Graphene is a one atom thick layer of sp2 hybridized carbon atoms which are arranged in a honeycomb type lattice. It has unique physical, electrical, chemical, mechanical and optical properties including large surface area, exceptional electrical conductivity, very high mechanical strength, and unmatchable thermal conductivity. It is capable of absorbing a large number of bio molecules by π-π stacking bonding or electrostatic bonding, due to these properties, it is considered as one of the best materials for the fabrication of biosensors and for loading drugs. It is the parent of other important allotropes of carbon i.e. 0D fullerenes, 1D carbon nanotubes, and 3D graphite. Graphene and graphene based materials are making huge impact in many applications including graphene field effect transistors, sensors, catalysis, and bioscience /biotechnologies (bio imaging, gene delivery, cancer therapy, tissue engineering and drug delivery). On 5 October, 2010, Andre Geim and K. Novoselov received the Noble Prize in Physics for their remarkable work on graphenewhereingraphene was synthesized by mechanically exfoliating graphite. Since then, a number of methods have been discovered for the synthesis of graphene such as chemical exfoliation, chemical vapor deposition and epitaxial growth on Silicon carbide.Inspite of have a large number of applications, graphene has certain limitations i.e. limited solubility and difficulty in functionalization, due to which it has limited applications in the field of biomedical engineering. In order to overcome these limitations, the derivatives of graphene such as graphene oxide (GO), graphene quantum dots (GQD),etc have been considered to be more suitable.

GO, a suspended form of graphite is synthesized by oxidizing graphite to graphite oxide and then exfoliating graphite oxide to GO. It is a single graphitic monolayer with randomly distributed aromatic regions (sp2 carbon atoms) and oxygenated aliphatic regions (sp3 carbon atoms) containing hydroxyl, epoxy, carbonyl and carboxylic functional group. The presence of functional groups in GO provides a remarkable hydrophilic character and facilitates immobilization of enzymes . Thus GO is much more advantageous than graphene in the field of biosensors and biomedical engineering.

Carbon allotropes

Carbon is one of the most abundant atoms on earth, which naturally occurs in many forms. Carbon has a melting point of about 3500oC and atomic radius of 0.077nm. It is the basis in all organic compounds. Materials made up of only carbon atoms are known as allotropes of carbon. Graphite and diamond are its most commonly used allotropes. It can either occur as amorphous carbon (unordered structure) or glassy carbon (semi ordered structure). Crystalline carbon exists in three forms: Diamond, Graphite, and Fullerene. At nano scale carbon can be broadly divided into four structures:

1. Fullerene
2. Tubes
3. Cones
4. Carbon black

Diamond lattice is sp3 hybridized in which each carbon atom is joined with four other carbon atoms whereas graphite has flat hexagonal carbon atom layers with 3.35 Å (angstroms) separation, and the two nearest carbon atom distance of 1.42 Å in the layer. Variations on the structure of graphene can conceptualize the other allotropes of carbon. Till now graphite is the most common form of carbon which can be used as lubricant, in the core of pencil etc. Graphite structure is composed of many layers of graphene which are stacked together on the top of each other. Another important nano scale allotropes of carbon are carbon nano tubes (CNTs). They can be thought of graphene sheets rolled into tubes. Owing to extraordinary strength and unique electrical properties, they can be used in many applications such as in electronics, optics, etc. They can possess both metallic and semiconducting electrical properties. Lastly, buckyballs are zero-dimension carbon allotropes which are named after Richard buckminister fuller, an architectural engineer who designed geodesic dome with the similar shape. They have many proposed applications such as encapsulation of reactive compounds, isolation of quantum system, etc.

Graphene

Graphene is one of the most durable and finest monolayer which is capable of free existence. The endurance of graphene is much greater than any other material and is the strongest and thinnest material known till date. Graphene was discovered in 2004 by Novoselov and Geim. They separated the graphene sheets by exfoliating graphite flakes using adhesive tape. After that many other techniques for the synthesis of graphene have been developed. Owing to its unmatchable properties, graphene can be used in a large number of applications.

Properties of graphene

Graphene is the purest form of carbon and it has a large surface area. Apart from this, it has high intrinsic mobility, high Young’s modulus and excellent thermal and electrical conductivity . It has an optical transmittance of about 97.7%. Graphene has a robust and flexible membrane, which provides essentially infinite possibilities for the modification or functionalization of its carbon backbone. It is the slimmest known material in the world and the strongest known till date. Its charge carriers have very high intrinsic mobility, zero effective mass, and can travel up to micrometers without being scattered at room temperature.

Methods of graphene synthesis

The properties of graphene are determined by the process of its fabrication. Mechanical exfoliation is the traditional technique for the synthesis of graphene. However, the graphene flakes synthesized by this method are not scalable and are of very small size. Also, this process is time consuming when fabricating small size graphene. This method is less efficient. Epitaxial growth on SiC is the process in which silicon carbide is decomposed at high temperature where Si is evaporated and the left over free carbon atoms form graphene layer on to the surface. This method is expensive and the graphene thus produced cannot be transferred to the other substrate. Chemical vapor deposition (CVD) is a scalable technique which can produce defect free graphene layers successfully. It is a low cost process for the production of graphene. CVD is a bottom-up process that has evolved as a scalable and reliable method for the production of graphene. It is inexpensive and produces large-area graphene. Among all the strategies to produce graphene, CVD on transition metal substrates has become the most promising approach. Growth of graphene by chemical vapor deposition was first sited on copper and nickel substrate, then it was done on various transition metals like Pt, Pd etc. CVD on nickel mainly synthesize poly graphene and monolayer is difficult to achieve since the growth process is uncontrollable however CVD on copper is a self-limiting process. Methane is used as a source of carbon atom since its decomposition occurs at very high temperature (>1200oC). Various thermodynamic parameters play an important role in the synthesis of mono or few layer graphene and among all, cooling rate is an important factor in avoiding the few layer graphene formation. CVD on Cu produces high quality uniform graphene since the process is self-limiting and no carbides are formed on the copper substrate. CVD involves formation of solid deposit on a suitable substrate by the activation of various gases. Two types of reactions can occur during the deposition process i.e. homogeneous gasphase reactions (occurs in the gas phase) and heterogeneous chemical reactions (occurs on heated substrate). Different processes which occur during the CVD technique as soon as the reactants are fed into the reactor are as follows:

1. The reactants are fed into the reactor.
2. Thermal activation.
3. Transportation of the reactants from the main gas stream.
4. Adsorption of reactants on to the surface of substrate.
5. 1. Dissolution/ bulk diffusion of species.
6. Thermal activation-mediated-surface processes, including chemical decomposition (catalytic), reaction, surface migration to attachment sites, incorporation, and other heterogeneous surface reactions
7. Desorption of by-products
8. Transport of the byproducts away from the substrate
9. Transport of by-products away from the deposition region

In the CVD process, copper plays dual role, i.e. it acts as a catalyst and a substrate. Methane is the main precursor gas and is of high quality (99.9995%) pre-diluted with Ar (5%). Due to high decomposition temperature and presence of one carbon atom per molecule, methane is used as the best precursor in the process. The temperature of the furnace should be maintained just below the melting point of copper i.e. 1083 degree C. Highly pure copper foil yields highly pure graphene, hence the copper foil is cleaned properly with acetone, isopropanol and methanol before using it. The CVD process consists of various steps depending on the temperature of the furnace but before that the substrate surface should be modified through cleaning process. The first step in CVD process is the heating stepwhere the catalyst-substrate is heated in hot-wall reactors up to the desired temperature. Next step is annealing step during which, the chemical reactions starts. It is done to clean the surfaces of the catalyst and to modify, the surface morphology including roughness, crystalline orientation, and grain size of the catalyst. Annealing step is done below the melting point of the metal substrate so that the substrate does not melt. This step is followed by Growing step where, the precursor gases are fed into the CVD chamber and the graphene growth initiates on the surface of the substrate. The pressure, gas flow and temperature can be modified as required. The nature of the catalyst determines whether the graphene will grow in this step or in the proceeding step. Next is the Cooling step which is very important for the synthesis of graphene. In this step, the pressure and is kept constant and the temperature is reduced to 200oC so that the surface of the substrate which is not covered by graphene should not get oxidized. In the Final step, the chamber is Backfilled with inert gases (Ar, N2) up to atmospheric pressure and the reactor chamber is opened. Hydrogen plays a dual role in the process of graphene growth by CVD on copper with methane as the main precursor. It not only acts as a co-catalyst in formation of graphene but also controls the grains shape and dimensions by eliminating the weak bonds. By reducing the amount of active sites on the copper surface, oxygen decreases the graphene nucleation density. In order to grow single crystal graphene domains, the surface oxygen should be controlled

Transfer of graphene layer on other substrate

Wet etching technique can be used to transfer the as synthesized graphene on to other substrates such as SiO2, glass coated ITO etc.In this process, the copper substrate decorated with graphene is first spin coated with polymer to support the graphene. After that, nitric acid is used to etch away the copper, leaving behind the graphene suspended on the polymer. The graphene on polymer is then fished away on the arbitrary substrate and then washed with hot acetone. Finally, the acetone is used to remove the polymer leaving behind the graphene layer on the desired substrate.

Applications of graphene

Graphene due to its excellent electrical, thermal, mechanical and electronic properties can be used in various applications such as graphene field effect transistors, transparent electrodes, Li-ion capacitors, sodium ion batteries, heat dissipation systems, ultra-capacitors, solar cells and fuel cells, integrated circuits, creation of multi wall nanotubes, anti-microbial for purification of water, electro chromic devices, polymers and composites and in the detection of gas molecules. It can also be used in various biomedical applications such as cell adhesion, detection of bio molecules, fabrication of biocompatible scaffold, electrochemical biosensors, optical biosensors, bio imaging, gene delivery, drug delivery,cancer therapy and tissue engineering.