JUNCTION CHARACTERISTICS OF CHEMICALLY DERIVED GRAPHENE-ON-SEMICONDUCTORS

Kamal Batra4th Year Undergraduate StudentIIT Kharagpur -721302

DATE : 14 July 2013

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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 hexagonal pattern

* basic building block for graphite material.

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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.

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FAMILIAR CARBON STRUCTURES Fullerene [0D]: 20 Hexagons, and 12

Pentagons (Physicists Awarded Nobel Prize for Discovery in 1996)

Carbon Nanotubes [1D]: Quasi- one dimensional form of carbon - Single walled nanotubes known since 1993

Graphite [3D], known since … a long time

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NOBLE PRIZE IN PHYSICS (2010)

Konstantin Novoselov Andre Geim

** Isolated large sheets in order to identify and characterize graphene and verify 2D properties

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PROPERTIES OF GRAPHENE1. 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.

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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, E=ћvFk. where ħ is the reduced Planck’s constant and vF (≈106 m/s) is the electron Fermi velocity in graphene.

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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 suppressed.

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 .

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OPTICAL PROPERTIES (CONTD.)*OPTICAL MEASUREMENT

-Take prepared macroscopic membranes of graphene

-Shine light through the membrane

-Detector measures light intensity

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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, and OH).

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.

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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.

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PROPERTIES OF GRAPHENE (CONTD.)

5.THERMAL PROPERTIES

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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 than copper).

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 %

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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 agglomeration

The agglomeration is dispersed into DI water using ultrasonication

GO can be reduced by chemical method.

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CHEMICALS/ EQUIPMENTS REQUIRED

Sr. No Chemical/ Equipments Specifications

1. Graphite powder SDFCL (100 micron)

2. Sodium Nitrate Sigma Aldrich

3. Potassium permanganate Merck

4. Hydrogen peroxide Sigma Aldrich, 3 wt% in H2O

5. Acetone Merck

6. Sodium Borohydride Merck

7. Sulfuric Acid Merck (98%)

8. ITO coated glass slidesSigma Aldrich (thickness =150-300 A0 ; Rs= 70-100 ohm/sq)

9. Spin coater Milman-model SPN 2000

10. Si (1 0 0 ) waferP-type, diameter= 4”, thickness =500 µm, resistivity=1-100ohm cm , one side mirror-polished

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SCHEMATIC OF FABRICATION METHOD

Graphite powder Graphite Oxide

Graphene Oxide (GO)

KMnO4/ H2SO4 for oxidation, 2hrs

Ultrasonication

Reduced GOReduction

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AQUEOUS DISPERSION OF GO AND RGO(1 WT %)

0 Hour 1 Hour

12 Hour

R-GO GOR-GO

R-GO

GO

GO

24 hour

R-GO GO

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HOW TO MAKE GRAPHENE THIN FILM ?

Spin Coating- * 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).

Parameters :

* Nature of fluid (viscosity,

surface tension etc )

* Rotation speed

* Time of RotationFig: spin coater unit

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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 specimens.

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.

Fig: SEM

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SEM IMAGES OF GRAPHITE, GO AND R-GO (CLOCKWISE)

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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 spectra:

-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 graphene.

-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….

Raman spectroscopy

Fig : Raman spectroscopy

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1000120014001600180020002200240026002800300032003400-500

0

500

1000

1500

2000

2500

3000

Inte

nsi

ty (a.

u)

wavenumber (cm-1)

D

G

2D

D+G

GO

RAMAN CHARACTERIZATIONS (514 NM)

-Graphite

1000120014001600180020002200240026002800300032003400-500

0

500

1000

1500

2000

2500

Inte

nsi

ty(a

.u)

wavenumber (cm-1)

D

G

2D

D+G

rGO

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RAMAN CHARACTERIZATIONS…

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

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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.

FTIR SPECTROSCOPY

Functional Group

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

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FTIR CHARACTERIZATION

0 500 1000 1500 2000 2500 3000 3500 4000 4500-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

Tra

nsm

itta

nce

(a.

u)

Wavenumber (cm-1)

3685

.75

1487

.46

1660

.47

1246

.7611

09.4

8

0 500 1000 1500 2000 2500 3000 3500 4000 4500

0.2

0.4

0.6

0.8

1.0

1.2

Tra

nsm

itta

nce

(a.

u)

wavenumber (cm-1)

0 500 1000 1500 2000 2500 3000 3500 4000 4500-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

Tra

nsm

itta

nce

(a.

u)

Wavenumber (cm-1)

3674.4

6

1498.7

4

1258.0

4

Graphite GO

rGO

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FTIR CHARACTERIZATION…

Functional Group

Characteristic absorption (cm-

1 )

Type of vibration & Intensity

O-H (alcohol) 3685.75 Stretch free, sharp

C=C (alkenes) 1660.47 Stretch, variable

C=C (aromatic) 1487.46 Stretch, medium

C-O (acid/ester) 1246.76 Stretch, strong

C-O (alcohol ) 1109.48 Stretch, strong

Functional Group

Characteristic absorption (cm-

1 )

Type of vibration& Intensity

O-H (alcohol) 3674.46 Stretch free, sharp

C=C (aromatic) 1498.74 Stretch, weak

C-O (acid) 1258.04 Stretch, strong

GO

rGO

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UV-VIS CHARACTERIZATION

200 400 600 800 1000 1200 1400-10

0

10

20

30

40

50

60

70

Tra

nsm

itta

nce

(a.

u)

wavelength (cm)

GO-2 GO-3 GO-1

200 400 600 800 1000 1200 1400

0

20

40

60

80

Tra

nsm

itta

nce

(a.

u)

wavelength (cm)

RGO-1 RGO-2 RGO-3

*1- GO/RGO represents “thickness of thin film” =10nm*2- GO/RGO represents “thickness of thin film” =50nm

*3- GO/RGO represents “thickness of thin film” =100nm

Graphene layer

(V) P-type Si (1 0 0 ) wafer

Aluminium

Silver (Ag)Sunlight

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To demonstrate the fabrication of a solid state heterojunction PV device with solution–processed

Graphene & P-type Silicon substrate.

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.

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EQUIVALENT CIRCUIT & BAND DIAGRAM OF THE GRAPHENE / P-SI HETERO JUNCTION

(a)Fig (a). Represents the equivalent circuitdiagram of the fabricated GO/n-Si hetero-junction device.

*Current density is greatly influenced bythe series resistance (Rs)

Fig (b). Energy diagram of G/p-Si schottkyjunction upon light illumination.

ΦG = 4.7 eV, χp-Si = 4.05 eVΦBP = Eg - ΦG + χp-Si = 0.47 eV Vp = 0.12Vbi = ΦBp – Vp= 0.47 – 0.12 = 0.35 eV

R-GO/p-Si (V)

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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 (2011)

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 (2013)

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

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Thank You !!