OVERVIEW ON THE MAJOR RESEARCH ACTIVITIES ON CARBON NANOTUBES BEING DONE IN AMERICA, EUROPE AND ASIA - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 300

of SWCNTs is inversely correlated with the tube diameter and it is sensitive
to molecular encapsulation. However, since RBM frequencies of SWCNTs are
also deeply inluenced by tube chirality, excitation wavelength and mixture
of tubes with different diameters, it is necessary to measure the Raman
spectra by using several laser excitation lines. In particular, the encapsulation
of fullerene C 60 molecules inside SWCNTs (the complex formed is called
nanopeapod) under laser excitations tuned from 1.19 to 1.65 eV was recently
investigated. 196
Figure 9.31 depicts 2D RBM intensity maps of SWCNTs both as control
tubes in solution (9.31a) and as nanopeapods (9.31b). Typical 2 n + m family
patterns were observed between 175 and 185 cm -1 under the excitation of
1.23-1.27 eV and between 181 and 199 cm -1 under excitation wavelengths
of 1.31-1.36 eV. On the basis of the photoluminescence excitation (PLE) and
Raman results reported in a previous study, 197 the observed signals were
attributed to the 2 n + m = 28, 29, 31, 32 and 34 families. In the case of the C 60
nanopeapods, the RBM frequencies between 170 and 183 cm -1 showed lower
shifts of approximately 1-7 cm -1 . Moreover, two new peaks were observed
at 193 and 198 cm -1 under the excitation energies of ∼ 1.28 and ∼ 1.24 eV,
respectively. It was important to note that the RBM phonon frequency on
encapsulation was observed only in the case of SWCNTs with diameters
greater than 1.25 nm, which corresponded to the smallest limit for C 60
encapsulation. In other words, only tubes with a diameter bigger than 1.25
nm were able to encapsulate the fullerene, with a diameter dependence trend
that was totally comparable to the RBM phonon frequencies: such signals
were upshifted in small tubes after C 60 encapsulation because of the steric
hindrance provided by the C 60 molecules. In contrast, larger-diameter tubes
showed downshifts in RBM phonon frequency, which could be attributed to
the hybridisation of the electronic states of the SWCNTs and those of C 60 on
the fullerene encapsulation.
Figure 9.31 2D Raman intensity maps of RBM phonon regions of (a) SWCNT control
samples and (b) C 60 nanopeapods in SDBS-D 2 O solution. Reproduced with permission
from Joung et al . 196 Copyright (2009) by the American Physical Society. See also Colour
Insert.