Macroscopic Freestanding Nanosheets with Exceptionally High

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Macroscopic Freestanding Nanosheets with Exceptionally High Modulus Meshal Alzaid, Abu M. N. Taufique, Salim A. Thomas, Clay Carufel, John M. Harris, Alex J. B. Waters, Amal Altayyar, Sylvio May, and Erik K. Hobbie Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01025 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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Macroscopic Freestanding Nanosheets with Exceptionally High Modulus Meshal Alzaid,† Abu M. N. Taufique,† Salim A. Thomas, Clay Carufel, John M. Harris,‡ Alex J. B. Waters, Amal Altayyar, Sylvio May, Erik K. Hobbie* North Dakota State University, Fargo, North Dakota 58108

RECEIVED DATE () ABSTRACT: Macroscopic single-wall carbon nanotube (SWCNT) films of nanoscale thickness have significant potential for an array of applications that demand thin, transparent, conductive coatings. Using macroscopic micrometer-thick polystyrene sheets as a reference, we characterize the elastic response of freestanding multifunctional SWCNT nanosheets possessing both exceptionally high Young’s modulus and good durability. Thin SWCNT films (20 nm to 200 nm thick) asymmetrically ‘doped’ with dilute concentrations of super-paramagnetic colloids were suspended in ethanol as freestanding nanosheets. Through repeated and controlled deformation in an external magnetic field, we measure the temporal relaxation of nanosheet curvature back to equilibrium. From the relaxation time and its dependence on nanosheet thickness and length, we extract the SWCNT nanosheet modulus through a simple viscoelastic model. Our results are consistent with nearly ideal SWCNT rigidity percolation with moduli approaching 200 GPa and limited plasticity for sufficiently thick sheets, which we attribute to the screening of van der Waals interactions by the surrounding solvent and the macroscopic nature of the deformation.

KEYWORDS: nanoparticles, multifunctional films, rigidity, durability

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INTRODUCTION Macroscopic multifunctional nanosheets show considerable promise for a broad range of potential applications, from electronic devices1-3 and mesoporous membranes4 to battery electrodes5 and specialty coatings for biomedical applications.6 They are also critical to applications in flexible electronics.7-9 Thin films and freestanding nanosheets assembled from single-wall carbon nanotubes (SWCNTs), in particular, have unique potential utility for a wide array of applications related to flexible electronics, sensors, and solar energy conversion,1-3,10-19 but there are a number of intriguing unresolved questions related to such things as the nature of the interaction potential between colloidal SWCNTs,20,21 the optimization of SWCNT solubility in common fluids and solvents,22,23 the complexity of SWCNT suspension rheology24,25 and self-assembly,26,27 and the elastoplasticity of flexible SWCNT films and polymer nanocomposites.28-34 Residing at the core of this potential are the outstanding electrical and mechanical attributes of SWCNTs,35 where the former can be selectively tuned through chemical doping.1-3,12-19 Because of the polymer-like morphology of individual nanotubes, the mechanical characteristics of SWCNTs are particularly appealing for applications that seek to engineer flexible conductive films with exemplary rigidity and durability. Nonetheless, a full exploitation of the TPa elastic modulus ascribed to individual SWCNTs has remained unrealized, due in large part to the complexity of the elastoplastic response exhibited by thin SWCNT films and SWCNT-polymer nanocomposites.28-34 In an ideal description, we can relate the elasticity of the sheet to the modulus of an individual nanotube and the morphology of the network, which most often resembles a nanoscale version of the common fibrous structure exhibited by paper.29 Indeed, using

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paper as a reference, the modulus of a mesoporous SWCNT network well inside the regime of rigidity percolation30 can be modeled as = /3, where φ is the SWCNT volume fraction and E0 = 1.2 TPa is modulus of an individual SWCNT.36 Taking φ = 0.5,30 we obtain the estimate E ≈ 200 GPa for an ideal SWCNT nanosheet. In contrast, wrinkling measurements on compressed SWCNT films on elastic polymer substrates suggest a modulus that is much less than this, on the order of 10s of GPa, with a strain dependent value that necessitates a model-based extrapolation to extract a measure of the pristine film modulus.30-32 Here,

we

characterize

the

macroscopic

elastic

recovery

of

freestanding

multifunctional SWCNT nanosheets exhibiting both exceptionally high Young’s modulus and good durability. Thin SWCNT films 20-200 nm in thickness are asymmetrically seeded or ‘doped’ with dilute concentrations of magnetic colloids and deformed repeatedly in ethanol using an external magnetic field. By measuring the temporal relaxation of nanosheet curvature back to the equilibrium profile, we extract an elastic recovery time and its dependence on nanosheet thickness and length. Using micrometerthick freestanding polystyrene sheets as a point of reference, we then extract a direct macroscopic measure of the SWCNT nanosheet modulus by invoking a simple model for the viscoelastic recovery of the suspended sheet. Our results are consistent with nearly ideal SWCNT rigidity percolation, with moduli approaching 200 GPa and limited plasticity for a range of functional SWCNT film thicknesses. We attribute this behavior to the screening of van der Waals (vdW) interactions by the surrounding solvent and the macroscopic nature of the deformation, with our results suggesting that the full

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mechanical potential of SWCNTs can be realized in thin-film geometries under appropriate conditions.

Figure 1. (a) UV-Vis absorption spectra of a SWCNT nanosheet, both intrinsic and chemically doped, deposited as a film on a glass slide. (b) TEM image of the nanosheet morphology (50 nm scale). (c) Sheet resistance as a function of thickness for both intrinsic and chemically doped SWCNT nanosheets. (d) IV characteristics of a solar cell (11.5 % PCE) made from a SWCNT nanosheet deposited on n-type crystalline silicon and chemically doped to be p-type, where the inset shows macroscopic SWCNT nanosheets (1 mm on a side) suspended in ethanol awaiting deposition.

RESULTS AND DISCUSSION Details related to the experiments can be found in the Materials & Methods section and in the Supporting Information. As motivation for the utility of multifunctional thin SWCNT films in general, we first summarize the practical optical and electronic properties of the SWCNT nanosheets in Fig. 1. Optical absorption spectra (Fig. 1a) of the SWCNT films show resonant absorption peaks associated with interband optical transitions across the Fermi level between sharp van Hove singularities in the density of

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states.21 For the mixed type/chirality CoMoCat SWCNTs used here, the dominant feature near 1000 nm corresponds to the S11 transition of the most prevalent (6,5) semiconducting species.35 After doping the SWCNTs with thionyl chloride and then with gold chloride dissolved in nitromethane, the interband absorption peaks are suppressed, which is consistent with the appearance of doping induced mid-gap states.18,37 As noted above, the morphology of the SWCNT films is fibrous and mesoporous (Fig. 1b). The same chemical doping protocol leads to a reduction in sheet resistance,18 which is plotted as a function of film thickness, before and after doping, in Fig. 1c. The doping protocol used here leads to a p-type SWCNT film that forms a p-n heterojunction with ntype silicon (Fig. 1d), where the photoconversion efficiency (PCE) of the device depicted in Fig. 1 (11.5 %) is typical for the SWCNTs used in this study. The record PCE for this type of SWCNT-silicon heterojunction is around 15 % and relies on the deposition of a TiO2 antireflective coating on top of the SWCNTs.38 Details related to device fabrication, doping, testing and PCE calibration can be found in the Supporting Information. Our motivation for the current study comes directly from the approach we use to fabricate these solar cells, which relies on suspending the top SWCNT layer as a macroscopic freestanding nanosheet in ethanol (inset, Fig. 1d) prior to fluid-mediated transfer to the silicon substrate and subsequent chemical doping. Despite being thin (20 nm to 100 nm), we observe that the suspended SWCNT nanosheets are remarkably robust under aggressive pipette handling and rapidly unfurl back to a flat configuration upon release from a crumpled state. The objective of this work is to quantify the elastic recovery of the SWCNT nanosheets, suspended in ethanol, in a controlled and reproducible fashion, in an attempt to gain insight into their rigidity and durability.

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To this end, we cut SWCNT films of varied anticipated specified thickness h (20 nm < h < 200 nm), still adhered to their filter paper supports, into strips of fixed width and varied total length, where both are on the scale of a few to several millimeters. One end of the filter-paper-supported SWCNT film was then ‘doped’ with a drop of a dilute suspension of super-paramagnetic colloids dispersed in methanol and allowed to dry. The SWCNT film was then released as a freestanding suspended nanosheet by dissolving the filter paper backing in acetone, and the nanosheet was then rinsed thoroughly via pipettemediated handling in acetone and ethanol. Finally, the clean freestanding SWCNT nanosheet was transferred to a quartz cuvette filled with clean ethanol and the nonmagnetic end was trapped at the top of the cuvette using an air bubble (Fig. 2a).

Figure 2. (a) Schematic of the experiment showing the length and width of the nanosheet. (b) AFM image of the edge of a SWCNT nanosheet (5 µm scale). (c) AFM step height of the SWCNT film on wafer silicon for the projection indicated by the horizontal dashed

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line in panel (b). The nanosheet has a mean thickness of 33.4 nm with RMS roughness just under 4 nm. (d) A 50 nm thick SWCNT nanosheet dispersed in ethanol and magnetically held in an initial deformed configuration (left) and the same film after release and subsequent relaxation back to the equilibrium profile (right, 1 mm scale). (e) Skeletonized snapshots of relaxation for the experiment depicted in (d). (f) (top) Falsecolor plot of sheet curvature in the plane of contour length (vertical) and time (horizontal) for the SWCNT nanosheet depicted in (d). The time scale bar is 2 s and the contour length scale bar is 0.5 mm. (bottom) A similar curvature plot for a 950 nm thick PS sheet (0.5 s horizontal scale and 0.5 mm vertical scale). (g) Mean local curvature vs. time averaged over the region of maximum initial curvature for the two sheets depicted in (f), where the curves are single-exponential relaxations.

Although some magnetic material is removed during nanosheet rinsing, enough remains statically adhered to the nanosheet such that the ‘doped’ end of the film responds to a weak external magnetic field. Using a small external magnet, the film was carefully pulled back into a curved configuration, held in place for several minutes to allow hydrodynamic transients to decay, and then released by rapidly pulling the magnet away (Fig. 2a, supporting movies). The time dependent profile of the nanosheet as it relaxes back to its original shape was digitally recorded using a CCD camera equipped with a macro-zoom lens to generate a sequence of images depicting the relaxation of the ‘backbone’ (Fig. 2b). We view the suspended sheet in the plane of maximum deformation, or projected along the nanosheet width, w, such that the deformed sheet and its relaxation appear to be quasi-1D. For a given film, this process is typically repeated many times for many different contour lengths, L, defined here as arc length between the free and trapped

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ends of the sheet, where L > w and w is kept constant. As a point of reference, similar measurements were also performed on macroscopic polystyrene (PS) microsheets of the same size but with much greater thickness (0.5 µm < h < 2 µm). From digitized image sequences like those shown in Fig. 2a, we numerically compute the sheet curvature as a function of time (t) and contour length (s) through the definition = | / |, where r is the position vector locating an element along the projected backbone of the sheet and s is the apparent contour length. This analysis was carried out using a custom video-processing algorithm written in MatLab. Two examples are shown in Fig. 2c, where κ  is represented as a color plot in the plane of time and contour length. By taking a mean projection of the curvature as a function of time over the region of maximum initial curvature, we extract ̅ , from which the relaxation time for over-damped elastic recovery can be determined based on a simple exponential fit (Fig. 2d). Note that we require much thicker PS sheets to achieve comparable relaxation times because the modulus of the PS sheets is much less than that of the SWCNT nanosheets.

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Figure 3. (a) Relaxation time as a function of projected sheet contour length L for the two different materials and a range of sheet thicknesses. The inset shows mean relaxation time as a function of inverse thickness for both PS (closed markers) and SWCNT (open markers). All of the lines represent power-law fits with the indicated exponents. (b) Modulus vs. SWCNT film thickness using micron-scale thick PS films as a reference, where the curve is the percolation fit detailed in the text. The filled blue markers at lower thickness are based on wrinkling. The inset demonstrates film durability as a function of thickness, as detailed in the text.

Our first task is to experimentally query the dependence of recovery time, τ, on nanosheet thickness and length. We fully anticipate that τ will depend not only on L, h, w, and the viscosity η of the host fluid (and hence its purity), but also on higher-order factors such as the size of the bubble used to trap the nanosheet and the dimensions of the cuvette. For these reasons, such additional factors were kept as constant as possible (e.g., by keeping w fixed, keeping the bubble size the same and using the same cuvette) and the final ethanol bath used to suspend the nanosheets for measurement was kept as pure as possible. Figure 3a summarizes the length and thickness dependence. For the latter, we used an ensemble average over large L datasets for a few of the films. To vary L, we manually adjusted the point on the nanosheet where it contacted the air bubble. Our results support the empirical scaling laws ∝ ℎ (inset, Fig. 3a) and for sufficiently thin films, ∝ (Fig. 3a). For thicker films (e.g., 1 µm and 2 µm PS) the L dependence is weaker, with ∝ , which we suggest could be due to faster relaxation, stronger reactionary flows, and hence finite size effects associated with the dimensions of the sheet with respect to those of the cuvette.

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For the SWCNT-film thickness regime of practical interest here (h < 200 nm), the scaling is ∝ /ℎ . Because the relaxation is modeled as exponential, and because we observe no dependence of recovery time on the magnitude of the initial deformation, we consider a linear model of over-damped elastic recovery. For a sheet of modulus E, length L, width w and thickness ℎ ≪ subject to a deformation of characteristic size ℓ, the elastic energy density is ℎ /24, where = 2/ℓ is the local curvature and the coordinate z represents the local displacement from the equilibrium (flat) geometry. The Rayleigh dissipation function is ℓ ! /2, where B is a constant of order unity, η is the viscosity of the suspending fluid (ethanol), and ℓ is the characteristic length scale of the hydrodynamic response.39 Force balance then gives ℓ ! + ℎ /3ℓ = 0 ,

(1)

with the solution = exp−/ and the characteristic recovery time = 3 ℓ ℓ /ℎ . Our model is linear viscoelastic, with all the elasticity associated with the nanosheet and all the viscous effects arising from the fluid. It gives the correct h-3 dependence and it gives the correct L4 dependence with the assumption ℓ ≈ ℓ ≈ . The latter is a reasonable approximation, given that L is the only varying macroscopic length scale in the system. Under the assumptions outlined above, the nanosheet modulus is = 3 /ℎ , where the only unknown in this expression is the dimensionless constant B. To determine B, we used macroscopic PS microsheets as a point of reference because the modulus of thin PS films has been well characterized.40,41 We chose the thickness of the reference PS sheet such that the scaling with L and h has the desired form and the recovery time is comparable to the SWCNT nanosheets (e.g., compare the L dependence of τ for the 150

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nm SWCNT sheet with that of the 500 nm PS sheet in Fig. 3a). Because the modulus of PS is much smaller than that of the SWCNTs, much thicker PS films are required to achieve comparable relaxation rate. As a specific example, we focus on the ‘0.5 µm PS’ film in Fig. 3a (actual h = 465 nm) because it is thick enough to have a modulus characteristic of bulk PS40,41 but thin enough to have the right response (Fig. 3a). Using the wrinkling approach detailed in Ref. [31], we independently obtained EPS = 3.5 GPa for PS films of this thickness ex post facto, in agreement with previous modulus measurements for PS films.30,40,41 Using this value in combination with the ‘0.5 µm PS’ data in Fig. 3a gives B = 4.5. Figure 3b shows SWCNT nanosheet modulus vs. h after calibration, where the error bars represent two standard deviations over an L dataset of around 40. The light blue markers are based on the wrinkling approach detailed in Ref. [31] and the black curve is the percolation fit ∝ ℎ/ℎ* − 1 , with hc = 10.8 nm and β = 1.30,42 Although a better fit can be obtained by including higher-order corrections that introduce more free parameters,30 there is a slight but reproducible drop in E above 100 nm. Interestingly, there is remarkable agreement between the measurements and our simple prediction = /3, and to our knowledge, this is the first time that this type of ideal behavior has been reported for a macroscopic SWCNT nanosheet. There is also good agreement between these results – which represent a direct macroscopic measurement of nansoheet mechanical response – and those deduced from the microscopic wrinkling patterns exhibited by the same materials on elastomeric substrates. However, it is important to note that the modulus based on wrinkling is extrapolated as a fitting parameter in a strongly strain dependent response.30,31,43

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A natural question is why this particular system would exhibit such an ideal strain response, especially given the elastoplastic behavior that has been previously reported for compressed nanoscale SWCNT thin films. Our explanation for this is twofold. First, previous measurements of SWCNT thin-film modulus were based on wrinkling in response to compression, which can adversely impact a mesoporous fibrous material with strong vdW attraction and limited sliding friction. In contrast, the measurements reported here are for macroscopic bending. Second, the previous measurements were performed in air, but the system here is dispersed in ethanol. An understanding of the importance of this distinction is most easily accessed through a consideration of vdW interactions and Hamaker constants in the context of Lifshitz theory.21,44 In that description, the interaction potential - is proportional to ./ − .0 /./ + .0 , where ./ is the dielectric constant of the SWCNT nanosheet and .0 is that of the surrounding medium. Because the dielectric constant of the fluid (around 20 for ethanol) is much larger than that of air (1), the presence of the fluid greatly reduces the overall strength of the attractive potential by simultaneously decreasing ./ − .0 and increasing ./ + .0 . Presumably, this screening would be most pronounced for thinner films where more nanotube surface area is exposed to the fluid, which might explain why the modulus starts to drop for thicker films (h > 100 nm). We also tested the freestanding SWCNT nanosheets for mechanical durability, as shown in the inset of Fig. 3b. Three SWCNT nanosheets of varied thickness (around 30 nm, 50 nm and 75 nm) were first measured and analyzed to assess modulus (the ‘pre’ curve in the inset to Fig. 3b). Each individual nanosheet was then collected in a pipette tip from an ethanol bath and quickly expelled back into the bath, where we repeated this

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‘catch-and-release’ process 100 times for each film before measuring nanosheet modulus again (the ‘post’ curve in Fig. 3b inset). While the thinnest film (h = 30 nm) showed a loss of rigidity as a result of excessive handling, the two thicker films showed good durability (inset, Fig. 3b). Previous studies that relied on wrinkling measurements to query SWCNT film mechanics suggest significant plasticity, with a yield strain that can either increase or decrease with increasing film thickness, depending on sample history.30,31 However, the most common picture that we have espoused is one in which the thicker films yielded at lower strains, an effect that we have attributed to stronger strain induced bundling under compression at higher SWCNT volume fractions.30 The results reported here depart from this picture, with the 50 nm and 75 nm nanosheets showing limited or no plasticity. Again, we attribute this difference to both the new deformation scheme used here (macroscopic bending vs. compression induced microscopic wrinkling) and the screening effect of the solvent, where the latter suppresses strain induced bundling and agglomeration as a mechanism of yielding. In the case of SWCNTs, the thin-film wrinkling approach is complicated by surface roughness (Fig. 2c),45 a feature that reflects the disordered mesoporous structure of the SWCNT nanosheets. However, the approach we use here avoids this complexity because the characteristic length scale of the deformation is macroscopic instead of microscopic.

CONCLUSIONS In conclusion, we have characterized the macroscopic viscoelastic recovery of freestanding multifunctional SWCNT nanosheets exhibiting both exceptionally high elastic modulus and good durability. Thin freestanding SWCNT films 20 nm to 200 nm in thickness have been deformed repeatedly in ethanol using an external magnetic field.

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From the temporal relaxation of nanosheet curvature back to its equilibrium profile, we have extracted an elastic recovery time associated with the curvature of the suspended nanosheets, and from the dependence of this relaxation time on nanosheet thickness and length, we have formulated a simple viscoelastic model for elastic nanosheet recovery. Using freestanding polystyrene microsheets as a point of reference, we then extracted a direct, macroscopic measure of SWCNT nanosheet modulus as a function of thickness and processing history. Our results are consistent with nearly ideal behavior, with nanosheet moduli approaching 200 GPa and limited plasticity for a range of useful film thickness. The ideal response of the SWCNT nanosheets is attributed to the screening of vdW interactions by the surrounding solvent and the macroscopic nature of the bending deformation. Our results suggest that the full mechanical potential of thin SWCNT films can be realized if a supporting medium is used to ‘screen’ the strength of SWCNT-SWCNT vdW interactions, and we expect these results to have important implications for the fabrication and processing of freestanding multifunctional SWCNT nanosheets for a variety of potential applications. MATERIALS AND METHODS CoMoCat SG65i SWCNTs were dispersed at 1 mg SWCNT/mL in a 2 % aqueous solution of sodium deoxycholate (DOC) through tip sonication (Thomas Scientific, 0.64 cm tip, 1 W/mL, 1 h, 0 °C) and the suspension was centrifuged for 2 h at 21000g to remove impurities and large bundles. The resulting supernatant contained primarily individualized SWCNTs with a small number of few-SWCNT bundles. SWCNT length distribution was measured with AFM (Agilent Technologies Model 5500), where details

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are given in a previous publication.31 The distribution is approximately log normal with a mean SWCNT length of 670 nm. Using a diameter of 0.75 nm, the aspect ratio is 103, which corresponds to a percolation threshold near 10 nm.31 SWCNT films were prepared on cellulose-ester filter paper through vacuum filtration,31 although other approaches could be equally used.46 Freestanding SWCNT films were produced by dissolving the paper support in an acetone bath, followed by three successive baths in clean acetone and three successive baths in clean ethanol to facilitate removal of residual paper and surfactant.31 The freestanding SWCNT nanosheets were transported and manipulated in ethanol using a pipette.31 Details related to the doping and characterization measurements in Fig. 1 can be found in the Supporting Information. The PS (Sigma-Aldrich, Mw = 192,000) was suspended in toluene stained with a small amount of soluble black die extracted from a Sharpie pen and the PS films were spin-coated onto mica. Black pigment was added to enhance video contrast for the PS microsheets and facilitate imaging of the microsheet backbone and we independently confirmed through wrinkling measurements that the modulus of the PS was unaffected. Details related to the wrinkling approach can be found elsewhere.30,31,32 The PS films on mica were scored into sheets and then floated onto the surface of a DI water bath, where they were recovered and transferred to an ethanol bath. The data in Fig. 3b represent two sets of separate experiments, one with a mean bubble width of 5.5 mm (B = 4.5) and one with a mean bubble width of 3.5 mm (B = 0.97). The experiments were performed in a standard quartz cuvette (12.5 mm × 12.5 mm × 45 mm). For magnetic manipulation, we relied on amino superparamagnetic microparticles obtained from Polysciences, which are iron-containing colloids with a polystyrene shell

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and a surfactant surface (2.5 % solids by weight/volume in water). Prior to use, we rinsed the colloids through repeated suspension and centrifugation in methanol until the supernatant was clear. The final methanol suspension used for labeling had a colloid concentration of 0.75 % by mass, with a mean individual particle size of 1.2 µm based on optical microscopy. The cleaned methanol suspension was bath sonicated prior to use, with a small volume (~ 5 µl) deposited on one end of a sheet from a micropipette. For the PS, the same approach was applied prior to floating the film from its mica support onto DI water. Thickness was measured ex situ at the end of each experiment using both AFM and optical absorption spectroscopy for the SWCNT films (on either wafer silicon or a glass cover slip) and AFM for the PS films (deposited on wafer silicon).31 The sheets were deformed using a stack of neodymium magnets, which for these films is akin to ‘cocking’ a spring in a viscous medium. The deformed film was held in place for roughly a minute by slowly and slightly pulling back the magnet before suddenly (an interval of roughly 0.25 s) releasing the film. The relaxation of the film was recorded at 17 frames per second using a CCD camera equipped with a macro-zoom lens and digitized to extract local curvature as a function of time as well as the contour length L of the film. Curvature in the plane of time and contour length was computed in Matlab and the mean time dependent curvature was computed with ImageJ. We used η = 1.095 cP for the viscosity of ethanol. The sheets were always deformed such that recovery opposed gravity to limit any gravitational contribution to the relaxation time. Particularly for larger L and h, the finite weight of the sheet/microbead system can induce curvature in the equilibrium sheet profile. However, because we model the sheets as an

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overdamped harmonic oscillator, the effect of gravity will be to simply alter the equilibrium shape of the sheet.

SUPPORTING INFORMATION AVAILABLE The Supporting Information contains videos of nanosheet relaxation, additional details related to materials and fabrication, and additional information related to the measurements presented in Figure 1. This information is available free of charge via the Internet at http://pubs.acs.org. †

These two authors contributed equally to this work

‡

Appareo Systems, Fargo, North Dakota 58108

* Corresponding author: [email protected]

REFERENCES 1. Nasibulin, A. G.; Kaskela, A.; Mustonen, K.; Anisimov, A. S.; Ruiz, V.; Kivisto, S.; Rackauskas, S.; Timmermans, M. Y.; Pudas, M.; Aitchison, B. et al. Multifunctional Free-Standing Single-Walled Carbon Nanotube Films. ACS Nano 2011, 5, 3214-3221. 2. Li, X. K.; Gittleson, F.; Carmo, M.; Sekol, R. C.; Taylor, A. D. Scalable Fabrication of Multifunctional Freestanding Carbon Nanotube/Polymer Composite Thin Films for Energy Conversion. ACS Nano 2012, 6, 1347-1356. 3. Zhao, S.; Gao, Y.; Li, J.; Zhang, G.; Zhi, C.; Deng, L.; Sun, R.; Wong, C.-P. Layer-byLayer Assembly of Multifunctional Porous N-Doped Carbon Nanotube Hybrid Architectures for Flexible Conductors and Beyond. ACS Appl. Mater. Interfaces 2015, 7, 6716-6723.

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Macroscopic Freestanding Nanosheets with Exceptionally High

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