**3. Carbon allotropes**

The most prominent allotropes of carbon are graphite, diamond and lonsdaleite. Each one has its specific properties which very varying from that of another one [16].

Graphite was named by German mineralogist and chemist A.G. Wenner in 1789 [11]. It is a naturally occurring material found in a metamorphic rocks. Its structure made by holding many layers via weak forces. Each layer consisting carbon atoms arranged themselves in a honeycomb lattice [16]. Graphite is characterized by its

dark opaque appearance with soft texture and well electrical conduction property. Within its structure, three electrons of each carbon atom shearing with another neighbor by single bonds while the fourth electron become delocalized overall the whole sheet of atoms in each layer. The delocalized electrons are not fixed to a specific carbon atom and can be move freely on the sheet's surface leading to induce a temporary dipoles which induced an opposite dipoles above and below each sheet that leads to hold the overall sheets of graphite structure [17].

Diamond was derived its name from the Greek word "adams". Diamond is the cubic high pressure phase of elemental carbon [18]. It is occurred naturally and it's the hardest known material [18], with a transparent appearance [11] or can be exist in different colors depending on the presence of specific elements or some structural defects with a very low electrical conduction [18]. In diamond, carbon atoms forming 3-D lattice structure in which each carbon atom shears its four electrons with another carbon atom by four single bonds [11].

Lonsdaleite, the hexagonal diamond [11], it is named in honor of Kathleen Lonsdale. It is found naturally in a graphite containing meteorites that struck earth. It is a transparent material with hardness much higher than that of diamond [11, 16].

Amorphous carbon [19], is an interested allotrope, in which carbon atoms can be exist with a wide variety of hybridization states. The most familiar form of amorphous carbon is diamond-like carbon (DLC) due to its structure which is amorphous with many fraction of sp3 bonding so it exhibit some important characteristics of diamond without the necessary for complicated synthesis techniques [20]. It is characterize by its high hardness, low friction property [20] and chemical inertness [20].

Furthermore, there are several allotropes of carbon atom exist in nano- scale featured by their unique structures that led to a great explosion in materials science and technology. Here we will give a summarize description about the structure of some of these materials [21].

Graphene, attractive nano- scale allotrope, is an atomic thick sheet made up of carbon atoms arranged themselves in a honeycomb lattice. So, it is a 2-D allotropic form within carbon family. It is represents the structure of other allotropes of carbon due to its structure flexibility. By rolling its structure, carbon nanotubes is obtained and by warping up a fullerene allotrope is obtained while the stacking of several sheets leads to produce graphite structure. So it is called a mother of all other carbon allotropes, see **Figure 2**. Due to its attractive properties it is candidates for many critical applications in industrial [4] and health care fields [22].

Carbon nanotubes is a cylindrical structure nano allotrope of carbon with a diameter of several nanometers and micro- scaled length, consisting of rolled graphene sheets. Its structure can be vary by its diameter or, its length or its layers number.

There are two main types of CNTs, single walled SWCNTs and multi walled MWCNTs. Several studies had been clarified that SWCNTs have about (1–3) nm diameter and few micrometers in their length, while MWCNTs have about (5–40) nm diameter with length around 10 micrometers. This type of carbon allotropes have unprecedented aspect ratio property due to their unique structure together with high thermal and electrical conductivity compared to other conductive materials [4, 23].

Fullerene and nanodiamond another allotropes of carbon atom, have unique and novel properties due to their unique nanostructure. These two carbon nanomaterials will be discussed later.

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*Fullerenes and Nanodiamonds for Medical Drug Delivery*

*DOI: http://dx.doi.org/10.5772/intechopen.97867*

**4. Hybridization of carbon atoms**

tion characteristics [24, 25].

*Representation of carbon allotropes.*

carbyne- general family (sp1

family (sp2

**Figure 2.**

family (sp3

carbon [14, 27].

The recent materials discoveries at nano scale open new horizons in all science and engineering fields. One of the most important point of view reported in several literatures is depending on classify carbon materials according to their hybridiza-

The electron hybridization determines the ability of carbon atoms to arrange themselves in a wide variety of structures with linear, planar and tetrahedral symmetry forming different allotropes, such as carbyne/carbyte, graphene/ graphite and diamond. So, depending on hybridization type carbon structures can be classified from sub-molecular to macroscopic scales into three general families:

hybridization state from its nanostructured like carbyne or cyclo-carbon to its macrostructural crystalline form [26]. The second class called graphene general

in their nano as well as macro-crystal structures with hexagonal and rhombohedral structures like graphene and graphite. The third class called diamond general

For more complexation, carbon atoms can be exist with other hybridization states called intermediate hybridizations with different degrees. The degree of

family), this family includes carbon atoms with sp2

family), the carbon within this family existing sp3

leading to produce strained C–C bonds. Usually carbon atoms with pure sp2 hybridization state shows an ideal flat structure. While in the case of atoms with curved structure, their hybridization degree should be >2 as in the case of fullerene

themselves in hexagonal rings as well as in a various other polygonal rings. The non-hexagonal rings can leads to curving the flat sheet or keeping its flatness if the

like diamond structure in its nano and macro scales [27].

hybridization in this case caused by the curvature of the sp2

On the other side, usually carbon atoms in sp2

polygon arrangement fulfils certain symmetry rules [14].

family), this family includes carbon atoms with sp1

hybridization state

hybridization state

hybridized structure,

hybridization state arrange

*Fullerenes and Nanodiamonds for Medical Drug Delivery DOI: http://dx.doi.org/10.5772/intechopen.97867*

*Materials at the Nanoscale*

diamond [11, 16].

inertness [20].

care fields [22].

ers number.

materials [4, 23].

als will be discussed later.

amorphous with many fraction of sp3

some of these materials [21].

dark opaque appearance with soft texture and well electrical conduction property. Within its structure, three electrons of each carbon atom shearing with another neighbor by single bonds while the fourth electron become delocalized overall the whole sheet of atoms in each layer. The delocalized electrons are not fixed to a specific carbon atom and can be move freely on the sheet's surface leading to induce a temporary dipoles which induced an opposite dipoles above and below each sheet

Diamond was derived its name from the Greek word "adams". Diamond is the cubic high pressure phase of elemental carbon [18]. It is occurred naturally and it's the hardest known material [18], with a transparent appearance [11] or can be exist in different colors depending on the presence of specific elements or some structural defects with a very low electrical conduction [18]. In diamond, carbon atoms forming 3-D lattice structure in which each carbon atom shears its four

Lonsdaleite, the hexagonal diamond [11], it is named in honor of Kathleen Lonsdale. It is found naturally in a graphite containing meteorites that struck earth. It is a transparent material with hardness much higher than that of

Amorphous carbon [19], is an interested allotrope, in which carbon atoms can be exist with a wide variety of hybridization states. The most familiar form of amorphous carbon is diamond-like carbon (DLC) due to its structure which is

acteristics of diamond without the necessary for complicated synthesis techniques [20]. It is characterize by its high hardness, low friction property [20] and chemical

Furthermore, there are several allotropes of carbon atom exist in nano- scale featured by their unique structures that led to a great explosion in materials science and technology. Here we will give a summarize description about the structure of

Graphene, attractive nano- scale allotrope, is an atomic thick sheet made up of carbon atoms arranged themselves in a honeycomb lattice. So, it is a 2-D allotropic form within carbon family. It is represents the structure of other allotropes of carbon due to its structure flexibility. By rolling its structure, carbon nanotubes is obtained and by warping up a fullerene allotrope is obtained while the stacking of several sheets leads to produce graphite structure. So it is called a mother of all other carbon allotropes, see **Figure 2**. Due to its attractive properties it is candidates for many critical applications in industrial [4] and health

Carbon nanotubes is a cylindrical structure nano allotrope of carbon with a diameter of several nanometers and micro- scaled length, consisting of rolled graphene sheets. Its structure can be vary by its diameter or, its length or its lay-

There are two main types of CNTs, single walled SWCNTs and multi walled MWCNTs. Several studies had been clarified that SWCNTs have about (1–3) nm diameter and few micrometers in their length, while MWCNTs have about (5–40) nm diameter with length around 10 micrometers. This type of carbon allotropes have unprecedented aspect ratio property due to their unique structure together with high thermal and electrical conductivity compared to other conductive

Fullerene and nanodiamond another allotropes of carbon atom, have unique and novel properties due to their unique nanostructure. These two carbon nanomateri-

bonding so it exhibit some important char-

that leads to hold the overall sheets of graphite structure [17].

electrons with another carbon atom by four single bonds [11].

**88**

**Figure 2.** *Representation of carbon allotropes.*
