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<sup>TS</sup>/O<sub>2</sub>C–R<sub>1</sub>–R<sub>2</sub>–H//Ga<sub>2</sub>O<sub>3</sub>/EGaIn, where Ag<sup>TS</sup> is template-stripped silver and EGaIn is the eutectic alloy of gallium and indium; R<sub>1</sub> and R<sub>2</sub> refer to two classes of insulating molecular units(CH<sub>2</sub>)<sub><i>n</i></sub> and (C<sub>6</sub>H<sub>4</sub>)<sub><i>m</i></sub>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<sub>1</sub> and R<sub>2</sub> in the junction does not alter the overall rate of charge transport. By using the Ag/O<sub>2</sub>C interface, this system decouples the highest occupied molecular orbital (HOMO, which is localized on the carboxylate group) from strong interactions with the R<sub>1</sub> and R<sub>2</sub> units. The differences in rates of tunneling are thus determined by the electronic structure of the groups R<sub>1</sub> and R<sub>2</sub>; these differences are <i>not</i> influenced by the order of R<sub>1</sub> and R<sub>2</sub> in the SAM. In an electrical potential model that rationalizes this observation, R<sub>1</sub> and R<sub>2</sub> contribute independently to the height of the barrier. This model explicitly assumes that contributions to rates of tunneling from the Ag<sup>TS</sup>/O<sub>2</sub>C and H//Ga<sub>2</sub>O<sub>3</sub> interfaces are constant across the series examined. The current density of these series-tunneling junctions can be described by <i>J</i>(<i>V</i>) = <i>J</i><sub>0</sub>(<i>V</i>) exp­(−β<sub>1</sub><i>d</i><sub>1</sub> – β<sub>2</sub><i>d</i><sub>2</sub>), where <i>J</i>(<i>V</i>) is the current density (A/cm<sup>2</sup>) at applied voltage <i>V</i> and β<sub><i>i</i></sub> and <i>d</i><sub><i>i</i></sub> are the parameters describing the attenuation of the tunneling current through a rectangular tunneling barrier, with width <i>d</i> and a height related to the attenuation factor β.