Raman studies of suspensions and solutions of singlewall carbon nanotubes

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. INTRODUCTIONHomogeneous 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.