Highly Conductive Single-Walled Carbon Nanotube Thin Film

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Highly Conductive Single-Walled Carbon Nanotube Thin Films Preparation by Direct Alignment on Substrates from Water Dispersions Lisa D. Pfefferle Langmuir, Just Accepted Manuscript • Publication Date (Web): 29 Dec 2014 Downloaded from http://pubs.acs.org on January 5, 2015

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Highly Conductive Single-Walled Carbon Nanotube Thin Films Preparation by Direct Alignment on Substrates from Water Dispersions

Journal: Manuscript ID: Manuscript Type: Date Submitted by the Author: Complete List of Authors:

Langmuir la-2014-03919u.R1 Article 23-Dec-2014 Azoz, Seyla; Yale University, Chemical Engineering Exarhos, Annemarie; University of Pennsylvania, Physics and Astronomy Marquez, Analisse; Yale University, Chemical & Environmental Engineering Gilbertson, Leanne; Yale University, Chemical & Environmental Engineering Nejati, Siamak; Yale University, Chemical & Environmental Engineering Cha, Judy; Yale University, Mechanical Engineering and Materials Science Zimmerman, Julie; Yale University, ; Yale University, Kikkawa, James; University of Pennsylvania, Physics & Astronomy Pfefferle, Lisa; Yale University, Department of Chemical Engineering

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Highly Conductive Single-Walled Carbon Nanotube Thin Films Preparation by Direct Alignment on Substrates from Water Dispersions Seyla Azoz1, Annemarie L. Exarhos2,†, Analisse Marquez1,†, Leanne M. Gilbertson1, Siamak Nejati1, Judy J. Cha‡3, Julie B. Zimmerman1, James M. Kikkawa2, Lisa D. Pfefferle1,* 1

Department of Chemical and Environmental Engineering, Yale University, New Haven,

Connecticut 06511, USA 2

Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania

19104, USA 3

Department of Mechanical Engineering and Materials Science, Yale University, New Haven,

Connecticut 06511, USA † These authors contributed equally. *Correspondence to: Lisa Pfefferle [email protected]

KEYWORDS Single Walled Carbon Nanotubes; SWNT; highly aligned; high conductivity; highly dispersed; Mayer rod rolling. 1 ACS Paragon Plus Environment

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ABSTRACT A safe, scalable method for producing highly conductive aligned films of single-walled carbon nanotubes (SWNTs) from water suspensions is presented. While microfluidic assembly of SWNTs has received significant interest, achieving desirable SWNT dispersion and morphology in fluids without an insulating surfactant or toxic superacid is challenging. Different from previous fabrication techniques, we present a method, uniquely producing a non-corrosive ink that can be directly applied to a device in situ. Functionalized SWNTs (f-SWNTs) are dispersed in an aqueous urea solution to leverage binding between the amine group of urea and the carboxylic acid group of f-SWNTs and obtain urea-SWNT. Compared with SWNTs dispersed using conventional methods (e.g. superacid and surfactants), the dispersed urea-SWNT aggregates have a higher aspect ratio with rod-like morphology as measured by light scattering. The Mayer Rod technique is used to prepare urea-SWNT, highly aligned films (2D nematic order parameter of 0.6, 5 micron spotsize, via Polarized Raman) with resistance values as low as 15-1700 ohms/sq in a transmittance range of 2-80% at 550 nm. These values compete with the best literature values to date for conductivity of SWNT-enabled thin films. The findings offer promising opportunities for industrial applications relying on highly conductive thin SWNT films.

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Single-Walled Carbon Nanotubes (SWNTs) show remarkable electronic, mechanical and thermal properties and have been the subject of significant attention owing to their unique characteristics. Because of their exceptional electronic properties, SWNTs are being researched extensively for innovative applications including transparent electrodes, electrodes in organic electronic devices, transistors, supercapacitors, solar energy applications and batteries.1, 2, 3, 4, 5, 6 Facile methods to orient and align SWNTs are critical to the performance of these SWNT-enabled technologies and are necessary for the advancement of these large-scale applications. Recent research has shown that SWNT alignment can reduce the resistivity as compared to a randomly oriented film7, 8 because it decreases the tube-tube junctions that cause high resistance. 9, 10 Therefore, the resistance values obtained from SWNT films are expected to decrease within ordered SWNT samples. Many studies have investigated the efficacy of aligning carbon nanotubes (CNTs) 2,11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21

utilizing methods such as: chemical vapor deposition (CVD), 13, 14 vacuum

filtration,22, 23 embedding CNTs in liquid crystals,15 spin-coating,2 surfactant-aided alignment,16 magnetic field alignment,17 electric field alignment,18 growth on patterned surfaces,19 concentration-dependent shear alignment,20 and microfluidic alignment techniques using superacids.21 However, most of these methods are too difficult to scale up for large-scale fabrication (spin-coating),24 costly to transfer to suitable substrates in industrial applications (vacuum filtration),24 require specialized equipment (magnetic alignment)25, highly dope the SWNTs (superacid)26, are hard to handle due to very high reactivity (superacid)27 and/or require additional post processing steps to remove the insulating dispersing agents to obtain conductive films(most surfactants)28, 29. In addition, methods that use corrosive solutions or require harsh post-treatment can not be used to directly print a SWNT film onto a device. Therefore, the


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development of an efficient, cost effective and easily scalable method to create highly conductive SWNT networks that are directly printable on a device is of crucial importance. Excellent dispersion is required for all solution and microfluidic techniques used to align SWNTs. Previous studies use surfactants to improve nanotube dispersibility in aqueous solvents. Although dispersion in a surfactant solution has been shown to preserve the intrinsic properties of the nanotubes,30 the resulting films often suffer from an insulating layer of surfactant sorbed to the tubes, which decreases their conductivity. In order to remove the insulating surfactants, a concentrated acid soaking method is commonly used, 31, 32 which does not allow a direct assembly of the film on a desired device, instead requiring additional post-process treatment steps. Research on alternative dispersion methods has shown that superacids, such as fluorosulfuric acid and chlorosulfonic acid, can be used to replace the surfactants and obtain dense networks of SWNTs in films. 26, 33, 34 However, the electronic properties of SWNTs, which are critical to the performance of the film, can be significantly changed by the use of superacids resulting in high level of p-doping.26 Also, the industrial use of superacids is costly, raises safety and environmental concerns27, and similarly precludes in situ deposition on a device. In this study, results from a newly developed method to overcome these limitations are presented. This novel method utilizes the amine group of urea to disperse functionalized SWNTs (f-SWNTs) in water. Fluidic alignment was used to create highly ordered SWNT conductive thin films with results, in terms of dispersion and electrical conductivity, comparable to previously described superacid and surfactant aligned SWNTs. Unlike surfactants, the urea does not electrically insulate SWNTs while still allowing for enhanced dispersion and alignment in the prepared films. This enables the direct deposition of the film on any device in situ, a significant advantage over films made by surfactant and superacid aided dispersions of SWNTs.


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Furthermore, static and dynamic light scattering experiments indicate that dispersed urea-SWNT aggregates are more polydisperse with a rod-like morphology (fractal dimension of unity) compared with more monodisperse plate-like aggregates of acid or surfactant treated SWNTs. 35, 36, 37

To obtain an aligned SWNT film, the dispersed urea-SWNTs were rolled onto a substrate using the Mayer Rod technique.38 Reproducible films were produced by controlling the solution volume and rolling speed. The coating fluid surface tension was optimized to be low enough to facilitate spreading on a wide range of substrates without defects, yet high enough to prevent colloidal agglomeration after deposition, as also noted by Bao and coworkers.39 This method provides a robust technique to fabricate SWNT films with high conductivity and low sheet resistances in a manner that is scalable, reproducible, efficient and environmentally friendly. Our study shows combination of high conductivity and good transparency in thin films. The sheet resistance values reported here range from 15 to 1700 ohms/sq over a wide range of transmittance values (up to 80%), which is competitive with the best values reported in the literature to date. 10, 24, 26, 31, 32, 34, 38, 39, 40, 41, 42, 43, 44

Results and Discussion Nitrogen Content after Urea Functionalization: The urea functionalization of SWNTs was characterized (Figure 1) using Electron Energy Loss Spectroscopy (EELS), Energy-dispersive X-ray Spectroscopy (EDX), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Figure 1a shows the proposed reaction schematic for the binding of urea via carboxylic acid groups on functionalized SWNTs. The high angular dark field STEM image in Figure 1b shows uniform distribution of


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carbon, oxygen and nitrogen in the urea-SWNTs film and indicates the presence of all three elements. EELS and EDX were used to determine the reaction efficiency by studying f-SWNT solutions before and after the addition of urea. Figure 1c shows the EELS comparison with C, N and O k edges plotted after background subtraction and normalization to the carbon k edge. The inset spectra show increased nitrogen presence when the f-SWNTs are treated with urea in comparison to untreated f-SWNTs. Likewise, the nitrogen peak appears in EDX for urea-SWNTs but not for untreated f-SWNTs (Figure 1d). FTIR is used to further investigate the urea bonding. FTIR spectra indicate covalent binding of nitrogen, from urea, to carboxylic groups of the fSWNTs (Figure S1). The band at ~1460cm-1 assigned to the asymmetric CN stretches in urea is absent in the urea-SWNT spectrum.45 Looking at the NH stretch it is clearly evident that the intensity of asymmetric NH2 vibration at ~3430 cm-1 is reduced, which can be attributed to the reduction of primary amine concentration of urea after attachment to the SWNT. The broadening of the NH peaks in this region is indicative of hydrogen bonding or possible polymerization of the isocyanate group.46, 47 Also, the clear downshift of the peaks in between 1500-1800 cm-1, particularly for the C=O vibration of the carboxylic group (~1760 cm-1), further supports our proposed change in the chemical environment of the SWNT surface.48 XPS was utilized to quantify the amount of C, O and N present in the urea-SWNT sample (Figure S2). Deconvolution of the C1s, O1s and N1s peaks enabled identification of the presence of certain O and N functional groups as well as N-O, N-C and O-C-N bonding. The nitrogen content of the urea-SWNT sample is ~5% based on integration of the peak area in the survey scan. Figure S2b shows peaks for different carbon moieties typically observed in the carbon nanotube XPS spectrum,49 namely C-C (sp2) (284.1-284.4 eV), C-C/C-H (285.0-285.3 eV), C-O (286.0-286.5 eV), C-C=O (287.1-287.5eV) and O-C=O (288.8-289.2 eV). However, the most


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interesting peak in our studies, which is labeled (A) in Figure S2b, can be also assigned to amide (N-(CO)-C) and diamide (N-(CO)-N), which typically can be found in between 288-289 eV.50 To further elucidate the change in the C1s environment after urea treatment, the C1s spectra before and after the treatment are compared in Figure S3. There is a clear increase in the peak intensity around ~288.8 eV (indicated by * in Figure S3), which can be explained by addition of amide and diamide species due to covalent attachment of the urea to the f-SWNT. This change is further accompanied by the relative increase in the C=O peak at ~532 eV 50 in the O1s signal (Figure S4). The change in the carbon environment due to the covalent attachment of urea to the f-SWNT translated to ~3.5% added nitrogen to the SWNT which is slightly lower than the 5% total nitrogen concentration estimated from the N1s. This mismatch in the nitrogen concentration suggests that nitrogen is present in different forms. By looking at the N1s (Figure S2c), we observe four different environments for the nitrogen in the urea-SWNT. The peak located at the 399.4eV can be attributed to the amine group (-NH2) on the SWNTs.51 The appearance of clear shoulder at ~402 eV can be assigned to the nitrogen in the framework of the SWNT, the peak located at ~403 eV can be assigned to pyridine-N-oxide species, and lastly the peak at ~406 eV can be assigned to nitrogen oxide environment.52, 53 SEM was used to characterize the relative alignment of films prepared with differentially treated SWNTs as shown in Figure 2. These images show the difference between aligned films (Figures 2a and b) and unaligned films (Figures 2c-d). Significant alignment of carbon nanotubes is observed for the urea-SWNTs (Figures 2a-b). On the other hand, significant bundling and disorder is observed in films prepared with f-SWNTs dispersed in water alone (Figure 3c) and pristine SWNTs dispersed in urea-water solution (Figure 2d). Large regions of unreacted urea wrapping and coating the SWNTs are noticeable in Figure 2d indicating that urea cannot bind


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with pristine SWNTs. The compiled SEM results elucidate the necessity of a) oxygen functionalization on the surface of SWNTs and b) the presence of urea as a dispersing agent to obtain well aligned films. Analysis of SWNT Aggregation State Using Light Scattering: The extent of dispersion is critical for the use of SWNTs in numerous applications, including conductive thin films as discussed in this study. While surfactants and superacids have been shown to successfully disperse SWNTs, they introduce certain challenges for applications to conductive thin films, including decreased conductivity due to an insulating layer (surfactants) and significant inherent hazard (superacid). Urea, as shown here, is a promising and effective alternative. For comparison, we additionally studied SWNTs dispersed with chlorosulfonic acid as a model for superacid-SWNTs, as well as SDS dispersed SWNTs as a model for surfactantbased SWNT dispersion (SDS-SWNT). The concentration of the SWNTs across the superacid-, SDS-, and urea-SWNTs are diluted down to 17 µg/mL for light scattering experiments. Combined light scattering techniques were used in this study to quantify differences in the aggregation state of urea-SWNTs compared to SWNTs treated using the conventional methods. Specifically, dynamic light scattering (DLS) was used to evaluate the relative dispersed aggregate size and polydispersity via probability distributions of aggregate diffusion time while static light scattering (SLS) was used to evaluate the dispersed aggregate morphology. DLS measures the scattered light intensity of the dispersed sample at a fixed angle (90°) while SLS measures the scattered light intensity at multiple angles simultaneously taking advantage of the relationship between intensity and scattering angle (Equation 2 and 3) to characterize aggregate morphology.35, 37, 54, 55


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Since SWNTs are inherently non-spherical and to avoid assuming aggregate homogeneity, the raw decay functions of the scattered light intensity, from DLS, are analyzed using the CONTIN algorithm.56 These autocorrelations, g(∆t), were fit to a Laplace transform:

Δ = /

(1)

to obtain a probability distribution, , for the diffusion time, τ (Figure S5). The value of τ at the peak of indicates the most probable aggregate size; smaller values of τ are associated with smaller aggregates and larger values associated with larger aggregates. In addition, the width of the indicates the aggregate polydispersity; narrow distributions represent a more monodisperse sample and broader distributions a more polydisperse sample. The peak location is not significantly different for any of the SWNT samples. Yet, the superacid treated SWNT aggregates are significantly more monodisperse than SWNTs treated using other dispersing agents, with the urea-SWNT dispersion containing the most polydisperse aggregates (Figure S5). This indicates that the superacid-SWNT aggregates are more homogenous in size while ureaSWNT aggregates are the most heterogeneous in size. SLS enables simultaneous measurement of the scattered light intensity over multiple angles, covering a range of 17-153°. Here, measurements were obtained every 1° using 8 detectors and a 20 s collection time over the range of 0.00516 < q < 0.03397 nm-1. Df is determined from the slope of the best fit line to the log-log plot of the scattered light intensity verses the scattering angle, shown in Figure 3.35, 37, 55 Several studies have used light scattering methods to evaluate the dispersed aggregate morphology of carbon nanotubes.35, 37, 54, 55 Results from those studies identified Df values ranging between 1, less compact rod-like morphology, and 3, more compact sphere-like morphology. While all three treated SWNTs studied here have


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Df values between 1 and 2, indicating aggregate morphologies between rod-like and plate-like, the urea-SWNT aggregates exhibit the lowest average Df (1.035 ± 0.008) compared to SDSSWNT and superacid-SWNT with Df of 1.354 ± 0.004 and 1.632 ± 0.006, respectively. Combined, the DLS and SLS data indicate that the dispersed urea-SWNT aggregates are more polydisperse and less compact than the aggregates prepared using SDS or superacid. In addition to light scattering data, the concentration of each sample suspended in the supernatant was evaluated to determine the relative extent of dispersion. All SWNT solutions were prepared using the same initial SWNT mass concentration and sonication technique to disperse in water. The solutions were then centrifuged and the supernatant used for SWNT film preparation. The mass of SWNTs in the supernatant was estimated by lyophilizing and weighing the residual mass. A plot of the relative supernatant concentration for differentially treated SWNTs is shown in Figure S6. Due to the inherent hazard of handling superacid, a supernatant concentration value is not reported for this sample. The urea-SWNT dispersion resulted in greater concentration of SWNTs in the supernatant compared to the SDS-SWNT based on the crude mass balance calculations. This indicates that urea is more effective at dispersing and stabilizing the f-SWNTs than SDS. These combined results, heterogeneous aggregate size, rodlike aggregate morphology, and greater SWNT mass in the supernatant, provide supporting evidence that explains the superior SWNT alignment and electronic performance achieved for urea-SWNTs. Analysis of Tube Alignment using Polarized Raman Spectroscopy: Polarized Raman spectroscopy serves as a reliable method for characterizing and quantifying SWNT alignment within our films. Figure 4a shows angle-dependent polarized Raman scattering of the SWNT G-band peak at ~1580 cm-1, which is known to be highly


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polarized along the nanotube axis.57, 58, 59, 60 Orientational alignment can be quantified by computing a 2D nematic order parameter, S. For a sample of 0.03% transparency at 550 nm and 5 µm spotsize, the G band intensity of the urea-SWNT films shows strong changes with rotation angle (Figure 4a). For this sample, three sets of spot sizes were measured at five different locations within the sample (covering 1 cm2 area) and the nematic order parameter, S, for each spot size was calculated. For each spot size, the phase of all the scans has been shifted and the data was averaged to determine Savg. Figure 4b shows the relationship of integrated G-band intensities and sample rotation for a sample of 0.03% transparency at 550 nm for various spot sizes. As observed in Figure 4b, alignment is higher over smaller areas. For a 65 µm spot size, the average order parameter Savg was 0.37. For smaller spot sizes of 5 µm and 2 µm, the average order parameter Savg was approximately 0.6. As the spot sizes get smaller, the alignment became more pronounced as would be expected for alignment whose direction changes over distance. S = 1 represents a perfectly aligned sample, however, because the SWNT are flexible there is already intrinsic orientational disorder within each SWNT, limiting the ordering value to be less than unity. Analysis of Thin Film Conductivity using Four Probe Measurements: The conductivity of films prepared with different transmittances obtained by changing the film thickness is measured and reported. The transmittance of the films was calculated by using their absorption values at λ = 550 nm and the optoelectronic properties of the SWNT films are assessed and shown in Figure 5. Our films showed comparable results with previously reported conductive SWNT thin films made using variety of techniques, including (i) surfactantbased SWNT films obtained by Mayer rod rolling,32 line patterning on PET, 31 Mayer rod coating


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and doping by fuming sulfuric acid,38 spray coating and nitric acid treatment,42 filtration method and multistep purification process,44 vacuum filtration of surfactant based solutions,40, 41vacuum filtration of SWNTs with tuned semiconducting to metallic tube ratios10, 40(ii) superacid-based SWNT films obtained by glass sliding,34 films on silica slide, with ether treatment to remove superacid,

24

filtration/transfer method

26

and (iii) polymer-SWNT films obtained by loss-yield

stress gels made by various film applicators,39 all of which require surfactants or superacids coupled with film making techniques. For sheet resistance measurements and comparison of results with earlier studies, it is important to note that the SWNTs used in various studies (Figure 5) contain a mixture of chirality, diameters, metallic and semiconducting tubes, which can differentially influence the final conductivity of the film. Although all of the results from these different groups utilizing different techniques follow a comparable trend, the starting SWNT material is different and this can make an important difference.10, 32, 43 Conclusion In this study, we demonstrate a facile and more benign method to produce highly conductive single-walled carbon nanotube thin films prepared directly on substrates from SWNT dispersion in water without the use of surfactants or superacids. This is achieved through a novel, cost effective and reproducible method to align SWNTs from water dispersion utilizing urea and the Mayer Rod technique for highly conductive thin film preparation. The key advantages of this method are that: i) enhanced dispersion and alignment collectively yields thin films with high conductivities comparable to or exceeding those currently published; ii) noncorrosive inks yield SWNT sheet resistances on par with those reported for superacid and surfactant methods, while also avoiding hazardous acids and allowing direct device application 12 ACS Paragon Plus Environment

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as the surfactant washing step is not needed; iii) the procedure is cost effective, facile and environmentally friendly. Therefore, our method to create highly conductive aligned SWNT networks that are directly printable on a device opens up new research interests and is of industrial importance. Dynamic and static light scattering measurements indicate that surface functionalization with carboxylic acid and urea effectively disperses SWNTs producing aggregates with rod-like morphology. Polarized Raman spectroscopy results confirm the enhanced ordering within the urea-SWNT films. Materials and Methods Functionalization of SWNTs with Urea Acid functionalized SWNTs (f-SWNTs) were purchased from Carbon Solutions, Inc. (Riverside, CA, batch number P3-SWNT, >90% purity, 1±0.5 µm length, nitric acid purified SWNTs containing 1.0-3.0 atomic% carboxylic acid groups as provided from manufacturer). Absorption bands are consistent with a mix of metallic and semiconducting nanotubes with an average diameter of 1.5 nm (Figure S7). We used f-SWNTs rather than pristine SWNTs in order to provide reaction sites to bind the amine groups in urea. We have shown that SWNT can be oxygen functionalized without acids using ozonation. While Ford, et. al.,46 used a melted urea to dissolve SWNTs in water, this method poses challenges of washing away the excess urea. As such, a method of ultrasonicating small amounts of urea with SWNTs was developed and used in this work. Purified f-SWNTs (20 mg) were added to 20 mL water-urea solution (0.1 wt %). (Water-urea ratio was optimized by the SEM images of the urea-SWNT films prepared with different urea concentrations and a comprehensive study is given in Supporting Information Figure S8.) The prepared solution was then tip sonicated (Misonix, set to a power of 52 W, ice bath was used to control temperature) for 20 minutes to allow sufficient time for the urea to 13 ACS Paragon Plus Environment

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react. Prior to aligning the SWNTs, the resulting urea-SWNT solutions were centrifuged at 12,000 rpm for 1 hour to obtain a homogeneous supernatant solution, which was used to fabricate the films. Phenylenediamine was also used to treat the SWNTS, in order to exploit the possibility of SWNT alignment with other water-soluble amine-containing molecules. The same procedure of tip sonicating f-SWNT in phenylenediamine-water solution (0.1%wt) was used. The phenylenediamine-SWNT films were characterized using Polarized Raman and SEM and the results are discussed in Supporting Information Figure S9. Mayer Rod Rolling Technique Using a Mayer Rod (R. D. Specialties, Lab Rod, 1/4'', #3) the SWNT-water-urea solution was rolled onto a glass substrate. The desired thickness (and thus transparency) of the SWNT film was controlled by the amount of solution poured onto the substrate and the thickness of the Teflon tape wrapped around the Mayer rod. The rolling procedure was performed in an oven set at 85 °C and the as-prepared films were allowed to dry in the oven overnight. Once dry, the films can further be peeled off to obtain free-standing SWNT films, if desired, or just left in a device format. Scanning Transmission Electron Microscopy (STEM) An FEI Tecnai Osiris operating at 200 kV, equipped with a quadrant EDX detector for highcount chemical mapping and electron energy-loss spectrometer for analytical work was used to collect STEM images. The SWNT solution in water was drop cast onto a lacey carbon TEM grid and allowed to dry in ambient air. Fourier Transform Infrared Spectroscopy (FTIR)


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Thermo Nicolet 6700 FTIR with SmartOrbit Diamond ATR accessory running Omnic version 7 was used for FTIR measurements. The solutions of SWNTs and urea in water were drop cast on the diamond accessory and dried using a heat gun. X-Ray Photoelectron Spectroscopy (XPS) Measurement and Peak Fitting XPS was performed on SWNT films using a Physical Electronics PHI 5000 VersaProbe with a scanning monochromatic source from an Al anode with dual beam charge neutralization. Survey XPS spectra were acquired at 100 W with pass energy of 117 eV over the range of 0-1100 eV with 0.5 eV resolution and 50 ms dwell time averaged over 3 scans. High resolution XPS spectra of C1s, N1s and O1s core electrons were acquired over an approximately 200 µm spot with 50 W beam power and averaged 15 times with 0.1 eV resolutions at 11.75 pass energy with 200 ms dwell time. The spectra were shifted to correct for charging. The carbon sp2 peaks were fitted by applying constraints over its full width at half maximum (FWHM, 0.8 eV) and for the rest of the carbon species peak FWHM were limited in between to 0.8-1.5 eV. An iterated Shirley background 61 was used to subtract the background from the spectrum and the concentration of the elements were calculated using the peak intensity and sensitivity factors previously calibrated for the 1s orbitals of the elements of interest at the 45 degree take-off-angle. N1s and O1s spectra were also fitted by subtracting the background and applying a 1.5 eV limit for the FWHM of their respective peaks. Sheet Resistance Measurements Sheet resistance measurements are performed using a Signatone four-point probe with 10 mm radius contacts and 40 mm spacing. A Keithley Model 2400 SourceMeter is used to apply current (V) and measure the resulting potential (I) drop. Voltage drop is measured with applied currents and converted to a sheet resistance using [π/ln 2] × V/I ≅ 4.53 × V/I. An average of 7 15 ACS Paragon Plus Environment

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measurements are conducted for each film, and the results are averaged. Sheet resistance measurements for samples with transmissivities of 0.03%, 2.4%, 35%, and 85% at 550 nm were performed at the University of Pennsylvania using a 4-probe configuration with 10 µm Au-plated micromanipulator probes at ~1 mm spacing and a Keithley 237 Source-Measure Unit. Data was converted to a sheet resistance in the same manner. Data was taken at different positions and orientations over a roughly 1 cm2 area in the center of each rolled film in order to avoid edge effects. Sheet resistance measurements were taken 30 times for each sample (except for the 0.03% transmittance sample, where 140 measurements were taken) in order to determine an average sheet resistance for each sample. An additional sheet resistance measurement was taken on a sample with 85% transmittance in a 4-probe configuration using lines of silver paste as contacts rather than probe tips. In this measurement, the entire 1.5 cm x 1.2 cm sample was spanned (spacing between contacts was ~4 mm). Light Scattering Data Collection and Analysis The stock sample SWNT solutions prepared as described above were diluted to 17 µg/mL for light scattering measurements (100 µL stock in 6 mL DI water). The superacid solution was prepared separately with 6 mL chlorosulfonic superacid without water dilution due to its reactivity with water. All samples, except pristine SWNTs, remained stable against aggregation for the extent of data collection. Since pristine SWNTs are not stable in solution, light scattering data is not reported for this sample. Both dynamic (DLS) and static light scattering (SLS) data was collected with a multi-detector light scattering unit (ALV-GmbH, Germany) with a Nd:vanadate laser (Verdi V2, Coherent, Inc., Santa Clara, CA) operating at 532 nm wavelength.


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Static Light Scattering: The structural morphology of the differentially treated SWNTs was determined using SLS. The relationship between the scattering intensity, I(q), and the scattering vector, q, enables quantification of the fractal dimension, Df, as follows: ~

(2)

where =

#

! "

(3)

with $ being the solvent index of refraction, % the wavelength of incident light, and & the scattering angle.35, 36 For rod-like materials, this relationship is valid when 2π/L < q < 2π/D (L and D are the rod length and diameter, respectively) and has thus been used to study the morphology rod-like SWNT dispersions.35, 37, 54 Multiple iterations were performed correcting the photodetector sensitivity each time to optimize the linear fit of the data. Data was collected in duplicate for each sample. The duplicate data was plotted together and the fractal dimension, Df, obtained from the best-fit line on the log-log plot of I/Io vs q (Figure 4).35, 36, 37, 55 The slope and standard deviation values were determined using linear regression. Dynamic Light Scattering: The scattered light from the dispersed SWNT samples was collected at a fixed angle (90°) for 200 10-second measurements. The raw correlation functions were exported and analyzed using the CONTIN algorithm to obtain a probability distribution of diffusion time (Figure S5).56 The relative location and shape of the probability distribution represents the aggregate size and aggregate polydispersity, respectively.55 Raman Scattering Normally incident Raman scattering data was collected using a home-built, polarized, confocal Raman microscope at 532 nm excitation wavelength. Excitation and emission polarizations were 17 ACS Paragon Plus Environment

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held parallel in a VV geometry while the sample was rotated relative to the V axis. Care was taken to reduce orbiting effects by aligning the rotation axis of the stage with the center of the collection field of view. The VV response of a single nanotube is given by & = cos & , where & is the angle between the nanotube axis and the polarization axis for Raman scattering measurements.62 Data are modeled as a stretched 2D distribution obtained by starting with a random distribution of orientations in the plane and supposing that one axis is stretched by a factor a.

The ensemble averaged response 〈&〉 corresponding to a is then obtained by

averaging the single nanotube response over this distribution, and finally a is varied to fit the observed data. A 2D nematic order parameter , = 〈2cos &. − 1〉 is then computed. 63


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FIGURES 1a)

1b)

1c)

1d)

Figure 1. a) Schematic showing proposed urea functionalization via N-C bonding with the oxygen functional groups. b) High angle annular dark field -STEM image showing the distributions of C, O and N. Corresponding EELS (c) and EDX (d) data for functionalized and urea-SWNT samples. The EELS inset is multiplied by 10 and compares the two curves after background subtraction was done on the N-K edge. EDX data is normalized to the O k edge. The spectra are displayed with no offset.


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Figure 2. SEM images of a) and b) urea-SWNTs prepared by sonicating f-SWNTs in the ureawater solution, c) f-SWNTs dispersed in water in the absence of urea, and d) pristine SWNTs sonicated with urea in water.


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Figure 3. A log-log plot of the scattered light intensity (I/Io) versus scattering angle (q) for the urea-SWNT compared with a conventional surfactant (SDS-SWNT) and superacid-SWNT. The plots represent duplicate data collections plotted together to obtain a best-fit line. The slope of the best fit line is used to evaluate the sample fractal dimension (Df) based on the relationship in Eqn. 2.


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a)

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b)

Figure 4. Raman spectroscopy results for urea-SWNT. a) G-band peak intensity versus energy at various rotation angles and b) intensity versus sample orientation angle at spotsizes 2, 5 and 65 microns. Savg = 0.37, 0.6 and 0.6 for 65 µm, 5 µm and 2 µm, respectively.


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Figure 5. Sheet resistance value comparison of urea-SWNT Mayer rod method to that of SWNT films from other work. The SWNT film sheet resistance versus transmittance at 550 nm of SWNT films i) urea-SWNT technique (this work) ii) Surfactant-SWNT on MCE membrane obtained from tuning semiconducting to metallic SWNT ratios10 iii) Ether treated SuperacidSWNT films on silica slide24 iv) Superacid-SWNT films obtained by a filtration/transfer method 26 v) Surfactant-SWNT line patterning31 vi) Surfactant-SWNT Mayer rod technique32 vii) SWNT-superacid sliding method34 viii) Surfactant-SWNT Mayer rod coated film, doped by fuming sulfuric acid38 ix) Polymer-SWNT films made by various film applicators.39 x) SWNT vacuum filtering method40, 41 xi) Surfactant-SWNT films obtained by spray coating and HNO3 treatment42 xii) Monodisperse metallic surfactant-SWNT films by vacuum filtration43 xiii) Surfactant-SWNT films by filtration method obtained after multistep purification process44


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ASSOCIATED CONTENT Supporting Information. XPS detailed spectra, normalized probability distributions of diffusion time for different functionalized SWNTs, UV-VIS/NIR characterization for as-received SWNTs. This material is available free of charge via the Internet at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author *Correspondence to: Lisa Pfefferle [email protected] Present Addresses 1

Department of Chemical and Environmental Engineering, Yale University, New Haven,

Connecticut 06511, USA 2

Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania

19104, USA 3

Department of Mechanical Engineering and Materials Science, Yale University, New Haven,

Connecticut 06511, USA Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. † These authors contributed equally. S.A. designed and carried out the experiments and analyzed the data. A.M. participated in sample preparation and carried out experiments. A.L.E. performed polarized Raman and additional sheet resistance experiments with supervision of J.M.K. A.M. and A.L.E. contributed to the work equally.


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L.M.G. carried out SLS and DLS measurements under supervision of J.B.Z. S.N. carried out XPS measurements and analyzed the data and compared with FTIR data collected by S.A. J.J.C. did TEM imaging and EDX/EELS. L.D.P supervised the project and suggested the use of urea. S.A wrote the manuscript with advice from L.D.P. All authors discussed results and revised the manuscript. This work was performed in partial fulfillment of the requirements for a PhD degree by S.A. Funding Sources National Science Foundation. (NSF DMR and NSF CBET) ACKNOWLEDGMENT Seyla Azoz and Lisa D. Pfefferle gratefully acknowledge financial support from NSF DMR0934520. Annemarie L. Exarhos and James M. Kikkawa thank NSF MRSEC DMR-1120901 for support. Leanne M. Gilbertson and Julie B. Zimmerman acknowledge NSF CBET-0854373 and NSF Graduate Research Fellowship Program (GRFP). Authors thank Prof Andre Taylor and Xiaokai Li for use of their facility and help in measuring conductivity values as well as providing superacid-SWNT solutions. We thank Dr Gayatri Keskar and Prof Gary Haller for critical discussions, Dr Fang Ren for help in UV-Vis measurements and Fjodor Melnikov for assisting with the statistical analysis. ABBREVIATIONS SWNT, single-walled carbon nanotube; SEM, scanning electron microscopy; EELS, electron energy loss spectroscopy; EDX, energy-dispersive X-ray spectroscopy; XPS, X-ray photoelectron spectroscopy; FTIR, Fourier transform infrared spectroscopy; CNT, carbon


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nanotube; CVD, chemical vapor deposition; SLS, static light scattering; DLS, dynamic light scattering.

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Highly Conductive Single-Walled Carbon Nanotube Thin Film

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