For many hundreds of years, diamond and graphite (Figure 1) were the only known crystalline allotropic forms of carbon. The discovery in the 1980’s of fullerenes, a family of crystalline Cn compounds (with n even), was rewarded with the 1996 Nobel Prize for Chemistry to Robert F. Curl Jr., Sir Harold W. Kroto and Richard E. Smalley.

    

  

   

   

Fullerenes are spherical molecules, the smallest of which composed of 60 carbon atoms that are arranged like the edges of the hexagons and pentagons on a football.

Therefore, fullerenes in general form an interesting class of compounds that surely will be used in future technologies and applications. Before the first synthesis and detection of the smaller fullerenes C60 and C70, it was generally accepted that these large spherical molecules were unstable. However, some Russian scientists already had calculated that C60 in the gas phase was stable and had a relatively large band gap.

As is the case with numerous, important scientific discoveries, fullerenes were accidentally discovered. In 1985, Kroto and Smalley found strange results in mass spectra of evaporated carbon samples. Herewith, fullerenes were discovered and their stability in the gas phase was proven. The search for other fullerenes had started.There are many other fullerenes of different shapes and sizes, such as C70, C82 etc. A model of a C60 molecule is shown on theReight.

Since their discovery in 1991 by Iijima and coworkers, carbon nanotubes have been investigated by many researchers all over the world. Their large length (up to several microns) and small diameter (a few nanometres) result in a large aspect ratio. They can be seen as the nearly one-dimensional form of fullerenes.

Therefore, these materials are expected to possess additional interesting electronic, mechanic and molecular properties. Especially in the beginning, all theoretical studies on carbon nanotubes focused on the influence of the nearly one-dimensional structure on molecular and electronic properties.

Nanotubes can be described as a rolled-up tubular shell of graphene sheet , which is made of benzene-type hexagonal rings of carbon atoms. The body of the tubular shell is thus mainly made of hexagonal rings (in a sheet) of carbon atoms, whereas the ends are capped by half-dome shaped half-fullerene molecules.

Single Walled Nanotubes (SWNT) can be considered as long wrapped graphene sheets. As stated before, nanotubes generally have a length to diameter ratio of about 1000 so they can be considered as nearly one-dimensional structures.

Carbon nanotubes may be classified into three different types: armchair, zigzag, and chiral nanotubes, depending on how the two-dimensional graphene sheet is "rolled up".

The first of the three structural categories is zigzag, which is named for the pattern of hexagons as one move circumferentially around the body of the tubule (Figure 2.3(a)). The second of these nanotube structures is termed armchair, which describes one of the two conformers of cyclohexane, a hexagon of carbon atoms, and describes the shape of the hexagons as one move around the body of the tubule (Figure 2.3(c)). The third form is known as chiral (Figure 2.3(b)) and is believed to be the most commonly occurring SWNT. The name chiral means handedness and indicates that the tubes may twist in either direction. The geometry of the chiral SWCNT lies between that of the armchair and zigzag SWCNTs (see Figure 2.3 (b)).

Multi walled carbon nanotubes (MWCNT) are nanotubes with more than one graphene cylinder nested one into another . The spacing of intershell by using image of TEM with high resolution; spacing found to vary from 0.34, augmenting with the diameter of tube diminishing. According to theoretical calculations the distance between two layers is d = 3.39 Å, slightly bigger than in graphite. Based on TEM images, the interlayer separation of d = 3.4 Å is commonly reported for MWCNT

CNTs & DRUG DELIVERY 17

Figure 3. An animation of a rotating CNT shows its 3D structure.

Figure 1. A 3D model of single-wall carbon nanotube (SWNT).

Figure 2. A 3D model of multi-wall carbon nanotube (MWNT).

carbon NanoBuds are a newly discovered material combining two previously discovered allotropes of carbon: carbon nanotubes and fullerenes. In this new material fullerenes are covalently bonded to the outer sidewalls of the underlying nanotube.

Consequently, NanoBuds exhibit properties of both carbon nanotubes and fullerenes. For instance, the mechanical properties and the electrical conductivity of the NanoBuds are similar to those of corresponding carbon nanotubes, however, because of the higher reactivity of the attached fullerene molecules, the hybrid material can be further functionalized through known fullerene chemistry

Additionally, the attached fullerene molecules can be used as molecular anchors to prevent slipping of the nanotubes in various composite materials, thus improving the composite’s mechanical properties.

Nanofibers consist of the graphite sheet completely arranged in various orientations. One of the most outstanding features of these structures presence of a plenty of sides which in turn make sites, with readiness accessible to chemical or physical interaction, especially adsorption. Carbon nanofibers change from 5 to several hundred microns on length and between 100- 300 nm in diameter.

In this special case, a plate of graphite oriented itself in a “herringbone” classification. Surface science studieshave revealed that certain faces prefer precipitation of the carbon in the form of the graphite.

It was pointed out that the distance between graphite is 0.34 nm separate from layers one of other one.

Examination of the high-resolution electron micrographs clearly indicates that the graphite platelets in these two examples are aligned in directions perpendicular (Figure 2.9 a) and parallel (Figure 2.9 b) to the fiber axis.

(a) [b)Fig. 2.9: High resolution electron micrographs and schematic representation of carbon nanofibers with their graphite platelets, (a) "perpendicular" and (b) "parallel" to the fiber axis

StrengthCarbon nanotubes are one of the strongest

and stiffest materials known, in terms of tensile strength and elastic modulus respectively. In 2000, a multi-walled carbon nanotube was tested to have a tensile strength of 63 Gpa. In comparison, high-carbon steel has a tensile strength of approximately 1.2 GPa. CNTs have very high elastic moduli, on the order of 1 TPa. Since carbon nanotubes have a low density for a solid of 1.3-1.4 g/cm³, its specific strength of up to 48,462 kN·m/kg is the best of known materials, compared to high-carbon steel's 154 kN·m/kg.

Electrical Because of the symmetry and unique

electronic structure of graphene, the structure of a nanotube strongly affects its electrical properties. For a given (n,m) nanotube, if n - m is a multiple of 3, then the nanotube is metallic, otherwise the nanotube is a semiconductor. Thus all armchair (n=m) nanotubes are metallic, and nanotubes (5,0), (6,4), (9,1), etc. are semiconducting. In theory, metallic nanotubes can have an electrical current density more than 1,000 times greater than metals such as silver and copper.

Thermal All nanotubes are expected to be very good

thermal conductors along the tube, exhibiting a property known as "ballistic conduction," but good insulators laterally to the tube axis. It is predicted that carbon nanotubes will be able to transmit up to 6000 watts per meter per kelvin at room temperature; compare this to copper, a metal well-known for its good thermal conductivity, which only transmits 385 W/m/K. The temperature stability of carbon nanotubes is estimated to be up to 2800 degrees Celsius in vacuum and about 750 degrees Celsius in air.