Defect-induced vibrational response of multi-walled carbon nanotubes using resonance Raman spectroscopy S.A. Curran a) and J.A. Talla Physics Department, New Mexico State University, Las Cruces, New Mexico 88001 D. Zhang Physics Department and Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, New Mexico 88001 D.L. Carroll Department of Physics, Wake Forest University, Winston-Salem, North Carolina 27109 (Received 27 April 2005; accepted 30 August 2005) We systematically introduced defects onto the body of multi-walled carbon nanotubes through an acid treatment, and the evolution of these defects was examined by Raman spectroscopy using different excitation wavelengths. The D and D modes are most prominent and responsive to defect formation caused by acid treatment and exhibit dispersive behavior upon changing the excitation wavelengths as expected from the double resonance Raman (DRR) mechanism. Several weaker Raman resonances including D and L 1 (L 2 )+D modes were also observed at the lower excitation wavelengths (633 and 785 nm). In addition, specific structural defects including the typical pentagon-heptagon structure (Stone–Wales defects) were identified by Raman spectroscopy. In a closer analysis we also observed Haeckelite structures, specifically A g mode response in R 5,7 and O 5,6,7 . I. INTRODUCTION Carbon nanotubes have attracted much attention due to their unique dimensional, electrical, mechanical, and thermal properties. 1–4 However, the application of car- bon nanotubes relies on their potential for being incor- porated into various existing technologies, such as poly- mer composites, 5 solar cells, 6 and a variety of sensor devices. 7 Typically, a successful incorporation of a new component requires a good interfacial connection to fully exploit the properties of the new component and achieve the best overall performance. 8 A good interfacial inter- action can be achieved by making various components compatible at the molecular level through chemical func- tionalization. Thus, much effort has been devoted to the functionalization of carbon nanotubes. 9–12 Chemical functionalization causes structural defects on carbon nanotubes and can alter the electronic structure of the carbon tubes. 13,14 Thus, it is important to detect and char- acterize the formation and type of defects generated dur- ing functionalization. Raman spectroscopy has been extensively used in characterizing graphitic materials such as highly ordered pyrolytic graphite, 15,16 pyrolytic graphite, 16 graphitic whiskers, 17 and carbon nanotube. 18–22 As for carbon nanotubes, several Raman modes have been identified as disorder-induced modes, such as D mode (∼1350 cm -1 ) and D mode (∼1620 cm -1 ). 19,23–25 They have been ex- plained by a double resonance Raman mechanism, 24,26 which essentially involves elastic and inelastic scattering between two resonance states around the K point in two- dimensional (2D) Brillouin zone for graphite. This theory was first proposed by Thomsen and Reich 27 in explaining the curious excitation-energy dependence of the graphite D mode and was further extended to all six phonon modes in 2D graphite by Saito and Dresselhaus et al. 23,24 Many previously unassigned phonon modes have been successfully assigned, and new dispersive phonon modes have also been predicted. 23,24 Raman studies on the evolution of defects have previ- ously been reported for plasma bombardment of multi- walled carbon nanotubes (MWCNTs) and showed a significant devolepment of the defect mode (D) at 1620 cm -1 , which is linked to the E 2g in-plane vibrational response. 28 Further chemical treatment of MWCNTs with dye molecules allowed a spectroscopic analysis of the vibrational changes along the nanotube by examin- ing both the electronic and spectroscopic changes in the D line. 29 However, most chemical functionalization started with simple acid treatment, 30 which serves to a) Address all correspondence to this author. e-mail: shay@physics.nmsu.edu DOI: 10.1557/JMR.2005.0414 J. Mater. Res., Vol. 20, No. 12, Dec 2005 © 2005 Materials Research Society 3368