posted on 2016-03-09, 00:00authored byTrevor
A. Petach, Apurva Mehta, Ronald Marks, Bart Johnson, Michael F. Toney, David Goldhaber-Gordon
One prominent structural feature
of ionic liquids near surfaces
is formation of alternating layers of anions and cations. However,
how this layering responds to an 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
(SrTiO3) 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 an applied gate bias
of 4.5 V the first two layers become significantly more cation rich
than at zero bias, accumulating about 2.5 × 1013 cm–2 excess charge density.