ja5b00448_si_001.pdf (1.41 MB)

Download file# Molecular Series-Tunneling Junctions

journal contribution

posted on 13.05.2015, 00:00 authored by Kung-Ching Liao, Liang-Yan Hsu, Carleen M. Bowers, Herschel Rabitz, George M. WhitesidesCharge transport through junctions
consisting of insulating molecular
units is a quantum phenomenon that cannot be described adequately
by classical circuit laws. This paper explores tunneling current densities
in self-assembled monolayer (SAM)-based junctions with the structure
Ag

^{TS}/O_{2}C–R_{1}–R_{2}–H//Ga_{2}O_{3}/EGaIn, where Ag^{TS}is template-stripped silver and EGaIn is the eutectic alloy of gallium and indium; R_{1}and R_{2}refer to two classes of insulating molecular units(CH_{2})_{n}and (C_{6}H_{4})_{m}that are connected in series and have different tunneling decay constants in the Simmons equation. These junctions can be analyzed as a form of series-tunneling junctions based on the observation that permuting the order of R_{1}and R_{2}in the junction does not alter the overall rate of charge transport. By using the Ag/O_{2}C interface, this system decouples the highest occupied molecular orbital (HOMO, which is localized on the carboxylate group) from strong interactions with the R_{1}and R_{2}units. The differences in rates of tunneling are thus determined by the electronic structure of the groups R_{1}and R_{2}; these differences are*not*influenced by the order of R_{1}and R_{2}in the SAM. In an electrical potential model that rationalizes this observation, R_{1}and R_{2}contribute independently to the height of the barrier. This model explicitly assumes that contributions to rates of tunneling from the Ag^{TS}/O_{2}C and H//Ga_{2}O_{3}interfaces are constant across the series examined. The current density of these series-tunneling junctions can be described by*J*(*V*) =*J*_{0}(*V*) exp(−β_{1}*d*_{1}– β_{2}*d*_{2}), where*J*(*V*) is the current density (A/cm^{2}) at applied voltage*V*and β_{i}and*d*_{i}are the parameters describing the attenuation of the tunneling current through a rectangular tunneling barrier, with width*d*and a height related to the attenuation factor β.