Understanding interface (odd–even) effects in charge tunneling using a polished EGaIn electrode

Charge transport across large area molecular tunneling junctions is widely studied due to its potential in the development of quantum electronic devices. Large area junctions based on eutectic gallium indium (used in the form of a conical tip top electrode) have emerged as a reliable platform for delineating structure–property relationships. Discrepancies, however, arise from different tip-morphologies and fabrication techniques. It can be, therefore, challenging to make reliable conclusions based on molecular features. Of particular note is the discrepancy between the behaviors of hydrocarbons containing odd and even numbered carbons across different EGaIn electrodes. Moreover, inconsistencies in tip roughness and oxide thickness can lead to more than a 100× increase in current densities with narrow distribution in data. Besides effects on the precision vs. accuracy of data, a theoretically predicted length-dependent limit to observation of the odd–even effect has not been realized experimentally. We developed a method to chemically polish the EGaIn tip to allow formation of smooth conformal contact due to re-establishment of liquid character at the point of contact though tension-driven reconstruction of a thin oxide layer. To evaluate the polished tip, we measured charge transport behavior across n-alkanethiolate SAMs and observed good correlation in the odd–even oscillation behavior to that observed from wetting studies. Since these molecules are homologues of each other, only differing in the orientation of the terminal CH2CH3 moiety, the odd–even effects are governed by orientation induced differences in the absences of SAM (gauche) defects. Comparison of obtained data with the literature shows significant difference between odd-numbered SAMs across Ag and Au.