Molecular Series-Tunneling Junctions

Charge 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/O2C-R-1-R-2-H//Ga2O3/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-(CH2)(n) and (C6H4)(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/O2C 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 this,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/O2C and H//Ga2O3 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(-beta(1)d(1) - beta(2)d(2)), where J(V) is the current density (A/cm(2)) at applied Voltage V and beta(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 beta.