When it was first discovered, Graphene promised to be a revolutionary, technological breakthrough. But, as years passed by, it seemed graphene would never live up to its potential. In 2010, after the Nobel Prize was awarded for discovering the material, Professor Geim issued statements of caution in the media a number of times. The graphene flagship to which Graphenea belongs, was awarded a billion euros and Professor Andrea Ferrari, from the University of Cambridge, stressed that a large amount of work is needed before graphene is able to become commercial.
Graphene Has No Bandgap
One of the main reasons for the apparent misunderstanding in the expectations of the public compared to those of the experts, was the lack of a band gap in graphene. A band gap includes a range of energies which charge-carrying electrons cannot occupy. This results in a significant difference in electron behavior of electrons below the gap to those that are above the gap. Electrons that are below the energy band gap are fixed to their positions and do not carry current. Those above the gap have sufficient energy to move around making the material conductive. Materials possessing a band gap are known as semiconductors with silicon being the most commonly used semiconductor.
All the electrons in graphene are made mobile by the absence of a band gap. While mobile electrons are excellent for carrying electricity and showing off a range of spectacular scientific breakthroughs, since the current cannot be turned off, this is a major contrast to the requirements of the traditional transistor, the basic element of electronic circuit logic.
Graphene Transistors are Real
A new class of graphene transistors was created by bombarding part of a graphene sheet with helium ions. That section of the sheet is then modified by the introduction of defects. The ion-irradiated section obtains a much larger charge carrier density than the remaining sheet creating an insulating region. Current flow is prevented by the insulating region between the electrodes of the device. The graphene transistor can be made to again conduct current by the application of an electric field to the pristine graphene parts now defaulting to the “0” state. An additional pair of electronic gates are applied to the field.
However, the graphene switch turned out to be of a low quality. The switching efficiency was poor when compared to silicon. Hence the slightly partial success of the first graphene transistors have caused a lot of skepticism.
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Figure 1. Printed circuits are about to become fast and flexible. Source: sxc.hu.
MIT researchers demonstrated a new way of increasing the charge carrier concentration in graphene. Chlorine was introduced to a graphene surface such that the sheet was not damaged in the process. Teams by Mildred Dresselhaus and Graphenea’s scientific adviser Tomas Palacios, changed the surface of graphene with chlorine plasma in a reactive ion etcher; a plasma-inducing chamber with tightly controlled conditions. After the process is tweaked carefully, they could retain the high charge carrier mobility of 1500 cm2/V, commonly obtained in untreated graphene.
The process helps graphene to be uniformly coated with coverage up to 45% of the surface area. According to therories, the all-important band gap will emerge in graphene. At just 5% more, it will be possible to have graphene transistors as conductive as silicon except that these would be ultrathin, transparent, printable, and flexible.
A Novel Kind of Graphene Logic
Almost simultaneously UC Riverside researchers have come up with a new approach to graphene transistors. Instead of making changes to graphene, Alexander Balandin and colleagues decided to change the logic. The team made use of regular band gap-less graphene and used the material’s unique property of negative differential resistance. Under certain conditions, with negative resistance, the electrical current increases with decreased applied voltage. A new type of transistor was achieved by biasing parts of the graphene sheet into the "negative" regime and other part normally. Here the researchers showed a logic gate made of graphene. At operating speeds as high as 400 GHz, the approach could usher in a new era of graphene information processing.
Conclusion
Graphene may have been hyped to a large extent but there is no denying that it truly is a wonder material and with the large amount of research being done, graphene will surely be used in a large number of applications. Even though much work is needed to fulfill the promise that this material holds, researchers and industrialists are aiming to achieve a common goal; new graphene-enabled technologies.
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