## Molecular Series-Tunneling Junctions

This paper explores tunneling current densities in self-assembled monolayer -based junctions with the structure AgTS/O2C‒R1‒R2‒H//Ga2O3/EGaIn, where AgTS 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 m―that are connected in series and have different tunneling decay constants in the Simmons equation

Kung-Ching Liao

2015

#### Scholarcy highlights

• Charge transport by tunneling through metal‒molecule‒metal junctions―junctions whose electronic features are modeled by a potential barrier1-9 and by molecular orbitals10-19― cannot be described adequately by classical diffusion, or by drift transport of charge.20-23 The classical circuit law states that the total resistance of two or more Ohmic resistors connected in series is the sum of the resistance of each resistor; that is, the sequence in which these resistors are assembled does not influence the overall current across the circuit
• This paper explores tunneling current densities in self-assembled monolayer-based junctions with the structure AgTS/O2C‒R1‒R2‒H//Ga2O3/EGaIn, where AgTS 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 andm―that are connected in series and have different tunneling decay constants in the Simmons equation
• 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 at applied voltage V, and βi and di 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
• This paper describes experiments that test the relationship of tunneling current to the order of aromatic and aliphatic groups in junctions of the form AgTS/O2C‒R1‒R2‒H//Ga2O3/EGaInn and
• In the context of experiments in which we consider the Ag/O2CR and H//Ga2O3 interfaces to be constant, we examined the sensitivity of tunneling currents to the permutation of R1 and R2 in the junction, and derived a form of the modified Simmons equation, J(V) = J0(V)exp(–β1d1 – β2d2), to describe the rate of charge transport across these junctions
• Using a potential barrier model that explicitly assumes constant contributions to rates of tunneling from the interfaces between the SAMs and the electrodes, we found thatn andm segments contribute independently but differently to the shape of the tunneling barrier, and that the values of β forn andm are independent of the order in which they are assembled
• The chemical interactions between the molecular units in the junction or at the self-assembled monolayer–metal interfaces, induces significant changes in the electronic structure of the individual units or their assembly, we detect changes in both the topography of the tunneling barrier and the rate of charge transport

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