'Superatoms' Open Window to Nanoparticle Chemistry
The principles behind the stability and electronic properties of metallic gold nanoclusters have been described, opening the door to new applications for materials, superconductivity and even self-assembly.
Alexander E. Braun, Senior Editor -- Semiconductor International, 8/7/2008
The principles behind the stability and electronic properties of miniscule nanoclusters of magnetic gold have been analyzed and described in a seminal study by researchers from the Georgia Institute of Technology (Atlanta), Stanford University (Palo Alto, Calif.), the University of Jyväskylä (Finland) and Chalmers University of Technology (Göterborg, Sweden). Gold and sulfur atoms aggregate in specific numbers and extremely symmetrical geometries, and can mimic the chemistry of single atoms of a different element. Researchers discovered that these clusters are stable because they behave like “superatoms” and exhibit a “divide and protect” bonding structure.
According to Robert Whetten, a professor at Georgia Tech’s School of Physics and School of Chemistry and Biochemistry, although gold nanoparticles are widely used in many fields, nobody fully understood their molecular structures and physiochemical properties. Researchers use gold nanoparticles because of their stability and distinct optical, electronic, electrochemical and biolabeling properties.
In 2007, Stanford researchers initially reported the first determination of a 102-atom gold cluster. Their X-ray structure study showed that pairs of organic sulfur (thiolate) groups extract gold atoms from the gold layer to form a linear thiolate-gold-thiolate bridge while weakly interacting with the underlying metal surface. This formed a “protective crust” around the nanoparticles, contradicting the belief that the sulfur atom merely sat atop the uppermost gold layer, bound to three adjacent metal atoms. This was the first time that atomic coordinates — the location of all of the atoms in one of these clusters — became available.
The 102-atom gold cluster superatom has a core of 79 atoms arranged into a truncated decahedron. Around this core, 23 gold atoms joined the thiolates. Results confirmed the “divide and protect” structure, showing that gold atoms divided themselves into two groups — those making up the metal core and those that protected it.
| Spacefilling (a) and ball-and-stick (b) representations of a 102-atom gold nanoparticle. A 79-atom gold core surface with a 23-atom protective layer is shown in c and d. Close-ups of protective layer units (e) and a 79-atom core (f, g). (Source: Georgia Institute of Technology) |
In the cluster, each gold atom donated one valence electron. Forty-four of these electrons were immobilized in bonds between gold atoms and thiolates, leaving 58 to fill a shell around the superatom. Because the cluster does not have to add or shed electrons, its structure is stable. A major energy gap was also exhibited. “The calculated energy gap between the highest occupied molecular orbit and the lowest unoccupied molecular orbital states for the 102-atom compound was significant: 0.5 eV. Metals typically have a gap of zero,” Whetten said, adding that this indicates an atypical electronic stability.
There had been speculation for years about the nature of the chemical bonding, structure and electronic principles holding such a structure together. Nobody fully understood how common sulfur-containing molecules bind to metallic gold; it was not known that such a large metallic cluster could attain molecular precision instead of just forming into an arbitrary aggregate. Amazingly, little was understood of the common and practical metallic chemistry of films like gold or silver. As Whetten put it, “The work provides a basis for the understanding of distinct electrical, optical and chemical properties of the stable monolayer-protected gold nanoclusters. We now have a model for nanoengineering ligand-protected gold clusters for application in catalysis, sensing photonics, biolabeling and molecular electronics.”
The research opens a door into another startling possibility. “The 58 electrons in this complex behave as if orbital details didn’t matter and as if they were part of a large — super — atom that follows its own electronic shell structure,” Whetten said. “These electrons fall into a nice pattern and open a large gap between the highest occupied energy level and the lowest unoccupied. Similar to what one finds in the Periodic Table if you go from neon with its 10 electrons to sodium with 1; there’s greater electronic stability.”
Simplicity emerges out of complexity. It may be possible to produce for superatoms composing metallic nanoparticle clusters something akin to the familiar Periodic Table. This would lead to unparalleled precision in the use of these structures. Copper and silver immediately come to mind. The principle also applies to aluminum, indium and gallium clusters, as well as their alloys, intermetallic compounds, plated or gilded combinations. Platinum and palladium would work well in a minority combination with higher conductivity metals.
Semiconductor applications for areas such as interconnect are obvious, particularly when dealing with fundamental limitations when shrinking lines. Work so far carried out by the researchers indicates that elements such as copper and gold have characteristic IR oscillation frequencies, which means they can strongly absorb or radiate IR energy. If the radiating frequencies are determined, it may be possible to suppress this.
Whetten expects that it will be possible to reach a level of precision of application with these metallic systems, as the one attained after buckyballs and fullerenes were finally produced. This resulted in superconductivity advances, carbon nanotubes, and more. Researchers now expect to gain understanding of the chemistry and electronic properties of related structures, such as flat crystalline surfaces of gold. Thousands of atoms may become defined.
According to Whetten, some think that crystalline clusters of niobium and gold are superconducting at much higher temperatures than what is available. This has been inferred from sophisticated calculations, and a decent sample of this alloy is yet to be generated. “There’s no way to determine what’ll come of it,” Whetten said. “There might even be unthought-of self-assembly applications.”
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The finding that large gold chalcogen clusters have unique stable sizes and structures is very...
Don Rasmussen - 2008-7-8 19:32:00
