Graphene: the magic carpet made of carbon
TWENTY YEARS ago Drs. Kroto and Smalley discovered a new allotrope or naturally occurring form of elemental carbon, called buckyball or fullerene. It was found to be a soccer ball-shaped molecule of nanometre size.
Fullerene formally launched the nanorevolution. Fullerene and similar nanoparticles can easily slip through the membranes of biological cells. Chemists and material scientists next made nanocigars and nanotubes. These take us from the `zero dimension' of fullerenes and nanoballs (which are closed-in surfaces that cannot be extended in any direction) to the one-dimensional step.
Being hollow, nanotubes can act as storage bags; but since the diameter is nanosized, they can take in only small sized molecules. Many think they are ideal light-weight vessels for storing hydrogen gas in future cars and other vehicles.
Electrical conduction
Now comes the extension into two dimensions — from ball to tube to sheets or carpets. Not that carbon sheets are not known. Graphite is not only carbon sheet, but also conducts electricity.
In contrast, diamond and fullerene do not; carbon nanotubes do when appropriately processed. But graphite is macroscopic — comes not in nano, not even micro, but in milli or macro-sizes. We can peel graphite sheets layer after layer and bring the sheet thickness down — but how far down?
The answer is: down to one atom thick.
This has now been done and the sheet or carpet is called graphene. It is a single planar sheet of carbon atoms — a single layer of carbon atoms densely packed into a benzene-ring structure. It is however not an allotrope because (a) the sheet is finite in size and (b) it contains at least one more element, namely hydrogen at the edges.
A typical graphene has the chemical formula C{-6} 2H{-2}O. In effect, it is a flake of graphite with hydrogen atoms used to terminate the dangling bonds. Graphene is thus a sheet of interlocked hexagons of carbon. The thickness of the sheet is the diameter of just a single carbon atom, namely 350 picometres or 0.35 nanometres.
Graphene may thus be described as the `Dhake ki Malmal' of material sciences. Recall how the textile weavers of yesteryears from Dacca, Bangladesh used to produce gossamer - thin muslin that was much coveted and specially ordered by royalty across the world since centuries.
A full sari or dupatta of this muslin would slip through the ring of milady's finger. A sheet of C{-6}2H{-2}O graphene, when tightly rolled should be able to pass right through the gaps or channels of biological cells.
Just as Dhake ki Malmal was a deceptively tough fabric, graphene too is incredibly strong, stable and flexible.
Latest addition
Graphene is the latest addition to the family of the fullerenes and nanotubes. Indeed, carbon nanotubes may be thought of as cylinders or tubes made of graphene. Fullerene (or buckyball or C{-6}O) can be made upon introducing 12 pentagons to the graphene hexagon.
Addition of less than 12, say one or two, will convert graphene into a nanotube. Thus, in principle, one can do molecular origami using graphene.
How is graphene made? One way is to mechanically peel small pieces of graphite that has been oriented in a chosen fashion.
This was the method adopted by researchers of the University of Manchester, U.K. They have been collaborating with colleagues at the Institute of Microelectronics Technology and High Purity Materials at Chernogolovka in the Moscow region of Russia.
The team has been focusing on the electronic properties of graphene and its potential use in microelectronic devices. Concurrently and independently, the team of Dr. Walt de Heer at the Georgia Institute of Technology in the U.S. has been doing the same, and have a `proof of principle' electronic device made of graphene.
They start with a wafer of silicon carbide and heat it under high vacuum. This drives the silicon atoms from the surface, leaving a thin continuous layer of graphene. This group again is interested in the novel properties of graphene, which make it an exceedingly attractive material for micro-electronics.
Indeed, Dr. de Heer says "we expect to make devices of a kind that do not have an analogue in silicon-based electronics, so this is an entirely different way of looking at electronics". Working on graphene since 2001, his group has now come out with a prototype electronic device made of a stack of about 10 layers of graphene.
What makes graphene such an attraction in microelectronics? First of all, a sheet of graphene has an `electron cloud' of pi electrons hovering around the sheet of carbon atoms. Since the material itself is of atomic and nanometre dimensions, effects special to quantum theory on one hand and relativity on the other become important.
No rest mass
The electrons in graphene behave like they have no rest mass at all. This makes them travel as fast as a million metres per second. In effect, this would allow manipulation of electrons as waves rather than as particles, much like photonic systems control light waves. We thus have a novel form of carbon that is an excellent conductor with very little resistance.
And unlike carbon nanotubes, where consistent sizes and electron properties have been problematic, and also where electrical resistance is higher, graphene turns out to be the ace material.
This very high velocity of electron flow in graphene plus the fact that it goes without any collisions on the way has made people suggest that graphene would act as a `ballistic' conductor/transistor. Professor Andre Geim there says, "People have been trying to make transistors faster and smaller.
There is a Holy Grail of electronics that engineers call ballistic transistors — ultimately faster than anything." The Manchester team's experiments suggest that such a ballistic transistor is possible. It appears that graphene is full of surprises and offers greater promise than one could reasonably hope for in a new experimental system.
D. BALASUBRAMANIAN
dbala@lvpei.org
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