Excited electrons "run" in a circle

Size of excitons in nanotubes experimentally determined for the first time

Professor Tobias Hertel and two of his graduate students, Sabine Himmelein and Thomas Ackermann, show the model of a carbon nanotube. © Robert Emmerich / University of Würzburg
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Whether in photovoltaics, computer technology or the life sciences: Tiny tubes made of pure carbon are suitable for many applications. New findings on the energy flow in these tubes are now described by researchers from Würzburg and Milan in the journal Nature Physics. For the first time in the world, they have succeeded in measuring the size of so-called excitons.

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"Put simply, these are energetically excited electrons that move in circular orbits and are mobile within the nanotube, " explains Professor Tobias Hertel from the University of Würzburg.

Small but powerful

Nanotubes made of carbon are a thousand times thinner than hair and at least a thousand times as exciting - at least for scientists. Some are stronger than steel, and generally their electrical conductivity is so high that only superconductors are better. No wonder many researchers expect big things from the small tubes.

Hertel is particularly interested in the optical properties of nanotubes. For the way in which they absorb, transmit and release light energy. "The knowledge of these processes is fundamental for later applications, such as in photovoltaics or fluorescence microscopy, " says the owner of the Department of Physical Chemistry II. Display

Together with colleagues from Milan he has investigated a special tube type, the (6, 5) nanotubes. If they are bombarded with energy in the form of laser pulses, so-called excitons are formed in the carbon skeleton.

World premiere: the size of excitons measured

So far, the expansion of excitons in solids has only been calculated theoretically. But for the first time ever, the researchers from Milan and Würzburg have succeeded in experimentally determining the size of excitons. A world premiere and reason enough for the journal "Nature Physics" to report on the study.

"The excitons in the nanotubes are larger than expected, " says Hertel, "two millionths of a millimeter". This is slightly larger than the diameter of the tubes and has the consequence that the excitons in the tubes can only move in two directions - like a big ship that can only travel forwards or backwards in a narrow channel, but not sideways.

Important: the mobility of the excitons

The mobility of the nanotubes is of special interest to the researchers from Würzburg. If the excitons are very mobile, they are very likely to reach the end of the tubes - which the scientists would not like. Because in this case, the excitons emit almost all of the previously absorbed energy in the form of heat.

Hertel's team, however, wants to make the excitons emit their energy as light, that they fluoresce. After all: "One of our goals is to develop fluorescent dyes from nanotubes for biomedical research." Such dyes could be used, for example, to demonstrate the functionality of proteins in cells.

Next: the next research steps

The size of excitons in nanotubes can now be measured. Now the researchers can begin to change the accompanying circumstances in the experiment. Can we influence the size and mobility of the excitons in our sense if we place the nanotubes in special fluids? We want to answer that question next, "says Hertel. As a result, the fluorescence of the tubes can be further improved.

(idw - University W rzburg, 17.12.2008 - DLO)