Defect-induced vibrational response of multi-walled carbon nanotubes using resonance Raman spectroscopy
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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 L1 (L2) + 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 Ag mode response in R5,7 and O5,6,7.
I. INTRODUCTION
Carbon nanotubes have attracted much attention due to their unique dimensional, electrical, mechanical, and thermal properties.1–4 However, the application of carbon nanotubes relies on their potential for being incorporated into various existing technologies, such as polymer 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 interaction can be achieved by making various components compatible at the molecular level through chemical functionalization. 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 characterize the formation and type of defects generated during functionalization. Raman spectroscopy has been extensively used in characterizing graphitic materials such as highly ordered
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0414 3368
J. Mater. Res., Vol. 20, No. 12, Dec 2005
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 explained by a double resonance Raman mechanism,24,26 which essentially involves elastic and inelastic scattering between two resonance states around the K point in twodimensional (2D) Brillouin zone for graphite. This theory was first proposed by Thomsen and Reich27 in explaining the curious excitation-energy dependence of the graphite D mode and was further extended t
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