Carbon nanotubes are promising elements for optoelectronic components. However so far there were no electronic methods to analyze the ultra fast optoelectronic dynamics of the nanotubes. A team of physicists headed by Professor Alexander Holleitner from the Technische Universitaet Muenchen (TUM) has now come up with a new method to directly measure the dynamics of photo-excited electrons in nanoscale photodetectors. This new measurement technique is about a hundred times faster than any existing method. It allowed the scientists headed by Professor Alexander Holleitner to measure the precise speed of electrons. In the carbon nanotubes the electrons only cover a distance of about 8 ten-thousandths of a millimeter or 800 nanometers in one picosecond.
At the heart of the photodetectors in question are carbon tubes with a diameter of about one nanometer spanning a tiny gap between two gold detectors. The physicists measured the speed of the electrons by means of a special time-resolved laser spectroscopy process – the pump-probe technique. It works by exciting electrons in the carbon nanotube by means of a laser pulse and observing the dynamics of the process using a second laser.
We introduce coplanar stripline circuits to resolve the ultrafast photocurrent dynamics of freely suspended carbon nanotubes (CNTs) in the time domain. By applying an on-chip pump−probe laser spectroscopy, we demonstrate that CNTs, contacted by metal electrodes, exhibit a picosecond photocurrent response. We find a combination of an optically induced ultrafast displacement current, transport of photogenerated charge carriers at the Fermi velocity to the electrodes, and interband charge-carrier recombination processes to dominate the ultrafast photocurrent of the CNTs.