Self-Assembled Honeycomb

A random splattering of anthraquinone molecules onto a copper surface coalesces into a classic, large-pored honeycomb pattern. The behavior is a striking, unprecedented example of molecules' ability to form complex shapes seemingly without outside direction (Science 2006, 313, 961).

An immediate practical use for the self-assembly, which could be tailorable, might be mask-free templating for lithography or nanoparticle growth, say the researchers who discovered the phenomenon???associate professor of chemistry Ludwig Bartels, graduate student Greg Pawin, and colleagues at the University of California, Riverside.

Scanning tunneling microscopy reveals that the honeycomb has large, empty pores with diameters greater than 50 ??. Each pore exposes more than 200 of the surface copper atoms.

Calculations by Pawin indicate that the patterning results partly from short-range attractions of the hydrogen bonds between the molecules. But what makes the large pore size possible are long-range repulsions mediated by the surface, Bartels says. "That additional constraint of long range repulsions forming an ordered pattern has not been observed before."

Peter Beton, head of the school of physics and astronomy at the University of Nottingham, in England, calls the work "fascinating." He tells C&EN, "It will be very interesting to see if more general design rules for these arrangements will emerge." Such rules "might lead to the identification of other molecular species, or other substrates, which could be used to form similar arrangements."

The UC Riverside group had been studying surface "walking" of dithioanthracene molecules (C&EN Online Latest News, Sept. 27, 2005), which have a backbone similar to anthraquinone. But the anthraquinone discovery was unexpected. The pore size and shape should be controllable, the team members say, and they are continuing to experiment, adding groups such as methyl to the anthraquinone molecules to alter their attractive and repulsive properties.

Theodore L. Einstein, chair of the physical sciences program at the University of Maryland, who has studied interactions of atoms on metal surfaces for decades, tells C&EN, "I was genuinely delighted to read Bartels's manuscript and see this exciting development."