TABLE OF CONTENTS
1.1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.1 Carbon Materials . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.2 Forms of carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1.3 Mass production of graphene . . . . . . . . . . . . . . . . . . . . 4
1.1.4 Problem statement . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.5 Scope of research . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1.6 Research Objectives . . . . . . . . . . . . . . . . . . . . . . . . 5
2 6
2.1 SCOPE OF THE PRESENT INVESTIGATIONS . . . . . . . . . . . . 6
2.2 GRAPHENE: AN OVERVIEW . . . . . . . . . . . . . . . . . . . . . . 8
2.3 Synthesis Methods of Graphene . . . . . . . . . . . . . . . . . . . . . . 9
2.3.1 Mechanical exfoliation of graphite crystals . . . . . . . . . . . . 10
2.3.2 Arc discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.3.3 Epitaxial growth on silicon carbide . . . . . . . . . . . . . . . . 10
2.3.4 Exfoliation of graphite oxide . . . . . . . . . . . . . . . . . . . . 11
2.3.5 Chemical Vapor Deposition (CVD) . . . . . . . . . . . . . . . . 11
2.4 Types of graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.4.1 Stability of graphene . . . . . . . . . . . . . . . . . . . . . . . . 14
2.5 Structure of Graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.5.1 Electronic structure of Graphene . . . . . . . . . . . . . . . . . 15
2.5.2 Phonon in graphene . . . . . . . . . . . . . . . . . . . . . . . . 19
2.5.3 Raman Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . 21
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2.5.4 Quantum Hall Eect . . . . . . . . . . . . . . . . . . . . . . . . 26
2.5.5 Ballistic conductivity . . . . . . . . . . . . . . . . . . . . . . . . 28
2.6 Properties of graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.6.1 Electrical and electrochemical properties . . . . . . . . . . . . . 30
2.6.2 Electronic transport . . . . . . . . . . . . . . . . . . . . . . . . 31
2.6.3 Magnetic properties . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.6.4 Polymer composite . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.6.5 Surface area of graphene . . . . . . . . . . . . . . . . . . . . . . 33
2.6.6 Surface and sensor properties . . . . . . . . . . . . . . . . . . . 35
2.6.7 Electrodes in solar cells . . . . . . . . . . . . . . . . . . . . . . . 37
2.6.8 Hydrogen Storage . . . . . . . . . . . . . . . . . . . . . . . . . . 38
2.6.9 Support membrane for transmission electron microscopy . . . . 39
2.6.10 Binding of DNA nucleobases and nucleosides . . . . . . . . . . . 39
2.6.11 Molecular sieves . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2.6.12 Graphene the emergence of new silicon . . . . . . . . . . . . . . 40
2.6.13 Graphane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3 42
3.1 EXPERIMENTAL AND RELATE ASPECTS . . . . . . . . . . . . . . 42
3.1.1 Synthesis and characterization of graphene . . . . . . . . . . . . 42
3.2 Characterization Techniques . . . . . . . . . . . . . . . . . . . . . . . . 43
4 45
4.1 RESULTS AND DISCUSSION . . . . . . . . . . . . . . . . . . . . . . 45
4.1.1 Synthesis and characterization of graphenes . . . . . . . . . . . 45
4.1.2 FESEM analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.1.3 Raman spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . 49
4.1.4 TEM analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
5 54
5.1 CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
5.2 Future Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
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CHAPTER ONE
1.1 INTRODUCTION
1.1.1 Carbon Materials
Group IVA, consists of carbon (C), silicon (Si), germanium (Ge), tin (Sn) and lead (Pb).
Carbon is the chief constituent of coal, and it forms the backbone of the hydrocarbon
molecules in oil and natural gas. The element, carbon, is one of the most versatile
elements in the periodic table in terms of the number of compounds it may form.
Carbon also occur widely in carbonate rocks, such as limestone, dolomite and marble.
Basically, carbon has 3 allotropes i.e. diamond, carbon nanotubes and fullerene. Each
of these carbon allotropes has dierent features due to the bonding between carbon
atoms. Carbon has four valence electrons with an electronic conguration of 1s22s22p2.
It may form virtually an innite number of compounds. This is largely due to the types
of bonds it can form and the number of dierent elements it can join in bonding.[1]
Carbon Bonding
Bonding in any element will take place with only the valence shell electrons. Carbon
may form single, double and triple bonds. The valence shell electrons are found in
the incomplete, outermost shell. In the ground state (lowest energy state), two of the
electrons are in the 1s orbital (K shell), two are in the 2s orbital (L shell) while the
third pair is in the 2p orbital (L shell). The 1s electrons are considered to be the core
electrons and are not available for bonding. There are two unpaired electrons in the
2p orbitals, so if carbon were to hybridize from this ground state, it would be able to
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Figure 1.1: Orbital diagram of carbon at ground state.
form at most two bonds. Recall that energy is released when bonds are formed, so
it would be to carbon’s benet to try to maximize the number of bonds it can form.
For this reason, carbon will form an excited state by promoting one of its 2s electrons
into its empty 2p orbital and hybridize from the excited state. By forming this excited
state, carbon will be able to form four bonds. Since both the 2s and the 2p orbital are
half-lled, the excited state is relatively stable.[1]
1.1.2 Forms of carbon
Carbon sits directly above silicon on the periodic table and therefore both have 4
valence electrons. However, unlike silicon, carbon’s 4 valence electrons have very similar
energies, so their wavefunctions mix easily facilitating hybridization. In carbon, these
valence electrons give rise to 2s, 2px, 2py, and 2pz orbitals while the 2 inner shell
electrons belong to a spherically symmetric 1s orbital that is tightly bound and has
an energy far from the Fermi energy of carbon. For this reason, only the electrons in
the 2s and 2p orbitals contribute to the solid-state properties of graphite. This unique
ability to hybridize sets carbon apart from other elements and allows carbon to form
0D, 1D, 2D, and 3D structures.[2]
Diamond
The three dimensional form of carbon is diamond. It is sp3 bonded forming 4 covalent
bonds with the neighboring carbon atoms into a face-centered cubic atomic structure.
Because the carbon-carbon covalent bond is one of the strongest in nature, diamond has
a remarkably high Youngs modulus and high thermal conductivity. Undoped diamond
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has no free electrons and is a wide band gap ( 5:5eV) insulator. The exceptional
physical properties and clever advertising such as \Diamonds are forever\ contribute
to its appeal as a sought after gem. When properly cut and polished, it is set to make
beautiful pieces of jewellery. One of the most famous of these is the Hope Diamond. For
many of the large, high quality crystals used to make jewelry, diamond must be mined.
The smaller defective crystals are used as reinforcement in tool bits which utilize its
superior hardness for cutting applications. [2] The high thermal conductivity of dia-
mond makes it a potentially useful material for microelectronics where heat dissipation
is currently a major problem. However, diamonds scarcity makes this unappealing.
To this end, scientists and engineers are trying to grow large diamond wafers. One
method to do so is chemical vapor deposition (CVD) where solid carbon is deposited
from carbon containing gases such as methane or ethylene. By controlling the growth
conditions, it is possible to produce defect free diamonds of limited size.[2]
Fullerenes and carbon nanotubes
More exotic forms of carbon are the low dimensional forms known as the fullerenes
which consist of the 0 dimensional C60 molecule and its 1 dimensional derivative, carbon
nanotubes. A single walled carbon nanotube is a single layer of graphite, referred to
as graphene, rolled into a cylindrical tube with a 1nm diameter Carbon nanotubes
can be metals or semiconductors and have mechanical properties similar to diamond.
They attracted a lot of attention from the research community and dominated the
scientic headlines during the 1990s and early 2000. This interest in nanotubes was
partly responsible for the resurgent interest in graphene as a potentially important and
interesting material for several applications.[2]
Graphene and Graphite
Graphene and Graphite are the two dimensional sp2 hybridized forms of carbon found
in pencil lead. Graphite is a layered material formed by stacks of graphene sheets sepa-
rated by 0:3 nm and held together by weak van der Waals forces. The weak interaction
between the sheets allows them to slide relatively easily across one another. This gives
pencils their writing ability and graphite its lubricating properties, however the nature
of this interaction between layers is not entirely understood. Another frictional eect
3
believed to be important is the registry of the lattice between the layers. A mismatch
in this registry is believed to give graphite the property of superlubricity where the
frictional force is reduced considerably. A single 2-D sheet of graphene is a hexagonal
structure with each atom forming 3 bonds with each of its nearest neighbors. These
are known as the bonds oriented towards these neighboring atoms and formed from 3
of the valence electrons. These covalent carbon-carbon bonds are nearly equivalent to
the bonds holding diamond together giving graphene similar mechanical and thermal
properties as diamond. The fourth valence electron does not participate in covalent
bonding. It is in the 2pz state oriented perpendicular to the sheet of graphite and
forms a conducting band. The remarkable electronic properties of carbon nanotubes
are a direct consequence of the peculiar band structure of graphene, a zero bandgap
semiconductor with 2 linearly dispersing bands that touch at the corners of the rst
Brillouin zone .[16] Bulk graphite has been studied for decades but until recently there
were no experiments on graphene. This was due to the diculty in separating and
isolating single layers of graphene for study.[2]
1.1.3 Mass production of graphene
The main problem with graphene is to nd a way to produce graphene in/on a large
scale and at a low cost, consequently the full potential of graphene for applications
will not be realized until their growth can be further optimized and controlled. Re-
producibility of the graphene production is also another problem studied by many
researchers. Among the dierent techniques that have been applied for the mass pro-
duction of graphene, chemical vapor deposition (CVD) appears to be the most promis-
ing method owing to its relatively low cost and potentially high yield production. The
CVD method seems to be the best because of the lower reaction temperature. Their
future use will also strongly depend on the development of simple, ecient and inex-
pensive technologies for large scale production.
1.1.4 Problem statement
There has been great progress in both the production and application of graphene since
it discovery in 2004. Till now, graphene has been commonly synthesized using four
4
dierent methods namely, arc discharge, epitaxial growth on silicon carbide, exfoliation
of graphite oxide and mechanical exfoliation. The chemical vapor deposition (CVD)
method has shown to be a promising method to synthesize graphene on a large scale.
However, the problems encountered in the CVD method are the many factors that
in uence the production of the dierent forms of graphene such as types of catalyst,
carbon source, ow rate of precursors and the operating temperature. Among these
parameters, the types of catalyst and carbon source are the most critical factors in u-
encing the types and structures of graphene produced. Hence, a detailed study on the
eect of the types of catalyst and carbon source on the formation of dierent types
and structure of graphene will be undertaken.
1.1.5 Scope of research
The scopes of this study are listed as below:
1. To prepare series of substrate supported catalysts.
2. To synthesize graphene from dierent precursors via chemical vapor deposition
(CVD).
3. To characterize the as-synthesized graphene using:
a Field Emission Scanning Electron Microscopy (FESEM),
b Atomic Force Microscopy (AFM),
c Raman spectroscopy,
d Transmission Electron Microscopy (TEM).
1.1.6 Research Objectives
The objectives of this research are:
1. To synthesize graphene using dierent types of transition metals as catalysts and
dierent carbon sources by the chemical vapor deposition (CVD) method.
2. To characterize the as-synthesized graphene samples.
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