Molecule Conductance in a Junction



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Thiophenes are conducting molecules that when placed between two electrodes under applied voltage a current can be observed. Theoretically, conductance should follow an exponential decay with increasing molecule length. However, experimentally the conductance shows anomalous behavior. We hypothesize that the anomalous behavior arises from increasing flexibility with increasing length, which results in increasing vibrionic coupling and a new transport mechanism. We want to show a transition between quantum coherent current through short chains to incoherent vibration enabled current in longer chains. Gaussian16 was used to run the molecules (N1-N6) through an optimization/frequency calculation. A Python code was used to output small deviations along the normal modes. Using Gaussian16 again we ran the deviations through an energy calculation. By using the energy data and taking a derivative the coupling parameter was calculated. The coupling data gathered showed that for N1 there is only higher energy vibrations with weak coupling, while N5 shows low energy vibrations with strong coupling. Furthermore, the model created shows that the N1 and N2 chains have no coupling at low energies. Starting at N3 the model shows coupling. This research gives insight into the anomalous behavior but further research is needed to choose a primary mode.