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discovered 02_2012

FOCUS// The HZDR Research Magazine WWW.Hzdr.DE 32 33 elementary particles much like the fast, charged particles boosted to near light speed in an accelerator. Physicists like Stephan Winnerl of the Helmholtz-Zentrum Dresden-Rossendorf already have a very clear understanding of electron mobility in the crystal lattices of semiconductors. Electrons can be accelerated inside a semiconductor by applying electric potentials, where, as we know, the especially fast and mobile electrons act as charge carriers to transport the electric current and thus ensure the current flow. How fast electrons can become depends greatly on their effective mass. Every semiconductor possesses its own particular crystal structure, which physically determines the effective mass of its electrons. For fast electronic components, such as transistors in computers or mobile telephones, one naturally wants materials whose electrons can be accelerated very easily to high speeds. While graphene actually belongs in the class of semiconductor materials, its electrons behave radically differently. Their effective mass seems to disappear altogether, meaning they race through the one-atom-layer plane incredibly fast. It seems only natural to exploit this in future electronics. Aside from the effective mass of their electrons, the various semiconductor materials differ by another important property: their so-called band gap. Quantum mechanical laws dictate that electrons reside only within specific energy ranges, known as bands. Physicists speak of a valence band and a conduction band. Between these bands is a forbidden zone, which the electrons are not allowed to enter. This band gap, or energy gap, is exploited to produce light in LEDs, for example. As an electron jumps from the higher-energy conduction band “down” to the valence band, it gives off its energy in the form of light. The colour of the emitted light depends on the size of the energy gap. Electrons without limits Inside the extraordinary semiconductor graphene, there is no forbidden zone between the energy bands, meaning there are practically no limits set to the mobility of its electrons. They streak around at high speeds through the essentially two-dimensional space of the super-thin carbon layer. What is more, the valence band and conduction band meet in a special way in graphene: “The bands cross over in a Dirac cone, which I like to imagine as two ice cream cones, one standing with its apex touching the apex of the other, inverted cone,” Stephan Winnerl tries to illustrate the concept, which cannot be described in simple terms. Indeed, electrons do not actually move through space and time in the familiar sense, and the energy bands we are talking about, being a quantum mechanical property of electrons, belong in so-called momentum space. Because there is no band gap, graphene cannot be used to build the transistor designs familiar from silicon technology. Instead, it offers completely novel approaches for using its fast electrons in high-speed switching processes. Nor can graphene be used for building light-emitting diodes or other optoelectronic technologies that otherwise exploit the light that electrons emit as they jump from the conduction band to the valence band. Yet, the very fact that it lacks a band gap and that the energy bands cross over in the special “ice cream cone shape” has a unique consequence for graphene’s optical properties: Graphene absorbs about two percent of the energy of incident light, while the remaining 98 percent passes through the graphene layer unhindered. That makes graphene almost completely transparent. Interestingly, the colour of the light does not matter, and the law applies even to invisible infrared radiation. This unique optical effect makes graphene very attractive as a transparent material for electrodes, in flat screens or solar cells, for instance. Large companies have already developed successful prototypes for this. One major advantage of substituting graphene for the conventional transparent electrode materials is that the scarce – and therefore expensive – raw material indium can be omitted. Carbon is almost unlimitedly available for making graphene, and graphene is furthermore completely harmless. Manfred Helm, Director of the HZDR’s Institute of Ion Beam Physics and Materials Research, also sees future applications in optoelectronics, especially for converting infrared light into electrical signals. Furthermore, the flexibility and stability of the material allows it to be used in new applications such as electronic circuits that can be printed easily and cheaply onto flexible films. MIRACULOUS STUFF: The unique properties of graphene make it interesting to both basic and applied science research. The figure shows the energy of electrons according to their wavenumber. The “occupied“ (shown in yellow-green) and “unoccupied“ (shown in blue-red) states are in seamless contact with each other at exactly six exceptional points. Image credit: © Paul Wenk – Wikipedia