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Polymer-Functionalized Carbon Nanotubes Investigated by Solid-State Nuclear Magnetic Resonance and Scanning Tunneling Microscopy L. S. Cahill, Z. Yao, A. Adronov, J. Penner, K. R. Moonoosawmy, P. Kruse, and G. R. Goward* Department of Chemistry & Brockhouse Institute for Materials Research, McMaster UniVersity, 1280 Main Street West, Hamilton, Ontario, L8S 4M1 Canada ReceiVed: February 23, 2004; In Final Form: May 17, 2004
Carbon nanotubes are an intriguing new form of carbon, comprising molecular-scale cylinders of nanometer diameter and micrometer to centimeter lengths. They exhibit many extraordinary mechanical and electrical properties and have a wide variety of anticipated applications. However, to realize these potential applications, chemists need to develop means by which to manipulate these nanotubes in a predictable and controllable way. Novel sidewall-modified carbon nanotubes functionalized with polymers, such as poly(methyl methacrylate) (PMMA), have been prepared to gain control over the properties of nanocomposites on the molecular level. Characterization of these materials has been limited by their insolubility in organic solvents. Here the interaction between the carbon nanotube and the polymer has been studied through the use of solidstate nuclear magnetic resonance (NMR) and scanning tunneling microscopy (STM). Fast magic-angle spinning (30 kHz), to achieve high-resolution 1H NMR, together with advanced pulse sequences such as 1H double quantum NMR with the BABA (back-to-back) sequence, and heteronuclear 1H-13C sequences, are used to demonstrate the association of the initiator moieties and polymers with the surface of the nanotubes. The findings are supported by STM data of nanotubes before and after functionalization with the initiator groups.
Introduction Since their discovery in 1991,1 carbon nanotubes have been the focus of numerous investigations. In particular, single-walled carbon nanotubes (SWNTs)2 hold technological promise due to their many extraordinary mechanical and electronic properties.3 They are known to have high tensile strength and high thermal stability, ideal for the preparation of high-strength fibers. Moreover, nanotubes are able to carry current densities up to 109 A/cm2, which is 2-3 orders of magnitude higher than in metals such as aluminum and copper.3 On the basis of these properties, carbon nanotubes have numerous potential applications in areas such as molecular electronics,4 conducting layers in light-emitting and photovoltaic devices,5 and as sensors.6 Additionally, development of carbon nanotube field effect transistors (CNFETs) has excited much research, as these materials not only allow for the required miniaturization of FETs but also will enable a bottom-up approach to device fabrication.3 However, while these ideas pique the imaginations of scientists and engineers, the practical use of carbon nanotubes has so far been restricted by the inability to effectively manipulate and process them in solution. One potential way to overcome this problem is through chemical functionalization of the nanotubes.7 Several groups have addressed this potentiality, derivatizing nanotubes to achieve solubility in both organic and aqueous media.7,8 Functionalization of carbon nanotubes not only can provide a handle for manipulation but also allows the unique properties of the nanotubes to be coupled to those of other types of materials.7 Recent studies in the Adronov group9 have demonstrated the possibility of functionalizing single-walled carbon nanotubes * Corresponding author: e-mail
[email protected]; phone (905) 5259140, ext 24176; fax (905) 522-2509.
(SWNTs) along the sidewall, generating initiator sites for atom transfer radical polymerization (ATRP). Further, it was shown that these initiator sites can be used to polymerize both methyl methacrylate and tert-butyl acrylate monomers in a “grafting from” approach to nanotube functionalization.9 Solid-state nuclear magnetic resonance (NMR) has been used by several groups to characterize carbon nanotubes and their derivatives.10-15 All studies to date, however, utilize onedimensional 13C NMR, which is encumbered by the large relaxation times, requiring extremely long acquisitions, particularly under slow magic-angle spinning (MAS) conditions.11,12 These studies provide only circumstantial evidence of covalent functionalization through the observation of additional carbon resonances, attributable to the organic functional groups. Solution-state 1H NMR has also been used to characterize solubilized nanotube derivatives.16,17 As yet, however, the real power of solid-state NMR for characterizing conformation and dynamics in materials has not been brought to bear on functionalized nanotube materials. In this paper we demonstrate the utility of solid-state NMR, particularly high-resolution, solid-state 1H NMR under fast MAS and multidimensional spectroscopy, for characterizing the functionalization steps used in the synthesis of these materials. In general, we propose that high-resolution 1H and multinuclear solid-state NMR can be used to characterize the derivatized nanotubes. Using 2D double quantum filtered (DQF) 1H NMR and 2D heteronuclear 1H-13C correlation spectroscopy, we show the close through-space correlation between the organic moieties and the nanotubes. This is the first spectroscopic evidence for the intimate spatial proximity of the nanotubes and functional groups. Our evidence confirms the indirect evidence obtained through methods such as atomic force microscopy (AFM), transmission electron microscopy (TEM), and Fourier transform
10.1021/jp0491865 CCC: $27.50 © 2004 American Chemical Society Published on Web 07/09/2004
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infrared spectroscopy (FT-IR), which illustrate the presence of both nanotubes and organic components, but do not directly provide proof of the bonding between the nanotubes and functional groups.9 Raman spectroscopy provides the most comparable data, illustrating the conversion of a fraction of the carbon electronic environments from sp2 to sp3 hybridization but without showing the nature of the new interaction.7,9,18 Scanning tunneling microscopy (STM) uniquely allows atomically resolved imaging of surface structures. It has been used to record atomically resolved images of pure SWNTs and of structural defects contained in them.19-22 However, individual functional groups on sidewall chemically modified SWNTs have not previously been imaged with STM. Other techniques such as AFM are easier to use and interpret but do not have the needed spatial resolution to image functional groups on modified nanotubes directly, requiring indirect ways of detecting functionalization.23 In this paper, we demonstrate that it is possible to directly image functional groups at the sidewalls of SWNTs.
SCHEME 1: Synthetic Strategy for Functionalization of Shortened Nanotubes
Experimental Section 1H
MAS NMR experiments were performed on a Bruker DRX 500 spectrometer at a 1H Larmor frequency of 500.13 MHz, by use of a double-resonance MAS probe supporting rotors of 2.5 mm outer diameter with a spinning frequency of up to 30 kHz. The spectra are referenced to adamantane (1.63 ppm, 1H). 1H NMR spectra were acquired with a 90° pulse length of 2.0 µs, a recycle delay of 2 s, and with the bearing gas at room temperature. DQ MAS experiments were performed with four cycles of the back-to-back (BABA) recoupling sequence.24 The t1 increments were set equal to one rotor period. 13C detection was achieved by use of MAS and the cross polarization sequence on a Bruker DRX 500 spectrometer operating at a 13C Larmor frequency of 125.77 MHz. The spectra are referenced to glycine (176 ppm, 13C). The 1H-13C dipolar recoupling correlation experiment was carried out with an excitation time of τexc ) 6τrotor. The number of slices in the indirect dimension was 24, with 512 transients averaged per slice. Phase-sensitive detection in t1 was achieved through States time-proportional phase incrementation (TPPI). The STM images were taken with mechanically formed Pt/ Ir wire tips under ambient conditions with a Digital Instruments Nanoscope II STM, type D head. Separate samples of clean and initiator-functionalized nanotubes in dimethylformamide (DMF) were sonicated for 30 min, the suspension was immediately dropped onto a freshly peeled HOPG surface, and the solvent was left to evaporate. The images were reproducible with a number of tips and over several days on the same sample as well as for different batches of nanotubes. Typical conditions were a tunneling current of IT ) 1.5 nA and a sample bias of USB ) +20 mV. All STM images are unprocessed. Results and Discussion Single-walled carbon nanotubes (SWNTs) purchased from Carbon Nanotechnologies Inc. (Houston, TX), were received in the form of purified “bucky pearls” having a metal content of less than 8%. These nanotubes were shortened to lengths of approximately 350 nm according to published procedures.9 The shortened nanotubes were subsequently derivatized according to a previously published synthetic procedure, as shown in Scheme 1.9 Briefly, the nanotubes were shortened to lengths of approximately 350 nm and were then treated with (4-hydroxyphenyl)glycine and octanal in a 1,3-dipolar cycloaddition to yield phenol-functionalized SWNTs (1). These phenols were then treated with 2-bromoisobutyryl bromide, resulting in
SWNTs functionalized with atom transfer radical polymerization (ATRP) initiators (2). Exposure of the resulting structures to acrylate monomers, such as methyl methacrylate (MMA), and appropriate catalyst/ligand systems allowed for polymerization from nanotube sidewalls, resulting in structures such as 3. High-Resolution 1H NMR under Fast MAS. High-resolution solid-state 1H NMR has emerged as a facile technique for characterizing material in the solid state.25 With advances in probe design, fast MAS is sufficient for obtaining reasonable 1H resolution, even in materials as complex as the derivatized SWNTs discussed here. To achieve spectral resolution in solidstate 1H NMR, magic-angle spinning (MAS) is applied, which enables us to study protons directly with excellent sensitivity and resolution. For rigid 1H systems, usually MAS frequencies of 30 kHz are used. 1H-1H double quantum (DQ) MAS spectroscopy provides information on 1H-1H dipolar interactions via dipolar recoupling pulse sequences (such as “backto-back,” or BABA).24 1H 2D DQ MAS spectra recorded in a rotor-synchronized fashion allow us to create double quantum coherences, as well as to convert them into observable single quantum coherences. BABA is a robust sequence that is suitable for the fast spinning speeds of 30 kHz. The ability of the 1H DQ MAS experiment to identify proton-proton proximities lies in the fact that both the excitation and subsequent reconversion of double quantum coherences relies on the presence of a dipolar coupling between a particular two spins. Since the dipolar coupling is proportional to the cubed power of the internuclear distance, a peak is only observed in the DQ MAS spectrum if the corresponding two protons are close together in space. In general, the presence of a peak in a 1H DQ MAS spectrum implies a proton-proton proximity of under 0.35 nm. This tool often enables us to assign a local solid-state structure to the materials of interest. 25 1H MAS NMR spectra obtained for the pure, shortened nanotubes, the phenol-functionalized NTs (1), the initiator NTs (2), and the PMMA-NTs (3), are shown in Figure 1. It is important to note that these spectra were obtained for less than 10 mg of sample, with data acquisition times of only 10 min. This is in stark contrast to the acquisition times for 13C NMR data and illustrates the utility of solid-state 1H NMR for evaluating the functionalization process. The spectrum of the as-received carbon nanotubes is highly distorted and not included. The spectrum contains a broad, high-frequency resonance, as well as sharp overlapping features, which are
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Figure 1. 1H MAS NMR spectra acquired under 30 kHz magic-angle spinning: (a) purified, shortened nanotubes; (b) phenol nanotubes (1); (c) initiator nanotubes (2); and (d) PMMA-functionalized nanotubes (3).
assigned to impurities on the surface of the NTs. We note this to emphasize the point that the observation of two species, in this case NTs and impurities, does not prove any interaction between the materials. The “sum of the parts” argument used to demonstrate functionalization is not, of itself, proof of having obtained the objective. These impurities are removed by the purification process, as evident in the spectrum of the purified, shortened NTs (Figure 1a). The shortened nanotubes show only a single, very weak signal at 6.9 ppm, attributed to the protons associated with the carboxylic acid groups introduced at the ends of the shortened nanotubes. The 1H spectrum of 1, Figure 1b, exhibits an aromatic resonance at 6.4 ppm, attributed to the four protons on the phenol ring, as well as a strong resonance at 0.5 ppm, assigned to the protons in the aliphatic heptyl chain. The latter resonance is shifted about 1 ppm lower in frequency, relative to a normal aliphatic chain. This shift, caused by a ringcurrent effect,26 gives the first evidence of the spatial proximity of the chain to the surface of the nanotubes. Comparable ringcurrent effects on 1H resonances in carbon nanotubes have previously been observed.10,18 The comparison to the initiator-functionalized nanotubes, 2, in Figure 1c, shows a marked broadening of the aliphatic resonance, consistent with the added contribution of two methyl groups that are not influenced by ring-current effects. Figure 1d shows the 1H MAS spectrum of the PMMA-functionalized NTs, 3, in which the dominant resonances belong to the two types of aliphatic protons in the polymer backbone and side chains, namely, the methylene protons at 0.9 ppm and the methoxy protons at 3.6 ppm. A very weak signal from the phenol groups of the initial functionalization is still visible at 7 ppm; however, the dominant PMMA signals indicate the relatively high molecular weight of the polymers grown from the comparatively small number of initiator sites. As well, application of a one-dimensional double quantum filter (DQF), (which removes intensity from resonances of protons that are motionally averaged) reduced the dominant PMMA signals due to side-chain and backbone mobility. In this case, the phenol resonances were more visible but still at less than a tenth the relative intensity of the PMMA resonances. This is consistent with the expected, very small number of initiator sites per
Figure 2. Two-dimensional 1H DQF NMR spectra of (a) PMMAfunctionalized nanotubes (3) and (b) initiator nanotubes (2). Data were acquired under 30 kHz MAS, with 64 scans/slice, and 32 t1 increments, for a total acquisition time of 2 h.
polymer. On the basis of the molecular weight of the polymers, which was estimated to be greater than 300 000 g/mol,9 the observable intensity from the phenols may indicate that some initiator sites are present but have not initiated polymerization. Further evidence for this interpretation is provided by the 13C NMR spectra of the materials, presented below. The application of 2D 1H DQF NMR sequences has been shown to provide valuable structural and conformation data for many classes of solid materials.25 The method, which probes through-space interactions utilizing the homonuclear dipolar coupling constant, has been used here to investigate the functionalized nanotubes. In Figure 2 we show a pair of 2D DQF 1H NMR spectra, acquired for the PMMA-NT sample (3) and the initiator NTs (2). The first shows the two resonances assigned to the methylene and methoxy protons, with strong diagonal intensity due to the autocorrelation among like protons, as well as off-diagonal intensity, due to the through-space interaction between the methylene backbone and methacrylate side chain protons. No influence from the NTs is observed. In contrast, the 2D 1H DQF spectrum of 2 shows diagonal intensity only, with no observable cross-peaks between the protons of the aliphatic side chain and the aromatic ring. This indicates a lack of spatial proximity between the CH2 groups of the aliphatic
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Figure 3. 13C NMR spectra of (a) raw nanotubes (bucky pearls), (b) shortened nanotubes, (c, d) phenol nanotubes (1), and (e) PMMAfunctionalized nanotubes (3). All spectra were acquired under 30 kHz MAS, with acquisition times of 48 h. Spectra a-c and e were acquired with direct excitation of the 13C nuclei and high-power proton decoupling during the acquisition time. Spectrum d was acquired under cross-polarization from 1H to 13C with a contact time of 6 ms.
chain and the phenol protons. A plausible interpretation is that the aliphatic chain is aligned along the surface of the nanotube, consistent with the observed ring-current effect on the chemical shift of these protons. This indirect evidence is further supported by our 2D 1H-13C correlation studies. 13C NMR and Heteronuclear Correlation Spectroscopy. In addition to homonuclear 1H spectroscopy, we used heteronuclear 1H-13C correlation spectroscopy to aid our assessment of the nanotube-functional group structure. In a fashion similar to the homonuclear double-quantum-filter sequence, we selectively recouple the dipolar coupling of interest (1H-13C) using a rotor-synchronized pulse sequence, TEDOR, optimized for fast-MAS conditions.27,28 This sequence allows us to generate a two-dimensional map of the proton-carbon through-space correlations within the material. Again, the method relies on the existence of a dipolar coupling to generate the desired coherence, which scales with the cubed power of the internuclear distance. Thus, the resulting 2D spectra allow for direct interpretation of the local conformation of many materials.25 Figure 3 shows the 13C NMR spectra obtained under 30 kHz MAS for the as-received nanotubes, the shortened NTs, and products 1 and 3. Several features are notable. First, the spectra of all four materials (Figure 3a-c,e) were acquired under direct excitation, with high-power proton decoupling. The first spectrum agrees with other published studies, in which a relatively
Figure 4. 1H-13C 2D correlation spectra acquired by a variant of TEDOR, with 30 kHz MAS, and six rotor periods of recoupling. The number of slices in the indirect dimension was 24, with 512 transients averaged per slice. (a) PMMA-NTs (3); (b) phenol nanotubes (1).
narrow 13C resonance at ∼125 ppm is observable, with a line width of 10 ppm.11-13,15,18,29,30 Surprisingly, the shortening process resulted in significant line broadening, to a full-width half-maximum line width of 25 ppm, as well as the appearance of a strong shoulder at about 160 ppm. The broadening of the 125 ppm 13C line shape reflects a significant change in the electronic structure of the carbon nuclei, which may result from the formation of defects that give a broadened chemical shift distribution.11 The shoulder at 160 ppm is attributed to the formation of carbonyl groups at the ends of the shortened nanotubes. The 13C NMR spectrum of 1, acquired under high-power proton decoupling, at 30 kHz MAS, exhibits the characteristic 13C nanotube resonance, shifted to slightly lower frequency at 120 ppm, but only a weak, broad resonance in the aliphatic region, corresponding to the addition of the functional groups. The 13C spectrum of 2 exhibits strong baseline distortion, possibly resulting from trace paramagnetic materials following the synthesis, and is therefore not included. The 13C spectrum of the fully functionalized product, 3, shows strong, sharp resonances corresponding directly to the expected resonances of PMMA.31 This spectrum was also acquired under direct
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Figure 5. STM images of (a, b) clean and (c, d) initiator-functionalized nanotube bundles on HOPG. Images a and b were taken shortly after each other of the same bundle. Single functional groups are clearly resolved in images c and d, which were taken on different days of different samples and with different tips. IT ) 1.5 nA, USB ) +20 mV for images a-c, USB ) +25 mV for image d. In images c and d, arrows highlight several of the functional groups. All images comprise raw, unprocessed data.
excitation with high-power proton decoupling, to ensure an appropriate comparison with the pure NTs. Power levels, pulse lengths, and recycle delays are matched in the four spectra, Figure 3a-c and e. The resonance attributable to the nanotubes is nearly invisible in the PMMA-functionalized sample. Without confirmation from the comparison with pure nanotubes, the weak resonance at 126 ppm in the spectrum of 3 would be assumed to be noise. The relative intensities of the resonances in these spectra give us clues as to the nature of the nanotube functionalization. The weak intensity of the nanotube signal in 3 suggests that the molecular weight of the polymer is quite large, such that the spectrum is dominated by 13C nuclei in the polymer chains. Comparison with the 13C spectrum of 1 suggests moreover that the number of initiator sites per carbon nanotube is very low, indicated by the weak signal of the aliphatic 13C signals of 1. Given that the spectra were acquired under identical conditions, the relative intensities can be taken as a measure of the abundance of each type of nucleus. For comparison, the 13C spectrum of 1 was acquired under cross-polarization with a contact time of 6 ms, chosen to maximize the spatial volume being probed (Figure 3d). Excellent resolution was achieved for the aliphatic region, and a strong, broad signal attributed to the nanotubes is still evident. The relative intensity of the aliphatic region increases dramatically compared to the spectrum acquired by direct excitation, consistent with the efficient transfer of magnetization driven by the dipolar interaction between proton and carbon, for the organic functional groups
as compared to the nanotubes. We note that our interpretation here differs from that of other groups, who have claimed that the weak intensity of the functional group 13C resonances obtained under high-power proton decoupling conditions is due to ring-current influences.18 We argue that if this were the case, the cross-polarization sequence would give comparably weak, broad aliphatic 13C intensity. We have taken advantage of the dipolar coupling to probe the spatial proximity of the nanotubes and functional groups in the 2D correlation spectroscopy described below. Heteronuclear correlation spectroscopy is well-known from solution-phase NMR, in which case through-bond or J couplings are used to map out the bonding structure of organic and biological molecules.32 These methods have also been applied in the solid-state, and a recently developed sequence based on the TEDOR (transferred-echo double-resonance) sequence,28 in which, rather than the J coupling, through-space dipolar couplings drive the correlation between nuclear pairs. With the aid of fast MAS, proton resolution in the indirect dimension together with 13C spectra free from spinning sidebands can be acquired.27 In Figure 4 we present the first 2D correlation spectra of functionalized carbon nanotubes, in which a clear correlation between the 13C resonance of the nanotubes and the 1H resonance of the aliphatic functional group is observed [δ(13C) ) 121 ppm, δ(1H) ) 0.5 ppm]. This represents the strongest spectroscopic confirmation of the interaction between the two components of the material (Figure 4a). The strong group of
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resonances at 18-33 ppm in the 13C dimension correlates with the aliphatic 1H chemical shifts, as expected. The nanotube resonance shifted to lower frequency at 118 ppm represents 13C nuclei that have changed their hybridization from sp2 to sp3, as expected in the functionalization process, and that are clearly correlated with the aliphatic protons. Also important, the 13C resonance, which exhibits a correlation, is much narrower than the broad feature of the nanotube sample itself, consistent with the site specificity expected from NMR studies, and demonstrating that a comparatively small number of 13C sites have been functionalized along the nanotube length. For comparison, a 2D correlation spectrum of 3 is shown in Figure 4a. The strong, sharp signals of the PMMA are clearly evident in the 13C dimension, and the correlations of these resonances to the two types of 1H in the polymer are observed, as expected. The notable feature of this spectrum, as in the 1D 13C spectrum in Figure 3e, is the absence of a 13C resonance attributable to the nanotubes. In some cases, a 2D study would enable us to observe such weak resonances; however, the absence of such a resonance in this case further supports our contention that very large polymer chains grow from a comparatively small number of initiator sites. Imaging with STM. Figure 5 shows the comparison of STM images, of clean nanotubes (a, b) and of nanotubes that have been functionalized with the initiator group 2 (c, d). As a guide to the eye, several of the functional groups have been highlighted with arrows. First, it should be noted that in both cases the nanotubes lay down on the graphite surface in bundles. Nanotubes organize into bundles due to the strong π-π interactions between the rolled graphene sheets, just as in graphite. Graphite is also a very good substrate for imaging nanotubes, because the π-π interactions between the nanotubes and the graphite surface aid in fixing the nanotubes on the surface during imaging under ambient conditions. Under our conditions, it is typically observed over time that bundles flatten on the surface with all nanotubes of a bundle settling down next to each other. Before this happens, there is a higher chance of a collision between the tip, often resulting in movement, blurry images and tip damage. The flattened bundle of functionalized nanotubes in Figure 5c gives us an opportunity to observe the uneven distribution of initiator groups between the different tubes in a bundle. This is presumably due to a different degree of exposure of the different nanotubes to the reagents, as bundles break apart and re-form during the reaction. The overall density of functional groups on this image is estimated to be about 1 group every 20 nm of nanotube, corresponding to a functionalization of 1 in 3000 carbon atoms along the wall of the tube and consistent with the previously reported ratio of polymer side chains to shortened nanotubes.9 Figure 5d presents a very close look at functional groups along two parallel nanotubes in a different bundle. They are more densely functionalized than the overall average, but this small sample should not be considered statistically relevant. Individual functional groups are visible while the nanotubes themselves have become almost invisible in this image. The heavy functionalization has modified their electronic structure to increase the band gap and make them more insulating, an effect that has been previously described by Raman spectroscopy.33 This observation serves as a reminder that STM measures electronic structure, not necessarily topography. The initiator groups show up as clear features on the functionalized nanotubes. They appear as dark pits along the tubes due to their electronic structure which offers no states for electrons to tunnel into under our conditions (1.5 nA, +20 or
+25 mV). Each group strongly interacts with the tip, as is evidenced in Figure 5c by the bright streaks. These streaks originate from each functionalized site and extend in the direction of the tip movement during image acquisition. Physisorbed entities would not stay reproducibly in place under these conditions. Hence this observation lends credibility to the claim of covalent bonding between the nanotubes and the initiator groups. In the high-resolution image (Figure 5d), the streaking is diminished by the slower scan rate chosen for this image. The chemical identity of the features along the nanotubes cannot be determined by STM; rather, we can infer their nature from their absence on the clean nanotubes and the sequence of events leading to their appearance. They have been identified by the NMR data presented above. Thus, the argument for direct attachment of the initiator groups to the nanotubes is strengthened by our STM data. While these data do not provide spectroscopic information about the nature of the attached groups, the spacing of the individual groups along the nanotubes and their distribution between different tubes can clearly be observed. The observed density of initiator groups is also consistent with our NMR data. No STM data is available for the polymerized samples, because the STM is limited to phenomena on conducting surfaces. Conclusions In summary, we have characterized a series of functionalized carbon nanotubes using solid-state NMR and STM. Our results demonstrate the intimate spatial proximity of the functional groups and the nanotubes spectroscopically. Solid-state 1H NMR confirmed the functionalization process, while the multidimensional 1H DQF spectra and 1H-13C correlation spectra clearly illustrate the correlation of the aliphatic protons with the carbon nanotubes surface. As well, 2D double quantum 1H NMR spectroscopy, acquired under fast MAS conditions, provided an initial assignment of the orientation of the heptyl alphatic chain of the functional group along the surface of the nanotubes. In addition, heteronuclear 1H-13C TEDOR spectroscopy confirmed this interpretation, via the clear correlation of the aromatic nanotube 13C resonance with the alphatic 1H chemical shift of the functional groups. STM data further supports the finding of covalently bound initiator groups spaced out over the nanotube bundles. Although a strong interaction between the groups and the scanning tip is evident, the groups remain in place along the nanotubes, due to their covalent bond with the tubes. Our studies of the series of functionalized nanotubes also provide the general conclusion that the synthetic procedure generates a small number of initiator sites, from which very large polymer chains are grown. This is in agreement with previous estimates of the molecular weight of the PMMAfunctionalized nanotubes. Acknowledgment. We are grateful to Brian Sayer and Alex Bain for helpful discussions. References and Notes (1) Iijima, S. Nature 1991, 354, 56-58. (2) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603-605. (3) Avouris, P. Acc. Chem. Res. 2002, 35, 1026-1034. (4) Collins, P. G.; Avouris, P. Sci. Am. 2000, 283, 62-69. (5) Kymakis, E.; Amaratunga, G. A. J. Appl. Phys. Lett. 2002, 80, 112114. (6) Kong, J.; Franklin, N. R.; Zhou, C. W.; Chapline, M. G.; Peng, S.; Cho, K. J.; Dai, H. J. Science 2000, 287, 622-625. (7) Hirsch, A. Angew. Chem., Int. Ed. 2002, 41, 1853-1859. (8) Bahr, J. L.; Tour, J. M. J. Mater. Chem. 2002, 12, 1952-1958.
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