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TT8.05 - Metal Oxide/Electrolyte Interfaces: Unprecedented Insight by Electrochemistry and nanoIR 
April 8, 2015   2:45pm - 3:00pm

Electrolyte-gated (EG) thin film transistors make use of electrolytes, such as ionic liquids and ion gels, to replace conventional dielectrics, such as SiO2. The low driving voltage (< 2 V) and printable nature of EG-transistors render them interesting for flexible, large-area applications [1-2]. The working principle of EG transistors can be explained by, at least, two mechanisms: (i) field-effect, where the channel conductivity is controlled by the electrostatic doping of active layer and (ii) electrochemical, where the channel conductivity is modulated by ions insertion/removal in/from the film. Apart from their technological potential, EG transistors are valuable platforms to investigate fundamental processes at electrolyte/semiconductor interfaces. Indeed, their planar architecture gives direct access to optical, chemical, and morphological characterizations. However, despite significant advancements achieved in the past few years in the field of electrolyte gating, the fundamental chemicophysical processes governing the doping of metal oxide materials of primary importance for energy conversion, energy saving, and display applications are yet to be fully understood. This is particularly true for EG transistors using, as the electrolyte, ionic liquids and ion gels due to their mechanical properties. A few examples of the questions not yet answered are: how the size of the ions constituting the ionic liquid (ion gel) affects the mechanism of doping (electrostatic vs electrochemical)? Is it possible to electrostatically (without ion insertion) induce an optical density change in electrochromic films? Here, using a combination of cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), transistor, and nanoIR (Atomic Force Microscopy (AFM) coupled to spatially resolved Infrared (IR) spectroscopy) measurements we studied doping/dedoping processes in EG transistors based on sol-gel synthesized WO3 and TiO2 thin films and ionic liquid or ion gel as the electrolyte. EIS measurements provided key insight on the interfacial capacitance of electrolyte (ionic liquid)/thin film (WO3 or TiO2), in turn permitting the calculation of the charge accumulated in the films. Our EG metal oxide transistors can be operated at gate-source biases as low as 1-1.5 V, have onset voltages of ca 0.5 V, and charge density of ca 1014-1015 charge carriers cm-2. The nanoIR technique, combining AFM imaging with chemical characterization by infrared spectroscopy resolved at the nanoscale, gives unprecedented insight on the evolution of the film morphology and chemical composition as a function of the advancement of the degree of doping. 1. Hong, K., et al., Printed, sub?2V ZnO Electrolyte Gated Transistors and Inverters on Plastic. Advanced Materials, 2013. 25(25): p. 3413-3418. 2. Tarabella, G., et al., New opportunities for organic electronics and bioelectronics: ions in action. Chemical Science, 2013. 4(4): p. 1395-1409.

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