimental evidence suggests they are faceted with a particular gem-stone shape, as reported in a forthcoming paper by Whetten, Dr. Uzi Landman, and their co-workers in the Zeitschrift fer Physik.
"The surrounding chains can be of any length, and can be modified to confer particular chemical properties, so that they can be incorporated into various solid-state and solution structures," Whetten noted. "Most importantly, each member of the series behaves as a substance composed of infinitely replicated molecules, which can be separated from other members of the series to yield pure substances with precisely defined properties."
The gold cluster molecules emerge spontaneously during the decomposition of certain gold-thiolate polymers of the type commonly used in decorative gold paints and in gold anti-arthritis drugs. With sufficient control of the decomposition process, this series can be isolated without concurrent production of larger gold crystals. It is then relatively easy to separate the principal members of the series from each other to obtain the necessary homogeneity. Once purified, the molecules spontaneously assemble into crystalline thin films, powders, or macrocrystals, while preserving the discrete properties of the individual gold nanocrystal cores.
Gold is important technically not only for its inertness -- once made, the clusters are immune to corrosion -- but also for its highly stable surfaces, which find application as junctions in critical microelectronic applications.
"The main fascination of very small metal crystals, and the foundation for their envisioned use in future electronics, arises from the fact that their conduction electrons are quantized both in their number -- charge quantization -- and in the states they can occupy -- energy quantization," Whetten added. "In crystals larger than a few nanometers, these effects can only be observed and used at very low temperatures, such as that of liquid helium, near absolute zero. The new series of nanocrystals are both sufficiently small that these effects are prominent even at ordinary temperatures and yet are large enough to have the robust crystalline properties of the bulk metal."
The electromagnetic and conduction properties of the clusters are extremely sensitive to charging, and somewhat less so to energy level. Whetten believes these states can be used in a proposed electronic circuitry known as "single-electronics."
The new gold cluster materials are the first to exhibit the charge-quantization effect in a macroscopically obtained material, for which every cluster behaves identically. The first measurements were conducted in the laboratory of Dr. Phil First at Georgia Tech by observing the step-like changes in the current passing from a scanning tunneling microscope tip to a gold plate through a single gold cluster molecule as the voltage was increased.
The highly regular spacing between these steps, known as the "Coulomb staircase," showed that the molecules' gold core is charging like a small metal sphere in a series of discrete steps by adding or removing single electrons.
The quantization of the energy levels of the conduction electrons is observed separately in optical spectroscopy experiments -- the spectrum of transmitted visible or infrared light -- that reveals discrete level structure, even at room temperature.
Research in the area of nanometer-scale molecular materials is highly interdisciplinary, requiring the skills of many diverse researchers and facilities. The molecular gold materials have been developed in Whetten's Georgia Tech laboratory, as guided also by the theoretical predictions and modeling of Landman's Center for Computational Materials Science.
They were characterized in the Georgia Tech Facility for High Resolution Microscopy, directed by Dr. Z. L. Wang, in the X-ray facilities at Georgia Tech of Dr. Angus Wilkinson and the National Synchrotron Light Source by Dr. Peter W. Stephens of the State University of New York-Stony Brook. The research in Whetten's laboratory has been carried out by a team of graduate students including Marcos M. Alvarez, Joseph Khoury, Greg Schaaff, Marat Shafigullin, Brian Salisbury, and Igor Vezmar.
VISUALS: Color photographs of laboratory work in isolating the gold clusters are available