Carbon is the key element in all living things and its ability to bond with itself has led to form oligomers and polymers that are vital to origin and sustainability of life. This remarkable property has led to myriad of other structures that has made carbon an important commodity element in both natural and manmade constructions. The most common allotrope of carbon is graphite, which is an abundant natural mineral. The second, rarer allotrope, diamond is also a naturally occurring mineral.
Carbon can make four bonds with other atoms. The way these bonds are made plays an important role in determining the chemical and structural properties of the final substance. In the simplest case, carbon can bond to each other by a single bond (C-C bond or sp3 hybridization). However, carbon can also bond to itself with double bonds and triple bonds (C=C: sp2 hybridization and C≡C: sp hybridization) producing completely different chemical and physical properties. This ability of carbon to bond to itself in multiple forms has given carbon many of its unique properties. This means that wide variety of structures is possible that has completely made of carbon. This ability to make all carbon compounds, particularly with double bonds, is one of the key driving factors of current nanotechnology revolution.
Apart from this carbon has many other advantages. Carbon is a lighter material and twelfth element in the periodic table. Covalent bonds between carbon, especially double bonds and triple bonds, can be particularly strong. This gives carbon nanostructures an amazing strength. In some cases multiple times higher than even stainless steel. Carbon can act as an electrical conductor or an insulator depending on the type of bonds present in the material. Same is true for the thermal conductivity of the carbon structures.
Carbon nanostructures are all carbon compounds that at least one of the dimensions is in the range of 1 to 100 nm. Scientists have observed number of carbon nanostructures including 0D ( carbon nanoparticles, fullerenes, carbon onions, carbon helices ) , 1D ( single and multi wall carbon nanotubes, carbon nanofibers) , 2D ( graphene) and 3D ( carbon nanohorns, carbon nanoforms). In this section we will primarily discuss on fullerenes, graphene and carbon nanotubes.
Graphene is a one atom thick planer sheet of sp2 bonded carbon atoms that are organized in to a hexagonal lattice, much similar to a honeycomb structure. Graphene, is often referred to as the wonder material due to number of extraordinary mechanical, structural, electrical and thermal properties it shows. Graphene is almost transparent yet 200 times stronger than steel. It’s one of the best heat and electric conductor known to man. Due to these reasons, graphene is under intense scientific exploration to exploit these amazing properties to real world applications.
Although, conceptualized many years ago, it was measurably produced and analyzed in the lab in 2003. These investigations revealed information about composition, structure and many of the properties it shows. Andre Geim and Konstantin Novoselov from the University of Manchester won the Nobel Prize in Physics in 2010 for the pioneering the experiments in this interesting two dimensional material.
Graphene is the mother of all graphitic nanostructures such as graphite, carbon nanotubes, fullerenes, etc. As all these nanostructures can be made by folding graphene in a specific way. The most common graphitic material, graphite, consist of stacked graphene layers on top of each other.
There are two distinct methods of fabricating graphene: mechanical exfoliation of graphene from graphite and growing graphene epitaxially on surfaces such as SiC crystals. The mechanical exfoliation technique was introduced and elaborated mainly by the Manchester group. This method involves, peeling of the graphite using a simple scotch tape to its primary component. This extremely simple can produce graphene with very high quality. However, this fabrication method has limitations in scaling up for large scale manufacturing of graphene. This is mainly due to breakage of graphene to small sizes during the process. Also, presence of few layer graphene structures are not uncommon in this method, that would impair the quality of the final product.
An alternative method to fabricate graphene is to grow it on special crystal surfaces using precursor materials. The method involves, of exposing an epitaxially grown hexagonal Silicone carbide crystal to temperature around 1300 degrees to evaporate out loosely bound silicone atoms from the surface. The remaining carbon atoms on the surface then rearrange to form a graphene layer. Another increasingly popular method is to grow graphene on copper surfaces using chemical vapor deposition method. Usually methane and hydrogen is used as the precursor gasses and the deposition is carried out under inert gasses at temperatures around 1100 degrees.
Graphene is a subject of great interest in number of different fields. Due to high transparancey and good electrical conductivity, graphene can serve as a promising mater in optoelectronic field. Specifically, in applications like transparent conductive electrodes, liquid crystal displays, touchscreens, etc. Due to high electrical conductivity, innertness, surface area, thinness and strength graphene based materials are investigated for bioengineering applications as well. Graphene has also made its entrance to the composite science. The high surface area, high strength, flexibility and light weight are all plus points for this materials.
There are number of applications of graphene in energy strorage, water purification, agriculture, textiles, coatings, etc.
Carbon nano tube (CNT)
Another, one of the highly explored allotrope of carbon is carbon nanotubes. These are cylindrically arranged structures of carbon in which the diameters and length in the range from 1-100 nm and 10 nm to few centimeters, respectively. When observed from a macroscopic magnification, CNTs are viewed as an interwoven strands of fibers, much like a cooked noodles. There are occasionally (depending on the synthesis procedure), nanotubes with structural anomalies such as junctions in the tubular structure. Y-junctions and T-Junctions are quite common.
One can view the CNTs as a seamless cylinders of rolled up graphene. CNTs can be broadly classified in to three categories, depending on the number of rolled up graphene sheets; single wall CNT (CWCNT), double walled CNT (DWCNT) and multi walled CNT (MWCNT). The graphene seets can be rolled at specific and distinct angles. Which is commonly referred to as “chiral” angles. This angle and the width of the nanotube are quite important parameters in determining the properties of a nanotube; for an example, whether the nanotube shows metallic or semiconducting properties.
Carbon nanotubes (although this name was not used) have been reported in scientific literature, 60 years earlier, but not picked up by the scientific community at that time. In 1952 L. V. Radushkevich and V. M. Lukyanovich published clear images of 50 nanometer diameter tumbes made of carbon in the Soviet Journal of Physical Chemistry. In the 1970’s Morinobu Endo, observed CNTs again, produced by a gas phase process. In 1987, Howard G. Tennett of Hyperion Catalysis received a US patent for a production method of CNTs which he called “cylindrical discrete carbon fibrils” with a dimater between 3.5 to 70 nm. With aspect ratios around 100. In 1991 Sumio Iijima of NEC observed multiwall nanotubes formed in a carbon arc discharge process, and two years later, he and a scientist named Donald Bethune at IBM independently observed single-wall nanotubes. By this time all the carbon forms were observed and in the years followed many scientists got on to the bandwagon.
Generally, CNTs are fabricated by heating a carbon containing precursor in the presence of a catalyst. The process generates a black carbon soot, which contains CNTs. Changes in the reaction conditions, such as pressure, temperature and changes in precursor gases play critical role in determining the end dimensions of the resulting product. There are also number of other procedures developed by the scientists. There are other three major methods of fabricating CNTs: 1) arc-discharge method 2) laser ablation method 3) catalyst assisted chemical vapor deposition.
As previously noted, catalyst assisted chemical vapor deposition is widely used technique as it generally produce, high yield of CNTs with high quality and purity. Also, it can produce ordered CNTs in aligned manner. The reaction is carried out in a horizontal reactor typically made of quartz. The carbon precursor is supplied in to the reactor and passed through the catalyst bed. The temperature of the reactor is typically maintained the range of 600-1100 degrees. At these conditions, precursors and their derivatives build on the catalyst resulting a CNT. The catalyst can also be introduced to the reaction chamber with the precursor or inert gas carrier. This method is called floating catalyst method and it further increase the catalyst surface area. In this method, no subsequent processes are required to remove the CNTs from a support bed.
CNTs like graphene, has been a focus of extensive research and scientific applications because of vast array of advantages properties it shows. The major properties of CNTs, among others, are high electrical conductivity, high tensile strength, high thermal conductivity, high surface area, etc. As a result of these extraordinary properties, it serve as a key nanomaterial in number of arenas such as, electronics, display, composites, microscopy, sensors, textiles and variety of applications in biomedical field.
Fullerene is an all carbon molecule that follows the structure and shape similar to that of a soccer ball. The most well-known and widely explored fullerene, C60 is a molecule with 60 carbon atoms linked together to form one atom thick, hollow sphere. This requires that all the carbon in the molecule need to be double bonded (sp2 hybridized) to form 12 pentagons and 20 hexagons. Similarly, other higher order fullerenes can be hypothesized, by keeping the pentagons constant and changing the number of hexagons. Later these structures were also experimentally produced and verified as C72, C80, etc.
Discovery of fullerene in 1985 by Richard Smalley, Robert Curl, James Heath, Sean O’Brien, and Harold Kroto at Rice University is considered to be a landmark incident in nanotechnology as it sparked the study of nanocarbon structures and motivated the emergence of nanotechnology field. First methods of making fullerenes were complex, and only produce small quantities of the material. However, in 1990, German physicist, Wolfgang Krätschmer found a simple procedure to prepare fullerenes in isolable quantities.
The first method of production of fullerenes was laser ablation of carbon in an inert atmosphere. This method only produced fullerenes in microscopic amounts. Later, an improved method for fullerene production was invented by Kratschmer and Huffmann through arc assited vaporization of graphite. This method is still practiced today and remain as a major fullerene production technique. Recent years, much simpler system for fullerene was invented through combustion of aromatic fuels such as benzene, toluene, etc. The method was pioneered at MIT and increasingly finding popularity among the scientists. All these methods produce fullerene containing soot that needed to be extracted using a suitable solvent. Usually toluene is used as it can dissolve most of the fullerenes. Then the fullerenes needed to be separated to yield pure substances. The technique usually used for this is chromatography and largely determines the cost of the final product. It’s believed that the carbon atoms coalesce to give small aromatic structures that are later assembled themselves to form stable structures. However, the exact mechanism of fullerene production is not yet clearly understood.
Fullerenes are heavily exploited in photovoltaic devices, due to its high electron affinity and superior charge transport capabilities. Fullerenes and their modified counterparts are widely used as photon acceptor components in polymer solar cells. They are also explored for applications in optical devices, chemical sensors, gas separation devices, hydrogen storage media and as polymeric fillers. Some fullerenes with or without modifications were identified to have antiviral activities and potential materials for drug delivery.