Carbon has many useful properties which can be explored by suitably arrange the carbon atoms. It is one of the most versatile elements present in the earth. It is also a unique element among electronic materials as regards to its structure or property relations. It can be used as a metal (quasi-2D graphite), a semiconductor, a superconductor, a polymer, a quantum dot fullerene, or a quantum wire CNT. Carbon based nanomaterials are defined as materials in which the "nanocomponent" is pure carbon. Table 6.1 lists the carbon based nanomaterials which are under investigation by the scientific community.
Carbon has been used for the reduction of metal oxides since B.C. 4000. In 1779, graphite, the allotropic form of carbon, was discovered. Ten years later another allotropic form of carbon, diamond was discovered. It was then found that both of these forms belong to a family of chemical elements. Kroto, Smalley and Curl discovered buckminsterfullerenes or fullerenes in 1985. They awarded 1996 Nobel Prize in Chemistry for this work. Sumio Iijima luckly discovered carbon nanotubes (CNTs) in 1991, while searching for new carbon structures in the deposits formed on graphite cathode surfaces during the electric arc-discharge.
6.2 Allotropes of Carbon
Allotropes of carbon are very stable due to the unique chemical bonding properties of carbon. Both the graphite and diamond are its natural allotropes, while fullerenes and their derivatives are manufactured in the laboratory. Allotropes of carbon are shown in figure 6.1. Physical and chemical properties of the allotropes of carbon are presented in table 6.2. Carbon's allotropes have a wide range of amazing properties like high tensile strength and high melting points.
Fullerenes are molecules of varying sizes composed entirely of carbon in the shape of a cylindrical tube, hollow sphere, or ellipsoid. Spherical fullerenes are popularly known as buckyballs and have the formula C60. Fullerenes have found tremendous applications in nanotechnology.
Diamond Graphite C60 CNT
Fig. 6.1: Allotropes of Carbon
Table 6.1: Carbon based nanomaterials.
Single Nanostructures
Films/ Coatings/
Nanostructured
surfaces
Nanostructured
bulk material
Particles
Nanotubes
Carbon Black*
SWNT
Carbon films
Nanostructured carbon
Fullerenes
MWNT
Diamond like carbon (DLC)
Nanoporous Carbon
Graphite
Nanohorn
Covalent carbides like SiC
Carbon foams
Nanocluster
Nanowires
Metallic carbides like TiC
Carbon Aerogels
Nanorods
Nano Carbon Nitrides
Carbon Nanocrystals
* Carbon black is the most widely used carbon nanomaterials now-a-days, it has applications
in car tyres, antistatic textiles and is used for colour effects.
Carbon nanotubes have cylindrical shape and hence are also called buckytube. One end of the CNT is closed with a hemisphere of the buckyball structure. The length of the CNT is several centimeters whereas its diameter is only a few nanometers. CNTs are of two types: single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs). Their remarkable properties like high strength, excellent electrical and thermal properties offer potential applications in nanoelectronics, optics, sensors, energy storage and so on.
Table 6.2: Comparison of physical and chemical properties of the allotropes.
Properties
Diamond
Graphite
C60
CNTs
Colour
Colourless
Steel black
to grey
Black solid/
Magenta in solution
Black
Density (g/cm3)
3.515
1.9 - 2.3
1.69
1.33 - 1.4
Specific Gravity
3.52
2.2
1.7 - 1.9
2
Hardness
(Moh's Scale)
10
1 -2
1 -2
1 -2
Melting Point(0C)
3550
3652 - 3697
>800 (sublimes)
3652 - 3697
Boiling Point (0C)
4827
4200
--
--
Electrical Conductivity
Insulator
Conductor
Semiconductor
Conductor -
Semiconductor
Hybridization
Sp3-tetrahedral
Sp2-trigional planar
Sp2-trigional planar
Sp2-trigional planar
Crystal Structure/
Shape
Cubic
Tabular
Truncated icosahedron
Cylindrical
6.3 Fullerene C60
The fullerene C60 was discovered accidently. Smalley and Curl developed a technique to analyze atom clusters produced by laser vapourization with time-of-flight mass spectrometry (Figure 6.2), which caught Kroto's attention because he was interested in the study of long-chain polyynes formed by red giant stars. When they used a graphite target, they could produce and analyze the long chain polyynes. In September, 1985, they experimented with the carbon plasma, confirming the formation of polyynes. They observed two mysterious peaks at mass 720 and 840, corresponding to 60 and 70 carbon atoms, respectively as shown in figure 6.3. Further reactivity experiments determined a most likely spherical structure, leading to the conclusion that C60 is made of 12 pentagons and 20 hexagons arranged to form a truncated icosahedron. These investigators found Cn clusters, where n is even numbers and n>20. Fullerenes with larger number of carbon atoms such as C70, C76, C80, C240 and even up to C540, were also discovered.
Fig. 6.2: Schematic diagram of the pulsed supersonic nozzle used to generate carbon cluster beams.
Number of carbon atoms per cluster
Fig. 6.3: Mass spectra of carbon clusters.
During the synthesis of fullerenes by any fabrication techniques, the most abundant among the fullerene species is C60. Its stability is high and posses greatest symmetry. Almost all C60 molecules are identical. However in some cases, 13C isotope (1.1% natural abundance) can substitutes randomly for 12C (98.9% abundance) in the caged molecule. The C60 molecule has monodisperse nanostructure with high (icosahedral Ih) symmetry. The fullerene C60 is the most interesting fullerene nanostructure due to its simplicity and relatively high abundance. The constituent fullerene molecules in a fullerene solid (fullerite) determine the structure or property relationships of fullerites.
6.3.1 Structure of C60
The fullerene C60, the molecule with the highest symmetry (Ih), is built of carbon atoms with mostly sp2-hybridization. The atoms are assembled in the form of a truncated icosahedron as shown in figure 6.4. All the 60 carbon atoms are positioned at the vertices of a regular truncated icosahedron (0.710 nm diameter). It is important that all carbon sites are identical, consistent with a sharp line in the NMR spectrum. The average distance between nearest carbon-carbon (C-C) atoms (ac-c) is extremely small about 0.144 nm, almost identical with graphite ac-c distance (0.142 nm).
Fig. 6.4: Structure of the fullerene C60 molecule.
In fullerene C60 molecule, every carbon atom at each corner is triagonally bonded to three other nearby carbon atoms, as in graphite. Out of the 32 faces of the regular truncated icosahedron, 20 faces are hexagons and the remaining 12 faces are pentagons. Thus, C60 molecule may be considered as a "rolled-up" graphene sheet (a single layer of crystalline graphite), which forms a closed shell molecular nanostructure, obeying Euler's theorem. Euler's theorem states that a closed surface consisting of hexagons and pentagons has exactly 12 pentagons, and an arbitrary number of hexagons. The introduction of pentagons gives rise to curvature in forming a closed structure of the molecule as shown in figure 6.5 (a). To minimize local curvature, the pentagons become separated from each other in the self-assembly process, giving rise to the isolated pentagon rule (IPR), an important rule for stabilizing fullerene clusters. IPR states that a stable and non-reactive fullerene is only formed when the pentagons at its surface are separated. Thus, C60 is the smallest stable fullerene formed, and it has only one isomer. Smaller, non-IPR structures like C20 and C36 have been produced successfully. However, they can only be studied in the gas phase or in an oligomerized/polymerized form in the solid due to their high reactivity. The smallest possible fullerene structure that obeys Euler's theorem is C20 which would form a regular dodecahedron with 12 pentagonal faces. But this non IPR structure is unfavourablebecause of its high local curvature and strain. The addition of a single hexagon adds two C atoms, thus all fullerenes Cnc contain an even number of carbon atoms (nc), in agreement with the observed mass spectra for fullerenes.
The diameter (di) of a fullerene can be estimated from the relation for an icosahedral fullerene,
di = ac_c [(15nc)1/2]/2Ï€, (6.1)
where ac-c = 0.144 nm is the average nearest-neighbour carbon-carbon distance. If nc < 103, the diameter of the fullerene Cnc nanostructure di ≤ 3 nm. Although all C atoms in C60 are equivalent, the bonds between different atoms are not equivalent. The valency of a carbon atom is four. These valence electrons of every carbon atom is engaged in covalent bonds, so that the two bonds on the pentagon perimeter are electron-poor single bonds (ac-c = 0.146 nm), and the bond between two hexagons is an electron-rich double bond (ac=c = 0.140 nm). Since each carbon atom has completed octet, the C60 molecule is likely a van der Waals bonded crystal which is nonconducting (an insulator or a semiconductor). Upon condensation into a solid, the C60 molecules form a close-packed structure with face centred cubic (f.c.c.) symmetry and a lattice constant (a0 = 1.4198 nm) under ambient conditions.
Fig. 6.5: Examples of closed shell fullerene configurations: (a) C60, b) C70 and (c) an armchair carbon nanotube.
6.3.2 Structure of Higher Fullerenes
During the synthesis of fullerene C60, larger molecular weight fullerenes Cnc (nc > 60) are also formed. The C70 is the most abundant among them. It is possible to isolate significant amounts of higher mass fullerenes like C70, C76, C78, C80, C82 and so on. These higher mass fullerenes also posses a close-packed crystal structure. Because of the lower molecular symmetries of high fullerenes, their symmetry, molecular orientation and temperature dependence are more complicated than that of C60. Some higher fullerenes show deviation from the quasi-spherical shape. The fullerene C70 molecule shows a rugby-ball shape, and is formed by adding a ring of 10 carbon atoms or a belt of 5 hexagons around the equatorial plane of the C60 molecule normal to one of the five-fold axes. Higher fullerene molecules (> C76) exhibit a further degree of freedom because for each carbon atom there exists more than one structural isomer of the fullerene. The lattice constant a of higher fullerene solids is proportional to √nc, as depicted in figure 6.6. This behaviour arises from the fact that the average radius of the molecules is roughly proportional to √nc since the C-C bond lengths do not change significantly with the size of the fullerene molecules.
Fig. 6.6: Lattice constant (af c c) of some higher fullerenes (C60, C70, C76 and C84) as a function of √nc.
Fullerenes often form isomers because a closed cage molecule Cnc can form different geometrical structures. Each different structure corresponds to a distinct isomer. As nc increases, the number of isomers obeying the isolated pentagon rule increases rapidly. For example, C78 has 5 distinct isomers and C80 has 7 isomers. However, fullerenes C60 (Ih), C70 (D5h) and C76 (D2) have only a single isomer each, which obeys the isolated pentagon rule.
6.3.3 Doping of Fullerenes
There are mainly three doping routes in which the electronic properties of a fullerene can be modified in a controlled manner. The doping methods are (a) doping from outside the molecules (intercalation), resulting fullerene salts, (b) doping from inside the molecule (endohedral), resulting metallofullerenes and (c) changing the molecular structure itself by doping on the buckyball called heterofullerenes, as illustrated schematically in figure 6.7. These doping methods are successful and can produce a number of fullerene intercalation compounds, endohedrally doped fullerenes (metallofullerenes) or heterofullerenes which substitute C atoms with heteroatoms like N or B. Besides, it is also possible to dope from outside via charge injection in a field effect device. All these techniques are successful in property engineering and thereby fullerene based materials offer a variety of properties like ferromagnetism, metallic conductivity, superconductivity, non-linear optical properties and so on.
Fig.6.7. Doping fullerenes: (a) from outside (intercalation), (b) on-ball (heterofullerenes) and (c) from inside (metallofullerenes).
If alkali (A) or alkaline earth (AE) metals dope with fullerene C60, a number of intercalation compounds, called fullerides, are being constructed. These fullerides usually take up face centred cubic (f.c.c), body centred cubic (b.c.c.) or body centred tetragonal (b.c.t.) crystal structures. The interstitial sites of the fullerene solid are occupied by the metal atoms. The metal atoms contribute their outer electron(s) partly or completely to the C60 molecules. The the schematic representations of intercalated compounds such as A3C60, A6C60 and A4C60 are shown in figure 6.8. The large and small spheres represent the C60 molecules and the alkali ions respectively.
Fig. 6.8: Schematic representation of intercalated compounds: (a) A3C60, (b) A6C60 and (c) A4C60 compounds. Source: Fleming et.al.(1991)
In 1991, heterofullerenes molecules, such as C60 - nBn and CmNn molecules were produced using molecular beam experiments. The heterofullerene C59N was the first one made available in large scale for solid state spectroscopic studies. It forms dimers ((C59N)2) and crystallizes in a lattice with monoclinic symmetry. Besides, the intercalation of alkali metal with C59N is also possible, which combines two fullerene doping mechanisms simultaneously (combinational doping). K6C59N was the first intercalated phase, where the dimer bonds were broken and the structure was b.c.c. similar to that of K6C60.
6.3.4 Metallofullerenes
As discussed in 6.3.3., Metallofullerenes are endohedrally doped fullerenes in which metal dopant is doped from inside. Metallofullerene nanostructures having a metal dopant within the fullerene cage is shown in figure 6.9. Different metal species can be introduced into the interior hollow core of the endohedrally doped C60 molecule. Usually one, two, or three metal species can be put inside a single fullerene cage. The endohedral fullerene configuration is usually denoted as M@Cnc. For example, La@C60 denotes one endohedral lanthanum in a C60 molecule and Y2@C82 denotes two Y atoms inside a C82 fullerene molecule. It is important that the stability of an endohedral fullerene depends on many significant parameters such as the dopant species, the number of dopant atoms or ions, , the number of carbon atoms, the amount of charge transfer between the dopant and the fullerene cage, and the shape of the fullerene isomer.
The location of the dopants inside the cage may not essentially be at the centre of the molecule as shown in figure 6.8. Metallofullerenes commonly exhibit dipole moment because the interaction between the dopant and the fullerene molecule may result charge transfer between them. Besides, these endofullerenes have large dipole moment (2-4 Debye) due to relatively large size of the fullerene shell. The high dipole moment may affect the solubility of specific metallofullerenes in solvents. This property would be exploited in isolating and purifying metallofullerenes. Moreover, other endohedral dopants (e.g. He and Ne) of fullerenes are also possible even with lower concentrations. Possible structural models of M@C60, La@C82 and Sc3@C82 metallofullerenes with dopants at different locations are shown in figure 6.8
(a) M@C60
(b) La@C82
(c) Sc3@C82
Fig. 6.9: Structural models of (a) M@C60, (b) La@C82 and (c) Sc3@C82 metallofullerenes (black balls represent the dopant).
6.3.5 Metal Coated Fullerenes
Metal coated fullerene clusters can be fabricated by vapour synthesis method provided the metal exohedral dopants satisfy size limitations. Monolayer coatings of metal atoms are suitable for alkali metal coated fullerenes. For example, in Li12C60, Li is supposed to locate in the centres of each pentagonal face of fullerene C60. Also alkaline earth atoms like Ca, Sr, and Ba are suitable to form multilayer metal structure over the C60 surface. For example, in Ca32C60 one Ca is placing over each of the 12 pentagonal and hexagonal faces. The multilayer metal structures of ordered Ca atoms over the fullrerene C60 surface are shown schematically in figure 6.10. The Ca atoms positioned over the icosahedral vertices of C60 are darkened and N denotes the number of metal layers.
N = 1
C32 C60
N = 2
C104 C60
N = 3
C236 C60
N = 4
C448 C60
Fig. 6.10: Multilayer metal (Ca) covered fullerenes.
6.3.6 Superconductivity in C60
Superconductivity is a state of matter in which the electrical resistance of a sample becomes zero, and in which no magnetic field is permitted to penetrate the sample. This phenomenon is observed in several metals, ceramic materials and nanoscale materials. Kraus et al. (1995) have reported superconductivity in Ba intercalated C60. Magnetization measurements of samples with a Ba6C60 show superconductivity at a critical temperature (Tc) of 6.5 K. The superconductivity in bulk chemically intercalated fulleride salts with a critical temperature of 33 K and in hole doped C60 derivatives in field effect transistor (FET) configurations with a critical temperature of 117 K are reported in the literature.
6.3.7 Applications of Fullerenes
Fullerenes have found tremendous applications in nanotechnology. Fullerenes draw lots of attention from researchers and technologists world wide because of their unique structures and properties. The scientific community has been trying to generate novel applications of these new carbon structures. Fullerenes and their derivatives foud potential use in chemical sensors, such as quartz crystal microbalance (QCM) and surface acoustic wave sensors (SAW). Piezoelectric crystal is the main component of both QCM and SAW. This crystal is highly sensitive to mass variations on its surface, and hence it can be applied for trace quantitative analysis. Since fullerene C60 and its derivatives show affinity to organic molecules, they can be coated onto piezoelectric crystal and used as a chemical sensor for organic molecules like propanol, butanol, ethanol and methanol. The frequency of oscillation of piezoelectric crystal decreases when the organic molecules adsorbed onto the fullerene derivative film. This change of frequency is proportional to the concentration of the adsorbent. Thus, the concentration of the adsorbent can be measured.
Fullerene can be used in photovoltaic cells as organic photovoltaics (OPV). The efficiency of the fullerene/polymer blend bulk heterojunction polymer solar cell is high. In OPV, n-type fullerene is used in combination with a p-type polymer, usually a polythiophene. They are blended and cast as the active layer to create a bulk heterojunction. The semiconducting properties of C60, C70 or C84 can be used for the construct of Organic Field Effect Transistors (OFETS). OFETS made with C84 exhibit greater mobility and stability than that made with C60 or C70.
Fullerenes have very useful applications in health care, where prevention of oxidative cell damage is desirable and in non-physiological applications where oxidation and radical processes are destructive. Fullerenes based drugs can be used in controlling neurological damages caused by diseases such as Alzheimer's disease, which are the result of radical damage. Pharmaceutical companies are also trying to explore the use of fullerene drugs for atherosclerosis, photodynamic therapy and anti-viral agents.
Fullerenes and fullerenic black are chemically reactive and, hence can be mixed to polymer structures to make new copolymers and nanocomposites with enhanced physical and mechanical properties.They can also be added to make composites. Other applications of fullerenes include catalysts, effective water purification and biohazard protection.
6.4 Carbon Nanotube Structures
Iijima was the first to recognize that nanotubes were concentrically rolled graphene sheets with a large number of potential helicities and chiralities rather than a graphene sheet rolled up like a scroll. Iijima initially observed only MWNTs with around 2-20 layers. Later he confirmed the existence of single walled carbon nanotubes (SWNTs) and elucidated their structure.
Single walled carbon nanotubes consists of single sp2 hybridized carbon sheets rolled into seamless tubes and the way the sheet is rolled determines the fundamental properties of the tube. They have diameters ranging from 0.7 to 3 nm and have a length to diameter ratio of about 1000. Thus, CNTs are generally considered to have one-dimensional structures. A SWNT consists of two parts, the sidewall of the tube and the end cap. These two regions have different physical and chemical properties. The structure of the end cap is similar to fullerene C60. The structural details of C60 are discussed in detail on section 6.3.1.
The side wall tube of SWNT is generated when a graphene sheet of suitable dimension is wrapped in a specific direction. One of the two atoms chosen in the graphene sheet can be treat as the origin. The sheet is rolled up till the two atoms coincide. The vector pointing from the origin to the second atom is called the chiral vector. The length of the chiral vector is equal to the circumference of the nanotube (Fig. 6.10) and its direction is perpendicular to the axis of the nanotube. SWNTs with different chiral vectors show different physical properties, such as optical, mechanical and electrical properties.
Fig. 6.11: Two-dimensional graphene sheet showing the circumference Ch, and the period T of the (n, m) nanotube, together with the chiral angle θ.
A nanotube is produced when a graphite sheet is rolled up about the axis T, as shown in figure 6. 11. To describe fundamental characteristics of the nanotube, two vectors, Ch and T, can be introduced. Ch is the vector that defines the circumference on the surface of the tube connecting two equivalent carbon atoms, Ch= nâ1 + mâ2, where n and m are integers, and â1 and â2 are the two basis vectors of graphite.
The chiral angle θ = tan-1[ïƒ-3(n/(2m + n))] (6.2)
Fig: Schematic illustration of rolling graphite sheet to create a CNT.
Source: Image gallery nanotechnology team NASA.
The chiral angle is used to separate carbon nanotubes into three classes differentiated by their electronic properties: armchair (n = m, θ = 30°), zig-zag (m = 0, n > 0, θ = 0°), and chiral (0 < |m| < n, 0 < θ < 30°), as shown in figure 6.12. Armchair carbon nanotubes are metallic (a degenerate semimetal with zero band gap). Zig-zag and chiral nanotubes can be semimetals with a finite band gap if (n - m)/3 = l (l = 0, 1, 2, … and mn) or semiconductors in all other cases. The unique electronic behaviour of each nanotube depends on its band gap. The band gap of the semimetallic and semiconductor nanotubes depends inversely as their diameter. Moreover, the electronic properties of CNts depend on diameter and chirality.
The diameter of the nanotube can be expressed as
dt = ïƒ-3[ac-c(m2 + mn + n2)1/2/Ï€] = Ch/Ï€ (6.3)
where Ch is the magnitude of Ch, and ac-c=1.42 Å, the C-C bond length. Combining different diameters and chiralities may result several hundred individual nanotubes, each with its own distinct mechanical, electrical, piezoelectric and optical properties.
Armchair structure: n = m
Zigzag structure: (n, 0)
Chiral structure: (n, m) m0
Fig. 6.12: Three types of SWNTs identified by the integers (n, m).
6.4.1 Physical Properties of Carbon Nanotubes
6.4.1.1Electrical Properties
CNTs have remarkable electrical properties. The suitable combination of the structural parameters n and m can create metallic CNTs of appropriate conductivity. The values of n and m determine degree of twist of the nanotube. As discussed in section 6.4, the electrical conductivity of nanotube depends on both diameter and degree of twist. As a result, CNTs can be made metallic or semi-conducting, which depends on chirality. Even a small variation of chirality can change a metallic CNT into a semiconductor one. Carbon nanotubes with different twisting angles are shown in figure 6.13a and the effect of twisting on the properties of a metallic nanotube is shown in figure 6.13b.
0 1 2 3 4
(a)
(b)
Fig. 6.13: (a) CNTs with different twisting angles and (b) Variation of band gap energy with twisting angle.
A plot of energy gap of the semiconducting nanotubes versus diameter is shown in figure 6.14. The graph shows that as the diameter of the tube increases, the band gap decreases. The expression for band gap
Egap= (2y0acc)/d, (6.4)
where y0 is the C-C tight bonding overlap energy (2.7 ± 0.1 Ev), acc is the nearest C-C distance (=0.142 nm) and d is the diameter of the nanotube. The fundamental energy gap ranges from 0.4 eV-0.7 eV. Various investigations have established that the band gap of a semiconductor nanotube changes with small variations of diameter and bond angle.
Fig. 6.14: Variation of energy gap with diameter.
The conductance study of nanotubes found that the nanotube behaved as a ballistic conductor with quantum behaviour. The value of conductance, G0 was found to be 1/12.9 kΩ-1, where G0= 2e2/h. In the metallic state the conductivity of the nanotube is very high. The electrical properties of ropes of SWNTs can be investigated by connecting electrodes at different parts of the nanotubes. The resistivity measured for metallic SWNT is approximately 10-6 Ω-m at room temperature. Thus, SWNT ropes are the best conductive carbon fibres. Besides, the current denisty reported for CNT is greater than 1011 A/m2. A single carbon nanotube can carry up to 25 μA of current. This corresponds to an extraordinarily high current density of 1013 A/m2. A typical V-I characteristic for a metallic carbon nanotube is shown in figure 6.15. There is a linear increase of current at low bias voltage (the ballistic regime), but then rolls over and saturates at high bias.
MWNTs show superconductivity with interconnected inner shells, at a critical temperature (Tc) of 12 K. However, the critical temperature is lower for ropes of SWNTs and MWNTs without interconnected shells. Ultra small SWNTs exhibit superconductivity at a transition temperature of about 20 K.
Fig. 6.15: V-I Characteristics of a metallic carbon nanotube.
It is interesting that individual SWNTs contain defects and these defects help the SWNTs to work as transistors. Besides these, connecting CNTs together may help to develop new transistor like devices. For example, if straight metallic section of a SWNT is joined to a chiral semiconducting section, a diode will be formed. When SWNTs are used as interconnects on semiconductor devices, they can transmit electrical signals at an amazing speed of about 10 GHz. It is worth to note that semiconductor SWNTs can be used in field effect transistors (FETs) instead of silicon. Furthermore, the energy dissipation in nanotubes will be very small because of their low resistance.
6.4.1.2 Mechanical Properties
Carbon nanotubes have very good mechanical properties because of the strength of the sp2 carbon-carbon bonds. CNTs are considered to be the strongest and stiffest materials in terms of their tensile strength and elastic modulus. In addition, they are extremely resistant to damage and can withstand physical forces. When stress is applied on the tip of a nanotube, it will bend without any damage to the tip. The tip regains its initial state just after the deforming force is removed. This reamarkable property of CNTs makes them suitable for probe tips in high resolution scanning probe microscopy.
The typical value of Young's modulus of nanotubes is 1-2 TPa, about 5 times higher than steel. The tensile strength of nanotubes is around 150 GPa, about 50 times higher than steel. These two properties along with the lightness of CNTs, give them great potential in applications like aerospace. The fabrication of a super hard material with a hardness of 62-152 GPa by compressing SWNTs over 24 GPa at 300 K has been reported in the literature. The bulk modulus (K) of the super hard material is 462-546 GPa, whereas the K value of diamond is 420 GPa only. These exceptional mechanical properties are ideal for reinforced composites, nanoelectromechanical systems (NEMS) and for probes in scanning force microscopes.
Table 6.2: Physical properties of carbon nanotubes
Material
Density
(g/cm3)
Young's Modulus
(TPa)
Tensile strength (GPa)
Elongation at break (%)
Thermal Conductivity
W/m/K
SWNT
-
~1 (from 1 to 5)
13-53
16
~2000
Armchair SWNT
1.33
0.94
126.2
23.1
-
Zigzag SWNT
1.34
0.94
94.5
15.6-17.5
-
Chiral SWNT
1.40
0.92
-
-
-
MWNT
-
0.8-0.9
11-150
-
-
6.4.1.3 Magnetic properties
Carbon nanotubes, either metallic or semiconducting, possess novel magnetic properties. It is significant that by applying suitable magnetic field along the axis of a carbon nanotube to control its metallicity. A CNT can be made either semiconducting or metallic, depending on the strength of the applied field. Its band gap will be an oscillatory function of magnetic field with a period Φ0 = h/e (the magnetic flux quantum). Thus, metallic tubes can be made semiconducting by applying a (even infinitesimally small) longitudinal magnetic field. But, the semiconducting nanotubes can become metallic in ultrahigh magnetic fields.
Systematic investigation on the magnetic susceptibilities of multiwalled carbon nanotubes as a function of temperature or magnetic field has found that the susceptibility of CNT was only around half that of graphite. It is also found that per carbon basis the susceptibility of CNT bundles is even larger than that of graphite. Studies of the magnetic properties of aligned multiwalled carbon nanotubes at various temperatures and magnetic field orientations found that the nanotubes are diamagnetic and anisotropic. Carbon nanotubes show variation in its resistance with a DC magnetic field at low temperature. This effect is called magnetoresistive effect. The transition of the samples from positive to negative magnetoresistance is also reported.
6.4.1.4 Aspect Ratio
CNTs exhibit high aspect ratio (about 1000:1), and hence can be used as conductive additive materials for all types of plastics. This large value of aspect ratio helps to provide electrical conductivity in plastics at small concentrations. Furthermore, CNTs have the potential to become an outstanding additive to offer electrical conductivity in plastics.
6.4.2 Applications
The novel properties of carbon nanotubes make them ideal candidate for many potential applications such as energy storage, actuators or artificial muscles, AFM probe tips, batteries, biosensors, drug delivery, data storage, hydrogen storage, microelectromechanical (MEMS) devices, nanolithography, super capacitors, thermal protection, waste recycling, composite materials, nanoporous filters and so on. A few potential applications of carbon nanotubes are presented in this section.
6.4.2.1 Molecular Electronics and Integrated Circuits
In any electronic circuit, particularly at nanoscale dimensions, the interconnections between various active and passive components are very difficult. The geometry, electrical conductivity and other unique characteristics of CNTs make them suitable for the interconnections in molecular electronics. Moreover, they can be used as switches themselves. The field effect transistor (FET) can be constructed with a semiconducting SWNT. When a suitable voltage is applied to the gate of the FET, the conducting nanotube transforms to a non-conducting state. Such CNT transistors can be coupled together to function as a logic gate, the basic component of computers. In 1998, the first carbon nanotube field effect transistors (CN-FETs) were fabricated at Delft and at IBM.
CNTs have very high thermal conductivity and hence, they can be used to develop CNT based heat sinks to eliminate heat from ICs and microprocessors. Since CNTs have novel conducting and semiconducting characteristics, they are robust for electronic devices rather than silicon based electronic devices. Hence, individual CNT based electronic devices may replace silicon devices.
6.4.2.2 Field Emission and Flat Panel Displays
When a small electric field is applied along the nanotube axis, the ends of CNT emit electrons with a large emission rate like water being pushed through a high powered hose. This effect is called field emission. The emission of electrons increases as the strength of the electric field enhances. The emission also depends on work function of the material. Carbon nanotubes are the best field emitters of any known materials because of the tip sharpness and large electrical conductivity. Even a small p.d. is applied across the tip and an electrode that is close to the tip, a very high electric field (nearly millions of volts/cm) set up near the tip. This high electric field is due to the tip sharpness and the electric field generated is inversely proportional to the radius of curvature of the tip. These large electric fields pull an enormous number of electrons out of the tip. For example, CNT films emit nearly 4 A/cm2. Furthermore, the emission current is extremely stable. These significant properties of CNTs are employed in the construction of field emission flat panel displays. For each pixel in the display of a flat panel, separate electron guns are used instead of a single electron gun in the conventional CRT display. The unique features of CNTs such as, high current density, low operating voltage, steady and long life make them ideal field emitters for flat panel displays. All these features make CNTs appropriate for many other applications also, such as AFM tips, microwave amplifiers, lightning arrestors, electron microscope cathodes, high resolution X-ray sources and cold-cathode lighting sources.
Researchers are looking ahead to explore suitable technology to replace conventional electron gun with CNT electron gun. Figure 6.16 shows the schematic diagram of a CNT based flat panel display. Such flat panel displays consume less power; exhibits speed and excellent brightness; offer wide viewing angle and a large operating temperature range.
Fig. 6.16: Schematic illustration of a CNT based flat panel display. (ITO: indium tin oxide)
6.4.2.3 Medical Application
Nanotubes have a wide range of significant applications in medical field. They found profound impact on biomedical and pharmaceutical fields. Numerous investigations have established that CNTs could be used as a grug delivery vehicle to carry drugs to infected cells. In the near infrared (NIR) range, single walled carbon nanotubes show strong light absorbance. When this light illuminates SWNT, localized heat will be produced due to the absorbance of NIR light. The heat thus produced will help to stimulate the release of drugs or genes from the nanotube surface. Multi walled nanotubes can easily pass through the cell membrane even without cell damage, as they act like nanoneedles. CNT based nanobiosensors can be used for diagnostics and drug discovery. Besides, CNTs can act as a powerfull tool to explore basic information about targets for therapies, since they can detect small amounts of molecules through electrical, optical, or mechanical means.
Actuators or Artificial Muscles
CNT actuators convert electrical energy to mechanical energy, causing the nanotube to move. Usually, a CNT actuator consists of two pieces of paper made from CNTs and are placed on either ends of a piece of tape that is attached to an electrode through which electric current is passed. When current is applied, the electrons will be pumped into a piece of CNT paper. Hence, nanotubes on that side of the paper expand and causing the tape to curl in one direction. This movement is similar to the way an artificial muscle works. Nanotube actuators will have applications in robotics and prosthetics.
