4.1 Introduction
Carbon is one of the most abundant elements in nature and has long been known to
exist in three forms: amorphous carbon, graphite, and diamond. However, after the
discovery of buckminsterfullerene C60 in 1985 an entirely new branch of carbon
chemistry has been created opening up a new era in materials science and nanotechnology. The discovery of fullerenes [1] and carbon nanotubes (CNTs) [2] has
attracted considerable attention in recent years because of their unique properties
such as small dimensions, relative low density, high strength-to-weight ratio, and
high electrical and thermal conductivity. Since then, carbon nanomaterials have
been demonstrated to be useful, for example, as energy materials, in the development of sensors [3, 4] or for drug delivery in medicine [5]. These exceptional
properties of CNTs have opened a wide range of promising applications like their
use in composite materials [6–9], where CNTs are expected to be one of the most
useful materials for improving the various properties of polymer matrix composites.
Another use of it has been suggested for hydrogen energy storage materials and
others [10–12]. Tremendous progress has been achieved in developing carbon
nanomaterials for high-performance energy conversion and storage systems [13].
Other aspects are concerned with their characterization by several analytical
methods when CNTs are found in environmental and biological samples; see recent
review [14]. Besides Raman, optical absorption spectroscopy within the ultraviolet,
visible and near-infrared ranges can be listed for their analysis. For morphological
studies, different microscopy techniques have been utilized like scanning and
transmission electron microscopy or atomic force microscopy. CNTs have seen
diverse commercial applications, where such materials have also been released into
the environment, posing a safety threat to those people who are exposed to these
nanomaterials. Therefore, monitoring and identification in particulate matter comprising CNTs become an important task, where toxicology aspects have also been
discussed [15]. On the other hand, therapeutic applications against cancer cells have
been suggested [16]. Other applications of CNTs can be found as sorbents for
solid-phase extraction and microextraction [17, 18] or as filters [19].
Nanotubes are one-dimensional wires having either metallic or semiconducting
nature, depending upon the orientation and chirality of the tube [20, 21]. A good
characterization of their structural and electronic properties is therefore of great
importance in material science. Raman spectroscopy has proven to be a nondestructive and nondissipative tool to characterize lower dimensional materials such
as nanostructures that have electronic and phonon properties different from the bulk
[22–24]. Raman spectroscopic measurements of CNTs provide a quick and simple
way to characterize them in terms of purity, diameter, chirality, conductivity, and
defects [25–29]. The first-order Raman spectra of multi-walled (MWCNT) and
single-walled CNTs (SWCNT) show a strong G-band corresponding to the E 2g
symmetry and two defect dependent D- and D’-bands, appearing due to the double
resonance effect, which is associated with elastic phonon scattering close to the
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A. K. Ojha and H. M. Heise
Carbon is one of the most abundant elements in nature and has long been known to
exist in three forms: amorphous carbon, graphite, and diamond. However, after the
discovery of buckminsterfullerene C60 in 1985 an entirely new branch of carbon
chemistry has been created opening up a new era in materials science and nanotechnology. The discovery of fullerenes [1] and carbon nanotubes (CNTs) [2] has
attracted considerable attention in recent years because of their unique properties
such as small dimensions, relative low density, high strength-to-weight ratio, and
high electrical and thermal conductivity. Since then, carbon nanomaterials have
been demonstrated to be useful, for example, as energy materials, in the development of sensors [3, 4] or for drug delivery in medicine [5]. These exceptional
properties of CNTs have opened a wide range of promising applications like their
use in composite materials [6–9], where CNTs are expected to be one of the most
useful materials for improving the various properties of polymer matrix composites.
Another use of it has been suggested for hydrogen energy storage materials and
others [10–12]. Tremendous progress has been achieved in developing carbon
nanomaterials for high-performance energy conversion and storage systems [13].
Other aspects are concerned with their characterization by several analytical
methods when CNTs are found in environmental and biological samples; see recent
review [14]. Besides Raman, optical absorption spectroscopy within the ultraviolet,
visible and near-infrared ranges can be listed for their analysis. For morphological
studies, different microscopy techniques have been utilized like scanning and
transmission electron microscopy or atomic force microscopy. CNTs have seen
diverse commercial applications, where such materials have also been released into
the environment, posing a safety threat to those people who are exposed to these
nanomaterials. Therefore, monitoring and identification in particulate matter comprising CNTs become an important task, where toxicology aspects have also been
discussed [15]. On the other hand, therapeutic applications against cancer cells have
been suggested [16]. Other applications of CNTs can be found as sorbents for
solid-phase extraction and microextraction [17, 18] or as filters [19].
Nanotubes are one-dimensional wires having either metallic or semiconducting
nature, depending upon the orientation and chirality of the tube [20, 21]. A good
characterization of their structural and electronic properties is therefore of great
importance in material science. Raman spectroscopy has proven to be a nondestructive and nondissipative tool to characterize lower dimensional materials such
as nanostructures that have electronic and phonon properties different from the bulk
[22–24]. Raman spectroscopic measurements of CNTs provide a quick and simple
way to characterize them in terms of purity, diameter, chirality, conductivity, and
defects [25–29]. The first-order Raman spectra of multi-walled (MWCNT) and
single-walled CNTs (SWCNT) show a strong G-band corresponding to the E 2g
symmetry and two defect dependent D- and D’-bands, appearing due to the double
resonance effect, which is associated with elastic phonon scattering close to the
124
A. K. Ojha and H. M. Heise