Biomedical Engineering Reference
In-Depth Information
The Raman spectrum of multi-walled carbon nanotunes (MWNTs) only shows a
single G band at
1360 cm 1 . Most of the spectral char-
acteristics that are used to distinguish single-walled carbon nanotubes from graphite
are not so obvious in MWNTs. The G -G splitting for large diameter MWNTs is
small and smeared out; as a result the G feature predominantly exhibits a weakly asym-
metric band, with a peak appearing at the graphite frequency of 1580 cm 1 . Therefore
it is diffi cult to differentiate the Raman signal of MWNTs from that of graphite.
1580 cm 1 and a D band at
15.4.1.2 Anisotropy of SWNT
The isolated SWNT acts as a dipolar antenna, polarized along the tube axis. Polarization
effects are very important for assessing aligned nanotubes. Generally the highest
Raman signal is observed when the incident light is polarized in the direction paral-
lel to the tube axis while almost no signal is observed when polarized perpendicular to
the axis (see Fig. 15.20) [135-137]. This anisotropic feature (highly effective for
nanotubes with a diameter within 0.4-2 nm) becomes less effi cient as the tube diameter
increases. The anisotropic feature of SWNT can be used to determine the orientation of
the tubes.
15.4.1.3 Single nanotube characterization
Designing nano-sized SWNT-based sensors requires the characterization of individual
nanotubes. Using the techniques of electron-beam lithography and nano-fabrication
Cronin et al. [138] have performed electrical and optical measurements on the same
individual nanotube. Figure 15.21 shows an atomic force microscope image of a single
carbon nanotube contacted by two metal electrodes. The white circle in the fi gure indi-
cates the approximate size and location of the laser spot when the spectrum on the right
was taken. By performing the in-situ measurements on the same nanotube the response
of a nanotube to changes in electrochemical potential was measured.
A 9 cm 1 upshift of the tangential mode (G band) vibrational frequency as well as a
90% decrease in intensity was observed by applying 1.0 V between an individual nano-
tube and a silver reference electrode in a dilute sulfuric acid solution.
15.4.1.4 Raman spectroscopy of modifi ed CNTs
Chemical modifi cation of CNTs is an essential step towards the fabrication of CNT-
based electrochemical sensors. Raman spectroscopy provides an effective way to mon-
itor the modifi cation process and to characterize the functionalized CNTs.
Modifi ed CNTs feature various spectral changes depending on the methods and the
location of modifi cations. These changes include variations in band frequencies, width,
and intensities. For example, aryldiazonium salts [139] were used to modify individual
sodium dodecyl sulphate (SDS) coated SWNTs with aryl group. The Raman spectrum
of functionalized (SDS-free) SWNTs shows a disorder mode much higher than pristine
SWNT; the radial breathing modes are nearly unobservable.
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