Graphene-based material lights the way to ultralow-power transistors

An international team of scientists has discovered what they believe to be a new route to ultralow-power transistors. Published in the journal Physical Review Letters, the key to their research lay in the use of a graphene-based composite material.

As transistors are squeezed into ever smaller areas within computer chips, the semiconductor industry struggles to contain overheating in devices. Researchers from the University of York and Roma Tre University believe the solution lies in composite materials built from monolayers of graphene and the transition metal dichalcogenide (TMDC).

“For many years, we have been searching for good conductors allowing efficient electrical control over the electron’s spin,” said the University of York’s Dr Aires Ferreira, lead researcher on the study, referring to the electron’s tiny compass needle.

“We found this can be achieved with little effort when two-dimensional graphene is paired with certain semiconducting layered materials. Our calculations show that the application of small voltages across the graphene layer induces a net polarisation of conduction spins.

The electron’s spin is like a tiny, point-like magnet which can point only in two directions: up or down. In materials where a major fraction of electrons’ spins are aligned, a magnetic response is produced, which can be used to encode information.

‘Spin currents’ — built from ‘up’ and ‘down’ spins flowing in opposite directions — carry no net charge and therefore, in theory, produce no heat. The control of spin information would therefore open the path towards ultra-energy-efficient computer chips.

“The current-induced polarisation of the electron’s spin is an elegant relativistic phenomenon that arises at the interface between different materials,” said University of York PhD student Manuel Offidani, who carried out most of the complex calculations in the study.

“We chose graphene mainly because of its superb structural and electronic properties. In order to enhance the relativistic effects experienced by charge carriers in graphene, we investigated the possibility of matching it with recently discovered layered semiconductors.”

The researchers showed that when a small current is passed through the graphene layer, the electrons’ spins polarise in plane due to ‘spin-orbital’ forces brought about by the proximity to the TMDC base. They also showed that the efficiency of charge-to-spin conversion can be quite high even at room temperature, with the unique character of electronic states in graphene enabling charge-to-spin conversion efficiency of up to 94%. This opens up the possibility of a graphene-based composite material becoming the basis for ultracompact and greener spin-logic devices.

“We believe that our predictions will attract substantial interest from the spintronics community,” said Dr Ferreira. “The flexible, atomically thin nature of the graphene-based structure is a major advantage for applications. Also, the presence of a semiconducting component opens up the possibility for integration with optical communication networks.”