Electrochemical sensors based on carbon nanotubes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 482

G

RBM

G

2 nd

D

0 1000 2000 3000

Frequency (cm 1 )

FIGURE 15.17

Raman spectrum of SWNT. (Reprinted with permission from [134]. Copyright (2004)

The Royal Society.)

The radial breathing mode (RBM) appearing in the 160 to 300 cm 1 region is associ-
ated with a symmetric movement of all carbon atoms in the radial direction (Fig. 15.18a)
[128]. The frequency of the radial breathing mode is inversely proportional to the diam-
eter of the individual nanotube through the relation

ν RBM A/d t B , where the A and

248 cm 1 nm and

B parameters are determined experimentally. For isolated SWNTs, A

B 0 have been found [129]. In nanotube aggregates, the weak intertubule coupling via

van der Waals interactions normally results in a 6-20 cm 1 shift to higher frequencies
due to space restrictions imposed by neighboring tubes [130]. For typical SWNT bundles
in the diameter range d t 10 cm 1 (where B

refl ects the tube-tube interaction) have been found [131]. For large diameter tubes ( d t

234 cm 1 nm and B

1.5 0.2 nm, A

2 nm) the intensity of the RBM feature is weak and is hardly observable.
The tangential G-band (or high energy mode) around 1580 cm 1 also provides a
signature of carbon nanotube. The G mode in SWNTs gives rise to a multi-peak fea-
ture which originates from the symmetry breathing of the tangential vibration when
the graphite sheet is rolled to make a cylindrically shaped tube (see Fig. 15.18b). The
two most intense G peaks are labeled G , for modes with atomic displacements along
the tube axis, and G , for modes with atomic displacement along the circumferential
direction (see Fig. 15.18b). The G bands for semiconductive and metallic SWNTs
are different in the line shape and frequencies (Fig. 15.19), i.e. broadened for metallic
SWNTs while sharp and weak for semiconductive SWNTs [128]. This feature can be
used to distinguish between the two types of SWNTs. The G mode shifts to lower
frequencies with decreasing diameter, and this decrease becomes larger as the curva-
ture of the sheet increases [132, 133].