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Analytical Chemistry

Self-assembling Nano Corrals

Study reveals spontaneous reactions that alter surface electronic properties

by Mitch Jacoby
February 27, 2006 | A version of this story appeared in Volume 84, Issue 9

OK Corral
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Credit: Photo By Brenda Bury
Polanyi (far right), his group, and Hofer (foreground) relax following their recent discovery.
Credit: Photo By Brenda Bury
Polanyi (far right), his group, and Hofer (foreground) relax following their recent discovery.

It seems like a suburban fantasy that a homeowner would be able to buy a sturdy yard fence that sets itself up automatically. But self-assembling fences are now available in the lab, although in nanometer sizes only.

No Assembly Required
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On a silicon crystal, chlorododecane dimers form spontaneously (top), encircling an individual raised silicon atom (b) and altering its energy level, causing the atom, which would otherwise appear bright in the micrograph (a), to appear dark. Longer molecules (bottom) can corral more than one atom (b, b′). The dark appearance of other surface features (x) is unrelated to the corrals. (Blue = carbon, white = hydrogen, green = chlorine.)
On a silicon crystal, chlorododecane dimers form spontaneously (top), encircling an individual raised silicon atom (b) and altering its energy level, causing the atom, which would otherwise appear bright in the micrograph (a), to appear dark. Longer molecules (bottom) can corral more than one atom (b, b′). The dark appearance of other surface features (x) is unrelated to the corrals. (Blue = carbon, white = hydrogen, green = chlorine.)

Researchers in Canada have demonstrated that quantum corrals, fencelike collections of atoms or molecules that encircle other atoms and modify their properties, can form readily through self-assembly. The study deepens understanding of basic surface chemistry and may lead to new methods for constructing molecular electronics devices.

In a handful of investigations during the past several years, researchers showed that electronic properties of select atoms on a surface can be modified by surrounding the chosen atoms with a corral constructed of well-placed atoms. In one highly publicized experiment, scientists at IBM used a corral to conjure an atomic mirage, the appearance of a phantom atom inside the corral where no atom actually existed. The mirage resulted from the corral's influence on surface electronic waves. Other studies showed similar effects on electromagnetic waves.

A common feature in the earlier studies is that the corrals were painstakingly assembled on cryogenically cooled surfaces by manipulating the atoms one at a time with the tip of a scanning tunneling microscope (STM).

In contrast, the new work, a combined experimental and theoretical investigation, demonstrates that, at room temperature, haloalkane molecules deposited on a crystalline silicon surface can assemble spontaneously into circular dimers that function as corrals and remain stable up to 100 °C (Surf. Sci. 2006, 600, L43).

The study was conducted by chemistry professor and Nobel Laureate John C. Polanyi, Sergey Dobrin, K. Rajamma Harikumar, Peter A. Sloan, and their coworkers at the University of Toronto, and Serge Ayissi and Werner A. Hofer of the University of Liverpool, in England.

After exposing the silicon sample to 1-chlorododecane, the researchers imaged the surface with an STM and found that the molecules had formed dimers that encircled individual silicon adatoms, or raised atomic features found on the bumpy yet crystalline surface.

"The corrals capture surface electrons and pile up charge on the enclosed adatoms," Polanyi explains. The upshot is that the energy levels of the corralled atoms shift and thereby disappear from the STM's view. The effect also was observed with larger molecules that form dimers that encircle more than one atom.

The results "open exciting new perspectives for the fabrication and use of corrals," says Gérald Dujardin, a senior scientist at the University of Paris-South, in Orsay. Dujardin, whose remarks appear in an accompanying commentary in Surface Science, notes that the method may provide a way to trigger controlled adsorption of organic molecules on a semiconductor surface.

On the basis of low-temperature measurements, the Toronto-Liverpool team proposes a novel mechanism for self-assembly in which the haloalkane molecules "ski" across the surface in an upright configuration with the halogen end down, until they collide with other haloalkanes and form dimers. Like skiers, once the molecules collide, they fall over and become immobilized, Polanyi says.

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