Self-Assembled Organic Circuits

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The semiconductor molecules arranged themselves in a densely packed single layer about three nanometers thick on top of the silicon-dioxide insulator between the source and drain. "The molecules attach as long as there is an open space, so there's truly a mono-molecule layer," says paper coauthor Stephan Kirchmeyer, who is vice president of H.C. Starck, the chemical company based in Leverkusen, Germany, that designed and produced the molecules.

In previous work by other groups, circuits were first dipped in an anchor chemical and then coated with a semiconductor solution, causing the semiconductor to attach to the anchor molecules. In the new molecules, the anchor and semiconductor are already strung together. "This becomes a one-step manufacturing process," says Yang Yang, a materials-science and engineering professor at the University of California, Los Angeles. "[It's] a smart approach."

Producing a well-ordered semiconductor layer creates a high-performance device. It improves a transistor's electron mobility, which in turn determines how much current it can carry and how fast it can switch on and off. "The performances of the devices [are] comparable to bulk transistors based on similar materials," Smits says.

The researchers finally combined their transistors into functioning circuits. In their Nature paper, they demonstrate several important logic components such as inverters and ring oscillators. They also demonstrate a complicated circuit called a code generator, using 300 transistors.

Kymissis admits that faster, better circuits have previously been made using self-assembly. But he says that the simplicity of the one-step assembly method and the ability of these transistors to function in such complex circuits "is a terrific advance."