Deformation Behavior of the Raman Radial Breathing Modes of Single-Wall Carbon Nanotubes in Composites
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Deformation Behavior of the Raman Radial Breathing Modes of Single-Wall Carbon Nanotubes in Composites Marcel Lucas and Robert J. Young Manchester Materials Science Centre, University of Manchester and UMIST, Grosvenor Street, Manchester M1 7HS, United Kingdom
ABSTRACT Raman spectroscopy is a technique widely used to study the vibrational modes of carbon nanotubes. The low-frequency Radial Breathing Modes (RBMs) are frequently used to characterize carbon nanotube samples. We report a Raman spectroscopic study on the straininduced intensity variations of the RBMs of Single-Wall Carbon Nanotubes (SWNTs) in epoxy/SWNT composites. The RBM intensities have been found to vary significantly over a range of strain between -0.3% and 0.7%. The trend (increase or decrease) as well as the magnitudes of the intensity variation depends on the nanotube diameter and its chirality. Using tight-binding calculations, we have shown that these intensity variations can be explained entirely by resonance theory. Electronic density of states calculations confirm that the energy separation between the Van Hove singularities shifts with strain. The nanotubes are thus moved closer or further away from resonance, causing the intensity variations. It is demonstrated that through the use of resonance theory, a tentative chirality can be assigned to each type of SWNT from knowledge of its RBM position and the effect of strain upon the RBM intensity, thus determining its entire structure.
INTRODUCTION The interest for Single-Wall Carbon Nanotubes (SWNTs) has increased steadily since their discovery in 1993, due to their exceptional electronic and mechanical properties [1]. Their outstandingly high Young's modulus and their structure-dependent electronic properties make them ideal candidates for nanoscale sensors. Raman spectroscopy has proved to be a very powerful and non-destructive method to determine the nanotube structure [2]. The position of the Radial Breathing Modes (RBMs), which correspond to the collective radial movement of the carbon atoms, is inversely proportional to the nanotube diameter [2] and is frequently used to characterize the structure and quality of raw carbon nanotube materials [1]. According to resonance theory the intensity of the Raman bands depends on the laser excitation energy (Elaser). It reaches its maximum when Elaser matches the energy separation Eii between the Van Hove singularities (VHS) in the nanotube electronic density of states (DOS) [2]. Raman spectroscopy was recently reported to be a very effective tool to monitor the deformation of carbon nanotubes. The position of the G' band shifts to lower wave number under a uniaxial tensile strain [3] and to higher wave numbers upon the application of uniaxial compression or with pressure [4]. Under pressure, the RBM positions also shift to higher wave numbers [5], and there are also relative intensity variations of the RBMs not commented upon by
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the authors [5]. In such experiments, mechanical deformation induces changes in the electronic prope
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