Self-assembling designer molecules could lead to nanoscale advances

By David Brand

Some are cylindrical, some look like a double sandwich and some are continuous three-dimensional cubic structures. All are generated by a class of designer macromolecules that could lead to improvements in solar-cell and fuel-cell technology, as well as advances in ultra-miniaturization of electronic devices.

A schematic illustration of supramolecular architectures of self-assembled extended amphiphilic dendrons shows (A) cubic micelles, (B) two-dimensional lamellar layers, (C) hexagonally arranged cylindrical columns and (D) three-dimensional continuous cubic structures. Cornell Center for Materials Research

These synthesized molecules self-assemble themselves into structures with dimensions on the order of 10 nanometers, an unusual process that mimics nature's most fundamental system of organizing living tissue. (One nanometer is about the width of three silicon atoms.)

The development of the new class of molecules is reported in the Sept. 10 issue of the journal Science by Ulrich Wiesner, professor of materials science and engineering at Cornell, and his research team consisting of postdoctoral researcher Byoung-Ki Cho and Ph.D. candidate Anurag Jain, as well as physics professor Sol Gruner, director of the Cornell High Energy Synchrotron Source.

The Cornell team designed the molecules to imitate nature's system of self-assembly: Our bodies, for example, are made up of functional assemblies of molecules, like cell walls, that form spontaneously. By mimicking this natural system, the researchers say, it is possible to design nanoscale structures that otherwise would be impossible to manufacture, ultimately leading to the construction of devices and machines whose dimensions are measured in nanometers. In just the past two years, the microelectronics industry has shown an interest in self-assembly as it searches for techniques to create devices with dimensions smaller than those possible with lithography.

Essentially, these new designer macromolecules can be programmed. "We can encode information about the self-assembly behavior into their molecular architecture," Wiesner said. "It is an exciting research direction known as molecular engineering."

The Science paper describes how researcher Cho took molecular design concepts from two known classes of synthetic macromolecules and combined them into a single one. The first was block copolymers, first synthesized in the 1950s. The second concept was dendrimers, tree-like macromolecules synthesized by techniques developed in the 1980s and 1990s, partly by Cornell researchers. The resulting molecule, called an extended amphiphilic dendron, shows a combination of behaviors, something that had been sought by researchers worldwide for the past decade.

The four researchers who have developed a new class of designer macromolecules in their Bard Hall laboratory are, from left, Sol Gruner, professor of physics and director of the Cornell High Energy Synchrotron Source; Ph.D. candidate Anurag Jain; Ulrich Wiesner, professor of materials science and engineering; and postdoctoral researcher Byoung-Ki Cho. Nicola Kountoupes/University Photography

One of the most curious characteristics of the new class of macromolecules is that they exhibit what the researchers call "a rich phase behavior": They change their structure several times as the temperature rises, and each stable phase produces a different behavior. Their self-assemblies range from one-dimensional cylinders to two-dimensional lamellae to three-dimensional continuous cubic structures. What's more, said Wiesner, when doped with lithium salts, ion transport -- the transport of electrical charges along nanometer-sized channels self-assembled by the molecules -- can be observed in these phases. This, he said, would make the continuous cubic structure "particularly relevant" for applications such as batteries, fuel cells and solar cells.

Wiesner noted that one interesting property of the novel materials described in the Science paper is that of a "supra-molecular switch." Electrical conductivity, he said, can change suddenly and dramatically with very small changes in temperature. "The materials thus could have immediate applications in temperature sensing," he said.

He described the materials as the result of an ongoing evolution. "People have studied small molecules for a long time; then they started to make linear, then blocked macromolecules and then dendrimers," he said. "Studies of their behavior generated a toolbox. We now know how certain structural design elements give rise to certain behaviors. We took two design features that have been studied for a long time and combined them into a new molecule, and interesting stuff happened."

The work was supported by the National Science Foundation's Cornell Center for Materials Research.