Scientists at the University of Manchester have come one step closer to building usable graphene transistors using boron nitride to isolate the material from external effects.
"Leaving the new physics aside, technologically important in our demonstration is that graphene encapsulated within boron nitride offers the best and most advanced platform for future graphene electronics," said Manchester graphene researcher Professor Andre Geim. "It solves several nasty issues about graphene's stability and quality that were hanging for long time as dark clouds over the future road for graphene electronics."
"We did this on a small scale but experience shows that everything with graphene can be scaled up. It could be only a matter of several months before we have encapsulated graphene transistors with characteristics better than previously demonstrated," added Geim.
Graphene has the potential to produced incredibly fast transistors.
However, if supporting substrates are in anyway conductive, they squash its performance by stealing the electron clouds that hover over graphene.
To trick graphene into behaving as though it was isolated, the Manchester researchers have sandwiched it between two layers of boron nitride (BN, see below).
"Creating the multilayer structure has allowed us to isolate graphene from negative influence of the environment and control graphene's electronic properties in a way it was impossible before," said fellow Manchester scientist Dr Leonid Ponomarenko.
The structure chosen in this research has the BN-graphene-BN sandwich attached to a substrate and topped with a second graphene layer.
Perhaps surprisingly for graphene - a material touted as a conductor to replace ITO in displays - this structure turns it into an insulator.
"So far people have never seen graphene as an insulator unless it has been purposefully damaged, but here high-quality graphene becomes an insulator for the first time," said Ponomarenko.
Either layer can be insulating, although the sandwiched graphene layer usually has better quality and therefore better insulating properties.
"Insulating behaviour in graphene was never observed because of charged impurities on graphene surface," Ponomarenko told Electronics Weekly. "These impurities are responsible for formation of so-called electron-hole puddles. Within each puddle graphene is a good conductor and a network of such puddles makes graphene sheet conducting too. In our devices second graphene layer screens these impurities making puddles much shallower and turning first layer into insulator."
Compared to a proper insulator like boron nitride, insulating graphene is less compromising.
"Graphene becomes insulating in our experiments at rather low temperatures, and when the concentration of charge carriers in the graphene sheet is quite low," said Ponomarenko. "At high concentrations it is a pretty good conductor at any temperature, including at very low temperature."
Without second graphene layer, the other graphene layer cannot become insulating. Would the remaining sandwiched layer behave like single-layer graphene in free-space?
"Not quite," said Ponomarenko. "Support plays its important role. Suspended graphene has been studied by our and other groups. It behaves differently."
Rather than grow graphene and boron nitride on a substrate, the sandwich was built by hand.
"We used our famous 'sticky tape' method to isolate graphene and thin flakes of boron nitride and then transfer them on top of each other," said Ponomarenko.
A paper, "Tuneable metal-insulator transition in double-layer graphene heterostructures" has been published in the journal Nano-letters.
Why boron nitride?
Graphene is a chicken wire-like lattice of carbon atoms that can conduct electricity because a thin cloud of electrons hover either side of the mesh.
Boron has a slightly smaller atom than carbon, and nitrogen a slightly larger atom. Together they can also form a chicken wire mesh.
"Boron nitride a similar lattice constant [atom spacing] to graphene and the same hexagonal structure. Graphene has six carbon atoms in a ring," University of Cambridge graphene researcher Robert Weatherup told Electronics Weekly. "Boron nitride has six atoms in a ring alternating boron-nitrogen-boron-nitrogen. It is know as 'white graphene' because it is white in bulk."
But boron nitride lacks the electron clouds.
"Boron nitride is a proper wide-band-gap insulator. At room temperature it doesn't conduct electricity at all," said University of Manchester graphene researcher Dr Leonid Ponomarenko.
