Public Release: 

Interfacing organic semiconductors to metal

Cornell University

ITHACA, N.Y. -- A team of Cornell University researchers has received $1.6 million in grants to develop technology that could lead to computers that are not only smaller and cheaper, but more flexible -- literally.

Officially, the project is to investigate "inorganic-organic interfaces." In simpler language, the problem is: How do you connect wires to organic transistors?

The project is funded by a $1.3 million, four-year grant from the National Science Foundation and $300,000 from the Semiconductor Research Corp., a consortium of electronics manufacturers. James R. Engstrom, Cornell associate professor of chemical engineering, is principal investigator on the project.

Organic semiconductors are drawing wide interest as a way to make very cheap electronic devices, perhaps even devices in which single molecules can act as switches. "Organic" here doesn't necessarily mean "living." Organic chemistry is the study of molecules built around chains of carbon and hydrogen atoms. Currently popular materials for making organic semiconductors are sexithiophene, made up of six five-sided groupings of carbons, and pentacene, made up of five six-sided rings. In such molecules some electrons are free to jump from one ring to another, making it possible to use the materials as transistors. Some organic semiconductors also can be made to emit light, making them useful as light-emitting diodes (LEDs) or in lasers. Organic LEDs already are in use in some cell phones and hand-held video games.

These and other semiconducting organic molecules can be made to assemble in long chains or polymers -- which are the basis of plastics -- so they can be used in inexpensive devices, such as "smart cards." Such devices can be made very cheaply using "wet chemistry" approaches rather than expensive nanofabrication equipment. Organics can form tough, flexible thin films and perhaps someday may be printed onto fabric or paper. They also offer a promising approach to computing at the molecular level, where single molecules act as transistors.

But as these devices grow smaller, making connections to them becomes more difficult. When metals come in contact with organics, metal atoms tend to diffuse into the organic material, muddying up the contact. "Currently you evaporate the metal onto the organic and cross your fingers," Engstrom says. He describes the problem as one of making "molecular solder."

Under the new grant, the Cornell researchers will study in detail the chemistry of the bond formed when organic films are deposited on metals (or in some cases, insulators) and, most importantly, the inverse -- where metals are deposited on the organic. Their approach involves "self-assembly," where a metal or insulating substrate is masked to form a pattern, such as the pattern of wires to which circuit elements are connected, and a film of organic material is allowed to deposit on the unmasked areas. Later they will examine additional chemical reactions in which metallic thin films are deposited on top of the organic layers to make a second "contact" with the organic layer. All useful electronic components are either two- or three-terminal devices, Engstrom explains.

The researchers plan to test layers of various metals and metal nitrides for these contacts. One novel approach will be the synthesis of molecules containing both the metal and the organic where the inorganic-organic interface is "prefabricated," already built into the molecular structure.

The eventual goal, Engstrom says, is to produce testable devices and demonstrate that they have useful properties, but not to make fully functional circuits. "All of the easy problems have been solved. I think we're tackling very difficult problems," Engstrom says. "There has been a lot of interest in using organics for molecular-based electronics, but they really haven't attacked the problem of the interface."

Engstrom will study the surface chemistry and develop fabrication methods. Co-principal investigators are Peter T. Wolczanski, the George W. and Grace L. Todd Professor of Chemistry, who, Engstrom says, will be doing "designer chemistry"; Paulette Clancy, professor of chemical engineering and the recently named director of the School of Chemical and Biomolecular Engineering, who will create computer simulations of the interface at the molecular level; George G. Malliaras, assistant professor of materials science and engineering, who will test the resulting devices; and Ronald R. Kline, professor of electrical and computer engineering and a member of the Department of Science and Technology Studies, who will develop a workshop on the underlying ethics of the research.

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