September 10, 2004

Journal Publishes C.U. Nano Research

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Cornell research to be published today in the Journal of Science features a new engineering technique that allows the synthesis of unprecedented small nanostructures using self-assembling macromolecules.

Inspired by the self-assembly of organic structures such as animal cell membranes from simpler organic compounds, the technique requires only the synthesis of the macromolecule polymers as well as a controlled increase in temperature to produce a variety of minute structures ranging from tiny conducting cylinders to rigid polygonal structures.

The research, compiled by a team led by Prof. Ulrich Wiessner, materials science & engineering, is expected to be applicable to various engineering industries. In particular, “self-assembly is now being discussed in the framework of the microelectronics industry, which will hit a wall in about 10 years in terms of miniaturizing even further,” Wiessner said.

The procedure offers the industry an alternative to the traditional top-down approach, in which nanostructures are assembled from large objects through miniaturization. Instead, “you can use bottom-up approaches,” Wiessner added.

In addition, many of the final structures produced by the self-assembling molecules have portions that are highly conductive and sections that are effective insulators. Consequently, according to Wiessner, these structures are particularly useful in a variety of devices such as solar cells.

The technique begins with the synthesis of a large macromolecule, or polymer, that is comprised of a linear molecule and a tree-like molecule. The linear molecule is extremely hydrophyllic, meaning it readily dissolves in water, while the tree-like molecule is quite hydrophobic. The combination of the two is then heated.

“When you heat the material up, [it] changes structure,” Wiessner explained. Basically, “all parts that like water come in contact, and all parts that dislike water come in contact.”

The shape of the final structure is highly dependent on temperature and is usually only a few nanometers — a millionth of a centimeter — in width. Consequently, the structures “can be utilized as an on/off switch,” explained Prof. Byoung-Ki Cho, material science and engineering, who synthesized many of the tree-like polymer portions and also participated in much of the research. In essence “this on/off switch can be [controlled] through the temperature,” Cho added.

Since much of this research was inspired by similar processes that occur in nature, the self-assembling polymers are essentially organic compounds consisting mainly of carbon, nitrogen, oxygen and hydrogen.

“Every living organism is full of self-assembled material,” Wiessner explained, “For example, through a spontaneous process, the cell walls of blood cells are formed — A wonderful example in which self-assembly takes place every day a million times.”

Synthesized nanodevices, however, are not yet stable enough to be used in most engineering applications. For instance, in most circuit devices components need to retain their structure and conductivity over a wide range of temperatures and humidity levels. Because most nanostructures created using this process change form readily with small changes in temperature, this is not guaranteed.

“We are currently optimizing the material, the structure, the conductivity value … [as well as] broadening the temperature,” Cho said.

Other areas of future research include the integration of non-organic molecules into the self-assembling polymer structure as well as reforming the macromolecule’s structure itself. For instance, Cho plans to test new polymer structures that vary from the current polar setup with the linear portion on one end and the treelike structure on the other end.

“He is extending [the structure] to an ABA kind of system, with a linear chain in in the middle and the [tree-like] structure on both sides,” said Surbhi Mahajan grad, who synthesized most of the linear portions and also contributed to the research.

The paper will appear in today’s issue of Science and received funding from the National Science Foundation. Other contributors to the research included Anurag Jain.

Archived article by David Andrade
Sun Staff Writer