Graphene The wonder material. Applications in electronics discussed.

1. Graphene-The Wonder Material
w.r.t
Application in Electronics
Submitted to
Dr.A.Kasi.Vishwanath
Reader
Center for Nanoscience &
Technology Zaahir Salam

2. Contents
Introduction.
Salient Features.
Impediments to Silicon Design.
Graphene Vs CNT’s.
Graphene in Electronics.
Current Prototypes Of Graphene.
 What to expect from Graphene -Major Applications
Conclusion.
References.

3. 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.

4. 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).

5. 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.

6. 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.

7. Some statistics for Graphene and CNT’s

8. Electronic Properties of Graphene
For physicists and device engineers, the most important
behavior of graphene comes from its electronic
properties !
How do the pz electrons (1 for each atom, 2 for each
hexagon) in graphene behave?

9. Like electrons in atoms, Quantum Physics tells us that electrons
can occupy states that are grouped in energies, i.e., energy
bands! [This is Solid State Physics
Metal Insulator
E.g. sodium, potassium, … E.g. diamond (gap = 6 eV)
[Consequence of quantum physics]

10. Semiconductor [E.g., GaAs, silicon, germanium, …]
y-axis is energy and x-axis is
proportional to momentum

11. Historical Note: Dirac (1928) suggested the
existence of antiparticle. In addition to
looking for antiparticles in cosmic ray and
huge particle accelerators, semiconductors
provide a table-top realization of
antiparticles in the form of holes (missing
electrons) in the valence band! [Crossover
of relativistic quantum physics and solid
state physics!]

12. Conclusion Of Dirac
[Lighter electron effective mass, higher curvature, faster
electronics!
Key idea behind the whole semiconductor industry! Electrons
in semiconductors behave as “free electrons” but with a
different mass!]

13. Graphene has unusual energy bands
• The Pz electrons (2 in each hexagon) completely fill the lower
band.
[Ordinary semiconductors or insulators] Graphene (gapless semiconductor)

14. Dirac Equation in Graphene
Quite Fast!
Electrons in Graphene behave as if they are massless!
[Expect fast electronic devices from graphene!]

16. Ambipolar Electric Field –Good Semiconductor
when a pulse of excess electrons and excess holes are created at a particular point in a
semiconductor an induced internal electric field will be present between them. This internal
electric field will cause the negatively charged electron and positively charged hole to drift or
diffuse together with a single effective mobility or diffusion coefficient.

17. 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.

18. Current Prototypes Of Graphene
 Graphene Transistors(Tunable Transistors)
 Ultra Capacitors
 Graphene Solar Cells
 Graphene based Sensors.
 Non-Volatile Memory
 Transparent Display Screens
 Heat Dissipation

19. 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.

20. 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 crystal's topmost layer, leaving behind just
carbon in the form of graphene
Graphene transistors have been made before, but they have not achieved graphene's full potential.

21. 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

22. Characteristics of GMOSFET’s
Direct-current behaviour of graphene MOSFeTs with a large-area-graphene channel. a, Typical transfer
characteristics 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 output
characteristics (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. At
sufficiently large VDS values, the output characteristics for different VGS,top values may cross75, leading to a
zero or even negative transconductance, which means that the gate has effectively lost control of the current.

23. 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.

24. Non Volatile Memory Contind..
Briefly, after the patterned GO film on a Si/SiO 2 substrate, prepared by “scratching” method (Step 1), was
annealed at high temperature (1000 ° C) (Step 2), the obtained hrGO patterns were transferred onto a
poly(ethylene terephthalate) (PET) substrate by a modifi ed transfer process (Steps 3–6), i.e., the poly(methyl
methacrylate) (PMMA) film was removed by UV irradiation and subsequently using the developer of isopropyl
alcohol (IPA):methyl isobutyl ketone (MIBK) (Step 6), instead of the commonly used acetone. It is worth mentioning
that 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. The
ultrathin 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 stackable
memory devices for ultrahigh density data storage.

25. 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.

26. 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.

27. 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.

28. 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.

29. 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).

30. 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.

31. 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.

32. 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%.

33. 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.

34. Transparent Touch Screens
Advancements in touch
screens
When mixed into plastics,
graphene can turn them into
conductors of electricity
Stiffer-stronger-lighter plastics

35. 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).

36. What to expect from Graphene --Major Applications

37. 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.

38. 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