INFLUENCE OF VAN DER WAALS INTERACTIONS ON THE RAMAN MODES IN SINGLE WALLED CARBON NANOTUBES A. M. Rao 1 , J. Chen 2 , E. Richter 3 , U. Schlecht 4 , P. C. Eklund 5 , R. C. Haddon 6 , U. D. Venkateswaran 7 , Y.-K. Kwon 8 and D. Tománek 9 1 Department of Physics & Astronomy, Clemson University, Clemson, SC 29634 2 Zyvex Corporation, Richardson, TX 75081 3 Department of Physics & Astronomy, University of Kentucky, Lexington, KY 40506 4 Max-Planck Institute for Solid State Research, 70569 Stuttgart, Germany 5 Department of Physics, Pennsylvania State University, University Park PA 16802, 6 Department of Chemistry, University of California, Riverside, CA 92521 7 Department of Physics, Oakland University, Rochester, MI 48309 8 Department of Physics, University of California, Berkeley, CA 94720 9 Department of Physics and Astronomy & Center for Fundamental Materials Research, Michigan State University, East Lansing, MI 48824 (e-mail: arao@clemson.edu) Raman spectroscopy has been used extensively to characterize SWNT bundles that are synthesized by the pulsed laser vaporization (PLV) or the electric arc (EA) methods [1-4]. A typical SWNT bundle contains on the order of 100 well-aligned SWNTs that are held together by the van der Waals (vdW) forces in a closed- packed triangular lattice. The room temperature Raman spectrum of the EA- derived SWNT bundles obtained using the 1064 nm excitation wavelength exhibits two prominent features at ω R ~160 cm -1 (radial band) and ω T ~1590 cm -1 (tangential band) (see Fig. 1). Three modes, previously identified with A 1g , E 1g and E 2g symmetries and frequencies that are nearly independent of the tube diameter, are expected near the 1590 cm -1 band for achiral tubes [5]. For the chiral tubes, six modes are Raman-active with A 1 , E 1 and E 2 symmetries [5]. In contrast to the high frequency band at 1590 cm -1 , the low frequency band centered ~160 cm -1 is identified with A 1g radial breathing modes whose frequency is strongly dependent on the tube diameter [5]. Consistent with the calculated mode frequencies and intensities, other weak Raman-active features have been observed in the intermediate phonon frequency region between ω R and ω T [1]. The position and the lineshape of these bands have been used extensively to determine the SWNT diameter distribution [3] and semiconducting/metallic nature of SWNTs [6]. In this paper, we present a Raman study of bundled and isolated SWNTs, and discuss the influence of vdW interaction on the vibrational and electronic properties of SWNT bundles as they are separated into isolated SWNTs. Recently, Chen et al. [4] reported the synthesis of soluble shortened and full-length single-walled carbon nanotubes. Details of the preparation and characterization of the solubilized tubes (S-SWNTs) are described in Ref. 4. Atomic force microscope (AFM) images showed that the majority of the bundled SWNTs were separated into small bundles (2-5 nm in diameter) and isolated tubes during the solubilization process [4]. In Fig. 1, the Raman spectrum of S-SWNTs in CS 2 is compared to that obtained for bundled SWNTs, the latter in powder form. The S-SWNTs and bundled SWNTs referred to in Fig. 1 are from the same as- prepared EA material. For an isolated SWNT, theoretical calculations have shown that the radial mode frequency ω R ’ exhibits a particularly simple dependence on the tube diameter d’ as [3], ω R ’ ~ 224 cm -1 ·nm / d’. (1)