Equivalent Circuits of a Self-Assembled Monolayer-Based Tunnel Junction Determined by Impedance Spectroscopy

The electrical characteristics of molecular tunnel junctions are normally determined by DC methods. Using these methods it is difficult to discriminate the contribution of each component of the junctions, e.g., the molecule–electrode contacts, protective layer (if present), or the SAM, to the electrical characteristics of the junctions. Here we show that frequency-dependent AC measurements, impedance spectroscopy, make it possible to separate the contribution of each component from each other. We studied junctions that consist of self-assembled monolayers (SAMs) of n-alkanethiolates (S(CH2)n−1CH3 ≡ SCn with n = 8, 10, 12, or 14) of the form AgTS-SCn//GaOx/EGaIn (a protective thin (∼0.7 nm) layer of GaOx forms spontaneously on the surface of EGaIn). The impedance data were fitted to an equivalent circuit consisting of a series resistor (RS, which includes the SAM-electrode contact resistance), the capacitance of the SAM (CSAM), and the resistance of the SAM (RSAM). A plot of RSAM vs nC yielded a tunneling decay constant β of 1.03 ± 0.04 nC–1, which is similar to values determined by DC methods. The value of CSAM is similar to previously reported values, and RS (2.9–3.6 × 10–2 Ω·cm2) is dominated by the SAM–top contact resistance (and not by the conductive layer of GaOx) and independent of nC. Using the values of RSAM, we estimated the resistance per molecule r as a function of nC, which are similar to values obtained by single molecule experiments. Thus, impedance measurements give detailed information regarding the electrical characteristics of the individual components of SAM-based junctions.