A long-sought goal of scientists has been to shrink the transistor, the basic building block of electronic circuits, to smaller and smaller size scales. Scientists at Cornell University believe they have now reached the smallest possible limit: a transistor in which electrons flow through a single atom.
The Cornell researchers have created a single-atom transistor by implanting a ‘designer’ molecule between two gold electrodes, or wires, to create a circuit. When voltage was applied to the transistor, electrons flowed through a single cobalt atom within the molecule. Paul McEuen, professor of physics at Cornell, describes the process by which electrons pass from one electrode to the other by hopping on and off the atom as ‘a virtual dance of electrons.’
McEuen cautions that the device cannot yet be described as having all the functions of a traditional transistor, such as amplification. But he sees a potential application for the new transistor as a chemical sensor because a change in the environment around the molecule could cause a measurable alteration of the conductance of the device.
At the heart of the Cornell group’s transistor is the ‘designer molecule’ synthesised by Hector Abruna, professor of chemistry and chemical biology, and graduate student Jonas Goldsmith.
At the molecule’s centre is a cobalt atom surrounded by carbon and hydrogen atoms, which is held in place on either side by molecular ‘handles’ made of pyridine. On their outer side, the ‘handles’ are attached to sulphur atoms, which bond the molecule to the gold electrodes.
Two different molecules were studied, one with longer ‘handles’ than the other. The shorter molecule was found to be a more efficient conductor of electrons.
The challenge faced by the Cornell researchers was to place a molecule less than two nanometers long between two gold electrodes. To do this they used a technique called electromigration, by which an increasingly large current is run through a gold wire, forcing the atoms to migrate until the wire breaks.
The molecule is then sucked into the gap by the high electric field present, and the sulphur bonds the molecule to the gold. ‘Using this technique you can very reliably get wires with a gap on the order of one nanometer,’ or about three silicon atoms, said McEuen.
Although the single-atom transistor demonstrates the potential for shrinking the size of components well beyond what is possible using conventional lithographic techniques, said McEuen, there are major technological hurdles to be overcome in order to build such a transistor for electronic applications. One problem to be solved is the ability to amplify a small signal.
The Cornell group plans next to focus on engineering a molecule with two different shapes that could act as a switch, changing between the two forms with the application of a voltage. ‘No one has yet put a single molecule in a circuit and activated it electronically,’ said McEuen.