Chemistry Reference
In-Depth Information
Fig. 35 Absorption spectra
of SWCNTs produced by
HiPco, laser, and arc
methods. Reprinted with
permission from [
140
].
Copyright 2001 American
Chemical Society
two main features at 868 cm
1
and 1,590 cm
1
, very close to the A
2u
and E
1u
modes
of graphite, respectively [
143
]. In the last decade, having available theoretical
calculations on freestanding CNTs, it has been possible to assign IR active phonon
modes to the correspondent chiral indices, highlighting defined CNT structures.
For instance, semiconducting chiral nanotubes present one typical phonon mode at
5,386 cm
1
(0.67 eV), while metallic tubes are characterized by two signals at
9,791 cm
-1
(1.21 eV) and 9,172 cm
-1
(1.14 eV) [
144
].
Concerning the analysis of functionalized CNT samples, IR is particularly useful
for the characterization of oxidized tubes, since carboxylic acid or carboxylate
C
O stretching vibrations are characterized by strong absorptions at 1,719 cm
1
and 1,620 cm
1
respectively. Others chemical fragments easily identified by IR
spectroscopy on CNTs are the amide group (between 1,660 and 1,640 cm
1
), the
C-H bond with a vibration band around 2,900 cm
1
, the C-Cl stretching mode peak
at 798 cm
1
, and the peak between 3,200 and 3,500 cm
1
correspondent to the O-H
stretching [
145
].
Apart from the introduction of peaks typical of the organic appendage, other
relevant modifications can appear in the IR spectrum after CNTs functionalization.
For instance, covalent chemistry on the nanotubes perturbs the periodicity of the
lattice, opening gaps at the Fermi level and changing a metal CNT to a semicon-
ductor. As a consequence, the simultaneous weakening in the strength of the inter-
band transitions is itself a signal of covalent functionalization, both in semicon-
ducting and in metallic CNTs. Alternatively, ionic doping of CNTs with electron
acceptors leads on one hand to the complete removal of the peak at 5,386 cm
1
and
on the other to enhanced metal-like intraband transitions visible in the far-infrared
(FIR), by creating partially filled metallic bands [
145
].
ΒΌ
