Asymmetric electron transport and highest occupied molecular orbital assisted tunneling through Zn-porphyrin molecular junctions

We report electron transport measurements from gold-zinc-porphyrin-gold molecular junctions formed in an electromigrated nanogap. Asymmetric current-voltage (I-V) behaviors about the zero bias voltage were observed at room temperature and 4.2 K. These observations are in contrast to measurements from a nanogap without any molecules, which are dominated by tunneling and display symmetric I-V characteristics. In addition, increasing the gate voltage suppressed the current through the junction at room temperature, indicating electron tunneling proceeded through the highest occupied molecular orbital. Density of states calculations were performed to explain these findings and understand the microscopic origins of the observations.