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Graphene and Its ApplicationsMATERIALS

Hailed a “rapidly rising star on the horizon of materials science,” graphene holds the potential

to overhaul the current standards of technological and scienti!c e"ciency and usher in a new era of #exible, widely applicable materials science. Graphene is the name given to the monolayer, honeycomb lattice of carbon atoms (1). $e two-dimensional carbon structure is characterized by sp2-hybridization, yielding a continuous series of hexagons, as represented in Fig. 1 (2). Until its discovery in 2004, graphene had been hiding in plain sight–tucked away as one of millions of layers forming the graphite commonly found in the “lead” of pencils. A team of researchers from the University of Manchester was the !rst to demonstrate that single layers of graphene could be isolated from graphite, an accomplishment for which team members Andrew Geim and Konstantin Novoselov were awarded the Nobel Prize for Physics in 2010. (3). Since then, the !eld of graphene research has exploded, with over 200 companies involved in research and more than 3000 papers published in 2010 alone (4). Many proclaim graphene as the 21st century’s “miracle material,” as it possesses powerful properties that other compounds do not: immense physical strength and #exibility,

unparalleled super-conducting capabilities, and a diverse range of academic and mainstream applications.

Physical AttributesGraphene boasts a one-atom-thick,

two-dimensional structure, making it the thinnest material in the known universe (2). A single layer of graphene is so thin that it would require three million sheets stacked on top of one another to make a pile just one millimeter high (4). In fact, graphene is so thin that the scienti!c community has long debated whether its independent existence is even possible. More than 70 years ago, the band structure of graphite was discovered, revealing to the scienti!c community that graphite was composed of closely packed monolayers of graphene held together by weak intermolecular forces. However, scientists at the time argued that two-dimensional structures, like that of graphene, were thermodynamically unstable and thus could exist only as a part of three-dimensional atomic crystals (1). $is belief was well established and widely accepted until the experimental discovery of graphene and the subsequent isolation of other freestanding two-dimensional crystals in 2004 (1). With its very discovery, graphene began to push the limits of traditional materials science.

Conventional wisdom dictates that “thin implies weak,” and most would agree that it is more di"cult to break through a brick wall than a sheet of paper. Yet, graphene de!es expectations. According to mechanical engineering professor and graphene researcher James Hone of Columbia University, “Our research establishes graphene as the strongest material ever measured, some 200 times stronger than structural steel,” (3). Recent research has also shown that it is several times tougher than diamond and supposes that it would take “an elephant balanced on a pencil” to break through a sheet of graphene the thickness of a piece of plastic wrap (4). $e enormous strength of graphene is attributed to both the powerful atomic bonds between carbon atoms in the two-dimensional plane and the high level of #exibility of the bonds, which allows a sheet of graphene to be stretched by up to 20% of its equilibrium size without sustaining any damage (4).

With the development of a new “wonder material” with properties like those of graphene, one might expect exorbinant prices and relative inaccessibility for mainstream applications. However, one of graphene’s most exciting features is its cost. Graphene is made by chemically processing graphite—the same inexpensive material

SCOTT GLADSTONE

!e Miracle Material of the 21st Century

Image courtesy of AlexanderAIUS retrieved from http:/ /en.wikipedia.org/wiki/File:Graphen.jpg (accessed 10 May 2012)

Figure 1: Monolayer model of sp2-hybridization of carbon atoms in graphene. The ideal crystalline structure of graphene is a hexagonal grid.

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that composes the “lead” in pencils (3). Every few months, researchers develop new, cheaper methods of mass-producing graphene and experts predict prices to eventually reach as low as $7 per pound for the material (4). $e thinnest, strongest material in the universe may be closer to commercial applications than initially imagined.

ConductivityGraphene’s record-setting properties

also enter the realm of thermal and electrical conduction. A team of researchers led by Michael Fuhrer of the University of Maryland’s Center for Nanophysics and Advanced Materials recently performed the !rst measurements regarding the e%ect of thermal vibrations on the conduction of electrons in graphene (5). All materials are characterized by an intrinsic property known as electrical resistance, which results from the intrinsic vibrations of atoms due to non-absolute-zero temperatures. When the atoms vibrate in place, they block the #ow of electrons through the material. $e only way to eliminate the vibrations is by reducing the temperature of a substance to absolute zero, a practical and scienti!c impossibility (5). Fuhrer’s research showed that thermal vibrations have an extraordinarily small e%ect on the electrons in graphene, yielding a resistivity that is about 35% less than the resistivity of copper. Fuhrer attributes this di%erence, in part, to the fact that graphene has far fewer electrons than copper, so electrical current in graphene is carried by a few electrons moving much faster than the electrons in copper (5). Before the discovery of graphene, copper was thought to be the material with the lowest resistivity at room temperature. For this reason, the overwhelming majority of electrical wiring is composed of copper. $is strongly implies a practical use of graphene in high-frequency electrical systems for which the use of copper limits overall performance.

Graphene’s powerful conducting ability also makes it an ideal candidate as a material for the next generation of semiconductor devices. Moore’s Law states that the number of transistors that can be !t on a single processing chip doubles approximately every 18 months, which translates to faster, more advanced devices that utilize high speed transfer of electric charge, such as computers and televisions (6). Graphene’s #exibility allows the single #at carbon sheets to be “rolled” into

semiconducting carbon nanotubes (see Fig. 2). Recent research shows that graphene-based nanotubes have the highest levels of mobility, a measure used to quantify how fast electrons, and thus electric current, move. $e limit to mobility of electrons is graphene is about 200,000 cm2/Vs at room temperature, compared to about 1,400 cm2/Vs in silicon, a staple in computer processing chips, and 77,000 cm2/Vs in indium antimonide, the highest mobility conventional semiconductor known (5). $e practical impact of this result is well stated in a review of semiconductor research: “Mobility determines the speed at which an electronic device (for instance, a !eld-e%ect transistor, which forms the basis of modern computer chips) can turn on and o%. $e very high mobility makes graphene promising for applications in which transistors must switch extremely fast, such as in processing extremely high frequency signals” (5).

Graphene’s conductive capabilities are also being utilized in the development of high power e"ciency capacitors. Electrochemical capacitors, commonly known as supercapacitors or ultracapacitors, di%er from the capacitors normally found in electronic devices in that they store substantially higher amounts of charges (7). $ese capacitors have recently gained attention because they can charge and discharge energy faster than batteries; however, they are limited by low energy densities where batteries are not (7). $erefore, an electrochemical capacitor that could combine the high energy density of a battery with the power performance

of a capacitor would be a signi!cant advance in modern technology (Fig. 3). While this ideal capacitor is not yet within reach, researchers at UCLA have produced capacitor electrodes composed of expanded networks of graphene that allow the electrodes to maintain high conductivity while providing highly accessible surface area (7). Further developments on this technology could lead to innovations such as credit cards with more processing power than current smartphones and computers (3).

Diverse Applications$e creation of the !rst man-made

plastic, Bakellite, in 1907 allowed for inventions like the plastic bag, PVC pipe, and plexiglass, which many now take for granted in daily life. Dr. Sue Mossman, curator of materials at the Science Museum in London, notes that graphene closely parallels Bakellite, saying: “Bakellite was the material of its time. Is [graphene] the material of our time?” (4). Dr. Mossman is one of many to compare graphene to plastics, citing the variety of applications and diversity of use as the strongest ties between the materials. Graphene has the potential to transform many di%erent !elds, covering a broad range of subject matter that encompasses everything from computational development to water puri!cation.

Current touch screen technology could see a massive overhaul with the introduction of graphene-based innovations. Modern touch-sensitive screens use indium tin oxide, a substance that is transparent but

Image courtesy of Arnero retrieved from http:/ /en.wikipedia.org/wiki/File:Carbon_nanotube_zigzag_povray.PNG (accessed 10 May 2012)

Figure 2: Monolayer model of sp2-hybridization of carbon atoms in graphene. The ideal crystalline structure of graphene is a hexagonal grid.

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carries electrical currents. However, indium tin oxide is expensive and, as some iPhone and other touch-screen gadget users have experienced !rsthand, is likely to shatter or crack upon impact (4). Replacing indium tin oxide with graphene-based compounds could allow for #exible, paper-thin computers and television screens. One researcher proposes the following scenario: “Imagine reading your Daily Mail on a sheet of electronic paper. Tapping a button on the corner could instantly update the contents or move to the next page. Once you’ve !nished reading the paper, it could be folded up and used afresh tomorrow,” (4). Samsung has been one of the biggest investors in graphene research, and has already developed a 25-inch #exible touch screen that uses graphene. Companies like IBM and Nokia have followed suit. IBM recently created a 150-gigahertz (GHz) transistor; in comparison, the fastest comparable silicon device runs at about 40 GHz (3). Even though graphene-based technology is beginning to emerge, scientists are faced with a fair share of problems. One of the biggest issues for graphene researchers is the fact that graphene has no “band gap,” meaning that its conductive ability can’t be switched “on and o%,” like that of silicon (2). For now, silicon and graphene operate in di%erent domains, but as Nobel Prize winner professor Geim states, “It is a dream,” (3).

$ere is good reason to believe that graphene research will be well worth the struggle. Most recently, researchers at the University of Manchester showed that

graphene is impermeable to everything but water. It is the perfect water !lter. In an experiment, the researchers !lled a metal container with a variety of liquids and gases and then covered it with a !lm of graphene oxide. $eir most sensitive equipment was unable to register any molecules leaving the container except water vapor–even helium gas, a molecule that is particularly small and notoriously tricky to work with, was kept at bay (8). Dr. Rahul Nair, leader of this research project, claims that this ability is due to the fact that, “graphene oxide sheets arrange in such a way that between them there is room for exactly one layer of water molecules. If another atom or molecule tries the same trick, it !nds that graphene capillaries either shrink in low humidity or get clogged with water molecules,” (8). It is hard to understate the importance of graphene oxide’s potential as an ideal !lter, as it could quickly and inexpensively replenish rapidly decreasing clean water supplies.

More powerful than a steel beam, tougher than a diamond, a better conductor than copper and the best water !lter possible– these are but a few of what Nobel Prize winners Geim and Novoselov claim to be a “cornucopia of new physical and potential applications” of graphene (1). $e potential uses of graphene are innumerable, and run the gamut from supercomputers that process at over 300 GHz to super-distilled vodka with zero percent water. Some have gone so far as to suggest iPhones that users can roll up and tuck behind their ears like a pencil, car tires

that can ride any terrain and never break, or batteries with lifetimes ten times as long as current models as practical applications for graphene technology. $e potential is there, but it is now up to those on the forefront of materials science to leverage these discoveries in the progression of mankind.

CONTACT  SCOTT  GLADSTONE  AT  

References

1. A.K. Geim, K.S. Novoselov, $e rise of graphene. Nature Materials 6, 183-191 (2007).2. M.J. Allen, V.C. Tung, R.B. Kaner, Honeycomb carbon: a review of graphene. Chem. Rev. 1, 132-145 (2010).3. A. Hudson, Is graphene a miracle material? BBC News (2011). Available at http://news.bbc.co.uk/2/hi/programmes/click_online/9491789.stm (May 2011).4. D. Derbyshire, $e wonder stu% that could change the world: graphene is so strong a sheet of it as thin as cling!lm could support an elephant. Daily Mail, Science & Tech (2010). Available at http://www.dailymail.co.uk/sciencetech/article-2045825/Graphene-strong-sheet-cling!lm-support-elephant.html (October 2011).5. Graphene – the best electrical conductor known to man. AZOM (2008). Available at http://www.azom.com/news.aspx?newsID=11679 (March 2008).6. G.E. Moore, Cramming more components onto integrated circuits. Electronics. 8, 4-7 (1965). 7. Graphene capacitors to increase power e"ciency. Times of India (2012). Available at http://articles.timeso!ndia.indiatimes.com/2012-03-20/infrastructure/31214537_1_graphene-capacitors-electrodes (March 2012).8. S. Anthony, Graphene: the perfect water !lter. ExtremeTech (2012). Available at http://www.extremetech.com/extreme/115909-graphene-the-perfect-water-!lter (January 2012).

Image courtesy of Stan Zurek from Maxwell Technologies retrieved from http:/ /en.wikipedia.org/wiki/File:Supercapacitors_chart.svg (accessed 10 May 2012)

Figure 3: Comparison of energy density and power output in batteries and capacitors.