Surrounding effects in single-walled and multi-walled carbon nanotubes
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Surrounding effects in single-walled and multi-walled carbon nanotubes Jean-Pierre Buisson, Olivier Chauvet, Serge Lefrant, Christophe Stephan, Jean-Michel Benoit* Institut des Matériaux Jean Rouxel, Nantes University 2 rue de la Houssinière, BP32229, Nantes, France * also at Trinity College, Dublin, Ireland ABSTRACT The low frequency RBM observed in SWNTs has been proved to probe efficiently their diameter distribution. We have built a model to estimate the interactions between individual nanotubes when arranged in bundles which leads to a RBM upshift of 10 to 20 cm-1. In a PMMA-SWNTs composite, our model shows that an upshift can be also predicted as due to the stress applied by the polymer on the bundles upon breathing. Finally, the interaction between concentric layers in MWNTs can lead to low frequency modes originating from the RBM of individual tubes as observed experimentally. INTRODUCTION Carbon nanotubes are fascinating materials which are extensively studied nowadays, both from the fundamental point of view or for their potential applications [1]. Several techniques are used to characterized them. Indeed, electron or near field microscopies are well adapted to the study of these objects at the nanometric scale. On the other hand, one frequently needs to characterize the materials at a macroscopic scale. Raman spectroscopy is very powerful for this purpose[2]. First order Raman spectra of carbon nanotubes can be divided into three parts. The high frequency region (1500-1600cm-1) reveals the intense tangential modes of nanotubes. The 1200-1400 cm-1 part of the spectra shows the so-called D-band of graphitic materials which is sensitive to the degree of disorder. In this paper, we will focus on the low frequency region (100250 cm-1) of the spectrum which reveals the radial breathing mode (RBM) of single walled nanotubes (SWNTs). The RBM are especially interesting since they seem to be a unique signature of SWNTs. Furthermore their eigenfrequencies scale inversely with the nanotube diameters. The experimental low frequency Raman spectra give thus a picture of the nanotube diameter's distribution in macroscopic samples. Actually, the situation may be more complex. It was suggested recently that the breathing vibrations of SWNTs are affected when they are arranged in bundles [3-4]. Furthermore, low frequency modes have also been observed in multiwalled nanotubes (MWNTs) samples [5]. In this paper, we present theoretical considerations supported by experimental results which show that the RBM is affected by the surrounding of the individual tube. First we propose a simple model to reveal the bundle effect. We show how this effect is affected when embedding the SWNTs in a polymer matrix. It gives an indirect experimental proof of the bundle effect. Then we use the same model to explain why low frequency modes are also observed on MWNTs.
A14.12.1
EXPERIMENTAL SWNTs and MWNTs were produced by the arc discharge technique in GDPC Montpellier and TCD Dublin respectively. The SWNTs sample were purified according to
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