JUNCTION CHARACTERISTICS OF
CHEMICALLY DERIVED GRAPHENE-ON-
4th Year Undergraduate Student
IIT Kharagpur -721302
DATE : 14 July 2013
WHAT IS GRAPHENE ?
The term graphene was coined by Hanns-Peter
Boehm in 1962 as [Graphite + ene ]
Graphene is ,
* an atomic scale honey-comb lattice made up
of carbon atoms.
* a first truly 2D material with regular
* basic building block for graphite material.
A CLOSER LOOK AT GRAPHENE
Graphene has :
* covalent bonding
* single planar sp2 hybridization
* Carbon-carbon distance of 0.142 nm
Similar 2D structures: Boron-Nitride (BN) and
Molybdenum-disulphide (MoS2), which have both
been produced after 2004.
FAMILIAR CARBON STRUCTURES
Fullerene [0D]: 20 Hexagons, and 12 Pentagons
(Physicists Awarded Nobel Prize for Discovery in
Carbon Nanotubes [1D]: Quasi- one
dimensional form of carbon - Single walled
nanotubes known since 1993
Graphite [3D], known since … a long time
NOBLE PRIZE IN PHYSICS (2010)
** Isolated large sheets in order to identify and
characterize graphene and verify 2D properties 5
PROPERTIES OF GRAPHENE
1. ELECTRONIC PROPERTIES
One of the hottest areas of graphene research focuses on
the intrinsic electronic properties; how electrons
flows through a sheet – only one atom thick – while
under the influence of various external forces.
The graphene lattice structure is
characterized by the two C-C bonds
(sigma, pi) constructed from
four valence orbital's ( 2s, 2px, 2py, 2pz)
where the z- direction is perpendicular to the sheet .
Here 3 electron per carbon atom in graphene are
involved in formation of strong covalent sigma bonds
& one electron per atom yields the pi bond.
*The only pi –electrons are responsible for the
electronic properties at low energy. 6
ELECTRONIC PROPERTIES (CONTD.)
In addition the C-C bonding is enhanced by fourth bond associated with
the overlap of pz (or pi) orbital's , the electronic properties of graphene
are determined by the bonding pi & anti bonding pi* orbital's that forms
electronic valence & conduction bands
Acc. to this band structures, graphene can be describe as a zero-gap
semiconductor. Also the pi-band electronic dispersion of grpahene at
six corner of the 2D hexagonal brillouin zone is found to be linear,
where ħ is the reduced Planck’s constant and vF (≈106 m/s) is the electron
Fermi velocity in graphene.
PROPERTIES OF GRAPHENE (CONTD.)
2. OPTICAL PROPERTIES
Graphene , despite being the thinnest material ever made , is still
visible to the naked eye. Due to its unique electronic properties, it
absorbs high 2.3% of light that passes through it, which is enough
that you can see it in air.
To help enhance the visibility of graphene flakes we deposit them on
to silicon wafers which have a thin layer of silicon dioxide. Light
shining on these three layers structures will be partially
transmitted & partially reflected at each interface.
This leads to complex optical interference effects such that,
depending on the thickness of silicon dioxide (which we can control to
high degree of accuracy) some colors are enhanced & some are
This technique takes advantage of the same physics which cause the
“ rainbow effect” that you see when you have a thin layer of oil
floating on water. In this case different colors corresponds to longer/
shorter optical path length that the light has had to travel
through the oil film .
OPTICAL PROPERTIES (CONTD.)
-Take prepared macroscopic
membranes of graphene
-Shine light through the
-Detector measures light
PROPERTIES OF GRAPHENE (CONTD.)
3. CHEMICAL PROPERTIES
Similar to the surface of graphite, graphene can adsorb and
desorb various atoms and molecules (for example, NO2, NH3, K,
Weakly attached adsorbates often act as donors or acceptors and
lead to changes in the carrier concentration, so graphene
remains highly conductive. This can be exploited for
applications as sensors for chemicals.
Other than weakly attached adsobates, graphene can be
functionalized by several chemical groups (for instances OH-, F-)
forming graphene oxide and fluorinated graphene. It has also
been revealed that single-layer graphene is much more
reactive than 2, 3 or higher numbers or layers.
Also, the edge of graphene has been shown to be more
reactive than the surface. Unless exposed to reasonably harsh
reaction conditions, graphene is a fairly inert material, and
does not react readily despite every atom being exposed and
vulnerable to it's surroundings.
PROPERTIES OF GRAPHENE (CONTD.)
4. MECHANICAL PROPERTIES
To calculate the strength of graphene, scientists used a technique
called Atomic Force Microscopy. By pressing graphene that
was lying on top of circular wells, they measured just how far you
can push graphene with a small tip without breaking it.
It was found that graphene is harder than diamond and about
300 times harder than steel.
Even though graphene is so robust, it is also very stretchable..
It is expected that graphene’s mechanical properties will find
applications into making a new generation
of super strong composite materials and along combined with
its optical properties, making flexible displays.
PROPERTIES OF GRAPHENE (CONTD.)
OTHER PROPERTIES OF GRAPHENE
Density 0.77 mg/m2
Breaking strength 42 N/m.
Conductivity 0.96x106 Ω-1cm-1 ( > copper).
Thermal Conductivity 5000 Wm−1K−1 , (10x greater
Graphene height 0.34 nm, almost one million
times thinner then human hair
Lightness 0.7 mg for 1 m2
High electron mobility 15,000 cm2V−1s−1.
Gapless semiconductor (zero gap )
Yet flexible doesn’t break easily, can support
4 kg for 1 m sq. graphene
Transparent 97.7 %
SYNTHESIS OF GRAPHENE OXIDE USING
MODIFIED HUMMER’S METHOD
Fabrication Flow Chart
Mixing of Graphite Powder with Strong Oxidizing Agent
Maintain Temperature (20 oC) and Stirring for 2 Hours
Washing of the suspension to remove Mn based Oxides and Metal Ions and filtered
Paste collected from the filter paper is dried at 50-60 oC until it becomes
The agglomeration is dispersed into DI water using ultrasonication
GO can be reduced by chemical method.
SCHEMATIC OF FABRICATION METHOD
Graphite powder Graphite Oxide
Graphene Oxide (GO)
KMnO4/ H2SO4 for
AQUEOUS DISPERSION OF GO AND RGO
(1 WT %)
0 Hour 1 Hour
HOW TO MAKE GRAPHENE THIN FILM ?
* Method for applying thin films.
A typical process that involves depositing a small puddle of a fluid
material on to the centre of a substrate & then spinning the substrate
at high speed (~3000 rpm).
* Nature of fluid (viscosity,
surface tension etc )
* Rotation speed
* Time of Rotation
Fig: spin coater unit
CHARACTERIZING CHEMICALLY-DERIVED GRAPHENE
Scanning Electron Microscopy
(SEM) uses a focused beam of high-energy electrons
to generate a variety of signals at the surface of solid
Secondary electrons and backscattered electrons are
commonly used for imaging samples.
SEM analysis is considered to be "non-destructive";
that is, x-rays generated by electron interactions do
not lead to volume loss of the sample. So it is possible
to analyze the same materials repeatedly.
SEM IMAGES OF GRAPHITE, GO AND R-GO (CLOCKWISE)
Distinguish the Quality of graphene.
Determine the no, of layers for n –layer graphene by
the shape, width & position of 2D peak.
*Features of graphene material in Raman
-D peak [1350 cm-1]: To describe disorders & local
defects at the edges of graphene & graphite platelets.
-G peak [1580 cm -1]: To asses the quality of graphene.
-2D peak is correlated to the carrier mobility of the
-ID/IG gives the metric of disorder in graphene
* The I2D/IG ratio is a better criterion in selecting
high quality single layer graphene
CHARACTERIZING CHEMICALLY-DERIVED GRAPHENE….
Fig : Raman spectroscopy
RAMAN CHARACTERIZATIONS (514 NM)
Material D-band G-band 2D- band D+G band ID/IG
Graphite 1350.2 1588.2 2701.8 - 0.425
GO 1347.0 1597.8 2666.9 2941.7 1.182
rGO 1341.0 1603.8 2672.9 2947.7 1.158
CHARACTERIZING CHEMICALLY-DERIVE GRAPHENE…
Easy way to identify the presence of certain functional groups in a molecule.
Also, one can use the unique collection of absorption band to confirm the identity of pure
compounds, or to detect the presence of specific impurities.
Type of vibration Characteristic
absorption (cm-1 )
O-H Stretch, H-bonded 3200-3600
C-O Stretch 1050-1150
C-H Stretch 2850-3000
C=C Stretch 1620-1680
C=O Stretch 1670-1820
C-OH Stretch 1200-1300
Fig: FTIR spectroscopy
(V) P-type Si (1 0 0 ) wafer
To demonstrate the fabrication of a solid state heterojunction
PV device with solution–processed Graphene & P-type Silicon
In this representative device, incident light
was transmitted through the thin graphene
film to reach the junction interface & thereby
PV action was observed.
Also by applying electric potential at the
G/p-Si junction, photo excited electrons &
holes can be separated, transported &
collected at the electrodes.
EQUIVALENT CIRCUIT & BAND DIAGRAM OF
THE GRAPHENE / P-SI HETERO JUNCTION
(a)Fig (a). Represents the equivalent circuit
diagram of the fabricated GO/n-Si hetero-
*Current density is greatly influenced by
the series resistance (Rs)
Fig (b). Energy diagram of G/p-Si schottky
junction upon light illumination.
ΦG = 4.7 eV, χp-Si = 4.05 eV
ΦBP = Eg - ΦG + χp-Si = 0.47 eV
Vp = 0.12
Vbi = ΦBp – Vp= 0.47 – 0.12 = 0.35 eV
LITERATURE SURVEY ON GRAPHENE-BASED
HETERO-JUNCTION SOLAR CELL
Sr. No. Reference VOC JSC FF (%) Efficiency (%)
1 ACS Nano, Vol. 4, p. 5633-5640 (2010) 0.43 V 3.5 mA/cm2 41 0.61
2 Advanced Materials, Vol. 22, p. 2743–2748 (2010) 0.48 V 6.50 mA/cm2 56 1.70
3 Proceedings of the Conference on China
Technological Development of Renewable Energy
Source, Vol. I, p. 387-390 (2010)
0.517 V 13.2 mA/cm2 58 3.93
4 ACS Appl. Mater. Interfaces, Vol. 3, p. 721-725
0.462 V 9.20 mA/cm2 30.0 1.25
5 Appl. Phys. Lett., Vol. 99, p. 233505 (2011) 0.487 V 16.03 mA/cm2 45.0 3.55
6 Appl. Phys. Lett., Vol. 99, p. 133113 (2011) 0.19 V 154.5 µA/cm2 25.0 2.15
7 Nano Lett., Vol. 12, p. 2745–2750 (2012) 0.54 V 25.3 mA/cm2 63.0 8.60
8 Physica status solidi (RRL), Vol. 7, p. 340-343
0.254 V 4.28 mA/cm2 23 0.25
9 J. Phys. Chem. C, Vol. 117 p. 11968–11976 (2013) 0.49 V 31.4 mA/cm2 63 9.73
10 Carbon, Vol. 57, p. 329-337 (2013) 0.51 V 24.28 mA/cm2 60.4 7.5
11 J. Mater. Chem. A, Vol. 1, p. 6593-6601 (2013) - - - 10.30