13150
J. Phys. Chem. C 2008, 112, 13150–13155
Two-Phonon Combination Raman Modes in Covalently Functionalized Single-Wall Carbon Nanotubes C. Fantini,*,† M. A. Pimenta,‡ and M. S. Strano§ Centro de DesenVolVimento da Tecnologia Nuclear, Belo Horizonte, MG, 31270-901 Brazil, Departamento de Fı´sica, UniVersidade Federal de Minas Gerais, Belo Horizonte, MG, 31270-901 Brazil, and Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 ReceiVed: May 1, 2008; ReVised Manuscript ReceiVed: June 4, 2008
Single-wall carbon nanotubes covalently functionalized with chloro-phenyl groups were investigated by Raman spectroscopy. We examine the features observed in the spectral ranges 350-1200, 1650-2000, and 2400-3200 cm-1. The Raman modes associated with the presence of defects, strongly enhanced by the functionalization are clearly distinguished from those defect independent. We indicate that some features highly enhanced by the electron-phonon coupling are very sensitive to charge transfers between the nanotubes and functional groups. A feature at ∼2900 cm-1 is observed in the spectra of functionalized nanotubes, and its origin is explained here as a combination of two disorder-induced modes. 1. Introduction The functionalization of carbon nanotubes1,2 with different chemical moieties is an efficient route to controlling solvent dispersion,3,4 electronic doping,5 electronic separation,6,7 or molecule-nanotube binding energies.8 The attachment of molecular groups to the nanotube side-wall and its influence on the nanotube electronic structure has been subject of large interest to the nanotube research in the last years.9–12 The understanding of the impact of the surface modifications on the nanotube electronic properties is essential for a large range of applications such as the development of nanotube-based sensors.13 Beside the electronic structure, the binding of functional groups on the nanotube surface also influences the phonon structure. Different effects on the electronic and vibrational properties are expected when the functional group is bound covalently or noncovalently to the nanotube surface. Inside this context, Raman spectroscopy is well established as a powerful experimental tools for the investigation of electronic and vibrational properties of carbon nanotubes, and recently it has presented itself as very useful to characterize and investigate the effects of chemical functionalization on the carbon nanotubes.14–17 A direct evidence of how a chemical moiety modifies or modulates the nanotube electronic structure is still an open question, and resonance Raman spectroscopy can contribute for this subject because excitonic states associated with optical transitions between van Hove singularities in nanotube density of electronic states can be probed with this experimental technique.18–20 Recently, it has been reported that the presence of functional groups causes shifts in the optical transition energies (Eii) observed by both resonant Raman and optical absorption spectroscopies.17 In the case of the covalent functionalization, an sp3 defect is created when the functional group is bound to the nanotube surface. The presence of defects allows one to observe, by Raman spectroscopy, phonon modes far from the Γ point of * To whom correspondence should be addressed. † Centro de Desenvolvimento da Tecnologia Nuclear. ‡ Universidade Federal de Minas Gerais. § Massachusetts Institute of Technology.
the Brillouin zone.21 Although many low intensity dispersive and nondispersive features can be observed in the Raman spectra of carbon nanotubes, most of the studies of functionalized nanotubes using Raman spectroscopy reported so far are devoted to the investigation of the first-order modes and the influences of the functional groups on them.16 Few effort has been devoted to the investigation of the low intensity second-order Raman modes, strongly sensitive to defects and charge transfers. In this work, we describe the influence of the covalent functionalization on the several different vibrational modes present on the carbon nanotube Raman spectra, including some second-order, two phonon Raman modes. Carbon nanotubes were functionalized with chloro-phenyl groups bonded covalently to the nanotube side-wall through a reaction of carbon nanotubes individually suspended in aqueous solution with 4-Clbenzene diazonium salt. By controlling the concentration of diazonium salt during the reaction, we can control the number of defects on the nanotubes sidewall and thus investigate the dependence of the different first and second order Raman modes as a function of the amount of defects. Some of these secondorder modes are disorder-induced modes and some of them are defect independent two phonon combination modes. The controlled introduction of defects by a step functionalization process allows us to describe the behavior of the second-order modes when the amount of defects is increased. 2. Experimental Details Functionalization of HiPco carbon nanotubes was performed by a reaction with 4-chlorobenzenediazonium salt in an aqueous SWNT suspension.2 Films of SWNTs functionalized with Clphenyl groups were prepared on SiO2 substrate, and the elemental composition analysis of these reacted SWNT films were performed by X-ray photoelectrons spectroscopy (XPS) to obtain the relative amount of chlorine atoms for each sample.17 The samples used here are identical to those ones functionalized with Cl-phenyl groups studied in ref 17 where the summary of elemental analysis by XPS is reported. MicroRaman measurements were performed using a Dilor XY triplemonochromator exciting the samples with laser lines 1.92, 2.18, 2.41, 2.54, and 2.71 eV from an Ar:Kr laser. A single
10.1021/jp803855z CCC: $40.75 2008 American Chemical Society Published on Web 08/05/2008
Covalently Functionalized Single-Wall Carbon Nanotubes
J. Phys. Chem. C, Vol. 112, No. 34, 2008 13151
Figure 1. Raman spectra of carbon nanotubes, functionalized with Cl-phenyl groups, recorded with Elaser ) 1.96 eV. Enlargement of the spectral ranges associated with (b) IFM, (c) 2oTO and iTOLA, and (d) G′ bands.
monochromator with an He:Ne (1.96 eV) and a diode (1.54 eV) laser was also used for the measurements. The samples were focused by a 50× objective and the laser power on the samples was kept around 0.5 mW. The heating of the sample by laser fluency can be neglected for this low laser power.
TABLE 1: Description of the Raman Modes Whose Dependence on the Covalent Functionalization Is Discussed in This Work wavenumber (cm-1) 350-500
3. Results and Discussions Figure 1a presents the Raman spectrum of a functionalized carbon nanotube sample where the well-known radial breathing modes (RBM), disorder-induced mode (D-band), and tangential modes (G-band) are observed. Beside these first-order modes, some two-phonon modes are observed in the regions indicated by rectangles and enlarged in Figures 1b-d. The intermediate frequency modes (IFM) region, composed by both first and second-order modes22 is shown in Figure 1b. The feature observed near 1750 cm-1 (Figure 1c) has been assigned as an overtone of the out-of-plane (oTO), infrared active mode of graphite, and the feature observed at ∼1950 cm-1 has been attributed to a two-phonon combination associated with the sum of the in-plane transverse optical (iTO) and the longitudinal acoustic mode (LA) of graphite in an intravalley doubleresonance Raman process.23 Figure 1d shows the G′ -band (overtone of the D-band) and two weak features at 2900 and 3200 cm-1 that will be further discussed in this text. Table 1 presents a summary with the description of the two-phonon and defect-induced modes observed in the Raman spectra of nanotubes whose behavior after covalent functionalization is the subject of this work. The D-band is originated from a double-resonance process24,25 involving an inelastic scattering by a phonon and an elastic scattering by a defect. Its intensity is strongly enhanced when structural disorder is increased. Thus, the ratio between the integrated areas of the D and G bands are associated with the amount of defects present on the nanotubes and it is frequently used to describe the disorder degree in graphite related materials. In our case it must be related to the amount functional groups covalently bonded to the nanotube surface. Figure 2a shows the ratio between the D and G bands integrated areas as a function of the number of functional groups per nanometer length of the nanotube, estimated from the percentage of chlorine atoms in the samples reported in ref 17. These values were
600-1100
845 1300-1400 1620
1750 1900
2600-2750
2900 3200
description steplike dispersive intermediate frequency modes associated with a combination of two phonons32 IFM: steplike dispersive intermediate frequency modes originated by a combination of an optical and an acoustic-like phonon22 originated from the oTO branch of graphene.29 D-band: disorder-induced band originated from an intervalley double-resonance process24,25 D′-band: disorder-induced band usually observed for disordered graphite27 originated from an intravalley double-resonance process24,25 M-band: overtone of oTO phonon.23 iTOLA and LOLA: combination of the iTO and LO optical phonons, respectively, with the LA acoustic phonon23,34 G′-band: overtone of the D-band, originated from a double-resonance process involving two phonon and independent of disorder combination of two defect-induced modes (first explained here) overtone of G-band
estimated for a (7,7) armchair or a (12,0) zigzag nanotube, whose diameters are close to 0.95 nm (the mean diameter in a HiPco sample). These nanotubes possess around 112 carbon atoms per nanometer length. In the case of the covalently functionalized nanotubes we can define a nanotube characteristic length (lc) as the distance between two adjacent functional groups bonded to the nanotube surface (see inset of Figure 2a). Thus, we can see from Figure 2a that the ID/IG ratio is inversely proportional to lc. From now on, the samples will be numbered from 1 to 6 from the nonfunctionalized sample to that one with
13152 J. Phys. Chem. C, Vol. 112, No. 34, 2008
Figure 2. (a) ID/IG ratio for the six samples used in the experiments, as a function of the number of functional groups per nanometer length of the nanotube, obtained from data in ref 17. (Inset) Schematic representation of a functionalized nanotube. (b) ID/IG ratio as a function of Elaser for the samples 2, 3, 5, and 6.
highest concentration of functional groups. Although the functionalization of carbon nanotubes depends on the diameter and metallicity,17 a homogeneous distribution of the functional groups around the sample was considered here as a first approximation to describe the dependence of ID/IG ratio on the distribution of functional groups along the nanotube. The result observed in Figure 2a is analogous to that one for disordered graphite materials first reported by Tuinstra and Koenig26 where the ratio between D and G bands presents a linear dependence on the inverse of the crystallite size (La). In that case, the borders of the crystallites make the same rule as the sp3 defect sites in the covalently functionalized nanotubes, and they are responsible to the loss of translational symmetry allowing the D-band becomes Raman active. The data shown in Figure 2a were obtained by exciting the sample with leaser excitation energy Elaser ) 2.54 eV, but, a dependence of the ID/IG ratio on Elaser is observed and presented in Figure 2b, where the ID/IG ratio for four different samples with different degrees of functionalization is plotted as a function of Elaser. This dependence of the ID/IG ratio on Elaser can be understood by considering the results previously reported for nanostructured graphite.27 In that case, the reduction of the crystallite size (La) is responsible to the increase in the ID/IG ratio, and the dependence of IG on Elaser while ID is Elaser independent causes the dependence of ID/IG on Elaser.28 The Elaser dependence of the ID/IG ratio is possibly related to the fact that the cross section for the D-band is independent of the Elaser. Figure 3 shows the Raman spectra in the range 300-1200 cm-1 associated with the intermediate frequency modes (IFM). Plots a and b represent spectra obtained with Elaser ) 1.96 and 1.58 eV, respectively. The sample associated with each spectrum is indicated by numbers in the figures. It has been observed in the IFM spectral range 600-1200 cm-1 some nondispersive
Fantini et al.
Figure 3. Raman spectra of functionalized nanotubes in the IFM region. The spectra were recorded by exciting the samples with Elaser ) 1.96 (a) and 1.58 eV (b). The dotted line indicates the first-order oTO mode, black arrows indicate the steplike dispersive modes, and gray arrows indicate the two bands originated from phonons at the M point in graphite Brillouin zone. The spectra are numbered according to the sample in which each spectrum is related.
first-order features and some steplike dispersive features explained by a combination of two phonon modes in a highly selective resonance process where two different excitonic states are connected by phonons.22,29 Recently, the same two-phonon model has been applied to explain the features observed between 350 and 500 cm-1, which also exhibit a steplike dispersive behavior.32 The black arrows in Figure 3 indicate the steplike dispersive peaks observed in ref 29. The intensities of these peaks are strongly suppressed after functionalization. As previously explained, the origin of these peaks is associated with the resonance with the optical transitions between the electronic states in metallic nanotubes.29 Because the displacement of the Fermi level and relaxation of resonance conditions is caused by the covalent functionalization, the intensities of these steplike dispersive IFMs are strongly reduced due to the lack of the resonance after functionalization. On the other hand, the broadband observed around 600 cm-1 (gray arrows) increases its intensity after functionalization. This band does not present a steplike dispersive behavior such as the peaks discussed above,29 and it is probably originated from the iTA and oTO phonon branches of the graphite phonon dispersion at the M point, where a singularity in the density of phonon states in graphene is predicted.30 In this case, because this is an out of Γ point band, the existence of defects is required for the momentum conservation. Thus, the increase in the intensity of this band after functionalization can be explained by the presence of sp3 defects created where the functional groups are bounded to the nanotube surface. A similar result has been previously observed when defects are created on nanotube surface by Ar+ ion irradiation.33 The peak centered around 845 cm-1 (dotted line) is a first order mode originated from the oTO phonon brunch at Γ point. The intensity of this peak is slightly reduced after functionalization, because it is not related to the presence of defects on
Covalently Functionalized Single-Wall Carbon Nanotubes
J. Phys. Chem. C, Vol. 112, No. 34, 2008 13153
Figure 4. Raman spectra from 1650 to 2000 cm-1 obtained with Elaser ) 1.58, 1.96, 2.19, and 2.54 eV from bottom to top, respectively. Lorentzian fit is shown by the gray curves. (b) Raman frequency as a function of Elaser for samples number 1 (solid symbols) and number 4 (open symbols). The frequencies of M-, M+, iTOLA, and LOLA features are represented by squares, circles, triangles, and diamonds, respectively. (c) Raman spectra obtained with Elaser ) 2.54 eV. Spectra from bottom to top corresponds to sample nos. 1, 3, and 5, respectively.
the nanotube. The oTO phonon branch presents a small negative dispersion, and thus the frequency of this mode in carbon nanotube will be dependent on the nanotube diameter.30,31 This explain the doublet structure observed for this band when Raman spectra is obtained with Elaser ) 1.58 eV. This is the energy of the ES22 transitions for the nanotubes in the family 2n + m ) 19, whose mean diameter is 0.94 nm, as well as the ES11 transitions for the nanotubes in the family 2n + m ) 16, whose mean diameter is 0.65 nm. This difference in the diameter is responsible for the two features inside the oTO band in Figure 3b. Finally, the intensities of the peaks observed between 350 and 500 cm -1 (dashed gray arrows) increase after functionalization, such as it was observed for SWNTs irradiated with Ar+ ions.33 It has been recently reported that these peaks also follow the steplike dispersive behavior and they are related to resonances with optical transitions between ES11 and ES22.32 According to the stepwise dispersion observed for these features, the acoustic phonon branch responsible for the origin of these modes should be the oTA phonon branch, that present a high phonon density of states close to the M point in the graphene Brillouin zone. Thus, in contrast with the steplike dispersive modes observed above 600 cm-1, these low-frequency modes depend on the presence of defects causing an increase in their intensities after functionalization because the formation of defects. Figure 4a shows the Raman spectra of the nonfunctionalized carbon nanotube sample in the range between 1650 and 2100 cm-1 where two bands associated with combination of phonons are observed. The spectra from bottom to top in Figure 4a were obtained with excitation energies 1.58, 1.96, 2.19, and 2.54 eV, and a Lorentzian analysis is shown in the spectra. The band observed at ∼1750 cm-1 has been assigned as an overtone of the oTO phonon branch of graphite.23 This 2oTO band, also called M-band is composed by two features identified in Figure 4 by M- and M+. The band observed around 1950 cm-1 is composed by two features originated by combinations of the transversal (iTO) and longitudinal (LO) optical phonons with the longitudinal acoustical one (LA). The lower and higher
frequency features of these bands are assigned by iTOLA and LOLA modes. The iTOLA and LOLA modes are highly dispersive while no significative dispersion is observed for the M-band. The same dispersions are observed for the pristine and functionalized samples as we can see in Figure 4b where solid and open symbols are related to the pristine sample no. 1 and a functionalized sample no. 4, respectively. When functionalization level increases, a reduction in the intensity of both 2oTO, iTOLA, and LOLA band is observed, and the spectrum completely disappears when more than 2 functional groups/nm are bonded to the nanotube surface. This can be observed in Figure 4c where Raman spectra in the range of these two-phonon modes for sample nos. 1, 3, and 5 are presented. No splitting of the LOLA feature such as reported for acid-treated nanotubes has been observed.34 This result shows that these two-phonon modes originated from double-resonance process are not defect-induced and their intensities are probably reduced because the lack of resonance conditions caused by charge transfers in the functionalization. Figure 5a shows the Raman spectra in the range from 2300 to 3300 cm-1 of carbon nanotube samples with different level of functionalization, where two features at 2900 and 3200 cm-1 are observed beside the G′ -band. The sample associated with each spectrum is indicated in Figure 5a by the numbers close to each spectrum. In principle, the band at 3200 cm-1 can be assigned as an overtone of the D′ -band or as an overtone of the G-band. The D′ band, which is around 1620 cm -1, is another defect-induced band present in the Raman spectra of nanostructured graphite, originated from an intravalley double-resonance process.24,25 Despite the D′ band to be defect induced, the presence of defects is not necessary for the observation of its overtone. The band at 3200 cm-1 is thus not induced by disorder, independent if it is assigned as an overtone of G or D′ -band, and the spectra in Figure 5a can be normalized by its intensity. An interesting observation in the second-order spectra in Figure 5a is the presence of a feature with frequency at ∼2900 cm-1, whose intensity becomes stronger for the samples with higher
13154 J. Phys. Chem. C, Vol. 112, No. 34, 2008
Fantini et al.
Figure 5. Raman spectra of G′, D + D′ and 2D′ bands. (a) Spectra obtained using Elaser ) 2.54 eV for samples with different level of functionalization. (b) Spectra obtained for the sample no. 4 excited with different laser excitation energies. The values of Elaser in eV are shown for each spectra in the figure.
concentrations of Cl-phenyl groups, suggesting that this band is associated with the presence of defects. An Elaser dependence of these second-order Raman modes is observed in Figure 5b where the Raman spectra of the sample no. 4 obtained with different laser excitation energies is presented. The well-known G′-band and the band at ∼2900 cm-1 present dispersions of ∼100 and ∼50 cm-1/eV, respectively, while the band at 3200 cm-1 is nondispersive. A small dispersion would be expected for the feature at 3200 cm-1 if it was originated from an overtone of the D′-band, because a small dispersion is observed in the frequency of the D′-band in nanostructured graphite.27 Thus, this band will be assigned here as an overtone of the G-band instead of an overtone of the D′band. It is observed for the disorder-induced band at ∼2900 cm-1 a dispersion of ∼50 cm-1/eV, similar to the dispersion of the D-band. Considering the observations of the dependence of the band at ∼2900 cm-1 on the functionalization level and its dispersion, the origin of this band can be described by considering the double-resonance mechanism developed to explain the origin of the D band 24,25 in graphite-related materials. The doubleresonance mechanism applied to the disorder induced band at ∼2900 cm-1 is shown in Figure 6a. The linear electron dispersion close to the K point of graphene Brillouin zone has been considered and it was calculated by E ) pVFk, where k is the electron wavevector and VF is the Fermi velocity of the electrons near the K point, whose value is 1.0 × 106 m/s.35 In the double resonance process for the feature observed at 2900 cm-1, an electron is scattered by a phonon associated with the D-band, whose wavevector will be called by qD, in an intervalley process. Afterward, the electron is scattered again by another phonon of wavevector qD′, associated with the D′-band phonon, in an intravalley process. For the momentum conservation requirements, the electron must be elastically scattered back by a defect to the state with k ) 0, to recombine with the hole in k ) 0 and to emit a scattered photon. According to the model proposed in Figure 6a, this band observed at 2900 cm-1 must be assigned as a combination of the two defect induced bands D and D′. The van Hove singularities in the nanotube electronic structure are represented by the intersection of the cutting lines with the graphene electron dispersion in Figure 6. The electron wavevector associated with each cutting line are given by k ) 2p/3dt,
Figure 6. (a) Double resonance process to explain the origin of the feature at ∼2900 cm-1. The electron dispersion and cutting lines wavevectors in the model were obtained by considering VF ) 1.0 × 106 m/s and dt ) 1.14 nm, respectively. (b) Ratio between the intensities of the D + D′ and 2G bands, obtained with Elaser ) 2.54 eV. RBM spectra for the nonfunctionalized sample at the same excitation energy (inset). (c) Ratio between the intensities of the D + D′ and G′ bands as a function of Elaser for sample nos. 4 (circles) and 7 (triangles). S 36 and d is the where p ) 1, 2, 3, 4,... for ES11, ES22, EM 11, E33,..., t nanotube diameter. The modulus of the electron wavevector (k) corresponds to the distance from the K point of graphene Brillouin zone to the corresponding cutting line.36 The notation E(SorM) corresponds to the i-th optical transition for semiconductii ing (S) and metallic (M) nanotubes. The nanotube diameter considered in the model was obtained form the RBM spectra in the inset of Figure 6b. The ωRBM ) 207 cm-1, resonant with Elaser ) 2.54 eV, corresponds to dt ) 1.14 nm, considering the relation between ωRBM and dt from ref 18. We observe that the double resonance process occurs when the electronic states ES33 and ES44 are connected by the two phonons. The D + D′ phonon energy (0.36 eV) is close to the energy difference between the transitions ES33 and ES44 given by E ) 2pVF/3dt, considering VF ) 1.0 × 106 m/s and dt ) 1.14 nm. Therefore, for highest excitation energies, when these optical transitions ES33 and ES44 are reached, a larger intensity for this second order feature is expected. In order to check the assumptions obtained from the discussion above, we describe here the dependence of the intensity of the D + D′-band on the presence of defects and on Elaser. The ratio between the integrated area of this band, assigned as D + D′, and the overtone of the G-band (2G) presents an almost linear dependence with the ID/IG ratio as shown in Figure 6b. This result shows that such as the D-band, the D + D′ band is strongly defect induced, following the same behavior as the D-band when the number of functional groups bonded to the nanotube surface is increased. The intensity of the D + D′ band also presents a dependence on the laser excitation energy. Figure 6c shows a dependence of the ratio between the intensities of D + D′ and the G′ band as a function of Elaser. As the intensity
Covalently Functionalized Single-Wall Carbon Nanotubes of G′-band is independent of Elaser,28 Figure 6c reflects basically the dependence of the intensity of the D + D′-band on Elaser. By comparing results obtained with different laser excitation energies, we observe that the ratio of the intensity of this band to the G′ band increases by increasing the laser energy (Figure 6c). The maximum intensity for the D + D′ band is observed for laser energies above 2.5 eV, where ES33 and ES44 optical transition energies connected by the two phonon participating of the process are resonant, in agreement with the double resonance model proposed. These observations corroborate the assumption of these second order features be assigned as a combination of two defect induced modes (D and D′ bands), originated from a double resonance mechanism involving two inelastic scattering by the two phonon modes and a elastic scattering by a defect. 4. Conclusions The presence of functional groups bounded to the nanotube surface causes significant changes in the Raman spectra of nanotubes due to both charge transfers and formation of defects. In particular, interesting changes were observed in the twophonon combination modes. In the spectral range associated with the IFM, the features originated from phonons far from the Γ-point are enhanced due to the increase in the amount of defects on the nanotubes. On the other hand, the intensities of the steplike dispersive features associated with a strongly selective resonance process are reduced due to the relaxation of the resonance condition caused by the charge transfer and displacement of Fermi level. The same was observed for the features in the range 1650-1900 cm-1, which are not related to the presence of defects. The feature observed at 2900 cm-1 was first assigned here as a combination of the D and D′ phonons, and in this case a defect is necessary to the momentum conservation. The ratio between this feature and the defect free overtone of D′ is strongly enhanced when the concentration of functional groups is increased. Moreover, this feature becomes more intense for higher excitation energies, because in this case ES33 and ES44 transition energies will be reached and connected by the phonons. In summary, the results reported here show that second-order modes can be very useful for the investigation and characterization of functionalized nanotubes and the results can be further extended to different functional groups and noncovalent functionalization. Acknowledgment. The authors thank the support of the “Rede Nacional de Pesquisa em Nanotubos de Carbono”, Ministry of Science and Technology, Brazil. C.F. acknowledges the fellowship from the Brazilian Agency CAPES for the postdoctoral research at University of Illinois (UIUC) where this work was partially developed. M.S.S. is grateful for funding from Beckman Foundation Young Investigator Award, a 3M Untenured Faculty Grant and the National Science Foundation for funding of this work.
J. Phys. Chem. C, Vol. 112, No. 34, 2008 13155 References and Notes (1) Sun, Y.-P.; Fu, K.; Lin, Y.; Huang, W. Acc. Chem. Res. 2002, 35, 1096. (2) Strano, M. S. J. Am. Chem. Soc. 2003, 125, 16148. (3) Price, B. K.; Tour, J. M. J. Am. Chem. Soc. 2006, 128, 12899. (4) Tchoul, M. N.; Ford, W. T.; Lolli, G.; Resasco, D. E.; Arepalli, S. Chem. Matter. 2007, 19, 5765. (5) Rafailov, P. M.; Thomsen, C.; D.-Weglikowska, U.; Roth, S. J. Phys. Chem. B 2008, 112, 5368. (6) Chen, Z.; Du, X.; Du, M.-H.; Rancken, C. D.; Cheng, H.-P.; Rinzler, A. G. Nano Lett. 2003, 3, 1245. (7) Usrey, M. L.; Nair, N.; Agnew, D. E.; Pina, C. F.; Strano, M. S. Langmuir 2007, 23, 7768. (8) Lee, C. Y.; Strano, M. S. J. Am. Chem. Soc. 2008, 130, 1766. (9) Strano, M. S.; Dyke, C. A.; Usrey, M. L.; Barone, P. W.; Allen, M. J.; Shan, H.; Kittrell, C.; Hauge, R. H.; Tour, J. M.; Smalley, R. E. Science 2003, 301, 1519. (10) Pan, H.; Feng, Y. P.; Lin, J. Y. Phys. ReV. B 2004, 70, 245425. (11) Veloso, M. V.; Souza Filho, A. G.; Mendes Filho, J.; Fagan, S. B.; Mota, R. Chem. Phys. Lett. 2006, 430, 71. (12) Cho, E.; Kim, H.; Kim, C.; Han, S. Chem. Phys. Lett. 2006, 419, 134. (13) Qi, P.; Vermesh, O.; Grecu, M.; Javey, A.; Wang, Q.; Dai, H.; Peng, S.; Cho, K. J. Nano Lett. 2003, 3, 347. (14) Souza Filho, A. G.; Jorio, A.; Samsonidze, Ge. G.; Dresselhaus, G.; Saito, R.; Dresselhaus, M. S. Nanotechnology 2003, 14, 1130. (15) Kim, U. J.; Furtado, C. A.; Liu, X.; Chen, G.; Eklund, P. C. J. Am. Chem. Soc. 2005, 127, 15437. (16) Graupner, R. J. Raman Spectrosc. 2007, 38, 673. (17) Fantini, C.; Usrey, M. L.; Strano, M. S. J. Phys. Chem. C 2007, 111, 17941. (18) Fantini, C.; Jorio, A.; Souza, M.; Strano, M. S.; Dresselhaus, M. S.; Pimenta, M. A. Phys. ReV. Lett. 2004, 93, 147406. (19) Telg, H.; Maultzsch, J.; Reich, S.; Hennrich, F.; Thomsen, C. Phys. ReV. Lett. 2004, 93, 177401. (20) Haroz, E. H.; Bachilo, S. M.; Weisman, R. B.; Doorn, S. K. Phys. ReV. B 2008, 77, 125405. (21) Saito, R.; Jorio, A.; Souza Filho, A. G.; Dresselhaus, G.; Dresselhaus, M. S.; Pimenta, M. A. Phys. ReV. Lett. 2002, 88, 027401. (22) Fantini, C.; Jorio, A.; Souza, M.; Ladeira, L. O.; Souza Filho, A. G.; Saito, R.; Samsonidze, Ge. G.; Dresselhaus, G.; Dresselhaus, M. S.; Pimenta, M. A. Phys. ReV. Lett. 2004, 93, 087401. (23) Brar, V. W.; Samsonidze, Ge. G.; Dresselhaus, M. S.; Dresselhaus, ¨ nlu¨, M. S.; Goldberg, B. B.; Souza Filho, G.; Saito, R.; Swan, A. K.; U A. G.; Jorio, A. Phys. ReV. B 2002, 66, 155418. (24) Baranov, A. V.; Bekhterev, A. N.; Bobovich, Y. S.; Petrov, V. I. Opt. Spectrosk. 1987, 62, 1036. (25) Thomsen, C.; Reich, S. Phys. ReV. Lett. 2000, 85, 5214. (26) Tuinstra, F.; Koenig, J. L. J. Chem. Phys. 1970, 53, 1126. (27) Canc¸ado, L. G.; Takai, K; Enoki, T.; Endo, M.; Kim, Y. A.; Mizusaki, H.; Jorio, A.; Coelho, L. N.; Magalhaes-Paniago, R.; Pimenta, M. A. Appl. Phys. Lett. 2006, 88, 163106. (28) Canc¸ado, L. G.; Jorio, A.; Pimenta, M. A. Phys. ReV. B 2007, 76, 064304. (29) Fantini, C.; Jorio, A.; Souza, M.; Saito, R.; Samsonidze, Ge. G.; Dresselhaus, M. S.; Pimenta, M. A. Phys. ReV. B 2005, 72, 085446. (30) Jishi, R. A.; Venkataraman, L.; Dresselhaus, M. S.; Dresselhaus, G. Chem. Phys. Lett. 1993, 209, 77. (31) Dubay, O.; Kresse, G. Phys. ReV. B 2003, 67, 035401. (32) Luo, Z.; Papadimitrakopoulos, F.; Doorn, S. K. Phys. ReV. B 2007, 75, 205438. (33) Ska´kalova´, V.; Roth, S.; Maultzsch, J.; Osva´th, Z.; Biro´, L. P. Phys. Status Solidi RRL 2007, 1, 138. (34) Ellis, A. V. J. Chem. Phys. 2006, 125, 121103. (35) Mafra, D. L.; Samsonidze, G.; Malard, L. M.; Elias, D. C.; Brant, J. C.; Plentz, F.; Alves, E. S.; Pimenta, M. A. Phys. ReV. B 2007, 76, 233407. (36) Saito, R. Dresselhaus, G. Dresselhaus, M. S. Physical Properties of Carbon Nanotubes; Imperial College Press: London, 1998.
JP803855Z
