Growing Graphene via Chemical Vapor

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Growing Graphene via Chemical Vapor

Deposition

Benjamin Pollard

Department of Physics, Pomona College

May 2, 2011

Graphene, a two-dimensional nanoscale allotrope of carbon, is a promising

material with many useful properties, including those of light transparency and

electrical conductivity. Over the past few years research on graphene increased

dramatically because of new methods to produce and study it. Since then re-

searchers have proposed uses for graphene ranging from flexible touch screens to

vacuum membranes. Many of these proposals rely on graphene grown via chemi-

cal vapor deposition (CVD), a relatively new technique for producing large-area

films of contiguous, multi-domain graphene. Once created, CVD graphene is

transferable to diverse substrates, making the technique versatile for many ap-

plications. One such application is as an electrode in an organic solar cell. This

investigation explores the growth of graphene via chemical vapor deposition on

copper and the subsequent transfer of that graphene to a silicon substrate, keep-

ing in mind a potential application as a transparent electrode in an organic solar

cell.

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Contents

1 Background 5

1.1 Carbon Allotropes . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2 Properties of Graphene . . . . . . . . . . . . . . . . . . . . . . . 8

1.3 Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.4 Previous work at Pomona College . . . . . . . . . . . . . . . . . . 12

2 Theory 15

2.1 Structure of Graphene . . . . . . . . . . . . . . . . . . . . . . . . 15

2.2 Graphene Growth Domains . . . . . . . . . . . . . . . . . . . . . 16

2.3 Electrical Properties of Graphene . . . . . . . . . . . . . . . . . . 17

2.4 Optical Properties of Graphene . . . . . . . . . . . . . . . . . . . 19

3 Fabrication and Measurement Techniques 20

3.1 Exfoliation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.2 Chemical Growth Methods . . . . . . . . . . . . . . . . . . . . . 21

3.3 Chemical Vapor Deposition . . . . . . . . . . . . . . . . . . . . . 23

3.4 Transfer Process . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.5 Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4 Experimental Methods 28

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4.1 CVD Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4.2 Transfer Process . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

4.3 Characterization and Results . . . . . . . . . . . . . . . . . . . . 31

5 Discussion and Future Work 43

6 Acknowledgements 44

List of Figures

1 Macroscale photos and atomic structure diagrams of diamond

and graphite. Image in the public domain, under the Creative

Commons License, commons.wikimedia.org. . . . . . . . . . . . 6

2 Depictions of graphene, graphite, carbon nanotubes and bucky-

balls (adapted from [15]). . . . . . . . . . . . . . . . . . . . . . . 7

3 Diagram of a solar cell. . . . . . . . . . . . . . . . . . . . . . . . . 11

4 Diagram of the band structure of ITO. . . . . . . . . . . . . . . . 13

5 Diagram showing the graphene lattice unit cell. [14] . . . . . . . 15

6 Diagram showing armchair and zig-zag cuts along a graphene

lattice. [13] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

7 Fermi surface showing Dirac Cones and the zero-gap nature ofgraphene [14]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

8 Diagram of CVD growth on copper. [26] . . . . . . . . . . . . . . 23

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9 Schematic diagram of the transfer process to an arbitrary substrate. 26

10 Photo of copper foil after graphene growth. . . . . . . . . . . . . 28

11 Photographs of growth furnace setup. . . . . . . . . . . . . . . . 29

12 Optical microscope image of graphene on copper foil. . . . . . . . 31

13 Optical microscope image of graphene on copper foil. . . . . . . . 32

14 SEM image of copper domains, 200m. . . . . . . . . . . . . . . 32

15 SEM image of copper domains, 20m. . . . . . . . . . . . . . . . 33

16 SEM image of graphene domain boundary, 2m. . . . . . . . . . 34

17 SEM image of copper steps, 2m. . . . . . . . . . . . . . . . . . . 34

18 SEM image of black spots, 5m. . . . . . . . . . . . . . . . . . . 35

19 Raman Spectrum of graphene on copper, UC Riverside. . . . . . 36

20 Raman Spectrum of graphene on copper, Pomona. . . . . . . . . 36

21 Raman Spectrum of graphene on SiO2, UC Riverside. . . . . . . 37

22 Optical thickness measurement of drop-cast PMMA. . . . . . . . 38

23 Optical thickness measurement of spin-cast PMMA. . . . . . . . 39

24 Microscope images of SiO2 wafer with evaporated PMMA over

time. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

25 Photo of SiO2 wafter after graphene transfer. . . . . . . . . . . . 42

26 Microscope image of SiO2 wafer after graphene transfer. . . . . . 42

27 Microscope image of SiO2 wafer after graphene transfer. . . . . . 43

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1 Background

1.1 Carbon Allotropes

Carbon, one of the most common atoms on Earth, occurs naturally in many

forms and as a component in countless substances. However, there are only a

handful of materials made solely of carbon and nothing else. These are called

allotropes of carbon. Two of these carbon allotropes have been collected from

nature and used by humans for centuries; they are graphite and diamond (see

Figure 1). Carbon can also occur as an unordered mess of atoms; this is called

amorphous carbon and will not be covered here. A related form, also outside the

scope of this investigation, is glassy carbon which has a semi-ordered structure

with bonds resembling other forms. Finally, there are three nanoscale forms

of carbon that have attracted widespread attention over the last half-decade

because of their novel properties. These carbon nanostructures are called buck-

yballs, carbon nanotubes, and graphene.

Diamond is the most stable form of pure carbon. Formed under high tem-

peratures and pressures under the earths crust, diamond is a tetrahedral lattice

with a carbon atom at each vertex. Each carbon atom thus forms four covalent

bonds with four neighboring atoms, completely filling its outer electron shell and

resulting in one of the hardest and most valued substances in human history.

Pure diamond has a wide bandgap and thus acts as a transparent insulator,

which as a single crystal gives diamond its dazzling optical properties. Impuri-

ties and dopants in diamond lead to other colors and bandgaps, giving rise to

rarer gemstones and specialized research respectively.

All other allotropes of carbon can be conceptualized as variations on the

lattice structure of graphene (see Figure 2). Graphene is actually the most recent

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Figure 1: Macroscale photos and atomic structure diagrams of diamond andgraphite. Image in the public domain, under the Creative Commons License,commons.wikimedia.org.

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Figure 2: Depictions of graphene, graphite, carbon nanotubes and buckyballs(adapted from [15]).

carbon nanomaterial to be widely studied, but its basic structure is simple.

Consider a 6-carbon ring of atoms, and then tessellate that hexagon to form a

2D hexagonal lattice similar to the surface of a honeycomb. Such a 2D sheet is

known as graphene. Graphenes properties are striking in a number of respects,

but perhaps most notable is that a single graphene sheet is quite stable and

mechanically resilient, as well as very electrically conductive.

By far the most common form of pure carbon is graphite. Graphite is

simply many layers of graphene stacked on top of each other. While each sheet

is tightly bound, only weak bonds exist between layers. This enables the layers

to slide laterally, making graphite slippery. Thus, graphite is commonly used

as a lubricant. Graphites other common usage is as the core of a pencil, where

flakes of graphite slide off the bulk material and remain as a mark on paper.

Carbon nanotubes (CNTs) are another nanoscale allotrope of carbon. They

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can be thought of as ribbons of graphene that have been rolled into a tube. While

only nanometers in diameter, CNTs can grow to millimeters in length. Due to

the strength of the bonds in a hexagonal carbon lattice, nanotubes are one of the

strongest fibers ever discovered. Additionally, due to the extra quantum con-

finement imposed on electrons along the circumferential axis, carbon nanotubes

can display both metallic and semiconducting electric properties. The electrical

nature of a nanotube stems from its physical shape, making CNTs intriguing

materials for pure research and numerous electromechanical applications.

Lastly, a buckyball is created by collapsing yet another dimension. Con-

ceptually, a buckyball is a small segment of a carbon nanotube that has been

pinched together at both ends to form a hollow sphere of carbon atoms. Named

after Buckminster Fuller, a architectural engineer and science-fiction writer who

designed domes with a similar shape, the 60-carbon buckyball was the first car-

bon nanomaterial to gain widespread attention. Buckyballs have many pro-

posed uses, such as encapsulation of reactive compounds in chemistry, isolation

of quantum systems to make a functional qubit, and fundamental quantum

experiments in which an entire buckyball acts as both a particle and a wave.

1.2 Properties of Graphene

Graphene is a nanoscale allotrope of carbon. Unlike graphite, the most common

allotrope, graphene is quasi-two-dimensional, since electrons can only move be-

tween carbon atoms in the 2D lattice. The extra quantum confinement of the

electrons due to the lack of a third dimension gives graphene various novel prop-

erties. For example, electrons interact with carbon atoms in the lattice to create

a system that acts like a single mobile charge carrier. The carrier moves ballisti-

cally over the graphene surface, enabling graphene sheets to conduct electricity

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very well. [15] Other complex interactions between electrons and the hexago-

nal lattice make graphene transparent, flexible and strong. [6] These properties

and others have compelled many researchers over the last half-decade to study

graphene for a diverse array of uses.

While graphite has been used for ages in a range of purposes from lubri-

cant to pencils, researchers only began widely studying graphene around the

year 1990. For the first decade, research was hindered due to the difficulty of

producing it in an electrically isolated environment and without defects. How-

ever, in 2004, two researchers named Andre Geim and Konstantin Novoselov at

Manchester University discovered a new method for producing graphene through

mechanical exfoliation. [19] Called the scotch tape method, the procedure is

detailed below under Exfoliation. Geim and Novoselovs discovery gave re-

searchers access to pure graphene in a number of desirable environments for

experimentation. Furthermore, they found that if silicon dioxide was used as a

substrate for the graphene flakes, the flakes appeared as a discoloration under

any optical microscope. These properties made it possible to perform electrical

and nanomechanical experiments on graphene that began to showcase the ma-

terials novel properties. The scotch tape method sparked widespread research

on graphene in many areas of physics and materials science, and won them the

Nobel Prize in Physics in 2010.

Aside from its use as a transparent electrode in an organic solar cell, graphene

has been considered for a huge range of purposes over the last half-decade. Its

mechanical strength makes it attractive as a optically transparent membrane.

Experiments on gases or substances in vacuum are envisioned in which grapheneis used as a window, preserving a closed environment while still allowing precise

optical measurements. [6] Graphene has also been proposed as a protective coat-

ing on metals such as copper to prevent corrosion. While not noticeably altering

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the appearance of the underlying metal, graphene effectively prevents oxidation

from the surrounding environment. [27] Graphene is also of interest as a simple

2D quantum system. Experiments have already been completed that show in-

teresting quantum phenomena in a clear and definitive way. The Quantum Hall

effect, now a common test for graphene purity, is one such example. [13] Fur-

ther research using graphene promises to test predictions from applied quantum

theories and provide insight into fundamental quantum physics.

1.3 Solar Cells

Among all the many exciting applications of graphene, use in organic solar

cells stands out as both important and accessible. Therefore, solar cells have

guided recent work on graphene at Pomona College as a context and end-goal

of graphene growth and transfer processes.

Fundamentally, solar cells convert photons of light into electric current.

Otherwise known as photovoltaic cells, they work by transferring the energy of

an incoming photon to a valence electron. This electron then has enough energy

to escape the confines of the atom it was bound to, leaving behind a hole of

positive charge. The electron and hole must be forced to separate spatially and

enter electrodes on opposite ends of the cell. They will then create a voltage

difference between the two electrodes, which generates a current when connected

to any electrical device drawing power.

Solar cells are commonly fabricated by depositing different materials one

after another, creating a stack in which each layer performs a specific function(see Figure 3). Special materials are required to perform each required function.

Semiconductors with precisely-tuned band gaps create an environment in which

electrons are excited by incoming photons and yet do not recombine with their

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holes before they have a chance to separate. This is known as the active layer.

Electrodes with specific work functions sandwich the active layer and pick up

electrons or holes. Charge separation is best achieved if the contact area between

the active layer and the electrodes is maximized, so it is best (at least for organic

cells) if the electrodes completely cover the area of the cell. However, light

must still be able to pass through the top electrode to interact with electrons

in the semiconductor. Thus, the top electrode must be both transparent and

conducting.

Figure 3: Diagram of a solar cell.

Materials that are transparent and conducting are uncommon, since typical

conductors are metals, which in turn are typically opaque. In the language of

semiconductors, this is because metals lack a band gap between valence and

conduction levels. More simply, the Fermi level in a metal is surrounded by

available electron states, so that any excited electrons can easily transition into

accessible higher-energy states and move between atoms in the crystal lattice

[11]. This is why metals are good conductors of heat and electric current. That

same freedom of electron mobility also makes metals opaque. When a photon

of light enters a metal, valence electrons can easily absorb that photon and

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temporarily enter a higher-energy state, ultimately settling back to the valence

band and releasing a phonon. Therefore light is quickly absorbed by the metal

and turned into heat energy instead of passing through uninterrupted.

Conventional organic solar cells use thin films of a material called indium tin

oxide (or ITO) as a transparent conducting electrode. ITO is a wide-bandgap

n-type semiconductor. Its bandgap is larger than the energy of visible light, so

electrons in the material cannot absorb those photons and thus ITO is visibly

transparent. Conduction in ITO is achieved through loosely-bound electrons

from oxygen atoms creating vacancies which are filled by nearby electrons in

the main indium lattice, creating energy levels called impurity states close to

the conduction band (see Figure 4). [8] Because of these special properties, ITO

is widely used in liquid crystal displays and touch screens in addition to solar

cells. Indium, however, is a rare metal, and given its increasing use due to the

rise of these technologies, many fear a worldwide shortage of indium in the near

future. Even now, the price of indium is increasing drastically. Furthermore,

ITOs transparency and conductivity are imperfect, and an increase in one comes

at the cost of the other. [22]

1.4 Previous work at Pomona College

Students under the direction of Prof. David Tanenbaum and his colleagues have

been investigating carbon nanostructure synthesis and properties for about a

decade. The lab started by investigating carbon nanotubes grown via chemical

vapor deposition with the work of Matthew Ferguson, James McFarland, Elias

Penilla and Ajoy Vase. Ian Frank (class of 2008), now a graduate student at

Harvard, transitioned to graphene by looking at exfoliated graphene with Paul

McEuen at Cornell University. Using the cheese grater approach (detailed be-

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Figure 4: Diagram of the band structure of ITO.

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low), Ian investigated suspended graphene flakes using both electromechanical

resonance and the tip of an atomic force microscope (AFM). By applying reso-

nant electric fields or pushing on the flakes with an AFM tip, Ian was able to

measure the mechanical properties of pure few-layer graphene. Subsequently,

Ian did a senior thesis at Pomona College on the scotch tape method of graphene

exfoliation (also detailed below). After establishing a setup for creating exfoli-

ated graphene at Pomona, Ian investigated the potential for optical and electron

beam lithography on it with the aim of producing graphene devices of any shape

and reasonably small size. I had the privilege of assisting Ian with the optical

lithography aspect of this work during my first year at Pomona. [13]

Scott Berkley (class of 2009) also spent two summers at Cornell University

working with Paul McEuen and David Tanenbaum. During the first summer,

Scott learned the cheese grater approach (below) of graphene exfoliation and

used the resulting suspended graphene flakes to further investigate the mechan-

ical properties of graphene with an AFM tip. [4] During his second summer at

Cornell, Scott transitioned to the scotch tape method (below). He characterized

the number of layers that could result from that method and began investigat-

ing the different interactions with laser light which arose from various numbers

of layers. [3] Scott went on to complete his thesis on organic solar cells, during

which time I worked separately on investigating an e-beam lithography system

for the electron microscope at Pomona College.

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2 Theory

2.1 Structure of Graphene

Graphene is a 2D sheet of carbon atoms arrayed in a hexagonal honeycomb

lattice. The sheet is held together with sp2 bonds between the carbon atoms

separated by a distance of about 1.4 angstroms, making the sheet quite strong. A

few such layers stacked on top of each other is still considered graphene; it takes

at least 10 layers (and in some respects more like 100) before a sample becomes

bulk graphite. There are about 3.4 angstroms between stacked sheets. [28]

Figure 5: Diagram showing the graphene lattice unit cell. [14]

The honeycomb lattice can be analyzed with a two-atom unit cell as a Bra-

vais lattice (see Figure 5). By mentally duplicating and translating this cell by

a set amount along set translation vectors, the entire lattice can be constructed.

There are two possible cuts along a honeycomb lattice; these are entitled arm-

chair and zig-zag due to the appearance of the resulting jagged edge along

such a cut (see Figure 6). The orientation of a lattice, specifically whether

a cut or a current is along the armchair or zig-zag direction, has interesting

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fundamental effects on the electronic behavior of graphene. [13]

Figure 6: Diagram showing armchair and zig-zag cuts along a graphene lattice.[13]

2.2 Graphene Growth Domains

This investigation focuses on the growth of graphene via chemical vapor depo-

sition, as described in detail below. In this process, carbon atoms adhere to

the surface of a metal substrate under high temperatures. Once a carbon atom

occupies a position on the surface of the substrate it pushes other carbons to the

side, creating a one atom thick layer of carbon. As the temperature is lowered

the carbon crystallizes into a layer of graphene. [19]

Unavoidably, the graphene crystallization will start at various places on the

surface of the substrate before the entire area has formed a lattice. Each initial

crystallization is referred to as a nucleation site, and establishes an orientation

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for the lattice that grows from it. As various crystal regions grow out from

nucleation sites, their borders will meet and a discrepancy will probably oc-

cur between the lattice orientations of each region. This will create a definite

boundary between regions. Growth stops when every region is surrounded by

such boundaries (or the edge of the substrate). At this point the regions are

called domains.

In a sense, domain boundaries represent defects in the crystal structure of

the graphene, since along those lines the bonding of the carbon atoms does not

follow the simple Bravais lattice from a repetition of the unit cell. This acts as a

barrier for charge transport phenomena and an exception to graphenes optical

properties (both discussed below). Therefore, it is desirable to maximize the

size of domains to limit the frequency of domain boundaries.

2.3 Electrical Properties of Graphene

In most conductors, the valence and conduction bands overlap, giving excited

electrons many states to occupy as they move throughout the material. Ma-

terials with this property are known as metals. Graphene, while an excellent

conductor, is not a metal but rather a zero-gap semiconductor. While the va-

lence and conduction bands do not overlap in graphene, they touch at the Fermi

level. [29] This can be seen by visualizing the Fermi surface of a 2D graphene

lattice, as in Figure 7. The Fermi surface for a lattice material is the energy

border between the valence and conduction bands in momentum space. For this

border to be defined the Fermi energy must fall inside an energy band and not

in a band gap; otherwise the valence and conduction bands do not touch at

all. Thus, Fermi surfaces only exist for conductors. Graphenes Fermi surface

consists of six double cones with the Fermi energy at the intersection of those

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cones. Because the cones are linear near this intersection the effective mass of

electrons in this region is zero (since effective mass is given by the curvature

of the energy bands in momentum space, and thus the curvature of the Fermi

surface). [12] This leads to an entirely new transport mechanism in graphene

compared to metals. The specifics of this regime rely on quantum electrodynam-

ics and Diracs relativistic equation of state. Without going into the details of

these theories, the results can be conceptualized by thinking of charge carriers in

graphene not as individual electrons, but as interacting groups of electrons that

behave as an entirely different type of particle. Called a Dirac fermion, these

charge carriers travel ballistically over the 2D surface at relativistic speeds. [25]

Because of this fundamentally different transport regime, pure graphene is able

to conduct electricity better than metals, with room-temperature resistivity on

the order of 106cm. [15]

Figure 7: Fermi surface showing Dirac Cones and the zero-gap nature ofgraphene [14].

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2.4 Optical Properties of Graphene

Aside from being incredibly conductive and strong, graphene is even more

attractive to work with because of its optical transparency. Simply because

graphene is thin, photons easily pass through it. In actuality, graphene has a

surprisingly high absorption rate for being only one atomic layer thick: 2.3% of

incident white light is absorbed by a single graphene sheet. Intriguingly, this

value is exactly equal to , where is the fine structure constant (e2/c).

This can be derived using quantum mechanical principles applying to 2D Dirac

fermions. [24]

All carbon allotropes have a particular affinity to light at specific wave-

lengths. These wavelengths correspond to the vibrational modes of sp2 carbon-

carbon bonds, such that when any higher-energy light excites the carbon ma-

terial, photons are re-radiated at those wavelengths. The technique of exciting

molecular vibrational modes and measuring re-radiated light is known as Ra-

man spectroscopy, and is the easiest and most reliable method of determining

the presence of graphene. Graphene produces two strong optical peaks in Ra-man spectra: the G peak and the D peak. The G peak is due to individual

bonds stretching and compressing, while the D peak is due to breathing modes

of the hexagonal rings of carbon atoms. They occur at 1560 and 1360 cm1

respectively. [9] Peaks can also be observed at twice those values due to the next

harmonic mode of the oscillation.

While graphenes transparency makes it difficult to see on most substrates,

a particular interaction occurs between a graphene sheet and a silicon substrate

with a 100nm layer of silicon dioxide on top that makes the graphene observable

under an optical microscope. Silicon dioxide naturally forms on the surface of a

pure silicon wafer, but a silicon dioxide layer of any desired thickness can be de-

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liberately grown thermally on a silicon wafer for use in the lab. When graphene

is deposited or transfered onto such a substrate, the index of refraction at the

surface of the silicon dioxide changes due to the graphene film. This results in

a slight discoloration, from pink to purple, in the places where graphene exists

on the wafer. [13,18]

3 Fabrication and Measurement Techniques

3.1 Exfoliation

Graphene has been the subject of intense widespread research for less than a

decade. Most of this work used graphene created by a process of mechanical

exfoliation called the scotch tape method. In this procedure, pure samples

of bulk graphite are placed on the sticky side of common adhesive tape. The

tape is pressed on a desired substrate and then peeled away. Flakes of graphene

around 50 microns wide are left on the substrate, along with chunks of graphite

and adhesive residue. The flakes can be discerned under an optical microscope

due to thin-film interference, appearing as a region of slight discoloration. [15]

The graphene left by the scotch tape method is pure and clean, which en-

ables researchers to measure its electrical and mechanical properties exactly.

However, a fair amount of time and luck are required to manually locate an ap-

propriate flake on the region exposed to the tape. This difficulty is compounded

if the graphene flake must be positioned in a certain way above or around an

existing feature on the substrate, as is commonly desired for many nanoscale

experiments. Lastly, for graphene to be used as an electrode on a solar cell it

must cover the entire surface area of the cell, which is much larger than the area

of a single flake.

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Aside from the scotch tape method, there is another method of graphene

exfoliation used extensively by Paul McEuen at Cornell University to create

suspended graphene sheets. I will refer to this method as the cheese grater

method. For this technique, pure bulk graphite is attached to the end of a

rod for support; a toothpick is sometimes used. [4] A silicon wafer (with a

silicon dioxide layer around 0.25 microns) is also prepared with trenches etched

into it using radio frequency plasma etching. The graphite is then dragged

across the trenches. The trenches, acting like a cheese grater, pull off pieces of

the graphite their corners, and flakes of few-layer graphene are pulled over the

trench. This results in exfoliated graphene suspended over a trench (up to 0.5

microns deep) which is ideal for performing mechanical and electromechanical

measurements. [7]

3.2 Chemical Growth Methods

While exfoliation produces very pure single-domain graphene with nearly ideal

mechanical and electrical properties, it has one large disadvantage. That is,

exfoliation results in graphene flakes scattered randomly on a substrate. Each

flake is on the order of only microns in size, and much of the substrate remains

uncovered. For many applications of graphene discussed earlier (including trans-

parent conducting electrodes for an organic solar cell), a contiguous covering of

graphene is needed. To produce contiguous graphene films, exfoliation cannot

be used and chemical methods are needed instead to grow graphene from carbon

atoms in another form. Common methods for chemical growth of graphene in-

clude reduced graphene oxides, molecular beam epitaxy, plasma-enhanced CVD,

and chemical vapor deposition. The first two will be briefly discussed below,

while chemical vapor deposition and plasma-enhanced CVD will be covered in

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the next section.

The technique of reduced graphene oxides is really the intersection of exfo-

liation and chemical growth methods. Exfoliated graphene flakes are oxidized,

enabling them to be suspended in aqueous solution. This solution is then passed

through a filter membrane with pores around 25 nanometers. The graphene ox-

ide flakes get caught by the membrane until the entire surface of the filter is

covered with graphene sheets. This covering can then be transfered to a more

desirable substrate. While the purity of the resulting graphene film is high,

the coverage of the film is often nonuniform. Parameters must be carefully

controlled to get the entire filter area covered. Additionally, the result of this

method is a film of graphene oxide as opposed to just graphene. Graphene oxide

films must be further treated chemically to to make them electrically conducting

instead of insulating. [2]

Epitaxial graphene is a commonly used technique for creating high qual-

ity monolayer graphene. Originally, epitaxial graphene was grown from silicon

carbide (SiC). When bulk SiC is heated to around 1500C some of the silicon

sublimates, leaving a layer of carbon behind on the surface. [10] Another method

of creating epitaxial graphene from SiC is that of molecular beam epitaxy. A

graphite filament is loaded into an ultra-high vacuum. As the filament is heated,

carbon atoms sublimate off of the graphite. These carbons form a molecular

beam in the vacuum, traveling through free space without interacting until they

land on a metallic substrate (such as iridium) and form a graphene layer. [23]

While molecular beam epitaxy produces high-quality uniform films over a large

surface, it requires an ultra-high vacuum which makes the process tedious andinaccessible to smaller groups.

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3.3 Chemical Vapor Deposition

A more recent alternative to the scotch tape method is that of chemical vapor

deposition, or CVD. In CVD, a metal substrate such as copper is put into

a furnace and heated under low vacuum to around 1000C. The heat anneals

the copper, increasing its domain size. [1] Methane and hydrogen gases are then

flowed through the furnace. The hydrogen catalyzes a reaction between methane

and the surface of the metal substrate, causing carbon atoms from the methane

to be deposited onto the surface of the metal through chemical adsorption (see

Figure 8). The furnace is quickly cooled to keep the deposited carbon layer from

aggregating into bulk graphite, which crystallizes into a contiguous graphene

layer on the surface of the metal. [19]

Figure 8: Diagram of CVD growth on copper. [26]

The graphene produced by this method is more likely to carry impurities

due to the various materials required for CVD. However, research has shown

that such impurities can be sufficiently minimized to create graphene as pure

as exfoliated flakes. [20] Additionally, the graphene from CVD tends to wrinkle

due to the difference in thermal expansion between graphene and copper. This

is decreased via proper annealing, but is still an ongoing research challenge. [1]

Most importantly, graphene from CVD is a contiguous film as large as the

underlying metal substrate, in stark contrast to the random micron-sized flakes

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from the scotch tape method. CVD thus allows graphene to be used as a layer

in a solar cell.

There are many ways to affect the outcome of a CVD graphene growth run.

Since the growth dynamics of carbon deposition and domain growth are not yet

fully understood, finding the proper balance of these controls is a largely exper-

imental task. [16] Perhaps the most natural variable to affect a CVD outcome

is the amount of the various reaction gases. Increased methane provides more

carbon atoms to deposit (and more nucleation sites leading to more domains),

while increased hydrogen promotes the reaction and also increases chemical pro-

cesses on the copper and surrounding environment. The temperature also affects

the rate of reaction, as does the speed of changes in temperature. Impurities

in the copper substrate detract from the growth process by encouraging nu-

cleation sites and thus hindering the formation of contiguous carbon domains,

so proper chemical cleaning of the copper is essential. Annealing time of the

copper also affects the level of impurity for the same reason. The geometry of

the growth chamber affects the deposition rate of carbon due to its effect on gas

flow patterns, specifically because of turbulent (instead of laminar) flow regimes.

Finally, any leaks in the vacuum system further detract from the growth, as oxy-

gen from the air oxidizes the copper, making the carbon atoms unable to adhere

to the copper surface and ruining the deposition.

Copper is not the only substrate which can be used in graphene CVD; in

fact many transition metals can be used. For example, graphene CVD on nickel

is somewhat common, and cobalt has also been used. [5] The main differences

between metal substrates come from differences in the metals ability to absorbcarbon. Nickel and cobalt absorb carbon more than copper, and this leads to an

overabundance of carbon on foils which crystallizes into discrete graphite chunks

instead of a single graphene sheet. For that reason, nickel and cobalt foils cannot

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be used and instead thin films (< 300nm for nickel) must be evaporated onto

a silicon substrate before growth. [19] Copper, on the other hand, attracts less

carbon and does so only at the surface rather than absorbing it into the bulk of

the material, since the weak bonds that hold the carbon atoms to the copper can

only be formed with open bonding sites at the surface of the lattice. Therefore

copper foils can be used in graphene CVD, simplifying the production process

as a whole and making it more robust.

While not used in this investigation, it should be noted that a fairly common

variant on CVD is that of plasma-enhanced CVD. PECVD works in much the

same way as has already been described, but in addition to using a furnace to

provide the heat energy for substrate annealing, an RF frequency AC current

is passed through the substrate. This spark ionizes the gases in the chamber,

enhancing the deposition onto the substrate. [21] While PECVD can be done

at much lower furnace temperatures than regular CVD, it requires additional

equipment beyond the system available at Pomona College.

3.4 Transfer Process

Another key advantage to CVD graphene growth is the ability to transfer the

graphene to an arbitrary substrate (see Figure 9). Once the graphene/copper

foil has been removed from the furnace and cooled, a polymer such as poly-

dimethylsiloxane (PDMS) or polymethyl methacrylate (PMMA) can be spin-

coated onto the graphene as a support, and then the copper removed using an

etchant such as ferric chloride (FeCl3). This leaves the graphene attached only

to the polymer, which can be positioned onto any other substrate (such as a

solar cell). A solvent can easily dissolve the polymer, leaving just the graphene

on any desired substrate. [1,19]

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Figure 9: Schematic diagram of the transfer process to an arbitrary substrate.

Various substrates are useful for specific purposes and stages of research.

As with the scotch tape method, silicon dioxide allows otherwise-transparent

graphene to be seen under an optical microscope. Thus silicon dioxide is a useful

substrate to investigate the uniformity of a growth procedure. Silicon dioxide

is also a good substrate to perform electrical measurements on the graphene,

further measuring its purity and checking its conductive properties. Once the

grown graphene has been verified and characterized on silicon, it can just as

easily be transfered to substrates involved in producing a solar cell such as glass

slides or organic films.

3.5 Measurement

Since graphene is transparent, verifying with the naked eye that it has indeed

grown on a metallic foil is difficult. Raman spectroscopy, however, can be used

to quickly verify the presence of graphene. In Raman spectroscopy, a laser is

directed at the material in question, and the re-emitted light is measured. The

incoming laser light excites characteristic molecular vibrations in the sample,

which emit photons at characteristic frequencies. Thus, if the frequencies asso-

ciated with carbon-carbon bonds are observed, there is graphene on the copper

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sample. [9] Raman spectroscopy is a quick procedure, but requires a special

tool for performing the measurement. These tools are usually purchased as a

single unit. A Raman tool is set up in the Chemistry Department at Pomona

College, but better tools exist for measuring graphene because of the spectral

range which they cover.

While not as conclusive, optical and e-beam microscopy can also indicate

the presence of graphene on copper by revealing graphene domain boundaries.

Once graphene has been transferred to a silicon substrate with a dioxide layer, it

can be seen as a slight discoloration under an optical microscope. Atomic force

microscopy can also be performed on graphene that has been transfered to a

flat rigid substrate to directly measure the thickness and thus number of layers

of the graphene film. Both techniques take under an hour and are available in

the Pomona Physics Department.

It is also useful to measure the thickness of a PMMA film to investigate

the transfer process. The thickness of thin films on a reflective substrate can be

measured using a simple spectrometer and light source. If light is shone on such

a thin film, it will reflect only at wavelengths equal to integer half multiples of the

thickness of the film, due to interference between the wavefronts reflected from

the top and bottom of the film. The intensity of reflected light R is governed

by the proportionality

R cos(4dn

),

where d is the thickness of the film, n is the index of refraction of the film, and

is the wavelength of that light. By taking a reflectance spectrum of a thin film

illuminated by white light and fitting the resulting curve to a function of this

form, the parameter d can be extracted as a measure of the films thickness. [17]

Taking about five minutes, this technique can be performed at Pomona College,

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though the proper equipment might need to be gathered and assembled each

time.

4 Experimental Methods

4.1 CVD Growth

The growth recipe used in this investigation follows that of the McEuen group

(see [30]). Graphene was grown on .025mm copper foil from Alfa Aesar. Apiece of foil was cut approximately 2cm x 3cm, and a small cut was made in the

lower right corner for positional reference later on (see Figure 10). The piece

was sandwiched between two glass slides and clipped with plastic alligator clips,

and left to flatten.

Figure 10: Photo of copper foil after graphene growth.

Before insertion into the furnace, the copper foil was cleaned by dipping it

in various solvents. The order of these dips was:

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1. Acetone (10 seconds)

2. Water (< 10 seconds)

3. Acetic Acid (10 minutes)

4. Water (< 10 seconds)

5. Acetone (10 seconds)

6. Isopropyl alcohol (IPA) (10 seconds)

The remaining IPA was removed using compressed air, and the copper then

loaded into the furnace tube (see Figures 11a and 11b). Multiple foils were

often loaded for a single run.

(a) Photo of flow regulators, gas tanks andgrowth furnace.

(b) Photo of growth furnace and pump.

Figure 11: Photographs of growth furnace setup.

Once all the foils were loaded, the furnace vacuum system was closed off

from the gas tanks and pumped down to around 30 mTorr. Hydrogen was then

flowed at a pressure of 150 mTorr and the furnace heated to 1000C (taking

about 20 minutes). The furnace was held at this temperature for 15 additional

minutes to allow the copper to anneal. Then, methane was flowed at a rate

measured by the coarse flow meter at the 89 mark (approximately 6 Torr) for

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13 minutes. The furnace heating system was then turned off, and the door

opened 2-3 inches. When the temperature reached 450C, the door was opened

completely. When the temperature reached 150C, argon flow was started at

a rate measured by the flow meter at the 120 mark, and the hydrogen and

methane flows were stopped. This left the pressure at around 1.8 Torr. After 2

minutes of argon flow, the pump was turned off and the pump valve closed. The

pressure was allowed to gradually rise to room pressure by the flow of argon,

after which the argon was stopped and the copper removed. [30]

4.2 Transfer Process

Once graphene was grown on a copper foil, it could be transferred to any other

substrate. First, PMMA (4% in Anisole) was drop-cast on the copper. This took

about 1.5 hours to evaporate, after which it was cured on a 145C hot plate for

3 minutes. A piece of scotch tape was placed sticking to the bottom edge of the

PMMA to act as a handle later. The sample was then O2 plasma cleaned for

2 minutes to remove the graphene on the opposite side of the copper. It was

then placed in a ferric chloride bath overnight to remove the copper. After the

copper etch the sample was rinsed in dH2O thoroughly until no ferric chloride

color could be seen in the water. The graphene could be stored in this way

floating in a glass jar of water.

To transfer to a substrate, the substrate was first cleaned. The PMMA

was then placed on the substrate (graphene side down) directly from a bath of

dH2O. Surface tension in the water held it on the surface, however care had to

be taken to avoid wrinkles. The sample was placed in a covered acetone bath

overnight to remove the PMMA. It was then removed (with a bubble of acetone

still covering the surface) and placed in an IPA bath for 1 hour. The sample

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was then placed in a desiccator box to dry, after which it was ready for use.

4.3 Characterization and Results

Figure 12: Optical microscope image of graphene on copper foil.

After graphene was grown on copper foils using the procedure described

above, many techniques were used to characterize it. They were all aimed

primarily at verifying that graphene was indeed grown on the copper, and to get

a sense of the uniformity of graphene coverage. First, optical microscope images

were taken of graphene grown on copper foils at 50x and 100x magnification

(see Figures 12 and 13). These images show two salient features. First, the

ridges in the copper substrate are clearly visible. They are due to the milling

process of copper foils, and are expected. Secondly, faint boundary lines can

be seen separating regions of slightly different shade. Those are believed to be

boundaries between domains of graphene growth.

Scanning Electron Microscope (SEM) images were also taken of graphene

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Figure 13: Optical microscope image of graphene on copper foil.

Figure 14: SEM image of copper domains, 200m.

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Figure 15: SEM image of copper domains, 20m.

grown on copper foils, illustrating many features of these samples. At low

magnification (around 500x), copper domain boundaries are clearly visible as

patches of varying shade (see Figures 14 and 15). These were only observed on

some samples, possibly due to a difference in copper annealing between the side

facing up and the side facing down while in the furnace. At higher magnifications

(10000x), dark lines criss-cross the surface of the sample (see Figure 16). I

believe those are edges between graphene regions; possibly tears in the graphene

resulting from a difference in thermal expansion between graphene and copper.

Also at 10000x, steps or ridges in the copper substrate can be seen (see Figure

17). This is evidence that the graphene is covering the copper uniformly, as the

copper would oxidize in the furnace as it cooled (and the steps would disappear)

if the graphene was not there to protect it. Lastly, black spots can be seendotting the surface at 5000x (see Figure 18). It is unclear what these spots are,

but they seem to follow the graphene tears to some extent. It is possible that

these spots are partially oxidized copper that is exposed through breaks in the

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Figure 16: SEM image of graphene domain boundary, 2m.

Figure 17: SEM image of copper steps, 2m.

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Figure 18: SEM image of black spots, 5m.

graphene. It is also possible, however, that these are clumps of graphite that

coalesced as the graphene formed.

To ensure that graphene does in fact exist on the copper foil, Raman spec-

troscopy data was collected at UC Riverside (see Figure 19). Peaks in the

graphene spectrum can be seen that are distinct from the spectrum for the foil

without graphene. They match the expected values for the G peak (1560 cm1)

and twice the D peak (the 2D peak, 2720 cm1), indicating that graphene is

present on the foil after growth. Similar data was taken using Pomonas Raman

tool (see Figure 20), but the lower wavelength range of that tool and our relative

inexperience at measuring graphene compared to the Riverside group made the

data less conclusive. However, a rough D peak can still be seen around 1360

cm1.

Raman spectra were also taken at Riverside of a first attempt to transfer

graphene to a SiO2 substrate (see Figure 21). Firstly, the steady sloping back-

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Figure 19: Raman Spectrum of graphene on copper, UC Riverside.

Figure 20: Raman Spectrum of graphene on copper, Pomona.

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Figure 21: Raman Spectrum of graphene on SiO2, UC Riverside.

ground of the copper substrate is visibly absent. Spectra were taken at various

points on the surface to characterize different optical features; this is shown in

the inset. Two types of spectra can be differentiated. The pink interior point is

similar to raw SiO2, while the patchy and yellow points have additional peaks. I

conclude that those additional peaks are due to graphene on the surface, which

is supported by the fact that these peaks lie at the G, D and 2D positions. The

peaks common to all spectra are therefore from the substrate itself. The smaller

peaks on either side of the 2D peak remain mysterious, but might be due to

multiple layers of graphene interacting and shifting the vibrational energy levels.

One of these peaks is close to the 2G position (3120 cm1); the difference might

also be due to these multi-layer interactions. All of this information suggests

to me that the graphene has torn and folded up on itself during the transfer

process, leaving areas of the substrate exposed.

Turning to the process of graphene transfer to another substrate, I inves-

tigated an alternative method for removing PMMA film once the transfer is

performed. The first few times I tried the procedure I removed the PMMA in

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(a) Original drop-cast

(b) After 2.5h evaporation

Figure 22: Optical thickness measurement of drop-cast PMMA.

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(a) Original spin-cast

(b) After 2.5h evaporation

Figure 23: Optical thickness measurement of spin-cast PMMA.

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an Acetone bath overnight; this sample was used in the Raman data above.

However, the Acetone bath seems to cause the graphene to float, tear and fold.

An alternative approach is to set the substrate on a hot plate at 300C and

allow the PMMA to evaporate for many hours. To test this process, I deposited

a PMMA film on a fresh SiO2 substrate and measured its thickness before and

after evaporation. I tried both a drop-cast of 4% PMMA in anisole and a spin-

cast of 8% PMMA in anisole at 2000rpm (both annealed at 145C for 1 min).

Using the optical film thickness measurement described in the Theory section

above, I measured the thicknesses of these films to be 4385nm for the drop-cast

and 856.5nm for the spin-cast, with uncertainties smaller than the significant

digits of the values (see Figures 22a and 23a). After setting these samples on the

hot plate for 2.5 hours, I repeated the measurement (see Figures 22b and 23b).

This yielded thicknesses of 258.0nm for the drop-cast and 47.8nm for the spin-

cast. Clearly 2.5 hours is not long enough to completely evaporate the PMMA

film, so I repeated the measurement after 5.5 hours. At this point, however,

the spectra were practically flat and no useful thicknesses could be extracted,

indicating that the film had largely evaporated.

To further investigate the thickness of PMMA over evaporation time, I used

the optical microscope to observe thin-film interference between the PMMA and

the silicon dioxide layer. I observed both a drop-cast and a spin-cast sample

using the same parameters as before. The drop-cast was too thick to see any

thin-film interference: it went from discolored at the beginning to appearing

completely gone after 11 hours. The spin-cast sample however yielded some

interesting (and somewhat pretty) images (see Figure 24). The four subsequent

images were taken after 3.5, 5.4, 11 and 23 hours on the hot plate. The first

two show complete coverage, while the third and forth show some clean areas

but still some PMMA spots. This indicates that even after almost a day on

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Figure 24: Microscope images of SiO2 wafer with evaporated PMMA over time.

the hot plate, spin-cast films do not completely evaporate, while drop-cast films

probably do.

Finally, optical images were taken of the transfered graphene onto a SiO2

substrate. A simple camera image shows a visible distinction where the film was

transfered (see Figure 25), but that could be due to residual PMMA as much as

to graphene. Two optical microscope images were also taken (see Figures 26 and

27). Figure 26 shows a wrinkled and folded sheet which could either be a large

graphene segment or PMMA. In both cases, this indicates that the graphene-

PMMA layer ripped and folded at some point during the transfer. Figure 27

shows a more common region of the sample. Patchy regions of discoloration

match the observed color from exfoliated graphene on silicon dioxide wafers,

suggesting that this is more graphene that has torn and folded.

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Figure 25: Photo of SiO2 wafter after graphene transfer.

Figure 26: Microscope image of SiO2 wafer after graphene transfer.

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Figure 27: Microscope image of SiO2 wafer after graphene transfer.

5 Discussion and Future Work

Many methods for observing graphene have confirmed that I have been able to

grow large-area graphene films on copper foils, and I have taken preliminary

steps in transferring this graphene to a silicon dioxide substrate. In the future,

the growth parameters of the CVD system should be optimized to create the

most reliable and uniform graphene films with the largest domain sizes possible.

The transfer procedure should also be investigated. Various techniques can be

used to remove PMMA from the final substrate, for example, an acetone bath

and a hot plate. They should be tried and tested, with the end goal of transfer

to an organic solar cell in mind. Once successful transfer to a silicon wafer has

been achieved, atomic force microscopy should be performed to measure the

thickness of the film. Electronic transport measurements should also be done

with a silicon substrate (or on ITO templates used in the production of solar

cells at Pomona) using a four-point probe setup to determine the graphenes

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purity. Lastly, the graphene should be included as a transparent conducting

electrode in the production of an organic solar cell and compared to cells made

with ITO.

6 Acknowledgements

I would like to recognize primarily Prof. David Tanenbaum for his interest and

dedication to my work over my entire time at Pomona College. He has provided

for me opportunity after opportunity to experience real research both at Pomona

and at large research centers, and guided me throughout my undergraduate

education and beyond.

Secondly I wish to acknowledge Matt Hasling for his help running the CVD

system and taking SEM images of our copper foils. I wish him the best of luck

continuing this research and look forward to working with him for a bit longer

before he becomes the student in charge!

Jenna deBoisblanc has also been a superb lab colleague and an excellent

resource on organic solar cells.

Thanks to Desalegne Teweldebrhan and Prof. Alexander Baladins group at

UC Riverside, and to Prof. Tyler Moersch in Pomonas chemistry department,

for their assistance in taking Raman measurements of my graphene samples.

I relied heavily on the work of Dr. Paul McEuen and his group at Cornell

University, especially Arend van der Zande.

Lastly, there have been many people connected to the Pomona Physics

department who have aided me in my work. Glenn Flohr, Tony Grigsby and

David Haley assisted me with machining and repairing lab equipment, while

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Profs. Alfred Kwok and Dwight Whitaker provided me with advice in lieu of

my primary advisor. And finally, thanks to Ian Frank and Scott Berkley, now

Pomona alumni who were my student mentors during my first two years in the

lab and whose work served as the foundation for my own.

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Growing Graphene via Chemical Vapor

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