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
, ,
ll i i i il i i l i l ll i i li i I i ll ll
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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 ex- foliation. 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 tempera- tures. 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 mono- layer 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.