Theory 'Stands Self-Assembly on Its Head'

PRINCETON, N.J., Jan. 4, 2006 -- A scientist at Princeton University is proposing a theory that could turn a central concept of nanotechnology on its head. If it bears out, it could have radical implications for the computer and telecommunications industries.
Salvatore Torquato and colleagues have outlined a mathematical approach that would enable them to produce desired configurations of nanoparticles by manipulating the manner in which the particles interact with one another.
The standard approach in nanotechnology is to devise new chemical structures through trial and error, by letting constituent parts react with one other as they do in nature, then seeing whether the result is useful. Nanotechnologists rely on "self-assembly," which refers to the fact that molecular building blocks do not have to be put together in some kind of miniaturized factory-like fashion. Instead, under the right conditions, they will spontaneously arrange themselves into larger, carefully organized structures.
As the researchers point out in a paper in the Nov. 25 issue of Physical Review Letters, biology offers many extraordinary examples of self-assembly, including the formation of the DNA double helix.
But Torquato and his colleagues -- visiting research collaborator Frank Stillinger and physics graduate student Mikael Rechtsman -- have taken an approach not seen in nature, which they call "inverse statistical mechanics."
"We stand the problem of self-assembly on its head," said Torquato, a professor of chemistry and a member of the Princeton Institute for the Science and Technology of Materials, a multidisciplinary research center devoted to materials science. Torquato is also a senior fellow at the new Princeton Center for Theoretical Physics.
Instead of employing the traditional trial-and-error method of self-assembly, the team starts with an exact blueprint of the nanostructure they want to build.
"If one thinks of a nanomaterial as a house, our approach enables a scientist to act as architect, contractor and day laborer all wrapped up in one," Torquato said. "We design the components of the house, such as the two-by-fours and cement blocks, so that they will interact with each other in such a way that when you throw them together randomly they self-assemble into the desired house."
To do the same thing using current techniques, a scientist would have to conduct endless experiments to come up with the same house. And in the end, that researcher may not end up with a house at all, but -- metaphorically speaking -- with a garage or a horse stable or a grain silo.
Torquato is a theorist rather than a practitioner, but his ideas may have implications for nanostructures used in sensors, electronics and aerospace engineering.
So far, the team has demonstrated the concept only theoretically, with computer modeling. They illustrated the technique by considering thin films of particles. Thinking of the particles as pennies scattered on a table, the pennies, when laterally compressed, would normally self-assemble into a pattern called a triangular lattice. But by optimizing the interactions of the "pennies," or particles, Torquato made them self-assemble into an entirely different pattern known as a honeycomb lattice (because it resembles a honeycomb). The honeycomb lattice is the 2-D analog to the 3-D diamond lattice -- the creation of which is somewhat of a holy grail in nanotechnology. Materials with diamond lattice structures are used in high-speed optical communications devices.
Diamonds found in nature self-assemble from carbon atoms that undergo a type of "directional bonding" that is hard to achieve in laboratory experiments. The researchers created their pattern with "nondirectional bonding," which was previously thought not to be possible. This advance should give experimentalists much more flexibility in creating useful structures, Torquato said. To create the honeycomb lattice, the researchers used techniques of optimization, which is essentially the science of inventing mathematical methods to make things run efficiently.
Torquato and his colleagues hope their efforts will be replicated in the laboratory using particles called colloids, which have unique properties that make them ideal candidates to test the theory. Chaikin said he is planning to do laboratory experiments based on the work.
They said they initially had trouble attracting research money for the idea. Colleagues "thought it was so far out in left field in terms of whether we could do what we were claiming that it was difficult to get funding for it," Torquato said. The work was ultimately funded by the Office of Basic Energy Sciences at the US Department of Energy.
For more information, visit: www.princeton.edu