Molecular Switches: Can a two-state conductance be controlled in single porphyrin molecules?

Tuesday, January 18, 2011 – 3:15pm
Reiss 502
Kim Michelle Lewis
Rensselaer Polytechnic Institute, Department of Physics

Realizing how one can manipulate conductance switching in molecular junctions can be essential in controlling the overall functionality of a molecule when it is wired into a circuit. There have been many reports of mechanisms responsible for conductance switching. Mechanisms include, switching that is electric field driven, variations in the contact geometries between bonds at the end of the molecule and substrate, charge bistability and conformational changes in the molecules. We show conductance switching in vertically aligned tunnel junctions formed by porphyrin molecules ligating a zinc or iron atom. Measurements were taken by forming single molecule junctions between a scanning tunneling microscope (STM) tip and a gold substrate. Peaks in the conductance histogram show molecules switched from a low to a high conducting state upon mechanical stretching of the molecule. The mechanical stretching we refer to is related to the change in the vertical alignment of the molecule with respect to the substrate as the molecule is pulled away from the surface. We believe that conformational changes, redox states, and electric field driven switching in the molecule may play a role in the two-state conductance we observe. Also, we explore possibilities to observe two-state conductance switching and vibrational modes in porphyrin molecules positioned between two fabricated electrodes separated by a nanogap.

Host: Paola Barbara