Tikalon Blog by Dev Gualtieri

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Piezomolecular Effect

February 22, 2011

Carbon has many useful properties beyond just burning it for heat. One reason for this is that it exists in several solid forms, known as allotropes . Diamond is a good abrasive , graphite makes great "lead" pencils , and fullerenes helped to launch nanotechnology . Graphite is a semiconductor , and it was very useful in early electronics as the material in resistors . It also has the useful property (in some cases) that its electrical conductivity increases quite a bit as temperature increases. This is a semiconductor property, and it's useful for making a primitive thermometer , like the liquid nitrogen level detectors I used to make to monitor our dewars in the lab.

Resistors, of course, are everywhere, but the next important use of carbon in electronics was as a microphone material. Thomas Edison discovered that sound pressure can change the resistance of carbon particles placed between conducting plates. Edison patented this device, later called a carbon button microphone , in March, 1877, and it found its way into transmitters and telephones .

The mechanism for action, which was something quite understandable in Edison's day, was that the pressure acting on the loose particles causes more contacts through which current could flow. The fact that carbon is a semiconductor was superfluous, except that it made the particles conducting. A variation in the carbon button microphone effect can be found in composite materials , typically polymers loaded with conducting particles, that change resistance with applied stress . Sometimes the materials are a bit more exotic, as when ruthenium oxide particles are mixed into glass to form a piezoresistive material with excellent properties (see figure).[1]

Fig. 3 of US patent no. 5,985,183 showing piezoresistive gauge factor as a function of ruthenium oxide content.

Fig. 3 of US patent no. 5,985,183 showing piezoresistive gauge factor as a function of ruthenium oxide content.[1]

A somewhat similar mechanical affect on conductance was found much earlier, in 1856, by William Thomson (Lord Kelvin), of the eponymous temperature scale . Kelvin found the piezoresistive effect , a change in resistance of metals with applied stress caused merely by a change in their shape. For a bar-shaped conductor, the resistance increases with length and decreases with cross-sectional area, and this was the effect that Kelvin saw.

Piezoresistance is much different for semiconductor crystals, as discovered by Charles S. Smith of Bell Labs in 1954.[2] The piezoresistive effect he saw was much greater than a geometry change could explain. As Smith conjectured, "This so called electron transfer effect arises in the structure of the energy bands of these semiconductors..." The situation is that the stress causes a redistribution of charge carriers of different mobilities .

Nowadays, "nano" is the name of the game, so it's no wonder that piezoresistance has been reduced to its smallest possible dimension. An international team from Arizona State University (Tempe, Arizona), the University of Barcelona (Barcelona, Spain), Fudan University (Shanghai, China) and the Max-Planck-Institute for Polymer Research (Mainz, Germany), led by Nongjian Tao of Arizona State University, has demonstrated the piezoresistance of a single molecule of pentaphenylene .[3-4]

I've termed this the "piezomolecular" effect, but you won't find anything piezomolecular if you search Google . It's a word that I invented, a neologism . Yes, what this research team found is piezoresistance, but I don't think the word, piezoresistive, adequately captures what they've done. We could call it the "LCAO piezoresistive effect," which is descriptive, but I don't think it would have popular appeal.

I'm not ashamed of my presumption, since physicists invent words all the time. There is, of course, the example of Gell-Mann's quarks . A recent example is David Mermin , who named the geometric pattern on the surface of one of the phases of superfluid ³He a "boojum ."[5] Mermin lifted the term from Lewis Carroll's "The Hunting of the Snark ," just like Gell-Mann found quark in Finnegans Wake by James Joyce . My term is much less creative.


A molecule of pentaphenylene held between two gold electrodes passes an electrical current. Source: Arizona State University.[4]	Mechanical strain applied to the molecule affects the overlap of atomic orbitals and the conductivity. Source: Arizona State University.[4]

The figure above shows the concept of the piezomolecular experiment, but the simplicity of the figure belies the difficulty of the experiment. Details of how the researchers were able to put molecules between electrodes can be found in the supplementary information (1.4 Mb PDF file) to their paper. The research group was able to vary the molecule's conductance over an order of magnitude. In the experiment, the molecule was stretched by varying the height of one electrode with respect to the other.

As could be expected, maximum conductance was obtained at minimum stretch when the p-orbitals of the carbon backbone had more of an overlap. As a control, the team used tetraphenyl molecules, which would not have this overlapping of p-orbitals, and there was no conductance change. Other tests were performed to ensure that they were not seeing any affects from the electrode contacts.

If you shoot a beam of light through the molecules captured between electrodes, I'm sure you'll see some changes to the light as the molecules stretch; perhaps in both polarization and intensity, but surely in the absorption spectrum . The signal may be too weak if the beam goes through just one molecule, but it might work with a stack of a few hundred. Now, there's a challenge!

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