Raman studies of suspensions and solutions of singlewall carbon nanotubes
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Raman studies of suspensions and solutions of singlewall carbon nanotubes N. Izard1, A. Pénicaud2, E. Anglaret1 1-Groupe de Dynamique des Phases Condensées, UMR CNRS 5581, Université Montpellier 2, Montpellier, France 2-Centre de Recherche Paul Pascal, CNRS-Université Bordeaux 1, Pessac, France
ABSTRACT Raman spectroscopy is used to probe the structure and electronic properties of nanotubes dispersed in a liquid phase. We show that the radial breathing modes are upshifted in suspensions due to the molecular pressure of the solvent. On the other hand, we directly probe charge transfer in solutions of nanotube polyelectrolytes and its reversibility after oxydation in air.
INTRODUCTION Homogeneous dispersions of single wall nanotubes in liquids are required for optical applications and processing in materials science. Functionnalization leads to changes in the structure of the nanotubes [1]. An alternative and popular route is the preparation of aqueous supensions with the help of surfactants [2]. On the other hand, a new route was recently explored to prepare solutions of nanotubes, via chemical protonation [3] or chemical reduction in polar organic solvents [4]. Raman spectroscopy is the most popular technique to characterize samples of single wall carbon nanotubes (SWNT), because it provides reliable informations not only on their vibrational properties, but also on their structural and electronic properties [5,6]. Raman is also an effective technique to study nanotubes dispersed in liquids. However, one must be aware of possible new structural interactions and changes in the electronic properties due to a new physical and chemical environment. In this paper, we study the Raman signatures of suspensions and solutions of SWNT. We especially focus on the changes in the radial breathing mode frequency between powders and suspensions. We also investigate charge transfers in the liquid phase for nanotube polyelectrolyte solutions.
EXPERIMENTAL Single wall carbon nanotube samples were prepared by the electric arc technique (from GDPC and Nanoledge, Inc.). Aqueous suspensions were prepared using ionic (SDS) or anionic (Triton X100) surfactants. After a few hours of low-power sonication, homogeneous and stable suspensions of bundles are obtained, as stated by electron microscopy and Raman measurements (not shown). By contrast, high power sonication (500 W during 15 minutes) leads to exfoliation of the bundles [7]. After an ultracentrifugation step (120000 g during 4 hours), aqueous suspensions of individual SWNT are formed, as stated by electron microscopy, photoluminescence [8] and photon correlation spectroscopy [9] techniques. Solutions of reduced SWNT (anionic form) were prepared from Na salts [10]. Indeed, it was shown recently that Na(THF)SWNT salts dissolve spontaneously in DMSO and other polar solvents to form a nanotube polyelectrolyte solution [4]. Since such solutions are not stable in air, we studied them both in sealed cells and after oxydation in air. The Raman experiments were carried out on a Jo
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