Transition Voltage Spectroscopy of Porphyrin Molecular Wires

Measurements of the current-voltage (I-V) characteristics of individual thiol-tethered porphyrin molecules (isolated in an alkanethiol matrix) and of self-assembled monolayers are presented. In both cases, it is found that I/V-2 displays a minimum at a characteristic transition voltage V-m. Repeated measurements of the transition voltage enable both its time development and statistical behavior to be determined. For isolated molecules, the transition voltage shows a multipeaked distribution of values, indicating the presence of a small number of distinct molecular/contact configurations, each having different transport characteristics. For self-assembled monolayers, in contrast, a single-peaked distribution is observed, which is consistent with parallel conduction through many molecules.