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–R₁–R₂–H//Ga₂O₃/EGaIn, where Ag^TS is template-stripped silver and EGaIn is the eutectic alloy of gallium and indium; R₁ and R₂ 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 R₁ and R₂ 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 R₁ and R₂ units. The differences in rates of tunneling are thus determined by the electronic structure of the groups R₁ and R₂; these differences are not influenced by the order of R₁ and R₂ in the SAM. In an electrical potential model that rationalizes this observation, R₁ and R₂ 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) = J₀(V) exp­(−β₁d₁ – β₂d₂), where J(V) is the current density (A/cm²) 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.