IBM create logic circuit within a single molecule

Researchers at IBM have created the first functional logic circuit within a single molecule, an achievement that could help to replace silicon in microchips. ‘We believe that carbon nanotubes are now the top candidate to replace silicon when current chip features just can’t be made any smaller,’ said Phaedon Avouris, manager of Nanometer Scale Science and Technology at the IBM T.J. Watson Research Centre.

The new circuit works on a miniature scale, using a hollow carbon tube approximately 1.4 nanometers in diameter.

The researchers changed the nanotube’s electrical characteristics so that some sections would allow the flow of electrons (called n-type, or negative, sections), while other sections would allow the flow of electric current using positive entities on the nanotube called positive holes (also known as p-type, or positive, sections).

The sections were turned into transistors that encode the ‘NOT’ logic function along the length of the nanotube, Avouris said.

The characteristics of the resulting circuit – its ability to propagate voltage, called gain – is said to allow for more transistors to be placed along the tube to make more complex circuits. Both p- and n-type sections are needed to build a logic circuit.

While working with their previously assembled p-type transistors, the IBM team discovered a simple way of producing n-type transistors. Avouris said this was achieved by simply heating a p-type transistor in a vacuum.

In the future, the researchers plan to create more complex circuits and to further improve the performance of the individual transistors, he added.

All information stored in a computer is made up of ones and zeros, which indicate whether a circuit is on (one) or off (zero). The circuit described in the research can switch the ones to zeros, and vice versa, according to Avouris.

The circuit, therefore, is called a ‘NOT gate’, one of two fundamental combinatorial logic circuits that computers use to perform computations.

Also known as a voltage inverter, the gate sends out the opposite voltage from the one it receives. Other logic circuits include the ‘AND’ and ‘OR’ gates, which perform other computations.

The ‘NOT’ gate, in combination with either the ‘AND’ or the ‘OR’ gates, can form all other logic circuits. All these functions are currently accomplished using silicon chips in modern computers.

Computers will eventually reach a maximum capacity with silicon that cannot be overcome, forcing the need for new materials capable of adding smaller computer circuits to maintain the advancements in the future, concluded Avouris.

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