GRAPHENEA Revolutionary Material In ELECTRONICS RAVI P. AGRAHARI (Science & Technology)
Introduction & HistoryWhat is GRAPHENE?• Graphene is a flat monolayer of carbon atoms tightly packed into a two- dimensional (2D)honeycomb structure ,which works on diffraction of electrons.• The Nobel Prize in Physics for 2010 was awarded to Andre Geim and Konstantin Novoselov "for groundbreaking experiments regarding the two- dimensional material graphene".• Carbon-the basis of all known life on the earth-has surprised the scientific community once again with its exotic properties.• Researchers have found that a form of carbon called graphene makes experiments possible that give new twist to electronics field.• This devices are predicted to be substantially faster, thinner & more efficient than current silicon based devices.
Graphene as a material• Graphene as a material is completely new –not only the thinnest ever but also the strongest.• It is the one-atom thick planar sheet of carbon atoms, which makes it the thinnest material ever discovered.• Carbon atoms in the sheet are densely packed in a two- dimensional(2D) honeycomb crystal lattice.• The carbon-carbon bond length in graphene is about 0.142nm.• Graphene is the basic structural element of some carbon allotropes including graphite, charcoal, carbon nanotubes & fullerenes.• It can be wrapped up into 0D fullerences, rolled into 1D nanotubes or stacked into 3D graphite.• Graphene is highly conductive-conducting both heat & electricity better than any other material, copper & stronger than diamond.• It is almost completely transparent, yet so dense that not even helium can pass through it.
˃High electron mobility- upto 25000cm² per volt-second .˃The theoretical mobility of electrons is 200 times that of silicon.˃The corresponding resistivity of the graphene sheet would be 10⁻⁶ Ω·cm.˃The charge storage time is less, so the operating frequency is high.˃The near-room temperature thermal conductivity of graphene was recentlymeasured to be between (4.84 0.44) 10^3 to (5.30 0.48) 10^3 Wm−1K−1.˃The breakover voltage is less than 0.3V.So , it can replace current devices, such as CMOS , and will give turning pointto the electronics.The below is the some allotropic structures of the carbon-
Graphene to SiliconChemical Properties of Graphene & Silicon : Structure of Graphene & Silicon : Two-D Graphene Silicon bond• Graphene is two dimensional allotrope of carbon, while silicon is singledimensional metalloid found in nature.Graphene has 6 electrons, while silicon have 14 number of electrons.• Three carbon-carbon covalent bonds [Trivalent] form a “Graphene”, while threesilica-carbon covalent bonds form a silicon as a metalliod.
• Graphene has two bands, one for empty particle & other for antiparticles[holes], & Silicon has two energy bands [conduction band & valance band] & an energy gap between them.• Graphene is considered as hybrid between a metal & a semiconductor, Silicon is pure semiconductor.• Doping of graphene is roughly divided into three categorie 1. hetero atom doping 2. chemical modification strategy 3. electrostatic field tuning• Doping of silicon is roughly divided into two categorie• 1. p-type• 2. n-type• both process are used for extrinsic not for intrinsic semiconductor.• Density of graphene sheet is > 1 g/cm3 , while silicon has 2.57 g·cm3
Electrical Properties of Graphene & Silicon :• Break-over voltage of graphene is less than 0.3V, & that of silicon is 0.3 Volts.• Transmission of electrons in graphene and silicon : ✔Dirac Fermions in Graphene ✔Electrons in Conventional Silicon
• Graphene is a Zero Gap Semiconductor. So it has Electron transfer is 200 times faster than Silicon.• Thermal conductivity of graphene was recently measured to be between (4.84 0.44) 103 to (5.30 0.48) 103 Wm−1K−1, & thermal conductivity of silicon is Thermal conductivity 149 W·m−1·K−1• Resistivity of the graphene sheet would be 10−6 Ω·cm, Electrical resistivity of the silicon 103 Ω·m.• On-off ratio of graphene is ~30 at room temperature & which is six times greater than silicon.• Graphene has very small voltage gain (typically, the amplitude of the output signal is about 40 times less than that of the input signal ) than silicon.• Graphene has produced structures just 15 to 40 nanometers wide that conduct current with almost no resistance, while silicon has such structures at some micrometer wide only.• Electrons in graphene move at an effective speed of light 300 times less than the• speed of light in a vacuum, allowing relativistic effects to be observed without• using particle accelerators.
GRAPHENE FABRICATIONConcept of FABRICATION• Usually Graphene can be fabricated by the methods of Chemical exfoliation, Thermal exfoliation.Graphene can be fabricated in the forms of :-(I) GRAPHENE SHEETS :- Fig: Graphene Sheets in layerFIG.- PROCESS FLOW SCHEMATIC FOR FABRICATION Fig: Single Graphene Sheet OF GRAPHENE SHEETS
(II) Carbon Nanotubes:- A Carbon Nanotube is a tube made entirely carbon with a diameter of about a nanometer (1/1,00,00,00,000th of a meter). It is hard to imagine something so small, but if you zoomed in so that a nanotube was as wide as one of your hairs, then your head would be about the width of Cayuga Lake! As shown in the illustration, a carbon nanotube is a rolled tube of carbon atoms in a honeycomb arrangement FIG-SCHEMATIC OF CARBON NANO TUBE FBRICATION FABRICATION
How can we see a nanotube with an atomic force microscope (AFM) ?A sharp needle attached to a diving board shaped cantilever ( a lot like a recordplayer needle) is scanned across the surface of the sample. Whenever the needlehits a bump, the whole board moves up and down, and this deflection is measuredby a laser. Using our AFM, we can determine how many nanotubes are connectedbetween a pair of electrodes, and their diameters. Here you see an illustration of anAFM, a real AFM , and an AFM in the CNF clean room:
Applications Graphene Transistors-• Graphene exhibites a pronounced effect to perpendicular external electric fields, allowing us to build FETs.• Researchers demonstrated & built an experimental graphene chip of single graphene transistor known as frequency multiplier.• However, these graphene transistor show a very poor on- The fabricated graphene field off ratio, small voltage gain & operating frequencies less effect transistor. than 25kHz.• Recently researchers have been able to create graphene transistors with an on-off ratio rate of 100GHz.• IBM had developed 10000 top gated transistors on 0.24 centimeter square chip.
Integrated Circuit-• Graphene has the ideal properties to become an excellent component of integrated circuits.• Its high carrier mobility & low noise allow it to be used as a channel in FETs.• Besides, the researchers have demonstrated the first functional graphene integrated circuit-a complementary inverter consisting of one p- & one n- type graphene transistor. Ultra capacitors:-• Due to incredibly high surface-area-to-mass ratio of graphene, its one potential application is in the conductive plates of ultra capacitors.• Graphene could be used to produce ultra capacitors with a greater energy storage density than is currently available. Anti-Bacterial:-• Sheets of graphene oxide are highly effective at killing bacterias such as Escherichia coli.• So, graphene could be useful in hygiene products or packaging.
Strength Applications:-• When graphene sheets are incorporated into composites, we could come up with a material that’s many times stronger than Kevlar.• The Chinese are already working on carbon- nanotube yarn for space suits & bullet proof vests. T-Ray Scanners:-• Terahertz radiation, or T-ray is used for detecting hidden objects at security checkpoints without the health risk posed by X-rays.• The fast frequencies generated by graphene circuits are the basis for chemical sensors & generators of THz-range light. Heat Dissipation to cool electronics:-• Overheating in laptops & other electronic gadgets is a major technological hurdle to the speed & energy efficiency of electronic products.• Graphene behaves as a strong heat conductor, which helps chip manufacturers to rich higher speeds with relative lower temperatures.• Scientists from University of California found that multiple layers of graphene show strong heat conducting properties, which can help in removing dissipated heat from electronic devices.
Advantages Limitations• Higher electron mobility • Single sheet of graphene is hard to• Works on principle of diffraction of produce. electrons. • The new fabrication & manufacturing• Superb electron & heat conductivity, methods has to be evolved for normal greater than copper. use in electronics.• Very less breakover voltage, less than • Due to small voltage gain, practical 0.3V use is limited.• It is transparent, yet so dense as even an • While graphene can be considered atom of Helium can’t pass through it. semiconductor like silicon, it lacks one• Stronger than diamond & steel crucial property- the ability to act as a• Can be used to make anti bacterial switch. materials as well as biodevices. • Graphene research has discovered• Can make very light weight parts for hidden interactions that will affect the auto bodies & armours way components are designed from the superfast material.
Conclusion From above discussion it is concluded that Graphene is a material which has the capability to eliminate the current semiconductors such as silicon and form a new era of superfast micro electronics. From recent researches it is observed that , the most likely applications for Graphene will be in analogue systems , such as radar , satellite communication and imaging devices. There are many agencies which are working very hardly on graphene and they have founded the new graphene devices. Some of them are- Defense Advanced Research Projects Agency of U.S. D.O.D., Indian agencies such as D.R.D.O., Nanoscale science and technology group at the IBM Watson Research Centre in Ossining, NY, etc.