Raman Features of Linear-Carbon-Chain and Multiwall Carbon Nanotube Composites

Structural and electronic properties of multiwall carbon nanotubes (MWCNTs) containing linear carbon chains (LCCs), which were produced by arc-discharge between carbon electrodes in an atmospheric pressure, have been studied by Raman spectroscopy as well as electron microscopy. Spectral features of Raman scattering from the LCC/MWCNT composites were reviewed with emphasis on the spectra obtained with a low energy photon (1.58 eV, 785 nm) excitation, which have not been described in detail so far. Characteristic frequencies of LCC stretching modes with the 785 nm laser excitation are observed at around 1740, 1759, and 1789 cm−1. In a low frequency region, radial breathing modes (RBMs) of the innermost tube within MWCNTs are observed at specific frequencies of 293, 341, 402, and 510 cm−1; the highest RBM frequency is tentatively assigned to a tube with the chiral index (4,3), whose diameter is expected to 0.50 nm. LCC bands observed with various excitation wavelengths from 488 to 785 nm show that the band consists of several peaks, and the relative intensities of constituent peaks change with the excitation wavelengths due to the resonance effect; the higher the excitation photon energy is, the higher the intensity of high-frequency LCC modes.