As semiconductor manufacturers continue to push down the size of their products' wiring, a number of research labs have started looking into whether they can simply take the process to its logical conclusion: a transistor made from a single molecule. A number of these items have been demonstrated, and they do manage to control the current flow through the molecular transistor, but they do so through a variety of tricks that have nothing in common with the methods used for the semiconductors in our electronics. In today's issue of Nature, an international team reports producing the first voltage-gated molecular transistors.

The basic principle behind a transistor is simple. All it needs is two electrodes, a source and a sink, and a gate that controls the flow of current between them. In semiconductor transistors, the gate contains a semiconductor and another electrode: raising or lowering the voltage in this electrode controls whether current can flow across the semiconductor between the source and sink.

For molecular transistors, the semiconductor is replaced by a single molecule. Electrons can flow through a variety of molecules, but controlling that process is not the easiest thing. A few of the past efforts have switched currents on and off by changing the charge on the molecule or playing with the spin of the electrons that pass through it, but these are difficult challenges in their own right, and far more complex than simply applying a voltage to the gate.

The new work involved creating a nanoscale gap in a gold wire that was placed directly above an aluminum oxide electrode that controls the gate. The gold had been covered with one of two types of molecules in advance and, once the gap was created, there was a chance that one of those molecules dropped into the newly vacated space, bridging the gap and enabling the molecule to conduct currents between the two gold electrodes.

By demonstrating they could detect the actual molecule in question responding to the gate voltage, the new results leave little room for doubt.

The authors used two different types of molecules. One was 1,8-octanedithiol, which is essentially a pair of sulfur atoms separated by a linear chain of eight carbons. The carbon chain allows electrons to transit, and the efficiency of the process was influenced by the voltage in the gate: applying a negative voltage brought the energy of the carbons' orbitals closer to the tunneling energy of the gold electrodes, and increased the current travelling across the molecule.

The team was able to get a larger effect by replacing the linear chains of carbons with 1,4-benzenedithiol, in which the sulfurs are separated by a benzene ring (shown above). Those of you who remained awake through high school chemistry may remember that the benzene molecule's mix of single and double bonds creates a single, diffuse electron orbital that extends around the ring. The energy of that orbital, as it turns out, is much easier to influence with an applied voltage, creating a more effective transistor.

The tour de force in the work, however, was the fact that the authors imaged the molecule sitting within the gate, and tracked the effect of the changes in gate voltage. A technique called inelastic electron tunneling can detect the vibrational modes available to the atoms in the molecule; the researchers used it to demonstrate that applying a voltage to the gate changes the energy of the orbitals, with a corresponding impact on the vibrational energy.

As an accompanying perspective points out, this is far from a trivial confirmation of the expected. Because of the size of the objects involved in producing molecular-scale gates and the high failure rate of their production, there's a very real risk of researchers being led astray by a contaminant or malformed structure of some sort that ends up acting like a gate. By demonstrating they could detect the actual molecule in question responding to the gate voltage, the new results leave little room for doubt.

Where this new research doesn't differ from past work is that high failure rate during production. Out of 418 devices they produced, only 35 behaved as if a molecule had fallen into place in the gate once the gap between its electrodes was created. Clearly, a success rate of less than 10 percent isn't anywhere close to being ready for manufacturing.

It's also worth pointing out that the wiring on either side of the gate would still be subjected to the same issues that Intel faces in terms of feature shrinks; the gate may be smaller, but that's only one part of a chip's wiring.

Still, the basic physics of the new gate are well understood and easy to control, which may make these devices easier to integrate into some sort of future circuitry. The results mean that the researchers can now shift their focus to application issues, like finding a long-lived molecular gate that attaches to the supporting wiring with high efficiency.

Nature, 2009. DOI: 10.1038/nature08639