Graphene

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Graphene

  1. 1. GRAPHENE: AN OVERVEIW OF WONDER MATERIAL Submitted by MALIHA KHATUN ELA DEPARTMENT OF PHYSICS UNIVERSITY OF DHAKA 1
  2. 2. ABSTRACTThe recent discovery of Graphene has sparked much interest, thus far focused onthe exceptional electronic structure of this particle, in which charge carriers mimicmass less relativistic particles. However in physical structure Graphene is one-atom-thick planar sheet of carbon atoms densely packed in a honeycomb crystallattice, which is the thinnest material and also the strongest material ever measured.As a conductor of electricity it performs as well as copper, as a conductor of it outperforms all other known materials. It is almost completely transparent, yet sodense that not even helium, the smallest gas atom, can pass through it. Carbon, thebasis of known life on earth, has surprised the world once again. Graphene hasemerged as an exotic material of the 21st century and has grabbed appreciableattention due to its exceptional optical, thermal and mechanical properties. The aimof this review article is to present an overview of the production process, propertiessuch as electronic, optical, thermal and mechanical along with their potentialapplications in various fields. The limitations of present knowledgebase and futureresearch directions have also been highlighted. 2
  3. 3. TABLE OF CONTENTS1. INTRODUCTION2. STRUCTURE OF GRAPHENE3. SYNTHESIS OF GRAPHENE 3.1 DRAWING METHOD 3.2 THERMAL DECOMPOSITION ON SiC 3.3 GRAPHITE OXIDE REDUCTION 3.4 CHEMICAL VAPOUR DEPOSITION4. PROPERTIES 4.1 ELECTRONIC PROPERTIES 4.2 OPTICAL PROPERTIES 4.3 THERMAL PROPERTIES 4.4 MECHANICAL PROPERTIES 4.5 QUANTUM HALL EFFECT IN GRAPHENE5. APPLICATIONS 5.1 GRAPHENE TRANSISTOR 5.2 GRAPHENE NANORIBBONS 5.3 TRANSPARENT CONDUCTING ELECTRODES 3
  4. 4. 5.4 ULTRACAPACITORS 5.5 OPV SOLAR CELLS 5.6 INTEGRATED CIRCUITS 5.7 GRAPHENE BIODEVICE 5.8 LIMITATIONS6. FUTURE ASPECTS7. CONCLUSIONREFERENCES1. INTRODUCTION: Materials are the basis of almost all new discoveries in science. The development of newmaterials can lead to the uncovering of entire new fields of study, as well as new solutions toproblems that may have been thought to be unsolvable. One such material is graphene, adeceptively simple arrangement of carbon atoms. This new material has a number of uniqueproperties, which makes it interesting for both fundamental studies and future applications. TheNobel Prize in Physics for 2010 was awarded to Andre Geim and Konstantin Novoselov "forgroundbreaking experiments regarding the two-dimensional material graphene". 4
  5. 5. Fig-1: Graphene is an atomic-scale honeycomb lattice made of carbon atoms.Graphene is the name given to a flat monolayer of carbon atoms tightly packed into a twodimensional (2D) honeycomb lattice, and is a basic building block for graphitic materials of allother dimensionalities. Theoretically, graphene (or “2D graphite”) has been studied for sixtyyears, and is widely used for describing properties of various carbon-based materials. Forty yearslater, it was realized that graphene also provides an excellent condensed-matter analogue of(2+1)-dimensional quantum electrodynamics, which propelled graphene into a thrivingtheoretical toy model. On the other hand, although known as an integral part of 3D materials,graphene was presumed not to exist in the free state, being described as an “academic” materialand was believed to be unstable with respect to the formation of curved structures such as soot,fullerenes and nanotubes. Suddenly, the vintage model turned into reality, when free-standinggraphene was unexpectedly found three years ago and especially when the follow-upexperiments confirmed that its charge carriers were indeed mass-less Dirac fermions. So, thegraphene “gold rush” has begun.In the second chapter the basic structures of graphene has been discussed including differentforms of carbon. 5
  6. 6. The third chapter presents the synthesis of graphene and various characterization techniquespertaining to 2D structures.Many extraordinary properties of graphene such as electrical, mechanical, thermal and opticalare discussed in the forth chapter. These properties have generated tremendous interest amongmaterial researchers.The recent applications in various fields such as in large scale assembly and transistors, sensors,transparent electrodes, solar cell, energy storage devices, integrated circuits will be reviewedwith a brief update in the fifth chapter.In the sixth chapter some of future scopes of graphene has been discussed.2. STRUCTURE OF GRAPHENE:Graphene is a single layer of carbon packed in a hexagonal (honeycomb) lattice, with a carbon-carbon distance of 0.142 nm. Although isolated graphene was reported for the first time only in2004, the progress it made over these years is enormous, and it rightly has been dubbed "thewonder material". Graphene sheets stack to form graphite with an interplanar spacing of0.335 nm, which means that a stack of three million sheets would be only one millimeter thick.Graphene is the basic structural element of some carbon allotropes including graphite, charcoal,carbon nanotubes and fullerenes. It can also be considered as an indefinitely large aromaticmolecule, the limiting case of the family of flat polycyclic aromatic hydrocarbons. 6
  7. 7. Carbon is arguably the most fascinating element in the periodic table. It is the base for DNA andall life on Earth. Carbon can exist in several different forms. However, carbon’s 4 valenceelectrons have very similar energies, so their wave functions mix easily facilitating hybridization.In carbon, these valence electrons give rise to 2s, 2px, 2py, and 2pz orbitals while the 2 inner shellelectrons belong to a spherically symmetric1s orbital that is tightly bound and has an energy farfrom the Fermi energy of carbon. For this reason, only the electrons in the 2s and 2p orbitalscontribute to the solid-state properties of graphite. This unique ability to hybridize sets carbonapart from other elements and allows carbon to form 0D, 1D, 2D, and 3D structures.A new form of molecular carbon is the so called fullerenes. The most common, called C60,contains 60 carbon atoms and looks like a football (soccer ball) made up from 20 hexagons and12 pentagons which allow the surface to form a sphere. The discovery of fullerenes was awardedthe Nobel Prize in Chemistry in 1996. 7
  8. 8. (a) (b) (c)Fig- 2: Graphene can be (a)wrapped up into 0D C60 fullurences (b)rolled into 1D nanotubes or (c)stacked into3D graphite.A related quasi-one-dimensional form of carbon, carbon nanotubes, have been known for severaldecades and the single walled nanotubes since 1993.These can be formed from graphene sheetswhich are rolled up to form tubes, and their ends are half spherical in the same way as thefullerenes. The electronic and mechanical properties of metallic single walled nanotubes havemany similarities with graphene.It was well known that graphite consists of hexagonal carbon sheets that are stacked on top ofeach other, but it was believed that a single sheet could not be produced in isolated form suchthat electrical measurements could be performed. It, therefore, came as a surprise to the physicscommunity when in October 2004, Konstantin Novoselov, Andre Geim and their collaborators1showed that such a single layer could be isolated and transferred to another substrate and thatelectrical characterization could be done on a few such layers. In July 2005 they publishedelectrical measurements on a single layer.8. The single layer of carbon is what we call graphene.Graphene and Graphite are the two dimensional sp2 hybridized forms of carbon found in pencillead. Graphite is a layered material formed by stacks 41 of graphene sheets separated by 0.3 nmand held together by weak vander Waals forces. The weak interaction between the sheets allowsthem to slide relatively easily across one another. This gives pencils their writing ability andgraphite its lubricating properties, however the nature of this interaction between layers is notentirely understood. A single 2-D sheet of graphene is a hexagonal structure with each atomforming 3 bonds with each of its nearest neighbors. These are known as the sigma bonds orientedtowards these neighboring atoms and formed from 3 of the valence electrons. These covalentcarbon-carbon bonds are nearly equivalent to the bonds holding diamond together givinggraphene similar mechanical and thermal properties as diamond. The fourth valence electrondoes not participate in covalent bonding. It is in the 2p z state oriented perpendicular to the sheetof graphite and forms a conducting sigma bond. The remarkable electronic properties of carbonnanotubes are a direct consequence of the peculiar band structure of graphene, a zero band gapsemiconductor with 2 linearly dispersing bands that touch at the corners of the first Brillion zone.Bulk graphite has been studied for decades but until recently there were no experiments on 8
  9. 9. graphene. This was due to the difficulty in separating and isolating single layers of graphene forstudy.3. SYNTHESIS OF GRAPHENE:In 2004, Andre Geim and Knowstantin Novolselov came up with an ingenious method afteryears of effort to isolate monolayer graphene flakes. As discovered in more detail later, theydeveloped the ‘scotch tap’ or ‘drawing method’ which relies on taking a large crystal of graphiteand peeling the crystal repeatedly by using an adhesive tape to generate a large number of thincrystals. Soon after, a group headed by Philip Kim at Columbia University in the US confirmedthe existence of graphene using the same drawing tecknique, while Walt de Heer and ClairBerger at Georgia Tech developed an epitaxial growth process that may be suitable for mass-producing graphene for industrial applications.[22]In 2008, graphene produced by exfoliation was one of the most expensive materials on Earth,with a sample that can be placed at the cross-section of a human hair costing more than $1000 asof April 2008. Since then, exfoliation procedures have been scaled up, and now companies sellgraphene in large quantities. On the other hand, the price of epitaxial graphene on SiC isdominated by the substrate price, which is approximately $100/cm2 as of 2009. Even cheaper 9
  10. 10. graphene has been produced by transfer from nickel by Korean researchers, with wafer sizes upto 30 inches reported.[22]In 2011, the institute of Electronic Materials Technology and Department of Physics, WarsawUniversity announced a joint development of acquisition technology of large pieces of graphenewith the best quality so far. In April, the same year, Polish scientist with support from PolishMinistry of Economy began the procedure for granting a potent to their discovery around theworld.[22]Some methods of production of graphene are discussed in this section, they are: Drawingmethod, Thermal decomposition on SiC, Graphite oxide reduction, Chemical vapour deposition.3.1 Drawing method:In a time when cutting-edge scientific research is expensive and complex, it seems absurd that abreak though in physics could be achieved with simple adhesive tape. But in 2004, Andre Gein,Kostya Novoselov and co-workers at the University of Manchester in the UK did just that. Bydelicately cleaving a sample of graphite with sticky tape, they produced something that was longconsidered impossible; a sheet of crystalline carbon just one atom thick, known as graphene.[3]The basic ‘recipe’ for making graphene using “scotch tape” technique requires using 300nm ofSiO2-coated silicon wafer as a substrate and cleaning it with a mix of hydrochloric acid andhydrogen peroxide to remove any residue that is adhering to the wafer. Following this onepatiently peels graphite by sandwiching it between scotch tape repeatedly till the tape istranslucent. Dabbing the tape on the SiO2 wafer and peeling it off leads to deposition of anassortment of flakes of different thickness on the surface. Examining the surface with a simpleoptical microscope allows seeing plenty of graphite debris and locating the very thin flakes ofgraphene. Fig-3 shows an optical microscope image of such a deposition. One can clearly see 10
  11. 11. that there are several flakes with different colours. The thickest flakes at the bottom of the imagehas silver colour, like a typical metal, and is very thick (~500nm), whereas ones that have a darkblue colour are ~50nm thick and the triangular flake that is barely visible is monolayergraphene. This method is also referred to as drawing method. The later name appeared becausethe dry deposition resembles drawing with a piece of graphite.[2]Fig-3: An optical microscope image of graphene after peeling using the ‘scotch- tape’ technique. The 300nmthick SiO2-coated Si wafer has a purple color and the color changes where layers of graphene aredeposited. The triangular flap of mono- layer graphene is clearly seen. Other flakes of graphene showvarying colour.3.2 Thermal Decomposition On SiC:The drawing method, which is the simplest way to simply peel it of a piece of graphite, which isan easy way but it is terribly uncontrolled. The other way is to start with an electronic materialcalled silicon carbide. In this method, silicon carbide is heated to high temperature (1100°c) toreduce it to graphene. In this case it could be as little as one and as many as several dozen layersof graphene.[22]Producing graphite through ultrahigh vacuum (UHV) annealing of SiC surface has been asattractive approach especially for semiconductor industry because the products are obtained onSiC substrate and requires no transfer before processing devices. When SiC substrate is heatedunder UHV; silicon atoms sublimate from the substrate. The removal of Si leaves surface carbonatoms to rearrange into graphene layers. The thickness of graphene layer depends on theannealing time and temperature. More recently, vapour phase annealing has been used toproduced graphene on SiC. At the expensive of a higher temperature, this method leads to the 11
  12. 12. formation of few layer of graphene on SiC with improved thickness homogeneity. Althoughproducing graphene on SiC substrate is attractive, several hurdles prevent the real application.For example, control the thickness of graphene layers in the routine production of large areagraphene is very challenging.[8]Many important graphene properties have been identified in graphene produced by this method.For example, the electronic band-structure (so-called Dirac cone structure) has been firstvisualized in this material. Weak anti-localization is observed in this material and not inexfoliated graphene produced by the pencil-trace method. Extremely large, temperature-independent mobilities have been observed in SiC-epitaxial graphene. They approach those inexfoliated graphene placed on silicon oxide but still much lower than mobilities in suspendedgraphene produced by the drawing method. It was recently shown that even without beingtransferred graphene on SiC exhibits the properties of massless Dirac fermions such as theanomalous quantum Hall effect.The weak van der Waals force that provides the cohesion of multilayer graphene stacks does notalways affect the electronic properties of the individual graphene layers in the stack. That is,while the electronic properties of certain multilayered epitaxial graphenes are identical to that ofa single graphene layer in other cases the properties are affected as they are for graphene layersin bulk graphite. This effect is theoretically well understood and is related to the symmetry of theinterlayer interactions.Epitaxial graphene on SiC can be patterned using standard microelectronics methods. Thepossibility of large integrated electronics on SiC-epitaxial graphene was first proposed in 2004,and a patent for graphene-based electronics was filed provisionally in 2003 and issued in 2006.Since then, important advances have been made. In 2008, researchers at MIT Lincoln Labproduced hundreds of transistors on a single chip and in 2009, very high frequency transistorswere produced at the Hughes Research Laboratories on monolayer graphene on SiC. Band gap ofthe epitaxial graphene can be tuned by irradiating with laser beams; modified graphene has a lotof advantages in device application.(Laser Patterning of Epitaxial Graphene for SchottkyJunction Photodetectors).[22] 12
  13. 13. 3.3 Graphite Oxide Reduction:An alternative method for creating single sheets starting from graphite oxide (GO) has beensuggested. Graphite can be oxidized to produced GO and then exfoliated to create stable aqueousdispersions of individual sheets. After deposition, GO may be reduced to graphene eitherchemically or by means of thermal annealing. However, this method has drawbacks. First, manyof the resulting sheets are found to be wrinkled or folded when examined by AFM. Second,cross-sectional step heights of more than 1nm are often observed for a single sheet, which ismuch larger than the theoretical value of 0.34nm found in graphite. This increased thickness maybe attributed to unreduced surface hydroxyl and epoxide groups. Such functionality aredetrimental to the electrical properties of graphene. Third, aqueous dispersions are not ideal fordeposition as the high surface tension of water leads to aggregation during the evaporationprocess. Finally, even if GO is perfectly deposited, reduction method tend to neglect the area indirect contact with the substrate. Attempts have been made to complete the reduction stage insolution, but sheets tend to aggregate due to the attractive forces between layers and an overalldecrease in hydrophilicity.[4]Although this simple method has been applied on a large scale to commercially available sulfuricacid intercalated graphite, it never results in complete exfoliation of graphite to the level ofindividual graphene sheets. The extent of thermal expansion is dependent on the type of graphiteused and on the intercalation procedure. With few exception, the graphite nanoplates obtainedvia this process typically consist of hundreds of stacked graphene layers and average between 30and 100nm in thickness. In addition to the thermal expansion route, the delamination ofintercalated graphite can sometimes be achieved by inducing a gas-producing chemical reactionwithin its interlayer galleries (chemical expansion). For example, a low-temperature chemicalexpansion route to graphite nanoplates and nanoscrolls, based on potassium-intercalatedgraphite, has been reported. However, this approach could not be reproduced in laboratory evenwith a duplication of the expensive ultra-high-intensity ultrasonic equipment reported therein. 13
  14. 14. GO is produced by the oxidative treatment of graphite via one of three principle methodsdeveloped by Brodie, Hummers, and Staudenmeier respectively. It still retains a layeredstructure, but is much lighter in colour than graphite due to the loss of electronic conjugationbrought about by the oxidation. According to the most recent studies GO consist of oxidizedgraphene sheets having their basal planes decorated mostly with epoxide and hydroxyl groups, inaddition to carboxyl and carboxyl groups located presumably at the edge (Lerf- Klinowskimodel). These oxygen functionalities render the graphene oxide layers of GO hydropholic andwater molecules can readily intercalated into the interlayer galleries. GO can therefore be alsothrough of as a graphite- type intercalated compound with both covalently bound oxygen andnon-covalently bound water between the carbon layers. Indeed, rapid heating of GO results in itsexpansion and determination caused by rapid evaporation of the intercalated water and evolutionof gases produced by thermal pyrolysis of the oxygen-containing functional groups. Such themaltreatment has recently been suggested to be capable of producing individual functionalizedgraphene sheets.By nature, GO is electrically insulating and thus cannot be used, without further processing, as aconductive nanomaterial. In addition, the presence of the oxygen functional groups makes GOthermally unstable, as it undergoes pyrolysis at elevated temperatures. Notably, it has beendemonstrated that the electrical conductivity of GO can be restored close to the level of graphiteby chemical reduction. Such reaction of GO; however have not studied in great detail. To thatend, the chemical reduction of exfoliation graphene oxide sheets with several reducing agentsand found hydrazine hydrate (H2NNH2.H2O) to be the best one in producing very thin graphenelike sheets, consistant with previous reports. High-resolution scanning electron microscopy(SEM) also provided with evidence of thin sheets.[5]3.4 Chemical Vapour Deposition (CVD) :In this work, a technique is developed for growing few-layer graphene films using chemicalvapour deposition (CVD) and successfully transferring the films to arbitrary substrates withoutintense mechanical and chemical treatments, to preserve the high crystalline quality of the 14
  15. 15. graphene samples. Therefore, we expect to observe enhanced electrical and mechanicalproperties. The growth, etching and transferring processes of the CVD-grown large-scalegraphene films are summarized in Fig-4.It has been known for over 40 years that CVD of hydrocarbons on reactive nickel or transition-metal-carbide surfaces can produce thin graphitic layers. However, the large amount of carbonsources absorbed on nickel foils usually form thick graphite crystals rather than graphene films(Fig. 5a). To solve this problem, thin layers of nickel of thickness less than 300nm weredeposited on SiO2/Si substrates using an electron-beam evaporator and the samples were thenheated to 1,000 °C inside a quartz tube under an argon atmosphere. After flowing reaction gasmixtures (CH4 :H2 :Ar=50:65:200 standard cubic centimeters per minute), the samples is rapidlycooled to room temperature (~25 °C) at the rate of ~10°Cs -1 using flowing argon. It is found thatthis fast cooling rate is critical in suppressing formation of multiple layers and for separatinggraphene layers efficiently from the substrate in the later process.A scanning electron microscope (SEM) image of graphene films on a thin nickel substrateshows clear contrast between areas with different numbers of graphene layers (Fig. 5a).Transmission electron microscope (TEM) images (Fig. 5b) show that the film mostly consists ofless than a few layers of graphene. After transfer of the film to a silicon substrate with a 300-nm-thick SiO2 layer, optical and confocal scanning Raman microscope (CRM) images were made ofthe same area (Fig. 5c, d). The brightest area in Fig. 5d corresponds to monolayers, and thedarkest area is composed of more than ten layers of graphene. Bilayer structures appear topredominate in both TEM and Raman images for this particular sample, which wasprepared from 7 min of growth on a 300-nm-thick nickel layer. It is found that the averagenumber of graphene layers, the domain size and the substrate coverage can be controlled bychanging the nickel thickness and growth time during the growth process (Supplementary Figs 4and 5), thus providing a way of controlling the growth of graphene for different applications. 15
  16. 16. Fig-4: Sunthesis, etching and transfer processes for the large scale and patterned graphene films. (a)Synthesis of patterned graphene films on thin nickel layers. (b) Etching using FeCl3 (or acids) and transfer ofgraphene film using a PDMS stamp. (c) Etching using BOE or hydrogen fluoride (HF) solution and transferof graphene films. RT, room temperature (~25°C).Atomic force microscope (AFM) images often show the ripple structures caused by thedifference between the thermal expansion coefficients of nickel and graphene (Fig. 5c). Theseripples make the graphene films more stable against mechanical stretching, making the filmsmore expandable. Multilayer graphene samples are preferable in terms of mechanical strengthfor supporting large-area film structures, whereas thinner graphene films have higher opticaltransparency. It is found that a~300-nm-thick nickel layer on a silicon wafer is the optimalsubstrate for the large-scale CVD growth that yields mechanically stable, transparent graphenefilms to be transferred and stretched after they are formed, and that thinner nickel layers with ashorter growth time yield predominantly mono- and bilayer graphene film for microelectronicdevice applications (Supplementary Fig. 4c) 16
  17. 17. Etching nickel substrate layers and transferring isolated graphene films to other substrates isimportant for device applications. Usually, nickel can be etched by strong acid such as HNO3,which often produces hydrogen bubbles and damages the graphene. So, an aqueous iron (III)chloride (FeCl3) solution (1 M) was used as an oxidizing etchant to remove the nickel layers. Thenet ionic equation of the etching reaction can be represented as follows: 2Fe3+ (aq) + Ni (s) → 2Fe2+ (aq) + Ni2+ (aq)Fig-5: Various spectroscopy analyses of the large-scale graphene films grown by CVD. (a) SEM images of as-grown graphene films on thin (300nm) nickel layers and thick ( 1mm) Ni foils (inset). (b) TEM images of 17
  18. 18. graphene films of different thicknesses. (c) An optical microscope image of the graphene film transferred to a300nm thick SiO2 layer. The inset AFM image shows typical rippled structures. (d) A confocal scanningRaman image corresponding to (c). The number of layers is estimated from the intensities, shapes andpositions of the G-band and 2D band peaks. (e) Raman spectra (532nm laser wavelength) obtained from thecorresponding coloured spots in (c), (d)This redox process slowly etches the nickel layers effectively within mild pH range withoutforming gaseous products or precipitates. In a few minutes, the graphene film separated from thesubstrate floats on the surface of the solution and the film is then ready to be transferred to anykind of substrate. Use of buffered oxide etchant (BOE) or hydrogen fluoride solution removessilicon dioxide layers, so the patterned graphene and the nickel layer float together on thesolution surface. After transfer to a substrate, further reaction with BOE or hydrogen fluoridesolution completely removes the remaining nickel layers.[14] 18
  19. 19. 4. PROPERTIES:Graphene’s unique properties arise from the collective behavior of electrons. Through theinvestigation of pristine graphene many charming properties were discovered in the past fewyears including extremely high charge (electrons and holes) mobility with 2.3% absorption ofvisible light, thermal conductivity, quantum hall effect etc. This section will focus on thegraphene properties (Electronic Properties, Optical Properties, Thermal Properties, MechanicalProperties and Quantum Hall Effect in Graphene) which have been found in some of the recentreview articles.4.1 Electronic Properties:It is interesting to note that the basic structure that gives rise to graphite, carbon nanotubes andC60 is graphene with sp2 hybridized molecular orbital; however, it was the last to be isolated.Figure-6 shows the hexagonal lattice structure of graphene that results from Fig-6: The lattice structure of graphene has hexagonal symmetry as indicated by the red and blue colored‘atoms’ taken together .This class of lattice is not a Bravais lattice but can be constructed from twointerpenetrating lattices of equilateral triangles. 19
  20. 20. the sp2 hybridization of the molecular orbitals starting from the half-filled outer shell of 2s 1 2p3(one can think of this as an intermediate state from the outer shell 2s22p2 prior to hybridization).The p orbital that is not hybridized is the pz orbital and is oriented perpendicular to the plane ofthe two-dimensional sheet. The hexagonal lattice is not a Bravais lattice and that implies that onecan describe it only in terms of two interpenetrating lattices of equilateral triangles one atom ofred lattice at the centroid of the blue lattice (see Figure 6). The sp2 bonds between the nearestneighbour atoms have a strong wavefunction overlap and give rise to a very strong covalentbond. However, it is the half-filled shell of unhybridized pz that gives the state its uniqueelectrical property due to the overlap with nearest neighbours to form π orbital.In order to better understand the electronic properties of any material it is important tounderstand the energy (E) and momentum (k) relationship for different k also known as bandstructure of the material. The origin of the band structure is simply related to the fact thatunhybridized pz, perpendicular to the plane, overlap with nearest neighbours to form π orbitalsspread out in energy and give rise to a band of states extended over a range of energies. Theresult of such a calculation, using the tight-binding approach, involves starting with a linearcombination of wavefunctions at blue and red sites (shown in Figure 6) and finding the energymomentum relationship taking into account the crystal structures symmetry. Here it isincorporated by using the fact that a blue lattice point has three next nearest neighbours (in red)that have an angular spread of 1200.The result of such a calculation is shown in Figure 7(a). 20
  21. 21. (a) (b) (c)Fig-7: Understanding the bandstructure, essentially the energy and momentum relationship for the electronsof graphene.The key things to notice are the fact that there are two bands (lower one in Figure 7(a) is thevalence band and the upper one is the conduction band). The plane through the middle is theposition of the Fermi energy, indicating that the valence band is completely filled and theconduction band is empty. The reason behind a completely filled valence band and completelyempty conduction band is that the starting set of pz are exactly half-filled resulting in the finalbands of the solid being filled only upto the valence band. The second key feature is that theconduction and valence bands touch at six points. Figure 7(b) shows the contour plot of the bandgap the difference in energy between the conduction and valence band. We see that at six pointsthe bandgap is zero and the symmetry of the hexagonal crystal structure, in real space, isreflected in the symmetry in the momentum (k) space, as expected. The six points are also thepoints at which the Fermi energy cuts two bands and so the solid has six Fermi points. 21
  22. 22. To understand the electronic properties of any system one needs to only look at the excitationsclose to the Fermi energy because far away from EF (energies much larger than kBT with kBbeing the Boltzmann constant and T is the temperature), the states are either completely filled, orare completely empty, and hence unable to participate in `transitions between states required forthe spatial movement of electrons. Effectively the electronic properties are determined byexcitations, like waves, on the surface of the Fermi Sea; the states deep below or high above theFermi energy are mostly irrelevant for electrical transport. To understand graphenes electronicproperties one needs to `zoom-in in very close to the Fermi energy and check the energy andmomentum relationship of the electrons.Figure 7(c) shows the `view of the band structure close to one of the Fermi points and one findsthat the bands look like two upturned cones. This implies that E ∞ k for the excitations close tothe Fermi energy. As a result, the electronic excitations inside graphene behave as if they aremass less as their energy and momentum are linearly related, unlike electrons in most materials,where energy and momentum are quadratically related ~ E ∞ k2. This peculiar nature ofelectrons energy and momentum relationship makes them analogous to relativistic particles, sayphotons, where the energy and momentum are linearly related. However, the electrons ingraphene do not flow anywhere close to the velocity of light (they travel at roughly 106 m/s, i.e.,about 1/300 the speed of light); they are analogous only because of their linear energymomentum (E ∞ k) relationship. As a result graphene is a ‘wonderland for studying veryinteresting quantum mechanical phenomena that typically cannot be studied in solid-statedevices, but could only be seen in particle accelerators. This unique E ∞ k relationship impliesthat one cannot use Schrődinger equation to describe the quantum mechanics of electrons buthave to use Dirac equation - a more appropriate quantum mechanical description.[2]4.2 Optical Properties:Graphene, despite being the thinnest material ever made, is still visible to the naked eye. It canshow remarkable optical properties. For example, it can be optically visualized, despite beingonly a single atom thick. Its transmittance (T) can be expressed in terms of the fine-structureconstant. The linear dispersion of the Dirac electrons makes broadband applications possible. 22
  23. 23. Saturable absorption is observed as a consequence of Pauli blocking and nonequilibrium carriersresult in hot luminescence. Chemical and physical treatments can also lead to luminescence.These properties make it an ideal photonic and optoelectronic material.[9]Linear optical absorption:For identifying graphene on top of a Si/SiO2 substrate the optical image contrast can be used.This scales with the number of layers and is the result of interference, with SiO2 acting as aspacer. The contrast can be maximized by adjusting the spacer thickness or the light wavelength.The transmittance of a freestanding SLG can be derived by applying the Fresnel equations in thethin-film limit for a material with a fixed universal optical conductance G0 = e2/(4ħ) ≈ 6.08 × 10−5Ω −1, to give:T = (1 + 0.5 πα)-2 ≈ 1 – πα ≈ 97.7%where α = e2/(4πε0ħc) = G0/(πε0c) ≈ 1/137 is the fine-structure constant.Graphene only reflects <0.1% of the incident light in the visible region1, rising to ~2% for tenlayers. Thus, we can take the optical absorption of graphene layers to be proportional to thenumber of layers, each absorbing A ≈ 1 – T ≈ πα ≈ 2.3% over the visible spectrum . In a few-layer graphene (FLG) sample, each sheet can be seen as a 2D electron gas, with littleperturbation from the adjacent layers, making it optically equivalent to a superposition of almostnon-interacting SLG. The absorption spectrum of SLG is quite flat from 300 to 2,500 nm with apeak in the ultraviolet region (~270 nm). In FLG, other absorption features can be seen at lowerenergies, associated with interband transitions.[9]Saturable absorption:Graphene can be saturated under strong excitation over the visible to near infra-red region, due tothe universal optical absorption and zero band gap. Interband excitation by ultrafast opticalpulses produces a non-equilibrium carrier population in the valence and conduction bands. Intime-resolved experiments, two relaxation timescales are typically seen: a faster one of ~100 fsthat is usually associated with carrier–carrier intraband collisions and phonon emission, and a 23
  24. 24. slower one, on a picoseconds timescale, which corresponds to electron interband relaxation andcooling of hot phonons.[9]Luminescence: Another interesting property of grapheme is photo luminescence. It could be made grapheneluminescent by including a suitable band gap, following two routes. One is by cutting it intonanoribbons and quantum dots, the other is by chemical or physical treatments, to reduce theconnectivity of the π-electron network. Bulk graphene oxide dispersions and solids do show abroad photo luminescence. Though graphene nanoribbons have been produced with varying bandgaps as yet no photoluminescence. Individual graphene flakes can be made brightly luminescentby mild oxygen plasma treatment. It is possible to make hybrid structures by etching just the toplayer, while leaving underlying layers intact. This combination of photo luminescent andconductive layers could be used in sandwich light-emitting diodes. Luminescent graphene basedmaterials can now be routinely produced that cover the infrared, visible and blue spectral ranges.[9]4.3 Thermal properties:Graphene is a perfect thermal conductor. Carbon allotropes such as graphite, diamond, andcarbon nano tubes have shown higher thermal conductivity due to strong C-C covalent bonds andphonon scattering. Earlier carbon nanotubes are known to have very high thermal conductivity.The experimentally determined thermal conductivity with room temperature value for multiwallcarbon nanotube is ≈ 3000W/mK and for single-wall carbon nanotube is ≈ 3500W/mK. Recentlythe highest room temperature thermal conductivity ≈ up to 5000W/mK for the single layergraphene has been reported [3]. Methods of measuring thermal conductivity (k) can be dividedinto two groups: steady state and transient. In transient methods, the thermal gradient is recordedas a function of time, enabling fast measurements of thermal diffusivity (DT) over large T ranges.. If K determines how well a material conducts heat, DT tells us how quickly a material conductsheat. The first experimental study of heat conduction in graphene was made possible bydeveloping an optothermal Raman technique. The heating power ΔP was provided by a laser 24
  25. 25. light focused on a suspended graphene layer connected to heat sinks at its ends. Temperature rise(ΔT) in response to ΔP was determined with a micro-Raman spectrometer. The G peak ingraphene’s Raman spectrum exhibits strong T dependence. The calibration of the spectralposition of the G peak with T was performed by changing the sample temperature while usingvery low laser power to avoid local heating. The frequency of the G peak (ωG) as a function oftemperature — calibration curve ωG(T) — allows one to convert a Raman spectrometer into anoptical thermometer. The optothermal Raman technique for measuring the K of graphene is adirect steady-state method. It can be extended to other suspended films, for example, graphenefilms, made of materials with pronounced temperature-dependent Raman signatures.[8]4.4 Mechanical Properties:Generally, application of external stress on crystalline material can alter inter atomic distances,resulting in the redistribution, in local electronic charge. This may introduce a band gap inelectronic structure and modify the electron transport property significantly. After carbonnanotubes , graphene has been reported to have the highest elastic modulus and strength.Researchers have already determined the intrinsic mechanical properties of the single, bilayerand multilayer of graphene.A single defect free graphene layer is predicted to show the highest intrinsic tensileStrength with stiffness similar to graphite. One method to determine the intrinsic mechanicalproperties is to probe the variation of the phonon frequencies upon the application of tensile andcompressive stress. The Raman spectroscopy is one of the techniques which can monitorphonon’s frequency under uniaxial tensile and hydrostatic stress. It has been observed that thetensile stress results in the phonon softening due to decreased vibrational frequency modewhereas compressive stress causes the phonon hardening due to increased vibrational frequencymode. Thus in graphene, studying the vibration of phonon frequency as a function of strain canprovide useful information on stress transfer to individual bonds (for suspended graphene) andatomic level interaction of graphene to the underlying substrate (for suspended graphene).Compressive and tensile strain in graphene layer was estimated using Raman spectroscopy bymonitoring change in the G and 2D peaks with applied stress. Recently, the band gap tuning was 25
  26. 26. reported under uniaxial strain. The band gap of 0.25eV was detected under the highest strain(0.78%) for the single layer graphene. It was also suggested that the uniaxial strain affected theelectronic properties of graphene much more significantly as it breaks the bonds of C-C lattice.[8]4.5 Quantum Hall effect in Graphene:The quantum mechanical version of the Hall Effect is known as Quantum Hall effect (QHE)which is observed in two dimensional electron systems subjected to low υf temperatures andstrong magnetic fields. There are integer and fractional QHE. Both of these Hall effects haveseen in Graphene, a single layer of carbon atoms in a two dimensional hexagonal lattice, at roomtemperature. The carbon atoms bond to one another via covalent bonds leaving one 2p electronper carbon atom unbonded. The result is that the Fermi surface of graphene is characterized bysix double cones. In the absence of applied fields, the Fermi level is situated at the connectionpoints of these cones. Since the density of electrons is zero at the Fermi level, the electricalconductivity of graphene is very low. However, the application of an external electric field canchange the Fermi level causing graphene to behave as a semi-conductor. In this case, near theFermi level the dispersion relation for electrons is linear and the electrons behave as though theyhave zero effective mass (Dirac fermions). Because graphene exhibits this behavior even at roomtemperature, it is observed to exhibit both the integer and fractional quantum Hall effects.Fig-8: Energy level diagram for graphene showing the Fermi level in the absence of any applied fields. 26
  27. 27. The existence of the QHE at such high temperatures in graphene is due to the large energy gapscharacteristic of Dirac fermions. These energies are given by: En= υf √|2neħB|Where υf is the Fermi velocity (106m/s) and n is the Landau level quantum number. In a strongmagnetic field (B=45T), the energy level spacing is 2800K.Graphene has a large concentration of charge carriers which keeps the lowest Landau levelcompletely populated at high magnetic fields. Therefore, any carriers above the lowest Landaulevel will not be able to overcome the energy gap, and the quantum Hall effect is observed. Thequantum Hall effect has been observed for the integer values of the filling factor as well as thefractional filling factors because graphene exhibits both the integer and fractional quantum Halleffects, it is ideal for studying QHE, and may be proven useful in the development of quantumcomputers.[10]5. APPLICATIONS: 27
  28. 28. For many years it was believed that carbon nanotubes would create a revolution in nano-electronics because of their microscopic dimension and very low electrical resistance. Thesehopes however have not yet come to fruition because of various difficulties. These includeproducing nanotubes with well-defined sizes, the high resistance at the connection betweennanotubes and the metal contacts that connect them to circuits and the difficulty of integratingnanotubes into electronic devices on a mass-production scale.Walt de Heer argues that with graphene we will be able to avoid all of these problems. Grapheneis useful in so many areas, that it is hard to pick and choose. Physicist like them because they area playground for understanding how electrons behave when graphene get confined in two-dimension. Biologists are interested in using them as a way of probing biological systems.Graphene can be either metals or semiconductors and their electrical properties can rival or evenexceed the best metals or semiconductors known. Because of this engineers are interested inusing them as building blocks for smaller transistors. Material scientists want to mix grapheneinto more traditional substances to create hybrid materials that are much stronger or areconducting, while still being malleable.[3]Some major applications of graphene are discussed in this section.5.1 Graphene Transistor:Owing to its high carrier mobility and saturation velocity, graphene has attracted enormousattention in recent years. Graphene – a sheet of carbon just one atom thick- shows great promisefor use in electronic devices because electrons can move through it at extremely high speeds.This is because, they behave like relativistic particles with no rest mass. This , and other unusualphysical and mechanical properties, means that the wonder material could replace silicon as theelectronic material of choice and might be used to make faster transistors than any that existtoday. 28
  29. 29. Fig-9: Graphene transistor.The field effect transistor (FET) is a key element, where the current flowing through a thinchannel layer is controlled by gate electrodes. FET can be operated faster with a channel layer ofa higher electron mobility material, which is the very point the application of graphene to FET.In 2004, there are some reports on the characteristics of FET using graphene as a channel layermaterial. For the fabrication of graphene- based FET, graphene exfoliation from HOPG is oftenused. On the other hand, in 2010, IBM reported on 100 gigahartz operation of a FET based ongraphene by heat treatment of SiC, has been a major topic. The 240nm graphene transistor madeat IBM were made using extant silicon manufacturing equipment, meaning that for the first timegraphene transistors are a conceivable- though still fanciful- replacement for silicon. Theoperation speed is already more than twice higher than that of silicon-based FET which usessilicon as the channel layer with the same gate length. It strongly indicates the high potentialityof graphene application to FET.In 2011, the manufacturing technology for IBM’s graphene transistors has been improved andthe new 155 GHz transistor is made with 400nm technology, on par with the commercialsolution available today, which is also a record note in terms of gate length for graphene.[6] 29
  30. 30. 5.2 Graphene Nanoribbons:While many labs are trying to efficiently synthesize large two-dimensional sheets of graphene, ateam of researcher from Sweden and the UK is investigating the synthesis of very thin strips ofgraphene just a few atoms wide. In contrast to graphene, these graphene nanoribbons have aunique electronic structures including a non zero band gap, which makes them promisingcandidates for semiconductor application. But, as with graphene sheets, one of the greatestchallenges for now is finding a way to efficiently synthesize these graphene nanoribbons.[15]Graphene nanoribbons (also called nano-graphene ribbons), often abbreviated GNRs, are thinstrips of graphene or unrolled single-walled carbon nanotubes. Graphene ribbons were originallyintroduced as a theoretical model by Mitsutaka Fujita and co-authors to examine the edge andnanoscale size effect in graphene. Theoretical calculations on the band structure of GNRs haveshown that GNRS exibit metallic properties or semiconductor properties ie, the band gap islarger than 0, depending on the orientation of the ribbon. Two configurations of GNRs structuresare illustrated in Fig-10 (a) and (b) focusing on the edges of GNRs.Fig-10: Illustration of Two Types of Edges, (a) Armchair Edge and (b) Zigzag Edge, and (c) TheoreticallyCalculated Bandgap of Armchair-Edged Graphene Nanoribbon 30
  31. 31. The configuration illustrated in (a) is called the armchair type where the edge has a cyclicstructures of four carbon atoms, ie, two couples of carbon atoms. A GNRs of the armchair typeconfiguration exhibits semiconductor properties. On the other hand, the configuration in (b) iscalled the zigzag type where the edge zigzags. A GNRs of the zigzag type configuration exhibitszero band gap. However, recent DFT calculations show that armchair nanoribbons aresemiconducting with on energy gap scaling with the inverse of the GNR width. In zigzagconfigurations, this gap is inversely proportional to the ribbons width.A metallic nanoribbon inside an insulating nanotube can be a thin, insulated nanowire. Ornanoribbons can be used directly inside of single- wall carbon nanotubes (SWNTs) to generate aslight emitting diodes. Semiconducting nanoribbons colud be used for transistor or solar cellapplications. Or a metallic combination could produce a new kind of coxial nanocable for use intransmitting radio signals.Their 2D structures, high electrical and thermal conductivity, and low noise also make. GNRs apossible alternative to copper for integrated circuit interconnects.[6]5.3 Transparent Conducting Electrodes:As a critical component of optoelectronic devices, transparent conductive coatings pervademodern technology. The most widely used standard coating is indium tin oxide (ITO), used innearly all flat panel displays and microdisplays. Causing problems for manufacturers though,indium is expensive and scarce and demand is increasing. Recently, prices have fallen back. Butgeologists say the cost of indium may not matter soon, because the earth’s supply of this elementcould be gone within just a few years. This has made the search for novel transparent electrodematerials with good stability, high transparency and excellent conductivity a crucial goal foroptoelectronic researchers. Recent word by researchers in Germany exploits ultra-thintransparent conductive graphene film as window electrodes in solar cells.[16]These graphene films are fabricated from exfoliated graphite oxide, followed by thermalreduction. The obtain films exhibit a high conductivity of 550 Scm and a transparency of morethan 70% over 1000-3000nm. 31
  32. 32. Graphene’s high electrical conductivity and high optical transparency make it a candidate fortransparent conducting electrodes ,required for such application as touch screen, liquid crystaldisplays, organic photovoltaic cells and organic light emitting diodes. In particular, graphene’smechanical strength and flexibility are adventurous compared to indium tin oxide, which isbrittle and graphene films may be deposited from solution over large areas. Large-area,continuous, transparent and highly conducting few-layered graphene films were produced bychemical vapor deposition and used as anodes for application in photovoltaic devices. Theelectronic and optical performance of devices based on graphene are shown to be similar todevices made with indium-tin-oxide.[17]5.4 Ultracapacitors:2010, the ultracapacitors- the battery’s quicker cousin- just got faster and may one day helpmake portable electronics a lot smaller and lighter, according to a group of researchers.Ultracapacitors don’t store quite as much charge as batteries, but can charge and discharge inseconds rather than the minutes batteries take. This combinations of speed and energy supplymakes them attractive for things like regenerative braking, where the ultracapacitors would haveonly seconds to recharge as a car comes to a stop. But sometimes a second is still too long, usingnanometer-scale fins of graphene, the researchers built an ultracapacitor that can charge in lessthan a millisecond.[21]Recently , graphene has become a hot topic in ‘macroscopic’ applications as well, notably in thedesign and manufacture of ultracapacitors , which have recently become highly valued as powerstorage system in electrical light-rail vehicles, diesel-electric battery drive systems and forklifttrucks.[18] 32
  33. 33. Fig-11: ultra capacitor having graphene as conductive plateThe possibilities opened by graphene go beyond improvements in the energy density ofultracapacitors, as the team of John Miller, president of JME, an electrochemical capacitorcompany based in Shaker Heights, Ohio and Ron Outlaw of the college of William and Mary,Williamburg, VA, have recently shown. By using electrodes made from vertically orientedgraphene nano-sheets, the team of Miller was able to improve drastically on the RC timeconstant of extant ultracapacitors, opening up new possibilities for the miniaturization of ACfiltering and rectifier circuits. The graphene nano-sheets reamble ~600 nm tall “potato chips”standing on edge in rows. The novel ultracapacitors charge and recharge in 200 microseconds,compared to ~1 second for nano-pore –based designs. These exciting developments indicate thatgraphene-based ultracapacitors are paired to take a farther step in the graphene revolution, whichhas been ongoing since ground breaking work of Geim and Novoselov in 2004.[18]5.5 OPV Solar Cell :The most unique aspect of the OPV (organic photovoltaic cell) devise is the transparentconductive electrode. This allows the light to react with the active materials inside and create theelectricity. Now graphene sheet are used to create thick arrays of flexible OPV cells and they areused to convert solar radiation into electricity providing cheap solar power. Now research teamunder the guidance of Chongwu Zhoa, Professor of Electrical Engineering, USC Viterbi School 33
  34. 34. of Engineering has put forward the theory that the graphene – in its form as atom-thick carbonatom sheets and then attached to very flexible polymer sheets with thermo-plastic layerprotection will be incorporated into the OPV cells. By chemical vapour deposition, qualitygraphene can now be produced in sufficient quantities also. The traditional silicon solar cells aremore efficient as 14 watts of power will be generated from 1000 watts of sunlight where as only1.3 watts of power can be generated from a graphene OPV cell. But these OPV cells more thancompensate by having more advantages like physical flexibility and costing less.The flexibility of OPV’s gives these cells additional advantage by being operational afterrepeated bending unlike the Iudium-Tin-oxide cells. Low cost, conductivity, stability, electrodeorganic film compatibility and easy availability along with flexibility give graphene OPV cell adecidedly added advantage over other solar cells.[8]5.6 Integrated Circuits :Graphene has the ideal properties to be an excellent component of integrated circuits. Graphenehas high carrier mobility, as well as noise, allowing it to be used as the channel in a FET. Theissue is that single sheets of graphene are hard to produce, and even harder to make on top of anappropriate substrate. Researchers are looking into methods of transferring single gaphene sheetsfrom their source of origin (mechanical exfoliation on SiO2/Si or thermal graphitization of a Sisurface) onto a target substrate of interest. In 2008, the smallest transistor so far, one atom thick,10 atoms wide was made of graphene. In May 2009, a team from Stanford University,University of Florida and Lawrence Livermore National Laboratory announced that they havecreated an n-type transistor, which means that both n and p-type transistors have now beencreated with graphene. At the same time, the researchers at the Politecnico di Milanodemonstrated the first functional graphene integrated circuit a complementary inverter consistingof one p- and one n-type graphene transistor. However, this inverter also suffered from a verylow voltage gain.[1] 34
  35. 35. 5.7 Graphene Biodevices :These devices are based upon graphene’s large surface area and the fact that molecules that aresensetive to particular diseases can attach to the carbon atoms in graphene. For example,researchers have found that graphene, strands of DNA and fluorescent molecules can becombined to diagnose diseases. A sensor is formed by attaching fluorescent molecules to singlestrand DNA and then attaching the DNA to graphene. When an identical single strand DNAcombines with the strand on the graphene a double strand DNA it formed that floats off from thegraphene, increasing the fluorescent level. This method results in a sensor that can detect thesame DNA for a particular disease in a sample.[23] 5.8 Limitations:Despite so many fruitful promises in the field of electronics, the graphene based IC’s;microprocessor, etc are unlikely to appear for the next 10-15 years. For more practicalapplications one would like to utilize the strong gate dependence of graphene for either sensingor transistor applications. One of the major problems lies in the production of high qualitygraphene having sufficient reproducibility. Also despite being almost similar to silicon-even a bitbetter in times of most of the characteristics graphene lacks the ability work as switch. Withoutthis, a chip will draw electricity continuously, unable to turn off. Unfortunately, grephene has nobad gap and correspondingly resistivity changes are small. Therefore, a plagued by a low on/offratio. However one way around this limitation, is to carve graphene into narrow ribbons. Byshrinking the ribbons the momentum of charge carriers in the transverse direction becomesquantized which results in the opening of a band gap. This band gap is proportional to the widthof the ribbon. This effect is pronounced in carbon nanotubes where a nanotube has a band gapproportional to its diameter. The opening of a fond gap in graphene ribbons has recently beenobserved in wide ribbon devices lithographically patterned from large graphene flakes and innarrow chemically synthesized graphene ribbons.[1] 35
  36. 36. 6. FUTURE ASPECTS:Despite the reigning optimism about graphene-based electronics, “graphenium” microprocessorsare unlikely to appear for the next 20 years. In the meantime, one can certainly hopefor many other graphene-based applications to come of age. In this respect, clear parallels withnanotubes allow a highly educated guess of what to expect soon.The most immediate application for graphene is probably its use in compositematerials. Indeed, it has been demonstrated that a graphene powder of uncoagulatedmicron-size crystallites can be produced in a way scaleable to mass production. Thisallows conductive plastics at less than 1 volume percent filling, which in combination withlow production costs makes graphene-based composite materials attractive for avariety of uses. However, it seems doubtful that such composites can match themechanical strength of their nanotube counterparts because of much stronger entanglementin the latter case.Another enticing possibility is the use of graphene powder in electric batteries that are alreadyone of the main markets for graphite. An ultimately large surface-to-volume ratio and highconductivity provided by graphene powder can lead to improvements in batteries’ efficiency,taking over from carbon nanofibres used in modern batteries. Carbon nanotubes have also beenconsidered for this application but graphene powder has an important advantage of being cheapto produce.One of the most promising applications for nanotubes is field emitters and,although there have been no reports yet about such use of graphene, thin graphiteflakes were used in plasma displays (commercial prototypes) long before graphenewas isolated, and many patents were filed on this subject. It is likely thatgraphene powder can offer even more superior emitting properties.Carbon nanotubes were reported to be an excellent material for solid-state gas sensors butgraphene offers clear advantages in this particular direction. Spin-valve andsuperconducting field-effect transistors are also obvious research targets, and recent reportsdescribing a hysteretic magnetoresistance and substantial bipolar supercurrents prove 36
  37. 37. graphene’s major potential for these applications. An extremely weak spin-orbit coupling and theabsence of hyperfine interaction in 12C-graphene make it an excellent if not ideal material formaking spin qubits. This guarantees graphene-based quantum computation to become an activeresearch area. Finally, we cannot omit mentioning hydrogen storage, which has been an activebut controversial subject for nanotubes. It has already been suggested that graphene is capable ofabsorbing an ultimately large amount of hydrogen and experimental efforts in this direction areduly expected. 37
  38. 38. 7. CONCLUSION:The field of graphene-related research has grown at a spectacular pace since single-layer flakeswere first isolated in 2004. Graphene that began its journey as an exciting material forfundamental physics has now become the focus of efforts by scientists in a wide range ofdisciplines. Organic and material chemists are busily working on new route to produce highquality single layers, while engineers are designing novel devices to explore graphene’sextraordinary properties. This review paper briefly discusses the significant structural attributesof graphene in association with other closely related carbon forms. Starting from its discoverythe evolution in synthesizing process has been reviewed. Its unique characteristic propertiesthose made it an extraordinary material has been highlighted. A wide range of utility in differentsectors has been mentioned as well as giving idea of its future scopes and prospects. Reviewingthe characteristic properties and utilitarian values of graphene it is very much clear that grapheneis indeed a wonder material of present time and it holds great promises to become one of theprimary materials of the times to come. 38
  39. 39. REFERENCES:[1] Md. Nazre Imam- Medak College of Engineering & Technology- Graphene: The FuturisticElement, 2011. http://www.slideshare.net/imamnazre/graphene-the-futuristic-element[2] Mandar M Deshmukh and Vibhor Singh- Graphene- an exciting two dimensional material forscience and technology, March, 2011. Resonance 238-246.[3] Antonio Castro Neto, Francisco Guinea and Nuno Miguel Peres- Drawing conclusions fromgraphene, November, 2006.[4] Vincent C. Tung, Matthew J. Allen, Yang Yang and Richard B. Karner- High-throughoutsolution processing of large scale graphene, 9 Nov, 2008. Nature Nanotechnology.[5] Sasha Stankovich, Dmitriy A. Dikin, Richard D. Piner, Kevin A. Kohlhaas, AlfredKleinhammes, Yuanyuan Jia, Yue Wu, SonBinh T. Nguyen, Rodney S. Rouff- Synthesis ofgraphene- based nanosheets via chemical reduction of exfoliated graphite oxide, 6 Mar,2007,Carbon 45 (2007) 1558-1565.[6] Yasushi Iyechika- Nanotechnology and Material Unit- Application of graphene to high speedtransistors: expectations and challenges, Oct, 2010. Quarterly review no- 37.[7] Matthew J. Allen, Vincent C. Tung, Richard B. Karner- Honeycomb carbon: A review ofGraphene, 20 Feb, 2009. Chemical reviews, XXXX, Vol-xxx.[8] Virendra Singh, Daeha Joung, Lei Zhai, Soumen Das, Saiful I Khondaker, Sudipta Seal-Graphene based materials: past, present and future, 3 Apr, 2011. Progress in Materials Science56 (2011) 1178–1271.[9] F. Bonaccorso, Z. Sun, T Hasan and A. C. Ferrari[10] Daniel g Flynn- Quantum Hall Effect In Graphene[11] A.K. Geim and K.S. Novoselov- The Rise of Graphene, 2007. Nature Material.[12] The Royal Swedish Academy of Science- Graphene, 5 Oct, 2010. 39
  40. 40. [13] T. Lammert, L. Rozo and E. Whittier- Graphene: Material of the Future, in Review, July16, 2009.[14] Keun Soo Kim , Yue Zhao , Houk Jang , Sang Yoon Lee , Jong Min Kim , Kwang S. Kim ,Jong-Hyun Ahn, Philip Kim, Jae-Young Choi & Byung Hee Hong- Large-scale pattern growthof graphene films for stretchable transparent electrodes, 14 Jan, 2009.[15] www.physorg.com[16] www.nanowerk.com[17] www.pubs.acs.org[18]http://materialsforenergy.typepad.com/materials/2010/11/graphenethe-ultimate-ultracapacitor.html[19] http://www.green.autoblog.com/2010/12/01/supercapacitor-breakthrough-beats-batteries-with-graphene/[20] http://nextbigfuture.com/2010/12/graphene-and-other-ultracapacitors.html[21] http://spectrum.ieee.org/semiconductors/nanotechnology/graphene-ultracapacitor-could-shrink-systems[22] www.en.wikipedia.org[23] www.understandingnano.com 40
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