In addition to the C60 molecule, fullerene may have larger structures, such as C70, C76, and C78, and smaller structures, such as C28 and C36. In the 1990s, C60 and other fullerenes began to be produced in large quantities via the condensation of soot that is generated in graphite vaporization [10]. Fullerenes may also occur naturally in materials affected by high-energy events, such as lightning and meteors, and in geological samples [7,8].

One of the most widely used processes of synthesis is based on the Krätschmer–Huffman method in which an electric arc is generated on graphite electrodes in a helium atmosphere at a pressure of approximately 200 torr. To separate out the fullerene, carbon soot from evaporated graphite is dissolved in a nonpolar solvent. Subsequently, the solvent is dried, and the C60 and C70 fullerenes are separated from the residue. Under an optimum current and pressure of helium flow, yields of up to 70% of C60 and 15% of C70 are achieved. The laser vaporization method is also used to produce fullerene. In a typical system, a Nd:YAG pulsed laser operating at 532 nm and 250 mJ is used as the source, and the graphite target is maintained in an oven at 1200°C. Although the yields are low, fullerenes have also been produced via deposition of soot from flames involving, for example, the combustion of benzene and acetylene [8,11].

Fullerenes have excellent mechanical properties, resisting high pressures and returning to their original shape even after subjected to more than 3000 atm. Theoretical calculations indicate that a single C60 molecule has a bulk modulus of 668 GPa when compressed to 75% of its size. This property makes fullerenes harder than steel and diamond, whose bulk modules are 160 and 442 GPa, respectively. Fullerenes can withstand collisions of up to 15,000 mph against stainless steel and maintain its shape, demonstrating their high stability. The optical properties of the fullerenes are also renowned for the delocalized π electrons, which generate nonlinear optical responses and intensity-dependent refractive indices [11,12].

Solar cell efficiency steadily increases. Although the energy conversion efficiency of a polymer/fullerene heterojunction solar cell is still low (approximately 10%), compared to conventional silicon cells (20% efficiency), the production cost of this type of solar cell may be significantly lower. Furthermore, organic solar cells can be flexible, rolled up, and scattered over any surface [13–15]. Plastic photovoltaic devices can be used to cover internal and external walls of buildings. Additionally, cells of any color and texture can be manufactured. For example, a cell phone can be painted with this material, and its battery can be charged while a person walks with it on a sunny day. Organic solar cells are also expected to be used in advertising, such as on light banners, liquid crystal displays, and food packaging [13–15].

Fullerenes can be used in other organic electronic devices. In organic field effect transistors, they serve as an n-type semiconductor to increase charge mobility and stability [8,13]. Interdigitated capacitors with fullerenes were used in sensors that explore the electron acceptor properties of fullerene films [16]. In optical limiters, fullerenes induce a decrease in transmittance with an increase of incident light. This allows all of the light below an activation threshold to pass through, for example, maintaining the light transmission at a constant level below the damage threshold for the eye or an optical sensor [16]. Fullerene hydride shows promise for storing hydrogen gas in electrical vehicles that use a fuel cell. This is encouraging because the available hydrogen storage technologies, such as compressed gas or storage as metallic hydrides, are dangerous and/or have low hydrogen storage densities [16].

In the medical and pharmaceutical fields, fullerene derivatives can be used in photodynamic therapy for cancer, such as in antibacterial agents and in neuroprotective drugs [1,8]. Due to its ability to include atoms, fullerenes are suitable as drug carriers. Additionally, noble gases can be encapsulated in fullerenes for use in nuclear magnetic resonance. Fullerenes are also antioxidants and react with free radicals, thus preventing cell damage or death. For example, fullerene has an antioxidant performance that is 100 times more effective than vitamin E. In engineering, fullerenes are employed to strengthen metal alloys [1,8,12].

The properties of SWNTs are determined by the structure formed by the bonds between the carbon atoms of the graphene sheet [21,22]. SWNT structures depend on the chirality of the tube, which is defined by the chiral vector (${\overrightarrow{C}}_{\text{h}}$) and the chiral angle (θ), as shown in Fig. 9.4. The vector ${\overrightarrow{C}}_{\text{h}}$ is the linear combination of the base vectors a₁ and a₂ of a simple hexagon via the relationship ${\overrightarrow{C}}_{\text{h}}=n{\overrightarrow{a}}_1+m{\overrightarrow{a}}_2$, where n and m are integers and ${\overrightarrow{a}}_1$ and ${\overrightarrow{a}}_2$ are the single cell vectors of a two-dimensional matrix that is formed by the graphene sheet in which the direction of the nanotube axis is perpendicular to the chiral vector, as shown in Fig. 9.4A.

The chiral vector pair (n,m) indicates the direction in which the graphene sheet is rolled. The values of this pair allow three types of arrangements for SWNTs: zigzag, armchair, and chiral, as shown in Fig. 9.4B. The pair (n,m) determines the nanotube chirality and therefore its optical, mechanical and, in particular, electrical properties [21,22]. If (n,m) is a multiple of 3, the nanotubes are of the armchair type with metallic behavior, that is, they have a Fermi level in a partially filled band (Fig. 9.5A). When (n,m) is not a multiple of 3, the nanotubes are the zigzag type and have semiconductor behavior (Fig. 9.5B).

The (n,m) relationships that are formed with the nanotube structure, the chiral angle, and the type of nanotube are listed in detail in Table 9.1.

MWNTs are described by models developed from electron microscopy images. They generally have a coaxial cylindrical shape, are formed as a coaxial polygon, or are rolled-up graphene sheets. MWNTs are formed by several graphene sheets that are rolled around each other concentrically with a distance between the sheets of 3.4–3.6 Å [21,22]. The length and diameter of MWNTs are different from the length and diameter of SWNTs, which implies different properties. Another factor that defines the type of nanotube formed (SWNT or MWNT) is related to synthesis.

The integration of CNTs with biological compounds (proteins, enzymes, antibodies, DNA) in electrical devices, such as biosensors, is due to the size compatibility because electronic circuit components have dimensions that are comparable to biomolecules. Electrostatic interactions and charge transfer, which are typical in biological processes, can be detected via electronic nanocircuits, which is helpful in detecting biological species [23–30]. For example, the diameter of a SWNT is comparable to the size of molecules, such as DNA (1 nm), whereas its length is greater, which provides a convenient interface for micrometric scale circuits. Additionally, the high-reactivity and effective area of CNTs provide synergy with organic molecules [23–27].

In biosensors that contain CNTs, there are typically electrodes that are modified via the immobilization of a biomolecule, such as an enzyme, as the recognition element [28,29]. CNTs are suitable for facilitating charge transfer in reactions with electroactive species in solution, in addition to increasing the electroactive surface area of the electrodes. In many cases, there is also synergy in the use of CNTs with other nanomaterials or biomolecules, for example, a more efficient catalysis of redox reactions of analytes, surface functionalization, and the compatibility of interactions with other types of nanomaterials (e.g., metal nanoparticles and quantum dots). Concerning biosensors, CNTs form suitable matrices for immobilizing biomolecules without the loss of biological activity. These biosensors that contain CNTs have been used to diagnose diseases, such as some cancers and tropical diseases (e.g., Chagas and leishmaniosis), and in traditional clinical tests, such as the measurement of glucose, urea, cholesterol, and uric acid [5,23–30]. Fig. 9.6 shows a representation of different types of sensors and biosensors that include CNTs.

SWNT semiconductors have been used in field-effect transistors (FET) to detect biological species under ambient temperatures and conditions. Nanotube field-effect transistors (NTFET) are obtained with the replacement of the solid-state gate with a layer of adsorbed molecules that modulates nanotube conductance. There are two classical NTFET device designs. The first design uses a single CNT as an electron channel between the source and drain electrodes. In the second type, a set of CNTs serves as a collective channel between the source and drain. The nanotube–analyte interaction may have one of two effects. The first effect is the charge transfer from the analyte molecules to the CNTs. In the second mechanism, the adsorbed analyte in the CNT walls decreases conductance [25–27]. The difference between these two types of mechanisms is obtained via measurements in a transistor. If a charge transfer occurs, the voltage threshold becomes much more positive (removing electrons) or more negative (donating electrons).

In energy storage, CNTs have been used as supercapacitors because of their high electrical conductivity, large surface area, mechanical strength, and low weight. Additionally, the possibility of modifying CNTs in chemical synthesis and their combination with other nanomaterials to improve the performance of the supercapacitors are important [31–33]. Other advantages are flexibility and optical transparency, allowing the development of flexible electronic devices, such as more pliable cell phones and computers. For this type of application, supercapacitors that contain only CNTs exhibit excellent electrical conductivity and a large surface area but do not reach the expected performance because of the contact resistance between the electrode and the current collector. Therefore, metal oxides (including those in nanoparticles) are incorporated into nanotubes to reduce the contact resistance and increase the storage density of supercapacitors [31–33].

In addition to the interest in devices, CNTs are useful as engineering materials because of their mechanical properties. CNTs have an elasticity of 1000 GPa, which is 5 times that of steel, a rupture tension of 63 GPa, and tensile strengths 50 (SWNTs) and 200 (MWNTs) times higher than that of steel. Adding to these characteristics is the lower density of CNTs compared with metals. With these properties, CNTs can be useful for aerospace engineering, such as in aircraft fuselages [19]. CNTs can form ultraresistant foams that work as high-compression springs. Threads made with CNTs are being studied to make fabrics in clothing that may be stronger than Kevlar, a material used currently in bulletproof vests [34].

Regarding products that are closer to industrialization, CNTs can be used as reinforcement in composites in which one of the phases has a nanometric size. One possibility is to apply CNTs in a polymer matrix to improve electronic properties and increase mechanical strength, for example, in plastic containers that require electrostatic shielding, such as in cell phones [35]. CNTs can also modify the mechanical properties of biodegradable polymeric nanocomposites that are used for tissue engineering and to manufacture artificial bones, cartilage, muscles, and nervous tissue. Another advantage of CNTs is the assistance they provide to bone cell proliferation and bone formation from functionalized surfaces [36–38].

The great advance in research on graphene has occurred because, among other reasons, it is cheap to obtain in a high quality. The synthesis methods can be divided into two groups, the bottom-up approach and the top-down approach [44]. The bottom-up approach consists of the growth of graphene sheets by combining the basic structural units. In this approach, organic synthesis methods are used, enabling graphene growth directly via molecular organic precursors. These precursors usually consist of benzene ring molecules with highly reactive functional groups, enabling controlled two-dimensional growth of the graphene sheet. Another method to grow two-dimensional graphene sheets is based on the growth in situ on catalyst substrates (e.g., Cu, Ni, Fe), such as via CVD, arc discharge, or SiC epitaxial growth. The major limitation of these techniques is that they still do not produce graphene in large amounts and uniformly, and they are expensive [44].

The top-down approach involves the breaking of the interplanar Van der Waals bonds of graphite. In this approach, chemical and mechanical exfoliation methods are used on graphite. These approaches have limitations in obtaining the two-dimensional graphene crystal. The mechanical or chemical graphite exfoliations do not allow complete control of the number of carbon layers in the graphene and the two-dimensional crystal lattice quality, limiting performance. In particular, exfoliating graphite in solution requires surface modifications to break the Van der Waals bonds and the formation of structural defects on the graphene carbon lattice. An intermediate species in this type of processing is known as graphene oxide (GO) due to its high density of oxygen functional groups. With GO, it is possible to obtain colloidal solutions that are stable for long periods of time. This characteristic is important for the formation of nanocomposites by combining graphene with other types of materials, such as polymers, biomolecules, and nanoparticles. Surface charge measurements (zeta potential) of GO have indicated that GO is negatively charged when dispersed in water [44]. Fig. 9.7 shows a simplified representation of the graphene and GO structures.

GO can be converted into graphene via reduction processes, producing products with properties similar to those of graphene. However, reduced GO has significant structural differences compared to graphene [41–44]. The main purpose of the chemical reduction of GO is to obtain graphene with mechanical and conductive properties that are similar to graphene produced via the scotch tape method. GO reduction allows regeneration of the graphitic structure with deoxygenation and dehydration, restoring the conductivity of the material and restructuring the sp2 bonds in the carbon matrix [41–44].

The reduction of GO can be performed using several methods, including chemical methods with reducing agents, such as hydrazine, hydrogen plasma, dimethylhydrazine, hydroquinone, sodium borohydride, ascorbic acid, alcohols, and strong alkaline solutions, electrochemical methods, thermal methods, and ultraviolet radiation methods. Importantly, the conductivity of the reduced GO depends on both the methodology used to oxidize the graphite and the reduction methodology. The structural defects that appear during the oxidation of graphene also lead to the removal of carbon atoms from the aromatic structure of the carbon planes, creating nanometric zones of discontinuity that are impossible to recover via reduction [41–44].

Graphene has been studied for next-generation lithium-ion batteries and in cathodes with high electrical conductivity [4,42]. With its morphology similar to a sheet, graphene can act as a conductive membrane and form a core–shell or nanocomposite with a sandwich-type structure. The increase in electrical conductivity of these morphologies that contain graphene can help overcome a major limitation of lithium-ion batteries, which is low power density. Additionally, the high thermal conductivity of graphene may be helpful for high current charges that generate a significant amount of heat in a battery. As anodes, the nanosheets of graphene can be used to intercalate lithium crystals in the layers [4,42].

Supercapacitors, also called electrochemical capacitors or ultracapacitors, are hosts of energy whose mechanism is associated with the electrical double layer at the interface between the electrode and the electrolyte [31,32]. Graphene is suitable for supercapacitors because of its flexibility, transparency, high electrical conductivity, large surface area, mechanical resistance, and low weight, which can allow the development of flexible electronic devices that can be incorporated in clothing and cell phones and allow the development more pliable computers. Additionally, graphene can be combined with other nanomaterials in search of synergy, as shown in Fig. 9.8 [47–52].

One challenge to produce graphene electrodes on a large scale is to avoid agglomeration of the plates because the agglomerations decrease the effective surface area, causing negative effects on the properties. To control this agglomeration, the plates are separated with nanoparticles, which also favors the electrochemical properties [47–52]. For example, metal oxides (in bulk or nanoparticles) are incorporated into graphene to decrease the contact resistance and increase the specific capacitance and energy density of supercapacitors. The combination of graphene and oxide in nanocomposites can generate supercapacitors with high performances, but the oxide must have a low cost and toxicity [47–52]. In this aspect, magnesium oxides may be preferable to ruthenium oxide because of their lower cost [50,51].

The unique electronic properties of graphene provide a glimpse of high-performance logic circuits of future decades, for example, in logic transistors, high-frequency transistors, flexible electronic devices, such as touch screens and electronic paper (e-paper), and organic light emitting diodes [42]. Graphene can also be used in photonics, photodetectors, optical modulators, optical polarization controllers, mode-locked solid state lasers, and insulators [42].

Like CNTs, graphene has been widely used in several types of sensors and biosensors (e.g., optical, chemical, and piezoelectric). For biosensors, in particular, graphene can be even more useful than CNTs, mainly because its two-dimensional structure facilitates functionalization for the incorporation of biomolecules and nanoparticles [53–55]. Graphene also has a higher biocompatibility and larger surface area, which can produce higher performance than CNTs or other carbon materials in biosensors [53–55].

For biomedical applications, graphene is promising due to its large surface area, chemical purity, and possibility of functionalization for drug delivery systems [42]. Its mechanical properties suggest applications in tissue engineering and regenerative medicine. Due to its single atomic layer thickness, conductivity, and mechanical resistance, graphene is suitable as a support for biomolecules to perform transmission electron microscopy [42]. Additionally, functionalized graphene can generate ultra-sensitive devices that are quick and capable of detecting biological molecules, such as glucose, cholesterol, hemoglobin, and DNA [53–55]. Graphene compounds are biocompatible with different cell types (such as mammalian and bacterial cells) both in vitro and in vivo and produce antibacterial effects [46].