The Origin of the Odd-Even Effect in the Tunneling Rates across EGaln Junctions with Self-Assembled Monolayers (SAMs) of n-Alkanethiolates

Odd-even effects in molecular junctions with self-assembled mono-layers (SAMs) of n-alkanethiolates have been rarely observed. It is challenging to 2 pinpoint the origin of odd-even effects and address the following question: are the odd-even effects an interface effect, caused by the intrinsic properties of the SAMs, or a combination of both? This paper describes the odd-even effects in SAM-based tunnel junctions of the form AgA-Ts-SCn//GaOx/EGaIn junctions with a large range of molecular lengths (n = 2 to 18) that are characterized by both AC and DC methods along with a detailed statistical analysis of the data. This combination of techniques allowed us to separate interface effects from the contributions of the SAMs and to show that the odd-even effect observed in the value of J obtained by DC-methods are caused by the intrinsic properties of the SAMs. Impedance spectroscopy (an AC technique) allowed us to analyze the SAM resistance (R-SAM), SAM capacitance (C-SAM), and contact resistance, within the junctions separately. We found clear odd-even effects in the values of both R-SAM and C-SAM, but the odd-even effect in contact resistance is very weak (and not responsible for the observed odd-even effect in the current densities obtained by J(V) measurements). Therefore, the odd-even effects in AgA-Ts-SCn//GaOx/EGaIn junctions are attributed to the properties of the SAMs and SAM-electrode interactions which both determine the shape of the tunneling barrier.