Importance of impedance spectroscopy in self-assembled monolayer-based large-area tunnel junctions

Abstract

Two-terminal direct current (DC) measurements are generally used for the electrical characterization of self-assembled monolayer (SAM)-based large-area tunnel junctions. In DC measurements, the total current through the molecular junction is measured and it is thus impossible to separate molecular effects from interface effects. The charge transport parameters are derived indirectly from the current–voltage relationship. For instance, contact resistance at the interface between the electrode and SAMs is estimated by extrapolating the resistance versus chain length curve. On the other hand, through impedance spectroscopy along with equivalent circuit modelling, each component of the junction can be determined. This paper compares the contact resistance values estimated using both DC and impedance spectroscopic data of alkanethiolate SAM-based molecular junctions reported previously. We show that in the case of molecular tunnel junctions with a resistive protective layer (PL), the widely used DC measurements alone fail to show the actual molecular behaviour of the junction as the charge transport through the junction is dominated by the PL itself. We also show that contact resistance estimated from the DC measurements on defective SAM junctions can be wrong and leads to the incorrect transport mechanism, while the impedance measurements estimate the actual values of contact resistance and identify the bottlenecks in charge transport through such a defective molecular junction.