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

Abstract

Odd-even effects in molecular junctions with self-assembled monolayers (SAMs) of n-alkanethiolates have been rarely observed. It is challenging to 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 Ag(A-TS)-SC(n)//GaO(x)/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 Ag(A-TS)-SC(n)//GaO(x)/EGaIn junctions are attributed to the properties of the SAMs and SAM-electrode interactions which both determine the shape of the tunneling barrier.