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3-Dimensional Structure of a Prototypical Ionic Liquid-Solid Interface: Ionic Crystal-Like Behavior Induced by Molecule-Substrate Interactions Daniel Ebeling, Stephan Bradler, Bernhard Roling, and Andre Schirmeisen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02232 • Publication Date (Web): 17 May 2016 Downloaded from http://pubs.acs.org on May 23, 2016

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The Journal of Physical Chemistry

3-Dimensional Structure of a Prototypical Ionic Liquid-Solid Interface: Ionic Crystal-Like Behavior Induced by Molecule-Substrate Interactions Daniel Ebeling,∗,† Stephan Bradler,‡ Bernhard Roling,‡ and Andr´e Schirmeisen† †Institute of Applied Physics, Justus Liebig University Giessen, Heinrich-Buff-Ring 16, 35392 Giessen, Germany ‡Department of Chemistry, University of Marburg, Hans-Meerwein-Strasse 4, 35032 Marburg, Germany E-mail: [email protected]

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Abstract The molecular structure of the ionic liquid-electrode interface has been recently investigated by atomic force microscopy (AFM) methods focusing either on the vertical structure of the ion layers or on the lateral structure of the innermost layer. Here, we combine high resolution AFM imaging with atomic force spectroscopy measurements to elucidate the structure of the interface between the ionic liquid propylammonium nitrate (PAN) and highly ordered pyrolytic graphite (HOPG). The lateral structure of the innermost layer of adsorbed molecules (i.e., the Stern layer) is resolved on the molecular scale by means of amplitude modulation atomic force microscopy (AM-AFM). A quasi (4x4)R0◦ overlayer is formed by the ionic liquid molecules on the HOPG surface. Additional dynamic mode force spectroscopy measurements reveal the existence of a layered structure of the ionic liquid normal to the surface plane and allow for a precise determination of the layer spacing. We are able to infer the three-dimensional structure of the PAN-HOPG interface from a combination of both information, i.e., the experimentally observed lateral and normal structures. The obtained 3D lattice structure is in accordance with a zincblende-type (ZnS) crystal structure.

Introduction Ionic liquids are promising candidates for different electrochemical applications due to their unique physical, chemical, and electrochemical properties, such as low vapor pressures, high electrochemical and thermal stability, and high ionic conductivity. 1–4 Consequently, ionic liquids have been tested as electrolytes in various types of electrochemical cells, such as batteries, supercapacitors, dye-sensitized solar cells, and fuel cells. 5 For these applications, the three-dimensional structure of the ionic liquid-electrode interface plays a crucial role, since it exerts a strong influence on the electrochemical processes in the electrochemical cell. Hence in the past years, large research efforts have been devoted to the experimental 2 ACS Paragon Plus Environment

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characterization and to a better theoretical understanding of the interface (see e.g., Ref. 6,7 for recent reviews). To this end, various experimental methods have been applied. Macroscopic impedance measurements and cyclic voltammetry have been used to determine the interfacial capacitance, 8–13 which governs the energy storage in supercapacitors. X-ray reflectivity measurements yielded electron density profiles at the ionic liquid-metal interface, 14,15 from which information about fractions of cations and anions within the interfacial layers were extracted. Scanning tunneling microscopy has been used for imaging the surface structure of single-crystalline electrodes in contact to an ionic liquid as well as the lateral structure of the innermost ion layer. 16–22 By means of surface-enhanced Raman spectroscopy and sumfrequency generation vibrational spectroscopy, information about the orientation of cations in the innermost layer with respect to the surface normal has been obtained. 23–28 Force spectroscopy methods have been applied for determining the spatial extent of the ion layer structure normal to the surface. 1,2,29–40 In particular, recent advances in the AFM instrumentation, control electronics, operation modes, and optimization of imaging parameters paved the way for investigating the lateral structure of the liquid-solid interfaces by in-situ AFM. 41–49 Recently, also the dynamic 3D-AFM technique, which was originally introduced for vacuum environment 50 and later implemented for measurements in liquid environment 51–54 has been applied to analyze the structure of the liquid-solid interface. However, in particular for an ionic liquid-solid interface a simultaneous detection of the normal and lateral molecular structure in one experiment to obtain the full 3D structure has not been shown so far. Here, we combine high resolution lateral imaging with dynamic mode force spectroscopy to analyze the three-dimensional structure of the interface between the ionic liquid PAN and HOPG. This interface was chosen, since it was shown previously that the innermost layer (i.e., the Stern layer) exhibits an ordered structure. 55 Our results reveal a hexagonal packing of molecules within the Stern layer, which is accompanied by the formation of several



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Figure 1: (Top row): Dynamic mode AFM topography images (9.5 x 9.5 nm2 ) obtained at three different amplitude setpoints (0.23 nm, 0.20 nm, 0.17 nm). The horizontal axis corresponds to the fast scan axis. All three images were scanned in the “downward” direction. (Bottom row): Corresponding images for the “upward” scan direction. The three in-plane directions of the observed structure are indicated by cyan and red lines. Parameters: rectangular silicon cantilever, PPP-NCSTAuD, Nanosensors, drive frequency = 31 kHz, nominal spring constant = 7 N/m, z-scale = 110 pm. molecular layers, residing at characteristic distances above the surface. Apparently, this structure exhibits a remarkable similarity with a zincblende (ZnS) ionic crystal structure, which is governed by ion-HOPG interactions.

Results and Discussion The lateral structure of the Stern layer of adsorbed ions at the PAN-HOPG interface was examined by in situ AFM within a hermetically sealed liquid cell under argon atmosphere


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(special care was taken to avoid any contamination of the system, further experimental details and information about the synthesis of the IL can be found in the methods part). Figure 1 shows a series of three topography images (9.5 x 9.5 nm2 ) obtained in the dynamic mode (amplitude modulation mode (AM-mode), often also referred to as tapping mode 56,57 ) at three different amplitude setpoints (0.23 nm, 0.20 nm, and 0.17 nm). For each amplitude setpoint two images are depicted corresponding to the “downward” and “upward” scanning directions, respectively. Herewith, the thermal drift of the system can be evaluated and the obtained lateral structure can be analyzed more precisely. This will be explained in detail further along the line (in Figure 3). For the time being we would like to record that the image contrast increases with decreasing setpoint values, clearly resolving a hexagonal structure at the lowest amplitude setpoint. The three in-plane lattice directions of the observed structure are indicated in each image by cyan and red lines. For the lowest amplitude setpoint, decent contrast is observed along all three directions. With increasing setpoint, however, the contrast weakens significantly leading to a faint stripe pattern which is barely visible. These faint lines are parallel to the in-plane lattice direction indicated by the red lines (see left images in Figure 1). A similar effect has been described by Kobayashi et al. for the interface between a mica substrate and aqueous KCl solution. 53 To obtain more information about the observed structure we also performed single point spectroscopy measurements, which are depicted in Figure 2. The three curves in the top, middle, and bottom part show the deflection, amplitude, and phase shift vs. tip-sample separation, respectively and were measured at an arbitrary sample location. The three signals were captured simultaneously in the AM-mode (see methods for further details). The different curves exhibit characteristic and reproducible features when approaching the sample surface. A closer look at the amplitude vs. distance curve (as well as the phase shift vs. distance curve) reveals an oscillatory behavior for decreasing distances indicating the existence of a layered structure normal to the surface. This layering effect was observed



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Figure 2: Deflection (top), amplitude (middle), and phase shift (bottom) vs. tip-sample separation curves. The different imaging conditions from Figure 1 are indicated in each graph by black, blue and green circles. These circles correspond to the three amplitude setpoints of 0.23 nm, 0.20 nm, and 0.17 nm, respectively. The HOPG surface is highlighted by the shaded area. The vertical dashed lines correspond to the spacing between the (111) planes of the proposed 3D structure of the ionic liquid (see Figure 4).


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before for other ionic liquids 29,30 and other liquids in general. 58,59 Please note that, depending on the operation mode, the oscillatory behavior of the tipsample interaction potential can lead either to discontinuities in the spectroscopy curves with characteristic jumps (usually observed when using soft springs/cantilevers in the static operation mode, see e.g. Refs. 29,30,58 ) or, as in the present case, to continuous curves with well-defined oscillations (usually observed in dynamic mode experiments where stiffer cantilevers are used, see e.g. Refs. 38,59 ). In both curves (amplitude and phase vs. tip-sample distance) we can clearly identify 3-4 oscillations with a characteristic spacing of approx. 4.5 - 5.0 ˚ A, each corresponding to one ordered layer of IL molecules above the HOPG surface. We will discuss this spacing in more detail further along the line. Another characteristic feature, which is observed in all three signals (deflection, amplitude, and phase) is a sudden, almost vertical increase/decrease in the curves at a tip-sample distance of approx. 3.0 ˚ A. At this point the deflection and phase vs. distance curves show a sudden increase, while the amplitude curve drops abruptly. This behavior can be attributed to a strongly bound innermost layer, i.e., Stern layer, of ionic liquid molecules adsorbed on the HOPG substrate, which keeps the tip from moving further towards the sample surface. As soon as the deflection (and hence the average loading force of the tip) exceeds a certain value the tip starts pushing through the Stern layer and the tip-sample distance continues to decrease until the HOPG surface is reached. No further movement of the tip with respect to the sample surface is observed, even when moving the cantilever base further towards the sample surface, leading to a vertical increase of the deflection curve at a distance of 0.0 ˚ A. Thus, we can exclude the existence of any other bound IL layers beneath. Please note that we simultaneously capture the deflection and the amplitude information during the dynamic mode spectroscopy measurements and use the deflection signal as a trigger channel for controlling the tip movement, allowing us to deduce at which point the tip touches the surface. In general the amplitude vs. distance curves alone are not sufficient to extract this information, since the amplitude signal can break



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down far before actually reaching the surface (see almost leveling behavior of the amplitude for distances < 3.0 ˚ A). Furthermore, the spectroscopy data reveals additional insight into the imaging process. The three amplitude setpoints (0.23 nm, 0.20 nm, and 0.17 nm), which were used to obtain the images in Figure 1 are marked in the spectroscopy curves by black, blue, and green circles, respectively. This indicates that the approximate tip-sample distance (i.e., the distance between tip and sample in the lower turning point of the oscillation cycle) during imaging was around 3.0 ˚ A in all three cases. Hence, in all three images the Stern layer was imaged, while the different appearance of this layer (i.e., the increasing image contrast in Figure 1 from left to right) is attributed to the different imaging conditions (i.e., setpoint amplitude), which determine the sensitivity of the system. 47,60,61 The measured thickness of the Stern layer (3.0 ˚ A) is in good accordance with values reported previously. 30 It was suggested that this corresponds to an, on average, flat orientation of molecules with respect to the substrate plane. Please note that a quantitative analysis of the tip-sample interaction forces relies on precise measurements of the system parameters, such as resonant frequency, spring constant, Q-factor, etc. (see e.g. Refs. 48,62–65 ), which is difficult for measurements in ionic liquids due to their high internal damping. Thus, we are using the directly measured signals (i.e., deflection, amplitude, and phase shift) for our study (see methods for further details). Still we would like to provide an upper limit for the average force acting on the tip during imaging. The average deflection of the cantilever for the three imaging conditions of 0.23 nm, 0.20 nm, and 0.17 nm was approx. 13 pm, 30 pm, and 90 pm, respectively (see black, blue, and green circles in Figure 2 top graph). For a spring constant of 14 N/m (which corresponds to 200% of the nominal spring constant) this relates to the following upper limits for the average tip-sample force during imaging: 0.2 nN, 0.4 nN, and 1.3 nN. In the next step we analyzed the lateral dimensions of the observed hexagonal lattice in the Stern layer. Since the obtained images are slightly afflicted by thermal drift the observed


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Figure 3: : (a) 2D-FFT filtered image for an amplitude setpoint of 0.17 nm (corresponding raw data is shown in Figure 1, right column). (b) 2D-FFT filtered image for same amplitude setpoint but obtained with reversed slow scan axis. Dimensions of the fitted lattices (see red circles) are given in the insets in the right bottom of each image (average periodicity = (0.58 ± 0.03) nm). The insets in the top right corner show the corresponding FFT images which reveal six bright spots each. In x-direction the positions of the bright spots coincide with their expected positions (see red dashed hexagon). In y-direction the structure is either elongated or compressed due to thermal drift during the measurement. (c) Sketch of the HOPG lattice with overlaid (4x4)R0◦ superstructure (see large red circles). Another sketch of the same overlayer for a simple hexagonal lattice is shown in the bottom right part. Balland-stick models of the propylammonium and the nitrate ions with surrounding circles (see top left part) allow for a comparison of lattice and molecule dimensions.



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structures in two consecutive scans can be either expanded or compressed along the slow scan direction. This is shown in Figures 3 a) and b) which depict 2D-FFT filtered images for “downward” and “upward” scans, respectively (both for imaging setpoint of 0.17 nm, see Figure 1). From fitting lattices to the filtered images (see red circles) we obtain for lattice vector a (vector close to fast scan direction, see Figure 3 a) and b)) an average lattice constant of (0.58 ± 0.03) nm. For lattice vector b (close to slow scan direction) the observed spread is much higher since the structure is either elongated or compressed in this direction due to the thermal drift. In this case the average value, however, is also 0.58 nm, indicating that the observed lateral structure is truly hexagonal. Figure 3 a) and b) also show 2D-FFT images (see insets in right top), which, as expected for hexagonal structures, clearly reveal six bright spots. The observed and the theoretical spot positions (see red dashed hexagon, drawn for periodicity on 0.58 nm, in inset) coincide very well for the x-direction (fast scan direction) while the structure is either compressed or elongated along the y-direction (slow scan direction). Please note that the periodicity of the observed hexagonal superstructure (0.58 nm) is significantly larger than the results published earlier by Page et al. 55 for a similar IL-substrate combination (there a periodicity of 0.48 nm was reported). A direct comparison of the presented images, however, reveals that the size of the imaged structures is in good agreement with the value reported here, which suggest an erroneous interpretation of the data in the literature case. In the following we want to identify if the observed structure resembles a commensurate or an incommensurate overlayer, since this will give an idea about the relationship between surface-molecule and molecule-molecule interactions. Essentially, this determines the mechanical and chemical properties of the adsorbed layer. Figure 3 c) shows a schematic structure of the HOPG(0001) surface (black honeycomb pattern with gray carbon atoms). The distance between two carbon atoms in the grid is 0.142 nm. 66,67 In a first step we simplify the complex HOPG lattice by considering a simple hexagonal grid of the same size (i.e. periodicity of 0.142 nm, see sketch in the right bottom). For this simple hexagonal grid a


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(4x4)R0◦ overlayer (see small red circles in the sketch) results in a theoretical lattice constant of 0.568 nm, which is in good accordance with the measured periodicity of the Stern layer ((0.58 ± 0.03) nm). √ √ The closest other possible overlayer structures would be either a ( 13x 13)R13.9◦ or √ √ a ( 21x 21)R10.9◦ superlattice (not shown). The periodicities of those two lattices are 0.512 nm and 0.651 nm, respectively deviating by 12% from our measured value. Now we refine our analysis to the more complex honeycomb-like structure of the HOPG surface (depicted in the middle of Figure 3 c). Every third C atom in each horizontal row of atoms is missing, corresponding to the hollow sites of the grid. Due to this, the (4x4)R0◦ overlayer for the simple hexagonal lattice does not resemble a perfect superlattice in the HOPG case (see large red circles, diameter = 0.568 nm). The centers of the large red circles are either located above hollow sites (1/3 in total) or above atom sites (2/3 in total). Thus, the observed hexagonal structure of the Stern layer is strongly determined by the underlying HOPG lattice while, at the same time, not representing a perfectly commensurate superstructure. This characteristic presumably originates from a complex interplay between molecule-substrate and molecule-molecule interactions. Figure 3 c) shows ball-and-stick models of the nitrate anion and the propylammonium cation (see left top), which are drawn to scale. The larger of the two ions (propylammonium) fits well into a circle of 0.568 nm diameter as indicated. The gray circle, which surrounds the nitrate ion, is significantly smaller and has a diameter of 0.25 nm. Thus, the observed hexagonal (4x4)R0◦ superlattice is in accordance with a flat orientation of the larger propylammonium ions as suggested previously (on the basis of the thickness of the Stern layer). Next, we are addressing the question if the observed 2D interface structure together with the vertical layer periodicity can be described by one of the standard 3D Bravais lattices. Two-element compounds (i.e., ionic crystals) are often based on the cubic crystal system and the most common structure types within this system are the cesium chloride (CsCl), the rock salt (NaCl), and the zincblende (ZnS) structures. For spherical ions the preferred



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structure (and therewith the coordination number) depends strongly on the radius ratio of the two ionic species according to Pauling’s rules. 68 In general, for an increasing difference in ion size a decrease of the coordination number is observed (CsCl: c.n. 8, radius ratio = (1.0 - 0.73); NaCl: c.n. 6, radius ratio = (0.73 - 0.41); ZnS: c.n. 4, radius ratio = (0.41 - 0.23)). Since the propylammonium and the nitrate ions have a distinct difference in size (and shape) the ZnS structure is the most likely candidate for the experimentally observed structure. To confirm our hypothesis, Figure 4 a) shows a scheme of the ZnS structure. This is composed of two interpenetrating face-centered cubic (fcc) lattices, which are each formed by one of the ionic species (gray and red spheres) and are offset by the vector (1/4, 1/4, 1/4). In the following we take a closer look at the (111) planes of the ZnS structure. A side and a top view of this crystallographic plane is depicted in Figure 4 b) (compare blue transparent triangles in Figure 4 a) and b). From the top view it becomes obvious that ions within the √ (111) plane are hexagonally close-packed. The periodicity is a/ 2, where a is the lattice parameter of the fcc lattice (see Figure 4 a). Since our measured lateral structure of the Stern layer is in good agreement with a quasi (4x4)R0◦ superstructure on HOPG (periodicity √ = 4 × 0.142 nm = 0.568 nm), we obtain a theoretical lattice parameter of a = 2 × 0.568 nm = 0.803 nm for our proposed ZnS structure. For direct visualization, Figure 4 c) shows a side and a top view of the proposed ZnS structure (a = 0.803 nm) with either one of the two ionic species assigned to the two different grid positions (cf. position of gray and red spheres in Figure 4 a) and b). The molecules were placed in a way that the ZnS grid positions coincide with the approximate center of charge, i.e., in each case the nitrogen atom of the molecule was placed at the respective grid location (please compare the area surrounded by a black triangle in Figure 4 c) top view with Figure 4 b) top view). Afterwards, the molecules were rotated arbitrarily around the [111] direction to avoid any collisions. In the side view of Figure 4 c) the proposed three-dimensional ZnS structure of PAN


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Figure 4: (a) Zincblende (ZnS) crystal structure with lattice parameter a. Gray and red spheres correspond to the different ionic species. (111) planes are indicated with transparent blue triangles. √ between (111) √ (b) Side and top view of the ZnS(111) plane. The distance planes is a/ 3. The distance between two atoms within a (111) plane is a/ 2. (c) Side and top view of the proposed 3D structure of the ionic liquid-HOPG interface. Molecular models of the two ionic species (propylammonium and nitrate) were arbitrarily assigned to either one of the two different grid positions within the ZnS structure (gray or red) and the lattice was oriented in a way that the [111] crystal direction is parallel to the [0001] direction of the HOPG surface (see text for details). For clarity, the nitrogen atoms of the nitrate and the propylammonium molecules are plotted in bright and dark blue, respectively.



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molecules is oriented in a way that the ZnS(111) planes are parallel to the HOPG(0001) basal plane. We have assumed a planar adsorption of the molecules as it was suggested in literature before. 30 From this we can infer that the first (neutral) layer of molecules has a thickness of approx. 3.0 ˚ A, which is in good agreement with the measured value from our spectroscopy experiments (see Figure 2). The spacing between (111) planes in a fcc lattice is √ A. Hence, the distance between the top of the 2nd , 3rd , or 4th molecular layers a/ 3 = 4.64 ˚ and the HOPG surface is expected to be at 7.6 ˚ A, 12.3 ˚ A, and 16.9 ˚ A, respectively (see Figure 4 c). We have highlighted these distance values in the spectroscopy graphs in Figure 2 by vertical dashed lines. There is a remarkable agreement between these theoretical distances and the measured oscillations in the amplitude and phase shift vs. distance curves. Each time the measured tip-sample distance coincides with the theoretical position of a molecular layer the amplitude signal drops, which results from stronger tip-sample interaction. We have performed the same analysis also for the NaCl and the CsCl structures (see Figures S1 and S2 in the Supporting Information). This shows clearly that the CsCl structure is not suited to describe the observed PAN-HOPG structure, since the spacing between (111) planes is too small. For the NaCl structure, however, the spacing is the same as for the ZnS structure, since NaCl is also composed of two fcc lattices. Though, due to the different offset (1/2, 1/2, 1/2) between the two fcc lattices, each layer of propylammonium is located in the middle between two nitrate layers and vice versa. Hence, we would expect a different oscillatory behavior in the spectroscopy curves, e.g. additional maxima/minima. Furthermore, the absolute values for the thickness of the Stern layer and the distances between the HOPG surface and the 2nd , 3rd , or 4th molecular layers differ from the experimental values. Hence, the proposed ZnS-type structure shows the best agreement with the observed normal and lateral structure of the PAN-HOPG interface. Please note that our model of the ZnS structure drawn in Figure 4 is not meant to be a one-to-one representation of the actual situation at the ionic liquid-solid interface. Since the experimental image resolution does not allow identification of the ionic species, it is possible


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that the positions of the cations and anions are in fact inverted. Further open questions are the exact orientation of molecules within the structure and the degree of order in the 2nd or higher molecular layers. The first question can already be answered partially since the thickness of the Stern layer was 3.0 ˚ A, which rules out that the propylammonium molecules stand upright on the surface. However, it was not possible to achieve molecular resolution within the 2nd , 3rd , or 4th molecular layer, which would permit a detailed study of the molecular arrangement further from the surface. Presumably, the forces which keep the molecules at the lattice positions weaken with increasing distance from the HOPG substrate leading to a blurred ZnS-type structure (or, from a different perspective, the ideal lattice structure is more easily perturbed by the AFM tip). But our results still provide a strong indication that the ions in the 2nd , 3rd , or 4th layer form a structure with a certain degree of order, showing on average the correct layer separation of the ZnS structure, while the ions are mobile and thus show liquid-like behavior. Thus, the proposed ZnS-type lattice is not meant as a rock solid crystal structure (in that case it would e.g., not be possible to perform AFM imaging inside this structure). Besides, another interesting question is if the observed structure is entirely determined by molecule-substrate and molecule-molecule interactions or if the confinement of the IL molecules beneath the AFM tip plays an additional role (see e.g. Ref. 69 ).

Conclusions To summarize, we have performed molecular imaging and force spectroscopy experiments of the PAN-HOPG interface at room temperature by dynamic AFM. To avoid contamination of the IL, experiments have been performed in a hermetically sealed liquid cell. High resolution imaging reveals a hexagonal close packing of adsorbed molecules in the Stern layer with a periodicity of (0.58 ± 0.03) nm. This measured value is in excellent agreement with a quasi (4x4)R0◦ superstructure of the HOPG lattice. Dynamic mode force spectroscopy



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measurements reveal the existence of a layered structure of ionic liquid molecules normal to the HOPG surface with a characteristic layer spacing of 4.5 - 5.0 ˚ A. The observed vertical spacing for the first molecular layers is consistent with the spacing of (111) planes of the proposed ZnS crystal structure. Hence, a combination of the experimentally observed lateral and vertical structures allows us to infer the three-dimensional structure of the PAN-HOPG interface, which exhibits a zincblende-type ionic crystal-like character whose dimensions are strongly influenced by the HOPG lattice.

Methods Synthetis of PAN PAN was synthesized similarly to Refs. 40,55 To a cooled (0◦ C) mixture of 5 ml propan-1-amine (Sigma-Aldrich, >99%) and 5 ml milli-Q water, 4 ml of concentrated nitric acid (AppliChem) were slowly added. The mixture was rotary evaporated at 40◦ C and 200 mbar for 150 min and then dried in vacuum for 8 h at 105◦ C and 1 h at 120◦ C. 1 H and 13 C NMR measurements show no water content or other impurities (see NMR spectra in Figures S3-S7 in the Supporting Information). The volumetric water content was determined via coulometric Karl-Fischer titration and is 70 ppm.

AFM measurements The AFM measurements were performed with a commercial instrument (Cypher ES, Asylum Research, US), which was equipped with a hermetically sealed liquid cell. All parts of the cell, which get in contact with the IL were thoroughly cleaned prior to the experiments by wiping, immersing, and rinsing with ethanol. The cantilevers (Nanosensors PPP-NCSTAuD, Au coating on detector side, nominal spring constant 7 N/m, nominal resonant frequency 160 kHz (in air)) were cleaned as well by immersing and rinsing in ethanol and an additional plasma treatment (ambient air plasma at 50 W for 3 min, Zepto plasma cleaner, Diener 16 ACS Paragon Plus Environment

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Electronic, Germany). Subsequently, all parts were introduced into a glove box (water and oxgen content below 1 ppm) for assembly. The HOPG substrate (Anfatec, Germany) was freshly cleaved inside the glove box directly prior to putting a droplet of IL onto the surface and sealing the liquid cell. Afterward, the sealed liquid cell was taken out of the glove box for performing the measurements. The temperature of the sample substrate was controlled to be (30 ± 0.1)◦ C. The tip was connected to ground potential while the sample was floating during the measurements. Imaging as well as spectroscopy experiments were conducted in the dynamic operation mode, i.e., in the amplitude-modulation mode (often also referred to as AC-mode or tapping mode). 56,57 During the spectroscopy experiments the amplitude, phase shift and deflection signals were captured simultaneously. The tip-sample separation was calculated using the following equation: “separation = piezo movement + deflection − amplitude”. The amplitude and deflection signals were calibrated by plotting the deflection signal vs. the calculated tip-sample separation (see Figure 2, top graph) and adjusting the calibration factor until an infinite slope in the region where the tip touches the surface is observed. The cantilever drive frequency (approx. 31 kHz) was adjusted by taking a thermal noise spectrum of the cantilever close to the sample surface. To check for reproducibility and possible changes in the tip morphology multiple spectroscopy curves have been taken at different sample locations before and after the imaging experiments. Please note that it is rather difficult to obtain accurate values for the system parameters (e.g. resonance frequency, quality factor, etc.) in ionic liquids due to their high internal damping, which leads to very broad resonance peaks (Q-factor on the order of 0.5 close to the surface). Hence, we refrain from analyzing the tip-sample interaction quantitatively (i.e. calculation of the tip-sample forces) since the reliability of this procedure relies on accurate input parameters (see e.g. Refs. 48,62–65 ). Nevertheless, the directly measured signals



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(i.e. deflection, amplitude, phase shift), which we use for our study still reveal meaningful qualitative information about the tip-sample interaction.

Supporting Information Available Figures S1 and S2 showing NaCl and CsCl structures, NMR spectra (Figures S3 - S7). This material is available free of charge via the Internet at http://pubs.acs.org/.

Acknowledgement This project was supported by the Laboratory of Materials Research (LaMa) of JLU. We acknowledge financial support within the LOEWE program of excellence of the Federal State of Hessen (project initiative STORE-E).

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Graphical TOC Entry AFM tip

HOPG


3-Dimensional Structure of a Prototypical Ionic ... - ACS Publications

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