# Atomic Force Spectroscopy on Ionic Liquids
## 6 Towards Three-Dimensional Mapping of the Interface Layer  
As the electric double layer not only exhibits a characteristic layered structure in the out-of-plane direction, as illustrated by the AFS examples above, but may also feature in-plane inhomogeneities, three-dimensional mapping is necessary in order to obtain a complete picture of electrochemical interface reactions. Using AFM-based techniques, two approaches to 3D mapping can be distinguished. Either force-separation curves can be recorded at different lateral positions using AFS, or classical AFM scans can be obtained at different separations between tip and surface, thus revealing slices through the liquid. These experiments have been primarily performed using the dynamic AFM modes, employing amplitude modulation (AM) or frequency modulation (FM), which are well-established techniques for mapping surfaces in liquids with atomic resolution [120]. Dynamic AFM has also been successfully applied to monitor the electric double layer in conventional electrolytes [121]. In order to map a solid-liquid interface by AFM, one must ensure a careful sample and tip preparation, an extremely low vibration and noise level, and sufficient thermal stability. Due to these experimental challenges, the number of publications showing 3D data of the interface layer obtained by AFM is small compared to those presenting single point AFS curves. Using contact-mode AFM, [PERSON] et al. [122] performed investigations of defects in the double layer structure between HOPG and [EMIm][TFSI], as shown in Figure 17. They measured 20 force-separation curves at positions separated by 25 nm from each other along a defined line across the HOPG surface (see Figure 17a). The resulting curves were compiled into a map providing a cross-section through the interface layer (see Figure 17b). The layered interface structure can be clearly identified here. It can also be seen that distortions of the structure are present in the liquid above the step edges of the HOPG  
Figure 16: Force–separation curves measured in [Py1,4][TIFSA] with different concentrations of Ag on the Au surface. Adapted from [PERSON] et al. [119]. substrate. In a small region, the ionic layers bend to compensate for the disturbance induced by the topographic defect. This becomes obvious with regard to the force-separation histograms obtained above the basal plane, which show up to five layers (Figure 17c) and above a step edge showing only one layer clearly (Figure 17d). To illustrate the interplay between the topography of the solid surface and the ordering of the ionic liquid, a second region of the HOPG surface was investigated by [PERSON] et al. [122], which contained no single step edges but few larger steps, as shown in the topography line scan in Figure 17e. The map of the force-separation curve measured along the line scan shown in Figure 17e reveals the presence of edge dislocations in the ionic liquid interface structure. Those dislocations are present above the basal plane of the substrate and are related to a distortion of the layered structure spreading over 50 nm to 80 nm. This behavior is in correspondence with observations of conventional liquid crystals [123] that also show topological effects, which can have an impact on the electrochemical performance when using ionic liquids. Figure 17: Influence of topological defects on the interface structure between highly oriented pyrolytic graphite (HOPG) and [EMIM][TFSI]. (**a**) Topography of HOPG; (**b**) force–separation curves measured at 20 points as marked in (**a**); (**c**) force–separation histograms obtained on the basal plane; and (**d**) from the step edge; (**e**) topography line profile with a length of 1 \(\mu\)m; (**f**) force–separation curves measured along the line in (**e**) showing different defect regions. Adapted with permission from [PERSON] et al. [122]. Copyright 2015, Elsevier. To map the solvation layers in ionic liquids, [PERSON] et al.