Molecular Circuits for Optoelectronics utilizing Carbon Nanotubes

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The photo-conductance across a single molecule shall be measured in a three-terminal device as a function of the wavelength, the source/drain voltage, a gate voltage on the third terminal and the temperature in the range of 4.2 K and room temperature. The molecules will be contacted by two single wall carbon nanotubes (SWNTs), which act as mesoscopic source/drain contacts to the molecule, via a self-assembly process. The organic circuit ¿SWNT-molecule-SWNT¿ has a length of several hundreds of nanometers, which allows contacting the device by standard e-beam lithography and in turn, patterning of local side-gates. By applying voltages to the side-gates, the energy levels of the electrons in the molecule can be altered electrostatically. For the photo-excitation of the molecule we utilize a pulsed, wavelengthtunable laser system, which allows measuring the photo-conductance close to the absorption bands of the molecule. Using a pulsed laser system instead of a continuous wave laser minimizes the influence of heating effects in the contacts on the transport properties of the molecule. In addition, atomic rearrangements in the SWNT-molecule contact region are expected to play only a minor role as compared to gold contacts. In order to give direct experimental evidence that only one molecule is being characterized, we plan transport measurements on photo-switchable molecules, such as azobenzene or stilbene, and also on metalloproteins. Photo-switches exhibit a conformational change induced by photo-excitation, which allows studying the effect of molecular conformation on the transport properties. At the same time, a successful and reversible switching of the molecular conformation demonstrates that just one molecule is being connected and characterized optoelectronically. Proteins, on the other hand, allow verification of their presence by scanning probe microscopy.