SYNTHESIS OF GRAPHENE

BY CHEMICAL VAPOUR DEPOSITION

Manish Shekhar, Nishit Taparia, Prabhakar Dwivedi,

Piyush Mohanty, Manohar Harsha Karigerasi, Prakhar Varshney

INTRODUCTION TO GRAPHENE

We are familiar with allotropes of carbon like diamond, graphite and Buckminster fullerene. The most recently

developed allotrope of carbon, which has managed to make waves across the scientific community round the

world is graphene. Graphene’s discovery was so important that Andre Geim and Konstantin Novoselov at the

University of Manchester won the Nobel Prize in Physics in 2010 "for groundbreaking experiments regarding

the two-dimensional material graphene".

Graphene is one layer thick, two dimensional hexagonal arrangement of atoms which contain planar

arrangement of sp2 bonded carbon atoms.

The carbon-carbon bond length in graphene is 0.142 nm and the interplanar distance between two layers of

graphene is 0.335 nm.

PROPERTIES OF GRAPHENE

Due to the unique structure of graphene, it has got many useful properties. Some of the properties are listed

below:-

ELECTRONIC PROPERTIES

One of the most important properties of graphene is its high electrical conductivity. Carbon has 4 valence

electrons in its last shell. In graphene each carbon atom is bonded to three other carbon atoms in a plane. This

leaves one electron freely in the third dimension which is responsible for electrical conductivity of graphene.

These free electrons form pi orbitals which are free to overlap and their bonding and anti-bonding dictates the

electronic properties of graphene.

MECHANICAL STRENGTH

Graphene is the strongest material ever discovered. Its ultimate tensile strength is 130 GPa while it is only 0.4

GPa for A36 structural steel. In addition to that, graphene is also very light. It is estimated that if a football field

is to be covered with one layer thick sheet of graphene, it would weigh less than 1 gram! Also graphene contains

elastic properties as it has a Young’s modulus of 0.5 TPa.

OPTICAL PROPERTIES

A single one-atomic graphene layer has the ability to absorb a spectrum as high as 2.3% of white light.

Graphene is quasi two-dimensional, since electrons can only move between the carbon atoms in the 2-D lattice.

This confinement of electrons in the 2-D structure gives rise to novel properties associated with graphene.

Conduction of electricity with graphene sheets acting as single charge carriers is caused due to the interaction of

these quantum confined electrons with the carbon atoms of graphene. Many complex interactions between the

electrons and the structure of graphene make it transparent, flexible and strong. These properties, among many

others, have compelled various researchers around the world to focus their efforts on unravelling the mystery

surrounding this element.

STRUCTURE OF GRAPHENE

Graphene has a hexagonal honeycomb structure which comprises of a 2-D sheet of carbon atoms, held together

by sp2 bonds, which are separated by an atomic distance of approximately 1.4 angstroms. To form graphite, at

least 10 to 100 sheets of graphene are stacked on top of each other with an interplanar distance of approximately

3.4 angstroms, and are held together by weak Van-der-Waal forces .

Analyzing the honeycomb lattice of graphene with a two atom unit cell as a Bravais lattice, we find that there

are two possible paths along which we can move on a honeycomb lattice - these are known as the ‘zig-zag’ and

‘armchair’ because of the appearance of the resulting jagged edge along the path.

The orientation of the lattice, especially whether the path of the electrons, is along the zig-zag direction or the

armchair direction has important effects on the properties of graphene.

GENERAL METHODS OF GRAPHENE SYNTHESIS

WHY CVD IS PREFERRED OVER OTHER PROCESSES

High quality graphene can be produced by mechanical exfoliation of graphite , but the yield is low and size of the

graphene flakes is comparatively small . Large area and high quality graphene can be produced by graphitization

of SiC crystals at elevated temperatures (1300 OC) ; however, the graphene films cannot be transferred to

arbitrary substrates and growth yields multi-layer as well as mono-layer graphenes. Graphene with large area

and high quality can be grown by thermal chemical vapor deposition (CVD) on catalytic transition metal

surfaces such as nickel and copper.

The synthesis of CVD graphene synthesis has following advantages:

It can be accomplished at a comparatively low temperature (i.e., ~1300 K or lower, which is very much

lower than the temperature needed for SiC sublimation, i.e., 1900-2300 K).

Due to catalyst-assisted defect healing , a very high quality mono-layered or multi-layered graphene is

easily synthesizable.

It can be readily synthesized on a very broad area (e.g., 100-1000 square inches).

Synthesized graphene can be readily transferred into other substrates for additional processing, utility and

accessibility in integrated circuits.

There are many tunable experimental parameters (type of catalyst, pressure and type of feedstock and

carrier gases, temperatures, etc.) . With these parameters, we can easily develop more than 10 millions of

combinations of them or specific experiments (for example, there are more than 20 discreet catalysts,

more than 10 discrete carrier gases, 10 discrete temperatures from 1000-1400 K, 10 discrete pressures

from ultra low limit to ambient pressure, and 10 discrete ratios between H2 gas in carrier and carbon rich

gas in the gas flow). It is practically impossible to search for the favorable condition of graphene CVD

growth in so large a subset of conditions. So, the favorable synthesis of graphene can only be lead by a

profound insight into the mechanism.

Therefore, we come to the conclusion that Chemical Vapor Deposition is the most effective way to synthesize

high quality graphene in large quantity.

In our case study, we focus on the growth of graphene using chemical vapor deposition. In this method, the

carbon atoms stick to the surface of the substrate metal at high temperatures. The carbon atoms which have

already occupied a position on the substrate surface prevent other carbon atoms from sticking on top and push

them aside, creating a one atom thick layer of atoms on the surface of the substrate. On lowering the

temperature, the carbon atoms crystallize to form a layer of graphene.

Recently a great boom is observed in the synthesis of graphene through Chemical Vapor Deposition on different

metal substrates. Among the metal substrates, Nickel(Ni) and Copper(Cu) are found to give best results.

DIFFERENCE BETWEEN CVD ON NICKEL AND COPPER

Graphene grown on Nickel using Chemical Vapor Deposition is non-uniform and multilayered. It contains

patches of thickness varying from 1 to 4 layers and size varying from 3-10 microns. The layers are stacked in AB

stacking order. During the process, carbon atoms are dissolved into the bulk and come to the surface while

cooling the nickel foil which leads to the formation of multilayered graphene.

Unlike graphene grown on nickel, graphene grown on copper is monolayered and uniform. Instead of

multilayers, on copper, graphene is grown as grains or domains with slightly different lattice orientations.

Solubility of carbon in copper is very less, so carbon atoms are adsorbed on the surface and join to form a single

layer of graphene.

THE PROCESS OF CVD

Since it is clear from the above discussion that Chemical Vapor Deposition on thin copper films is a better way to

synthesize graphene than on nickel films, here we discuss the Chemical vapor Deposition of graphene on Copper

films.

An oversimplified version of the process is explained below :-

1. Copper films are exposed to methane(CH4) and hydrogen.

2. Catalytic dehydrogenation (decomposition) of methane into CHx and finally into C and H.

3. Depending on partial pressure of methane and hydrogen, methane flow rate and temperature, the copper

surface is either super-saturated, saturated or undersaturated with CHx.

4. Nuclei are formed in the regions of local supersaturation of CHX.

5. In case of saturated and supersaturated copper surface, graphene islands are formed by the growth of

nuclei.

6. Cu surface is partially or fully covered with graphene depending on the temperature, pressure and flow

rate.

Undersaturated : When Cu surface is undersaturated, no nucleation takes place even if CHx is present in vapor

phase or on Cu surface.

Saturated : When Cu surface is saturated, graphene nucleates and grows to a certain island size and stops

growing because the amount of CHx available on the copper surface is not sufficient to promote the attachment

of C atoms to the growing graphene island leading to partial coverage of Cu surface. In this case, the Cu surface,

the graphene islands and the vapor phase are in equilibrium.

Supersaturated : When Cu surface is supersaturated, the amount of methane is always enough for to produce

enough CHx for promoting the formation of graphene islands all over the Cu surface.

EFFECT OF FLOW RATE AND PARTIAL PRESSURE OF METHANE ON DOMAIN SIZE

For the process to take effect, temperature of the metal substrate (here Cu) should be a little less than its melting

temperature. Generally, it is 1000o C for copper. Such temperature is required for decomposition of methane and

bonding together of C atoms. Also, it is assumed that crystallographic orientation does not have any effect on the

domain formation.

High temperature with low methane flow rate (JMe) and methane partial pressure (PMe) yields a low density of

graphene nuclei and large domain size.

However, JMe and PMe have a critical value depending upon the growth system below which graphene nuclei do

not form.

Above those values, in a specific range of values, graphene growth stops before the surface is fully covered even if

the surface is continuously exposed to methane. For complete coverage, partial pressure of methane must be

increased.

The above graph shows, for PMe=285 mTorr, the surface is fully covered with graphene domains in about

1.5 minutes.

Whereas, for PMe=160 mTorr, coverage reaches a maximum of 90% at around 3 minutes and never

reaches 100%.

If we define vcoverage as the graphene coverage rate defines as the graphene area Agraphene divided by the total

copper surface area ACu per unit time, then,

And if vdomain is the average area growth rate of graphene domains, then :

where n is the domain(or nucleus) density.

This graph shows that as the graphene surface coverage area increases, growth rate dramatically decreases. The

decrease in the growth rate can be attributed to the fact that carbon species are supplied by Cu catalyzed

decomposition of methane. So, if available Cu surface is less, less C species are available.

ROLE OF HYDROGEN

CVD employs a low-pressure mixture of methane and hydrogen flowing over Cu substrate heated to a

temperature slightly below its melting point (∼1000 C). It is the most suitable technique till now for controlled

and large amount of Graphene production and it is improving as researches are going on for further production.

Detailed analysis revealed that such samples are made of randomly oriented domains in which scattering at the

boundaries lead to lower charge carrier mobilities.

Despite further improvements, many details in this process remain mysterious and require examination for

control of the graphene quality. One of such mysteries in CVD is role of Hydrogen in it. In published method, its

concentration varies from zero to thousand times the amount of methane. So, to clarify these values and to

identify routes to grow large size single domain graphene monolayers, ambient pressure for growth on Cu foil

with Argon as a buffer gas and very low partial pressure of methane (30 ppm) allowing monitoring individual

graphene domains as a function of hydrogen pressure is studied.

As per experiments, graphene growth is strongly dependent on the hydrogen contribution. It mainly does two

jobs :-

It acts as an activator for surface-bound carbon which leads to monolayer growth.

It acts as an etchant that controls the size and structure of the resulting graphene domains.

So, as a result the growth of graphene depends upon the partial pressure of hydrogen and growth attains its

maximum at certain level of partial pressure. The morphology and size of domains vary with the pressure.

Following are some observations :-

No Graphene production is observed at very low hydrogen pressures.

At intermediate pressures, growth rates are good but domains are irregular in shape with irregular edges.

Nearly perfect hexagons are observed at high pressures and growth ceases at a size which is dependent

on the hydrogen pressure. Raman analysis shows that edged are of zigzag symmetry if structure have

high stability.

Following figure shows variation of shape with partial pressure of H2.

As solubility of carbon in Cu is very low, which makes it different from other catalytic metal surfaces.

Chemisorption of methane on Cu with formation of species like CHx. These species agglomerate into multimeric

species ultimately leading to graphitic carbon.

When Cu surface is already covered with carbon, next layer formation is very slow, almost at negligible rate.

Because of that and low concentration of methane in the feed, no growth of graphene is observed. So, some

additional co-catalyst is needed i.e. hydrogen.

The etching effect of hydrogen proceeds not only during growth but also during cooling of sample after

deposition. Etching noticeably occurs for graphene on Cu is no less than 850 C. Growth of graphene at low

hydrogen pressure has irregular shaped grains but their annealing for 30 minutes gives clear 1200 edges.

Thus, hydrogen plays a dual role in the process of graphene growth by CVD. Firstly, it acts as co-catalyst for the

formation of active surface bound carbon species required for graphene growth. Secondly, it controls the grain

shape by etching away the weak C-C bonds.

ROLE OF KINETICS

In this section , we focus upon the role kinetics has to play in the CVD processes. Particularly , we will observe

the pressure of the reaction chamber in the CVD synthesis of graphene . In brief the kinetic models of APCVD as

well as LPCVD are presented thereby providing insights for evaluating the difference between APCVD vs

LPCVD/UHCVD graphene synthesis.

Evaluation and differentiation between the Kinetics related processes between the aforementioned syntheses

APCVD and LPCVD for the growth of Graphene using Cu Catalyst :

We assume that the thermodynamic status of Graphene synthesis using the metal Copper as a base catalyst at a

particular temperature in a CVD is constant throughout , even though one may use the AP-,LP- or UHV

conditions. However , upon extensive scrutiny and experimental revelations , it has been established that the

kinetic processes are quite different. The kinetics of the CVD process has very important ramifications on

the rate of growth and deposition of the Graphene films, thickness uniformity across significantly large areas and

the very important part , the density of the defects that must have crept in.

To emphasize the differences in the kinetics associated with the aforementioned techniques (AP,LP,UHV) CVD

processes , we draw upon previous CVD models and modify them to account for synthesis of graphene using low

carbon solid solubility catalyst. We must mention and caution here that we are discussing have very low

carbon solubilities, resulting in diffusion of carbon either on surface or limited to a few nanometers below the

surface. An interesting fact worth mentioning here is that present development is undergoing with regards to

Kinetic models for Graphene synthesis using catalysts with intermediate to high carbon solid solubilities.

At the beginning of the analysis of the kinetic model we have a system in which a steady state gas flow of a

mixture of methane, hydrogen and argon gases on the surface of a Cu catalyst at a synthesis temperature is

taken .

The path which the carbon species endure are as listed below :

1) Diffusion through the boundary layer and reaching the surface, where they get

2) Adsorbed on the surface

3) Decompose to form active carbon species

4) Diffusion into the catalyst close to the surface and form graphene lattice

5) Inactive species get desorbed from the surface, form molecular hydrogen

6) And diffuse away from the surface through the boundary layer and eventually swept away by the bulk of

the continual gas flow.

The above mentioned processes can be classified into 2 regions :

mass transport regime which extensively involves diffusion through the boundary layer

the surface reaction regime.

Overall , we can also deduce from the above situation that there are two fluxes of the active species that coexist.

The equations for these fluxes are given by :

F( mass transport) = H(g)C(g)-C(s)

F(surface reaction)=K(s)C(s)

Where F(m t) is the flux of the active species through the boundary layer and F(s r) is the flux of the consumed

active species at the surface (*assuming first order kinetics), H(g) is the mass transport coefficient , C(g) is

the concentration of gas in bulk, and C(s) is the concentration of the active species at the surface. These fluxes

are in series, and the slower of the two processes is the rate-limiting step during graphene synthesis.

It can also be interesting to know that at steady state we can state the two fluxes are equal to each other and

the value of both of them being equal to the total flux.

The total flux can also be re-written, after a deliberate mathematical calculation as:

F(Total Flux)=K(s)H(g)/(K(s)+H(g))C(g).

Mathematical observations can bring about three conditionalities :

H(g)<<K(s)( mass transport limited region)

H(g)>>K(s)(surface reaction controlled region)

H(g) ∼K(s)(Mixed region) .

It has been determined that at high temperatures under typical APCVD conditions, mass transport

through boundary layer is limiting and vice versa for the other region under LP and UHV conditions.

Thus the role of kinetic processes in graphene syntheses using low carbon solid solubility catalysts under APCVD

and LPCVD conditions has been elucidated.

EFFECT OF CRYSTALLOGRAPHY AND ITS ENERGETICS

The carbon atoms available for building the graphene islands are obtained through decomposition of methane.

In this decomposition process, initial state is the adsorbed CH4 and final state is C atoms and four H atoms on Cu

surface.

As shown in the figure below, all the above steps are endothermic and the corresponding activation energy

barrier range from 1.0 eV to 2.0 eV.

For Cu(111), the schematic representation of decomposition of methane on Cu surface is shown in the diagram

below.

Activation energies, some key geometric parameters and reaction energies for methane decomposition on

Cu(111) are listed in the table below.

The final product (C+4H) has higher energy by about 3.60 eV than initial CH4, therefore atomic carbon is very

unfavourable on Cu surface energetically.

In most of the CVD experiments, copper foil is used. So, different crystallographic surfaces can be exposed.

Studying the dehydrogenation process on Cu(100), the geometric representation is shown below.

And the corresponding energies and geometric data is given below :

Adsorption energy of atomic carbon on the surface Cu(100) is 6.08 eV which is 1.23 eV more stable than that on

a Cu(111) surface. This is because the Cu coordination number of carbon atom is higher on Cu(100) surface

where it is 4 while on the Cu(111) surface it is 3.

NUCLEATION THERMODYNAMICS

Small carbon containing species are not stable without hydrogen. They only become stable without hydrogen at

a certain size. At this point, smallest graphene is formed. Here is a thermodynamic analysis to estimate when this

nucleation can occur.

References: Po=1 atm T=1300K ½ of the energy of H2 molecule

The expression for chemical potential of hydrogen at Po in eV,

Under arbitrary pressure,

For methane,

Taking ½ of H2 energy and C atom energy as a reference, the energy of CH4 is calculated as -9.234 eV. So,

If methane and hydrogen are fed simultaneously, then relation between chemical potential of carbon and

hydrogen atoms at CH4 and H2 equilibrium,

= ratio of methane and hydrogen partial pressure.

When chemical potential of a surface carbon species is higher than , it is not stable and will react with H2.

Considering stability on Cu (111) surface. Isolated atomic carbon atom has a chemical potential of -4.85 eV

(approximating by its adsorption energy). Under typical pressures, it is much higher than . Therefore, atomic

carbon is not stable, according to the energetics results. Also for Cu (100) surface, C is also unstable at typical

experimental conditions. Carbon chemical potential is -6.08 eV for atomic carbon on the (100) surface.

An adsorption energy correction is added to the chemical potential of a free graphene(7.85 eV) to get the

chemical potential of graphene adsorbed on copper surface. The lattice of graphene is extended to match the

lattice between Cu atoms. The calculated adsorption energy for (111) and (100) is 0.01 eV/C. So, in most cases

source gases have higher chemical potential than graphene. Thermodynamically, this is the driving force to grow

graphene. Also, graphene becomes unstable at very high pressures.

Infinite two dimensional carbon sheets are stable on the surface, whereas single carbon atoms are unstable.

However, the transition from single carbon atoms to large sheets happens via the nucleation growth process. The

potential energy of a cluster of ‘n’ carbon atoms adsorbed on the surface of Cu can be given as :

where Ecu is the energy of the adsorbed system; Esurf is the energy of the copper surface and EC is the energy of

the isolated carbon atom in vacuum.

Controlling the size of nucleation is an important factor in controlling the graphene sample quality and its

productivity. The growth of graphene rate decreases with increase in H2 concentration. This is caused due to the

smaller nucleation size at higher CH4/H2 ratio. Lower CH4/H2 ratio leads to lower nucleation density and

improved quality of single layer graphene.

TRANSFER ON ARBITRARY SUBSTRATE

Another advantage of graphene synthesis through CVD is that it is relatively easy to transfer it to any other

substrate. Once graphene has been formed on copper foil, it is cooled. Polymers such as polydimethylsiloxane

(PDMS) or polymethyl methacrylate (PMMA) are then spin coated onto the graphene to provide support, and

then FeCl3 acts as an etchant which removes copper. The graphene now is attached only to polymer, which helps

to position graphene on other substrate such as a solar cell. Polymer can be dissolved by a solvent, which leaves

only graphene on the substrate.

CONCLUSION AND FUTURE CHALLENGES

To wrap up our analysis, we can finally conclude that while on most metal surfaces graphene may grow using

monoatomic C atoms, but on Cu, graphene is synthesised in the form of small cluster of C atoms. This can be

attributed to the low solubility of C atoms in Cu surface, which leads to low density of C atoms available for

nucleation. Also, nucleation size at different experimental conditions should be manually controlled to improve

the graphene sample quality as it directly influences the productivity of the graphene being synthesize

There are many challenges which are being faced in the synthesis of graphene. Some of them are listed below

Required quality of graphene has to be made cheap enough for practical applications.

Such thin and delicate material requires technology to handle it.

Graphene is the strongest material discovered, but a lot of work is yet to be done on the technology for

using it in structural applications like air crafts.

Additional research is required for its application in fabrication, optical materials etc.

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