Synthesis of graphene

Materials Engineering

Presented By:Apratim Khandelwal

Different Methods For Synthesis of Graphene

Supervision By: Dr. Shuhui Sun

Contents

• Introduction to Graphene

• Properties

• Applications

• Methods Used- CVD (Chemical Vapor Deposition).

Mechanical Exfoliation.

Epitaxial Growth of Graphene on SiC.

Modified Hummers Method.

Electrochemical Exfoliation.

• N-doped Graphene

Introduction

• Graphene : Carbon in 2D

• 2-dimensional, crystalline allotrope of carbon.

• Carbon atoms are densely packed in a regular sp2-bonded atomic-scale hexagonal pattern.

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Fig1: High resolution TEM images of graphene

http://en.wikipedia.org/wiki/Graphene

Properties

• Electronic properties

– High electrical conductivity

– Semi metal/zero gap semiconductor

– High electron mobility

• Mechanical properties

– One of the strongest material

– Ultimate tensile strength is 130GPa (0.4 GPa-Steel)

– Very light

– High surface area to volume ratio.

– Has elastic properties too (Young’s modulus – 0.5TPa)

Properties

• Optical properties

– Single layer of graphene absorbs spectrum as high as 2.3% of white light

– Saturation absorption under strong excitation

• Wetting properties

– Super hydrophobic property

– Super hydrophilic property

Applications

• Electrochemical storage devices

Super-capacitors, graphene-enhanced lithium ion batteries.

• Biological Engineering

Tissue regeneration- potential biocompatibility.

• Aerospace applications

Integration into other composite materials.

• Water purification systems

Monolayer graphene- ultrafilteration.

Some of the major applications of graphene are:

Applications

• Opto-Electronic applications

Replacing Indium Tin Oxide.

• Photovoltaic Cells

Replacing Si Solar cells and ITO in DSSC.

• Various other applications which involve enhancing electrical, mechanical and various other properties of devices or materials as such.

Methods Used

• Chemical Vapour Deposition (CVD).

• Mechanical Exfoliation.

• Epitaxial Growth of Graphene on SiC.

• Chemical Methods such as Hummers Method.

• Electrochemical Exfoliation.

Various Methods used till date are as follows:

CVD (Chemical Vapour Deposition)

• A way of depositing gaseous reactants on a substrate.

• The process is similar to PVD (Physical Vapour Deposition) the only difference being the use of solid reactants instead of gaseous reactants as in CVD.

• The carrier gases are combined in a reaction chamber which is maintained at certain temperature and pressure (as required by reaction).

• The reaction occurs on the substrate on which one of the product (carbon) is deposited and the by products are pumped out.

• Substrate is usually a transition metal (Ni/Cu) or some ceramic such as glass.

• The selection of substrate depends upon the feasibility of transferring the graphene onto the required material.

• The gases used are generally Methane (source of carbon) Hydrogen and Argon are also used along with methane as reaction stabilizers and enhancing the film uniformity.

• Although there are various types of CVD processes but most modern processes (regarding CVD pressure) are as follows:

-> LPCVD (Low Pressure CVD): Carried out under sub-atmospheric pressures. Low pressure prevents unwanted reactions and also increase the uniformity of the films on the substrate.

-> UHVCVD (Ultra High Vacuum CVD): Carried out under extremely low atmospheric pressures (6-10 Pa).

CVD (Chemical Vapour Deposition)

CVD (Chemical Vapour Deposition)

Fig2: CVD schematic diagram of the process.Precision Fabricators Ltd Tutorial 2 - Chemical Vapour Deposition (© Rice University)

1) Diffusion of reactants through boundary layer.2) Adsorption of reactants onto substrate surface.3) Occurrence of chemical reaction on the surface.4) Desorption of the by products from the surface.5) Diffusion of by products through boundary layer.

CVD (Chemical Vapour Deposition)

ADVANTAGES-• High quality, impervious, and harder graphene is obtained.• Producing large domains of graphene is easy.• High growth rates possible.• Good reproducibility.

LIMITATIONS-• High temperatures (greater than 900 °C) leads to wrinkled

graphene due to difference in Coefficient of Thermal Expansion.• Complex process.• Production of corrosive and toxic gases.• Difficulty in controlling the thickness in some cases (number of

layers).• Difficulty in transferring the film to other surface (exfoliation).• Difficulty in achieving the uniform deposition of the carbon.

Mechanical ExfoliationProcess:• A fresh piece of Scotch tape is taken (about six inches long).• The adhesive side is pressed onto the HOPG (Highly Ordered

Pyrolytic Graphite) for about ten seconds.• The tape is gently peeled away with thick shiny layers of graphite

attached to it.• The part of the tape with layers from the HOPG was refolded

upon a clean adhesive section of the same piece of the tape and then the tape is unfolded.

• This process is repeated several times until the end of the tape is no longer shiny but becomes dark/dull and grey.

• These graphite layers on the tape are transferred onto the surface of the Si/SiO2 wafers by gently pressing them onto the tape for some time and then peeling off.

• The wafers are then examined using various characterization techniques.

Mechanical Exfoliation

(a) Adhesive tape is pressed onto the HOPG.(b) The tape is peeled off when some layers stick to the surface.(c) This tape is pressed onto the surface of the target substrate.(d) The tape is peeled off when the layers stick to the target surface.

Fig3: Schematic diagram for the mechanical exfoliation of graphene from graphite using Scotch tape.K S Novoselov and A H Castro Neto 2012 Phys. Scr. 2012 014006, Two-dimensional crystals-based heterostructures: materials with tailored properties.

Mechanical Exfoliation

Advantages:

• Safe and simple process.• Few layer graphene can be easily obtained.• The chances of impurity in the graphene so obtained are less.• Sample preparation is simplified.

Limitations:

• Yield obtained may not meet the requirements.• Requires skilled manual labour.• Despite the fact that tape residue does not seriously affect the

quality of the graphene flake samples, it does make those samples more difficult to find on the substrate.

• The main concept behind the process is that the vapour pressure of Silicon is higher, as a result on heating the SiC wafer, the Si evaporates leaving behind the graphene layers on the SiC.

• The theoretical studies show the various stable structures of carbon that grow on SiC which determines the mechanism for growth of carbon layers on SiC substrate.

• The evaluation of number of graphene layers is done by observing the quantized oscillations of the electron reflectivity.

• Multilayer, bilayer or single layer graphene can be grown on the SiC substrate by controlling various parameters such as SiCtemperature and pressure.

• Graphene single crystals can also be synthesized using this process since few layer graphene (FLG) always maintains it’s epitaxial growth with the SiC substrate.

Epitaxial Growth of Graphene on SiC

Epitaxial Growth of Graphene on SiC

Fig4: Schematic illustration of the thermal decomposition method.Hiroki Hibino†, Hiroyuki Kageshima, and Masao Nagase, “Graphene Growth on Silicon Carbide”, NTT Technical review.

• The SiC substrate is heated at a temperature (around 1200 °C) and the conditions of the chamber are set accordingly.

• UHV (Ultra High Vacuum) technique hinders the uniform growth of MLG (Multi Layer Graphene) and favor the bilayer graphene.

• The Si atoms evaporate due to thermionic emission leaving behind the carbon atoms on the remaining substrate.

• The carbon layers accumulating on the substrate are controlled by controlling the temperature and pressure.

• The final SiC substrate is covered with the carbon layers which can be either bilayer, monolayer or multilayer graphene.

• The type is distinguished by using the suitable characterization techniques (such as TEM), the shade of the layer in the images can be used as the classifying method for recognizing the type of graphene.

Epitaxial Growth of Graphene on SiC

Advantages:• High quality graphene.• Easy method for growing single crystals of graphene.• The layers of graphene can be controlled conveniently.• Higher temperatures ensure reproducible, clean and ordered graphene.• Patterning of graphene is easier due to the use of insulating SiC

substrate.• Further advantage is that SiC is already a large bandgap semiconductor

already used in electronic applications.

Limitations:• High temperature.• Difficulty in growing uniform MLG in UHV conditions (impurity

free conditions).• Lattice constant mismatch and difference in coefficient of

thermal expansion of SiC - C can lead to various defects.

• The Hummers method is used for producing graphene by oxidising graphite to GO by using suitable oxidising agents such as KMnO4.

• The GO so produced is again then chemically reduced to get graphene.

• The modified Hummers method introduces a way to get a more stable GO colloidal solution.

• Ultra-sonication is used for stabilizing the GO solution and enhancing the exfoliation in the GO solution.

Hummers Method (modified)

Fig5: A typical illustration of the difference between the graphite and the GO so formed.Mateusz CISZEWSKI, Andrzej MIANOWSKI, “Survey of graphite oxidation methods using oxidizing mixtures in inorganic acids”, CHEMIK International Edition, CHEMIK 2013, 67, 4, 267-274

The modified hummers method can be carried out in three major steps as follows:

Oxidation

• Natural graphite flake is mixed with a strong acid such as H2SO4 /HNO3 followed by continuous stirring in ice bath.

• Then KMnO4 is added and stirred at room temperature.

• Then the solution is kept overnight after adding DI water and H2O2.

• Centrifugation is used for dilution until the pH is around 7.

• Ultra-sonication is carried out to get monolayer GO.

Reduction

• Addition of certain reducing agents such as hydrazine or NaBH4 is made to the measured solution.

• The attached functional groups are removed and to enhance the exfoliation certain polar aprotic solvents can be used along with organic compounds.

• Although thermal reduction gives a better quality graphene but has its own disadvantages.

Post-treatment

• Then the solution is filtered and washed with DI water until neutrality.

• The product is dried and grinded.

• The graphene so produced can then be send for characterization tests.

Hummers Method (modified)

Hummers Method (modified)

Advantages:• High yield.• Scalable to industrial level.

Limitations:• The defects on graphene sheets are inevitable.• The process is time consuming and can be laborious.• The thickness control is not as promising as in the epitaxial graphene

growth or the CVD process.

Electrochemical Exfoliation• The electrochemical exfoliation method is an eco-friendly method fro producing high quality

graphene.

• The electrochemical exfoliation of graphite due to the ions present in the solution give the FLG or GO depending upon the nature of electrolyte.

• The choice of electrolyte is based on the requirement of the oxidizing environment and more importantly on the size of the intercalating ion.

• The setup includes two electrodes one of them being graphite or HOPG the other can be Cu, Pt or can be HOPG/graphite also.

• A voltage of +/- 10V is applied generally for an unequal period of time.

• The intercalating ion in the solution exfoliates the graphite into FLG by penetrating in between the sheets due to the applied voltage.

• The need for negative voltage is to bring the intercalated ions back into the solution along with the sheets, hence the need of unequal application time for the voltages.

• The electrolyte used is often diluted with DI water and then at the end of the process the solution is taken for centrifugation to separate the graphite particles from graphene/GO.

• The solution can also be subjected to ultra-sonication for enhancing the exfoliation and some stabilizers can also be used for the solution.

• The final solution is either dried or taken directly for characterization.

Electrochemical ExfoliationAC voltage/555 timer

Graphite rod Graphite rod

Electrolyte

Fig6: A basic setup for the process is illustrated in the following figure.

Fig7: Schematics of the exfoliation mechanism for the peroxide electrolyte. The mechanism may differ depending upon the nature of ions/electrolyte.

K.S.Rao , J.Senthilnathan, Y.F.Lui, M.Yoshimura, Role of peroxide ions in formation of graphene nanosheets by electrochemical exfoliation of graphite, Scientific reports 4, Article number:4237, 2014

Electrochemical Exfoliation

Advantages:• High yield.• Scalable to industrial level.• Easy to operate and relatively a faster approach.• Eco-friendly.• FLG can be easily obtained.• The graphene obtained can be functionalised depending upon

electrolyte and hence can be more compatible with certain organic compounds or polymers.

Limitations:• The impurities may be present in the form of unwashed salts in

between the graphene layers.• This may affect the conductivity of the graphene.• The thickness control is not as promising as in the epitaxial graphene

growth or the CVD process.

N-doped Graphene

• The electronic properties of graphene doped with nitrogen or boron are enhanced similar to the case of doped Si.

• The doping depends upon the type of graphene used, hence depends upon the initial process used for the synthesizing the graphene.

• Till now various methods are used for synthesizing N-doped graphene, some of them are as follows:

1) Co-growth of ammonia and methane by CVD.

2) Arc discharge of graphite.

3) Annealing of GO with ammonia.

4) Treating graphene with nitrogen plasma.

Thank You