Essay: Graphene

Graphene can be defined as one atom thick layer of carbon atoms arranged in a 2- dimensional hexagonal crystal lattice. The carbon atoms in the 2’dimensional planar structure is sp2 hybridized. It can be imagined as numerous benzene rings attached to each other without their hydrogen atoms being attached to them. It is the fundamental unit of other forms of carbon such as Carbon Nanotubes, graphite and fullerenes. Carbon nanotubes can be formed by rolling the grapheme sheets in a cylindrical form. Numerous layers of graphene when stacked on top of each other forms graphite. It was established theoretically in 1940 that graphene is the fundamental unit of graphite [1].

Figure. 1 showing the 0-dimensional graphene(fullerene), 1-dimensional graphene(carbon nanotubes) and 2-dimensional graphene monolayer sheet [1].

1.1 History of graphene

About 6000 years ago, graphite was used for the first time. In 1960’s, it was found that the conductivity of graphite intercalation compounds displayed a surprisingly higher conductivity. Since then the research of graphene has started. The scientists and researchers had no clue about the cause of high values of conductivity displayed by graphene. Scientists researched graphene with a viewpoint of having a lighter and cheaper substitute which acts as a conductor replacing heavy metals.The task of obtaining graphene sheets was a challenging for the researchers and hence the research work grew slowly in the late 20th century [4].
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1.2 Structure of Graphene

As we know graphene is a one layer thick carbon atoms arranged in a 2 dimensional hexagonal lattice. The bond length between 2 sp2hybridized carbon atom is 0.142nm[1]. The energy of interaction between two layers of graphene is 2eV/nm2. The number of carbon atoms per unit cell of graphene is 2. The lattice of graphene can be viewed as 2 interpenetrating sub lattices. The atoms of one sub lattice located at the center of triangles formed by the other sub lattice. The unit cell of graphene has 6-fold axis of symmetry.

The p-orbitals of carbon atom lie perpendicular to the plane of graphene sheet and they hybridize to form the ??* conduction band and also the ?? valence band. This leads to the planar conduction phenomena in graphene. The momentum of charge carriers within the Brillouin Zone determines the energy of these bands.

The center of the hexagonal aromatic ring sits on top of the carbon atoms of graphene layer below it. This shows that the symmetry of the sheets is trigonal rather than being hexagonal[5].

2. Synthesis

Graphite is a naturally occurring allotrope of carbon which is easily available in large quantities. We can produce grapheme sheets out of graphite both at a smaller scale for the experimental purposes as well as at large scales for the various application purposes[3].
In the late 20th century, when the research about graphene was gaining pace, various attempts for the synthesis of graphene sheets were done. On the same approach of producing Carbon Nanotubes, efforts were made to synthesize graphene using CVD on the metal surfaces and thermal decomposition of silicon carbide. These methods did not produce monolayer Graphene sheets but produced few stacked layers of graphene. So CVD approach is being used widely nowadays. The approach used by the people who isolated graphene for the first time in 2004, was peeling the graphite layer by layer. They used the adhesive tape to peel the graphite layers one by one till graphene monolayers were produced. With this approach a very high quality graphene having micron size thickness can be produced.

Exfoliation of Graphite

Graphene can be used as a substitute for other filler materials used in composites such as a CNT. The properties of graphene can be relevant only if the sheet is exfoliated properly at the Nano scale. One of the major problems faced in the synthesis of the materials like CNT, graphene etc. is their tendency to agglomerate due to the Vander-Waals forces.

The energy of interaction between 2 adjacent layers of graphene is around 2ev/nm2. This shows that the amount of energy required to exfoliate the graphene sheet is quite low which is about 300N/??m2. This amount of energy can be easily supplied by the help of an adhesive tape to exfoliate the graphene sheet. This process allows a thinning of graphite layer only up to few nanometers in dimension and a monolayer of graphene can’t be produced.

The method developed by Andre Geim and Konstantin Novoselov is to use a common adhesive tape to stick and peel repeatedly for several times. By this method they were successful in reducing the size of 1 ??m thick flake of graphite to a monolayer graphene sheet. The smooth and thin sheets of graphene are then observed by Optical Microscopy and then transferred to a clean substrate by a gentle press of tape.

One of the major drawbacks observed in this method is that after removal of graphene from the tape surface, some glue deposits are left as residue on the graphene sheet. The effect of this is seen in reduction in carrier mobility. The ways to remove the glue residues are –

By giving heat treatment in a reducing atmosphere.

By Joule heating under vacuum conditions at a temperature of 500??C.

Wet Chemical Routes

In the chemical method of extracting Graphene layer out of Graphite –

The Vander-Waals forces between the 2 layers of graphite is reduced by the action of reactants in the interlayer space.
After this, the reactant produces high gas pressure due to intercalant decomposition.

Subsequently, the layers of graphene are separated and there occurs a degradation of sp2 lattice into sp2-sp3 sheets which has less stability for ??-?? stacking.

The method of reducing of reducing the aromaticity of sp2-graphite has been known for past 150 years from now. This has been known after Brodie worked on the carbon atomic mass determination. The reaction between potassium chlorate, nitric acid and sulphuric acid is highly exothermic. When the mixture of above is reactants is placed as an intercalation material, it produces graphite which has different hybridization and which is modified. The modified graphite bears hydroxyl and carboxyl moieties.
The suspension containing graphite, potassium chlorate, nitric acid and sulphuric acid was named as graphitic acid but later it was commonly known as graphite oxide. A different method to produce graphite oxide which was faster as well as safer was developed by Hummers in 1958.

Hummer’s method

In this method, graphene is produced by the exfoliation of graphite which is treated chemically in order to expand volumetrically. Graphite is dispersed into a solution of concentrated sulphuric acid, potassium permanganate, sodium nitrate at 45??C for some hours. After this treatment, the expanded graphite was kept for x-ray diffraction and it was found that the peak obtained after diffraction was at 0.75nm. This value was different from the peak value of 0.34nm obtained after x-ray diffraction of graphite. This showed that the interlayer spacing for the expanded graphite was more than that of original graphite. The graphite obtained after treatment with the intercalation reactants is used widely in chemical as well electrochemical industry and it is popularly known as ‘Expandable Graphite’. The volume expanded graphite is 100 times greater than that of natural graphite used before.
After the intercalant reactants are added to the graphite layers, the single sheets of graphene are separated by separation of large amounts of gases between the layers of graphite called the Vander-Waal’s spaces. The production of gases can be done chemically as well as thermally after this, the suspension is annealed at 1050??C, which splits the graphite oxide into several sheets. To further separate the graphite oxide sheets into monolayer, Ultrasonication is used. After ultrasonication, the presence of non-oxidized graphene sheets is judged by the color of the solution. The solution containing graphite oxide in major concentration is yellow in color whereas the solution containing non oxidized graphite is greenish blue. Chemical reaction techniques are also used to recover monolayer graphene sheets from graphene oxide. The graphite oxide is reduced by the help of hydrazine vapors when graphite oxide is deposited on a substrate. The reduction oxide by the above method results in formation of reduced graphene flakes unless a stabilizing agent is added to the suspension. The above methods used for the reduction of results in an ill-defined intermediate between graphene and graphene oxide. However, the obtained product is easily processed. The recovery of the sp2 character graphene is partial. For this partial sp2 behavior also the reduction in resistance is 4 to5 times greater than that compared to graphite oxide [5].

Chemical Vapor Deposition

The method of producing single layer graphene on a substrate using CVD is one of the most optimum and widely used technique. This method has been used over decades to produce carbon nanotubes and graphene and it is not a new idea at all. The expensive method to produce high quality graphene monolayer is to grow them on Iridium and Ruthenium substrates by using low pressure conditions. One more expensive method is to produce monolayer graphene by thermal decomposition of carbides. Recently, it has been observed that few layers of graphene can be grown on nickel as well as it can also be grown on copper using CVD technique. These metals are cheap as well as standard material used for nanofabrication. The graphene that has been grown on substrate can then be transferred on any arbitrary substrate.

2.3.1 The setup of furnace

The setup of the CVD furnace can be seen in the figure shown below. The low molecular mass hydrocarbons such as methane is flown over the substrate. The temperature maintained is high and pressure used is low pressure. To dilute the hydrocarbon, we use inert gases such as argon. Hydrogen can also be used as a medium for dilution. The furnace used is Lindberg Temperature Controlled Furnace. A 1 inch diameter tube is used as a substrate. An oil pump with a cold trap to the gas flow exhaust is used to achieve low pressure conditions. The pressure conditions should be such that in presence of gas the pressure must not be greater than 1 Torr and with no gas conditions it must not be greater than 10 Torr.

Fig 2 showing the setup of the CVD furnace [6].

Requirements of the copper substrate

The copper substrate must have the following requirements

• It should be kept as flat as possible. If the foils are crumpled then it will lead to production of graphene layers which are cracked and it will lead to poor transfer on the substrate.
• The contamination level of copper should be as less as possible because contamination of copper leads to poor quality of graphene sheets.
• The grain size of copper must be of size almost equal to 100??m.
• The evaporated graphene deposited on a 700 nm thick substrate can also be used for the growth of graphene. The other forms of copper that can be used are single crystal copper or copper penny. The figure below shows some copper form that can be used.

Fig 3 showing copper that can be used as a substrate for CVD process [6].

The contaminated surface of copper leads to poor quality of graphene sheets produced. Surface of copper must be cleaned periodically. To clean the surface and deoxidise the copper surface the following methods are used ‘

• Firstly dip the copper in acetone for 10 seconds.
• Then wash with water.
• Then dip in acetic acid for 10 minutes.
• Again wash with water and dip in acetone for 10 seconds.
• After that dip in isopropanol for 10 seconds.
• After the following procedure, the copper is dried gently using a low flow nitrogen gun.

3.3.3 Procedure of growth of graphene sheet.

The step-wise-step process given below is used the growth of graphene on copper substrate.

• Initially 3 to 5 copper foils are placed on the substrate.
• Vacuum pump is used to bring down the pressure to 10mTorr.
• Then hydrogen gas is passed in the furnace. 6sccm of hydrogen gas is passed to obtain a gas pressure of 120mTorr.
• The furnace is then heated to 1000??C.
• After this, the sample is annealed at 1000??C in hydrogen atmosphere for 10 minutes.
• Then 157sccm methane is passed for 13 minutes.
• After this the system is allowed to cool down to 150??C.

After this the system is repressurised by replacing methane and hydrogen with argon at 200sccm by turning off the vacuum pump. It has been observe that even if we run the graphene growth process for a long time, it does not lead to multilayer graphene growth. This shows the self-suppressing behavior of graphene growth. If the growhprocedss is run for a very short time for example, less than 10 seconds then graphene islands are observed on the substrate. The same observation of the graphene islands is made if the pressure inside the furnace is very low.it has been observed that by very slow cooling down of furnace along with ambient pressure can lead to multilayer graphene sheets.

Transfer of graphene layer to the substrate

The procedure for the transfer of graphene layer to the substrate is as follows

Firstly, a thin polymer film is placed over the grown graphene.

After this, the foil is etched in acid leaving the graphene film attached to the polymer.

The graphene film is then brought into contact with anothersubstrate having the graphene deposited on the substrate [6].

3.3 Thermal decomposition of carbides

The other method to synthesize graphene is by thermal decomposition of carbides.when silicon carbide is heated at 1300?? C under vacuum conditions, sublimation of silicon atoms takes place. Due to this, a layer of carbon-enriched surface is developed on the surface of the substrate. On further heating of the silicon carbide, a layer of graphite is observed. If controlled sublimation is done, then graphene monolayer can be synthesized over the entire surface of silicon carbide wafers [5].

4. Purification of graphene

It is one of the major challenges to keep the graphene which has been grown free from all types of impurities. It poses several problems such as

• It prevents graphene from being a flat sheet.
• It prevents good contact resistance.
• It puts a barrier on imaging with electrons and photons.
• It destroys the unique graphene properties.

There are basically two methods used for cleaning graphite and they are

• By using solvents and
• By using annealing.

3.1Annealing

It can be done as follows

The sample is annealed at 400??C in an environment having 200sccm of hydrogen gas for 2 hours.

It can also be annealed in air at 350??C for 2 hours

3.2 Use of solvents

The sample for cleaning is soaked in acetone or dichloromethane for several hours.

It is then rinsed in isopropanol [6].

Functionalization of graphene

The functionalization of graphene is a necessary process that is done before it is used in various applications. Solvent assisted techniques are used for further processing.

This solvent assisted technique requires layer by layer assembly, spin coating and filtration.

Graphene has an inherent property to agglomerate.When the functionalization of graphene is done properly then the agglomeration is prevented during its reduction in solvent phase and that helps in maintaining its inherent properties. One of the widely used material in the production of graphene is graphite oxide. Processable graphene is generated from chemical treatments of graphene oxide. The Hummer’s method as described above in the synthesis of graphene one of the fruitful methods of preparing graphene. The graphite oxide surface contains a lot of functional groups such as carboxyl group, hydroxyl group, ketonic group and epoxide diols. These groups modify the inherent properties of graphene to a great extent and can affect the solubility of the compounds in nonorganic and organic solvents.

The presence of carbonyl as well as carboxyl groups on the graphite oxide sheets makes them strongly hydrophobic in nature. Due to this they easily get dispersed in water. The graphite oxide produced by hummer’s method can be reduced chemically, photo-chemically or thermally. If the reduction process of graphite oxide is done without any stabilizer, then this leads to aggregation of graphite particles at the bottom of the vessel used for the reduction process.it happens because the agglomeration of graphene is rapid and irreversible. Therefore to prevent this graphite oxide sheets are treatedfor surface modifications by 2 methods-

• Covalent modifications and
• Non-covalent modifications

The reduction is done after the surface modification has been done. The reduction of alkylamine modified graphene oxide sheets result in functionalized graphene having stable dispersions. As mentioned before, the groups which make graphite oxide hydrophilic such as carboxylic and sulphonate groups can be introduced into the graphite oxide to produce water-dispersible graphene sheets. Trials also have been made for producing dispersible graphene directly from graphite.

Covalent modification of graphene

The surface functionalization of graphene deals with modified hybridization of carbon atoms at the ends of the sheets and on the surface of the sheets as well. The hybridization changes from sp2 to sp3 for the carbon atoms which also results in structural modifications of the carbon atoms. There are four methods under the covalent modification and they are

• Nucleophilic substitution reaction
• Electrophilic substitution reaction
• Condensation reaction
• Addition reaction.

Nucleophilic substitution reaction

The epoxy groups on graphite oxide are the main reactive sites on graphite oxide in this type of reactions. The organic modifiers used for the nucleophilic substitution reaction bears an amino group (R-NH2). This amine functional group carries a lone pair of electrons on it. As compared to other methods, this reaction occurs very easily at room temperature and in aqueous mediums. This method is therefore used for large scale production of functionalized graphene. This method is shown below in the figure.
Primary amines as well as amino acids have been used for the surface treatment of graphite oxide. The primary amines having small chains can be grafted at room temperature. However, for long chain aliphatic amines, the heating of the reaction mixture must be done for 24 hours.

It has been observed with the help of X-ray diffraction methods that the interlayer distance between the 2 graphite oxide derivatives is dependent on the chain lengths and their orientation with respect to the layers.The graphite oxide is hydrophilic in nature and this leads to its tilted orientation.

Alkaline solution of amino acids is used for the reaction between amino acids and graphite oxide. R-NH2 groups attack on the group of graphite oxide.

Figure.4 showing the nucleophilic substitution reaction

Electrophilic substitution reaction

In the electrophilic substitution reaction, the hydrogen atoms of graphene are replaced by an electrophile. Aryl diazonium salt can be used to graft it on the surface of graphene. Another example of electrophilic substitution reaction is replacement of hydrogen atoms by a diazonium salt by para nitro aniline. The figure shown below shows that the surfactant used here is sodium dodecyl benzene sulphonate. When reduced at conditions having pH at 10 and at a temperature of 80?? C for 24 hours, the dodecyl benzene sulphonate functionalized is highly dispersible in N, N-dimethyl formamide.
On microscopic observations it revealed that chemically exfoliated graphene sheets are less than 5 layers thick. The electrical conductivity values of sulphonated graphene is higher as compared to the other functionalized graphene.

Figure.5 showing the functionalization of graphene oxide [7].

Condensation reaction

Condensation reaction is a type of chemical reaction in which 2 functional groups combine to form a larger molecule leaving behind a by-product which is a small molecule. The by-product can be water, hydrogen, hydrogen chloride etc. One example of condensation reaction that occurs between isocyanates, diisocyanates and amine compounds. This leads to the formation of amides and carbamate ester linkages.

The figure shown below is a schematic representation of the production of isocyanate functionalized graphene oxide.

Figure.6 showing the isocyanate treatment of graphene oxide

The N, N-dimethyl formamide compounds reacts in nitrogen atmosphere. Before the addition of N, N- dimethyl formamide, isocyanates and graphene oxide are taken in a flask. The resulting functionalized graphene oxide is highly dispersible in N, N-dimethyl formamide and is found to be very useful in preparation of polymer nanocomposites.

Addition reaction

It is a type of chemical reaction that occurs between 2 molecules of low molecular mass to form a high molecular mass molecule. An addition reaction that shows 1,3-dipolar cycloaddition of azomethine ylide on the surface of graphene is shown below in the figure.

Figure.7 showing the addition reaction1,3-dipolar cycloaddition of azomethine ylide on graphene [7].

A similar example of addition reaction is the cycloaddition of azidotrimethyl silane on the surface of epitaxial graphene the functionalized graphene sheets are easily dispersible.

Non covalent functionalization

The non-covalent functionalization of graphene sheets requires physical adsorption of suitable molecules on the surface of graphene. Thus noncovalent interaction involves Van Der Waal’s, hydrophobic and electrostatic forces.

It is achieved by

• Polymer wrapping
• Adsorption of surfactants or small aromatic molecules
• Interaction with polyphyrins or biomolecules such as peptides.

It is one of the most extensively used technique for the surface modification of carbon based nano materials. It has been used for the surface modification of carbon nanotubes [7].

Properties of graphene

5.1Electronic properties

Graphene shows excellent mobility of electrons at room temperature, which is more than 20,000cm2/V s. due to this property it shows immediate access to Quantum Hall Effect. The mobility of graphene is temperature independent between 10 and 100 K. it is observed that by annealing and improved sample preparation, the conductivity can be increased up to 25000cm2/V s. this high value of conductivity was not observed for any of the semiconductors or semimetals. It has been observed that the mobilities of electrons as well as holes are same for the graphene. Graphene bears the potential to be used in Field Effect Transistors.

5.2 Optical properties

Graphene exhibits excellent optical properties. The graphene monolayer obtained from black graphite soot or silverish graphite crystals becomes highly transparent. This makes it suitable for use in several applications. The transparency of graphene layers decreases when its thickness decreases. The 2-dimensional gapless behavior of structure of graphene is held responsible for decreasing transmittance with increasing depth. When one layer of graphene is present, it absorbs 2.3% of incident light and when 2 layers are present, it absorbs 4.6% of incident light. This linear dependence of absorption is observed till 5 layers the reflectance of graphene is less than 0.1%. Graphene absorbs light for wavelengths less than 400nm in the visible spectrum. Graphene hence bears the potential to be used as transparent electrode for solar cells, in liquid crystals and in transparent flexible electrode.

Mechanical properties

The allotropes of carbon such as graphite, carbon nanotubes and diamond are well known for their exceptional values of young’s modulus and hardness. Similarly graphene also displays excellent young’s modulus and hardness [5]. It is the thinnest and strongest known material discovered so far [3].

Thermal properties

The thermal properties of graphene are dominated by acoustic phonons. The graphene in thermal applications is not possible without large-scale inexpensive graphene production. At room temperatures, the thermal conductivity of graphene is found to be (4.84??0.44) ?? 103 to (5.30??0.48) ?? 103 W’m’1’K’1 [8].

Chemical properties

Graphene is the only allotrope of carbon in which the carbon can react from both the sides. Graphene is the only form of carbon that has the highest ratio of edgy carbons. Due to the presence of defects, the chemical reactivity is further increased [10]. Various types of defects within the sheet of graphene increases the chemical reactivity of graphene to a great extent [11]. The onset of reaction between the basal plane of a single layer of graphene sheet and the oxygen starts at a temperature of 260 degree Celsius. The burning of graphene atoms is observed at a temperature of 350 degree Celsius [12] and [13]. Since the availability of carbon atoms is possible in both the lateral directions, so it proves out to be the most reactive allotrope of carbon. Graphene is commonly modified with oxygen and nitrogen functional groups. These functional groups can be analyzed by Infrared Spectroscopy, but the analysis of functional groups prove to be a difficult task unless and until the structures are well controlled [14] and [15].

Characterization of graphene

The characterization of graphene can be done by various methods. Some of them are discussed here.

6.1 Raman Spectroscopy

Raman spectroscopy generally measures the shift in energy that is caused when grapheme lattice scatters the phonon vibrations. Raman spectroscopy has been proved to be a powerful technique for measuring various properties of graphene.
The relative and shape of these peaks are an indicator of layer number and the structure of graphene sheets. The d peak is usually invisible in the pristine graphene. The geometry of the structure can be destroyed by elements such as adsorbates, impurities or change in hybridization to sp3 at some specific location. This break in symmetry is demonstrated in d ring. The height in G peak increases as the number of layers increased in the graphene layer. There will be no apparent shape change in these peaks.
Similar to the g peak, 2g peak indicates the increase in no. of layers. However, this 2g peak is very high. The degenerate carbon-carbon atom vibration is responsible for the occurrence of g peak. The carbon ring breathing made is responsible for the d peak. The higher order double resonance of breathing made is responsible for the 2D peak.

Sensitive to the increase in number of layers for the layers greater than 5, the peak resembles the peak obtained for a graphite.
We can analyze the peaks using precise peak position and width for more information on graphene. As shown in figure, the d and e part of figure shows raman spectra images taken by re in via confocal raman microscope of the typical raman spectra of CVD grown graphene. Image d shows graphene grown on copper substrates where as e shows the image of graphene grown between gold electrodes.

Both the figures shows non uniform background and it is due to nearby presence of metal substrate which is copper and gold in this case.

In both the cases, the D peak is sharp and higher than the G peak. This shows an indication of graphene which is single layer.

6.2 Electron Microscopy:

These types of techniques are used for electron microscopy and they are

1. SEM
2. TEM
3. STEM

It is used to find the amount and type of contamination, integrity of graphene membranes and also the submicron structure of graphene.

In the process of SEM, a very thin beam of electrons, usually nano meter thick electronic radiation is flashed on the sample. The no. of electrons coming back is measured by the help of the instruments for example this ultra SEM can be used for this.

A low extraction voltage of 2 kV is used for electron microscopy. Using this low voltage, the contrast of the graphene is highlighted.

The integrity of graphene layers can be checked using SEM technique.

SEM images are shown below,

There are basically three methods by which the integrity of graphene is lost

• Partial Tearing
• Complete Tearing
• Stick down on to the substrate

This method can be used to check the quality of graphene
In this method, the intrinsic resistivity of graphene is measured using 4- probe method. A current namely direct current is made to flow through the graphene sample which flows from source to drain. The corresponding voltage difference is measured the resistivity of graphene can be found using the formula shown below

??= W/L(V1-V2/ISD)

Here,
P= resistivity
W= width of the graphene
L= length of graphene

This method proves to be advantageous because it makes us capable of ignoring the contact resistance of electrodes and measures the intrinsic resistivity.

The figure shown below depicts the graph between the top gate voltages. Resistivity graph. It shows a peak which is related to the conical band structure of graphene membrane. It has been found experimentally that the resistivity shows its maximum value when the Fermi Level is at the Dirac point, where the density of states comes out to be zero for graphene. Usually, the density of states is observed to be zero when the applied potential is nil. However, normally the peak voltage is deviated from the zero value since charges get trapped on the monolayer graphene membrane.

By this method, the amount of electrostatic doping can be found out by the peak position by the formula given below

N= (-C/e)(Vdirac)

Here,
N= carrier density
C= capacitance per unit area between gate and the graphene
Vdirac= position of the peak

The capacitance of the top gated device can be found as a parallel plate capacitor by the formula given below

Ctg= ‘t ‘o/d

is used in the formula above depends on the top gate dielectric material and also depends on the thickness d of sample

The gate dielectric for the sample used in fig shown below is evaporated Silicon Oxide which has relative permittivity equal to 3.9 and d= 90 nm. From the above data, we estimate that graphene was doped with hole having a density’ n’_0=5′??10’^11 /’cm’^2.

As we observe, the resistance is getting dropped effectively on both the sides of the peak. This occurs because of the fact the electron or hole concentration is changing and shifting away from the Dirac point.

The asymmetricity of the peak is due to hysteresis in trace diection, the hyseteresis occurs due to the trapping of mobile charges in the gate dielectric.

The electronic disorder of graphene can be measured by the amount scattering. The scattering of electrons is measured by the observed change in mobility is defined as the ratio of conductivity to the number of charge carriers. The formula given below can be used to measure the conductivity by the above technique.

6.3 Fluorescence Quenching Technique

This is a technique which was used recently to image graphene monolayers. This method can also be used to image graphene monolayers as well as graphene oxide mono layers for the same sample evaluation and manipulation which makes the synthesis process to be improved. It has been found that it is a time saving and low cost technique which can be used to visualize graphene as well as graphene oxide monolayers. The mechanism of imaging is such that it involves the quenching of emission from a dye coated graphene oxide and reduced graphene oxide. The dye can be subsequently removed by rinsing off the dye from the sample and this process does not harm the sample at all. The contrast used to see the sample is aroused as a result of chemical interaction between the graphene sheets and the dye molecules at a molecular scale.

This occurs because of charge transfer from a dye molecule to graphene oxide which results in quenching of fluorescence.

The contrast was measured and it was found to be 0.78 for 300nm SiO2. It was observed that the contrast was higher when the graphene sheets were deposited on 100nm SiO2. The quartz and glass substrates gave the contrast to be 0.2 and 0.07 respectively. These values helped the graphene sheets to be visualized clearly [2].

Figure 8. showing the optical microscopy image of single, double and triple layer of graphene on SiO2 with a 300 nm SiO2 over layer. This has been labeled as 1L, 2L and 3L [2].

6.4 X-ray Photo electron Spectroscopy of graphene oxide

S.Y.Toh et al. used XPS to characterize graphene oxide (GO) and electrochemically reduced graphene oxide (ERGO). This technique can be used to effectively determine the amount of carbon, oxygen and various other functional groups present over graphene oxide as well as electrochemically reduced graphene oxide. It is quite an accurate technique to determine the elements present as compared to the elemental analysis technique because in elemental analysis technique one needs to fully dehydrate a graphene sample which is quite a difficult task to do. In the figure 8 shown below, XPS spectrum of oxygenated graphene (GO & ERGO) is shown. It shows 2 peaks, one at 530 eV and the other at 284 eV both of which correspond to O1s and C1s spectra respectively. The ratio of O1s and C1s peak intensity shows the amount of oxygen content in the oxygenated graphene. The peaks O1s and C1s tells us about the functional groups present on the carbon back bone of graphene. The information given by O1s spectrum complements the C1s information.

The intensity of the peaks depend upon the potential applied when the mode applied is constant potential mode or the number of cycles [18].

Figure. 9 showing XPS spectrum of GO and ERGO [17]

Applications of Graphene

7.1 Electronic Application

One of the remarkable properties of Graphene include very high electron mobility i.e. 2000 ‘cm’^2/Vs at room temperature along with high sensitivity to field effect and large lateral extension makes it a perfect candidate serving an alternative to Carbon Nano Tubes(CNTs) for Field Effect transistors(FETs) also the mobility of Graphene does not change with Temperature from 10 K to 100 K.
More success could be achieved regarding the mob via improved sample preparations which greatly includes desorption of adsorbates by current annealing which could lead the mob up to 25000’cm’^2/Vs.

Augmentation in this regard could also be obtained where the devices are made up of Oxide supported Graphene as in this case the scatterers are trapped charges leading up to the value of mobility around 40,000 ‘cm’^2/Vs.

Further efficient screening or complete removal of the substrate can tremendously increase the mobility value and that’s why for suspended and annealed Graphene devices possess the value of mobility around 200000 ‘cm’^2/Vs which is the largest ever recorded value of the mobility for the devices which are frequently used in Field Effect Transistors (FETs), as the crux of the Field Effect Transistors lie in the mobility of the charge carriers Graphene based devices are the best substitute in this aspect.

The second remarkable property of the Graphene lies in the fact that it shows almost strict linear dispersion curve mean Dirac point where the Fermi Level can easily be modified by the help of manipulation in terms of doping level concentration and that also electrostatically this leads to the discovery of Berry’s phase or a Quantum Hall Effect with fourthly degenerated Landau Levels because of K and K’ Valley and Pseudo Spin Degeneracy except for the zero Landau level which is only twice degenerated due to strict electron- hole symmetry near the Dirac Point

7.2 Applications for its Mechanical Properties

The investigation and experimentation in the field of mechanical properties of graphene is less investigated but whatever the results that are obtained are phenomenal in its specific aspect like stiffness of the Graphene sheet lies in the range of 300-400 N/m having very high value of breaking strength. The intrinsic strength that is the specimen which is devoid of any defects shows the strength up to 42 N /m making it the material which can be a candidate in terms of devices which require great mechanical strength.

The Young’s Modulus of Graphene lies in the range of 0.5 to 1 TPa which is remarkably high making it the future of very high modulus application, as nowadays more experiment is carried out to have substitutes of iron in terms stiffness and strength Graphene based materials could be the one.

It is very interesting to note in Graphene Oxide Sheets that in spite of the defect concentration being too high it retains its intactness to fairly large degree and shows remarkably good mechanical performance with Young’s Modulus around 0.25 TPa.
The blending properties of Graphene Oxide are highly compatible to use it into the fabrication of matrices serving it as an ideal candidate for the mechanical reinforcements.

The single sheet of Graphene can sustain very high tension which is unlikely found in any other material known, making it highly potent for NEMs (nano electro- mechanical devices) like for sensors and resonators.

The ease to fabricate these devices by the use of Graphene also increases its potential as can be seen in the fact that mechanically exfoliated single and multilayer Graphene which are placed over trenches in silica (SiO2) substrate and these were contacted to fabricate the nano electro- mechanical systems

7.3 Graphene as a Sensor

The candidate for the making of sensors should be having a good stead in terms of properties like interface accessibility to transduction efficiency, molecular sensitivity, mechanical and electronic robustness, and here in Graphene we could have entire gamut which makes it a great candidate for the making of sensors.

The advantage lies in the fact that ‘sp’^2 hybridization gives it the benefit in terms of robustness which does not jeopardize the 2-D delocalized transport properties unlike Carbon Nano Tubes.

For the sensing action to be performed the interaction between sensors and environment is very crucial and in this case suspended Graphene provides pure interface where atoms can be very easily exposed to environment

These are not limited to chemical species but generally applied to any phenomena capable of inducing a local change in carrier concentration.

The production of Graphene and its oxide based objects are less costly than Carbon Nano Tubes making it more efficient in terms of cost management making it more famous in industrial aspect.

7.4 Application for its Optical Properties

The fact that Graphene based materials possess great optical properties as well as excellent electrical conductivity makes them an excellent substitute for ITOs (Indium Tin Oxides materials).

Combination of high film conductivity and optical transparency plus chemical and mechanical stability makes it highly suitable for the fabrication of transparent electrode for Solar cells and Liquid Crystal Display (LCDs).

The combination of high mobility, large ohmic area contact and metallic conductivity helps in limiting the background noise in transport experiment [5].

7.5 Graphene used for Device Applications

Graphene oxide as well as reduced graphene oxide can be synthesized by the processes given below

• Drop cast
• Spraying
• Dip coating
• Spin coating
• Vacuum filtrations deposition

Various devices which contain single layer, few layers (<10 nm) and thin films can be fabricated using graphene oxide and reduced graphene oxide layers. The throughput for the thin film deposition is high whereas the throughput for the thin film deposition is high whereas the throughput for single and few layers graphene can be low because the process of deposition for single and few layers graphene involves random placement. The methods such as drop cast, spray deposition and dip coating are convenient ways to build devices however, they show a limitation of non-uniform coating thickness on the device substrate which is caused due to aggregation of graphene oxide and reduced graphene oxide sheets. Minimal wrinkling can be achieved for the substrate deposition if the spin coating process is employed [2].

Future prospects of graphene

Graphene has been already put in use for various engineering applications. Its exceptional optical, electrical, thermal, properties have made it an attractive candidate for various engineering uses. The exceptional properties of graphene originated as a result of 2 dimensional structure of a single sheet graphene such as quantum Hall Effect, highest transport of charge and exceptional thermal conductivity provides potential for graphene to be researched for microelectronics application. The semi metal character of graphene makes it suitable to be used in transistors in future.

Graphene nano ribbons and bi layer graphene were developed which had suitable band gap to be used in field effect transistors (FETs). Still many obstacles are being observed for graphene synthesis, sonication, characterization as well as device fabrication now also. Defect free graphene exhibits exceptional properties and it can be isolated by scotch tape method. However, this method cannot be used for large scale production.

The chemical and mechanical modification of graphene affects the electrical, chemical and mechanical properties of graphene. In order to achieve desired band gap, electrical conductivity and mechanical properties, controlled oxidation as well as reduction is required.

Modification of graphite, graphene oxide and reduced graphene is required in a controlled manner so that it can be used in different applications.

The health risk which is posed by graphene by graphene needs to be evaluated by investigating its toxicity and bio compatibility of graphene and its derivatives.

Graphene based materials can be used in transparent flexible electrodes, graphene/polymer nano composites, organic electronics, energy storage as well as in mechanical parts. More research is required towards advancements in issues like homogenous distribution of graphene, interface bonding with the matrix material, gas barrier properties of graphene, distribution of graphene into the matrix in a homogenous manner.

It has been observed that the charge mobility of reduced graphene oxide is quite higher than amorphous silicon and polymers which are semi conducting. This makes graphene a potential material to be used in electronics. Folding, wrinkling, overlapping at macro scale and defects of reduced graphene oxide are the main obstacles which hinders its applications in electronics. Various researches are being done to overcome these obstacles.

To view monolayer graphene under an optical microscope, there is a need to have a suitable substrate and a suitable wavelength to create the required contrast to view the image. More research is done so that graphene mono layer can be simply detected without any substrate or support material. TEM with aberration correction has made it possible to examine the defect structure of graphene as well as graphene quantum dot at atomic resolutions [2].

References

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