Identification of the constituents of double-walled carbon nanotubes using Raman spectra taken with different laser-exci
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S.G. Chou Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Wencai Ren Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, People’s Republic of China
J.A. Gardecki George R. Harrison Spectroscopy Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
¨ nlu¨, and B.B. Goldberg A.K. Swan, M.S. U Department of Electrical and Computer Engineering, Boston University, Boston, Massachusetts 02215
Hui-Ming Chenga) Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, People’s Republic of China
M.S. Dresselhaus Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 28 November 2002; accepted 25 February 2003)
Double-walled carbon nanotubes (DWNTs), synthesized by the catalytic decomposition of methane, were explored by resonance Raman spectroscopy using different energies for laser excitation. Based on the radial breathing mode frequencies, the indices of the two layers of a DWNT were approximately assigned, depending on the interlayer separation of the two coaxial layers of the DWNT, which ranged from 0.34 to 0.40 nm. From the tentatively assigned results, it was found that the two walls of the DWNT are not strongly selected by chirality and diameter. The results, however, suggest that, for the tubes that are resonant with the available laser excitation energies, most of the outer layers of the observed DWNTs in our samples are semiconducting, while the inner layers of the observed DWNTs are either semiconducting or metallic based on the assembled DWNTs. The characteristics of the G, D, and G⬘ band of the DWNTs are discussed, and a double peak feature in the D and G⬘ band, originating from the inner and outer layers of the DWNTs, is reported. I. INTRODUCTION
In general, carbon nanotubes can be classified into multi-walled carbon nanotubes (MWNTs) and singlewalled carbon nanotubes (SWNTs). Double-walled carbon nanotubes (DWNTs), which can be considered as two coaxial SWNTs, are a limiting case between SWNTs and MWNTs, and are very important from both a theoretical and experimental standpoint.1 SWNTs can be either semiconducting (S) or metallic (M) depending on
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 5, May 2003
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their diameters and chiralities. Therefore, DWNTs can be divided into four I-O types: S-S, S-M, M-S, and M-M, where the sequence I-O refers to the inner and outer layers, respectively. Thus, an S-M or M-S DWNT can be, respectively, a molecular conductive wire covered by an insulator or a molecular capacitor in a memory device.1 Understanding the growth mechanism of DWNTs might help us find a way to control the growth process to obtain the desired DWNT for device applications. Recently, DWNTs have receiv
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