Raman spectra of SWCNTs

In the following sections, we first show the phonon dispersion relation of CNTs, and then the calculated results for the Raman intensity of a CNT are shown as a function of the polarisation direction. We also show the Raman calculation for a finite length of CNT, which is relevant to the intermediate frequency region. The enhancement of the Raman intensity is observed as a function of laser frequency when the laser excitation frequency is close to a frequency of high optical absorption, and this effect is called the resonant Raman effect. The observed Raman spectra of SWCNTs show resonant-Raman effects [5, 8], which will be given in the last section. 

Quantum effects are observed in the Raman spectra of SWCNTs through the resonant Raman enhancement process, which is seen experimentally by measuring the Raman spectra at a number of laser excitation energies. Resonant enhancement in the Raman scattering intensity from CNTs occurs when the laser excitation energy corresponds to an electronic transition between the sharp features (i.e., (E - ,)" type singularities at energy ,) in the ID electronic DOS of the valence and conduction bands of the carbon CNT. 

Raman spectra of SWCNTs as well as the most common experimental techniques of their characterization have been also thoroughly discussed in literature [3, 26]. The strongest Raman bands of SWCNTs are the RBM band (radial breathing mode in the range 100-300 cm ), and the G-band (tangential mode at around 1600 cm ) as shown in Fig. 7.2 [27]. Two more characteristic, but weaker bands are the D-band (disorder-induced band in the range 1300-1400 cm ) and the G -band (sometimes called D -band) at around 2600-2800 cm . ... 

In addition to many other publications dealing with the so-called G-band of the Raman spectra of SWCNT, we consider the structure of the fundamental bands together with the structure of their harmonics 2vg and 2vd, and their sums Vq + Vd and Vq + Vrbm- Figure 7.6a shows the review Raman spectra of SWCNT, single... 

Fig. 7.6 Review spectra of SWCNT (1), HOPG single crystal (2) and polyurethane/SWCNT (2%) composite (3) under excitation with 514.5 nm (a), and fragments of Raman spectra of SWCNT in the spectral region corresponding to D-band (b), G-band (c) and a region of the second-order harmonics 2vg, 2vd and Vq + (d) under excitation with 514.5 (1), 488 (2) and...

One can see that G-band is characterized with the highest intensity in the spectra (Fig. 7.6a). It should be noted as well that the SWCNT 2vd band is noticeably higher in intensity in comparison with the main D band, which can be seen from the lower curve 1. Broadband noise is typically not strong in Raman spectra of SWCNT but became more noticeable for the bulk systems (graphite and polymer-SWCNT composite, curves 2 and 3 in the Fig. 7.6a). SWCNT possess also a low-frequency vibration mode at Vrbm 160 cm corresponding to radial oscillations of carbon atoms in plane of the cross section, which exhibits a strong variation with nanotube diameter [7]. The SWCNT vibration spectra show a sum harmonic signal Vg+Vrbm with a spectral shape, which will be discussed below. 

Raman spectra of SWCNT in the range of D- and G-bands and the second-order bands under excitation with various wavelengths are shown in Fig. 7.6b-d,... 

Table 7.1 Spectral positions and half-widths of the main spectral components of the most intensive bands in Raman spectra of SWCNT and graphitic materials under study ...

Anomalous Changes of Vibration Modes Intensities and Anharmonicity in Raman Spectra of SWCNTs... 

Fig. 12.2 Comparison of high- and low-frequency Raman spectra of SWCNTs, DWCNTs, and small-diameter MWCNTs. Spectra were recorded using 633-nm laser excitation wavelength...

Carbon Nanotubes. An FT Raman band at 146 cm-1 was assigned as a radial breathing mode for bundles of SWCNT with a tube diameter of 1.53 nm.215 Oxidation of HipCO SWCNT (diameter 8-10 A) leads to irreversible disappearance of Raman breathing modes below 220 cm-1, in the temperature range 150 400 C.216 The Raman spectra of SWCNT produced by plasma-enhanced CVD gave evidence for the formation of single tubes at quite low temperatures (ca. 550°C).217... 

Resonance Raman excitation profiles were measured for individually dispersed SWCNT in aqueous solution (excitation 695-985 nm.).218 The IR and Raman spectra of SWCNT functionalised by long-chain hydrophobic groups show that the fundamental structure is unchanged on modification.219 A review has appeared of Raman data for SWCNT and DWCNT, especially the conversion of C60/SWCNT peapods into DWCNT at high temperatures.220... 

The Raman spectra of SWCNT (diameter 0.4 nm.) in a matrix of AlP04-5 zeolite single crystals gave dominant peaks due to (5,0), (4,2) and (3,3) structures.221,222 SWCNT have been grown in MCM-41 molecular sieve templated with C 2 or Ci6 alkyl chains. Their Raman spectra show that they are of uniform diameter (2.6 nm. for Ci2, 3.3 nm. for Ci6 systems).223 Raman spectra have been reported for superhard phases of SWCNT and C6o-224... 

Raman spectra of SWCNT/C03O4 film (upper line), C03O4 thin film (middle line), and the Si02/Si substrate (lower line). ... 

Kalbac, M., Kavan, L., Dunsch, L., and Dresselhaus, M.S. (2008) Development of the tangential mode in the Raman spectra of SWCNT bundles during electrochemical charging. Nano Lett, 8, 1257-1264. 

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