INTERDISCIPLINARY RESEARCH
Striking Gold
New materials are the first to exhibit charge-quantization effects in a macroscopically obtained material, for which every cluster behaves identically.
By John Toon
AN INTERDISCIPLINARY TEAM of researchers at the Georgia Institute of Technology has isolated a new series of highly stable and massive gold-cluster molecules possessing 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,"
photo by Stanley Leary
Graduate Students Igor Vezmar and Joseph Khoury
use a high-mass spectrometer to analyze the new series of gold clusters.
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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 to 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 7,500, 14,000, 22,000 and 28,000 protons, corresponding to about 38, 75, 110 and 145 gold atoms, respectively. They are thus in the same mass range as larger protein molecules, as reported by M. M. Alvarez, T.G. Schaaff and colleagues in papers published recently in Chemical Physics Letters and the Journal of Physical Chemistry. 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 gemstone shape, as reported in a recent paper by Whetten, Dr. Uzi Landman, and their co- workers in the Physical Review Letters.
"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, or even in a peptide modification, into aqueous media," Whetten notes. "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."
This optical microscope image shows a 150-micron
crystal grown from 1.7 nanometer gold nanocrystals.
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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 adds. "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 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," and will be tested for other applications in photoelectrochemistry and catalysis where their superior robustness may have advantages.
photo courtesy Robert Whetten
Each molecule in
the new series, an array of which is shown here via
scanning tunneling microscope (STM), has a compact, crystalline gold core. A
shell of tightly packed hydrocarbon chains surrounds the core and is linked to it
via sulfur atoms.
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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.
Subsequent electrochemical measurements on these clusters found an "ensemble Coulomb staircase" involving billions of identically behaving clusters, as reported by Whetten and collaborators at the University of North Carolina at Chapel Hill in the Journal of the American Chemical Society.
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 Electron Microscopy Facility, 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.
The work was reported April 16 at the 213th National Meeting of the American Chemical Society in San Francisco.
Further information is available from Dr. Robert Whetten, School of Physics,
Georgia Institute of Technology, Atlanta, GA 30332-0430.
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Last updated: Dec. 3, 1997