Far Infrared Characterization of Single and Double Walled Carbon Nanotubes
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Far Infrared Characterization of Single and Double Walled Carbon Nanotubes S. G. Chou1, Z. Ahmed1, G.G. Samsonidze2, J. Kong3, M. S. Dresselhaus3, 4, D. F. Plusquellic1 1 Physical Measurement Laboratory, National Institute of Standard and Technology, Gaithersburg, MD20899 2. Department of Physics, University of California and Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA94720 3. Department of EECS and 4. Department of Physics, Massachusetts Institute of Technology, Cambridge, MA02139 ABSTRACT High resolution far infrared absorption measurements were carried out for single walled and double walled carbon nanotubes samples (SWCNT and DWCNT) encased in a polyethylene matrix to investigate the temperature and bundling effects on the low frequency phonons associated with the low frequency circumferential vibrations. At a temperature where kBT is significantly lower than the phonon energy, the broad absorption features as observed at room temperature become well resolved phonon transitions. For a DWCNT sample whose inner tubes have a similar diameter distribution as the SWCNT sample studied, a series of sharp features were observed at room temperature at similar positions as for the SWCNT samples studied. The narrow linewidth is attributed to the fact that the inner tubes are isolated from the polyethylene matrix and the weak inter-tubule interactions. More systematic studies will be required to better understand the effects of inhomogeneous broadening and thermal-excitation on the detailed position and lineshape of the low frequency phonon features in carbon nanotubes. INTRODUCTION The detailed phonon structure of carbon nanotubes determines the mechanical and thermal transport properties for nanotube-based materials and devices1. At room temperature, the thermal transport properties of nanotubes are mostly determined by optical phonons, whereas at a temperature significantly lower than the Debye temperature, the detailed thermal properties of carbon nanotubes are largely determined by low-frequency, acoustic-branch phonons2, 3. In most vibrational spectroscopic studies of SWCNT, the energies for phonons could be derived from the zone folding scheme, whereby the phonon dispersion relation of 2D graphene is folded into the 1D Brillouin zone for the SWCNTs4. The approach, in most cases, yields predictions that are consistent with both resonance Raman and IR spectroscopic findings 5, 6. On the other hand, special theoretical considerations are required in the case of the lower frequency modes derived from the acoustic branches of graphene because some of the in and out of plane acoustic branch modes associated with graphitic translation in nanotubes cannot be expressed exactly in the zone folding scheme4. A number of theoretical models have been developed based on elasticity theory and mass-spring constants to predict the structures of these phonon modes in the low energy region for single walled carbon nanotubes, usually associated with circumferential vibrations, but the detailed experim
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