Voltage Controlled Interfacial Layering in an Ionic Liquid on SrTiO$_3$

TL;DR

This study investigates how ionic liquid layering on SrTiO3 surfaces responds to applied voltage, revealing increased layer thickness and charge accumulation under positive bias through x-ray reflectivity analysis.

Contribution

It provides the first detailed structural analysis of ionic liquid layering on SrTiO3 under applied potential using x-ray reflectivity and a modified crystal model.

Findings

Layer thickness increases with positive bias.

Significant cation enrichment occurs at the interface under bias.

Charge density oscillates near the surface with and without applied voltage.

Abstract

One prominent structural feature of ionic liquids near surfaces is formation of alternating layers of anions and cations. However, how this layering responds to applied potential is poorly understood. We focus on the structure of 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl) trifluorophosphate (BMPY-FAP) near the surface of a strontium titanate (SrTiO$_3$) electric double-layer transistor. Using x-ray reflectivity, we show that at positive bias, the individual layers in the ionic liquid double layer thicken and the layering persists further away from the interface. We model the reflectivity using a modified distorted crystal model with alternating cation and anion layers, which allows us to extract the charge density and the potential near the surface. We find that the charge density is strongly oscillatory with and without applied potential, and that with applied gate bias of 4.5 V the first two layers become significantly more cation rich than at zero bias, accumulating about $2.5 \times 10^{13}$ cm$^{-2}$ excess charge density.