Deutsch: Animation einer Kohlenstoffnanoröhre (Photo credit: Wikipedia)
For years, it was thought that the only organized and purely carbon substances that could exist were graphite and diamond. Within the last twenty years, the discovery of buckminsterfullerene molecules lead to the isolation and recognition of carbon nanotubes and most recently, two dimensional graphene sheets. These materials are a single atom thick but incredibly strong. They possess several unique qualities that could be applied in a surprisingly wide gamut of fields. The highly ordered hexagonal bond composition and high internal quantum efficiency give these new molecules purpose in areas ranging from electronics to biomedical engineering. The possible properties and applications in industry are a huge topic of interest in research, but there is still much to be learned about these once thought impossible carbon structures.
Graphene is the thinnest and strongest material ever discovered. It is literally a sheet of atoms organized in a honeycomb crystal lattice that maintains its shape if handled. If a sheet of kitchen cling film had the same strength as flawless graphene, it would require more than 20,000 Newtons to puncture it with a pencil [1]. An average car only exerts about 15,000 Newtons. This breaking strength has achieved the theoretical limit. Defects rarely form in the single layered structure. It is such a good quality conductor, electrons move through it as nearly the speed of light in a well-made sample [2]. Graphene is also malleable and pliable, and strain can transfigure its electronic, optical, and phonon properties. It exhibits elasticity beyond the reach of any other crystal, about 20% [3], and yet it is harder than diamond. It is also highly thermally conductive, and heat is transferred by acoustic phonons instead of electrons. However, this conductivity is reduced in multilayer graphene [5]. Some of these effects were once thought to exist only in the plasma surrounding very dense neutron stars. Therefore, is worth mentioning that all of these attributes can be observed at room temperature.
It had not previously been discovered because theoretically, it should not be able to exist without destruction by thermal fluctuations. The physicist who instigated graphene in 2004, A.K. Geim, first produced it by what is aptly named the Scotch Tape Method. Handcrafted by peeling away increasingly thin layers of graphite, this amazing and record breaking substance was never any further than the nearest basically equipped desk. Geim was awarded the Nobel Prize in Physics for this work in 2010. More sophisticated ways of developing graphene are being pioneered. Lithographic techniques can etch unwanted carbon atoms away. Graphite flakes can be thinned out by chemical insertion of acid molecules between their layers. These layers are then heated to vaporize these acids, and ultrasonic waves further fracture very long graphene ribbons [4].
Graphene would not have been discovered, however without its progenitor, the buckminsterfullerene molecule. The scientists who discovered it weren’t looking to develop a new molecule. Harold W. Kroto of Sussex University, was researching stellar atmospheres by vaporizing graphene with a laser. Richard E. Smalley of Rice University, was synthesizing clusters of metals and semiconductors such as Silicon and Gallium Arsenide, and vaporizing carbon in Helium. Kroto traveled to Smalley’s lab to use their laser to mimic the surface of a star, and Robert F. Curl, also of Rice University, joined them. Mass spectrometry of the resulting vapor carbon clusters revealed that C60 was the dominant species, especially if the graphite was subjected to longer cooking times [6]. At first, the team was puzzled as to why a structure containing 60 carbon atoms would be most prominent. Eventually, they realized that, while an open structure of 60 atoms isn’t anything remarkable, a closed structure of the same makeup would indeed be special [7]. The result was a truncated icosahedron- a polyhedron of 20 hexagonal faces and 12 symmetrically spaced pentagons. If that seems like a too much of a mouthful, a quick look identifies this shape, in the most spherical of all known molecules, as an American soccer ball. So close is the resemblance that the new molecule was almost called ‘soccerene,’ before the final decision was made to dub the new molecule buckminsterfullerene. This is a namesake of Buckminster Fuller, an architect who designed the geodesic sphere, which greatly resemble the molecule. The ‘buckyball,’ as it was affectionately nicknamed, is composed of closed covalent bonds that give this molecule stability and symmetry that had not yet been observed. Its bending strain is equally distributed over all vertices. In 1996, the Nobel Chemistry Prize was awarded to Smalley, Curl, and Kroto for the identification of the buckyball. This discovery was just the tip of the carbon iceberg. Smalley himself is quoted as saying, “C60 was the Rosetta Stone but the tubes is where it’s about to really flower [60].” What he was referring to was the next advancement in carbon allotropes- rolled up cylinders of graphene called carbon nanotubes.
Sumio Iijima, a physicist working in Japan, was inspired by the fullerene molecule and was determined to find another elemental form of carbon. In 1991, Iijima reported the observation of single and multi layer carbon “needles” grown from an electrode. These hollow tubes, the smallest with a diameter of 2.2 nanometers (that’s 0.0000000022 meters), were found open ended or capped with what can be visualized as half a buckyball at one or both ends [8]. Before the discovery of graphene, carbon nanotubes were the standout in their conductivity, tensile strength, flexibility, and light weight. Nanotubes are 100 times the strength of steel, but only a sixth the weight. Through polymerization, chains with an extremely high length to width ratio can be grown [9].
The most obvious appliances of these carbon allotropes come from their tiny size but extreme strength. Material scientists can use graphene as product augmentations to increase stability without adding weight. These enhanced materials can be used in construction to decrease cost. The inert nature of graphene would cause no harm to marine wildlife if used to build boat hulls. NASA could find considerable use in graphene in improving space shuttles. Besides its light and strong qualities, graphene’s thermal conductivity would make it more heat resistant and better able to handle re-entry than other materials. Nanotubes of graphene could make a viable tether in a space elevator whereas steel would collapse under its own weight. One end of the cable would be anchored to Earth, while the other was attached to a satellite in geosynchronous orbit and 35,900 km high.
Carbon nanotubes with a small enough diameter are a quasi one-dimensional system. The hollow inside could transport molecules or contain chemical reactions, similar to a tiny test tube, without interfering in the reaction, or act as quantum wires. Nanoscientists believe that they could be the base for nanoscale gears and bearings [7]. Nanotubes in a metal alloy would improve military technology- such a composite could be engineered to be difficult to detect electronically for stealth purposes, or to replace heavy Kevlar bulletproof vests. [9]. Graphene oxide has been studied, and submicrometer thick membranes of it are impermeable to liquids and gases, even the difficult to contain Helium, while facilitating the progression of water [10]. Alcohol has been distilled to a high proof using graphene oxide sheets.
Carbon is perfect for use in a solar cell, because it is a conductor, and lacks a defining characteristic of semiconductors called a bandgap. That means it can absorb light over any wavelength in the electromagnetic spectrum, or in flat panel displays [7]. The lack of bandgap is a hindrance, however, in transistors. If this obstacle can be overcome, graphene may be the best possible metal for transistor application- replacing silicon. It is not as difficult to produce consistent sizes and thicknesses, or to integrate into electronic devices as it is for carbon nanotubes [2]. Graphene would allow the miniaturization of these devices, because its resistivity does not increase with decreasing size as it does for copper. Graphene based transistors can run at higher frequencies and higher efficiencies than the current silicon transistors. One way to give graphene a bandgap is to tear it into ribbons a few nanometers in width. The size of the bandgap is inversely proportional to the width of the ribbon. This is an unwanted restriction, so Geim, the creator of graphene, has tried to make graphene more of a switch. A sheet of graphene mesh, with niches cut out, and placed on a silicon substrate demonstrated a much larger current flow in response to an applied voltage. The space between the holes in the graphene determined the transistor behavior [3].
The field of biology could benefit from new carbon-applied science. Graphene could be engineered to sequence a person’s genome in much less time and for thousands of dollars less than technology allows now. A strand of DNA is threaded through a single layer of graphene. Probing the transverse conductance of DNA nucleotides can identify the base sequence. This could give biologists and geneticists clues about evolution, disease causing genes, and disease prevention [11]. Inert, hollow, and stiff carbon nanotubes can be used to efficiently administer gene therapy to individual cells without toxicity. This could revolutionize the treatment of cancer while minimizing cell death. It is speculated that they can also be implanted under the skin, noninvasively, containing any of a whole range of medications to be administered regularly. Diabetes patients could be freed from multiple daily shots. Severe allergy sufferers could always carry an epinephrine shot under their skin in case of a life threatening reaction that would release upon necessity [12].
The potential applications for these three organizations of carbon are limitless. Only years of research will tell what new uses these materials will be put to. With such amazing properties, it is certain that the brilliant minds of our day will engineer new technology to improve daily life.
References
1. Dumé, Bella. Graphene has record breaking strength. Physicsworld.com, 2008.
2. Tretkoff, Ernie. Graphene’s Unique Properties Offer Much Potential. Americal Physical Society 15, (2006).
3. Geim, A. K. Graphene: status & prospects. Science 324, 1530–1534,
(2009).
4. Bullis, Kevin. Graphene Transistors. Technology Review- Published by MIT, 2008.
5. Ghosh, S.. Thermal conduction in graphene and graphene multilayers. Diss. University of California, Riverside, 2009. Dissertations & Theses: Full Text, ProQuest.
6. Schwarzschild, Bertram. Nobel Chemistry Prize Goes to Curl, Kroto and Smalley for Discovering Fullerenes. Phys. Today 49(12), 19-21 (1996).
7. Harris, P. F. J., Carbon Nanotubes and Related Structures: New Materials for the Twenty-first Century. (Cambridge University Press, Cambridge, 1999) pp. 1-14.
8. Sumio, I., Helical microtubles of graphitic carbon. Nature 354, 56, (1991).
9. Tománek, D., Morphology, Growth and Destruction of Carbon Nanotubes. Computational Nanotechnology Lab- Michigan State University, (2011).
10. Nair, R. R., Wu, H. A., Jayaram, P. N., Grigorieva, I. V., and Geim, A. K., Unimpeded Permeation of Water Through Helium-Leak-Tight Graphene-Based Membranes. Science 335, 442 – 444, (2012).
11. Scheicher, R. H., Prasongkit, J., Grigoriev, A., He, Y., Liu, M., and Ahuja, R., Graphene Nano-Electrodes for DNA Sequencing: an Ab initio Perspective. APS March Meeting 57 (2012).
12. Cheung, W., Pontoriero, F., Taratula, O., Chen, A. M., and He, H. DNA and carbon nanotubes as medicine. Rutgers University, (2010).
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