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In recent years, graphene has attracted a fair amount of attention in the electronic industry. The first isolation of single sheet of graphene from graphite using scotch tape brought a shock in the field of science in 2004 by the group of A K Geim in the Manchester University, UK (2). The discovery of two dimensional graphene has brought reasons terrific properties of graphene such as the ambipolar field effect(1), the quantum hall effect at room temperature(5), ballistic transport(2), long electron mean free paths(4), superb thermal conductivity(10), enormous mechanical strength(3) and outstanding flexibility(3).
Graphene is the new material's name given to the two dimensional sheet of -sp2 hybridized carbon with honeycomb network which has one atom layer thick Graphene, containing linear and isotropic energy dispersions in the low-energy region. As carbon itself, the electronic configuration is 1s22s22p2. All carbon atoms are all 2s, 2px, and 2py hybridized, orbital each contains one electron, and form the sigma () bond with neighbouring of hybridized carbon atoms which produce the hexagonal structure of graphene which is 120°between atoms. The sigma bonds of the carbon atoms influences and effects to the mechanical properties and structure of the graphene.(3) The pz orbital, perpendicular to the plane, has remained a half-filled electron system forming the pi () bond with other neighbouring of px orbital. The bonds are responsible for the unusual electronic properties of graphene.
Figure1. - It shows that Honeycomb network graphene is bonded with angle 120. (16)
The 2D-graphene can be loaded up to form 3D-graphite, rolled to form 1D-nanotubes, and wrapped up to form 0D-fullerenes. Graphite is made out of stacks of graphene layers that are weakly bonded by Van Der Waals force. Fullerenes can be visualized as wrapped-up graphene having pentagons layer as shown in figure 1.2..
Figure1. - Allotrope of carbon. (6)
Although many scientists have attempted to develop devices based on graphene ever since it was first observed 40 years ago, the graphene devices have not been commonly used. Due to the difficulty of manufacturing high quality and large quantity of samples and that the scientist assumed to be that "two-dimensional graphene crystal which is thermodynamically unstable at finite temperature." (2)
There are three main methods which are currently being used; the micromechanical exfoliation of layers, thermal treatment of SiC, and Chemical Vapour Deposition method on Nikel (Ni) or copper (Cu) substrates, as well as a variety of studies involving the use of chemically modified graphene (CMG).(1) Numbers of physical properties depends on how the graphene is produced. This is due to the influence of interfacial effects in epitaxial graphene, which are heavily dependent on both the silicon carbide substrate and several growth parameters. (2)
Graphene has tremendously high charge carrier mobility due to high quality of its 2D crystal lattice involving an unusually low density of defects and a ballistic transport at 300K which is only observed in graphene. In 2008, Kim's group at Columbia measured carrier mobility in excess of 200,000 cm2/ Vs at 300 K (1) for a single layer of suspended and annealed graphene which was enough to give a jolt in the field of science and was the largest value reported for a semimetal or a semiconductor. The discovered value of carrier mobility is much higher than that of silicon (1400 cm2/ Vs) and indium antimonide (77000 cm2/ Vs) which are typical materials for computer chips and solar cell respectively. The mobility in graphene remains high even at the highest electric-field-induced carrier concentrations and temperature. It looks like to be less affected by chemical doping (3). The silicon transistor has become nearly as small as it can and performs effectively. Hence the graphene can be ideal replacement for Silicon.
Graphene is known as the lowest resistivity among the material on earth. From conductivity experiments, the Im (mean free path) increases from ~150 nm to 1m in a suspended sample before and after annealing respectively. At room temperature with high carrier density, the ballistic transport on micrometer scale is proved, which results in major implications for the electronic industry.
Figure 1. - " Carrier mobility in graphene. (a) Conductance of a suspended graphene sample before and after annealing as a function of carrier density, n, at a temperature of 40K. (b) Mobility displayed as a function of n exceeds 200,000 cm2/Vs at n = 2Ã-1011 cm-2. The dotted line is the expected behaviour for ballistic transport." (4)
The fascinating part in graphene is the way the electrons are represented within the lattice.In condensed matter, "the quantum relativistic effects are usually minute in the known experimental systems that can be described accurately by the non-relativistic the Schrödinger equation." (16) However, the quasi-particles in graphene is described by Dirac Hamiltonian Equation which is H = -iÄvfÆ¡ (17) where is characterized by the Fermi velocity that is 1/300 of the speed of light with mass-less relativistic particles (4). The symmetric and asymmetric band structure of graphene encounter at the edge of the Brillouin zone, which leads to a cone-shaped energy spectrum at the Dirac point K and K' as shown figure 1.4. Due to the linear dispersion relation near Dirac Points, the graphene's quasi-particles behave differently from those in typical metals and semiconductors where the energy spectrum is illustrated approximately to the parabolic dispersion relation. The quasi-particle in graphene is called the Dirac fermions, where the electrons behave as relativistic quasi-particles, which are unable to slow down. However, in theory the Dirac fermion carries one unit of electric charge and can be controlled by electromagnetic fields, in contrast, neutrinos have no electric charge and do not interact strongly with any kind of material.
The Dirac point is the interception points where results in zero bands gap hence electrons exist not only in conduction band but also in valence band. The carriers can be altered constantly between holes and electrons by supplying the gate electric field in concentration which is known as ambipolar filed effect. The unique electrons behaviour leads beyond the restrictions of silicon-based semiconductor.
Figure 1. -a) shows K and K' Dirac points, b) electric structure of graphene and c) Linear dispersion relation in graphene. (17)
Another amazing electrical property is the quantum hall effect (QHE) in mono layer graphene at room temperature with a presence of magnetic field perpendicular to the graphene layer. The QHE is two-dimensional equivalent of the Hall Effect with quantized values of conductivity. (8) This Quantum Hall effect has been reported in typical semiconductors below about 30K because "thermal fluctuations wash out the delicate quantum effects that are responsible for it." (15) QHE behaviour at room temperature is only observed in graphene, this is due to the unexpected nature of charge carriers and the effective mass of the conduction electron disappeared in graphene. The hall conductivity is expressed as where N is the landau index. And mono-layer has a 1/2 phase shift as shown in the equation above. The energy quantization of the graphene electronic structure in a magnetic field with a field strength B is expressed as where refers to holes and electrons, vf is the Fermi velocity and N an integer Landau level number. The most important fact is the presence of the zero-energy state at N=0 which is shared by electrons and holes with massless behaviour.
Diamond, graphite, and carbon nano-tubes have been known for their tremendous mechanical property in terms of fracture strength and Young's modulus. Few layers of graphene have superior mechanical property than steel due to honey network structure. Interestingly, it can be also stretched by 20% of itself due to the way of carbon bond forms. In graphene, the bond length is 0.142nm with inter-planar length which is approximately 0.341nm. The chemical forces between layers are relatively weak.
Regarding the experiments, the graphene has fracture strength of 200 times larger than that of steel with 1 TPa. Other example with monolayer of graphene, which is 0.5 TPa, has been reported by atomic force microscopy. The young's modulus yielded approximately 0.5~1.0 Tpa which is very close to the accepted value for bulk.(4)
The mechanical property is dependent on the amount of defects present, types of defect and types of edge terminations. It can be enhanced by chemical cross-linking between two layer using ions and annealing. Compared to the great mechanical values reported the cost of graphene is cheap. Showing that graphene can be ideal candidates for mechanical reinforcement.
Graphite is opaque however, the graphene extracted from graphite is transparent when it is one atom-thick. Transparency of thin layer graphene decreases with increasing thickness by adding layers. For example, for a 2-nm thick layer graphene, the average transmittance measures about >95% and for a10-nm thick layer graphene, it is about 70%. Chemically exfoliated graphene based material with outstanding conductivity is an ideal material for replacement of ITO (40/sq at >80% transmittance) or carbon nano-tube mats (70/sq at 80% transmittance) such as transparent electrode material due to not only high transparency and low cost but also high mobility electrons within lattice structure with low resistivity.
The macroscopic linear dependence of the transmittance with the thickness of graphene films is intimately related to the two-dimensional gapless electronic structure of graphene. (5) For instance, the opacity of mono-layer and bi-layer is 2.3% and 4.6% respectively with a minor reflectance which is enough to be seen by the naked eyes. The transparency of graphene depends only on the fine structure constant, =, which describes the coupling between light and relativistic electrons, typically associated with quantum electrodynamics phenomena rather than conventional materials science (5).
Figure1. - The optical image shows how much the intensity of transmitted absorbed by graphene and part (b) shows that the graph has plotted transmittance against Effective filtration volume. (4)
Mechanical exfoliation is known as the best method to produce graphene in terms of electrical and structural quality. The adhesive tape is used to exfoliate directly a thin layer of graphene from graphite micro pillars. The layer of graphene can be deposited on a desired location by using glass tip. The method requires breaking Van der Waals bond cross linkage between two layers of graphene, which is 2eV/nm2. (2) The disadvantage of this method is that the thickness of graphene varies from a few nanometer or a mono-layer of graphene.
Improved methods have been investigated by Novoselov and Geim. Both groups used adhesive tape to repeat the stick and peel till bringing a 1m thick graphite flake to about a mono-layer thin sample on the SiO2 substrate. The disadvantage of this approach is that it can leave glue residues on the sample which can restrict the carrier mobility. In order to avoid the glue residues, typically, freshly cleaved graphite is brought into contact with the deposition substrate and sandwiched together between two electrodes. (5) By using high voltage, the graphite can be removed from the substrate.
Growth on substrate
There are two ways to produce graphene either by thermal decomposition of carbides or epitaxial growth of graphene on substrate.
The silicon atom in silicon carbide (SiC) is sublimated at 1300 C° under vacuum whilst the carbon-enriched surface experiences restructuring. The careful control of sublimation has recently led to the formation of very thin graphene coatings over the entire surface of SiC wafer. (5) The quality of the graphene layer can be improved by annealing under controlled temperature at 1650 C° since the sublimation of silicon occurs at 1500 C°. This method leads the mono-layered graphene with a mobility of 2000 cm2/vs at 27K for a carrier density of ~1013 cm-2.
The method for decomposition of hydrocarbons into graphitic material for a large production of carbon nano-tubes is known as Chemical Vapour Deposition (CVD). This method depends on precipitation of carbon onto the Ni layer on SiO2 substrate. Ni layer is used as a catalyst for the CDV growth of centimetre-sized continuous graphene films composed of one atom thick graphene domain. Ni layer is heated up to 1000 C° and exposed to a hydrocarbons gas environment. The carbon atoms are produced and diffused onto Ni. As the Ni substrate is cooled down, the carbon atoms are layered on the surface of the substrate. The layer of carbon (graphene) is separated from the substrate by hydrofluoric acid (HF) etching. However, HF vapour during etching can pose serious health problems. There are other etchant that can be used in place of HF. The quality of this production is much higher than the thermal decomposition method. The mobility reaches 4000cm2/Vs and can be produced as large scale integration of devices.
Figure 1. - (a) substrate before carbon precipitate, (b) substrate after graphene precipitated on Ni surface. (4)
Low-cost and easy
No special equipment needed
SiO2 Thickness is tuned for better contrast
Labor intensive (not suitable for large-scale production)
Most even films (of any method)
Large scale area
Difficult control of morphology and adsorption energy High-temperature process
Versatile handling of the suspension
Fragile stability of the colloidal dispersion
Reduction of graphene is only partial
Table - illustrates between advantages and disadvantages in the production method which currently is used for graphene layers. (4)
Due to the unique band structure which is zero band gap, graphene can be used in Field Effect Transistors. The experimental values of the field-effect mobility of graphene are one order of magnitude higher than that of Si (1). Using Cu instead of Ni substrate, graphene can achieve larger scale with uniform electrical properties. The high frequency conductance has reached 26 GHz with top gate. Another substrate is an organic polymer buffer layer, which produces optimized property that reaches up to 100 GHz. Its special electrical property could make a graphene the successor to silicon in the new generation of microchips, surmounting basic physical constraints limiting the further development of ever smaller, and ever-faster silicon chips if graphene can be switched off.
As graphene has unusual optical characterization and electrical conductivity with low thickness, it can be replaced to be used in light-based application such as solar cells and LED. The transmittance of application that is reported so far was measured at 70%. It is much lower than ITO (transmittance 90%). However, the film of graphene requires lower-cost production and is produced by a simpler method compared to ITO.
Another application for graphene is a replacing the copper connection between microchips and other devices due to its lower resistance and lower heat generation.
Due to the outstanding properties of Graphene, it attracts scientist to study with high passion for graphene based application. The graphene has curious electrical properties such as QHE, ballistic transport at room temperature with including the mechanical property that is stronger than other material ever known and also it has high transmittance.
Although it has superior properties, the complexity in production and some properties of graphene being uncontrollable is not simply being solved in recent time. In the future in order to use the graphene in industry fields, firstly, new technique for large scale, fast growth, and high quality need to be investigated in further. Secondly, the transferring graphene layer on substrate requires upgrading since it can increase the efficiency of graphene characterisations. Lastly, the method of graphene integrating with present electronic devices must be improved. Even though it has difficulty in many ways, the excellent electronic, mechanical and optical properties of graphene can be future material.