Graphene-The Wonder Material w.r.t Application in ElectronicsSubmitted toDr.A.Kasi.VishwanathReaderCenter for Nanoscience &Technology Zaahir Salam
ContentsIntroduction.Salient Features.Impediments to Silicon Design.Graphene Vs CNT’s.Graphene in Electronics.Current Prototypes Of Graphene. What to expect from Graphene -Major ApplicationsConclusion.References.
Introduction Graphene is an Allotrope of Carbon. Andre Geim and Konstanstin Novoslev won 2010 Physics Nobel prize for “Groundbreaking Experiments with 2D material Graphene”. Researches are testing various prototypes of Graphene which could replace Silicon based devices. Graphene could possibly replace or enhance current Silicon based devices.
Salient Features Thinnest imaginable material. Largest surface area(~3,000 m2 per gram). Strongest material ever measured(theoretical limit). Stiffest know material(stiffer than diamond) Most stretchable & pliable crystal(up to 20% elastically) Record thermal conductivity(outperforming diamond). Highest current density at room temp(million times of Cu) Completely impermeable (even He atoms cannot squeeze through) Highest intrinsic mobility(100 times of Si) Lightest charge carriers(zero rest mass) Longest mean free path at room T (micron range).
Impediments to Silicon Design After replacing Vacuum Tubes, Silicon is so far used for most of the electronic components. Conduction due to Diffusion in Silicon. Power consumption by Silicon devices is higher. Fragile and Non-Flexible. Temperature dependent properties. Limit on integration of circuits.
Graphene Vs CNT Electrical properties of CNT are dependent on Chirality that is width of tube. It is difficult to control Chirality on such a micro- scale. Graphene is Unzipped CNT. 2D structure of Graphene allows movement of Electrons with constant speed, regardless of individual energy level.
Electronic Properties of GrapheneFor physicists and device engineers, the most importantbehavior of graphene comes from its electronicproperties ! How do the pz electrons (1 for each atom, 2 for each hexagon) in graphene behave?
Like electrons in atoms, Quantum Physics tells us that electronscan occupy states that are grouped in energies, i.e., energybands! [This is Solid State Physics Metal Insulator E.g. sodium, potassium, … E.g. diamond (gap = 6 eV) [Consequence of quantum physics]
Semiconductor [E.g., GaAs, silicon, germanium, …]y-axis is energy and x-axis isproportional to momentum
Historical Note: Dirac (1928) suggested theexistence of antiparticle. In addition tolooking for antiparticles in cosmic ray andhuge particle accelerators, semiconductorsprovide a table-top realization ofantiparticles in the form of holes (missingelectrons) in the valence band! [Crossoverof relativistic quantum physics and solidstate physics!]
Conclusion Of Dirac[Lighter electron effective mass, higher curvature, fasterelectronics!Key idea behind the whole semiconductor industry! Electronsin semiconductors behave as “free electrons” but with adifferent mass!]
Graphene has unusual energy bands • The Pz electrons (2 in each hexagon) completely fill the lower band. [Ordinary semiconductors or insulators] Graphene (gapless semiconductor)
Dirac Equation in Graphene Quite Fast!Electrons in Graphene behave as if they are massless! [Expect fast electronic devices from graphene!]
Ambipolar Electric Field –Good Semiconductorwhen a pulse of excess electrons and excess holes are created at a particular point in asemiconductor an induced internal electric field will be present between them. This internalelectric field will cause the negatively charged electron and positively charged hole to drift ordiffuse together with a single effective mobility or diffusion coefficient.
For Use in Electronics: Generation of Bandgap• Graphene is an uniform structure with no Band Gap.• Band Gap can be induced by layers of Graphene sheets or application of external electrical field.• Electron mobility is 2500 cm^2/s, which is around 100 times faster than Silicon.• Graphene has great strength and is invisible.• It is Photosensitive.
Current Prototypes Of Graphene Graphene Transistors(Tunable Transistors) Ultra Capacitors Graphene Solar Cells Graphene based Sensors. Non-Volatile Memory Transparent Display Screens Heat Dissipation
Graphene Transistors• No Band Gap as Such, Very small band gap, 250meV.• Band gap is achieved by two layers of Graphene.• Difficult to distinguish between On and Off stages.• Problems Pure graphene is a particularly good conductor, it is a terrible semiconductor - the kind of material needed to make transistors. Adding metal contacts to graphene - to shuttle electric charges into and out of it - is tricky, and often results in damage.
GT Contd…….• To tackle both issues, researchers at Friedrich-Alexander University Erlangen-Nuremberg in Germany have enlisted the help of a somewhat lesser-known material called silicon carbide - a simple crystal made of silicon and carbon.• In 2009, several members of the same team reported in Nature Materials that when wafers of the material were baked, silicon atoms were driven out of the crystals topmost layer, leaving behind just carbon in the form of graphene Graphene transistors have been made before, but they have not achieved graphenes full potential.
GT Contd…….• A high-energy beam of charged atoms to etch "channels" into thin silicon carbide wafers defining where different transistor parts would be.• A bit of hydrogen gas in during this process. This affected how the top graphene layer was chemically joined to the underlying silicon carbide: either making a given region conducting or semiconducting, depending on the etched channels. Schematics of different graphene MOSFET types: back-gated MOSFET (left); top-gated MOSFET with a channel of exfoliated graphene or of graphene grown on metal and transferred to a SiO2-covered Si wafer (middle); top-gated MOSFET with an epitaxial-graphene channel (right). The channel shown in red can consist of either large-area graphene or graphene nanoribbons
Characteristics of GMOSFET’sDirect-current behaviour of graphene MOSFeTs with a large-area-graphene channel. a, Typical transfercharacteristics for two MOSFETs with large-area-graphene channels. The on–off ratios are about 3 (MOSFET 1)and 7 (MOSFET 2), far below what is needed for applications in logic circuits. Unlike conventional Si MOSFETs,current flows for both positive and negative top-gate voltages. b, Qualitative shape of the outputcharacteristics (drain current, ID, versus drain–source voltage, VDS) of a MOSFET with an n-type large-area-graphene channel, for different values of the top-gate voltage, VGS,top. Saturation behaviour can be seen. Atsufficiently large VDS values, the output characteristics for different VGS,top values may cross75, leading to azero or even negative transconductance, which means that the gate has effectively lost control of the current.
Non Volatile Memory• For the first time they demonstrate full solution-processed, flexible, all carbon diodes. Both the top and bottom electrodes are made of highly reduced GO (hrGO) films obtained by the high-temperature annealing of GO. The active material is made of lightly reduced GO (lrGO) obtained by low-temperature annealing and then light irradiation of GO. The fabricated diode shows electrical bistability with a non-volatile WORM memory effect. In particular, our all-solution procedure and all-rGO components enable a low-cost, environment-friendly, and mass-production manufacturing process of the devices.
Non Volatile Memory Contind..Briefly, after the patterned GO film on a Si/SiO 2 substrate, prepared by “scratching” method (Step 1), wasannealed at high temperature (1000 ° C) (Step 2), the obtained hrGO patterns were transferred onto apoly(ethylene terephthalate) (PET) substrate by a modifi ed transfer process (Steps 3–6), i.e., the poly(methylmethacrylate) (PMMA) film was removed by UV irradiation and subsequently using the developer of isopropylalcohol (IPA):methyl isobutyl ketone (MIBK) (Step 6), instead of the commonly used acetone. It is worth mentioningthat due to the effective UV irradiation and efficient lift-off process, PMMA was completely removed within 2 min.The thickness and sheet resistance of the hrGO electrode is ∼ 10 nm and ∼ 2000 Ω sq − 1 , respectively. Theultrathin feature of the hrGO electrode ( ∼ 10 nm), less than that of metallic electrodes (normally over 50 nm),benefits the subsequent fabrication of multi-layer lrGO films by spin-coating to achieve multi-layer stackablememory devices for ultrahigh density data storage.
Non Volatile Memory Contind.. All-rGO device with a sandwich configuration of hrGO/lrGO/hrGO. By applying a voltage to the top/bottom electrode, the resistance state of the memory device can be controlled. Initially, the diode had a high resistance state (HRS). The resistance gradually decreased with a negatively increased voltage (stage I), the current varied from 3.67 × 10 − 12 to 2.97 × 10 − 6 A.• When the voltage approached the switching threshold of ca. − 13.2 V (stage II), the current abruptly increased from 2.97 × 10 − 6 to 4.66 × 10 − 4 A, indicating the resistive switching from a HRS (i.e., OFF state) to a low resistance state (LRS, i.e., ON state). The switching from a HRS to a LRS is equivalent to the “write” process in the data storage operation.
Ultra Capacitor• Multiple layers of Graphene can hold greater charge in smaller area.• Graphene supercapacitors were created using a LightScribe DVD burner.• Power densities far beyond existing electrochemical capacitors, possibly within reach of conventional lithium- ion and nickel metal hydride batteries.
UC contnd..• The team, which was led by Richard Kaner of UCLA, started by smearing graphite oxide — a cheap and very easily produced material — films on blank DVDs. These discs are then placed in a LightScribe drive, where a 780nm infrared laser reduces the graphite oxide to pure graphene.• The laser-scribed graphene (LSG) is peeled off and placed on a flexible substrate, and then cut into slices to become the electrodes.• Two electrodes are sandwiched together with a layer of electrolyte in the middle — and voila, a high-density electrochemical capacitor, or supercapacitor as they’re more popularly known are created.
Graphene in Li-Ion battery• While computing power roughly doubles every 18 months, battery technology is almost at a standstill.• Supercapacitors, which suffer virtually zero degradation over 10,000 cycles or more, have been cited as a possible replacement for low-energy devices, such as smartphones.• With their huge power density, supercapacitors could also revolutionize electric vehicles, where huge lithium-ion batteries really struggle to strike a balance between mileage, acceleration, and longevity.• It’s also worth noting, however, that lithium-ion batteries themselves have had their capacity increased by 10 times thanks to the addition of graphene. Either way, then, graphene seems like it will play a major role in the future of electronics.
Solar cells with Graphene• The discovery - made by researchers at the Institute of Photonic Science (ICFO), in collaboration with Massachusetts Institute of Technology, Max Planck Institute for Polymer Research, and Graphenea S.L. Donostia- San Sebastian - demonstrates that graphene is able to convert a single photon that it absorbs into multiple electrons that could drive electric current.• We have seen that high energy photons are converted into a larger number of excited electrons than low energy photons.• The large scale production of highly transparent graphene films by chemical vapour deposition.• In this process, researchers create ultra-thin graphene sheets by first depositing carbon atoms in the form of graphene films on a nickel plate from methane gas. Then they lay down a protective layer of thermo plastic over the graphene layer and dissolve the nickel underneath in an acid bath. In the final step they attach the plastic-protected graphene to a very flexible polymer sheet, which can then be incorporated into a OPV cell (graphene photovoltaics).
Graphene-based Natural Dye-Sensitized Solar Cells• The counter electrode (CE) - catalytic Platinum (Pt) film deposited on TCOs like ITO or FTO.• Require high temperature processing, hindering the deposition on some substrates (e.g., polymeric substrates).• Moreover, they are brittle- flexibility is required. On the other hand, Pt tends to degrade over time when in contact with the (I- /I3-) liquid electrolyte, reducing the overall efficiency of DSSCs.• In this context carbonaceous materials(like Graphene) feature good catalytic properties, electronic conductivity, corrosion resistance towards iodine, high reactivity, abundance, and low cost.
DSSC Continued…• Dye - Transition metal coordination compound complexes, and synthetic organic dyes -based on tedious and expensive chromatographic purification procedures.• Natural dyes and their organic derivatives are non toxic, biodegradable, low in cost, renewable and abundant, so they are the ideal candidate for environmentally friendly solar cells.• The combination of Graphene and natural sensitizers opens up new scenarios for totally green, natural, environmentally friendly and low cost DSSCs.
DSSC Continued…• Indeed, graphene matches all the key requirements for replacement as CE:- • High specific surface area. • High exchange current density and • Low charge-transfer resistance.• Graphene thin films were produced by liquid phase exfoliation of graphite , and spin-casted on stainless steel, FTO and glass.• Indeed, DSSCs assembled with CE made of graphene deposited onto FTO (1.42%) outperform the total conversion efficiency of those based on Pt (1.21%).• Graphene can both catalyses the reduction of tri-Iodide and back transfers the electrons arriving from the external circuit to the redox system.• DSSCs assembled with graphene deposited onto glass as CE show efficiency ~0.8%.
Sensors With Graphene Many sensing approaches, such as electrochemistry, surface enhanced Raman spectroscopy (SERS) and surface plasma resonance (SPR), have been used to develop highly sensitive and selective, low cost sensing devices aiming at the detection of numerous toxic chemicals and in particular, biomolecules in the aqueous environment. Electronic sensors based on field-effect transistors (FETs)are favored due to their high sensitivity, simple device configuration, low cost, miniaturization of devices, and real-time detection. The realization of electronic detection is based on the conductance change of FET semiconducting channels upon adsorption of target molecules.
Transparent Touch ScreensAdvancements in touchscreensWhen mixed into plastics,graphene can turn them intoconductors of electricity Stiffer-stronger-lighter plastics
Heat Dissipation• Graphene is also a better conductor of Heat than Copper.• Innovative Micro Heat Sink designs are proposed which are fabricated on current Silicon on Insulator devices (SOI).
What to expect from Graphene --Major Applications
CONCLUSION• Graphene is a new hope for electronic devices and could possibly replace or rejuvenate Silicon based devices. It seems to be a better material than Silicon and CNT.• Lack of Natural Band Gap prevents Graphene to replace Silicon based devices very now.• Successful prototypes include Superconductor, Flexible Displays and Ultra-Capacitor.• It shall introduce new era of devices for electronics, space, bio-medical and energy harvesting.• Graphene devices might surround us very soon.
References• Graphene http://en.wikipedia.org/wiki/Grapheneaccessed on March, 29 2009• Graphene Confirmed the World’s Strongest Known Material http://gizmodo.com/5026404/graphene-confirmed-as-the- worlds- strongest-known-material accessed on March, 29 2009• Nanotechnology Reserchers go Ballistic Over Graphene http://www.nanowerk.com/spotlight/spotid=2340.php accessed on March 29, 2009• TR10: Graphene Transistors http://www.technologyreview.com/read_article.aspx?ch=specials ections&sc=emerging08&id=20242 accessed on March 29, 2009• Graphene: Charged Up http://www.natureasia.com/asia materials /highlight.php ?id=77 accessed on March 29, 2009