Graphene Electronics Report

8/9/2019 Graphene Electronics Report

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1.0 Background:

Graphene is the name given to a flat monolayer of carbon atoms tightly packed into a

two-dimensional (2D) honeycomb lattice, and is a basic building block for graphitic

materials of all other dimensionalities [1]. 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 material and 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-standing graphene was unexpectedly found

three years ago7, 8 and especially when the follow-up experiments9,10 confirmed that its

charge carriers were indeed massless Dirac fermions. So, the graphene gold rush has

begun.

What is a crystal?

Something is crystalline if the atoms or ions that compose it are arranged in a regularway.

What is 2D Crystal: Obviously, a single atomic plane is a 2D crystal, whereas 100layers should be considered as a thin film of a 3D material

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Figure 1Mother of all graphitic forms. Graphene is a 2D building material for carbonmaterials of all other dimensionalities. It can be wrapped up into 0D buckyballs, rolledinto 1D nanotubes or stacked into 3D graphite.


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Timeline of Graphene Gold Rush

2.0 Free Standing Graphene Discovery:

There exist numerous methods for graphene depending on low to high yield. Some of

these methods are discussed here as follows.

1. Chemical exfoliation,probably the closest to the micromechanical exfoliation

method is chemical exfoliation, which can be traced back to the original work of

Professor Brodie (Brodie, 1859) who treated graphite with acids and arrived at

graphon (or graphite oxide as we now know it). Graphite oxide can be thought

of as graphite intercalated with oxygen and hydroxyl groups, which makes it a

hydrophilic material and easily dispersible in water. This technique produces

extremely thin, sometimes even monolayer, flakes of this material which can then

subsequently be reduced, producing low-quality graphene. One can imagine an

even simpler path for chemical exfoliation. Although graphene is hydrophobic,

it can be dis- persed in other, mostly organic, solvents (Blake et al., 2008;

Hernandez et al., 2008). By repeating the exfoliation and purification

(centrifugation) process several times one can obtain 50% and higher fractions of

graphene in suspension.

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2. Catalytic cracking of hydrocarbons, or precipitation of dissolved carbon on a

metal surface with sub- sequent graphitization, has long been known to produce

high-quality graphene layers. (Grant and Haas, 1970; Gall et al., 1985, 1987;

Nagashima et al., 1993; Gall et al., 1997; Forbeaux et al., 1998; Affoune et al.,

2001; Harigaya and Enoki, 2002). A similar process is the graphitization of excess

carbon atoms after sublimation of silicon from the surface of silicon carbide (van

Bommel et al., 1975; Berger et al., 2004).

3. Micromechanical cleavage: The simplest implementation of this method for

graphitic materials is to use bulk graphite and exfoliate [8] it into individual

planes. Graphite is a layered material and can be consid-ered as a stack of

individual graphene layers. High-quality graphite typically requires growth

temperatures of above 3000 K, but exfoliation can be done at room

temperaturesan order of magnitude lower than the growth temperatures. In

fact, many of us have performed this procedure numerous times while using

pencils, as drawing with a pencil relies on exfoliation of graphite (though not up

to the monolayer limit,

which would be

practically invisible to the

naked eye).

Figure 1:The Micromechanical

Cleavage Technique (Scotch-tape method) for producinggraphene. Top row: Adhesivetape is used to cleave the top fewlayers of graphite from a bulkcrystal of the material. Bottom

left: The tape with graphitic flakes is then pressed against the substrate of choice. Bottom

right: Some flakes stay on the substrate, even on removal of the tape


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Figure 2:Thin Graphitic Flakes on a surface of

Si/SiO2 (300 nm of SiO2, purple color). Thedifferent colors correspond to flakes of differing

thicknesses, from $100 nm (the pale yellow ones)to a few nanometers (a few graphene layersthemost purple ones). The scale is given by thedistance between the lithography marks (200Micron).

The critical ingredient for success was the observation [1] that graphene becomes visible

in an optical microscope if placed on top of a Si wafer with a carefully chosen thickness

of SiO2, owing to a feeble interference-like contrast with respect to an empty wafer. If

not for this simple yet effective way to scan substrates in search of graphene crystallites,

they would probably remain undiscovered today. Indeed, even knowing the exact recipe,

it requires special care and perseverance to find graphene. For example, only a 5%

difference in SiO2 thickness (315 nm instead of the current standard of 300 nm) can

make single-layer graphene completely invisible.

Graphene can also be separated by micromechanical cleavage of graphite [5]. Alternative

procedures, such as exfoliation and growth, so far only produced multilayers [6], but it is

hoped that in the near future efficient growth methods will be developed.

Despite the wide use of the micromechanical cleavage, the identification and counting of

graphene layers is a major hurdle. Monolayers are a great minority amongst

accompanying thicker flakes. They cannot be seen in an optical microscope on most

substrates. They only become visible when deposited on oxidized Si substrates with a

finely tuned thickness of the oxide layer (typically, 300 nm SiO2) since, in this case, even

a monolayer adds to the optical path of reflected light to change the interference color

with respect to the empty substrate [1,4]. Atomic force microscopy (AFM) has been so


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far the only method to identify single and few layers, but it is low throughput.

3.0 Graphene Identification:

Raman Spectrum of Graphene and Graphene Layers

Graphenes electronic structure is uniquely captured in its Raman spectrum. Raman

fingerprints for single layers, bilayers, and few layers reflect changes in the electron

bands and allow unambiguous, high-throughput, nondestructive identification of

graphene layers [4].

Here the samples are prepared by micromechanical cleavage [1]. To provide the most

definitive identification of single and bilayers (beyond the AFM counting procedure) we

perform transmission electron microscopy (TEM) on some of the samples to be measured

by Raman spectroscopy. Samples for TEM are prepared following a similar process to

that previously used to make freestanding and TEM-compatible nanotube devices [7]. In

addition, this allows us to have freestanding layers on a grid easily seen in an optical

microscope, facilitating their location during Raman measurements, Fig. 2(a). Electron

diffraction is done in a Zeiss 912 ohm microscope at a voltage of 60 kV, and high-

resolution images are obtained with a Philips CM200 microscope at 120 kV. A high

resolution- TEM analysis of foldings at the edges or within the free- hanging sheets gives

the number of layers by direct visualization, since at a folding the sheet is locally parallel

to the beam, Figs. 2(b)2(e). Edges and foldings of one or two layers are dominated by

one or two dark lines, respectively. The number of layers is also obtained by a diffraction

analysis of the freely suspended sheets for varying incidence angles, and confirms the

number of layers from the foldings, Figs. 2(d) and 2(e). In particular, the diffraction

analysis of the bilayer shows that it is A-B stacked (the intensity of the 1120 diffraction

spots (outer hexagon) is roughly twice that of the 1100 (inner hexagon), Fig. 2(h), in

agreement with diffraction simulations obtained by a


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Figure 3: (a) TEM of suspended graphene. The

grid is also visible in optical microscopy. (b)High-resolution image of a folded edge of a

single layer and (c) a wrinkle within the layer.(d) Folded edge of a two layer, and (e) internalfoldings of the two layer. The amorphouscontrast on the sheets is most likely due tohydrocarbon adsorbates on the samples that werecracked by the electron beam. (f) Electrondiffraction pattern for close to normal incidencefrom single layer and (g) from two layers. Weakdiffraction peaks from the supporting metalstructure are also present. (h) Intensity profileplot along the line indicated by the arrows in(f),(g). The relative intensities of the spots in thetwo layer are consistent only with A-B (and notA-A) stacking. Scale bars: (a) 500 nm; (be) 2nm.

Figure 4:(a) Comparison of Ramanspectra at 514 nm for bulk graphiteand graphene. They are scaled tohave similar height of the 2D peakat 2700 cm-1. (b) Evolution of thespectra at 514 nm with the numberof layers. (c) Evolution of theRaman spectra at 633 nm with thenumber of layers. (d) Comparison ofthe D band at 514 nm at the edge of

bulk graphite and single layergraphene. The fit of the D1 and D2components of the D band of bulkgraphite is shown. (e) The fourcomponents of the 2D band in 2layer graphene at 514 and 633 nm.

(a)

(b) (c)

(d) (e)


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4.0 Why Graphene So Important:

The current interest in graphene can be attributed to three main reasons [4]. First, its

electron transport is described by the Dirac equation and this allows access to quantum

electrodynamics in a simple condensed matter experiment [15]. Second, the scalability

of graphene devices to nano- dimensions [610] makes it a promising candidate for

applications, because of its ballistic transport at room temperature combined with

chemical and mechanical stability. Remarkable properties extend to bilayer and few-

layers graphene [4 6,8,11]. Third, various forms of graphite, nanotubes, buckyballs, and

others can all be viewed as derivatives of graphene and, not surprisingly, this basic

material has been intensively investigated theoretically for the past 60 years [12].

A. Graphenes quality clearly reveals itself in a pronounced ambipolar electric field effect(Fig. 4) such that charge carriers can be tuned continuously between electrons and holes

in concentrations n as high as 1013 cm2 and their mobilities can exceed 15,000 cm2

V1 s1 even under ambient conditions710. Moreover, the observed mobilities weakly

depend on temperature T, which means that at 300 K is still limited by impurity

scattering, and therefore can be improved

significantly.

Figure 5: Ambipolar electric field effects insingle-layer graphene. The insets show itsconical low-energy spectrum E(k), indicatingchanges in the position of the Fermi energyEF with changing gate voltage Vg. Positive(negative) Vg induce electrons (holes) inconcentrations n = !Vg where the coefficient! " 7.2 # 1010 cm2 V1 for field-effectdevices with a 300 nm SiO2 layer used as adielectric79. The rapid decrease in resistivity$ on adding charge carriers indicates their

high mobility (in this case, "5,000 cm2 V1s1 and does not noticeably change withincreasing temperature to 300 K).

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B. A further indication of the systems extreme electronic quality is the quantum Hall

effect (QHE) that can be observed in graphene even at room temperature, extending the

previous temperature range for the QHE by a factor of 10

C. An equally important reason for the interest in graphene is a particular unique nature

of its charge carriers. In condensed- matter physics, the Schrdinger equation rules the

world, usually being quite sufficient to describe electronic properties of materials.

Graphene is an exception its charge carriers mimic relativistic particles and are more

easily and naturally described starting with the Dirac equation rather than the Schrdinger

equation.

4.1 Electrical Properties of Graphene [1]

The electrical resistivity of single crystals of graphite is about 4 to 6 x10 5 ohm-cm. This

corresponds to conductivity of the order of that of a poor metal. The temperature

coefficient of the conductivity is negative, as in the case of a metal. Polycrystalline

graphite, on the other hand, has a much higher resistivity which varies very strongly

according to the type of graphite used, and has a positive temperature coefficient of

conductivity' to about 1400'C, and negative thereafter. Since the crystals of commercial

graphites tend to be of the order of 10 ' cm, and it is quite porous (density 1.6 as against

2.25 for single crystals), it seems reasonable to attribute the high resistivity of

polycrystalline graphite to the crystal boundaries, on which may be lodged impurity

atoms.

From the point of view of its electronic properties, graphene [1] is a zero-gap

semiconductor, in which low-E quasiparticles within each valley can formally be

described by the Dirac-like hamiltonian

00kx+iky

kxiky= = kF FH

Where k is the quasiparticle momentum, % the 2D Pauli matrix and the k-independent

Fermi velocity &F plays the role of the speed of light. The Dirac equation is a direct

consequence of graphenes crystal symmetry.


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4.1.1 Chiral Quantum Hall Effects

At this early stage, the main experimental efforts have been focused on the electronic

properties of graphene, trying to understand the consequences of its QED-like spectrum.

Among the most spectacular phenomena reported so far, there are two new (chiral)

quantum Hall effects (QHEs), minimum quantum conductivity in the limit of vanishing

concentrations of charge carriers and strong suppression of quantum interference effects.

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Figure 6: Chiral quantum Hall effects. a, The hallmark of massless Dirac fermions is

QHE plateaux in %xy at half integers of 4e 2/h (adapted from ref. 9). b, Anomalous QHEfor massive Dirac fermions in bilayer graphene is more subtle (red curve56): %xy exhibitsthe standard QHE sequence with plateaux at all integer N of 4e2/h except for N = 0. The

missing plateau is indicated by the red arrow. The zero-N plateau can be recovered afterchemical doping, which shifts the neutrality point to high Vg so that an asymmetry gap("0.1eV in this case) is opened by the electric field effect (green curve60). ce, Differenttypes of Landau quantization ingraphene. The sequence of Landau levels in the densityof states D is described by EN !'N for massless Dirac fermions in single-layer graphene(c) and by EN !'N (N 1) for massive Dirac fermions in bilayer graphene (d). Thestandard LL sequence EN !N + 12 is expected to recover if an electronic gap is openedin the bilayer (e).

4.1.2 Conductivity Without Charge Carriers

Another important observation is that graphenes zero-field conductivity ! does not

disappear in the limit of vanishing n but instead exhibits values close to the conductivity

quantum e2/h per carrier type9. Figure 6 shows the lowest conductivity %min measured

near the neutrality point for nearly 50 single-layer devices. For all other known materials,

such a low conductivity unavoidably leads to a metalinsulator transition at low T but no

sign of the transition has been observed in graphene down to liquid-helium T. Moreover,


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suspended graphene has intrinsic ripples or not has been addressed by Monte Carlo

simulation and transmission electron microscopy (TEM) studies. The microscopic

corrugations as shown in Fig. 8 were estimated to have a lateral dimension of about 8 to

10 nm and a height displacement of about 0.7 to 1 nm. Sub-nanometer fluctuations in

height for graphene platelets deposited on an SiO2 -on-Si substrate were studied by

scanning tunneling microscopy (STM). Although some STM experiments indicated a

limited or negligible correlation between small (< 0.5 nm in height) corrugations and

local electrical properties, evidence has been presented for strain induced local

conductance modulations for bigger ripples (23 nm in height). Ripples can be induced,

suggesting that the local electrical and optical properties of graphene could be altered

through ripple-engineering for possible application in devices.

Figure 7: Schematics of the crystal structure, Brillouin zone and dispersion spectrum ofgraphene

Figure 8: Rippled graphene from a Monte Carlo simulation. The red arrows are ~ 8 nmlong


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5.0 Properties:

5.1 Electronic Properties:

Intrinsic graphene is a semi-metal or zero-gap semiconductor. Graphene differs from

most conventional three-dimensional materials. It was realized as early as 1947 by P. R.

Wallacethat the E-k relation is linear for low energies near the six corners of the two-

dimensional hexagonal Brillouin zone, leading to zero effective mass for electrons

and holes. Due to this linear (or conical") dispersion relation at low energies, electrons

and holes near these six points, two of which are inequivalent, behave

like relativistic particles described by the Dirac equation for spin 1/2 particles. Hence,

the electrons and holes are called Dirac fermions, and the six corners of the Brillouin

zone are called the Dirac points. The equation describing the E-k relation

is ;where the Fermi velocity vF~ 106m/s.

The band structure of graphene differs from that of a typical semiconductor in the

following points:

Around the point where the conduction band and the valence band meet each

other, the slope of the band structure is linear.

The conduction band is connected continuously with the valence band, which

means the band gap is zero.

The band structure of graphene is shown in Fig. 9 in comparison with that of a typical

semiconductor as shown in Fig. 10. An electron propagates as a wave in a crystal. A band

structure illustrates the relation of the wave number to the energy of electrons in a crystal.

Electrons in a crystal successively occupy from the lower states to the upper in the band

structure. As is shown in Fig. 11, the band structures of a typical semiconductor splits

into two bands, the upper and the lower. Usually, there are almost no electrons in the

upper band, and there are almost no vacancies of electrons (holes) in the lower band. The

upper and the lower bands are called the conduction band and the valence band,

respectively. Between the conduction band and the valence band, there exists an energy

zone where there are no states for electrons to occupy, which is called a bandgap. The


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bandgap and electron mobility at room temperature for several semiconductors in

comparison with Graphene are shown in Figure 5.

Figure 9: Band Structure of Typical Semiconductor

Figure 10: Band Structure of Graphene


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Figure 11: Electron Mobility and Bandgap

In the case of graphene, because the slope is linear, the effective mass of electrons is

zero. This means that graphene shows very high electron-mobility. A theoretical

expectation of the electron-mobility of graphene is 1,000 times higher than that of silicon,

and an electron mobility as high as 2#106cm2/V sec has been experimentally achieved,

which is 100 times higher than that of silicon. Because higher electron mobility leads to

shorter switching time for a transistor, graphene has been expected as a material that

could realize high-speed electronic devices which could break the speed records made by

conventional semiconductors such as silicon or compound semiconductors. In the case of

a typical semiconductor, the band structure at around the top of the valence band and the

bottom of the conduction band shows a parabolic shape and the slope changes gradually.

The larger the change of the slope of the band, the less the effective mass of the electrons.

5.2 Mechanical Properties:

Graphene appears to be one of the strongest materials ever tested. Measurements have

shown that graphene has a breaking strength 200 times greater than steel, with a tensile

strength of 130 GPa.

The mechanical properties of monolayer graphene including the Youngs modulus and

fracture strength have been investigated by numerical simulations such as molecular


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dynamics. The Youngs modulus of few layer graphene was experimentally investigated

with force-displacement measurements by atomic force microscopy (AFM) on a strip of

graphene suspended over trenches. Circular membranes of few-layer graphene were also

characterized by force-volume measurements in AFM. Recently, the elastic properties

and intrinsic breaking strength of free-standing monolayer graphene were measured by

nano indentation using an AFM Fig. 12 and Fig. 13. It was reported that defect-free

graphene has a Youngs modulus of 1.0 TPa and a fracture strength of 130 Gpa.

Figure 12: Scanning Electron Microscopy (SEM) image of graphene flake spanning an

array of circular holes

Figure 13:Illustration of Nanoindentation on Membranes

5.3 Optical Properties:


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Graphene's unique optical properties produce an unexpectedly high opacity for an atomic

monolayer, with a startlingly simple value: it absorbs !""2.3% of white light, where "is

the fine-structure constant. Due to the unusual low-energy electronic structure of

monolayer graphene that features electron and hole conical bands meeting each other at

the Dirac point the high opacity in graphene is observed. The constant transparency (~

97.7%) has been experimentally observed for graphene in the visible range and the

transmittance linearly decreases with the number of layers for n-layer graphene. A

deviation from this universal behavior has been found for incident photons with energy

lower than 0.5eV, which was attributed to the finite temperature and a doping-induced

chemical potential shift of the charge-neutrality (Dirac) point. Fig. 14 shows the

photograph of a 50m aperture partially covered by graphene and its bi-layer. The line

scan profile shows the intensity of transmitted white light along the yellow line. Inset

shows the sample design: a 20- m thick metal support structure has apertures 20, 30,

and 50 m in diameter with graphene flakes deposited over them

Figure 14: A 50m Aperture Partially Covered by Graphene and its Bi-layer


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6.0 Opening and Tuning of Bandgap:

Digital devices which require a high switching on/off ratio but the zero bandgap of

graphene makes it hard to design digital devices. Two techniques are developed to study

the opening of bandgap

6.1 Bi-layer Graphene

Bi-layer graphene is a lamination of two layers of graphene. Although the bandgap of bi-

layer graphene is still zero, it can be opened by applying an external electric field across

the bi-layer graphene. Adsorption of atoms such as potassium on the bi-layer graphene is

also effective for opening the bandgap similarly. The method where an electric field is

applied to bi-layer graphene is not only advantageous but also of great interest in that the

bandgap is tunable by the applied field strength. It has been reported that the bandgap

was tuned up to about 0.3 eV.

6.2 Graphene Nanoribbon

Second technique for opening the bandgap of graphene is to decrease the width of a

graphene sheet. Graphene is called a graphene nanoribbon when the width of the

graphene is several times the unit cell of graphene. Theoretical calculations on the band

structure of graphene nanoribbons have shown that graphene nanoribbons exhibit

metallic properties or semiconductor properties. Depending on the orientation of the

ribbon two configurations of graphene nanoribbon structure are shown in Fig 15 focusing

on the edges of graphene nanoribbons. The configuration illustrated in Fig. 15(a) is called

the armchair type where the edge has a cyclic structure of four carbon atoms. A graphene

nanoribbon of the armchair type configuration exhibits semiconductor properties. On the

other hand, the configuration in Fig. 15(b) is called the zigzag type where the edges are

zigzags. A graphene nanoribbon of the zigzag type configuration exhibits zero bandgap.

Relationship between the bandgap and the width of the armchair type graphene

nanoribbon obtained by a theoretical calculation is shown in Fig. 16. Although the

bandgap changes cyclically with the width, the general trend of bandgap is an increase

with decreasing the width. Here it should be noted that the bandgap can fluctuate largely

even by a slight change in the width. Therefore, it is thought that bandgap control by the


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nanoribbon width will require very fine fabrications of a nanometer or smaller precision

in both width and orientation.

Figure 15: (a) Armchair Edge Graphene Nanoribbon (b)Zigzag Edge Graphene

Nanoribbon

Figure 16: Theoretically Calculated Bandgap of Armchair Edged Graphene Nanoribbon

7.0 Applications:

Graphene's unique characteristics and behaviors are invaluable in the field of electronics.

Since silicon-based technology is reaching its limits, graphene is seen as the successor of

semiconductors, due to its highly mobile charge carriers and energy storage potential.

The fact that graphene is stable at a nanometer scale, and perhaps even down to a single

carbon ring, distinguishes it from other materials used in electronics. Nonetheless, its


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immediate use in the present day is not yet fully defined. While the Scotch tape method

that Geim and Novoselov employed is useful in extracting graphene in the laboratory, an

efficient procedure is still necessary for the industrial production of graphene. In addition,

reproducing the qualities from one graphene device to the next requires accurately

controlling each feature within the device. Such issues may pose challenges now, but

graphene continues to ignite a wide array of possibilities.

Its heat resistance, electrical conductivity, strength and transparency make graphene an

ideal candidate for various composite materials. It can be used to make efficient electric

batteries, lighter aircraft and automobile parts, and medical equipment. Graphene sheets

can act as gas sensors, used to detect the passage of harmful gases along pipelines; when

certain gas molecules become attached to a graphene layer, its electrical resistance will

change at that spot, so the locations of the molecules can be tracked. Stacks of oxidized

graphene sheets also have the capacity to store hydrogen, revealing significant

implications in developing fuel cells and in stabilizing hydrogen as a viable energy

source. Furthermore, with its flexibility and sensitivities to light, graphene can pioneer a

new generation of light-emitting devices, such as touch-screen displays, and more

efficient solar cells. There are numerous widespread applications that touch upon the

goals of many industries and scientists around the world, as they begin to implement

graphene to extend the bounds of progress. The features and the application fields are

schematically shown in Fig. 17.


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Figure 17: Distinctive Properties of Graphene and Possible Application FieldsField Effect Transistor

7.1 Field Effect Transistor:

The field effect transistor (FET) is a key element of digital systems, where the current

flowing through a thin channel layer is controlled by gate electrodes. FET can be

operated faster with a channel layer of a higher electron mobility material, which is the

very point of the application of graphene to FET. For the fabrication of graphene-based

FET, graphene exfoliated from HOPG is often used. The operation speed is already more

than twice higher than that of silicon-based FET which uses silicon as the channel layer

with the same gate length. It strongly indicates the high potentiality of graphene

application to FET.

Graphene FET with a four-terminal configuration, as depicted in Fig. 18. The FET has

two input terminals, both a top gate and a back gate, and the polarity of the FET can be

switched by switching the input to the back gate. A graphene flake was first deposited by

mechanical exfoliation of highly oriented pyrolitic graphite on a highly doped p-type Si

substrate, covered with 100 nm of thermally grown SiO2. The flake was identified as bi-

layer graphene by Raman spectroscopy. After the conventional photolithography process,

10 nm Ti/50 nm Au metal stack was deposited using a vacuum evaporator and lifted off

as source and drain contacts. The source-drain spacing was 5 )m, and the mean channel


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width was 4 m. Next, the Ti/Au back-gate electrode was formed on the back side of the

substrate. Just after the deposition, the back-gate FET showed p -type properties with a

field-effect mobility of about 1700 cm2 /V s, which was estimated from the

transconductance at the linear region. After a 50 nm Al2O3insulation layer was deposited

by means of the atomic layer deposition ALD technique at 300 C using tri-methyl-

aluminum and water as precursors, 10 nm Ti/50 nm Au top gate was evaporated on the

Al2O3layer and lifted off. Top-gate length was 1 )m. After the ALD deposition, the FET

showed ambipolar characteristics. Such polarity change has been observed in some

graphene transistors, covered with ALD-grown insulator, or annealed electrically. It is

thought that the shift is due to the removal of contamination adsorbed on the graphene

surface. The FET shows ambipolar proprieties whereas the top-gate FET is n-type at

positive Vbg, and p-type at negative Vbg in the measured Vtg range. Merit of this

structure is that back-gate electrodes are isolated from each other and the polarity of the

individual transistors can be controlled independently after completion of the fabrication

process.

Figure 18: Schematic structure of a four-terminal graphene FET with both a top gate anda back gate

7.2 Polarity Controllable Graphene Inverter:

Polarity-controllable inverter constructed using a four-terminal ambipolar graphene field

effect transistor (FET). The slope of the inverter transfer curves can be changed by

changing the back-gate voltage. The circuit of Polarity-controllable inverter is shown Fig.

19(a) and the transfer curve is shown in Fig. 19(b). When the control voltage Vc is larger

than a certain threshold,\ the circuit acts as inverter, since the polarity of the FET is n-


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type, whereas below the threshold, the output is not inverted. By constructing the back-

gate electrode only below the channel region like a double-gate semiconductor-on-

insulator MOS transistor,the inverter can operate at higher frequencies.

Figure 19: (a) Polarity Controllable Graphene Inverter(b) Transfer Curves

7.3 Sensors:

Graphenes conductance changing as a function of extent of surface adsorption, large

specific surface area, and low Johnson noise, recent experimental and theoretical research

has demonstrated that monolayer graphene is a promising candidate to detect a variety of

molecules, such as gases to biomolecules. Charge transfer between the adsorbed

molecules and graphene is proposed to be responsible for the chemical response. As

molecules adsorb to the surface of graphene, the location of adsorption experiences a

charge transfer with graphene as a donor or acceptor, thus changing the Fermi level,

carrier density, and electrical resistance of graphene. Fig. 20(a)shows a typical schematic

of a graphene FET device for sensing gas molecules. During the exposure of the device to

gas (e.g. NH3 ), the time evolution of source-drain current (Ids) versus gate voltage ( Vgs

) was recorded Fig. 20(b). Initially, the Dirac point ( VD) is close to the back gate bias of

0 V; after 5 minutes of exposure, the Dirac point appears at *20 V and slowly shifts to

its final position at about *30 V. These results suggest that NH3molecules adsorb on the

graphene surface and n-dope the graphene in the FET device. Based on the charge

transfer rate and the Dirac point shift, the concentration of the molecules on the graphene


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surface was estimated at 8 #1013cm*2after 30 minutes of exposure. Moreover, reduced

graphene oxide has been shown to be a good sensor, achieving sensitivities at parts per-

billion levels for detection of chemical warfare agents and explosives. CMG has been

used in biodevices as a sensor at both the biocellular and the biomolecular scale. It can

act as an interface to recognize single bacteria, a label-free, reversible DNA detector, and

a polarity-specific molecular transistor for protein/DNA adsorption.

Figure 20: (a) Schematic of a Graphene FET Gas Sensor Device(b) Evolution of Ids-Vgs

curves with exposure to NH3of the Graphene FET for different durations

7.4 Transparent conductive coating:

Graphene is optically active and absorbs a rather large fraction of incoming light for a

monolayer (2.3%), but this is still significantly smaller than the typical absorption

coefficient which could be achieved with a more traditional transparent conductive

coating material. In combination with its low electrical resistivity, high chemical stability

and mechanical strength, this absorption coefficient makes graphene an attractive

material for optoelectronic devices. Transparent conductors are an essential part of many

optical devices, from solar cells to liquid crystal displays and touch screens. Traditionally

metal oxides or thin metallic films have been used for these purposes, but with existing

technologies often complicated and expensive, there has been an ongoing search for new

types of conductive thin films. Furthermore, many of the widely used metal oxides

exhibit non uniform absorption across the visible spectrum and are chemically unstable;

the commonly used indium tin oxide (In2O3:Sn), for instance, is known to inject oxygen

and indium ions into the active media of a device. Graphene avoids all of these


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disadvantages. Moreover, it has recently been demonstrated that large areas of graphene

can be grown by the CVD method and transferred onto practically any surface. Devices

like solar cells and liquid crystal displays (LCD) which use graphene as a transparent

conductive coating have already been created.

8.0 Conclusion:

In this report we tried to analyze various aspects of graphene from its evolution to current

application. Though, it may look simpler, at first hand, to realize and use, but extraction

and detection of single layer graphene at industrial scale is the biggest challenge. The

preparation of graphene materials via chemical processing routes (e.g., oxidation of

graphite followed by reduction of the graphene oxide platelets obtained by exfoliation)

may be able to produce fairly large amounts of graphene cost effectively; however, the

chemical details (e.g., oxidation/reduction mechanisms and detailed chemical structures)

need to be more fully understood. Future efforts for graphene and n-layer graphene such

as achieving desired surface functionalization, and, e.g., the cutting or preparation into

desired shapes, could generate novel structures having many applications. Due to

grpahenes inherent properties of superfast transport phenomena, it is touted as one of the

newest materials with prospects of replacing hitherto semiconductor based devices.


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References:

1. A. K. Geim and K. S. Novoselov, The rise of graphene, Nature Materials 6, 183(2007) Manchester Centre for Mesoscience and Nanotechnology, University ofManchester, Oxford Road, Manchester M13 9PL, UK.

2.

P. R. Wallace, The Band Theory of Graphite, Phys. Rev 71, 662, (1947)

3. K. S. Novoselov, et al. Electric field effect in atomically thin carbon films.Science 306, 666 (2004).

4. A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S.Piscanec, D. Jiang, K. S. Novoselov, S. Roth, and A. K. Geim Raman Spectrumof Graphene and Graphene Layers, Physical Review Letters, PRL 97, 187401(2006).

5. K. S. Novoselov et al., Proc. Natl. Acad. Sci. U.S.A. 102, 10 451 (2005).

6.

Y. Zhang et al., Appl. Phys. Lett. 86, 073104 (2005).

7. J.C. Meyer et al., Ultramicroscopy 106, 176 (2006); Science 309, 1539 (2005).

8. K. S. Novoselov Nobel Lecture: Graphene: Materials in the Flatland theFlatland,Reviews of Modern Physics, vol. 83, JulySept. 2011.

9. M. Zhou , Y. L. Wang , Y. M. Zhai , J. F. Zhai , W. Ren , F. A. Wang , S. J.Dong, Chem. Eur. J. 2009 , 15 , 6116 .

10. W. Gao , L. B. Alemany , L. Ci , P. M. Ajayan ,Nat. Chem . 2009 , 1 , 403.

11.S. J. An , Y. Zhu , S. H. Lee , M. D. Stoller , T. Emilsson , S. Park , A. Velamakanni , J.

Ho , R. S. Ruoff ,J. Phys. Chem. Lett. 2010 , 1 , 1259 .

12.V. C. Tung, M. J. Allen, Y. Yang, and R. B. Kaner, Nat. Nanotechnol. 4, 25(2009).

13.H. Wang, D. Nezich, J. Kong, and T. Palacios, IEEE Electron Device Lett. 30,547 (2009).

14.Hiura, H., et al., 1994, Nature (London) 367, 148.

15.

Horiuchi, S., et al., 2004, Appl. Phys. Lett. 84, 2403.


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Annexure:

[01]In particle physics, a fermion (named after Enrico Fermi) is any particle which obeys

the FermiDirac statistics (and follows thePauli exclusion principle). Fermions contrast

with bosons which obey BoseEinstein statistics.

A fermion can be an elementary particle, such as the electron; or it can be a composite

particle, such as the proton. The spin-statistics theorem holds that, in any

reasonable relativistic quantum field theory, particles with integer spin are bosons, while

particles with half-integer spin are fermions.

By definition, fermions are particles, which obey FermiDirac statistics: when one swaps

two fermions, the wave function of the system changes sign. This

"antisymmetric wavefunction" behavior implies that fermions are subject to the Pauli

exclusion principle, i.e. no two fermions can occupy the same quantum state at the same

time. This results in "rigidity" or "stiffness" of states that include fermions (atomic nuclei,

atoms, molecules, etc.), so fermions are sometimes said to be the constituents of matter,

while bosons are said to be the particles that transmit interactions (i.e. force carriers) or

the constituents of electromagnetic radiation.

[02]Allotropy or allotropism is the property of some chemical elements to exist in two or

more different forms, known as allotropesof these elements. Allotropes are different

structural modifications of an element;[1]the atoms of the element are bonded together in

a different manner.

Take carbon for example: 4 common allotropes of carbon are diamond (where the carbon

atoms are bonded together in atetrahedral lattice arrangement), graphite (where the

carbon atoms are bonded together in sheets of a hexagonal lattice), graphene(single

sheets of graphite), and fullerenes (where the carbon atoms are bonded together in

spherical, tubular, or ellipsoidal formations).

The term allotropy is used for elements only, not for compounds. The more general term,

used for any crystalline material, ispolymorphism. Allotropy refers only to different


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forms of an element within the same phase (i.e. different solid, liquid or gas forms); the

changes of state between solid, liquid and gas in themselves are not considered allotropy.

[04]Ballistic transport is the transport of electrons in a medium with negligible electrical

resistivity due to scattering. Without scattering, electrons simply obey Newton's second

law of motion at non-relativistic speeds.

[05]A chiral molecule is a type of molecule that lacks an internal plane of symmetry and

thus has a non-superimposable mirror image.

[06] A relativistic particle is a particle which moves with a relativistic speed; that is,

a speed comparable to the speed of light. This is achieved by photons to the extent that

effects described by special relativity are able to describe those of

such particles themselves.

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