"Speedometer" developed for electrons

Ultrafast photodetectors made of carbon nanotubes

Carbon nanofibers between two gold electrodes © Alexander Holleitner / TUM
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Carbon nanotubes are promising candidates for optoelectronic components such as photodetectors or photoswitches. But so far, it is extremely difficult to analyze and influence the optical and electronic properties of nanotubes. However, physicists now present in the journal "Nano Letters" a method by which they can immediately determine how fast electrons move in such extremely small photodetectors.

The new measurement method developed by the team led by Professor Alexander Holleitner of the Technical University of Munich (TUM) enables the temporal resolution of the so-called photocurrent in photodetectors down to the picosecond range.

Very small time interval

"A picosecond is a very small time interval, " explains Holleitner. "If the electrons were traveling at the speed of light, they would almost reach the moon in a second. In one picosecond, on the other hand, they would only reach a third of a millimeter. "

According to the researchers, the new measuring technology is around a hundred times faster than the existing methods. So scientists can now accurately measure the speed of the electrons. In the carbon nanotubes, the electrons only cover about eight ten-thousandths of a millimeter or 800 nanometers in one picosecond.

Laser pulse stimulates electrons

At the core of the investigated photodetector are carbon tubes with a diameter of only about one nanometer, which are electronically integrated via metallic contacts. The speed of the electrons is determined by the physicists with the help of so-called coplanar strip lines, which they evaluate by means of a special time-resolved laser spectroscopy method, the pump-probe technique. display

Here, a laser pulse excites electrons in the carbon nanotubes and tracks the dynamics of this process with a second laser.

Many new analysis options

According to the researchers, the newly developed method provides numerous findings and new analysis options that are of interest for a number of applications. This includes above all the further development of optoelectronic components. (Nano Letters, 2011; doi: 10.1021 / nl1036897)

(Technical University of Munich, 09.03.2011 - DLO)