Chaitanya Gupta1, Mark A. Shannon2, and Paul J. A. Kenis1. (1) Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, 114 Roger Adams Laboratory, Box C-3, 600 S. Mathews Avenue, Urbana, IL 61801, (2) Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, 2132 Mechanical Engineering Lab MC-244, 1206 West Green Street, Urbana, IL 61801
We propose a methodology to quantify and measure charge transport through an alkane thiol monolayer film that is self assembled on a polycrystalline gold substrate and in contact with an aqueous electrolyte solution that contains no redox active species. The constitutive expression for the transport of charge through a dielectric medium is modified to yield a mechanistic description of the admittance of a gold-monolayer-electrolyte system, where transport constants like charge mobility and diffusivity that appear as unknowns are estimated using a scaling argument. This admittance expression, when fit to an experimentally obtained voltage spectrum of impedance values, enables the calculation of parameters that characterize charge transport in the monolayer. The nature of the charge transport process through the monolayer film immobilized on gold is dependent on the magnitude of the electric field that exists within the organic thin film and is shown to follow well characterized solid-state charge transport models of Ohmic conduction at low electric fields and space charge limited current flow at larger fields. At very large fields (~ 109 V/m), the transport of charge is limited by the quantum mechanical tunneling of electrons from the ground state energy level of the immobilized ions at the Stern layer to the LUMO of the monolayer phase. The effect of electrolyte properties of the various transport processes mentioned above is also examined.