Atomic Force and Scanning Tunneling Microscopy of Ordered Ionic Liquid Wetting Layers from 110 K up to Room Temperature.

Ionic liquids (ILs) are used as ultrathin films in many applications. We studied the nanoscale arrangement within the first layer of 1,3-dimethylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C1C1Im] [Tf2N]) on Au(111) between 400 and 110 K in ultrahigh vacuum by scanning tunneling and noncontact atomic force microscopy with molecular resolution. Compared to earlier studies on similar ILs, a different behavior is observed, which we attribute to the small size and symmetrical shape of the cation: (a) In both AFM and STM only the anions are imaged; (b) only long-range-ordered but no amorphous phases are observed; (c) the hexagonal room-temperature phase melts 30-50 K above the IL's bulk melting point; (d) at 110 K, striped and hexagonal superstructures with two and three ion pairs per unit cell, respectively, are found. AFM turned out to be more stable at higher temperature, while STM revealed more details at low temperature.

Ionic liquids (ILs) have received significant interest in surface and interface science during the last two decades.[1−3] They typically combine low melting points (often below room temperature) with very low vapor pressures,[4] which facilitates surface science studies in ultrahigh vacuum (UHV).[5−8] The large number of possible cations and anions with an enormous range of combinations allows for an excellent tunability of the properties of the ILs and enables tailoring them for specific applications.[9,10] Besides the fundamental interest in the interface properties of liquids, the gas/IL and IL/support interfaces are of specific importance for lubrication,[11−13] separation,[14−18] electrochemistry,[19−29] and catalysis[30−40] applications. In catalysis, ILs even stimulated the new concepts “supported ionic liquid phase” (SILP)[36,38,39] catalysis and “solid catalyst with ionic liquid layer” (SCILL).[40] For both, the inner surface of porous particles is covered with a thin IL film. In SILP, this IL film acts as a nonvolatile solvent for the reactants and the homogeneously dissolved catalyst, and in SCILL a heterogeneous catalyst—typically metal nanoparticles on porous oxide supports—is covered by the IL film. In particular for SCILL, it is of utmost importance to understand the interaction of the IL with the metal surface[40−42] and its influence on reactivity, selectivity, and catalyst poisoning.[40,43−45] This knowledge is required to optimize such catalysts for specific reactions and conditions.[3,41,46−50]

A variety of powerful tools have been applied to study the initial stages of the adsorption of ILs on surfaces under clean UHV conditions. These include X-ray photoelectron spectroscopy (XPS), UV photoelectron spectroscopy (UPS), reflection adsorption infrared spectroscopy (RAIRS), and scanning tunneling microscopy (STM).[3,41,51−58] In particular, angle-resolved XPS (ARXPS) in combination with physical vapor deposition (PVD)[51,59] of ILs yielded detailed insights into ultrathin IL films. One pioneering work was the identification of the so-called checkerboard structure of the very first layer of 1-methyl-3-octylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C8C1Im][Tf2N]) and 1,3-dimethylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C1C1Im][Tf2N]) on Au(111) at room temperature (RT). In this checkerboard structure, the cationic and anionic head groups are (on average) adsorbed next to each other, in direct contact with the metal surface.[52] This adsorption configuration was later confirmed by STM for frozen single layers of [C8C1Im][Tf2N] and of some other ILs on metal surfaces at temperatures well below the melting points of the bulk ILs (see Table S1 in the SI). At very low temperatures (typically 90–150 K), highly ordered IL layers were observed. To the best of our knowledge, the highest reported temperature where ordered structures have been observed so far was 227 K for [C2C1Im][Tf2N] on Au(111).[57] At RT, however, STM images of these ILs showed only strongly fluctuating stripy features along the fast scanning direction due to highly mobile ions in the 2D liquid (or 2D gas) state,[56,57,60] which are moved by the STM tip. Notably, for [C1C1Im][Tf2N] films, no such STM studies have been carried out yet. Furthermore, to the best of our knowledge, no UHV-based AFM studies on ultrathin IL films have been reported so far at all.

In addition to UHV-based studies of ultrathin IL films, there are a variety of studies of IL/solid interfaces that were performed in the liquid, that is, for macroscopic thick IL films. Such liquid-phase measurements with AFM[61−72] and STM,[69] X-ray or neutron reflectivity,[73,74] sum-frequency generation (SFG),[73,75] or RAIRS[58] allowed for studying systems at ambient conditions or even in operando under an applied potential in an electrochemical cell. However, these conditions are often challenging in terms of impurities, accumulation of contaminations, or potential-induced effects.[3,23,61,76−79] The majority of liquid-phase studies have been performed by AFM, mostly on HOPG, mica, or Au(111); for an excellent review on the structure of ILs down to the nanoscale in the liquid phase see Hayes etal. (and references therein).[1] By measuring force–distance curves, a vertical layering of the IL in the vicinity of the substrate surfaces was deduced that also depends on an applied potential.[61] In a recent liquid-phase AFM study of [C2C1Im][Tf2N] on HOPG, Elbourne etal. resolved the lateral structure of the interface layer with molecular resolution.[61] Recently, Umeda etal. showed atomically resolved 3D solvation structures.[80] In contrast, a much smaller number of liquid-phase STM studies were performed.[69,81−83] Some years ago, the Magnussen group was able to resolve the potential-dependent adlayer structure at the interface of [C4C1Pyrr][Tf2N], [C6C1Im][Cl], and [C6C1Im][Tf2N] with Au(111) by in situ video-STM.[84−86]

In this work, we combine AFM and STM in UHV to study the molecular arrangement of [C1C1Im][Tf2N] on Au(111), with a coverage up to a closed single wetting layer, from 400 K down to 110 K. Besides its inert nature, the Au(111) surface has the advantage of the herringbone reconstruction, which allows for an easy determination of the crystallographic main directions in microscopy.[87] Due to its small and symmetric cation, [C1C1Im][Tf2N] has a rather high bulk melting point of around 300 K within the [CC1Im][Tf2N] IL series;[88,89] thus, we expected the formation of ordered layers at or even above RT, which can be imaged with AFM and STM. Indeed, we are able to observe molecularly resolved STM images of a highly ordered ultrathin IL layer on a metal surface at RT and also demonstrate that molecularly resolved AFM images of ordered ultrathin IL layers can be obtained under these well-defined UHV conditions.

Results and Discussion

Imaging the Wetting Layer at Room Temperature (300 K)

The [C1C1Im][Tf2N] layers were prepared by PVD of a certain amount of IL onto clean Au(111) at temperatures between <170 and 322 K in UHV. Thereafter, they were annealed for several minutes at elevated temperatures (380 K at maximum) and cooled back to 300 K, in order to obtain maximum island sizes; for details see Experimental Section and the Supporting Information (SI). From a previous ARXPS study, we know that [C1C1Im][Tf2N] remains intact upon adsorption on the surface and forms a so-called wetting layer (WL), with cations and anions in contact with the support in the checkerboard structure.[52] In the following, the deposited amount of IL will thus be given in units of this WL; in other words, 1.0 WL means that the surface is fully covered by a wetting layer.

Figure a and b show overview STM (left) and noncontact AFM (right) images after deposition and annealing of [C1C1Im][Tf2N] on Au(111), with coverages of 0.5 and 0.75 WL, respectively; IL-covered (IL-free) areas are marked by green (yellow) dots. In these large-scale STM and AFM images (500 × 250 nm2 scanning area), we observe that the terraces of the Au(111) substrate with several hundred nm widths are partly covered with large IL islands of uniform height. The edges of the IL islands appear very stripy along the fast scanning directions; this effect was seen before in STM studies of other ILs below room temperature and was assigned to an equilibrium between a condensed solid phase forming the 2D islands and highly mobile ions in between; the latter are likely influenced by the moving probe tip.[54−57,90] Notably, this stripe effect is much more pronounced in STM than in AFM, and only very noisy large-scale images could be obtained by STM. This indicates that the interaction of the ions with the tip might be considerably stronger in STM than in noncontact AFM under our measurement conditions. Since the islands are confined in most cases by a Au step edge, we assume that the IL tends to nucleate preferentially at the step edge and grow from there on the terrace when cooling the sample after the annealing step. As will be discussed later, nucleation and formation of small IL islands can also occur directly on terraces when deposition and imaging is done at low temperatures.

Figure 1

STM (a, c, e) and AFM (b, d, f) images measured at 300 K, after deposition of [C1C1Im][Tf2N] on Au(111) between ∼170 and 322 K, followed by annealing before imaging. The coverages and annealing temperatures are 0.5 WL/322 K (a), 0.75 WL/300 K (b), 1.0 WL/379 K (c), 0.75 WL/300 K (d), 1.0 WL/376 K (e), and 0.5 WL/300 K (f). (Details of the preparation, coverages, and tunneling parameters are summarized in Table S2 in the SI). On top of the figure, a schematic sketch of the cation (left) and the anion (right) IL is depicted.

The medium-scale STM and AFM images (100 × 50 nm2) of closed islands in Figure c (1.0 WL deposited) and Figure d (0.75 WL), respectively, clearly show the herringbone reconstruction of the Au(111) surface through the IL layer, which evidence that the surface reconstruction of the Au substrate is not lifted upon IL adsorption. The WL islands themselves exhibit a regular hexagonal dot-like internal structure, and the long-range order reveals a high degree of crystallinity within the solid 2D islands.

The hexagonal arrangement of the RT phase is clearly seen by the round protrusions in the high-resolution (10 × 5 nm2) STM and AFM images of Figure e (1.0 WL deposited) and Figure f (0.5 WL), respectively. Note that the STM image (Figure e) required additional drift correction based on the underlying herringbone reconstruction of the Au(111) surface. Since we observed the hexagonal structure for all coverages investigated by us, we conclude that the densely packed IL islands are stabilized by intermolecular attraction. From the more stable AFM pictures, the next-neighbor distance between protrusions (that is, the length of the vectors of the green unit cell, see Figure f) is measured to be 0.99 ± 0.06 nm. In all high-resolution images, we were never able to achieve submolecular resolution at room temperature, and only round protrusions were observed for the RT phase (the apparent deviations from circular shape in Figure e are assigned to drift effects). As will be discussed later in detail for the low-temperature results, we assume that the protrusions represent the anions, which exhibit fast rotational movements, while the cations do not show up as distinct features in STM and AFM at RT.

The solid IL islands attached to the Au step edges are clearly visible in AFM up to temperatures between 330 and 350 K. This is evident from a series of large-scale AFM images with 0.5 WL coverage, measured successively while slowly cooling from 400 K to room temperature; see Figure S1 in the SI. Above 330–350 K, only blurred gold step edges are detected by AFM, which implies that about 30–50 K above the bulk melting point of [C1C1Im][Tf2N] of 295–299 K,[88,89] the wetting layer loses its solid character and transforms into a 2D liquid or 2D gas. Notably, in contrast to the AFM, STM turned out to be more challenging at these temperatures: Above RT, noise and stripe effects turned out to be too pronounced for stable imaging.

The fact that we find solid WL islands in AFM images at 330–350 K (and thus, even above the IL’s melting point) differs from UHV-based STM findings on Au(111) for [C2C1Im][Tf2N] and [C8C1Im][Tf2N]: The highest temperatures, at which imaging of a nearly closed IL wetting layer was possible so far are 240 K for [C2C1Im][Tf2N] and 200 K for [C8C1Im][Tf2N] on Au(111),[57] which is well below the respective bulk melting temperatures (for both ILs around 260 ± 10 K;[88,89] note that the values of the bulk melting temperatures for the same ILs from different studies can differ by up to 20 K; see Table S1 in the SI). We attribute this fact to the absence of longer side groups and the higher symmetry of [C1C1Im]+ compared to [C2C1Im]+ and [C8C1Im]+, which possibly leads to a better stabilization of the ions within the WL islands on Au(111).

Imaging the Wetting Layer at Low Temperature (110 K)

In order to reduce thermal motion and to obtain submolecular resolution, the WL is probed also at low temperatures. Figure a and b show overview STM images (left) and noncontact AFM images (right) measured at 110 K, after deposition of 0.5 WL [C1C1Im][Tf2N] onto Au(111), followed by annealing to 295 and 381 K, respectively (for details of the preparation see the Experimental Section and Table S2 in the SI), in order to form large islands. Similar to the measurements at RT, large WL islands of several hundred nm widths on the terraces of the Au(111) substrate are seen; IL-covered areas are marked by green dots; areas nearly free from IL are marked by yellow dots. On the IL-free Au(111) terraces, we observe some bright spots (marked by arrows) at the elbow kinks of the underlying substrate herringbone reconstruction, which are assigned to very small IL islands. Similar to step edges, these kinks apparently allow for island nucleation during cooling, as it has also been reported for other ILs on Au(111).[55,57,60]

Figure 2

STM (a, c) and AFM (b, d) images measured at 110 K, after deposition of [C1C1Im][Tf2N] on Au(111), annealing, and cooling. The large-scale images (a, b) show extended WL islands (marked by green dots); on the nearly IL-free areas (yellow dots), smaller IL islands are detected at the elbows of the herringbone reconstruction (some marked by arrows). The medium-scale images (c, d) reveal that the WL crystallizes in two distinct phases, a “striped” (marked “S”, stripe directions highlighted with yellow lines) and a “hexagonal” (“H”) one; the unit cells are indicated in red and blue, respectively, similar to Figures and 5 (for details see text). The coverages and deposition/annealing temperatures are 0.5 WL/295 K (a), 0.5 WL/381 K (b), 1.0 WL/370 K (c), and 0.5 WL/381 K (d) (details of the preparation, coverages, and tunneling parameters are summarized in Table S2 in the SI).

Figure 4

STM (a, c) and AFM (b, d) images at 110 K after deposition of [C1C1Im][Tf2N] on Au(111), annealing, and cooling. The high-resolution images show the molecularly resolved 2D crystalline structure of the striped (a, b) and hexagonal (c, d) phases including primitive and corresponding superstructure unit cells. The oval shape of the protrusions, particularly seen in STM, is attributed to the [Tf2N]− anions (indicated by white ovals) adsorbed in cis-configuration with the CF3 groups pointing toward the vacuum. Within the striped phase, double rows are occasionally interrupted by single rows (see yellow lines in a and b). The coverages and deposition/annealing temperatures are 1.0 WL/370 K (a), 0.50 WL/381 K (b), 1.0 WL/368 K (c), and 1.0 WL/390 K (d) (details of the preparation, coverages, and tunneling parameters are summarized in Table S2 in the SI).

Figure 5

Fourier transformations of single-phase images of [C1C1Im][Tf2N] on Au(111) of (a) the RT phase at 300 K with its primitive unit cell in green, (b) the hexagonal H phase at 110 K with its (√3×√3)R30° superstructure unit cell in blue, and (c) the striped S phase at 110 K with its (2×1) superstructure unit cell in red. All Fourier transformations exhibit an underlying roughly hexagonal pattern with a similar “primitive” reciprocal unit cell (green). (d) Scaled models with the [Tf2N]− anions of the experimentally determined real space unit cells of the RT structure at 300 K and the H and S phases at 110 K. The positions and orientations of the cations could not be determined and are therefore not shown. (a: FT of Figure f, b: FT of Figure c, FT of Figure S3a).

Probing large WL islands on a medium scale at 110 K (Figure c and d) clearly shows that the Au(111) herringbone reconstruction is not lifted by the IL also at low temperatures. Moreover, both STM and AFM reveal that the WL crystallizes in two distinct phases, a “striped” (marked “S”) and a “hexagonal” (“H”) one. Both phases are typically separated by sharp boundaries as indicated by white dashed lines; in rare cases, mixed phases are found as denoted “S,H” in Figure c. As will be discussed in more detail below, the “S” and the “H” phase have individual superstructures with slightly different primitive unit cells. The 3-fold symmetry of the underlying (111) substrate leads to domains with different orientation, which can even occur within one phase (e.g., see differently oriented unit cells for the H phase in Figure c). As indicated by the straight yellow lines in Figure c and d, the rows of the S phase typically do not run along the main crystallographic ⟨110⟩-axes of the substrate (indicated as white double arrows), but are rotated by an average angle of around 20°. Notably, we found no indications for amorphous phases in the [C1C1Im][Tf2N] WL at any temperature. This is different from STM studies on similar ILs with longer alkyl chains: after deposition at RT and subsequent cooling to measurement temperatures between 100 and 150 K, the WLs of [C2C1Im][Tf2N], [C8C1Im][Tf2N], and [C4C1Pyrr][Tf2N] on metal surfaces showed the coexistence of extended amorphous phases and well-ordered crystalline phases.[55−57,60] We attribute the absence of such amorphous phases for [C1C1Im][Tf2N] to the smaller size and higher symmetry of the cation, which facilitates a regular lateral arrangement of the ion pairs in the WL on Au(111).

The analysis of many STM and AFM images after annealing to 295–380 K, like those in Figure , showed that the S and H phases are observed with a similar probability in the extended WL islands, which indicates that both configurations must be very close in free energy. This is underlined by the occasional observation of a transformation from an S to an H phase and vice versa when rescanning the same area several times (an example is shown in Figure S2).

The situation is different for deposition at low temperature without an annealing step. In this case, (mostly) smaller WL islands are formed, with the S phase clearly dominating. This is evident from the AFM images in Figure a (3b), which were recorded at 110 (180) K after deposition of 0.5 (0.75) WL [C1C1Im][Tf2N] at <170 (<200) K, respectively. As mentioned above, the S phases are rotated by an average angle of around 20° (indicated by short colored lines) to the main crystallographic ⟨110⟩-axes of the substrate.

Figure 3

AFM images of [C1C1Im][Tf2N] on Au(111) measured (a) at 110 K, after deposition of 0.5 ML at T < 170 K, and (b) at 180 K, after deposition of 0.75 ML at T < 200 K (details of the preparation, coverages, and tunneling parameters are summarized in Table S2 in the SI). In both images, the S phase clearly dominates. Similar to the observation in Figure , the rows of the S phase (indicated as colored short lines) typically do not run along the main crystallographic ⟨110⟩-axes of the substrate (indicated as white double arrows), but are rotated by an average angle of around 20°.

High-resolution STM and—with lower quality—AFM images of areas within the annealed WL islands (Figures a–d) reveal that the S and H phases observed at 110 K are both composed of protrusions with oval shape, but with different orientations, as indicated by the white ellipses in Figure a–c. This oval shape contrasts the situation for the RT phase, where only round protrusions are found in the WL (see above). The superstructure unit cell of the H phase (blue) in Figure c and d contains three protrusions rotated by 60° relative to each other, while the superstructure unit cell of the S phase (red) in Figure a and b contains two protrusions of different orientation. The yellow lines illustrate that the S phase features alternating rows of identically oriented protrusions packed in an A-B-A-B fashion; the protrusions are rotated by ±30° (±7°) relative to the direction of the row. Occasionally, single-row line defects, that is, neighboring rows with the same orientation of the oval protrusions, are observed; see Figure a.

The differences between the RT phase at 300 K and the H and S phases at 110 K are also clearly visible in the Fourier transformations (FT) of Figures f, 4c, and S3a, which are shown in Figure a–c, respectively. All three FTs show an overall hexagonal or close-to-hexagonal lattice, as is indicated by the corresponding green reciprocal “primitive” unit cells in the FTs. At RT, no additional spots are observed in Figure a, which is in line with the green unit cell shown in Figure e and f. The additional spots in the FTs of the two phases at 110 K result from a (√3×√3)R30° superstructure (blue) for the H phase and a (2×1) superstructure (red) for the S phase, with respect to the “primitive” unit cells (green). The corresponding “primitive” and superstructure cells in real space are indicated in Figures , 2, and 4 using the same colors.

As pointed out above, the size and shape of the “primitive” unit cells (green) are similar for all three phases. Note that for the S phase and H phase the “primitive” unit cells are obtained by considering only the position of the protrusions but not their orientation. The corresponding reciprocal unit cells in Figure a–c exhibit angles of 60 ± 6°. The comprehensive analysis of the FTs and also the real space STM and AFM images reveals that the two real space “primitive” unit cell vectors of the RT phase and the H phase have a length of 0.99 ± 0.06 nm, which corresponds to an area of the “primitive” unit cell of 0.85 ± 0.10 nm2. In the S phase, the vector along the stripe direction (yellow line in Figure c and d) is shorter by about ∼15%, yielding an area of the “primitive” unit cell of 0.74 ± 0.10 nm2. A schematic sketch of proposed real space structures of the RT, H, and S phases is shown in Figure d.

As already mentioned above, we propose that the spherical protrusions of the RT phase at 300 K and the oval protrusions of the S and H phases at 110 K represent the [Tf2N]− anions only, while the [C1C1Im]+ cations are not imaged at all in STM and AFM. This interpretation is supported by STM work from Behm and co-workers for the checkerboard arrangement of the [C2C1Im][Tf2N] and [C8C1Im][Tf2N] WLs on Au(111) and Ag(111).[53] Their STM images show two small overlapping protrusions, which are assigned to the two upward CF3 groups of the [Tf2N]− anion in cis-conformation, which is adsorbed on gold via its SO2 groups. This geometry was earlier proposed by us for the same anion on Au(111) using ARXPS.[52] In addition, the cations were clearly visible in STM through their upward bent ethyl and octyl side chains, while their imidazolium rings could not be detected. For the [C1C1Im][Tf2N] IL studied here, the [C1C1Im]+ cation with its five-membered ring does not have protruding side chains. It is adsorbed flat on the surface and thus has a significantly lower height than the anion. This was independently proposed by ARXPS recently.[91] It is thus not surprising that we do not observe the [C1C1Im]+ cation in STM and AFM. The fact that the two CF3 groups of the anion appear only as one oval protrusion (which has the same footprint as the two small protrusions reported by Behm and co-workers)[55,56] is attributed to higher noise level in our images.

The arrangement of the anions and cations of [C1C1Im][Tf2N] in a checkerboard structure is further confirmed by analyzing the areas of the primitive unit cells of all three phases, which is roughly 0.9 nm2. Considering the bulk molecular volume of a [C1C1Im][Tf2N] ion pair of ∼0.4 nm3,[92] and the height of the wetting layer of ∼0.37 nm,[91] we can estimate the footprint F of cation and anion adsorbed next to each other to be roughly 0.40 nm3/0.37 nm ≈ 1.1 nm2. This is somewhat larger than the “primitive” unit cell areas of the adsorbed WLs found here, but one has to consider that in a confined ordered state a more dense packing is possible compared to the less ordered bulk liquid. A more dense packing at the interface is further supported by the fact that the WL is more strongly bound to the Au(111) surface compared to IL multilayers, as it was recently proven by temperature-programmed XPS.[59]

As a last point, we want to address why we image the anions not as ovals but only as circular protrusions in the RT phase at 300 K (Figure e and f). This observation is attributed to rotational movements of the anions around their adsorption sites, which at room temperature are too fast on the time scale of our STM and AFM measurements.

Summary and Conclusion

By combining STM and AFM in ultrahigh vacuum with physical vapor deposition of ILs, we are able to resolve the molecular structure of the [C1C1Im][Tf2N] wetting layer on Au(111) in the temperature range from 110 to 300 K. [C1C1Im][Tf2N] shows a well-ordered hexagonal pattern of circular protrusions in scanning probe microscopy even at its bulk melting point, which is near room temperature. This is attributed to the small size and high symmetry of the cation. The observed hexagonal phase at RT is assigned to the well-known checkerboard structure of anions and cations adsorbed next to each other. The bright protrusions represent the [Tf2N]− anions, which are imaged in STM and AFM due to their larger height as compared to the flat-lying (and, thus, invisible) cations. The observation of an ordered phase at RT is in contrast to previously studied ILs with lower symmetry, where ordered structures were only observed well below the ILs’ melting points and, thus, far below room temperature.

Upon cooling from RT to 110 K, the overall hexagonal structure is maintained, but superstructures with larger unit cells form containing two or three anions (as imaged by oval protrusions) with different orientations. These hexagonal and striped phases coexist in equal amounts and can transform into each other; presently, it is not clear whether this transformation is driven by tip effects or happens spontaneously. If [C1C1Im][Tf2N] deposition and measurement is performed at temperatures below 200 K, the striped phase clearly dominates.

In the ordered RT phase at 300 K, we only observe a primitive hexagonal structure with one protrusion per unit cell. To explain this observation, we propose that the thermal energy at RT is sufficient to overcome the energy barrier for rotation of the anions. Therefore, we can no longer distinguish anions with different orientations on the time scale of STM and AFM.

In conclusion, we could demonstrate that not only STM but also AFM is a highly suitable method to study the structure of ultrathin ionic liquid layers under UHV conditions with molecular resolution. We could show that well-ordered wetting layers of ILs are formed even at RT choosing the appropriate IL and can be imaged at this temperature. Such structural studies with molecular resolution allow for detailed insights in the adsorption process of ILs on solid surfaces, which is highly relevant not only for the SILP and SCILL concepts in catalysis but also for understanding wetting phenomena in all thin film applications of ILs.

The combination of noncontact AFM and STM turned out to be highly beneficial for temperature-dependent studies: for the system studied here, AFM turned out to be superior to STM at higher temperature, that is, at and above room temperature. On the other hand, at low temperatures, the quality of the images was better in STM, which in the present study allows for a better determination of the arrangement of the anions in the unit cells of the H and S phases.

Experimental Section

The scanning probe microscopy (SPM) experiments were carried out in a Scienta Omicron two-chamber UHV system; the base pressures in the preparation and analysis chambers are 1 × 10–10 and 3 × 10–11 mbar, respectively. The microscope is a Scienta Omicron VT-AFM-Q+-XA. The STM images were recorded in constant current mode with either a Pt/Ir or W tip. The given bias voltages refer to the sample. The AFM images were recorded in noncontact mode, and the given frequency shift refers to the respective cantilever resonance frequency, typically around 320 kHz. Details on preparation and measurement conditions for each image are given in Table S2 in the SI. To obtain high-resolution images at low temperature with submolecular resolution, STM was more suitable. However, for large images (1000 × 1000 nm2) and at higher temperatures, the STM measurements became quite susceptible to adsorbate–tip interactions, yielding instabilities, while AFM measurements remained more stable. The images were processed with WSxM[93] software, and moderate filtering (Gaussian smooth, background subtraction) was applied for noise reduction. If possible, the crystallographic main axes (⟨11̅0⟩, ⟨1̅01⟩, ⟨011̅⟩) in the images were determined from the surface reconstruction of Au(111).[87] The wetting layer is defined as the closed first layer of cations and anions arranged in a checkerboard fashion next to each other.[52]

[C1C1Im][Tf2N] was synthesized under ultrapure conditions according to previous publications.[94] Notably, the anion [Tf2N]− is also known as [NTf2]−[13,26,47] or [TFSA]−,[23,29] and the cation [C1C1Im]+ as [MMIm]+[43] in the literature. The Au(111) single crystal (MaTecK) was cleaned via Ar+ ion sputtering and annealing at 900 K. The IL was deposited with an effusion cell, developed in our group explicitly for IL deposition, at an evaporation temperature between 373 and 383 K.[95] The flux was checked with a quartz crystal microbalance, calibrated by determining the ratio of IL-covered and uncovered area on the single crystal in the SPM images.

The different layers were prepared by depositing a defined amount of IL (monolayer or multilayers) at a given temperature Tprep, sometimes followed by heating to Tmax (in some cases Tprep was Tmax); note that above 350 K no multilayer adsorption occurs due to desorption.[59] The measurements were performed at Tmeas. Details of the preparation can be found in Table S2 in the SI.