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18860

J. Phys. Chem. B 2004, 108, 18860-18865

Solubilization of SWNTs with Organic Dye Molecules Theresa G. Hedderman,* Sinead M. Keogh, Gordon Chambers, and Hugh J. Byrne Facility for Optical Characterisation and Spectroscopy (FOCAS)/School of Physics, Dublin Institute of Technology, KeVin Street, Dublin 8, Ireland ReceiVed: February 25, 2004; In Final Form: August 12, 2004

Single Wall Carbon Nanotubes (SWNTs) are insoluble in most organic solvents, such as toluene. Improvements in the solubility of the SWNTs are, however, seen as a result of interaction with dye molecules such as terphenyl and anthracene. The suspensions formed are stable for periods greater than 24 months. Spectroscopic analysis clearly shows interaction between the SWNTs and dye molecules. The fluorescence of the dye molecules is quenched on interaction with SWNTs, and, in the case of terphenyl, the spectrum is red shifted. Raman spectroscopy of the composites shows vibrations that are not present in either the SWNTs or dye powders. At the position at which these unique Raman peaks occur in the composite spectra, it was found that both the dye and SWNTs had infrared (IR) active vibrations at these wavenumbers. It is therefore thought that the new Raman peaks in the composite samples are possible IR modes that become Raman active on interaction between the dyes and SWNTs. The Radial Breathing Modes (RBMs) give detail as to how diameter selective the dye samples are when compared to the pristine SWNT modes. Red shifting of the RBMs for both composite spectra was observed. It is believed that such a result is due to the debundling of the tubes on interaction with the dye molecules.

Introduction Single Wall Carbon Nanotubes (SWNTs) are insoluble in all commonly used organic solvents. It has been observed that SWNTs in toluene precipitate out within 24 h.1 Several methods for enhancing the solubility of SWNTs have been reported;2-4 the most successful employ the use of conjugated polymer systems such as poly(m-phenylenevinylene-co-2,5-dioctyloxyp-phenylene) (PmPV).5,6 PmPV has been shown to purify and solubilize SWNTs in the diameter range 1.2-1.4 nm.4,5 It is proposed in this study that the dyes anthracene and terphenyl will also purify and solubilize the SWNTs, but, due to the small size of the dye molecules, diameter selectivity is not expected; but, there may be a degree of structural selectivity. The dyes anthracene and terphenyl each resemble the backbone structure of armchair and zigzag SWNTs respectively, and are therefore candidates for selective interaction via mapping with specific SWNTs. Armchair SWNTs are metallic while two-thirds of zigzag SWNTs are semiconducting, so selective interaction of the dye molecules may lead to separation of the SWNTs by electronic properties. This report is based on the observation that the solubility of SWNTs in toluene is substantially enhanced upon addition of dye molecules terphenyl and anthracene.7 After initial preparation, it was noted that solutions were stable for indefinite periods (greater than 24 months) and that the SWNTs are finely dispersed. This observation indicates an enhanced stability of the SWNTs solutions in the presence of the dye molecules. Presented in this paper are a number of spectroscopic studies that indicate that the dye molecules interact with SWNTs. The studies were performed in toluene due to its poor affinity for the retention of SWNTs, and thus can act as a good indicator of solubility due to the presence of the dyes.1 Laser-vaporization* Corresponding author. Phone: 00 353 1 402 7909, Fax: 00 353 1 402 7901, E-mail: [email protected].

produced SWNTs were the preferred tubes for this study, as they have a wide diameter range of 1.2-1.7 nm with a mean diameter of 1.4 nm.8,9,10 With a diverse diameter range, it can be investigated if the dyes are diameter selective. Experimental Section Three concentrations of the dyes terphenyl and anthracene in toluene were prepared. The concentrations prepared were 4.0 × 10-4, 1.5 × 10-3, and 2.5 × 10-3 M for terphenyl and 5.5 × 10-5, 2.0 × 10-4, and 3.0 × 10-4 M for anthracene. The anthracene concentrations chosen are approximately a factor of 10 less than those of the terphenyl due to the greater tendency of anthracene to aggregate. The dye solutions were characterized using UV-vis-NIR (Perkin-Elmer Lambda 900), fluorescence (Perkin-Elmer LS55), and Raman spectroscopy (Instruments SA LabRam 1B). Laser vaporization SWNTs obtained from Rice University (Tubes@ rice.edu, Houston, TX) were added to all dye solutions in a 1:1 ratio by weight (w/w) SWNT/dye molecules. The suspensions were sonicated using a Branson ultrasonic tip for 30 s and allowed to settle for 24 h, after which the supernatant liquid was carefully pipetted. In all cases, the suspensions were then allowed to settle for a further 24 h before being characterized by the various spectroscopic methods mentioned above. The precipitate was found to be relatively rich in SWNTs, so the solubilization is only partial. Throughout this paper, all the dispersion concentrations quoted are as prepared. For Raman measurements at 632.8 and 514.5 nm, samples were drop cast onto glass slides from suspension. Samples were allowed to dry for 24 h under ambient conditions. The absence of any characteristic toluene features in the Raman spectra indicates that all toluene has been removed. Removal of the dye molecules from the composite samples was achieved using filtration and subsequent flushing with toluene. The washing procedure entailed pouring the composite solution into

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Solubilization of SWNTs with Organic Dye Molecules

Figure 1. Fluorescence of terphenyl at concentrations (a), 2.5 × 10-3 M, (b), 1.5 × 10-3 M, and (c), 4.0 × 10-4 M before and after (d), (e), and (f) respectively, the addition of SWNT. The inset shows the red shifting of terphenyl (d) upon the addition of SWNT.

Figure 2. Fluorescence of anthracene at concentrations (a), 3.0 × 10-4 M, (b), 2.0 × 10-4 M, and (c), 5.5 × 10-5 M before and after (d), (e), and (f) respectively, the addition of SWNT. The inset shows there is no red shifting of anthracene (d) upon the addition of SWNT.

a 10 mL syringe lined with a Millipore HV filter disc with a pore size of 0.45 µm and purging the solution. The composite collected on the Millipore disc was then flushed with excess toluene. The Millipore disc containing the nanotubes was allowed to dry, and then it was removed from the syringe and placed in a sample bottle containing 10 mL of toluene. The contents of the sample bottle were placed in the sonic bath (ULTRA Sonick 57×) at medium power for 2 h, after which the Millipore disc was removed. The solution containing the washed nanotubes was then drop cast onto a glass slide and allowed to dry under ambient conditions for 24 h. Results and Discussion SWNTs are insoluble in toluene and precipitate out in a matter of hours, yet upon the addition of the dye molecules to the solution a stable suspension of SWNTs is formed. The addition of the SWNTs to the solutions in a 1:1 ratio by weight (w/w) results in a visible quenching of the dye fluorescence.7 The quenching of the fluorescence gives a strong indication that an interaction is occurring between the SWNTs and dye molecules. This visual observation was confirmed by fluorescence spectroscopy as shown in Figures 1 and 2. Figure 1 shows the fluorescence of terphenyl in toluene at concentrations (a), 2.5 × 10-3 M, (b), 1.5 × 10-3 M, and (c), 4.0 × 10-4 M before and after the addition of laser vaporization

J. Phys. Chem. B, Vol. 108, No. 49, 2004 18861 SWNTs (d), (e), and (f), respectively. Upon the addition of the SWNTs to terphenyl at a 1:1 (w/w) ratio, quenching occurred by 90% for (a), (b), and (c), respectively. This indicates that quenching is independent of concentration in this concentration range. Before the addition of SWNTs, the emission of terphenyl is seen to have a peak maximum at 345 nm. On addition of SWNTs, the emission spectrum red shifts by approximately 20 nm. The inset in Figure 1 clearly shows the dramatic red shift of terphenyl upon the addition of SWNT. The red shifted spectrum (d) has been multiplied by a factor of 10 for reasons of clarity and visibility. It should be noted that no red shifting of the fluorescence is observed in pure terphenyl solutions up to concentrations of 1.1 × 10-2 M. Therefore, the red shifting in terphenyl is most likely due to an increase in the effective conjugation of the molecule on interaction with the nanotubes. Terphenyl is composed of three phenyl rings with each connected to the other by a single bond. When terphenyl is dissolved solely in toluene, each of the phenyl rings rotates freely about the single bond and independently of the other phenyl rings, and, as a result, the π electrons in the individual phenyl rings do not fully communicate with each other. However, in order for mapping of terphenyl to the SWNT, the phenyl rings must align themselves in a planar fashion. On alignment of the three phenyl rings, the π electrons of each phenyl ring now communicate with each other, and this results in an apparent increase in conjugation. Figure 2 shows the fluorescence of anthracene in toluene before the addition of SWNTs at concentrations (a), 3.0 × 10-4 M, (b), 2.0 × 10-4 M, and (c), 5.5 × 10-5 M and after the addition of SWNTs (d), (e), and (f), respectively. Upon the addition of SWNTs to anthracene at a 1:1 (w/w), quenching occurred by 26% for (a) and 11% for (b) and (c), respectively. The fluorescence of anthracene indicated similar trends to terphenyl, but there was no significant spectral shifting observed. The slight shifting of the blue edge is concentration dependent due to reabsorption of the fluorescence emission. The peaks at ∼405 and 428 nm are unaffected by concentration or the addition of nanotubes. Anthracene, like terphenyl, is comprised of three phenyl rings, but the phenyl rings are fused so there is no free rotation of the rings. The anthracene molecule is a locked conformer and has a rigid structure. It is a planar molecule, and so its natural structure is ideal for mapping to SWNTs. For these reasons, when anthracene maps to the SWNT, red shifting of the peak maximum is not observed. This result is shown in the inset in Figure 2. The fluorescence studies point strongly to a mapping of the dye molecules onto the SWNTs’ surface via π-π stacking. As there is so far no evidence of charge transfer between the dyes and the nanotubes, the quenching of the fluorescence is most likely due to energy transfer to the nanotube via vibrational coupling. Raman scattering is a valuable tool to investigate the vibrational properties and thus characterize a sample of SWNTs. Figure 3a shows a typical Raman spectrum for a SWNT bundle in the range 100-1800 cm-1 taken with a 514.5 nm laser excitation line. There are two phonon modes that give a strong Raman scattering signal, Radial Breathing Modes (RBMs) in the region of 200 cm-1, and tangential carbon stretching modes (G-line) at ∼1580 cm-1.11 The RBM frequency is inversely related to the SWNT diameter.11,12 The tangential mode weakly depends on diameter.13 Its line shape depends strongly on whether the tube is metallic or semiconducting.11,12

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Figure 3. Raman spectra at 514.5 nm, (a) pristine SWNT, (b) pristine terphenyl, (c) composite spectrum of SWNT and terphenyl, and (d) washed composite sample.

Figure 4. Raman spectra at 514.5 nm, (a) pristine SWNT, (b) pristine anthracene, (c) composite spectrum of SWNT and anthracene, and (d) washed composite sample.

Figure 3 compares the Raman spectra of the pristine SWNT (3a), pristine terphenyl powder (3b), and a composite sample derived from the 2.5 × 10-3 M solution of SWNTs and terphenyl (3c), taken at a laser excitation of 514.5 nm. The composite spectrum in Figure 3, part c is not a combination of that of the pristine SWNTs and the dye material and is therefore a spectrum of a unique compound. Comparing Figure 3, parts a and c, the characteristic SWNTs peaks such as the G-line at 1591 cm-1 and the RBMs at ∼200 cm-1 are present in the composite spectrum. These peaks do not appear to be enhanced or damped due to interaction with the terphenyl. On comparison of Figure 3, parts b and c, a number of the pristine terphenyl peaks are significantly damped. The peaks in question are the ring vibrations and ring stretches at 770 and 1271 cm-1, respectively. These vibrations are restricted when the π electrons of the dye molecule interact with the SWNT. In the spectrum in Figure 3, part b, the peak at approximately 1600 cm-1, which represents the conjugated carbons in terphenyl, is damped by the tangential stretching of the SWNT in the composite spectrum in Figure 3, part c. Most interesting of all is that there are a number of new peaks present in the composite spectrum that are not present in the spectra in either part a or part b of Figure 3. These new peaks occur at 750, 1092, 1340, and 1440.9 cm-1 and are indicated by the arrows in Figure 3. A possible origin of the new peaks at 750, 1340, and 1440.9 cm-1 is that of infrared (IR) active dye or SWNT modes that become Raman active upon interaction. Terphenyl is known to have a very strong IR peak at 746 cm-1 that is due to C-H out of plane deformation vibrations.14 The dye also has an IR peak at 1340 cm-1 due to in plane vibrations. The 1440.9 cm-1 may result from IR CdC stretching vibrations of the aromatic ring in terphenyl. SWNTs also have IR active modes at ∼1085 and ∼1440 cm-1.15 The unique Raman peaks in the composite spectrum may thus be explained by IR modes that become Raman active when both the dye and the SWNT interact. Similar behavior is observed for the anthracene composite from the 3.0 × 10-4 M solution, as shown in Figure 4. The composite spectrum in Figure 4, part c again shows the characteristics of the nanotubes as well as some features attributable to the dye. However, there are many features of the composite spectrum that do not appear in the Raman spectrum of either the pristine nanotube (Figure 4, part a) or the dye (Figure 4, part c). As before, it is proposed that these unique peaks at 450, 1120, 1350, and 1440 cm-1 may be attributed to anthracene and SWNT IR modes that become Raman active upon interaction.14,15 The peaks at 450, 1350, and 1440 cm-1 may be assigned to anthracene IR modes, while those at 1120 and 1440 cm-1 may be assigned to SWNT IR modes.

In Figures 3 and 4, part d is the result of washing the composite sample in part c with excess toluene. In both cases, the spectra of the washed sample is similar to the pristine SWNTs’ spectra, indicating that the SWNTs can be recovered from the composites. In the case of anthracene, the process appears fully reversible, whereas in the terphenyl there is a remnant peak at 1440 cm-1. The process may thus be looked upon as a possible purification technique for SWNTs. It is also worth noting that for both composite samples, a similar behavior was observed in the Raman spectra using a 632.8 nm laser excitation line as a source. A more detailed analysis of the Radial Breathing Modes (RBMs) can yield further information as to the interaction of the dye molecules with the nanotubes. Assignments of SWNT diameters can be made according to Kuzmany et al.11 or Jorio et al.,16 including a damping factor due to tube bundling from the spectral positioning of the RBMs. Presented below are a number of spectra that show the effects on the SWNT as a result of interaction with the dye molecules. Figure 5, part a shows the RBM region for the pristine nanotubes, the terphenyl composite, and the anthracene composite at 632.8 nm. Figure 5, part b shows the RBMs for the pristine nanotubes, the washed terphenyl, and anthracene composite. From Figure 5, parts a and b, it can be seen that the pristine nanotube sample shows a dominant feature at 191 cm-1, with indications of further features at 165 and 215 cm-1. The profile is consistent with the resonance of larger metallic tubes.17,18,19 The profile of the anthracene composite shows similar features to the pristine sample in that it is dominated by the feature originating at 191 cm-1, but the features originating at 175 cm-1 become relatively more pronounced. Comparing the pristine material, the feature originating from 215 cm-1 is relatively stronger in the composite spectrum. The profile indicates that the medium diameter tubes as well as a few small tubes are preferentially solubilized by the anthracene. The upshift of the composite is ∼6 cm-1. The upshifting of the features is indicative of a change in the local surroundings of the tubes, and may be indicative of at least partial debundling of the tubes.20-24 A similar behavior is observed for the terphenyl composite. The feature originating from 191 cm-1 is upshifted to 197 cm-1. As is the case for the anthracene composite, the feature originating from 215 cm-1 is slightly weaker than in the pristine material and the feature originating at 175 cm-1 again has become more pronounced. The preferential solubilization of this medium to large diameter tubes and the loss of the small tubes may indicate a greater degree of selectivity by terphenyl. Figure 5, part b shows the pristine SWNT and the washed composite RBMs. The profiles are generally the same as in

Solubilization of SWNTs with Organic Dye Molecules

Figure 5. (a) RBMs of pristine SWNT, anthracene composite and terphenyl composite at laser excitation 632.8 nm. Figure 5b shows the RBMs of pristine SWNT, washed anthracene composite, and washed terphenyl composite at a laser excitation of 632.8 nm.

Figure 5, part a. The most notable feature is the increased upshift of the two composite spectra on washing. Comparing the washed composites with the pristine SWNT sample, there is a total upshift of ∼8 cm-1 for tubes selected by anthracene and terphenyl. This is a further shift of 2 cm-1 in both cases. A number of possibilities that may contribute to the RBM upshifting were investigated, the first being the solvent toluene. It is believed that the solvent is not responsible for the upshift, as it is removed in the sample preparation stage. Furthermore, no spectral characteristics of toluene were observed. The second possibility for an upshift is the presence of the dye molecules. It has been shown that a change in the local environment of the SWNT will cause a shifting of the RBM. The addition of polymers such as PmPV to SWNT solutions have been shown to induce a considerable upshift5,6,22 of 7 cm-1 comparable to the 6 cm-1 observed for anthracene and terphenyl. It is not until the polymer or dye molecules are removed that one can determine if there is any upshift resulting from debundling. If the bundle size has undergone little or no change, then removal of the polymer or dye molecule will cause a downshift equal to the RBM of the pristine sample of SWNT. If the bundle size has decreased, then one would expect to see a significant upshifted spectrum compared with the pristine SWNT material and, indeed, the composite after the removal of the polymer or dye molecules. In the case where PmPV is used to solubilize SWNT, a downshift25 of 7 cm-1 was reported on removing the polymer, suggesting that the polymer PmPV induces little or

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Figure 6. (a) RBMs of pristine SWNT, anthracene composite, and terphenyl composite at a laser excitation of 514.5 nm. Figure 6b shows the RBMs of pristine SWNT, washed anthracene composite, and washed terphenyl composite at a laser excitation of 514.5 nm.

no debundling of SWNT. This notion is not entirely unreasonable, since new research conducted by Coleman et al.26 has shown that only 13-14 carbons in the PmPV molecule interact with the tubes and they do not coil around individual SWNTs as previously thought. The downshift, along with Coleman’s research, suggests that perhaps the polymer is interacting with the backbone of a SWNT that is in a bundle and that PmPV does not act as a nanospacer27 as previously thought. The sharp RBM peak observed may suggest that the bundle is composed of SWNTs with a very similar diameter.8,21 In this study, removal of the dyes anthracene and terphenyl caused an increased upshift in the RBMs. These findings suggest that washing the composite samples leads to the release of the damping of the SWNT vibrations imposed by the dye molecules. Therefore, the most probable explanation for the observed upshift is debundling. Atomic Force Microscopy (AFM) is currently being conducted to further support and confirm the notion that the nanotubes are debundling. A further point to add is that the precipitates of both anthracene and terphenyl composites were compared to the pristine SWNT sample. Results showed that there is negligible shifting of the precipitate spectra compared to the pristine sample. At 514.5 nm, the incident light is predominantly resonant with nonmetallic tubes in the pristine material.28 In Figure 6, parts a and b, the RBM region is dominated by a feature at 184 cm-1 for the pristine SWNT spectrum, with evidence of

18864 J. Phys. Chem. B, Vol. 108, No. 49, 2004 further features on both sides. A point to note is that the RBM spectra at 514.5 nm do not upshift to the same extent as they do at 632.8 nm and that the profiles are not as well resolved. In the case of the anthracene and terphenyl composite spectra in Figure 6, part a, the RBMs upshift by ∼4 and ∼2 cm-1, respectively. The composite profiles are again dominated by the feature originating from 184 cm-1. In the case of the anthracene composite, the feature originating from 205 cm-1 no longer dominates the spectrum to the extent it did in the pristine sample. It may be said that in the case of the anthracene composite there is a loss of small diameter SWNT. The profile indicates that large and medium SWNTs are solubilized by anthracene. The profile of the terphenyl composite is similar to that of anthracene, but with the additional loss of the peak at ∼165 cm-1, the larger diameter SWNT. This result indicates increased selectivity by terphenyl. As was the case with 632.8 nm, the washed composite samples in Figure 6, part b further upshift by ∼2 cm-1. This gives a total upshift of ∼6 cm-1 for the anthracene composite and ∼4 cm-1 for the terphenyl composite sample when compared to the pristine SWNT spectrum. Examination of the precipitated SWNT from the composite solution showed no shifting when compared to the pristine sample, as was the case at 632.8 nm. It should be noted that no attempt has been made to assign diameters to the tubes according to the models of Kuzmany et al.11 or Jurio et al.13 The degree of bundling before interaction with the dyes is not known, and the degree of debundling and the degree of damping caused by interaction with the dyes can only be inferred by the shifting of the RBMs. Although some preferential solubilization by the dyes is inferred from the change in the RBM profiles, there is no clear differentiation between the dyes, and so a preferential solubilization of armchair by one and zigzag by the other cannot be inferred. It should be noted, of course, that there is a considerable number of chiral nanotubes present that may swamp the spectrum at the laser wavelengths employed. This study has, however, demonstrated that small dye molecules are candidates for the solubilization and purification of SWNTs. The choice of a 1:1 ratio was largely arbitrary, and concentration dependent studies may lead toward a fine-tuning of the specificity of the interaction. Temperature dependent studies may shed further light on the energetics of the interaction within the composite. To conclude this section on the RBMs, it is clear that the dyes are definitely solubilizing some nanotubes more than others. An upshifting of the composite has been observed and the change in the damping factor is quite considerable, between 4 and 8 cm-1. There is no qualitative deference between the two dyes and no evidence for a specific electronic interaction. Further studies on concentration dependence are currently under investigation to optimize the ratios of interaction. Conclusion The small dye molecules anthracene and terphenyl have been shown to solubilize SWNTs in toluene. The solutions are stable for prolonged periods of time. The spectroscopic investigations point toward a mapping of the dyes onto the nanotube surface, although examination of the RBM region of the Raman spectra does not show any differentiation between the two dyes, giving no evidence of selective interaction. Although the results so far do not indicate a high level of selectivity, there are two key results, the first being the solubilization of the tubes and the second being the strong indication that the ropes and bundles are being separated by the dye molecules. Further concentration dependent studies are currently underway to probe the specificity of the interaction.

Hedderman et al. Acknowledgment. FOCAS is funded under the Irish Government National Development Plan 2000 -2006 with assistance from the European Regional Development Fund. T.G. Hedderman acknowledges support from the DIT Scholarship Fund. References and Notes (1) Bahr, J. L.; Mickelson, E. T.; Bronikowski, J.; Smalley, R. E.; Tour, J. M. Chem. Commun. 2001, 193. (2) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95. (3) Niyogi, S.; Hu, H.; Hamon, M. A.; Bhowmik, P.; Zhao, B.; Rozenzhak, S. M.; Chenm J.; Itkis, M. E.; Meier, M. S.; Haddon, R. C. J. Am. Chem. Soc. 2001, 123, 733. (4) Duesberg, G. S.; Blau, W.; Byrne, H. J.; Muster, J.; Burghard, M.; Roth, S. Synth Met. 1999, 103, 2484. (5) Dalton, A. B.; Blau, W. B.; Chambers, G.; Coleman, J. N.; Henderson, K.; Lefrant, S.; McCarthy, B.; Stephan, C.; Byrne, H. J. Synth. Met. 2001, 121, 1217. (6) Dalton, A. B.; Stephens, C.; Coleman, J. N.; McCarthy, B.; Ajayan, P. M.; Lefrant, S.; Bernier, P.; Blau, W. B.; Byrne H. J. J. Phys. Chem. B 2000, 104, 10012. (7) Hedderman, T. G.; O’Neill, L.; Maguire, A.; Keogh, S. M.; Gregan, E.; McCarthy, B.; Dalton, A. B.; Chambers, G.; Byrne H. J. Proc. SPIEInt. Soc. Opt. Eng. 2002, 4876, 696. (8) Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Physical Properties of Carbon Nanotubes; Imperial College Press, World Scientific Publishing Co.: Singapore, 1998. (9) Alvarez, L.; Righi, A.; Rols, S.; Anglaret, E.; Sauvajol, L.; Munoz, E.; Maser, W. K.; Benito, A. M.; Martines, M. T.; de la Fuente, G. F. Phys. ReV. B 2001, 63, 153401. (10) Thess, A.; Lee, R.; Nikolaev, P.; Dai, H.; Petit, P.; Robert, J.; Xu, C.; Lee, Y. H.; Kim, S. G.; Rinzler, A. G.; Colbert, D. T.; Scuseria, G. E.; Tomanek, D.; Fisher, J. E.; Smalley, R. E. Science 1996, 273, 483. (11) Kuzmany, H.; Plank, W.; Hulman, M.; Kramberger, Ch.; Gruneis, A.; Pichler, Th.; Peterlik, H.; Kataura, H.; Achiba, Y. Eur. Phys. J. B 2001, 22, 307. (12) Rao, A. M.; Richter, E.; Bandow, S.; Chase, B.; Eklund, P. C.; Williams, K. A.; Fang, S.; Subbaswamy, K. R.; Menon, M.; Thess, A.; Smalley, R. E.; Dresselhaus, G.; Dresselhaus, M. S. Science 1997, 275, 187. (13) Jorio, A.; Souza Filho, A. G.; Dresselhaus, G.; Dresselhaus, M. S.; Swan, A. K.; Unlu, M. S.; Goldberg, B. B.; Pimenta, M. A.; Hafner, J. H.; Lieber, C. M.; Saito, R. Phys. ReV. B 2002, 65, 155412. (14) Socrates, G. Infrared and Raman Characteristic Group Frequencies; John Wiley and Sons, Ltd.: 2001. (15) Branca, C.; Corasaro, C.; Frusteri, F.; Magazu, V.; Mangione, A.; Migliardo, F.; Wanderlingh, U. Diamond Relat. Mater. 2004, 13, 1249. (16) Jorio, A.; Saito, R.; Hafner, J. H.; Lieber, C. M.; Hunter, M.; McClure, T.; Dresselhaus, G.; Dresselhaus, M. S. Phys. ReV. Lett. 2001, 86, 1118. (17) Pimenta, M. A.; Marucci, A.; Empedocles, S. A.; Bawendi, M. G.; Hanlon, E. B.; Roa, A. M.; Eklund, P. C.; Smalley, R. E.; Dresselhaus, G.; Dresselhaus M. S. Phys. ReV. B 1998, 58, 16016. (18) Duesberg, G. S.; Blau, W.; Byrne, H. J.; Muster, J.; Burghard, M.; Roth, S. Chem. Phys. Lett. 1999, 310, 8. (19) Kataura, H.; Kumazawa, Y.; Maniwa, Y.; Umezu, I.; Suzuki, S.; Ohtsuka, Y.; Achiba, Y. Synth. Met. 1999, 103, 2555. (20) Rao, A. M.; Chen, J.; Richter, E.; Sclecht, U.; Eklund, P. C.; Haddon, R. C.; Venkateswaran, U. D.; Kwon, Y. K.; Tomanek, D. Phys. ReV. Lett. 2001, 86, 3895. (21) Duesberg, G. S.; Loa, I.; Burghard, M.; Syassen, K.; Roth, S. Phys. ReV. Lett. 2000, 85, 5436. (22) Keogh, S. M.; Maguire, A.; Hedderman, T. G.; Gregan, E.; Farrell, G.; Dalton, A. B.; McCarthy, B.; Chambers, G.; Byrne, H. J. Proc. SPIEInt. Soc. Opt. Eng. 2002, 4876, 723. (23) Zhang, M.; Yudasaka, M.; Koshio, A.; Jabs, C.; Ichihashi, T.; Iijima, S. Appl. Phys. A 2002, 74, 7. (24) Keogh, S. M.; Hedderman, T. G.; Gregan, E.; Farrell, G.; Chambers, G.; Byrne, H. J. J. Phys. Chem. B 2004, 108, 6233. (25) Keogh, S. M.; Gregen, E.; Dalton, A. B.; Byrne, H. J. Final year thesis submitted 2001 to the Dublin Institute of Technology Dublin 8, Ireland.

Solubilization of SWNTs with Organic Dye Molecules (26) Coleman, J. N.; Maier, S.; Fleming, A.; O’Flaherty, S.; Minett, A. L.; Ferreira, M. S.; Hutzler, S.; Blau, W. J. J. Phys. Chem. B 2004, 108, 3446. (27) Dalton, A. B.; Chambers, G.; Byrne, H. J.; Coleman, J. N.; McCarthy, B.; Panhuis, M.; Blau, W. J.; Duesberg, G. S.; Roth, S.; Ajayan,

J. Phys. Chem. B, Vol. 108, No. 49, 2004 18865 P. M. Non-destructive methods to process and purify carbon nanotubes. Recent Res. DeV. Phys. Chem. 2002, 6, 327. (28) Pimenta, M. A.; Marucci, A.; Empedocles, S. A.; Brown, S. D. M.; Mathews, M. J.; Roa, A. M.; Eklund, P. C.; Smalley, R. E.; Dresselhaus, G.; Dresselhaus, M. S. J. Mater. Res. 1998, 13, 2396.