Coaxing atomic-scale building blocks into technologically useful shapes can be so frustrating that researchers are sometimes forced to rely on magic. One product they’re after is a crystal surface studded with evenly spaced, identical clusters of metal atoms for use in microelectronics, information storage, and nanoscale chemical reactors. Such clusters can spontaneously assemble themselves in small groups, but not normally in large quantities. Now a team reports in the 11 February print issue of PRL that they have employed a touch of magic to create one of the first large arrays of metallic “nanoclusters” that are both well ordered and uniform in size.
In the early 1980s, physicists discovered that gaseous metal atoms coalesce into globs of particularly stable, or “magic,” numbers of atoms. More recently, they have found a similar effect at work on surfaces, where vapor atoms can stick in clusters of specific sizes. In December, a group in Taiwan reported scanning tunneling microscopy evidence of large arrays of these nanoclusters. They used the highly ordered atoms of a silicon crystal as a kind of template to evenly spread magic-sized gallium clusters across a surface .
A team led by Qi-Kun Xue of the Chinese Academy of Sciences in Beijing found that tweaking the way they evaporated metal atoms onto the same kind of silicon crystal gave them arrays made of various elements. They estimate that perhaps 1011 uniform clusters formed over the entire 1.5 by 4 mm surface, according to images from a scanning tunneling microscope. The precise pattern depended upon the mix of elements the team used from among indium, manganese, and silver.
Each cluster formed in the same half of the silicon crystal unit cell, leaving the other half empty, but only if the surface temperature was between 100 and 200 degrees Celsius and the rate of arriving metal atoms was not too high. According to the team’s computer calculations, these conditions insured that the arriving atoms had time to move around on the surface and find their lowest energy configuration. If the surface was too cold or atoms arrived too frequently, some clumped and became trapped in the wrong half of the silicon unit cell.
The team originally thought they would need charged atoms to force all of the clusters into one half or the other. “To our surprise, before we tried the [charged atom] idea, they were already ordered correctly,” explains team member Zhenyu Zhang of Oak Ridge National Laboratory in Tennessee. “Then we realized most of the driving force is provided by this very amazing [periodic] surface structure.”
Arrays of such 1 to 2 nm clusters are ideal for advanced optical and electronic devices, Zhang says, because electrons confined in such spaces ought to produce photons of a useful wavelength. If the clusters are magnetized, they might be used for ultra-dense information storage media, spintronics devices, or catalysts for chemical reactions, he adds, though such devices are still futuristic.
Dongmin Chen of the Rowland Institute for Science in Cambridge, MA, says this marriage of the template and magic number concepts has resulted in a “remarkable” example of self-assembly. “I think it’s a significant step forward.” He speculates that there may even be some new physics involved: Individual silicon atoms guide the metal atoms, but perhaps whole clusters exert an influence on adjacent ones as well. While that question is too complicated for computer simulations to handle at the moment, he says, the new results are in themselves “a very nice gift that we got from nature.”
- M. Y. Lai and Y. L. Wang, Phys. Rev. B 64, 241404 (2001).