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GRAPHENES
Carbon is an interesting and important element not
only because it forms millions of organic compounds
with other elements but also due to its capability to
form a variety of allotropes.
Diamond and graphite, the two well-known
allotropes of carbon, were known from ancient
times.
Fullerenes, the third form of carbon were discovered
in 1985 and carbon nanotubes in 1991.
Though it was realized in 1991 that carbon
nanotubes were formed by rolling a 2D
graphene sheet, a single layer from 3D graphitic
crystal, the isolation of graphene was quite
elusive, resisting any attempt on its
experimental work until 2004.
Despite its lack of isolation, graphene is the best
theoretically studied allotrope of carbon from
the more than sixty years!
• In graphite the interlayer spacing is 3.35 Ao, the
van der Waals distance for sp2-bonded carbon.
• It is reported that the cohesive energy or
exfoliation energy for pyrolytic graphite is 61
meV/C atom.
• It has been estimated that a 1-nm square of
graphene contains about 38 carbon atoms and
the separation energy of two 1-nm squares of
graphene is over 2eV.
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To overcome this large cohesive energy and
achieve removal of a single layer form 3D
graphite, a top-down approach was followed by
physicists form Manchester University, UK led
by Andre Geim.
To obtain graphene they employed the
technique of micromechanical cleavage.
It involves mechanical exfoliation (repeated
peeling) of small mesas of highly oriented
pyrolytic graphite (HOPG).
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Studies on individual graphene sheets freely
suspended on a micro-fabricated scaffold in
vacuum or air have reported by Meyer et al.,
Although graphene could be obtained by
careful micromechanical cleavage, it becomes
almost impossible to find this 2D material using
modern techniques for studying atomically thin
materials.
Also, counting of graphene layers is a major
hurdle.
While scanning probe microscopy has too low a
throughput to search for graphene, scanning
electron microscopy is unsuitable because of
the absence of clear signatures for the number
of atomic layers.
Surprisingly, if graphene is placed on top of a Si
wafer with a carefully chosen thickness of SiO2
(typically 300 nm), it becomes visible in an
optical microscope owing to weak interferencelike contrast with respect to an empty wafer.
 However, Raman spectral studies of single, bilayer and
few-layer graphenes reflect changes in the electronic
structure and electron-phonon interactions, and allow
unambiguous, high-throughput and non-destructive
identification of graphene layers.
 Several reasons can be attributed to the current
interest in graphene.
 Graphene films are found to be 2D and semi-metallic,
with zero-overlap between conduction and valence
bands.
• They exhibit a strong ambipolar electric field
effect with electric concentrations up to 1013
cm-2.
• Their mobolities (µ) exceed 15,000 cm2 V-1 s-1.
Quantum Hall Effect is usually observed at very
low temperatures, typically below the boiling
point of liquid helium.
Efforts to extend the QHE temperature range
using semiconductors with small effective
masses of charge carriers have so far failed to
reach temperatures above 30K.
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However, quite recently, QHE has been
observed in graphene at room temperature,
thus opening up new vistas for graphene-based
resistance standards and for novel quantum
devices working at elevated temperatures.
Graphene is a promising candidate for several
future applications because of it ballistic
transport at room temperature, combined with
chemical and mechanical stability.
The stability of 2D crystals is attributed to gentle
crumpling in the third dimension, as revealed by
TEM studies.
Graphene is the building block for carbon
materials of all other dimensionalities and
therefore the mother of all graphitic materials.
Thus, the 2D material, rolled into 1D nanotubes
or stacked into 3d graphite.
 Collaborative research has been in progress for the
past two years between scientists at Manchester
University and Max Planck Institute for Solid State
Research in creating graphene-based transistors.
 The transistor operates at room temperature, making it
potentially viable for future electronic components
possibly by replacing silicon.
Recently, Schedin et al., have reported
graphene-based gas sensors that are capable of
detecting minute concentrations of various
active gases.
They also discerned individual events when a
molecule attaches to the surface of the sensor.
Though at present nanocomposite materials
employing carbon materials are dominated by
carbon nanotubes, several problems still need
to be solved.
The important issues are the fact that
nanotubes tend to clump together during
processing the difficulty of controlling their
diameter and the way the carbon sheet is rolled.
Stankovich et al., are of the view that many of
the above problems may be mitigated by
making composite materials of graphene sheets
and polymers.
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Their approach involves the preparation of
graphene-polymer composites via complete
exfoliation of graphite and molecular level
dispersion of individual, chemically modified
graphene sheets within polymer hosts.
Even 0.1% by volume of graphene in the
composite
is
sufficient
for
electrical
conductance
The conductivity increases by incorporating
more graphene, reaching 1S/m at a loading of
2.5% by volume.
Like fullerenes and carbon nanotubes which
have generated considerable interest in both
basic and applied research, one can expect a
similar revolution with graphene as well
PREPARATIVE METHODS OF
GRAPHENE
Reductive pyrolysis of camphor
Exfoliation of graphitic oxide
Conversion of nanodiamond
Arc evaporation of SiC
CAMPHOR GRAPHENE (CG)
To prepare CG, camphor was pyrolysed over
nickel particles under a reducing atmosphere.
The reaction was carried out in a two-stage
furnace and camphor was slowly sublimed
(170°C) by heating from the first furnace to
the second furnace held at 770°C where the
micron sized nickel particles were placed.
EXFOLIATED GRAPHENE (EG)
To prepare EG involved the thermal exfoliation
of graphitic oxide. In this method, graphitic
oxide was prepared by reacting graphite with a
mixture of conc. Nitric acid and sulfuric acid
with potassium chlorate at room temperature
for 5 days.
Thermal exfoliation of graphitic oxide was
carried out in a long quartz tube at 1050°C
under an Ar atmosphere.
DIAMOND GRAPHENE (DG)
Thermal conversion of nanodiamond (particle
size 4–6 nm, Tokyo Diamond Tools, Tokyo,
Japan) to graphene was carried out at 1650 C
in a helium atmosphere to obtain DG.
SILICON CARBIDE GRAPHENE (SG)
SG was obtained by arc evaporation of SiC
(arc melted mixture of Si and graphite) in a
hydrogen atmosphere (200 Torr) with a DC
current of 45 A and 38 V.
CHARACTERIZATION METHODS
• X-Ray Diffraction
• Transmission election microscopy
• Raman Spectroscopy
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X-Ray diffraction patterns of the CG, EG and DG.
(b) Lorentzian fit for (002) and (c) double Lorentzian fit for
(100) and 101) diffraction peaks for DG.
Sample
Number of layers from
(002) reflection
Crystallite size from
(100) reflection/nm
CG
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6.1
EG
3, 16
4.7
DG
6, 87
5.0
TEM images of graphene obtained by (a) reductive pyrolysis of camphor (CG), (b) thermal
exfoliation of graphitic oxide (EG), (c) thermal conversion of nanodiamond to graphene (DG) and (d)
arc
evaporation of SiC (SG).
Raman spectra
Sample
D
G
G’
2D
D+G
CG
(a) 1321
(b) 1342
1567
1569
1604
1605
2647
2687
2919
2920
EG
(a) 1324
(b) 1352
1569
1574
1605
1608
2652
2705
2908
2926
DG
(a) 1332
(b) 1330
1576
1576
1606
1608
2678
2682
2909
2905
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CONCLUSION
• Even though graphene samples prepared by different
methods, in terms of the number of layers, crystallite
size as well as surface area, graphene prepared by the
exfoliation of graphite (EG) seems to be best,
possessing high surface area and a smaller number of
layers.
• (DG) has greater thermal stability than EG. Both EG and
DG exhibit significant hydrogen uptake.
• EG shows electrochemical redox behavior similar to
that of the basal plane of graphite and can be used for
fabrication of electrochemical supercapacitors.
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