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Surface Confinement Induces the Formation of Solid-Like Insulating Ionic Liquid Nanostructures Massimiliano Galluzzi, Simone Bovio, Paolo Milani, and Alessandro Podestà J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12600 • Publication Date (Web): 21 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018

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The Journal of Physical Chemistry

Surface Confinement Induces the Formation of Solid-like Insulating Ionic Liquid Nanostructures

Massimiliano Galluzzi,1,2 Simone Bovio,2,3 Paolo Milani,2 Alessandro Podestà2,*

1

Shenzhen Key Laboratory of Nanobiomechanics, Shenzhen Institutes of Advanced Technology,

Chinese Academy of Sciences, Shenzhen 518055, Guangdong, China

2

CIMAINA

and Dipartimento di Fisica, Università degli Studi di Milano, via Celoria 16, 20133 -

Milano, Italy

3

Present affiliation: Reproduction et Développement des Plantes, CNRS UMR 5667 / PlATIM, SFR

Biosciences UMS3444/US8, ENS de Lyon, 15, Parvis Rene Descartes, F-69007, Lyon, France

* Corresponding author: Prof. Alessandro Podestà

Name: Prof. Alessandro Podestà Affiliation/Institute: C.I.Ma.I.Na and Dipartimento di Fisica, Università degli Studi di Milano, via Celoria 16, 20133 - Milano, Italy Address: via Celoria 16, 20133 - Milano, Italy E-mail: [email protected]

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ABSTRACT: We report on the modification of the electric properties of the imidazoliumbased [BMIM][NTf2] ionic liquid upon surface confinement in the sub-monolayer regime. Solid-like insulating nanostructures of [BMIM][NTf2] spontaneously form on a variety of insulating substrates, at odd with the liquid and conductive nature of the same substances in the bulk phase. A systematic spatially-resolved investigation by atomic force microscopy of the morphological, mechanical and electrical properties of [BMIM][NTf2] nanostructures showed that this liquid substance rearranges into lamellar nanostructures with a high degree of vertical order and enhanced resistance to mechanical compressive stresses and very intense electric fields, denoting a solid-like character. The morphological and structural reorganization has a profound impact on the electric properties of supported [BMIM][NTf2] islands, which behave like insulator layers with a relative dielectric constant between 3 and 5, comparable to those of conventional ionic solids, and significantly smaller than those measured in the bulk ionic liquid. These results suggest that in the solid-like ordered domains confined either at surfaces or inside the pores of the nanoporous electrodes of photoelectrochemical devices, the ionic mobility and the overall electrical properties can be significantly perturbed with respect to the bulk liquid phase, which would likely influence the performance of the devices.

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1 INTRODUCTION Due to their ionic nature, negligible volatility, and highly versatile design of the cation/anion pair,12

Room-Temperature Ionic Liquids (ILs) can replace aqueous electrolytes in many applications

aimed at conversion and storage of energy,3 such as electrochemical supercapacitors,4-5 solar cells6-7 and batteries.8-9 In these applications, ILs are in contact with the solid surface of the electrode. At this interface, the symmetry of the bulk liquid environment is broken, and this can induce structural organizations at the nanoscale, as reported in several experimental10-12 and theoretical13 publications. The interfacial interactions of ILs are likely very important also in the case of biological surfaces, such as the phospholipid bilayers and the biomembranes;14-15 a complete understanding of ILs behavior would shed light on their environmental impact and toxicity, fostering biotechnological applications.16-17 In the case of solid surfaces, the formation of ordered domains and nanostructures at the liquid-solid interface is well documented in the literature. In the case of pure ILs in contact with a solid surface (either charged non-metallic, biased metallic, or carbonaceous), force spectroscopy experiments by Atomic Force Microscopy (AFM)18-20 and Surface Force Apparatus (SFA)21-22 showed a stepwise profile in the force versus distance curves due to the organization of ions near the liquid-solid interface into solvation layers, whose amplitude correlates with the size of the cation-anion pair. These experimental results are in good agreement with the ionic density profile obtained by molecular dynamics simulations.23-25 The nanometric solvation layers formed at the interface show resistance to normal compression and viscosity 1-3 order of magnitude higher than in the bulk liquid, the ions still maintaining a good degree of mobility.11 The electrical and structural properties of the first few monolayers of IL in contact with the solid substrate can be crucial for the performance of devices employing ILs as electrolytes (as in 3



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supercapacitors,4-5 solar cells7, 26 and batteries8-9), gating media at electrified interfaces,27-29 or active fluids (as for electrowetting30-32 and tribology33-34). The study of nanometer-sized IL systems (very thin films and nanostructures) on solid surfaces represents therefore a very interesting topic, since this configuration is relevant not only to address the effects of extreme spatial confinement of ILs, but it is also paradigmatic of the use of ILs in advanced applications. Evidence has been reported of the surface-induced formation of ordered layered nanostructures of ILs extending up to several nm from the surface, at distances very large compared to the ion pair diameter.35-38 At odd with the case of the solvation layers observed at the bulk liquid/solid interface, which can be easily penetrated by a suitable probe and typically reassemble reversibly upon the relief of the perturbation, in the case of solid-supported IL thin films the layers typically possess solid-like behavior, and cannot be displaced if not by the application of intense mechanical stresses. Solid supported IL nanostructures with extended layering have been produced in different conditions: pure bulk IL confined at solid interface,39-41 IL diluted in solvent and drop-casted on solid surface,35, 37, 42

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and thin layers deposited by molecular beam deposition (electrospray or vacuum gradient).21,

. Different flat substrates have been investigated: insulator,42, 44-45 carbonaceous materials35,

and metals41. In all these cases, extended layering depending on the experimental conditions and methods of deposition has been observed. The reports on the formation of solid-supported ordered nanostructures mainly concern imidazolium-based ILs (the weak layering of an ammonium-based IL represents to our knowledge an exception46). Recently, our group reported a study of solid-like nanostructures of 1-Butyl-3methylimidazolium Bis(trifluoromethylsulfonyl)imide ([BMIM][NTf2]) deposited by drop-casting from a IL/methanol solution on solid insulating surfaces.24,

47-48

AFM analysis revealed highly

ordered lamellar structures extending tens of nanometers from the substrate, with high vertical 4


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regularity and resistance to normal compressive stresses, much higher than those required to penetrate the solvation layers found at the bulk liquid/solid interface. Experimental results were qualitatively in agreement with numerical simulations.24-25,

48

The mechanism behind the

formation of extended solid-like layers is still under debate, addressing a combination of causes: the high mobility of diluted ions, the driving force from solvent evaporation,49 the water layer adsorbed on the surface,42, 50 the IL/surface interaction (coulombic, hydrogen bonding),13 the role of the spatial confinement of ILs.12 ILs inside the nanometer-sized pores of a nanostructured electrode are strongly confined, therefore the layering phenomena can be highly amplified with respect to smooth surfaces, which can turn into an advantage or a disadvantage depending on the application. The spatial confinement likely induces a marked reduction of the ionic mobility at the electrode-electrolyte interface, and a perturbation of the electrostatic double layer; these effects, resulting in higher interfacial resistance, inhibited charge transfer, and reduced ability to store charge into the electric double layer, can be detrimental to the performance of supercapacitors, solar cells, and batteries. Moreover, while the reduced ionic mobility can be beneficial for applications where stable, well adherent, few nanometers thick protective layers are required (i.e. to reduce wear), it could negatively affect the lubrication in macro, micro and nano-devices.11,

23, 34

. So far,

researchers have focused their attention to the investigation of the structural and morphological properties of supported ILs thin films and nanostructures. To our knowledge, the electrical properties of ordered ILs domains on solid surfaces have not been directly characterized. In this work we have investigated how the surface-induced structural reorganizations of [BMIM][NTf2] nanostructures assembled at room temperature on a variety of solid insulating substrates influence the electric properties of the IL layers, by means of a combined morphological and 5



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electrical analysis by AFM. We have shown that a conductive to insulating transition accompanies the surface-induced liquid to solid-like structural transition of solid-supported ILs, with a marked reduction of the dielectric constant compared to the bulk phase.

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2 EXPERIMENTAL METHODS 2.1

Ionic Liquids, Solvents, Substrates. Ionic liquid [BMIM][NTf2] with 98.0%

purity grade was purchased from Sigma Aldrich and stored in a clean high-vacuum chamber before usage, in order to avoid water contamination. Methanol (purity 99.8%, HPLC) from Fluka, and ethanol and chloroform (purity 99.8%, HPLC) from Sigma Aldrich, were distilled twice before using for thin films deposition. Amorphous silica coverslips, single crystal silicon substrates, mica, single crystal polished MgO, TiO2 (rutile) and NaCl substrates were used as deposition surfaces for the morphological and structural characterization. The cleaning procedure for the substrates before deposition consisted in the fresh cleavage for mica and NaCl, while all other oxide substrates were cured in aqua regia solution (1h bath) for the removal of contaminants and the full re-hydroxylation of the surface. The electrical measurements were performed using n+ doped silicon substrates (with an approximately 1.5 nm thick native oxide layer on top) with evaporated Au micro-contacts (see Supporting Information, Figure S1). 2.2

Sample Preparation. Thin films of ionic liquids were prepared by the drop

casting deposition method. A 20 µl droplet of diluted (1 µg/ml) [BMIM][NTf2] solution in different solvents was deposited onto the clean substrate. For all the substrates considered in this work, the solvent was left to evaporate completely (typically 5-6 min were enough for all solvents) in standard laboratory conditions (T = 23°, RH = 42%). We checked the absence of solvents contaminations by performing AFM investigation of samples produced by depositing only the solvents without the IL, in the same experimental conditions. 2.3

Atomic Force Microscopy Characterizations.

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2.3.1 Morphological and Structural Characterization. Both a Multimode Nanoscope IV and a BioScope Catalyst AFMs (Bruker, Santa Barbara, USA) were used. Morphological analysis was performed in tapping mode using RTESP-300 probes from Bruker, with resonance frequency between 200 and 300 kHz, k = 40 N/m and radius below 10 nm. Morphological maps have been collected with a sampling resolution of 2048 x 512 points using a scan rate of 1 Hz. Upon image flattening by line-by-line subtraction of polynomials up to the third order, flat terraces produce sharp peaks in the height histograms. The statistical analysis of layered structures has been performed using Matlab (MathWorks, Natick, MA, USA) custom routines. The height distributions have been analyzed by multi-Gaussian fitting to extract thicknesses and errors of the different terraces. Considering these nanostructures as layered systems, we guess an internal structure consisting by terraces composed by a superposition of molecular layers having a fundamental step with height δ. The height of each terrace (with thickness hi) is then supposed to be a multiple of the basic monolayer height δ (see Figure 2C). The structural analysis aims at finding the best divider δ of terrace heights, and the series of best integers Ni such that hi = Ni δ. To this purpose, we minimize the χ2 function with respect to δ and Ni:

1 ℎ ℎ = − (1)

where wi = σi-2, σi being the error associated with the layer thickness hi; Ni = [hi/δ] represents the closest integer such that hi ≈ δ [hi/δ]; N is the number of elements of the specific data set. For each set of data, usually more than 20 images were analyzed.

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2.3.2 Mechanical Characterization. For the investigation of the mechanical resistance during imaging at different loads, and for the nanomechanical analysis, we have used contact mode probes, SNL-10, with force constant k = 0.3 N/m and tapping mode probes, RTESP-300, k = 40 N/m from Bruker, respectively. In Force Volume mode (FV), a force curve is recorded on every point of a grid spanning a finite area.51 A force curve represents the deflection of the cantilever, proportional to the applied force, as a function of the z-piezo displacement, during approaching/retracting cycles. The surface morphology is reconstructed from the ensemble of force curves, recording the height value at which the maximum force setpoint is reached, while the tip is in contact with the surface. FV allows the acquisition of topographic maps at different applied force, therefore testing the mechanical stability of the system under study. If disruption of surface features occurs during the acquisition of the topographic map at finite load, morphological changes can likely be detected during successive acquisitions. Nanomechanical tests of the IL layers have been performed using the Point & Shoot imaging mode, consisting in the acquisition of a set of force curves in defined locations (for instance on the bare substrate, on top of different terraces, etc.), selected in a previously acquired topographic map (a solid-like IL nanostructure, in our case) . The acquired topographic map also allows measuring accurately the thickness of the layer and its terraces. The nanomechanical analysis is then performed by fitting the Hertz model to the experimental force vs indentation curve (derived by the force vs piezo displacement curve, see Ref.51): 4 = ∗ √ ⁄ (2) 3 where δ represents here the indentation, E* is the reduced Young’s modulus, and R is the tip radius (the nanomechanical protocol is described in our recent publication Ref

52

). The Hertz 9



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model can be used to describe, to a variable degree of accuracy, the deformation on an elastic surface by a rigid sphere. The radius R of the sharp probe is one of the most critical parameter needed to extract a reasonably accurate value of the Young’s modulus of elasticity. The procedure for the calibration of the radius is described in Supporting Information, Figure S2. In the typical conditions of the present work, the Hertz model is a good approximation of the elastic response of the solid-like IL layer, since the total indentation is not large compared to both the probe radius and the IL layer thickness. In force vs indentation curves, penetration events are observed at higher forces (breakthrough events). In the case of layers with multiple terraces, several breakthrough events can be observed. The separation along the distance axis between adjacent breakthrough events is be well correlated to the thickness of the terraces.

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2.3.3 Electrical Characterization. For conductivity and nanoscale capacitance experiments, we have used doped diamond-coated probes CDT-CONTR (Nanosensors), with nominal elastic constant k = 0.3 N/m, and tip radius R = 150 nm. The experimental setup for the nanoscale electrical measurements consists of a high-resolution analog-to-digital acquisition board (DAQ PCI-6251, National Instruments); a current limiter placed in series with the tip in order to limit the maximum current to 150 nA to avoid damage to the AFM head; a custom low-noise transimpedence amplifier, designed to perform high-resolution current measurements with wide bandwidth;53 custom Virtual Instruments (VIs) developed in Labview environment to record the I-V curves (LABVIEW, National Instruments). The AC voltages were provided by a digital lock-in SR830 (Stanford Research System) using a breakthrough box placed between microscope and controller. The same lock-in amplifier was used to record the capacitive and resistive components of the measured AC currents. The electrical measuring system (AFM plus attached devices) was placed inside a grounded Faraday cage in order to shield effectively the electromagnetic noise (see Supporting Information Figure S5). The AFM was suspended by thick elastic cords onto a heavy marble plate to provide good insulation from vibrations. Additional details about electrical measurement setup are available in Supporting Information, Figure S6, S7.The dielectric constant of an insulating film at the nanoscale can be quantified using a two-step procedure, described in detail in Refs.54-58 First, the value of the effective radius R of electrical contact is obtained from capacitance vs distance curves measured on the bare conductive substrate. Then, the ratio of the thickness over the dielectric constant h/εr (an effective thickness) is obtained from capacitance curves recorded on the dielectric thin film. The following equations represent the capacitance vs distance curve acquired on a bare electrode (Equation 3) and on a thin dielectric film with dielectric constant εr (Equation 4):56-58 11



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!"# ($)

= % + '$ + 2()* log .1 +

/(0123(4⁄))

789: ($)

= % + '$ + 2()* log .1 +

/(0123(4⁄)) 6 (4) 5;

5

6 (3)

Here ε0 is the vacuum dielectric constant, θ is the tip cone angle, B represents the angular coefficient (in aF/nm) of the parasitic capacitance at large distances, and A all the parasitic contributions that do not vary with z. A sub-aF resolution in capacitance was achieved by averaging 100 curves acquired by applying an AC bias with amplitude of 500 mV and frequency of 90 kHz, setting the lock-in integration time to 100 ms and the approaching velocity to 7 nm/s. Topographic maps were always acquired before and after the electrical measurements (always performed using the Point & Shoot scheme), in order to select the most interesting locations, and to monitor the onset of disruptive events that could lead to tip contamination and, in general, to artefacts. Moreover, the topographic map allows measuring the thickness of the IL structures, an important parameter in the fitting procedure. The contamination of the AFM tip, i.e. upon interaction with a liquid IL droplet, can strongly influence the results of the electrical characterizations, for example through the modification of the effective radius of curvature of the tip. All measurements underwent therefore a test procedure. First, a second topographic map was typically recorded after the electrical measurement. If topographical deformations, delamination or ruptures were detected, the measure was considered unreliable. Second, the value of the electric radius measured after the measurement (RAFTER) was compared to the one measured before (RBEFORE), to check the apex status (See Table 1). Approximately 20% of the total measurements was considered reliable. Contamination artifacts are presented and discussed in the Supporting Information, Figure S11, S12.

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3 RESULTS AND DISCUSSION 3.1

[BMIM][NTf2] Forms Layered Structures on a Variety of Insulating

Substrates, Irrespective to the Solvent Used for the Deposition. When a small volume of highly diluted [BMIM][NTf2] solution (1 µg/ml) is cast on several insulating flat surfaces, a population of randomly distributed thin layers in coexistence with liquid micro- and nanodroplets is typically observed. This phenomenon is quite general, as long as one deals with insulating substrates, and independent on the solvent used for depositing the IL. We have demonstrated this by using different substrates, and three different solvents possessing good solvation for [BMIM][NTf2], high evaporation rate at room temperature, yet different polarity: methanol, ethanol, and chloroform, with dielectric constant r = 32.8, 24.6, 4.8, respectively.59 As shown in Figure 1, layered nanostructures with flat terraces and sharp edges were found on oxidized silicon when using all the three different solvents (Figure 1A-C), and on other insulating substrates: crystalline MgO, TiO2, and NaCl (Figure 1D-F). We have already reported about similar structures on mica and amorphous silica, using methanol as solvent.47 Interestingly, also the SEM analysis revealed that across the whole surface of the oxidized silicon substrate two different types of IL structures coexist, as suggested by the contrast in the secondary electrons map (see Supporting Information Figure S3). This contrast between the substrate and the two types of structures is due to different electric properties of the IL domains (insulator/conductor), as confirmed by the data presented in the following sections, and witnesses the co-existence of IL dielectric solid-like layers and conductive liquid droplets.

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Figure 1. Surface morphology of drop casted [BMIM][NTf2] on (A) oxidized silicon (methanol solution), (B) oxidized silicon (ethanol solution); (C) oxidized silicon (chloroform solution); (D) single crystal polished MgO (methanol solution); (E) single crystal polished TiO2 rutile (methanol solution); (F) single crystal NaCl (methanol solution). Following a procedure published in previous work,47 we have characterized the basic molecular step of the layered structures by means of a statistical analysis of AFM topographies (Figure 2A) under the hypothesis that each structure consists of terraces with different heights piled one on top of another (Figure 2B), and that each single terrace is formed by several basic molecular layers with thickness δ. By significantly expanding our previous analysis47 to other 14


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substrates and solvents, and in excellent agreement with it, we have found that the height of terraces is compatible with multiples of a basic molecular step δ = 0.60 ± 0.01 nm (Figure 2C). The result shown in Figure 2C is remarkable since it shows that [BMIM][NTf2], when deposited in such a way that tiny amounts of liquid are dispersed on an insulating surface, rearrange in ordered layered nanostructures, irrespective to the solvent used.

Figure 2. (A) 3D AFM morphology of a layered structure on oxidized Si (2.2 µm, vertical scale 35nm) and (B) profile section corresponding to the line in (A) showing thickness of terraces labelled as h1, h2, h3. (C) Fundamental step δ calculation through correlation between average 15



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heights of IL terraces prepared in different conditions (surfaces and solvents) and integer number of basic steps in each structure (error bars are comparable with marker sizes).

The value of δ found in our experiments is in agreement with the results of numerical simulations of thin [BMIM][NTf2] layers on silica, 24-25, 48 although the simulations could not predict the extended layering observed experimentally up to tens of nanometers due to the limited cell size and integration time. The deposition method by solvent casting and short simulation time are the main reason of differences between experimental and simulation results. The fact that the values of δ are all similar irrespective to the solvent used suggests that the solvent does not play a strong structural role in the formation of the ordered layers, although the role of residual water cannot be ruled out completely: controversially, positive42, 60 and negative61 effect on layering were reported. The fact that similar layering is observed when using solvents of different polarity (also performing deposition in solvent saturated atmosphere47) suggests however that the role of water in the observed layering is not predominant. Strong confinement of small amounts of IL can favor the transition to an ordered structure,12 and the drop casting method likely provides this fundamental condition. Similar long-range ordering (up to 60 nm) in ILs was obtained by Jurado et al.62 by inducing solid-like phase transition after cyclic confinement processes using an extended surface forces apparatus. Because of solvent evaporation and surface dewetting, indeed, the original macroscopic drop of solvent/IL solution deposited on the substrate fragments into a multitude of increasingly smaller droplets, down to the micro- and nano-scale. This effect provides the confinement eventually contributing to the layering at the solid interface.11-12, 62-63 Whenever the amount of locally deposited IL remains relatively large, approximately above 1 µm in size, the liquid phase is favored energetically and no further layering occurs. The observed co-existence of liquid and layered structures is thus obtained. The mechanism behind the formation of layered 16


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ordered IL structures could be schematically described as follow. Initially, evaporation and surface dewetting increase IL concentration inside small droplets. At this stage, ion-ion interactions (e.g. van der Waals, hydrogen bonding, π–π, solvophobic, Coulombic, and packing interactions) depend on the effective dilution, considering a progressive increase of ionic concentration.64 Later on, layers are formed, and stabilized, by a favorable surface interaction provided by fixed charges on insulating surfaces, voltage bias on metals, and π−π stacking on amorphous carbon.10, 13 The internal complex dynamics of an evaporating droplet (convective motion, temperature and concentration gradient)65 can provide the suitable conditions for a local annealing, favoring the stabilization of the structure. Although a similar vertical ordering is observed for different solvents (and surfaces), differences in polarity, the surface wettability and the evaporation rate of the different solvent/substrate combinations may influence the spatial distribution as well as the inplane geometry of the stacked layers. For instance, rounded, regular layers are mainly obtained using methanol (Figure 1A), while layers with irregular edges are obtained using chloroform on the same substrate (Figure 1C).

3.2 [BMIM][NTf2] Forms Mechanically Strong Ordered Structures. AFM images and height analysis suggest that [BMIM][NTf2] nanostructures behave like lamellar solids, which can be cleaved along preferential directions.48, 66 In fact, when imaging in contact mode at higher forces, delamination was observed, as well as erosion located preferentially at the terraces edges, where the mechanical strength of the film is expected to be reduced67. In our previous works, we reported evidence that the layered structures oppose a strong mechanical resistance to vertical compression by the AFM tip, while liquid IL droplets are easily displaced and/or penetrated.48, 66 New measurements carried out in Force Volume (FV) mode corroborate these conclusions (Figure S4 in the Supporting Information).

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The evidence collected so far suggests that [BMIM][NTf2] structures formed on oxidized silicon and other insulating smooth substrates are not only (vertically) ordered, but also possess a mechanical resistance to compressive (and to a good extent also to lateral) stresses that is typical of solid materials. In the same samples, IL droplets provide the counter-example of structures, which are neither ordered nor mechanically resilient. In order to study quantitatively the mechanical response of ordered IL layers to high compressive stresses in the range 50-500 nN we have recently coupled topographic imaging (in tapping mode, Figure 3A) to local indentation measurements. 40 In Figure 3 we present a detailed analysis of the experimental data collected so far. Upon contact with the surface in selected locations, the probe is pushed against the IL film, which is elastically deformed for the first 1.5-2 nm before a breakthrough event takes place. Several such events can be seen in the same indentation curve, demonstrating that several terraces are penetrated sequentially, down to the underlying silicon substrate. The elastic indentation is reasonably small compared to the film thickness (10-20 nm, measured from the topographic map), and also compared to the probe radius (25 nm, see Supp. Info section 2 for radius calibration). In these conditions, the Hertz model of a hard sphere indenting a flat infinite surface (described in Equation 2) allows a reasonably accurate estimation of the value of the reduced Young modulus E* of [BMIM][NTf2] layers: E* = 1.8 GPa (inset of Figure 3C), a typical value for plastics like Nylon and PET. Breakthrough events typically occur at forces of 80-100 nN, corresponding to a pressure of about 3.5 kbar.

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Figure 3. In panel (A) a tapping mode topographic map of a layered [BMIM][NTf2] island is shown. The locations where sets of indentation curves have been acquired are marked using different colored symbols (black disk: substrate; blue square: topmost layer 1; red triangle: layer 2). In (B), representative indentation curves collected in the different locations, including the substrate are shown. Panel (C) shows the distribution of tip-surface separation values from 30 force curves collected on layer 1. The inset in panel (C) shows the analyzed force curves, and highlights the fit

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of the elastic compression of the topmost terrace (layer 1) using the Hertz model of Equation 2. The correspondence of discrete breakthrough events with periodicities and sub-periodicities observed along the distance axis is also shown.

At a closer inspection, the indentation curves collected on the IL layers (a few representative are shown in Figure 3B) show reproducibly a saw tooth profile. The distances corresponding to the onset of indentation, where the force rises until the breakthrough point, are compatible with the heights of terraces observed in the topographic maps. Figure 3C shows the distribution of tip-surface separation distances from a set of 30 indentation curves (inset of Figure 3C) collected on the topmost terrace shown in Figure 3A (layer 1, blue square). Peaks in the distribution highlight the distances at with penetration of the layers occur. Peak to peak distances provide a rough estimation of the terraces heights. A main periodicity of about 5 nm (in agreement with the vertical spacing observed in the topography), and sub-periodicities of about 3 nm. There is no clear evidence outside the noise band of events occurring with a periodicity corresponding to the observed molecular step δ. The indentation experiments confirm that [BMIM][NTf2] nanostructures are made of stacked layers with a high degree of vertical order and remarkable mechanical resistance to compressive stresses. MD simulations of the indentation of thin IL layer near charged surfaces show a critical breakthrough pressure of 5 kbar in good agreement with our measured values,24-25 although due to the constraints of the numerical simulation the vertical order is localized within 3 nm from the surface. Our latest analysis confirms that the values of the breakthrough pressure are more than 1 order of magnitude intense than those measured by AFM during the penetration of solvation layers of pure bulk IL near charged surfaces.10,

50, 61, 63

Comparable breakthrough pressures were observed for a compact layer

adsorbed on a biased (2V) Au electrode.68 20


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3.3 [BMIM][NTf2] Layers are Resilient with Respect to Intense Electrostatic Fields. The mechanical resistance of ILs structures was further investigated by comparing topographic maps of layered structures before and after the application of intense electrostatic fields (> 108 V/m), localized between the conductive AFM tip and the sample. Bulk ILs are typically electro-sprayed at these field values.38, 69 Several images of the same structure were collected, alternatively keeping the tip at a fixed increasing electrostatic potential with respect to the substrate, or grounded. The effects of the application of the electric field were monitored by imaging the sample in the best imaging conditions, i.e. with both the tip and the sample grounded, after the application of the electric field. Figure 4A shows the initial sample morphology before the application of the electric field. The corresponding Gaussian fitting of height distribution (from the region delimited by the rectangle) is shown in Figure 4D. Peaks correspond to terraces. The area below each peak is proportional to the terrace area. For applied voltages below 4 V, no modifications were observed (data not shown).

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Figure 4. AFM topographic maps of thin [BMIM][Tf2N] films on n+ doped Si (110) acquired in neutral conditions (both tip and sample grounded). (A) before applying the electric field; (B) after a complete scan with a bias of +4V; (C) +8V. In (D) the Gaussian fitting of height histograms from the region delimited by the rectangles.

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Data in Figure 4B,C show that the application of the electric field strongly perturbs the liquid droplets, while the ordered layers are only slightly delaminated and/or eroded (this is witnessed by the appearance/disappearance of peaks in the height distribution, or by a change in peaks area). This in turn suggests that in the layers the ions are tightly bound in a stable and compact structure. The delamination is observed also on the substrate (see Figure 4C, and the peak at negative height values in Figure 4D), demonstrating that the oxidized silicon surface is fully covered by a thin film of layered ionic liquid with height h = 1.7 nm. By comparing Figure 4B to Figure 4A, it is evident that new terraces can form, especially near existing islands (see the appearance of a new peak at about 8 nm in Figure 4D). Evidently, the electrified AFM tip, while moving along the sample, can drag around weakly bound ions, most likely from IL droplets and terrace edges, and redeposit the material in shape of new layers. This effect is similar to the one observed by Kaisei et al.,70 who succeeded in the fabrication of 4 nm thick [BMIM][BF4] solid-like films on platinum using a polarized AFM tip as a nano-inkjet printer. We have so far collected experimental evidence supporting the conclusion that the surface-induced [BMIM][NTf2] layered nanostructures are not only ordered (in the vertical direction), but also possess mechanical resistance to vertical and lateral stresses, and resilience with respect to the application of very intense electric fields, which are typical of solid materials. The next step forward is then arguing whether the solid-like [BMIM][NTf2] nanostructures possess an insulating character, in analogy with conventional solid salts, where ions are tightly bound in a regular crystalline lattice.

3.4 DC Conductivity of [BMIM][NTf2] Solid like Nanostructures. The electrical conductivity of [BMIM][NTf2] nanostructures has been quantitatively investigated by measuring the local I-V characteristics. To this purpose, after recording a topographic map 23



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of the structure under investigation, the AFM conductive tip was kept in stable contact with the surface with a controlled maximum load, while the applied voltage was ramped in the interval -4V and 4V and the DC electric current was measured. Conductivity measurements were performed on 3 different samples for sake of comparison: evaporated Au micro-contacts in series with a 100MΩ resistor; a doped n+ silicon substrate with its native oxide layer; a thin layer of [BMIM][NTf2] with thickness of about 10 nm deposited on a similar silicon substrate.

Figure 5. I-V curves acquired in DC mode on Au micro-contacts (continuous blue line), n+ doped silicon capped by its native oxide (dashed red line), and a solid-like 10 nm thick [BMIM][NTf2] layer on the same silicon substrate (black dotted line). The I-V curve of the [BMIM][NTf2] nanostructure with expanded vertical axis is shown in the inset.

As shown in Figure 5 the behaviour of Au micro-contacts is Ohmic, the slope of the I-V curve being compatible with the resistance placed in series with the micro-contacts. The doped silicon substrate with its native oxide layer shows the typical behaviour of a metal-oxidesemiconductor (MOS) junction, with a sudden non-linear increase of the current after an absolute voltage threshold between 1 and 2 V. The I-V characteristics of the solid-like IL layer is almost flat, 24


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showing an extremely weak conductivity (inset in Figure 5), corresponding to an effective resistance of 400-500 GΩ estimated from the slope of the I-V curve on the positive voltage axis. Considering the height of the IL film (10 nm) and the underlying native oxide layer, the resistivity is in the range 1016-1018 Ω/cm, which demonstrates the insulating behaviour of the solid-like IL film. At the light of these results, the contrast observed in the SEM image shown in the Supporting Information (Figure S3) can be interpreted, as anticipated, as originating from the different electrical properties of the different structures coexisting on the substrate. The insulating solid-like IL islands appear brighter, because of the charge accumulation; the conductive IL nano- and microdroplet, appear darker; the conductive doped Si substrate capped by its thin native oxide layer, with intermediate contrast.

3.5.

Dielectric Behavior of Solid-like [BMIM][NTf2] Layers.

Because of the

insulating character of [BMIM][NTf2] layers, we performed local capacitance spectroscopy to characterize their dielectric properties. We have used the conductive AFM tip as one of the two electrode of a nanoscale capacitor, the other being the conductive n+ doped silicon substrate; the thin IL layer plays the role of the dielectric slab in between the capacitor plates. This is the standard configuration of AFM-based nanoscale impedancespectroscopy,54, 56-57 described in details in the Materials and Methods section as well as in Supporting Information Figure S5, S6, S7. We have acquired hundreds of capacitance vs distance curves on different solid-like IL nanostructures, previously imaged by AFM to be able to select the measurement locations, as well as to measure the IL layer thickness. The capacitance curves were acquired by applying an AC voltage oscillating at 90 kHz and representative curves are shown in Figure 6.

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Figure 6. Average capacitive curves acquired on top of a [BMIM][NTf2] solid-like layer and on the bare silicon substrate, with the fit by Equation 3 and Equation 4, respectively.

The weighted average value of the dielectric constant of [BMIM][NTf2] layers at 90 kHz (from capacitive measurements) is εr = 4.8 ± 0.8 (all the measured values and their errors are reported in Table 1). The dielectric nature of solid-like layers was confirmed by electrostatic force microscopy,71-72 by recording the deflection of the cantilever caused by the electrostatic interaction with the surface of the sample as a function of the separation distance.73-75 The results are reported in the Supporting Information (Section S6, Figure S8 and Table S1). The average value εr = 3.1 ± 0.2 obtained by DC electrostatic measurements is smaller than that obtained by AC capacitance spectroscopy. We attribute the observed discrepancy to sources of systematic errors, such as the subtraction of a strong irregular interference pattern (Supporting Information, Section S8, Figure S12J), or to a minor extent the uncertainty in the determination of the surface potential (Supporting Information, Section S7, Figure S9, S10).

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Table 1. Summary of the Experimental Results Obtained with Capacitive AC Nanoscale Impedance Spectroscopy Measurements RBEFORE(nm)a

RAFTER (nm)b

h (nm)c

h/εr (nm)d

εre

207 ± 11 210 ± 8 12.5 ± 0.2 2.6 ± 1.7 4.8 ± 2.3 211 ± 6 206 ± 5 9.5 ± 0.1 2.0 ± 0.8 5.3 ± 3.7 196 ± 5 201 ± 7 13.5 ± 0.2 2.3 ± 0.7 5.6 ± 1.8 215 ± 4 217 ± 5 10.6 ± 0.2 2.5 ± 0.5 4.2 ± 1.0 a RBEFORE is the radius of the calibration before the measurement on IL, bRAFTER represents the calibrated radius after the measurements; c h is the thickness of the IL layer extracted by the topographic AFM map. d The value of the reduced thickness h/εr and e value of the dielectric constant εr.

The dielectric constant values found in our experiments are similar to those of insulating materials, like SiO2 (εr = 3.9) or NaCl (εr = 5.3). The dielectric constant values measured on solidlike IL nanostructures are significantly lower than those reported in the literature for pure bulk ILs, either extrapolated from impedance spectroscopy measurements at high frequencies (εr ≈ 15.0),76 or by dielectric relaxation NMR (εr = 13.7).77 In bulk ionic liquids, the conductivity is directly connected to the translational mode of ions, masking other degrees of freedom (orientation, rotation, libration, vibration, atomic polarizability etc.) that contribute to the polarizability.13 The decrease of local dielectric constant could be justified by the fact that the mobility and degrees of freedom in the terraces are greatly reduced. The results of the mechanical and electrical tests reported in this work provide clear evidence that the ions in the layered nanostructures are trapped in a more regular structure compared to the liquid phase. A similar effect was experienced by Morikawa et al.78 by measuring dielectric constant of water confined in an extended nanospace (10-1000 nm), and showing a significantly reduced value approximately 3 times lower than that for the bulk.

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The surface-induced formation of thin solid-like insulating IL nanostructures may have a strong impact on the behavior of nanotechnological systems and devices, in particular because such structures can be present at the interface between a bulk ionic liquid and a solid surface. This is the case of the IL being used as gating medium in innovative field-effect transistors, where the voltage is controlling the formation of the dielectric gate.28-29 Similarly, ILs are used as active liquids in electrowetting applications.30-32 A liquid to solid-like transition accompanied by a dramatic reduction in conductivity of the interfacial ionic liquid layers in conditions of extreme confinement would affect strongly the performance of those devices where the IL is used as electrolyte impregnating nanoporous electrodes. This is the case of electrodes made of i.e. nanoporous carbon (electrolitic supercapacitors79-80), or silica or other oxides (for Gratzel cells,81 gas-capture82-83, and catalytic84-85 devices). The behavior of nano-confined ILs in porous matrices and nanostructures was recently reviewed.12, 86 The general feature of nanoconfined ILs is that the detailed local structure (ionic layer structure and orientation) and dynamics (ionic mobility) of the confined ILs are mainly governed by the distance and chemical interaction with the pore walls. ILs near to the pore walls are prone to form layered structures with solid properties and lowered dynamics because of the strong interaction between ILs and the pore walls.87 In the central region of pores, ILs structure and dynamics are less affected by the confinement, and the ILs maintain a character similar to the bulk. Varying the amount of ILs inside the pores and the pore size in the matrix can be the effective strategy to control the degree of confinement, and thus the dynamics and organization at interface.88-89

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4 CONCLUSIONS We have investigated by AFM the local morphological, nanomechanical, and electrical properties of [BMIM][NTf2] nanostructures on silica. We have shown that in conditions of strong surface interaction and nanoscale confinement, imidazolium-based ILs form vertically-ordered and rigid structures, which are highly resilient to intense electric fields, and possess an electrically insulating character, with a dielectric constant εr = 3-5, similar to that of conventional solid salts. These unexpected electrical and structural properties of ILs may have a strong influence on the performance of those devices, where ILs wet or impregnate surfaces and nanoporous electrodes, in conditions of extreme confinement.

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5 SUPPORTING INFORMATION DESCRIPTION 1) Substrate preparation and cleaning. 2) Characterization of the apical radius of curvature of AFM tips. 3) Scanning electron microscopy analysis of IL thin films. 4) Test of the resistance to vertical compression of solid-like [BMIM][NTf2] islands. 5) Details on the experimental setup for capacitive and electrostatic AFM measurements. 6) DC Electrostatic AFM measurements on thin [BMIM][NTf2] solid-like layers. 7) Measurement of the surface potential Vsp. 8) Artifacts in capacitive and electrostatic AFM measurements. 9) Bibliography.

6 Acknowledgements The authors extend a special thank to M. Sampietro and G. Ferrari from the Department of Electrical Engineering of Politecnico di Milano, for the valuable support in the implementation of the nanoscale impedance microscopy technique and the required electronic devices, and for providing the doped silicon substrates with integrated Au micro-contacts. The authors thank Davide Marchesi (Fondazione Filarete, Milan, Italy) for his assistance in scanning electron microscopy. M.G. acknowledges the Shenzhen Science and Technology Innovation Committee (KQJSCX20170331162214306) for support.

7 Conflict of Interest The authors declare no conflict of interest. 30


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Figure 6. Average capacitive curves acquired on top of a [BMIM][NTf2] solid-like layer and on the bare silicon substrate, with the fit by Equation 3 and Equation 4, respectively. 180x106mm (150 x 150 DPI)


Page 42 of 43

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Graphical abstract 42x22mm (300 x 300 DPI)


Surface Confinement Induces the Formation of ... - ACS Publications

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