Molecular Transistors

December 24, 2009

We are all familiar with the amazing progress in electronic technology — Moore’s Law, and all that — that has gotten us ever smaller, cheaper, and more powerful devices.  It is almost boring to say that the laptop on which I’m writing this is much more powerful, by virtually any measure, than the first mainframe computer I used back in 1970, which filled a good-sized room and costs several million dollars.  But how small can these things be made?

Ars Technica has a report on some new research being done by an international team that has led to the successful creation of a voltage-gated transistor using a single molecule.   (The paper has been published in the journal Nature; the abstract is here.)  A transistor is conceptually a simple device.  It consists of two electrodes, separated by a gate that controls the flow of current between them.  In a conventional transistor, the gate is a semiconductor attached to a third electrode; varying the voltage applied to this third electrode controls the flow of current through the device.

It is relatively easy to find chemical compounds whose molecules can conduct electricity.  The trick is figuring out how to implement the gate.  A few previous experiments have managed to produce something like a transistor, but only by fairly complex “trickery”, such as manipulating the spin of the electrons passing through the molecule.  Although this is a very nifty trick, it is not really practical, since doing the trickery is far too complicated.

In the new experiment, a very fine gold wire is coated with an organic compound.  The wire is placed above an aluminum oxide electrode, and then a nano-scale gap is introduced into the wire.  If the construction is successful, a molecule of the organic substance will remain in the gap to become the gate, controlled by the aluminum oxide electrode.

The team used tried two organic compounds containing sulfur, and had the best results with 1,4-benzenedithiol.  The benzene ring is hexagonal, and this compound has the sulfur atoms at opposite vertices of the hexagon.  If you took an introductory course in organic chemistry, you may remember that the geometry of the benzene ring, with its alternating double and single C-C bonds, means that the electron orbitals form a “cloud” around the ring.  This makes it much easier to affect the electron energy by applying an external voltage via the gate electrode.

What is particularly interesting about this work is that the experimenters were actually able to get a “picture”, of sorts, of the function of the molecular transistor:

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.

This is a much clearer result than any previously obtained, since it avoids potential red herrings due to contaminants or fabrication problems.

This is still, of course, just a research result; you should not expect to see it in your iPhone anytime soon (the Steve Jobs Reality Distortion Field notwithstanding).  But it is a demonstration of how much scope there is for further amazing technology.

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