Molecular Polarizability as a Descriptor for Molecular Conductance

• Other Authors: Vatan Meidanshahi, Reza (Author)
• Format: Dissertation
• Language: English
• Published: 2014
• Subjects: Chemistry
• Online Access: http://hdl.handle.net/2286/R.I.27439

abstract: We studied the relationship between the polarizability and the molecular conductance that arises in the response of a molecule to an external electric field. To illustrate the plausibility of the idea, we used Simmons' tunneling model, which describes image charge and dielectric effects on electron transport through a barrier. In such a model, the barrier height depends on the dielectric constant of the electrode-molecule-electrode junction, which in turn can be approximately expressed in terms of the molecular polarizability via the classical Clausius-Mossotti relation. In addition to using the tunneling model, the validity of the relationships between the molecular polarizability and the molecular conductance was tested by comparing calculated and experimentally measured conductance of different chemical structures ranging from covalent bonded to non-covalent bonded systems. We found that either using the tunneling model or the first-principle calculated quantities or experimental data, the conductance decreases as the molecular polarizability increases. In contrast to this strong correlation, our results showed that in some cases there was a weaker or none correlation between the conductance and other molecular electronic properties including HOMO-LUMO gap, chemical geometries, and interactions energies. All these results together suggest that using the molecular polarizability as a molecular descriptor for conductance can offer some advantages compared to using other molecular electronic properties and can give additional insight about the electronic transport property of a junction. These results also show the validity of the physically intuitive picture that to a first approximation a molecule in a junction behaves as a dielectric that is polarized in the opposite sense of the applied bias, thereby creating an interfacial barrier that hampers tunneling. The use of the polarizability as a descriptor of molecular conductance offers signicant conceptual and practical advantages over a picture based in molecular orbitals. Despite the simplicity of our model, it sheds light on a hitherto neglected connection between molecular polarizability and conductance and paves the way for further conceptual and theoretical developments. The results of this work was sent to two publications. One of them was accepted in the International Journal of Nanotechnology (IJNT) and the other is still under review in the Journal of Physical Chemistry C. === Dissertation/Thesis === Masters Thesis Chemistry 2014