Employing single molecules as active functional units in electronic devices is a promising new technological concept of fast growing interest [1-3]. For the development of such components it is crucial to better understand electron transport through single molecules. Transport measurements through single molecules which are immobilized by self-assembling techniques between two metallic electrodes have already proven the ability of organic molecules to act as functional parts in nano-scale devices. Three relevant methods to fabricate suitable electrodes have been established in recent years:
i) mechanical controlled break junctions (MCBJs) [4,5],
ii) on-chip electrodes with fixed distance (by electromigration and electrochemical techniques) [6,7].
iii) Scanning Probe Microscopy (SPM) .
The major part of the experimental designs are restricted to measure only the simplest transport properties (current vs. voltage) and no additional parameter is straightforward adjustable to influence the molecules between such extremely close electrodes. Reliable experimental data utilizing more control parameters is highly demanded to close the widening gap between theoretical predictions and experimental data. SPM techniques allow to measure other properties beside the conductance and provide at the same time several parameters to control the junction. They are easier adaptable to address a broad range of different experiments. Scanning Tunneling Microscopy (STM) has been a useful tool to test the conductivity of molecules on surfaces by means of tunneling-spectroscopy. It has recently advanced to a point where it is possible to perform direct conductance measurements of molecules linked between substrate and tip in a statistical manner . Examples to point-out the possibilities of SPM-techniques are Atomic Force Microscopy (AFM) that provides the force simultaneously and Scanning Near-field Optical Microscopy (SNOM) with its ability to expose the contact region to an external optical field. Optical spectroscopy as a powerful tool in identifying molecular species is limited in resolution by the so called diffraction barrier. SNOM is a popular technique to overcome this wavelength dependent limit and is capable of addressing small sample volumes.
We developed a novel method to contact single molecules utilizing a SNOM-tip as a counter electrode to perform Raman spectroscopy and conductance measurements at the same time. Analysis of the vibrational spectrum of a molecule in between two metallic electrodes could be used as an analytical tool to verify the structure of the molecular species between the electrodes and help to clarify the importance of inelastic processes in the non-equilibrium-situation of electron transport.
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