Graphene- A Noble 2D Nanomaterial

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Graphene- A Noble 2D Nanomaterial

  1. 1. JUNCTION CHARACTERISTICS OF CHEMICALLY DERIVED GRAPHENE-ON- SEMICONDUCTORS Kamal Batra 4th Year Undergraduate Student IIT Kharagpur -721302 DATE : 14 July 2013
  2. 2. 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. 2
  3. 3. 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. 3
  4. 4. 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 4
  5. 5. NOBLE PRIZE IN PHYSICS (2010) Konstantin Novoselov Andre Geim ** Isolated large sheets in order to identify and characterize graphene and verify 2D properties 5
  6. 6. 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
  7. 7. 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. 7
  8. 8. 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 . 8
  9. 9. OPTICAL PROPERTIES (CONTD.) *OPTICAL MEASUREMENT -Take prepared macroscopic membranes of graphene -Shine light through the membrane -Detector measures light intensity 9
  10. 10. 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. 10
  11. 11. 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. 11
  12. 12. PROPERTIES OF GRAPHENE (CONTD.) 5.THERMAL PROPERTIES 12
  13. 13. 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 % 13
  14. 14. 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. 14
  15. 15. 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 slides Sigma Aldrich (thickness =150-300 A0 ; Rs= 70-100 ohm/sq) 9. Spin coater Milman-model SPN 2000 10. Si (1 0 0 ) wafer P-type, diameter= 4”, thickness =500 µm, resistivity=1-100ohm cm , one side mirror-polished 15
  16. 16. SCHEMATIC OF FABRICATION METHOD Graphite powder Graphite Oxide Graphene Oxide (GO) KMnO4/ H2SO4 for oxidation, 2hrs Ultrasonication Reduced GO Reduction 16
  17. 17. AQUEOUS DISPERSION OF GO AND RGO (1 WT %) 0 Hour 1 Hour 12 Hour R-GO GO R-GO R-GO GO GO 24 hour R-GO GO 17
  18. 18. 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 Rotation Fig: spin coater unit 18
  19. 19. 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 19
  20. 20. SEM IMAGES OF GRAPHITE, GO AND R-GO (CLOCKWISE) 20
  21. 21. 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 21
  22. 22. 1000120014001600180020002200240026002800300032003400 -500 0 500 1000 1500 2000 2500 3000 Intensity(a.u) wavenumber (cm -1 ) D G 2D D+G GO RAMAN CHARACTERIZATIONS (514 NM) 22 -Graphite 1000120014001600180020002200240026002800300032003400 -500 0 500 1000 1500 2000 2500 Intensity(a.u) wavenumber (cm -1 ) D G 2D D+G rGO
  23. 23. 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 23
  24. 24. 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 24
  25. 25. FTIR CHARACTERIZATION 25 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 Transmittance(a.u) Wavenumber (cm -1 ) 3685.75 1487.46 1660.47 1246.76 1109.48 0 500 1000 1500 2000 2500 3000 3500 4000 4500 0.2 0.4 0.6 0.8 1.0 1.2 Transmittance(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 Transmittance(a.u) Wavenumber (cm -1 ) 3674.46 1498.74 1258.04 Graphite GO rGO
  26. 26. 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 26 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
  27. 27. UV-VIS CHARACTERIZATION 27 200 400 600 800 1000 1200 1400 -10 0 10 20 30 40 50 60 70 Transmittance(a.u) wavelength (cm) GO-2 GO-3 GO-1 200 400 600 800 1000 1200 1400 0 20 40 60 80 Transmittance(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
  28. 28. Graphene layer (V) P-type Si (1 0 0 ) wafer Aluminium Silver (Ag) Sunlight 28 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.
  29. 29. EQUIVALENT CIRCUIT & BAND DIAGRAM OF THE GRAPHENE / P-SI HETERO JUNCTION 29 (a)Fig (a). Represents the equivalent circuit diagram of the fabricated GO/n-Si hetero- junction device. *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 R-GO/p-Si (V)
  30. 30. LITERATURE SURVEY ON GRAPHENE-BASED HETERO-JUNCTION SOLAR CELL 30 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
  31. 31. Thank You !! 31

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