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Abstract
The field of nanoionics has rapidly emerged as a transfonnative science with potential to provide immense fundamental and technological insight into an array of ionic systems including devices for electrochemical energy conversion and storage. While knowledge of concepts related to ionic space charge layer influences and the resulting ability to manipulate ionic conduction have increased considerably in recent years, the pursuit of a complete fundamental understanding of nanoionic phenomena continues. With these motivations, the development and study of a solid state ionic field effect transistor (IFET) is presented as a platfom1 for the study of nanoionics.
An IFET, a parallel to a metal-oxide-semiconductor field effect transistor, utilizes an external electric field to introduce an ionic space charge layer in an ion conducting material such as the chosen proton conducting polymer, N afion. The material response, monitored as the current through the device, provides the desired insight into prope1iies of the ionic space charge layer. Implementation of double layer modeling has resulted in estimates of electric potential and charge carrier concentration profiles across the conduction channel for surface potentials up to 0.1 V and carrier concentrations to 1.0 M. The limited spatial extent of the ionic space charge layer has been calculated to be on the order of a few nanometers. As a result, equivalent circuit modeling of the device predicts small but potentially measurable device responses to a sinusoidal gating voltage.
Devices constructed from N afion membranes of thicknesses ranging from 600nml75 μm were characterized with gating voltages of 0.2-10.5 V in amplitude. Studies of several different device configurations show indications of ionic space charge layer manipulation but often along with significant undesirable current leakage. The most promising configurations examined were either geometrically complex or were found without explanation to be inconsistent or irreproducible. Experimental challenges have brought forth a series of recommendations for future work including consideration of device dimensions, material preparation, and characterization strategies. These valuable experimental insights and theoretical foundations provide new innovative and constructive progress toward the realization of an ionic field effect transistor.
