Gaining microscopic insight into molecular junctions by transport experiments

The properties of molecular junctions are investigated by the means of different transport experiments with mechanically controlled break junctions in liquid environments. First, the method of opening and closing cycles is introduced as a means to obtain conductance values of molecular junctions.
Then conductance fluctuations observed while not actively moving the electrodes are investigated. It is found that alkane dithiol molecules cause fluctuations of larger amplitude compared to measurements in pure solvent. These fluctuations are attributed to the binding and unbinding of molecules to the gold electrodes which demonstrates the limited lifetime of molecular junctions at room temperature.
To gain insight into molecular junctions beyond linear conductance measurements, a fast IV measurement method is developed which continuously records IV data at a rate of 100 curves/s. This allows for the observation of quick changes of the IV characteristics along single conductance plateaus. Both symmetric and asymmetric versions of OPE with different linker groups are measured and the data described by an analytic single level model. The coupling constant for the different molecule-metal bonds and the relative alignment of the dominant molecular energy level are obtained as fitting parameters of the model. It is found that transport through the symmetric molecules is dominated by different molecular orbitals depending on the linker group (HOMO vs. LUMO).
In a comparative study of biphenyl molecules with thiol, cyanide and isocyanide linker groups, isocyanide is found to form molecular junctions most reliably and at the highest conductance value. Additionally, the isocyanide molecules show jumps to contact in most of the closing curves as opposed to the other investigated linker groups where such jumps are very rare. The most interesting observation with isocyanide molecules is the additional plateaus in opening curves at lower conductance values which are attributed to the chaining of two or more molecules.