Probing Nanoscale Ion Dynamics in Ultrathin Films of Polymerized

Letter pubs.acs.org/macroletters

Probing Nanoscale Ion Dynamics in Ultrathin Films of Polymerized Ionic Liquids by Broadband Dielectric Spectroscopy Maximilian Heres,† Tyler Cosby,† Emmanuel Urandu Mapesa,‡ and Joshua Sangoro*,† †

Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States Institute of Experimental Physics I, University of Leipzig, Linnestr. 5, 04103 Leipzig, Germany

‡

S Supporting Information *

ABSTRACT: Continuous progress in energy storage and conversion technologies necessitates novel experimental approaches that can provide fundamental insights regarding the impact of reduced dimensions on the functional properties of materials. Here, we demonstrate a nondestructive experimental approach to probe nanoscale ion dynamics in ultrathin films of polymerized 1-vinyl-3-ethylimidazolium bis(trifluoromethylsulfonyl)imide over a broad frequency range spanning over 6 orders of magnitude by broadband dielectric spectroscopy. The approach involves using an electrode configuration with lithographically patterned silica nanostructures, which allow for an air gap between the confined ion conductor and one of the electrodes. We observe that the characteristic rate of ion dynamics significantly slows down with decreasing film thicknesses above the calorimetric glass transition of the bulk polymer. However, the mean rates remain bulk-like at lower temperatures. These results highlight the increasing influence of the polymer/substrate interactions with decreasing film thickness on ion dynamics.

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open challenge.21 Thus, there is an urgent need for novel experimental approaches capable of probing nanoscale ion dynamics without altering the materials under study. Alongside nuclear magnetic resonance spectroscopy, ellipsometry, and calorimetry, broadband dielectric spectroscopy (BDS) is one of the main experimental tools that has yielded a wealth of information regarding molecular dynamics in nonionic ultrathin polymer films in the recent past.8,23,24 BDS has two unique advantages; namely, (i) it enables the study of molecular/structural dynamics and charge transport in a wide frequency range, spanning more than 15 orders of magnitude, and a broad temperature range, and (ii) the sensitivity of BDS measurements increases with decreasing separation between the electrodes and hence decreasing amount of sample material under study. Various novel sample configurations such as interdigitated electrodes and evaporated electrodes have been used in combination with BDS in order to determine molecular dynamics in films of nonelectrically conducting polymers. However, measurement of dynamics in nanometric thin films of ion-conducting polymers by the conventional BDS is particularly challenging due to the fact that the electrical resistances decrease with film thickness resulting in electrical short circuits. This difficulty has impeded ion dynamics studies of electrically conducting thin polymer films.

olymerized ionic liquids (polyILs) are a new class of functional electrolytes that combine the desirable mechanical characteristics of polymers with the unique physicochemical properties of molecular ionic liquids in the same material.1−17 PolyILs make it possible to circumvent the key limitations of low molecular weight ionic liquids, namely, leakage and poor mechanical properties. These materials are promising electrolytes for use in dye-sensitized solar cells, lithium batteries, actuators, field-effect transistors, and electrochromic devices.2,7,14,18,19 Many of these technologies involve the use of polymerized ionic liquids confined in one dimension as thin films. Despite their prospects as ideal polymer electrolytes, the impact of nanoscale confinement on ion dynamics in polyILs has remained unexplored due to the technical difficulties associated with performing accurate and reliable experiments of ion dynamics in reduced spatial dimensions. To date, many techniques have been developed to probe local ionic transport and electrochemical processes on the nanoscale, the most common of which is the family of methods based on scanning probe microscopy.20 These tools have yielded significant insights in several areas of nanoscale science. For example, unprecedented access to the details of proton transport as well as polarization switching in ferroelectric and multiferroics at the nanoscale has resulted from the development of piezoresponse force microscopy and its variants.21,22 However, it is well recognized that this class of techniques faces severe limitations associated with tip-induced electrochemical reactions. Decoupling the material response of the unperturbed sample from those affected by the (often unwanted) reactions remains an © XXXX American Chemical Society

Received: August 3, 2016 Accepted: September 6, 2016

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The ε″der representation is necessary to suppress the contribution of the DC conductivity in the bulk polyIL measurements. However, incorporation of an air gap in the electrode configuration for measurement of the thin films accomplishes the same task experimentally. Therefore, ε″der and ε″ may be utilized for the bulk and nanoscale film, respectively, to obtain the same information as shown in Figure 2.

Here we show that a nanostructured electrode configuration with a noncontacting upper electrode enables unprecedented access to ion dynamics in ultrathin films of polymerized ionic liquids down to thicknesses of 7.5 nm, as measured using broadband dielectric spectroscopy. These studies reveal that ion dynamics below the calorimetric glass transition temperature of the bulk polymerized ionic liquids is unaltered under onedimensional confinement. At higher temperatures, dynamics are slower with decreasing film thickness. These results illustrate the increasing relative contributions of polymer/substrate interactions to ion dynamics with decreasing film thicknesses. The broadband dielectric spectra are usually presented in terms of the complex conductivity, σ*, and dielectric function, ε*, with respect to frequency. These quantities are expressed in real and imaginary parts, σ′, σ″, ε′, and ε″, respectively. The onset of frequency dependence in σ′ with increasing frequency marks the characteristic rate of ion transport in polymerized ionic liquids. Physically, successful ion motion is usually accompanied by rearrangement of the surrounding molecular environment in response to the changes caused by the mobile ions. This additional ionic polarization shows up in both formalisms of the dielectric spectra. The real part of complex conductivity is related to the dielectric loss through ε″(ω) = σ′(ω)/ωε0, where ω is the radial frequency and ε0 is the permittivity of free space.25 As shown in Figure 1, the process

Figure 2. Dielectric loss for bulk polymerized ionic liquid (a) obtained from the derivative formalism and the dielectric loss of 25 nm thin film (b) at selected temperatures, as a function of frequency. Lines indicate Havriliak−Negami fits. Insets in (a) and (b) show electrode configurations for bulk and film measurements for polyIL (blue), between brass electrodes (gold) and nanostructured electrodes (green), respectively.

To obtain accurate peak positions, ε″(ω) and ε″der(ω) for film and bulk samples, respectively, were fit using the empirical Δε Havriliak−Negami function ε* = ε∞ + β γ . Here, ε∞ (1 + (iωτHN) )

is the dielectric function at the high frequency limit; Δε is the dielectric relaxation strength; τHN is the characteristic time of the relaxation; and β and γ are shape parameters. The characteristic rate of charge hopping is then found from the peak maximum calculated from the Havriliak−Negami fitting parameters according to Figure 1. Real and imaginary parts of the complex conductivity, σ* = σ′ + iσ″, complex dielectric function, ε* = ε′ − iε″, and derivative representation of dielectric loss, ε″Der, as a function of frequency for bulk polymerized 1-vinyl-3-ethylimidazolium bis(trifluoromethylsulfonyl)imide at 230 K. The vertical dashed line indicates the onset of frequency dependence in σ′ as well as a peak in ε″Der attributed to ion dynamics.

ωc =

1/ β ⎤⎡ −1/ β ⎤ ⎡ 1 ⎢ ⎛ βπ ⎞ ⎥⎢ ⎛ βγπ ⎞ ⎥ sin⎜ sin⎜ ⎟ ⎟ ⎥⎦ τHN ⎢⎣ ⎝ 2 + 2γ ⎠ ⎥⎦⎢⎣ ⎝ 2 + 2γ ⎠

It is well established that the derivative analysis yielding ε″der(ω) alters the spectral shapes, and therefore, it is not the intent of the current work to analyze details of spectral shapes under confinement in comparison to the bulk. The focus here is on the characteristic time scales of ion dynamics. The mean ion dynamics rates obtained for bulk polyILs and films of 60, 25, 12, and 7.5 nm thickness are plotted versus inverse temperature in Figure 3. It is observed that ion dynamics remain unaltered by the degree of confinement below the calorimetric glass transition temperature (Tg) of the bulk polyIL, while the dynamics are systematically slower with decreasing film thickness above Tg. From a fundamental point of view, polymerized ionic liquids belong to the broader class of glass-forming polymeric materials which exhibit the dynamic glass transition characterized by continuous slowing down of molecular mobility by many orders

corresponding to ion dynamics is obscured by the dominant dc ionic conductivity contribution to the dielectric loss for the bulk polymerized ionic liquid. However, the dielectric loss spectra can also be well approximated by the derivative analysis proposed by Wübbenhorst and van Turnhout,26 ε″der(ω) = (−π/2)[∂ε′(ω)/∂ln ω]. This representation suppresses the contribution of dc conductivity to the dielectric loss. The frequency corresponding to the peak maximum in ε″der is comparable to the onset of dispersion from the dc plateau in σ′ as illustrated in Figure 1 for the bulk polymerized ionic liquids. The analysis of ion dynamics here will focus on the peak frequency in the dielectric loss spectra. 1066

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1-butyl-3-methylimidazolium side chains. One expects attractive interactions between the silica surface of the substrate and the contacting layers of imidazolium cations of the polymer chain. The bis(trifluoromethylsulfonyl)imide anion is not polymerized and is therefore free to move separately from the polymer chain. Recent broadband dielectric spectroscopy measurements of this class of polyILs have shown that ion dynamics are orders of magnitude faster than the corresponding polymer segmental dynamics which control the calorimetric glass transition temperature.44 Below the glass transition temperature, the motion of the anions controls ion dynamics. In this temperature region, the rate of ion dynamics is independent of film thickness since the polymerized cation is immobile in the experimental time scale. Above Tg the additional influence of polymer dynamics is evident by the onset of a Vogel−Fulcher−Tammann-type temperature dependence of ionic conductivity, as is well-known for these types of materials.44 The increasing relative contribution of the polymer/substrate interactions with decreasing film thickness is expected to decrease the rate of ion dynamics by hindering polycation mobility. The effect of surface interactions on the ion dynamics in bulk samples on the other hand, which are 4−5 orders of magnitude thicker than film samples, are expected to be negligible. This becomes evident since bulk samples, even though they have two bound interfaces, still exhibit faster ion dynamics above Tg than thin films which only have one unbound interface. The calorimetric glass transition temperature for thin-film samples might also change under confinement. The focus of this letter, however, lies in the investigation of ion dynamics and not the glass transition temperatures of ultrathin films of polyILs. A recent related study by Bocharova et al. reported interesting experiments illustrating the ability to form reproducible nanopatterns on thin films of polymerized ionic liquids by applying voltages as low as 1 V to the tip/polymer junctions of an atomic force microscope.20 These patterns are presumably due to tip-induced electrochemical reactions on the polyIL leading to the observed surface modifications. In contrast, broadband dielectric spectroscopy in combination with the nanostructured electrodes enables nondestructive investigation of ion dynamics in ultrathin films of polyILs. The high impedance of the air gap in the nanostructured electrode configuration employed in the current work effectively prevents dielectric breakdown of the sample. It should be noted that the polymerized ionic liquid reported in ref 19 is identical to the one studied in the current work. It is therefore remarkable that no changes in the characteristic ion dynamics rates are observed at lower temperatures down to 7.5 nm film thickness, while Bocharova et al. observed softening of the polymerized ionic liquids for films as thick as 300 nm. The disparity could be due to the differences in the measurement techniques employed. Instead of permitting direct currents through the sample as is typical for modern scanning probe techniques, the nanostructured electrodes only permit displacement currents through the sample, measured between the two electrodes. These currents reflect local ionic transport and enable one to probe their dynamics on the nanoscale, thereby limiting parasitic electrochemical reactions. Due to the versatility of broadband dielectric spectroscopy, the approach reported in the current work is applicable for investigating nanoscale ion dynamics in a wide range of ionic materials as well. In conclusion, ion dynamics in a systematic series of ultrathin films of a polymerized ionic liquid are investigated over broad

Figure 3. Mean ion dynamics rates for bulk and thin films of polymerized 1-vinyl-3-ethylimidazolium bis(trifluoromethylsulfonyl)imide as a function of inverse temperature. The red dashed line indicates the calorimetric glass transition temperature of the bulk polyIL. The error bars are smaller than the sizes of the symbols, unless indicated otherwise.

of magnitude as the temperature is lowered toward the calorimetric glass transition. The quest to understand the nature of the glass transition in polymeric systems remains an active field despite numerous decades of concerted effort by different researchers to explain the phenomenon.27−35 Some of the key models proposed to explain the dynamic glass transition emphasize the role of co-operative dynamics and the existence of a finite length scale on which structural relaxation takes place. These approaches include, among others, the Adam−Gibbs concept of “co-operatively rearranging regions”,36 the random first-order transition model proposed by Wolynes,37 and the two-order parameter model of Tanaka.38 Within the framework of these models, structural dynamics are expected to exhibit an inherent, strongly temperature-dependent length scale on which molecular fluctuations take place. This implies that the dynamics should be drastically altered and, in the case of geometrical confinement, faster below this length scale.39−43 In addition, since classical theories predict a coupling between structural dynamics and ionic diffusion, ion dynamics should be altered in sufficiently confined polymerized ionic liquids compared to their bulk counterparts. However, when considered in the broader context of recent studies of dynamics in ultrathin polymer films and ion transport in bulk polymerized ionic liquids, the results presented in the current work indicate that the inherent length scales of molecular fluctuations in the systems investigated are smaller than 7.5 nm. We recently demonstrated that ion dynamics are faster than structural dynamics in bulk polymerized ionic liquids, especially below Tg, where structural dynamics are effectively frozen. Nevertheless, there is considerable influence of polymer dynamics at higher temperatures. A Vogel−Fulcher−Tammann-type of thermal activation of transport properties is characteristic of ionic conductivity above Tg.44 The slowing down of ion dynamics above Tg with reducing film thicknesses is therefore justified, due to the additional influence of the polycations on the overall ion dynamics. The silicon wafers employed in the experiments reported in this work feature a thin layer (∼5 nm) of silica, which is known to possess dangling hydrogen bonds on the surface.23 It is also well established that the surface of silica is negatively charged, except in very acidic media.45 The polymerized ionic liquid under consideration consists of a polyethylene backbone with 1067

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ACS Macro Letters frequency and temperature ranges for the first time by broadband dielectric spectroscopy, using a nanostructured electrode configuration featuring an air gap. It is observed that the mean rates characterizing ion dynamics remain bulklike below the glass transition temperature down to 7.5 nm film thicknesses. Above the glass transition temperature, the ion transport rates slow down with decreasing film thicknessan effect attributed to the increasing relative contribution of the polymer/substrate interactions. This approach provides new opportunities for nondestructive experimental access to nanoscale ion dynamics in different materials over a broad range of time scales.

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MATERIALS AND METHODS

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ASSOCIATED CONTENT

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the National Science Foundation for financial support through the Polymers Program award DMR-1508394. The authors also thank Friedrich Kremer for technical help with the nanostructured electrodes. The authors are grateful to Veronika Strehmel for providing the samples of polymerized ionic liquids studied in this work.

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Samples of polymerized 1-vinyl-3-ethylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids reported here were synthesized according to the procedure reported in ref 42. Highly doped silicon wafers with orientation (100) and well-controlled thermally oxidized silica layers were purchased from commercial sources (Novocontrol Technologies GmbH). A layer of aluminum was then deposited onto the backside of the wafer in vacuum to improve contact with the electrical connectors from the dielectric spectrometer. In addition, a photoresist was then spin-casted onto the front side to protect the surface from contamination prior to film deposition. The wafers were then cleaved into 4 mm × 8 mm pieces and cleaned in acetone then rinsed in ethanol to remove the photoresist. As a final step, the wafers were dried under a flowing stream of high purity nitrogen dry gas. At this stage, the wafers were typically ready for spin coating with polymerized ionic liquid films. Varying concentrations of polymerized 1-vinyl-3-ethylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid in dimethylformamide were prepared and spincasted following a similar method established for ultrathin polymer films. The samples were then annealed in oil-free evacuated chamber (10−5 mbars) at 390 K for 24 h. The thicknesses of the films were determined by atomic force microscopy (AFM) measurements using a Nanoscope Multimode AFM across a scratch made through the film by a sharp blade. AFM images of the films taken after BDS measurements also revealed average sample roughness below 1 nm on a scale of 100 × 100 nm for areas not affected by the depth measurements, as shown in the Supporting Information. The nanostructured electrode configuration was employed to perform dielectric measurements at different film thicknesses from bulk down to 7 nm film thickness. In this electrode configuration, the total measured dielectric function is related to the dielectric response of the polymer film, air gap, and the native silica layers on which the spincoated sample rests. This electrode geometry is advantageous for ion-conducting materials which would otherwise be extremely difficult to measure with the required accuracy due to the low electrical resistance arising from ionic conduction in the nanometric length scales. The dielectric measurements of bulk and thin films of polymerized ionic liquids reported here were performed over a wide frequency (10−1−107 Hz) and temperature (260−420 K) range under dry nitrogen atmosphere using a Novocontrol High Resolution Alpha Dielectric Analyzer (Novocontrol Technologies GmbH) with an applied voltage of 1 V. S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00601. Thin-film quality analysis and Figures S1 and S2 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 1068

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