Investigation of Ion-Dipole Interaction in Ion-Gated Synaptic Transistors Incorporating Interfacial Fluorinated Molecules

Electrolyte-gated transistors (EGTs) have been emerged as candidates for synaptic transistors in analog computing due to their outstanding characteristic of high energy efficency, fast computing speed, and potential for a wide range of applications. However, EGTs encounter a critical limitation into the application in analog computing systems, characterized by abrupt changes in ion movement under external electric fields. Also, the gating effect by ions is determined by the ions present at the channel/electrolyte interface, and such analysis and understanding in devices are still lacking. Therefore, considering this inherent nature in EGTs is crucial and requires a comprehensive understanding of electrostatic interactions to precisely modulate ion movement at the interaface.<br/>In this study, we propose a novel solution focusing on ion-dipole interaction by introducing Heneicosafluorododecylphosphonic acid (DDPA) as a interfacial ion receptor on the InGaZnO oxide semiconductor channel. The advantage of using DDPA lies in its structure, which includes 9 fluoroalkyl chains and a trifluoromethyl group. This configuration creates a significant dipole moment, facilitating strong coulombic interactions with lithium ions, even with a molecular length of ~2 nm. This approach induces strong ion-dipole forces between lithium ions and the negatively charged DDPA at the channel/electrolyte interface. The incorporation of DDPA enables the trapping of lithium ions at the van der Waals (vdW) gap between each DDPA molecules. Consequently, lithium EGTs incorporating DDPA exhibit exceptional synaptic characteristics, such as nearly linear conductance switching and long-term retention exceeding 30 minutes, even at switching voltages below 3 V.<br/>Furthermore, chemical analysis and density functional theory (DFT) calculations demonstrate that DDPA can facilitate sequential lithium ion trapping at the interface through ion-dipole interaction. We found that lithium is most energetically stable when positioned within the vdW gap between DDPA molecules, overcoming the vdW forces within the gap. Additionally, this resulted in observable changes in the binding energy of F 1s, as measured by X-ray Photoelectron Spectroscopy. Therefore, we confirmed that analysis results were consistent with claims regarding the effects of DDPA. This presented novel method provides ideas for modulating ion movement in EGTs, enhancing their potential for use in applications for multimodal analog computing.