A team of scientists, led by Professor Philip Hofmann from Aarhus University in Denmark, set out to determine specifically whether bilayer graphene could be used as a semiconductor. Their results suggest that it could replace silicon transistors in electronic circuits, and serve as the basis for chips that are faster and consume less energy than at present.
Graphene is pure carbon in the form of a very thin, almost transparent, sheet just one atom thick. It is widely hailed as a ‘miracle material’ because of its remarkable strength and efficiency in conducting heat and electricity. In its current form, graphene is not suitable for transistors, which are the foundation of all modern electronics. For a transistor to be technologically viable, it must be able to ‘switch off’ so that only a small electric current flows through its gate when in a standby state. Since graphene does not have a band gap, it cannot switch off.
The latest research used a new material – bilayer graphene – in which two layers of graphene are placed one on top of the other, leaving a small band gap to encourage the transfer of energy between layers. Using STFC’s Artemis laser, which is based at the Rutherford Appleton Laboratory in Oxfordshire, the researchers fired ultra-short pump laser pulses at the bilayer graphene sample, boosting electrons into the conduction band.
A second short, extreme ultraviolet wavelength pulse then ejected electrons from the sample. These were collected and analysed to provide a snapshot of the energies and movement of the electrons.
“We took a series of these measurements, varying the time delay between the infrared laser pump and extreme ultraviolet probe, and sequenced them into a movie,“ said STFC’s Dr Cephise Cacho, one of the research team. “To see how the fast-moving electrons behave, each frame of the movie has to be separated by just a fraction of a billionth of a second.”
Professor Hofmann added that, “What we’ve shown with this research is that our sample behaves as a semiconductor, and isn’t short-circuited by defects,” alluding to the fact that there can be imperfections in bilayer graphene as the layers sometimes become misaligned.
The research team asserts that the results of this research, in which the graphene showed no defects, suggest that further technological effort should be carried out to minimise imperfections. Once this is done, there is a chance that the switch-off performance of bilayer graphene can be boosted enough to challenge silicon-based devices.
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