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 nanotubes 1,2 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 mo- lecular 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 sen- sors. 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 (E ii ) observed by both resonant Raman and optical absorption spectroscopies. 17 In the case of the covalent functionalization, an sp 3 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 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 co- valently to the nanotube side-wall through a reaction of carbon nanotubes individually suspended in aqueous solution with 4-Cl- benzene 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 second- order 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 Cl- phenyl groups were prepared on SiO 2 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. Micro- Raman measurements were performed using a Dilor XY triple- monochromator 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 * To whom correspondence should be addressed. † Centro de Desenvolvimento da Tecnologia Nuclear. ‡ Universidade Federal de Minas Gerais. § Massachusetts Institute of Technology. J. Phys. Chem. C 2008, 112, 13150–13155 13150 10.1021/jp803855z CCC: $40.75  2008 American Chemical Society Published on Web 08/05/2008