Grephene: A new form of Carbon M. V. N. Murthy, The Institute of Mathematical Sciences, Chennai "Imagine a sheet of material that's just one atom thick, yet super-strong, highly conductive, practically transparent and able to reveal new secrets of fundamental physics. That's graphene, isolated by Andre Geim and Konstantin Novoselov, 2010 Nobel Laureates in Physics." --From the official announcement of the Nobel Prize to Professors Andre Geim and Konstantin Novoselov of the University of Manchester, UK. Carbon is the most common element known from prehistoric times. So why is graphene, a new form of carbon, so exciting? To understand the importance of graphene, let us first look at this wonderful material called carbon and its many forms. Origin of Carbon Carbon is one of the most abundant elements in the Universe, after hydrogen, helium and oxygen. It is the chemical basis of all known forms of life--indeed of all organic matter. It is the second most abundant element by mass (18.5%) in the human body after oxygen. The name carbon comes from the Latin word "carbo" meaning coal or charcoal. In nature the element carbon is manufactured mainly during the fusion processes that occur inside massive stars (much more massive than the Sun) where effectively three helium nuclei combine to form carbon nucleus. This carbon is scattered into space during supernova explosions (death of massive stars). The dust containing heavy elements like carbon is captured by other stars and planets during their formation. Thus the birth of life as we know it---for which carbon is essential---has its origins in the death of massive stars. ------------------------------------------------------------ BOX: Different forms of Carbon Carbon is known from pre-historic times in the form of soot and charcoal whereas in the form of diamond it is known from as early as 2500 BC. It was Lavoisier who first showed in 1772 that diamonds are indeed a form of carbon. Carbon in the form of graphite was also known from early days although it was thought of as a form of lead---hence the name lead pencils when they are really filled with graphite. In the late 18th century, Carl Wilhelm Scheele showed that even graphite was a form of carbon. Indeed carbon is the main constitutent for the hardest (diamond) material to the softest (graphite) material known to us. Atomic carbon as we all know has the atomic number 6. There are six electrons which go around a nucleus which consists of 6 protons and 6 neutrons. Its standard atomic weight is 12.01 gms per mole. Carbon occurs in many stable forms or molecular configurations which are called "allotropes". The three most common naturally occuring forms are: 1. Amorphous carbon: Its common form is as a powder and is a combination of carbon atoms in a non-crystalline, irregular state. Substances such as charcoal, soot, etc., are made up of amorphous carbon with a density of 1.8-2 gms per cc. 2. Graphite: At normal pressure carbon takes the form of graphite in which the carbon atoms are arranged in planes. The planes are like sheets and are covered by an arrangement of hexagonal rings of carbon atoms. In graphite these sheets of carbon atoms are stacked one above the other with very weak bonds between the sheets so much so these sheets can slip easily past one another. This also gives graphite the softness. The density of graphite is about 2.27 gms per cc. Graphite is also a good conductor due to its electronic configuration. 3. Diamond: When carbon is subjected to very high pressures it combines to form diamond. Unlike graphite here the carbon atoms are arranged in a 3-dimensional network of tetrahedrons (pyramid-shape). Because the carbon-carbon bond is very strong, diamond is one of the hardest occurring substances. You cannot scratch a diamond easily. Diamonds are however not forever---diamond is unstable under normal conditions and changes to graphite, but very, very slowly. The density of diamond is also very high---about 3.6 gms per cc. In addition carbon can be prepared to form interesting structures. One such structure is the "bucky ball" which is actually like graphite with a hexagonal arrangement interspersed with pentagons. This can be folded to form a spherical football-like structure: it is essentially one large molecule with 60 carbon atoms. Carbon can also be made into tubes, so called buckytubes. Carbon nano-tubes are similar to buckyballs except they are bonded in a curved sheet to form a hollow cylinder. A large family of organic molecules (present in living things) are composed of hydrogen atoms bonded to carbon atoms. Furthermore, when combined with oxygen, carbon forms many groups of biological compounds such as sugars, alcohols, fats, etc. Carbon is the basis of all known organic life and is the fundamental basis of organic chenistry. END OF BOX -------------------------------------------------------------------- Amazing graphene We know that Carbon exists in various forms. Graphene is the latest addition to various forms of carbon and is a new material. It is carbon arranged in layers (planes) which are just one atom thick! In each layer, the atoms are arranged in a honey-comb lattice. You cannot have anything thinner than this but it is also the strongest of materials. It conducts electricity better than copper and conducts heat better than any other known material. While it is completely transparent to light, it is so dense that even helium (smallest gas atom after hydrogen) cannot pass through it. Graphene, as the name indicates, can be easily obtained from graphite. A typical one-millimeter thick graphite contains about three million layers of graphene stacked one on top of another. The layers are held together weakly and therefore can be easily separated. The Nobel winning work When we write with an ordinary pencil this is precisely what we are doing---peeling the layers off to leave the impression on paper. But one atom thick---that is where the Nobel Prize comes in. In a sense graphene has always been there as layers in graphite. In fact, all forms of carbon have always existed in nature. The challenge therefore was to be able to separate it and identify it which was akin to doing the impossible. That is precisely what Geim and Novoselov did. They did it in a remarkably simple fashion using adhesive tape to peel off thin layers from a larger piece of graphite. Many samples are obtained by peeling off flakes from graphite using the tape trick. Each sample may consist of many layers of graphene. The problem is then to identify the single layer graphene pieces among the peeled samples. Before 2004, even though many had tried, it was thought to be impossible, that is until Geim and Novoselov demonstrated it. In order to identify that they have indeed obtained a single atom thick layer of graphene, they attached the flakes to a plate of oxidised silicon and put it under a microscope to search for the single layer. This is the non-trivial part of it as it is like searching for a needle in a haystack. If the thickness of the substrate is just right (300 A or about 1/30 micrometres) then the single layer pieces look distinctly different from the multi-layer pieces and it is possible to locate them. This is the skill that was mastered by Noveslov and Geim and it won them the Nobel prize. The thickness of the substrate is crucial. If it is 315 Angstroms instead of 300 Angstroms, the graphene is not distinguishable. Under the microscope, with the right conditions, one sees a rainbow of colours which puts graphene in view as a truly two-dimensional crystalline material even at room temperature. Graphene's bizarre properties Initially Geim and Novoselov could obtain very small samples but they discovered the perfect composition of graphene---a neat ordering of hexagonal structures forming a lattice of carbon atoms. The lattice allows the electrons toetravel long distances without disturbance, making graphene a very good conductor of electricity. The lattice also makes the electrons feel much lighter, almost like the particles of light, namely photons. In a sense graphene's properties are bizarre and contradictory---it is flexible like plastic but is stronger than diamond; it conducts electricity like a metal but is transparent to light like glass. Applications The initial fascination about graphene stemmed from its importance to theoretical physics due to its bizarre properties. The exictement about graphene is, however, also due to its many possible applications. Because graphene is such a good conductor, scientists are dreaming of graphene transistors which will be faster than the silicon transistors which are widely used now. They also become much smaller since graphene components can be packed on a chip more tightly. Silicon based transistors (chips) ushered in the first revolution in miniaturisation. Now graphene transistors are likely to usher in the second revolution in miniaturisation. Scientists are also dreaming of paper-thin transparent computer monitors, touch screens, light panels or even solar cells that can be rolled up and carried easily. Given the strength of graphene layers, these devices may also be mechanically more robust. Sensors made of graphene could detect the smallest levels of pollution since even a single molecule adsorbed on the graphene surface can be detected. Indeed there is no limit to applications that one can dream of, given the extra-ordinary properties of graphene. Just as with silicon, the abundance of graphene means that it is easily available and cheap for manufacturing devices. The applications are endless! About the Scientists Andre Geim, now 51 years old, and Konstantin Novoselov, now 36, are presently professors at the University of Manchester in UK. Prof. Novoselov was a Ph.D student of Prof. Geim. They have always had fun playing with materials at their disposal, trying to create something new. Prof. Geim had in fact managed to make a live frog levitate (float) in a magnetic field which got him the IgNobel prize in 2000---a satirical prize awarded to work that "make people laugh first and think second". Prof. Geim has the distinction of being the first person to win both prizes.