**1. Introduction**

As a nondestructive chemical analysis technique, Raman spectroscopy has become a powerful research tool providing detailed information about chemical structure and identity, phase and polymorphism, molecular interactions and crystallinity. Raman spectrum is a distinct chemical fingerprint for a particular molecule or material and it can be used to quickly identify the sample, or distinguish it from others. Therefore, Raman spectroscopy may be used in any application where nondestructive, microscopic chemical analysis or imaging is required. The use of Raman spectroscopy initially originating in physics and chemistry analysis has now spread to a variety of applications in materials science [1] or even in biology for ultrafast reveling of bacteria [2] and medicine [3].

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

Recently, carbon materials have revolutionized the field of material science. In order to illustrate the importance of Raman spectroscopy in the field of carbon nanocomposites a short description of some representative carbon allotropes will be exposed. Due to its unique electronic structure, carbon is an element available in a variety of structural forms being able to form sp3 , sp2 and sp hybridization networks more stable than any other element. Carbon nanomaterials offer a wide range of useful properties such as large specific area [4], excellent electrical conductivity [5], high Young's modulus [6] and thermal conductivity [7]. Raman spectroscopy is an important tool for the characterization of carbon nanomaterials offering valuable information about the existence of structure defects or further functionalization.

carbon atoms [11]. Having excellent thermal, mechanical, electrical and barrier properties [12], graphene is recommended for many applications such as: electronics [13], antimicrobial materials [14], construction materials [15], battery [16] and supercapacitors etc. More details about

In order to differentiate these materials there is a strong demand for techniques that can be used to characterize them. As a remarkably sensitive technique, Raman spectroscopy suits perfectly on these demands, being highly responsive to symmetric covalent bonds with very little or no dipole moment. Raman spectroscopy is capable of discerning any slight changes

When monochromatic radiation is incident upon a carbon allotrope sample the light will interact with the sample in a specific way. Every bond in the Raman spectrum corresponds directly to a specific molecular bond vibration, including bonds such as C-C, C=C, C-H etc. As a chemical fingerprint of the material, the general spectrum profile (peak position and intensity) provides unique information which can be used to identify the material and distinguish

When comparing the Raman spectra of two carbon allotropes – diamond and graphite – significant differences between these two materials can be noticed even if both are entirely made of C-C bonds (**Figure 1**). The Raman spectrum of pure diamond exhibits an extremely sharp signal at ~1332 cm−1. The Raman spectrum of graphite shows different features compared to diamond. Two distinguishable peaks are revealed at ~1350 cm−1 (D band) and ~1580 cm−1 (G band). The G band arises from the stretching of the C-C bond in graphitic

the graphite spectrum reveals that graphite is not as uniform in structure as diamond. The D-mode in graphite is induced by disorder or defects and increases linearly with decreasing

carbon systems. The presence of an additional band in

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graphene and other graphene derivatives will be exposed in section 2 of this chapter.

in structure.

it from others.

materials and it is common to all sp2

graphite crystallite size (**Figure 2**).

**Figure 1.** Structure of the most representative carbon allotropes.

Although carbon materials are all entirely made of C-C bonds, the orientation of these bonds is different for each type of carbon allotrope. All these materials are exclusively composed of pure carbon but are different structural forms and exhibit quite different physical properties and chemical behavior. Carbon exists in two allotropic forms: crystalline allotropes (diamond, graphite, fullerene, carbon nanotubes) and amorphous allotropes (carbon black, coke, charcoal).

Diamond is the hardest material on earth and finds applications in cutting, drilling, and jewelry. In diamond structure each carbon atom undergoes sp3 hybridization and it is linked with four other carbon atoms in a tetrahedral structure. Diamond does not conduct electricity because it does not exhibit any delocalized electrons.

Graphite has a layered structure and all these layers are held by Van der Waals forces. In graphite structure each carbon atoms is sp2 hybridized and each layer is composed of hexagonal rings of carbon atoms.

Fullerenes are made by heating graphite in an electric arc in the presence of inert gas. These carbon allotropes are cage like structures, with all carbon atoms sp<sup>2</sup> hybridized.

Carbon nanotubes (CNTs) exhibit the form of cylindrical carbon molecules and exhibit unique features that make them extremely useful in a plethora of applications especially in nanotechnology, electronics, optics and many other fields of materials science. A carbon nanotube can be defined as a tube-shaped material, entirely made from carbon, having the diameter measuring on the nanometer scale. A carbon nanotube can be as thin as a few nanometers but as long as hundreds of microns [8]. CNTs are at least 100 times stronger than steel, but only one sixth as heavy, so nanotube fibers could strengthen almost any material [9]. Nanotubes may conduct heat and electricity better than copper. CNT are already incorporated in polymer composites to control or enhance conductivity. Carbon nanotubes may be classified as singlewall (SWCNTs) or multi-wall nanotubes (MWCNTs). SWCNTs can be simply envisaged like a regular tube entirely made of carbon atoms. In contrast to single-wall carbon nanotubes, the MWCNTs are an assemblage of an outer and at least one inner carbon tube separated one from another by interatomic forces.

Graphene is a thin single layer of pure carbon. Graphene was firstly isolated in 2004 by two researchers from The University of Manchester using the "scotch-tape" technique [10]. Basically graphene consists in a monolayer of graphite. In more complex terms, graphene is a two dimension honeycomb single layer crystal lattice formed by the tightly packed sp<sup>2</sup> bonded carbon atoms [11]. Having excellent thermal, mechanical, electrical and barrier properties [12], graphene is recommended for many applications such as: electronics [13], antimicrobial materials [14], construction materials [15], battery [16] and supercapacitors etc. More details about graphene and other graphene derivatives will be exposed in section 2 of this chapter.

Recently, carbon materials have revolutionized the field of material science. In order to illustrate the importance of Raman spectroscopy in the field of carbon nanocomposites a short description of some representative carbon allotropes will be exposed. Due to its unique electronic structure, carbon is an element available in a variety of structural forms being able to

nanomaterials offer a wide range of useful properties such as large specific area [4], excellent electrical conductivity [5], high Young's modulus [6] and thermal conductivity [7]. Raman spectroscopy is an important tool for the characterization of carbon nanomaterials offering valuable information about the existence of structure defects or further functionalization.

Although carbon materials are all entirely made of C-C bonds, the orientation of these bonds is different for each type of carbon allotrope. All these materials are exclusively composed of pure carbon but are different structural forms and exhibit quite different physical properties and chemical behavior. Carbon exists in two allotropic forms: crystalline allotropes (diamond, graphite, fullerene, carbon nanotubes) and amorphous allotropes (carbon black, coke,

Diamond is the hardest material on earth and finds applications in cutting, drilling, and jew-

with four other carbon atoms in a tetrahedral structure. Diamond does not conduct electricity

Graphite has a layered structure and all these layers are held by Van der Waals forces. In

Fullerenes are made by heating graphite in an electric arc in the presence of inert gas. These

Carbon nanotubes (CNTs) exhibit the form of cylindrical carbon molecules and exhibit unique features that make them extremely useful in a plethora of applications especially in nanotechnology, electronics, optics and many other fields of materials science. A carbon nanotube can be defined as a tube-shaped material, entirely made from carbon, having the diameter measuring on the nanometer scale. A carbon nanotube can be as thin as a few nanometers but as long as hundreds of microns [8]. CNTs are at least 100 times stronger than steel, but only one sixth as heavy, so nanotube fibers could strengthen almost any material [9]. Nanotubes may conduct heat and electricity better than copper. CNT are already incorporated in polymer composites to control or enhance conductivity. Carbon nanotubes may be classified as singlewall (SWCNTs) or multi-wall nanotubes (MWCNTs). SWCNTs can be simply envisaged like a regular tube entirely made of carbon atoms. In contrast to single-wall carbon nanotubes, the MWCNTs are an assemblage of an outer and at least one inner carbon tube separated one

Graphene is a thin single layer of pure carbon. Graphene was firstly isolated in 2004 by two researchers from The University of Manchester using the "scotch-tape" technique [10]. Basically graphene consists in a monolayer of graphite. In more complex terms, graphene is a

two dimension honeycomb single layer crystal lattice formed by the tightly packed sp<sup>2</sup>

hybridization and it is linked

hybridized and each layer is composed of hexago-

hybridized.

bonded

elry. In diamond structure each carbon atom undergoes sp3

carbon allotropes are cage like structures, with all carbon atoms sp<sup>2</sup>

because it does not exhibit any delocalized electrons.

graphite structure each carbon atoms is sp2

from another by interatomic forces.

nal rings of carbon atoms.

and sp hybridization networks more stable than any other element. Carbon

form sp3

180 Raman Spectroscopy

charcoal).

, sp2

In order to differentiate these materials there is a strong demand for techniques that can be used to characterize them. As a remarkably sensitive technique, Raman spectroscopy suits perfectly on these demands, being highly responsive to symmetric covalent bonds with very little or no dipole moment. Raman spectroscopy is capable of discerning any slight changes in structure.

When monochromatic radiation is incident upon a carbon allotrope sample the light will interact with the sample in a specific way. Every bond in the Raman spectrum corresponds directly to a specific molecular bond vibration, including bonds such as C-C, C=C, C-H etc. As a chemical fingerprint of the material, the general spectrum profile (peak position and intensity) provides unique information which can be used to identify the material and distinguish it from others.

When comparing the Raman spectra of two carbon allotropes – diamond and graphite – significant differences between these two materials can be noticed even if both are entirely made of C-C bonds (**Figure 1**). The Raman spectrum of pure diamond exhibits an extremely sharp signal at ~1332 cm−1. The Raman spectrum of graphite shows different features compared to diamond. Two distinguishable peaks are revealed at ~1350 cm−1 (D band) and ~1580 cm−1 (G band). The G band arises from the stretching of the C-C bond in graphitic materials and it is common to all sp2 carbon systems. The presence of an additional band in the graphite spectrum reveals that graphite is not as uniform in structure as diamond. The D-mode in graphite is induced by disorder or defects and increases linearly with decreasing graphite crystallite size (**Figure 2**).

**Figure 1.** Structure of the most representative carbon allotropes.

**Figure 2.** The Raman spectra for a) natural diamond and c) synthesized diamond compared with b) Raman spectrum of graphite (Yao K et al. (2017) copyrights) [17].

Having remarkable properties such as high electrical conductivity, very high tensile strength and low thermal expansion coefficient, CNTs have been investigated for many applications such as composite materials, microelectronics and electronic components, solar cells, energy storage devices [24] etc. In addition, their one-dimensional structure makes them an ideal platform for biomedical applications. Due to their important applications a significant number of methods to produce CNTs were developed: arc discharge method [25], laser method [26], chemical vapor deposition (CVD) [27], ball milling [28], etc. Final properties of CNTs are

**Figure 3.** Raman spectra for C60 and C70 films. The insets show the molecular structures for C60 and C70 (Zhang X et al.

In **Figure 4** a comparison of Raman spectra of graphite, SWCNTS and MWCNTs is depicted. The stretching of the C-C bond in graphitic materials gives rise to the so-called G-band Raman

and nanotubes but is not used for distinguishing one carbon nanostructure from another. The

any modification to the structure of graphene, such as the strain induced by external forces in multiwall nanotubes, or even by the curvature of the side wall when growing a SWCNT [29]. A prominent G band can be noticed in the graphite Raman spectrum at ~1580 cm−1. As one can see the G band is present also in the SWCNT and MWCNT spectra but with different width. The G-band of investigated SWCNTs splits in two band components because of large diameter

Another important band in the Raman spectra of the investigated nanotubes at ~1350 cm−1 was observed known as the D band. The D band is caused by disordered structure of gra-

Raman spectra as one can see in the Raman spectrum of CNTs making Raman spectroscopy

of CNTs the D band is significantly increased compared to graphite. The increase of the ratio

nanotubes and it can be used to distinguish metallic and semiconducting nanotubes.

one of the most sensitive techniques to characterize disorder in sp2

carbon systems. This spectral feature is similar for graphite

carbon materials and can be used to investigate

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carbon materials. In case

dependent on the production and purification methods.

feature which is common to all sp2

(2016) copyrights) [21].

G-band is highly sensitive to strain effects in sp<sup>2</sup>

phene sheets. The presence of disorder in sp2

More complex structures can be also investigated by Raman spectroscopy. Fullerenes have attracted much attention for their applications in non-linear optics [18] or biomedical devices [19]. C60 (also called Buckminster fullerene) and C70 have been investigated in a large number of experiments because of their potential applications fabrication of nanodevices such as field effect transistors and flat panel display devices based on field emission [20].

**Figure 3** compares the Raman spectra of C60 and C70. The Raman spectrum of C60 exhibited strong signals at 1467 cm−1 and 1567 cm−1. This fact reveals that C60 is composed of sp2 bonded carbon and the sharpness of the signal shows that C60 exhibits a uniform structure. On the contrary, the Raman spectrum of C70 exhibits numerous other peaks. In case of C70 film, the main peaks are located at 1564 and 1228 cm−1 due to a reduction in molecular symmetry which results in more Raman bands. Their relative intensities strongly depend on the excitation laser wavelength.

Until 1980 only four carbon allotropes were known: graphite, amorphous carbon, fullerenes and diamond. Since their discovery in 1991 by Dr. Sumio Ijima, carbon nanotubes (CNTs) have gained tremendous attention as a versatile nanomaterial with abundant applications. Like previously mentioned, CNTs are carbon allotropes with a cylindrical nanostructure.

CNTs are essentially rolled up graphene sheets that have been sealed to form hollow tubes (**Figure 1**). Depending on the number of concentrically rolled-up graphene sheets, CNTs are classified to single-walled (SWCNT), double-walled (DWCNT), and multi-walled CNTs (MWCNT). The structure of SWNT can be imagined by wrapping a one-atom-thick layer of graphene into a seamless cylinder. DWCNT is considered as a special type of MWCNT wherein only two rolled up graphene sheets are present. MWCNT consists of two or more numbers of rolled-up concentric graphene sheets [22]. The diameter of SWCNT is generally up to 2 nm. In case of MWCNTs the diameter varies from 5 to 20 nm, seldom exceeding 100 nm [23].

**Figure 3.** Raman spectra for C60 and C70 films. The insets show the molecular structures for C60 and C70 (Zhang X et al. (2016) copyrights) [21].

More complex structures can be also investigated by Raman spectroscopy. Fullerenes have attracted much attention for their applications in non-linear optics [18] or biomedical devices [19]. C60 (also called Buckminster fullerene) and C70 have been investigated in a large number of experiments because of their potential applications fabrication of nanodevices such as

**Figure 2.** The Raman spectra for a) natural diamond and c) synthesized diamond compared with b) Raman spectrum of

**Figure 3** compares the Raman spectra of C60 and C70. The Raman spectrum of C60 exhibited

carbon and the sharpness of the signal shows that C60 exhibits a uniform structure. On the contrary, the Raman spectrum of C70 exhibits numerous other peaks. In case of C70 film, the main peaks are located at 1564 and 1228 cm−1 due to a reduction in molecular symmetry which results in more Raman bands. Their relative intensities strongly depend on the excitation laser

Until 1980 only four carbon allotropes were known: graphite, amorphous carbon, fullerenes and diamond. Since their discovery in 1991 by Dr. Sumio Ijima, carbon nanotubes (CNTs) have gained tremendous attention as a versatile nanomaterial with abundant applications. Like previously mentioned, CNTs are carbon allotropes with a cylindrical nanostructure.

CNTs are essentially rolled up graphene sheets that have been sealed to form hollow tubes (**Figure 1**). Depending on the number of concentrically rolled-up graphene sheets, CNTs are classified to single-walled (SWCNT), double-walled (DWCNT), and multi-walled CNTs (MWCNT). The structure of SWNT can be imagined by wrapping a one-atom-thick layer of graphene into a seamless cylinder. DWCNT is considered as a special type of MWCNT wherein only two rolled up graphene sheets are present. MWCNT consists of two or more numbers of rolled-up concentric graphene sheets [22]. The diameter of SWCNT is generally up to 2 nm. In

case of MWCNTs the diameter varies from 5 to 20 nm, seldom exceeding 100 nm [23].

bonded

field effect transistors and flat panel display devices based on field emission [20].

wavelength.

182 Raman Spectroscopy

graphite (Yao K et al. (2017) copyrights) [17].

strong signals at 1467 cm−1 and 1567 cm−1. This fact reveals that C60 is composed of sp2

Having remarkable properties such as high electrical conductivity, very high tensile strength and low thermal expansion coefficient, CNTs have been investigated for many applications such as composite materials, microelectronics and electronic components, solar cells, energy storage devices [24] etc. In addition, their one-dimensional structure makes them an ideal platform for biomedical applications. Due to their important applications a significant number of methods to produce CNTs were developed: arc discharge method [25], laser method [26], chemical vapor deposition (CVD) [27], ball milling [28], etc. Final properties of CNTs are dependent on the production and purification methods.

In **Figure 4** a comparison of Raman spectra of graphite, SWCNTS and MWCNTs is depicted. The stretching of the C-C bond in graphitic materials gives rise to the so-called G-band Raman feature which is common to all sp2 carbon systems. This spectral feature is similar for graphite and nanotubes but is not used for distinguishing one carbon nanostructure from another. The G-band is highly sensitive to strain effects in sp<sup>2</sup> carbon materials and can be used to investigate any modification to the structure of graphene, such as the strain induced by external forces in multiwall nanotubes, or even by the curvature of the side wall when growing a SWCNT [29]. A prominent G band can be noticed in the graphite Raman spectrum at ~1580 cm−1. As one can see the G band is present also in the SWCNT and MWCNT spectra but with different width. The G-band of investigated SWCNTs splits in two band components because of large diameter nanotubes and it can be used to distinguish metallic and semiconducting nanotubes.

Another important band in the Raman spectra of the investigated nanotubes at ~1350 cm−1 was observed known as the D band. The D band is caused by disordered structure of graphene sheets. The presence of disorder in sp2 -hybridized carbon systems results in resonance Raman spectra as one can see in the Raman spectrum of CNTs making Raman spectroscopy one of the most sensitive techniques to characterize disorder in sp2 carbon materials. In case of CNTs the D band is significantly increased compared to graphite. The increase of the ratio

single layer of graphene on a silicon oxide substrate by peeling the graphite by micromechanical cleavage [10]. In 2010 the two researchers won the Nobel Prize in Physics for their pioneering study [31]. Several methods have been established for graphene production, such as micromechanical or chemical exfoliation of graphite [32], graphitization of silicon carbide [33], chemical vapor deposition (CVD) growth [34], and chemical, thermal or electrochemical

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Graphene is a one-atom-thick planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice [36]. Due to its 2D nature graphene exhibit a unique combination of characteristics not seen in other carbon allotropes. Graphene is the thinnest, strongest and stiffest material [37]. Graphene has extraordinary electrical properties due to the high electron mobility at room temperature [38]. In terms of mechanical properties graphene exhibit greater performances when single or few layer graphene are employed. Therefore, the superior mechanical properties of graphene and its derivatives make them the ideal candidates for incorporation into a large variety of materials in order to produce composites with

Graphene also exhibits other high characteristics such as large specific surface are, high transparency and high thermal conductivity. Because of its high surface area graphene also finds potential applications as support material in catalysis field as an electrode material in electro-

Due to these extraordinary properties, graphene found already a great number of important applications with potential used in touch screens displays, fuel cells, intelligent coatings, transparent conductive films and flexible electronics [40]. In addition, once functionalized with biomolecules like polysaccharides [41], proteins [42], etc. or other biological systems graphene can be integrated for developing new applications in biomedicine and bio-nanotechnology

It is worth noting that the electronic properties of graphene drastically depend on the number of graphene layers. For that reason, the graphene community distinguishes between monolayer graphene, bilayer or few-layer graphene. A structure composed of more than 10 graphene layers exhibits the electronic properties of graphite and therefore is considered as a thin film of graphite. Being transparent as well as a good conductor, graphene may replace

In order to integrate graphene into various functional structures or other materials for making performant nanodevices one preliminary condition is required: graphene sheets have to be exfoliated into individual or few-layer sheets and stabilized. Also, unwanted by-products and structural damage can be produced while synthesizing graphene. A quick and precise method for determining the number of layers of graphene sheets is essential for speeding up the research and exploration of graphene. In sp2-bonded carbon species, as a highly sensitive and non-destructive technique, Raman spectroscopy can be used to investigate the number of layers, the type and relative quantity of defects, mechanical strain, and any further functionalization. Therefore Raman spectroscopy is one of the most powerful tools

chemical applications such as supercapacitors and batteries [39].

such as biosensors [43], biocatalysis [44], biofuel cells [45], etc.

the electrodes in the indium used in touchscreens [46].

available for analysing graphene.

reduction of graphene oxide [35].

enhanced properties.

**Figure 4.** The Raman spectra of a) graphite, b) single wall carbon nanotubes (SWCNTs) and c) multiwall carbon nanotubes (MWCNTs).

between the intensity of D band and the intensity of G band (Id/Ic) indicates that some disorder of the graphene sheets is induced. As previously mentioned graphite consists of stacks of planar sheets of graphene and carbon nanotubes are basically rolled-up graphene sheets. Therefore the presence of the 2D band (also named G` band) is observed in all spectra of the investigated samples which features the arrangement and the number of graphene layers [30]. The importance of the 2D band of graphene will be better explained in section 2 of this chapter. The radial breathing mode (RBM) is particularly important for the determination of the diameter of CNT, its frequency being related to the aggregation state of SWCNTs. The RBM band is unique to SWNCTs and corresponds to the expansion and contraction of the nanotubes. Comparing the Raman spectrum of MWCNTs to that of SWCNTs some important differences can be easily noticed: the absence of RBM mode in MWCNTs spectrum and much sharper D peak in MWNCTs. The RBM band is not present in case of MWNCTs due to the outer tubes that restrict the breathing mode. The presence of a more outlined D band in case of MWCNTs is observed because of the multilayer configuration of nanotubes suggesting a more disordered structure. In addition, a sharper D+G combination peak strongly supports the presence of higher disorder in the MWCNTs, compared to SWCNTs.
