Processing and Properties of Highly Enriched Double-Walled Carbon Nanotubes: Supplementary Information Alexander A. Green and Mark C. Hersam* Department of Materials Science and Engineering and Department of Chemistry, Northwestern University, Evanston, IL 60208-3108, USA. *e-mail: m-hersam@northwestern.edu Redispersion of Sorted Carbon Nanotubes To facilitate optical absorbance and fluorescence measurements at wavelengths longer than ~1350 nm, the sorted DWNTs and small diameter SWNTs were transferred into 1% w/v SDS in D 2 O. These sorted carbon nanotubes, initially suspended in a mixture of SC, iodixanol, and H 2 O, were precipitated from solution by diluting the nanotube dispersions with water to bring the SC concentration to less than 0.1% w/v, and subsequently diluted with isopropanol to completely withdraw the SC from the carbon nanotube sidewalls. The precipitated solutions were then filtered through anodized aluminum oxide membranes (Whatman Anodisc), and rinsed with copious amounts of water to remove the remaining SC and iodixanol. The resulting carbon nanotube films were then immersed in acetone and freed from the membranes using bath sonication. The acetone in turn was removed by heating at 90 o C for two hours leaving solid, surfactant-free sorted carbon nanotube material. The separated DWNTs and SWNTs were both redispersed in 3 ml of 1% w/v SDS in D 2 O using a horn ultrasonicator (Fisher Scientific Model 500 Sonic 1
Dismembrator). The ultrasonicator was equipped with a 3 mm diameter tip and operated at 15% amplitude for 90 minutes while the samples were cooled in an ice bath. Poorly dispersed carbon nanotube bundles were then removed by ultracentrifugation of the suspensions for 14 minutes at 38 krpm in a Beckman Coulter TLA100.3 rotor. The top 2.5 ml of each solution was then carefully decanted for optical characterization. Transmission Electron Microscopy Measurements TEM samples were prepared on TEM grids coated with an ultrathin (< 3 nm) carbon film (Prod. #01824, Ted Pella, Inc.). A 5 µl droplet of sorted DWNTs in 1% w/v SDS in D 2 O was deposited on the grid. After ~30 s, the grid was dried using filter paper and rinsed in deionized water. The grid was then dried again using filter paper. TEM images taken on a JEOL JEM-2100F FAST TEM confirm that the DWNTs consist predominantly of double-walled species. Representative images are shown in Fig. S1. Diameter Determination Using Raman Spectroscopy The average diameters of the sorted DWNTs and large diameter SWNTs were determined from RBMs measured at multiple excitation wavelengths. Since SWNT diameters can be related to RBM frequencies with the equation S1 ω RBM = A/d t + B, we first determined the values of the parameters A and B using the Raman spectra of thin films of as-produced HiPco-SWNTs having a known chirality distribution. To account for differences in RBM frequency as a result of nanotube bundling and environmental effects, the films of HiPco-SWNTs were prepared in an identical manner to those used for sorted DWNT and SWNT measurements. As shown in Fig. S2a, the RBM 2
frequencies and diameters of the HiPco material were well described by a fit with A = 218.2 and B = 19.6. The RBM region of Raman spectra obtained from highly enriched DWNTs at multiple excitation wavelengths are presented in Fig. S2b. These spectra exhibit a bimodal diameter distribution at all wavelengths as expected for DWNTs, and a few weak peaks attributed to SWNT impurities. To evaluate the mean diameters of the DWNTs, we computed the average RBM frequency of the peaks associated with the inner and outer DWNT walls. These RBM frequencies were then converted to carbon nanotube diameters and corrected to account for differences in the laser power to arrive at mean inner and outer wall diameters of ~0.86 nm and ~1.61 nm, respectively. Similar analysis on sorted large diameter SWNTs revealed a mean diameter of ~1.60 nm with some small diameter carbon nanotube impurities. Optical Absorbance and Photoluminescence of Sorted DWNTs and SWNTs Prior to photoluminescence measurements, the relative absorbencies of the DWNTs and sorted small diameter SWNTs in 1% w/v SDS in D 2 O were adjusted to ensure similar concentrations of carbon nanotubes. This concentration matching was done by taking the optical absorbance of the two solutions (Fig. S3a) and subtracting the absorbance background, represented as a linear function of energy, to obtain the relative absorbance (Fig. S3b). The solutions were then diluted to equalize the peak relative absorbencies of both samples from ~900-1300 nm. The absorbance in this wavelength range arises from the S11 transitions of the inner DWNT walls and the small diameter SWNTs, and the S22 transitions of the outer DWNT walls. Although the contributions of 3
both inner wall and outer wall absorbance may cause us to overestimate the DWNT concentration, ph dependent measurements indicate the majority of absorption over ~900-1300 nm is due to the inner walls of the DWNTs (Fig. S4b). The photoluminescence of the two solutions was measured using a Horiba Jobin- Yvon Nanolog-3 fluorimeter S2 using identical acquisition parameters. For the display figures, the data were interpolated, and the effects of the absorption of the emission and the excitation beams were corrected. The separated DWNTs and small diameter SWNTs were also characterized in neutral and acidic environments to investigate the effects of carbon nanotube sidewall protonation. ph 7 and ph 1.9 samples were prepared by taking 840 µl of the carbon nanotube solution and diluting with 10 µl of deionized H 2 O and 10 µl of 1 M HCl in deionized H 2 O, respectively. The optical absorbance of these solutions was then measured using reference solutions containing the same mixture of H 2 O and D 2 O for background subtraction. In acidic conditions, the previously strong absorption of the S11 transitions was completely suppressed in the small diameter SWNTs (Fig. S4a). In contrast, the transitions corresponding to the S11 and higher-order transitions of the inner DWNT walls maintained most of their absorption strength following protonation (Fig. S4b). We also attempted to measure fluorescence from the SWNT and DWNT solutions at ph 1.9. However, we could not detect any signal from these solutions even when the photoluminescence of the corresponding neutral solutions could readily be detected using the same instrumentation. Lastly, we were unable to detect fluorescence from thin films of DWNTs, in which the carbon nanotubes were present as bundles. 4
Preparation of Unsorted DWNTs for Transparent Conductive Films Unsorted DWNT solutions were prepared from the same solution of sonicated carbon nanotubes used to produce the separated SWNT and DWNT material. A 1.5 ml volume of this starting solution was centrifuged for 30 minutes at 16,000 relative centrifugal force (Eppendorf Microcentrifuge 5424), similar to conditions used in previous unsorted transparent conductor work S3-5. The top 1.0 ml of solution, free of large bundles and poorly solubilised material, was carefully decanted and incorporated into control films of unsorted DWNTs using procedures identical to those employed to prepare the sorted carbon nanotube films. Figure S1 Representative TEM images of sorted DWNTs. Scale bars in the images are 4 nm. Figure S2 Supplementary RBM spectra. a, Plot of RBM frequency as function of inverse diameter for HiPco-SWNT thin films. Individual points represent chiralities with known diameters excited at different excitation energies and the solid line is a linear fit to the experimental data. b, RBM spectra of sorted DWNTs collected at six different excitation wavelengths. Peaks attributable to inner and outer DWNT walls are shaded red and blue respectively. Additional peaks due to impurity SWNTs are marked by grey asterisks. 5
Figure S3 Optical absorbance and relative absorbance of sorted DWNTs and small diameter SWNTs used in photoluminescence measurements. a, The optical absorbance spectra of the DWNTs and SWNTs are shown as solid blue and red curves, respectively. The absorbance background curves, modelled as linear functions of energy, are represented by dashed curves. b, The relative absorbance of the DWNTs (blue) and SWNTs (red) formed by subtracting the background absorbance from the solution optical absorbance. Figure S4 ph-dependent optical absorbance of sorted DWNTs and small diameter SWNTs. a, Absorbance of SWNTs at ph 7 (solid curve) and ph 1.9 (dashed curve). b, Absorbance of DWNTs at ph 7 (solid curve) and ph 1.9 (dashed curve). Shaded regions mark wavelength ranges associated with semiconducting (red) and metallic (blue) transitions. Supplementary References S1. Bachilo, S.M. et al. Structure-assigned optical spectra of single-walled carbon nanotubes. Science 298, 2361-2366 (2002). S2. Arnold, M.S. et al. Sorting carbon nanotubes by electronic structure using density differentiation. Nature Nanotech. 1, 60-65 (2006). S3. Artukovic, E. et al. Transparent and flexible carbon nanotube transistors. Nano Lett. 5, 757-760 (2005). S4. Hecht, D., Hu, L.B., & Gruner, G. Conductivity scaling with bundle length and diameter in single walled carbon nanotube networks. Appl. Phys. Lett. 89, 133112 (2006). S5. Zhou, Y.X., Hu, L.B., & Gruner, G. A method of printing carbon nanotube thin films. Appl. Phys. Lett. 88, 123109 (2006). 6
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