Graphene structures
Graphenes come in a variety of structures from atom-thick sheets to Buckyballs and Nanotubes.

Graphene: The Stuff Dreams Are Made Of

Once upon a time we though it was only good for pencil lead

By Rudolf Kraft

Graphite was discovered in the early 1500's in Borrowdale in the Cumberland Mountains in England. This metallic-like substance was mistakenly held for a lead ore and its first application served as a writing contriviance known until nowadays as a “lead pencil”. Graphite is comprised of nano dimensioned, irregular shaped and sized chunks of graphene. The common usage of graphite was restricted mainly to heat resistant applications and for lubrication purposes. Centuries were to pass until its true value and properites were discovered.

Hanns Peter Boehm first coined the term graphene in 1962 as a combination of graphite and the suffix –ene, describing a single-layer carbon foil. Many stacked graphene chunks comprise crystalline or flake forms of graphite; they are, so to say, the structural element of graphene and other carbon allotropes. There were several others who had toyed about with graphite but never proceeded further than skin deep.

It was Andre Geim and his assistant Konstantin Novoselov who intensified their combined efforts in 2004 to finally expose the true nature of graphene. In 2010, they were awarded the Nobel Prize for physics for their overwhelming discovery. The discovery of graphene has provided an unparalleled burst in many technical disciplines. This material had been before the very eyes of humanity for a long, long time but was never recognized for what it really is because no one had bothered to take a closer look.

Graphene is a one carbon-atom-thick two-dimensioned planar sheet / chunk (not 3- dimensional) of bonded carbon atoms that are densely packed into a honeycomb crystal lattice resembling a chicken wire structure. The carbon atoms are located in the corners of the hexagonal structure bonded via three connections to their neighboring carbon atoms.

Graphene, in its natural form graphite, does contain impurities. However, in its pure crystalline form, graphene reveals astonishing properties. Its tensile strength (bonding strength from atom to atom) is 217 times higher than that of steel and it is harder than diamonds. At normal temperature and pressure, it exhibits attributes that makes it nearly a superconductor; its heat and electric conductivity is 100x + better than that of copper.

It should be noted that the attributes of graphene are displayed on the nano scale but have been scaled up to the macro range with some success. It is difficult to maintain these properties on the macro scale but not impossible.

CNTs are cylindrically rolled up sheets of graphene; these cylinders can be produced in varying diameters and lengths. Production of CNTs, under controlled envorinmental conditions is self-assembling. The afore mentioned also applies to Fullerenes / Buckyballs. Buckyballs have been successfully used to deliver drugs in specific quantities to exact locations thus avoiding the “shotgun principle”.

Worldwide, scientists, universities, research centers etc. have pounced upon graphene and are scrutinizing the attributes of this amazing substance for possible future applications. Of all known elements none other besides carbon reacts chemically with so many other elements. A further astonishing property of graphene is its tuneability. When some of the carbon atoms in the crystalline lattice are replaced / doped with other elements it can be tuned over a broad band from an excellent conductor to a semiconductor up to an insulator. No other material displays even remotely such qualities. The packaging density with semiconductor functions (diodes, transistors, ICs etc) on graphene is much higher than on silicon. It is only a matter of time until semiconductor applications with silicon are replaced with such based on graphene. Graphene-based transistors can operate at much higher frequencies and more efficiently than the silicon transistors now in use. First transistor models have already established switching frequencies and efficiencies that can never be equalled from silicon types.

The structure of graphene is very stable and resistant to many aggressive acids and alkalis such as hydrofluoric acid and ammonia which makes it ideal as a coating for containers to harbor dangerous liquids or gases. The distance in the hexagonal structure is even so small that it proves to be an effective shield for hydrogen which cannot penetrate the graphene lattice.

Scientist and engineers at MIT have successfully grown multi-walled CNTs (MWCNTs) on graphene sheets.

The method employed by the MIT researchers enables a tremendous increase in surface enabling a high energy density capacitor without suffering a decrease on the power density potential of the electrodes. This is only the beginning but the perspectives look more than good. Such capacitors could equal and surpass the energy density of present LiBs with a power density enabling charging from a few seconds to minutes. Graphene as a literally indestructible material would suffer no capacity losses nor charging-cycle limitations. Such a capacitor without an eloctrolyte is not subject to temperature deviations as a battery is.

In the beginning of research on graphene, it was difficult to produce the material in the required purity and quantity. As it seems, this problem has also been solved by MIT researchers.

Here are some more interesting links reflecting the state of progress made on graphene research. http://www.media.rice.edu/media/NewsBot.asp?MODE=VIEW&ID=15053

Times Article Viewed: 18147
Published: 06-Jan-2011


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