# Atomic Force Spectroscopy on Ionic Liquids
## 6 Towards Three-Dimensional Mapping of the Interface Layer
[124] employed frequency modulation (FM)-AFM force mapping, using a quartz tuning fork as a qPlus sensor, which has a higher \(Q\) factor than standard cantilevers used for the optical lever technique and thus can also provide high resolution in viscous liquids. On the [Py1,4][FAP]/Au(111) interface, they were able to measure force spectroscopy series by recording frequency shift-separation curves at different positions of the surface. By applying bias to the substrate, they could monitor the changes to the electrical polarization. They found a characteristic ordered solvation structure detected as oscillations in the frequency shift signal, corresponding to the length of an ion pair. The same FM-AFM technique was applied to the [Py1,4][FAP]/KCl(100) interface by [PERSON] et al. [125]. In this study, an alkali-halide substrate was employed, as it forms a neutral surface, having the same quantity of anions and cations in the uppermost surface layer. Hence, a different interface layer structure than that on electrified metal electrodes will form. In Figure 18a, a series of frequency shift-distance curves measured along a line of 4 nm is shown as a 2D map. A distinct oscillation in the frequency shift signal with up to five stripes can be identified. This is illustrated by the cross sections displayed in Figure 18b-e, extracted from the 2D map at four different positions. The distance between the stripes is smaller than the ion pair size of [Py1,4][FAP] and rather corresponds to the lattice constant of KCl. The frequency shift-distance curves exhibit a sawtooth-like pattern with distinct jumps, which is a completely different behavior than that observed by FM-AFM on electrified surfaces and showing sinusoidal curves. Hence, [PERSON] et al. [125] concluded that those jumps are not related to the existence of solvation layers but are caused by a tip-induced dissolution of the KCl surface in the ionic liquid. This example shows that care must be taken when interpreting force-separation curves and that not all observed step-like features are indicative of the existence of interface layers. A further approach to image the interface structure was followed using AM-AFM. This technique has been successfully applied to image the HOPG/PAN or HOPG/[EMIm][TFSI] interface [126; 127]. Two-dimensional maps of the in-plane structure of the interface have been obtained with a high resolution comparable to STM, and a laterally heterogeneous interface layer has been identified. [PERSON] et al. [128] extended the AM-AFM investigations on HOPG/PAN to the third dimension by also performing dynamic force spectroscopy. The phase and amplitude were measured as a function of the separation, revealing a layered interface structure. [PERSON] et al. [47] applied the combination of AM-AFM mapping and spectroscopy of the five different ionic liquids, ethylammonium nitrate (EAN), propylammonium nitrate (PAN), ethanolammonium nitrate (EtAN), ethylammonium formate (EAF), and N,N-dimethylethylammonium formate (DMEAF). Phase- and amplitude-separation curves were measured on the mica surface. Complementary AM-AFM maps of the innermost and near-surface layers have been obtained using cantilevers of different stiffness, thus providing spatially-resolved information in three dimensions. The results, shown in Figure 19 provide evidence for a strong influence of molecular configuration of the cation on the interface structure. Figure 18: Frequency modulation (FM)-AFM investigation of the [Py1,4][FAP]/KCl(100) interface. (**a**) Frequency shift–distance curves measured along a distance of 4 nm; (**b–e**) cross-sections obtained at the lines marked in (**a**). Adapted with permission from [PERSON] et al. [125]. Copyright 2014, the American Chemical Society. It can be seen that cations with short alkyl lengths form a superstructure related to the ionic ordering of the mica surface, providing a template for structuring the innermost layer of the ionic liquid (see center column).