Advanced in Operando Atomic Force Microscopy Studies of the Gating Mechanisms at Metal Oxide Ion-Gated Transistors

Characterization of interfaces by advanced techniques <i>in operando</i>, including Atomic Force Microscopy (AFM), are gaining significance in the scientific investigation of electrochemical devices since they permit to follow materials’ dynamics, which in turn are linked to device performance.<br/>Ion-Gated Transistors (IGTs) enable the modulation of electronic conductivity in a semiconducting channel upon the application of an electrical bias between the channel material and the gate electrode, through an ionic medium. Metal oxides such as WO₃ and TiO₂ as channel materials interfaced with ionic liquids, such as 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]), shows great promise for low-power, large-area electronics but also, at the fundamental level, the study insulator-to-metal transitions. The presence of Li⁺ ions in the ionic liquid can enhance the versatility of IGTs in terms of the degree of doping and time response ^1,2.<br/>The doping mechanisms establishing the effectiveness of the ion gating mechanism are not well understood. Gaining insight into the structure of the electric double layer (EDL) forming at the channel ionic medium interface is expected to advance such understanding.<br/>We report on a nanoscale study at interfaces between WO₃ and TiO₂ films and ionic liquids, possibly including Li ions, by AFM, force-distance profiling. The aim of the study is to shed light on phenomena and processes such as ion layering, ion packing density and shrinking, and ion reorientations. Scanning probe studies have been paralleled by computational efforts (Molecular Dynamics, MD). AFM force-distance profiling in conjunction with simulation methods, allows the characterization and understanding of the structure of the EDL and its evolution with the applied bias, thus permitting optimizing device performance.