In 2004, Konstantin Novoselov, Andre Geim and their colleagues from Manchester, UK and Chernogolovka, Russia reported the existence of graphene, a two-dimensional sheet of carbon that is 1 atomic layer thick. This discovery took the world by surprise because, almost 70 years earlier, physicists had argued convincingly that materials like graphene would be too thermodynamically unstable to exist. Graphene was immediately hailed as the “next big thing”, and new information that has emerged about its electrical, chemical and physical properties have only added to the growing excitement. In 2010, Novoselov and Geim were awarded the Nobel Prize in Physics for their discovery and characterization of graphene.
Can 2D crystals be thermodynamically stable?
It was long believed that strictly two dimensional (2D) arrangements of atoms would be unstable, since thermal fluctuations (basically the same kind of force that causes ice to melt and lose its crystal structure above 0°C) would be as large as the force binding the atoms together, causing the structure to fall apart . The original discoveries in 2004 and recent research have shown that there are in fact means to create 2D crystal arrangements. Graphite, a familiar form of carbon found in pencil “lead”, is often described as being made of stacks of 2D grids of carbon atoms, with each atom bound to three others in a repeating hexagonal arrangement (Figure 1). Each of these sheets of carbon atoms is graphene (Figure 2). Structures formed by such sheets folding on themselves to form zero dimensional or one dimensional structures, such as buckyballs and nanotubes (Figure 1), were also known to exist for nearly a decade and had indicated that carbon might in fact be stable enough to exist in the 2D sheet form. In hindsight, physicists have discovered that carbon – carbon bonds are in fact strong enough and small enough that thermal fluctuations are not enough to destabilize graphene even at room temperature.
Figure 1. Different forms of carbon: graphite, or stacked sheets of graphene (left); buckyball (center); and nanotube (right). (Image credit: Wikimedia Commons user Mstroeck)
So what makes graphene special?
Perhaps the most exciting and most well understood properties of graphene are its very high electrical conductivity and its capacity to carry large currents at room temperature . Carbon atoms have four electrons available to make chemical bonds, however, graphene is one atom thick and every atom in the crystal is bound to only three others. Each atom thus has one free electron available for electronic conduction. The electrical properties of graphene far exceed those of metals, since each 2D lattice of graphene provides as many charge carriers as metals are only able to supply from bulkier 3D atomic arrangements, even when metals tend to have some electrons delocalized and shared in a “sea of electrons” among all atoms within a piece of metal. What makes graphene a real “dream material” for making electronics though is its chemical properties . Apart from its ability to attach various molecules on its surface, chemical changes can be made to parts of a large graphene sheet such that local electrical properties can be finely controlled and varied on the same surface (Figure 2). This allows for very exciting possibilities like assembling parts that act like different components of an electronics circuit all on a graphene scaffold! Such nanometer-sized circuitry may one-day enable faster and tinier computational and electronic devices.
Figure 2. On the left is the atomic structure of a graphene sheet, with each carbon atom binding to 3 of its neighbors, making hexagonal repeats. On the right is an artist’s impression of different localized chemical and electrical environments created on a single sheet. This possibilty can allow faster and smaller electronic circuits. (Illustration by Krista Shapton)
Graphene’s monoatomic layer also gives it physical properties that almost seem too contradictory to be found in the same material. It’s very malleable and absorbs only 2.3% of incident light, making it an excellent candidate for use in LCDs (for touchscreen displays, for instance) and solar cells. Its physical strength coupled with high conductivity also makes it a great choice for scanning probe microscopy, a technique that investigates atomic structures of other materials using a tiny, strong, and conductive tip. Another exciting prospect is the use of graphene powder for more efficient electric batteries.
Future of the dream material
Whether or not graphene lives up to its promise of commercial applications will be determined by how easily it can be assembled and manipulated without expensive laboratory equipment. The obvious advantage that graphene has over conventional materials currently used in electronics and solar cells is the low cost of raw materials. In most techniques, graphite and glass substrates are used, compared to the rare metals used in devices today. There has been rapid progress in the area of graphene production, and thanks to some recent research  using ultrasonic sound waves to blast graphite suspensions, it may soon be possible to make graphene in ways that industries would find easier to adopt. Synthesis of high quality graphene in bulk is thus a key engineering problem that is intriguing synthetic chemists and physicists.
In addition to applications to technologies, graphene’s exciting surface and chemical properties also lend a great system for studying basic science questions that were not possible to address before. For example, graphene has opened doors to studies of the quantum mechanical properties of 2D materials by providing a great system for basic laboratory research. Interesting topics being investigated include an understanding of how 2D crystals melt, the origins of the great physical strength observed, and the exact nature of their electrical and chemical properties . As research continues, engineers are bound to find applications for graphene beyond the obvious. Solar cells might be something we hope graphene will directly improve, but its indirect impacts are likely to be more far reaching as graphene becomes an intergral part of everyday electronics. Motivated by the successful discovery and application of graphene, scientists are now investigating the existence and properties of other 2D materials, and it is hard not to believe that we have opened a Pandora’s box of 2D materials!
Gairik Sachdeva is a PhD student at the Harvard School of Engineering and Applied Sciences.
1. Geim, A.K., & Novoselov, K.S. (2007) The rise of graphene. Nat. Mater. 6:183-191.
2. Novoselov, K.S. (2009) Graphene: The magic of flat carbon. ECS Transactions 19(5): 3-7.
3. Geim, A.K. (2009) Graphene: Status and prospects. Science 324:1530-1534.
Links of Interest:
Washington Post article on graphene: http://www.washingtonpost.com/wp-dyn/content/article/2010/10/25/AR2010102503505.html
Vega science video about graphene: http://vega.org.uk/video/programme/325