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Surface Oxygen Technology

for Graphene Production

Prepared by:

Vanissa B. Gale

Stephen G. Abelada

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Graphene: An unusual wonder material

One of the world’s strongest and flexible material, graphene is an ultimate two-dimensional

carbon allotrope that is just one atom-thick sheet of carbon atoms bonded together in a hexagonal

honeycomb lattice (de la Fuente, 2014; Nanowerk, 2014). As the whole of science realm boasts it,

graphene is the thinnest material ever made yet still visible to the naked eye; about one million times

thinner than the common paper. It is the lightest material known to man with one square meter

coming in at around 0.77 milligrams only (de la Fuente, 2014). It possesses exceptional electronic,

thermal, mechanical, optical and chemical properties as well (Nanowerk, 2014). Permitting a unique

combination of such superior properties, graphene is yet another promising material scientists and

researchers have aimed to extensively exploit for a wide spectrum of applications in the past few

years.

Graphene is a great heat and electrical conductor. It is yet the best conductor of heat at room

temperature, having thermal conductivities ranging from (4.84 ± 0.44) × 103 to (5.30 ± 0.48) × 10

3

W•m−1

•K−1

). This exceptional property secures graphene’s suitability for high-temperature

applications. Electrons are able to flow through graphene more easily than through copper and any

other metallic materials common to us. In fact, graphene studies reported electron mobility at values

more than 15,000 cm2•V

−1•s

−1, allowing electron movement at a rate of 1,000 km/s or roughly 200

times faster than that for silicon, a material fundamental to contemporary electronic devices (de la

Fuente, 2014). Electrons travel through the graphene sheet or network as if they carry no mass, as fast

as one hundredth that of the speed of light. In this regard, graphene shows potential as material for

flexible electronics, solar cells, batteries and high-speed transistors. Furthermore, the way its electrons

behave makes it very useful to study some fundamental physical properties and its near perfect crystal

lattice means a very apt or suitable system to experiment with, rendering less limitations and

inaccuracies for passionate researchers and scientists (Nanowerk, 2014).

Graphene’s tensile strength proudly exceeds 1 TPa due to its unbroken structural pattern and

relatively strong bonds between component carbon atoms, explicitly conceding that graphene is

relatively robust and harder than diamond and in fact about 300 times harder than steel.

Comparatively, it will take the weight of an elephant perfectly balanced on a needle point in order to

break one atom-thick graphene fabric. Despite its sturdiness, graphene is inherently stretchable; it can

be stretched up to 20% of its initial length. This unusual property is due to the abilty of the bonds to

Figure 1. Three-dimensional structure of a graphene sheet

(Guardian News and Media Limited, 2014).

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be twisted, pulled and curved to a certain extent without experiencing breakage (Brewster, 2013;

Nanowerk, 2014).

In chemical properties aspect, graphene, similar to graphite surface, can adsorb and desorb

various atoms and molecules including NO2, NH3, K and hydroxides. It can also be functionalized and

activated by several chemical groups, for instance, OH- and F- groups, creating graphene oxide and

fluorinated graphene, materials of varied applications especially in medicine and electronics

(AzoNano, 2014; Nanowerk, 2014).

An almost completely transparent material, graphene is able to optically transmit more than

90% of light and 2.3% of visible light that hits or strikes it. This is a major requirement for

optoelectronic applications. Since graphene is as well highly conductive, it would work very well in

optoelectronic devices particularly as an LCD touch screen material (Brewster, 2013; Nanowerk,

2014).

Applications and Relevance to Society

With the many excellent properties of graphene mentioned, its applications extend but are not

limited to numerous disciplines vital to current societal endeavors including bioengineering,

optolectronics and electronics, composite materials, energy technology and nanotechnology.

Figure 2. Conductive graphene-based material (Plastic

Electronics, 2012).

Figure 3. Graphene-based coating (Hsu, 2012).

Figure 4. Graphene-oxide paper (Graphene

Supermarket, nd).

Figure 5. Graphene in Nokia’s “HumanForm”

concept phone (xtremevbtalk, 2014).

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Graphene poses a vital part in the future of bioengineering. With graphene offering a large

surface area, high electrical conductivity, thinness and strength, it would make an excellent candidate

for the development of fast and efficient bioelectric sensory devices. These devices, if manufactured

with graphene, then will possess the ability to monitor crucial and sensitive parameters such as

glucose, hemoglobin and cholesterol levels in the blood and even DNA sequencing at more

convenient and accurate levels than existing ones (Nanowerk, 2014).

In the field of optoelectronics, indium tin oxide (ITO) is the current material utilized as

coating particularly for transparent LCD screens. However, recent tests have shown that graphene is

certainly able to match the properties of ITO (Nanowerk, 2014). With its high tensile strength,

thinness, flexibility and optical properties, existing optoelectronic devices may be replaced by more

robust, stretchable and transparent ones. It is in fact a prospective candidate for competition counter to

bendable ITO electronics that will soon hit consumer markets. As to electronics, research has mostly

focused on making computer chips more powerful by packing in more transistors while

simultaneously making such transistors more potent by hastening the electron flow. Computer chips

rely on billions of transistors to control the flow of electricity in their circuits. As science approaches

the limit for how small transistors can be, graphene could push the limit back by both moving

electrons in transistors faster and reducing their very size to a few atoms or less (Brewster, 2013).

Figure 6. Optical image of a graphene wireless

sensor biotransferred onto the surface of a tooth

(Berger, 2014).

Figure 7. Transistors manufactured by Intel at

32 nanometer-sizes (Brewster, 2013).

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With graphene being much stronger at the same time much lighter, the use of steel for

manufacture of aircraft structures and materials is gradually replaced by a composite material

employing graphene integrated to carbon fiber, significantly improving fuel efficiency, reducing

aircraft weight and hence increasing flight range. Due to its electrical conductivity as well, graphene

could even be used to coat aircraft surface material to prevent electrical damage resulting from

lightning strikes. In this application, the same graphene coating could be utilized to measure strain

rate, notifying the pilot of any changes in the stress levels that the aircraft wings are under. Moreover,

these characteristics can help in the development of high strength-requirement applications such as

body armor for military personnel and vehicles (Nanowerk, 2014).

In relation to energy technology, graphene is a prospective material for revolutionizing solar

cell mass production. Solar cells rely on semiconductors to absorb sunlight. Today’s semiconductors

are made of silicon having two electron layers. Solar cells work by transferring the energy from light

particles to the calm electrons in one layer, which eventually become excited and jump to the free-

flowing electrons layer, hence creating electricity. Graphene’s layers of electron networks overlap,

indicating that less light energy is needed to get the electrons to jump between layers. This property

perceptibly could raise solar cell energy efficiency. Using graphene would also allow manufacture of

solar cells that are hundreds of thousands of times thinner and lighter than those that rely on silicon

(Brewster, 2013).

Figure 8. Formula One car made of carbon

fiber with graphene (Autosavant, 2008).

Figure 9. Solar cell installation (Laden, 2013).

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Major Barriers to Graphene Production

With the numerous excellent properties and the seem-to-be endless possibilities for its

applications, the question remains as to why graphene commercialization is not fully realized; why

graphene remains behind laboratory bars and is not perceptibly competing in the global industrial

markets? The answer lies on the current graphene size, structure, quality and method of production.

One of the greatest challenges faced today in commercializing graphene is how to produce

large, single, high-quality crystals in an environmentally-friendly reproducible manner at low cost.

Graphene has always been grown in a polycrystalline form, that is, it is composed of many small

crystals that are joined together with irregular chemical bonding at their boundaries (“grain

boundaries”), something like a patch-work quilt. Large, single-crystal graphene is of great interest

because the grain boundaries in polycrystalline material have defects and may house several impurity

substances. Eliminating such defects hence implies a better material of superior qualities (Science

Daily, 2013). It may sound simple to just create graphene that exemplify what key player industries

nowadays consider as ideal. However, it is very difficult to produce large, single-crystalline graphene

thin films possessing very high electrical and thermal conductivities along with excellent optical

transparency, at least not with the current methods available to us.

Another issue of grave concern is that synthesis of graphene by conventional methods

involves the use of toxic chemicals that usually result in the generation of hazardous waste and

poisonous gases. Furthermore, the complexity of several of these methods entails associated high

production cost for graphene which, consequently, yields a high-cost end material, that is, graphene at

a very expensive cost (Nanowerk, 2014).

Current Methodologies for Graphene Production

As of today, common techniques available for the production of graphene include

micromechanical cleavage, chemical vapor deposition, epitaxial growth on SiC substrates, chemical

reduction of exfoliated graphene oxide, liquid phase exfoliation of graphite and unzipping of carbon

nanotubes. However, each of these methods can have its own advantages as well as limitations

depending on its target application or applications (Nanowerk, 2014). In order to surmount these

barriers in commercializing graphene, concerted efforts are being made by researchers at various

research and development institutes, universities and companies worldwide to develop new methods

for large scale production of low-cost and high quality graphene via simple and eco-friendly

approaches.

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Surface Oxygen Technology: An emerging solution to the problems

A recent paper entitled “The Role of Surface Oxygen in the Growth of Large, Single-Crystal

Graphene on Copper”, featured originally in the journal Science on November 8, 2013, might as well

be the answer to such graphene-production and quality dilemmas. A team led by postdoctoral fellow

Yufeng Hao and doctor Rodney Ruoff of the Department of Mechanical Engineering and the

Materials Science and Engineering Program of Cockrell School of Engineering in the University of

Texas at Austin, in collaboration with doctor Luigi Colombo, a material scientist from Texas

Instruments, worked extensively for three years on a major breakthrough graphene-growth method.

As opposed to the common notion of the graphene research community that oxygen may poison

graphene growth, the team found out that oxygen is in fact a critical factor for nucleation and growth

of centimeter-size, single graphene crystals but only when present at the right surface concentration

(Henderson, 2014; Science Daily, 2013).

Figure 10. Conventional methods commonly used for the synthesis of graphene along with their key

features and their current and future applications (Nanowerk, 2014).

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Graphene is typically grown in an oxygen-free environment because it requires a certain

substrate (previous pioneering work by the Ruoff group has shown copper foil to be a good choice) on

which to grow. The copper substrate catalyzes the nucleation and growth of graphene crystals when

heated within a high-temperature chemical vapor deposition (CVD) system, a tube furnace, to about

1000 °C in the presence of methane gas as carbon feedstock. Different from the previous works

however, the Cu foils were deliberately exposed to trace amounts of oxygen for seconds to up to 1-2

minutes before growth then gradually introducing small amounts of oxygen during growth runs. This

methodology caused a drastic reduction in the number of graphene nuclei that formed by intentionally

oxidizing most, but not all, of the copper substrate (i.e., the nucleation density or the density at which

individual crystals first start to grow is decreased). This allows the few crystals that grow further from

such nuclei to grow much larger, limited only by their maximum size rather than by adjacent

insignificant crystals that would just ‘get in the way’ of further growth. By controlling the

concentration of surface oxygen, the researchers have increased the crystal size from a millimeter to a

centimeter, about 10,000 times larger than the largest crystals produced from four years ago. Rather

than hexagon-shaped crystals with several grain boundaries, the addition of the right amount of

surface oxygen furthermore produced crystals with multi-branched edges, similar to a snowflake,

hence appreciably lesser number of grain boundaries. (Henderson, 2014; Science Daily, 2013; The

University of Texas at Austin, 2013). To put in simpler terms, the ultimate result of the study is a

smaller number of larger, non-polycrystalline crystals with less grain boundaries and hence far fewer

imperfections than the previous methods allowed.

Figure 11. Centimeter-size, single-crystalline graphene grown on a

copper substrate (The University of Texas at Austin, 2013).

Figure 12. Graphene crystals with multi-branched edges

similar to a snowflake (Henderson, 2013).

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The simplicity of the method whereby graphene is just allowed to grow on a copper substrate

by deposition of methane at high temperatures and exposing it to oxygen at the right concentration is

as well remarkably beneficial than the much more complex steps (much like their complex names)

involved in micromechanical cleavage, epitaxial growth on SiC substrates, chemical reduction of

exfoliated graphene oxide, liquid phase exfoliation of graphite and unzipping of carbon nanotubes. In

addition, neither expensive acids nor any other harmful substances are involved in the process; it’s

just methane which is virtually harmless if properly contained and handled upon. This new method

essentially as well offers less wastes (only unutilized methane vapor) compared to other conventional

methods. This indeed is a step towards more economically-viable, cleaner and ‘greener’ ways of

mass-producing graphene (Graphenea, nd).

So what are the tangible benefits this study led the researchers to? From the crystals formed, a

major finding by the team was that the “carrier mobility” of electrons (how fast the electrons move) in

graphene films grown in the presence of surface oxygen is exceptionally high. This is important

because the speed at which the charge carriers move is important for many electronic devices – the

greater the speed, the faster the device can perform. Yufeng Hao deliberately says that the knowledge

gained in this study could prove useful to industry. For instance, these single-crystal films will

eventually allow existing devices to perform tasks quicker and more efficient and later be used as

prototype materials for the evaluation and development of new types of devices with superb features

than could be achieved before. This, undeniably, will be a huge leap in the field of electronics

(Science Daily, 2013).

Much for the results of the current research, the study enabled only the determination of the

effects of varying oxygen surface concentration to the electrical properties (and implicitly

optoelectrical and thermal properties) of the graphene crystals produced. The effect of such variable

on other promising graphene properties including the mechanical and chemical ones are yet to be

exploited or discovered. Moreover, extensive years and effort have yet to be undertaken and several

enduring tests have to be performed before graphene crystals can be enlarged to approximately meter

sizes. Nevertheless, the surface oxygen method of graphene production is an evidence that better

innovations are not impossible to materialize for the resolute, determined individuals equipped with

resourcefulness and the will to manifest such.

Figure 13. Comparison of Graphene grown on oxygen-rich copper (left) and oxygen-free

copper (right) (The University of Texas at Austin, 2013).

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References

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From http://www.azonano.com/article.aspx?ArticleID=2934.

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http://www.autosavant.com/2008/05/28/carbon-fiber-for-the-common-man/.

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applications/.

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