**1. Introduction**

### **1.1 Nanostructures**

In the recent years, the synthesis and characterization of nanomaterials have been one of the most efficacious ways to produce new materials with improved or completely new properties [1]. Their physical dimensions can be used to classify the nanomaterials in subgroups. One-dimensional (1D) nanostructures are systems in which one of the spatial dimensions has less than 100 nm, such as carbon nanotubes, metallic nanowires, or zeolites having 1D cavities (**Figure 1**). Lamellar materials are classified as two-dimensional (2D) nanostructures, because there are formed by platelets piled up in one crystallographic direction, as the graphite and clays. For materials having nanocavities or structures those follow in all directions, they are named as three-dimensional nanostructures (3D; as some zeolites). When the material is symmetric in all directions, it is considered as zero-dimensional (0D) nanostructures, as found in quantum dots, fullerenes, or cyclodextrins (**Figure 1**).

Nanostructures are systems in which at least one of the spatial dimensions is smaller than 100 nm [1]. The synthesis of controlled dimensional nanostructures and the characterization of the intrinsic and potentially peculiar properties of these nanostructures are central themes in nanoscience. The study of different nanostructures has great potential to test and understand fundamental concepts about the role of particle dimensionality on their physicochemical properties. Among the various materials studied in the literature, undoubtedly,

**Figure 1.** *Schematic models for 3D, 2D, 1D, and 0D materials.*

carbon-derived materials, especially fullerenes and nanotubes, and more recently graphenes, are of particular note.

There are two central questions for the study of chemistry and physics of these nanostructures: (i) how controlled dimensionality nanostructures can be fabricated, and (ii) what are the intrinsic and potentially peculiar properties of these nanostructures. ID structures have great potential for testing and understanding fundamental concepts about the role of dimensionality and size over the properties. For example, 1D systems must have singularities in their electronic density of states. There are also several applications, as 1D systems are smaller structures that can be used for efficient electrical transport. Their selected properties can be explored, for example, in nanoelectronics. Among the carbon nanostructures, we highlight carbon nanotubes and graphenes [2–4].

#### **1.2 Carbon allotropes**

The discovery of fullerenes in 1985 opened a new field in chemistry [5]. Since this discovery, research with carbon structures has grown rapidly. In 1991, Sumio Iijima was the first researcher to observe some unusual carbon structures under a transmission electron microscope. Iijima called these structures as carbon nanotubes (CNTs) [2], because they consisted of many cylindrical coiled carbon layers. The layers have carbon atoms attached by six-membered rings and are stacked in the c-crystallographic direction, such as in graphite. CNTs can be open-ended or closed-ended with closed ends having five-membered carbon rings as in fullerenes.

The CNTs can be multiwalled carbon nanotubes (MWCNTs), double-walled carbon nanotubes (DWCNTs), or single-walled carbon nanotubes (SWCNTs). In fact, depending on the way, the graphite layers are coiled, i.e., the different combinations of the vectors (defined by the integers n, m) that define each tube geometrically, they can be classified as "armchair," "zigzag," and "chiral" (**Figure 2**). As consequence of this peculiar geometry, the electronic conductivity and optical properties, such as luminescence and light scattering, are dependent of the tube geometry [6–8].

CNTs attract great attention because they are model systems for nanoscience and have great potential for applications such as composite materials, batteries, sensors, and nanoscale electronics. Interest in CNTs is in their unique structure and properties, their small size (from 0.8 to 2 nm in diameter, see **Figure 1**), their ability to be metallic or semiconductor depending on their geometric structure (**Figure 2**), their exceptional ballistic transport properties and thermal conductivity, optical polarizability, and high structural perfection [6, 7].

SWCNTs have a relatively simple structure, allowing detailed calculations of their electronic structure [7, 9–12]. The unique optical properties of CNTs are due to the confinement of electronic states in one direction, resulting in the so-called

**95**

nanotubes [7, 14, 15].

*and n specify a symmetry and nanotube diameter [6].*

**Figure 2.**

*Raman Spectroscopy and Imaging of Carbon Allotropes DOI: http://dx.doi.org/10.5772/intechopen.90867*

van Hove singularities [10, 13]. The presence of a large number of electron states with very close values in energy leads to an intensification of corresponding photophysical processes, and it is possible through electron absorption, photoluminescence, resonance Raman, and photoelectron spectroscopy techniques to obtain detailed information about the electronic structure and vibration analysis of

*Schematic models for single-axis carbon nanotubes with normal axis in (a) direction θ = 30° [tube seat (n, n)], (b) direction θ = 0° [zigzag tube (n, 0)], and (c) direction 0° < θ < 30° [chiral tube (n, m)]. The values of m* 

Theoretical predictions about the one-dimensional electronic structure of nanotubes have been experimentally verified. The most conclusive evidence comes from Scanning Tunneling Microscopy/Spectroscopy (STM/STS) studies, which showed atomic resolution images and the corresponding electronic structures for metallic and semiconductor nanotubes, and verified the dependence of electronic properties on diameter and helicity [16]. Therefore, the electronic structure of nanotubes depends only on their symmetry, being quite peculiar to solid-state physics. Specifically, the electronic structure may be metallic or semiconductor, depending on diameter and chirality, although there is no difference in the chemical

Despite the unique properties of CNTs, and these are already produced in macroscopic quantities, allowing the study of their physicochemical properties, they still have low solubility in most solvents, as a consequence of their high aggregation, limiting the possibility of chemical manipulation and technological application. Thus, different approaches have been employed to separate or disaggregate CNTs, such as chemical modifications of nanotubes or through interaction with polymers. Graphite (3D), which is the most well-known allotropic form of carbon, has

carbon atom joining three other atoms forming a planar array of hexagons. The layers remain connected via the interaction of van der Waals forces. These twodimensional sheets have the thickness of a carbon atom, which allows them to have different properties that differ from graphite, such as high electrical, thermal conductivity, and mechanical stiffness [3, 4, 18–22]. These layers have been called graphene (2D), which were discovered by scientists André Geim and Konstantin

Graphene is the allotropic form of carbon most recently studied, due to its wide application in the scientific environment. This material is obtained via graphite

hybridization (**Figure 3**), one

bonds between carbon atoms in different nanotubes.

layers of several carbon atoms bonded with sp2

Novoselov at the end of 2004 at the University of Manchester.

*Raman Spectroscopy and Imaging of Carbon Allotropes DOI: http://dx.doi.org/10.5772/intechopen.90867*

#### **Figure 2.**

*Modern Spectroscopic Techniques and Applications*

graphenes, are of particular note.

*Schematic models for 3D, 2D, 1D, and 0D materials.*

**Figure 1.**

carbon nanotubes and graphenes [2–4].

**1.2 Carbon allotropes**

geometry [6–8].

carbon-derived materials, especially fullerenes and nanotubes, and more recently

There are two central questions for the study of chemistry and physics of these nanostructures: (i) how controlled dimensionality nanostructures can be fabricated, and (ii) what are the intrinsic and potentially peculiar properties of these nanostructures. ID structures have great potential for testing and understanding fundamental concepts about the role of dimensionality and size over the properties. For example, 1D systems must have singularities in their electronic density of states. There are also several applications, as 1D systems are smaller structures that can be used for efficient electrical transport. Their selected properties can be explored, for example, in nanoelectronics. Among the carbon nanostructures, we highlight

The discovery of fullerenes in 1985 opened a new field in chemistry [5]. Since this discovery, research with carbon structures has grown rapidly. In 1991, Sumio Iijima was the first researcher to observe some unusual carbon structures under a transmission electron microscope. Iijima called these structures as carbon nanotubes (CNTs) [2], because they consisted of many cylindrical coiled carbon layers. The layers have carbon atoms attached by six-membered rings and are stacked in the c-crystallographic direction, such as in graphite. CNTs can be open-ended or closed-ended with closed ends having five-membered carbon rings as in fullerenes. The CNTs can be multiwalled carbon nanotubes (MWCNTs), double-walled carbon nanotubes (DWCNTs), or single-walled carbon nanotubes (SWCNTs). In fact, depending on the way, the graphite layers are coiled, i.e., the different combinations of the vectors (defined by the integers n, m) that define each tube geometrically, they can be classified as "armchair," "zigzag," and "chiral" (**Figure 2**). As consequence of this peculiar geometry, the electronic conductivity and optical properties, such as luminescence and light scattering, are dependent of the tube

CNTs attract great attention because they are model systems for nanoscience and have great potential for applications such as composite materials, batteries, sensors, and nanoscale electronics. Interest in CNTs is in their unique structure and properties, their small size (from 0.8 to 2 nm in diameter, see **Figure 1**), their ability to be metallic or semiconductor depending on their geometric structure (**Figure 2**), their exceptional ballistic transport properties and thermal conductivity, optical

SWCNTs have a relatively simple structure, allowing detailed calculations of their electronic structure [7, 9–12]. The unique optical properties of CNTs are due to the confinement of electronic states in one direction, resulting in the so-called

polarizability, and high structural perfection [6, 7].

**94**

*Schematic models for single-axis carbon nanotubes with normal axis in (a) direction θ = 30° [tube seat (n, n)], (b) direction θ = 0° [zigzag tube (n, 0)], and (c) direction 0° < θ < 30° [chiral tube (n, m)]. The values of m and n specify a symmetry and nanotube diameter [6].*

van Hove singularities [10, 13]. The presence of a large number of electron states with very close values in energy leads to an intensification of corresponding photophysical processes, and it is possible through electron absorption, photoluminescence, resonance Raman, and photoelectron spectroscopy techniques to obtain detailed information about the electronic structure and vibration analysis of nanotubes [7, 14, 15].

Theoretical predictions about the one-dimensional electronic structure of nanotubes have been experimentally verified. The most conclusive evidence comes from Scanning Tunneling Microscopy/Spectroscopy (STM/STS) studies, which showed atomic resolution images and the corresponding electronic structures for metallic and semiconductor nanotubes, and verified the dependence of electronic properties on diameter and helicity [16]. Therefore, the electronic structure of nanotubes depends only on their symmetry, being quite peculiar to solid-state physics. Specifically, the electronic structure may be metallic or semiconductor, depending on diameter and chirality, although there is no difference in the chemical bonds between carbon atoms in different nanotubes.

Despite the unique properties of CNTs, and these are already produced in macroscopic quantities, allowing the study of their physicochemical properties, they still have low solubility in most solvents, as a consequence of their high aggregation, limiting the possibility of chemical manipulation and technological application. Thus, different approaches have been employed to separate or disaggregate CNTs, such as chemical modifications of nanotubes or through interaction with polymers.

Graphite (3D), which is the most well-known allotropic form of carbon, has layers of several carbon atoms bonded with sp2 hybridization (**Figure 3**), one carbon atom joining three other atoms forming a planar array of hexagons. The layers remain connected via the interaction of van der Waals forces. These twodimensional sheets have the thickness of a carbon atom, which allows them to have different properties that differ from graphite, such as high electrical, thermal conductivity, and mechanical stiffness [3, 4, 18–22]. These layers have been called graphene (2D), which were discovered by scientists André Geim and Konstantin Novoselov at the end of 2004 at the University of Manchester.

Graphene is the allotropic form of carbon most recently studied, due to its wide application in the scientific environment. This material is obtained via graphite

**Figure 3.**

*STM image obtained from a HOPG crystal and the schematic models for the crystalline graphite. The crystallographic parameters were obtained from Ref. [17].*

oxidation and exfoliation processes. Depending on the synthesis parameters (oxidant type, reactant proportions, temperature, etc.), graphene sheets or their oxide (GO) or graphene oxide will have distinct properties. Brodie performed the first documented synthesis in 1859 [23]. Since then, several other scientists have prepared graphene oxide (GO) synthesis experiments from graphite (G) in order to reduce the environmental hazards that this synthesis presents in the use of strong oxidants and concentrated acids. Modifications were later made by Staudenmaier (1898), who focused on improving the reaction introduced H2SO4 to the mixture and some aliquots of solid KClO3 throughout the reaction. With these modifications, the author achieved the synthesis of a more oxidized graphic material and a simplification in the reaction [24]. However, there is release of ClO2 gas, with risks of explosions during the process. In 1958, Hummers proposed some more modifications to this synthesis with the intention of making it safer and more profitable, replacing the oxidizing agent used in the Brodie method with MnO4 and an additive, NaNO3. All the methods mentioned so far make use of strong and toxic reagents for the production of GO, however the use of thermal expansion of graphite, or sonic spacing, those are more green routes, it takes around 6–12 h of preparation.

## **1.3 Raman spectroscopy and imaging**

Through the years, the infrared and Raman spectroscopies have been the techniques *par excellence* for the investigation of the vibrational structure of conjugated materials, such as dyes [25], metallic complexes [26–28], conducting polymers [29, 30], nanocomposites [31–33], and carbon allotropes [34–39]. The laser is the common source in Raman spectroscopy, the radiation interacts with the sample through the scattering process. The incident light (laser source) has much more energy than the vibrational levels, however, by the scattering process; the information about the vibrational modes can be accessed. This behavior is very different

**97**

**Figure 4.**

*Raman Spectroscopy and Imaging of Carbon Allotropes DOI: http://dx.doi.org/10.5772/intechopen.90867*

4

phores, just by selecting the appropriated laser line energy.

can be studied (**Figure 4**).

scattered frequency λ*<sup>s</sup>*

10<sup>−</sup><sup>6</sup>

from the IR vibrational spectroscopy, where the radiation energies have the same magnitude of vibrational levels, and by absorption process, the vibrational modes

electronic transition [40, 41]. This behavior changes dramatically when the laser energy is close to an electronic transition, this condition is known as Resonance Raman. Hence, the intensities of the vibrational modes associated with the excited electronic state in resonance are amplified by 105–7 times. For multichromophoric systems, like conducting polymers, it is possible to screen the different chromo-

In recent years, the use of Raman spectrometers coupled with different microscopes, from optical to force atomic type, has increased. In **Figure 5**, the configuration of an optical microscope coupled to a laser source used for Raman measurements is schematically illustrated. The laser lines reach to the sample on the microscope stage by optical elements. The Raman scattered radiation is collected in a scattering angle of 180° by the same microscope objective and captured by an opening of the spectrometer using a beam splitter. It is necessary that the instrument has a high lighting efficiency, and the collection of scattered radiation must be precisely done, owing to the very small Raman cross section (typically a factor of

 to 10<sup>−</sup>12 of the incident radiation) and the small volume of the sample. Raman microscopy can be considered a nondestructive technique; however, sometimes the laser power can destroy the sample or change its structure during the measurement. The use of microscopy opens the possibility to search different areas of the sample. In a conventional microscope, it is possible to investigate a very small part of the

*Schematic representation of two electronic states (ground and excited) and their respective vibrational levels (the electronic and vibrational levels are not in the same scale). The arrows indicated the types of transitions among the different levels. For Raman scattering, if the laser line (wavenumber is represented by ν0) has energy similar to one electronic transition of the molecule, the signal can be intensified by resonance process, known as resonance Raman effect. In the figure, ν0 and νs (the scattered frequency is composed of νev,gm and νev,gn, the stoke and antistoke components, respectively) are the laser line and the scattered frequencies, respectively (for illustration purposes, just the stokes, νs < ν0, component is shown in the diagram).*

The intensities of the Raman bands are proportional to the fourth power of the

when the laser energy is very far from a permitted molecular

*Modern Spectroscopic Techniques and Applications*

oxidation and exfoliation processes. Depending on the synthesis parameters (oxidant type, reactant proportions, temperature, etc.), graphene sheets or their oxide (GO) or graphene oxide will have distinct properties. Brodie performed the first documented synthesis in 1859 [23]. Since then, several other scientists have prepared graphene oxide (GO) synthesis experiments from graphite (G) in order to reduce the environmental hazards that this synthesis presents in the use of strong oxidants and concentrated acids. Modifications were later made by Staudenmaier (1898), who focused on improving the reaction introduced H2SO4 to the mixture and some aliquots of solid KClO3 throughout the reaction. With these modifications, the author achieved the synthesis of a more oxidized graphic material and a simplification in the reaction [24]. However, there is release of ClO2 gas, with risks of explosions during the process. In 1958, Hummers proposed some more modifications to this synthesis with the intention of making it safer and more profitable, replacing the oxidizing agent used in the Brodie method with MnO4 and an additive, NaNO3. All the methods mentioned so far make use of strong and toxic reagents for the production of GO, however the use of thermal expansion of graphite, or sonic spacing, those are more green routes, it takes around 6–12 h of preparation.

*STM image obtained from a HOPG crystal and the schematic models for the crystalline graphite. The* 

Through the years, the infrared and Raman spectroscopies have been the techniques *par excellence* for the investigation of the vibrational structure of conjugated materials, such as dyes [25], metallic complexes [26–28], conducting polymers [29, 30], nanocomposites [31–33], and carbon allotropes [34–39]. The laser is the common source in Raman spectroscopy, the radiation interacts with the sample through the scattering process. The incident light (laser source) has much more energy than the vibrational levels, however, by the scattering process; the information about the vibrational modes can be accessed. This behavior is very different

**96**

**Figure 3.**

**1.3 Raman spectroscopy and imaging**

*crystallographic parameters were obtained from Ref. [17].*

from the IR vibrational spectroscopy, where the radiation energies have the same magnitude of vibrational levels, and by absorption process, the vibrational modes can be studied (**Figure 4**).

The intensities of the Raman bands are proportional to the fourth power of the scattered frequency λ*<sup>s</sup>* 4 when the laser energy is very far from a permitted molecular electronic transition [40, 41]. This behavior changes dramatically when the laser energy is close to an electronic transition, this condition is known as Resonance Raman. Hence, the intensities of the vibrational modes associated with the excited electronic state in resonance are amplified by 105–7 times. For multichromophoric systems, like conducting polymers, it is possible to screen the different chromophores, just by selecting the appropriated laser line energy.

In recent years, the use of Raman spectrometers coupled with different microscopes, from optical to force atomic type, has increased. In **Figure 5**, the configuration of an optical microscope coupled to a laser source used for Raman measurements is schematically illustrated. The laser lines reach to the sample on the microscope stage by optical elements. The Raman scattered radiation is collected in a scattering angle of 180° by the same microscope objective and captured by an opening of the spectrometer using a beam splitter. It is necessary that the instrument has a high lighting efficiency, and the collection of scattered radiation must be precisely done, owing to the very small Raman cross section (typically a factor of 10<sup>−</sup><sup>6</sup> to 10<sup>−</sup>12 of the incident radiation) and the small volume of the sample. Raman microscopy can be considered a nondestructive technique; however, sometimes the laser power can destroy the sample or change its structure during the measurement. The use of microscopy opens the possibility to search different areas of the sample. In a conventional microscope, it is possible to investigate a very small part of the

#### **Figure 4.**

*Schematic representation of two electronic states (ground and excited) and their respective vibrational levels (the electronic and vibrational levels are not in the same scale). The arrows indicated the types of transitions among the different levels. For Raman scattering, if the laser line (wavenumber is represented by ν0) has energy similar to one electronic transition of the molecule, the signal can be intensified by resonance process, known as resonance Raman effect. In the figure, ν0 and νs (the scattered frequency is composed of νev,gm and νev,gn, the stoke and antistoke components, respectively) are the laser line and the scattered frequencies, respectively (for illustration purposes, just the stokes, νs < ν0, component is shown in the diagram).*

**Figure 5.** *Conventional Raman microscope.*

sample (1 μm approximately or smaller). The high lateral resolution and depth of field (the order of a few micrometers) are very useful for the study of multilayered polymeric thin films or other complex materials [42–44].
