Electronic Design

Researchers Demonstrate Single-Molecule Computer Circuit

Researchers at IBM's T.J. Watson Research Center in Yorktown Heights, N.Y., have created the world's first logic-performing computer circuit within a single molecule. Such circuits may someday lead to a new class of computers that are smaller and faster and consume less power.

Using a carbon nanotube, a tube-shaped molecule of carbon atoms that is 100,000 times thinner than a human hair, the researchers created a voltage inverter. Inside computer chips, a voltage inverter changes a logic "1" into a logic "0." Generally, voltage inverters contain two types of transistors with different electronic properties; while electrons carry the electrical current in n-type transistors, electron-deficient regions called holes carry the electrical current in p-type transistors. All previous carbon nanotube transistors have been p-type only.

In making the voltage-inverter circuit, also known as a "NOT" gate, the IBM scientists encoded the entire inverter logic function along the length of a single carbon nanotube. The carbon nanotube is positioned over gold electrodes to produce two p-type carbon-nanotube field-effect transistors in series. Then, the device is covered by an insulated layer called PMMA. E-beam lithography is used to open a window and expose part of the nanotube.

This project builds upon nanotube developments made earlier this year by IBM. In April, the same IBM research team led by Phaedon Avouris developed a technique to produce arrays of carbon nanotube transistors. This method bypasses the need to meticulously separate metallic and semiconducting nanotubes. Structures built using this process were utilized in the single-molecule circuit. This configuration is the world's first intramolecular (single-molecule) logic circuit.

To convert p-type nanotube transistors into n-type transistors, the IBM researchers heated p-type transistors in a vacuum. This process can then be reversed by exposing the transistors to air. In addition to converting an entire nanotube from p-type to n-type, the scientists were able to selectively convert part of a single nanotube to n-type. Potassium is evaporated through the nanotube's open window to convert the exposed p-type nanotube transistor into an n-type nanotube transistor. The other nanotube transistor remains p-type.

The output signal from the new nanotube circuit is stronger than the input. This gain is essential for assembling gates and other circuit elements into future microprocessors. Since this nanotube circuit has a gain of 1.6, the researchers project that even more complex circuits can be made along single nanotubes.

For more information about the single-molecule computer circuit, go to www.ibm.com.

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