Narrow nanoribbons for graphene transistors

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Graphene crystal.
Graphene crystal.

Using a simple chemical method, researchers have succeeded for the first time in growing graphene ribbons just a few nanometers wide on specially prepared surfaces. Graphene ribbons are seen as highly promising candidates for electronic applications, as their properties can be varied according to their width and the shape of their edges.

Graphene transistors are seen as possible successors to today’s silicon transistors. Formed from two-dimensional layers of carbon, graphene possesses a number of quite extraordinary properties: not only is it harder than diamond, extremely resistant to tensile stress and impervious to gases, it is also an excellent electrical and thermal conductor. However, as graphene is a semi-metal, it has no band gap, unlike silicon, and therefore no switching function, which is THE essential prerequisite for electronic applications. Researchers have now developed a new process for producing graphene ribbons with a band gap.

Extremely narrow graphene ribbons

Until now, graphene ribbons have been obtained by "cutting" graphene layers in much the same way as noodles are cut from rolled-out dough. Or carbon nanotubes were unrolled after being cut lengthwise. The result is a quantum band gap - an energy domain in which no electron can be found, and which determines physical properties such as commutability. The width and shape of the ribbon’s edges determine the size of the band gap, and thus influence the characteristics of the components built from it.

These researchers came up with the idea that, if it were possible to produce extremely narrow graphene ribbons - well under ten nanometres - and still with well-defined edges, it should be possible to use them to produce components with any number of variable optical and electronic characteristics. This is because, depending on requirements, it would be possible to vary the bandgap width of the ribbons and thus also the switching characteristics of the resulting transistors. However, this is a highly complex business, as the lithographic methods used up to now to cut graphene wafers, for example, came up against their fundamental limitations: they only delivered ribbons that were too wide and, what’s more, had diffuse edges.

Growing graphene ribbons

The researchers describe a simple method using surface chemistry to produce graphene ribbons of such narrowness without resorting to any cutting - bottom-up, from isolated molecules. To achieve this, they deposit special monomers on gold or silver surfaces under ultra-high vacuum. These monomers are substituted at "strategically" important positions by halogens, which polymerize in a first step to form polyphenylene chains.

In a second step, a reaction triggered by higher heating causes the removal of hydrogen atoms and the coupling of polymer chains into a graphene-like aromatic system. The resulting graphene ribbons have a thickness of one atom, a width of one nanometer and a length of up to 50 nanometers. These graphene ribbons are so narrow that they exhibit a bandgap, and thus, like silicon, possess switchability properties - an important first step towards the transition from silicon-based microelectronics to graphene-based nanoelectronics. But even better: depending on the monomers used, graphene ribbons are formed with different structures, either straight or zigzagged.

Other interesting perspectives

As the researchers are now able to produce graphene tapes (almost) at will, they propose in the next stage to study how the different shapes of the tapes’ edges influence their magnetic properties. Their surface chemistry method opens up further interesting prospects for the targeted doping of graphene strips: the use of monomers with nitrogen or boron atoms in exactly defined positions, or monomers with additional functional groups, should make it possible to produce positively or negatively doped graphene ribbons. The combination of different monomers is also possible, and could enable the creation of heterojunctions - in other words, junctions between graphene ribbons with different band gaps - which could be used in solar cells or high-frequency electronic components. The method developed lends itself to this, as the researchers have already demonstrated: with two suitable monomers, they succeeded in creating a junction node linking three graphene ribbons together.

To date, work has focused on graphene ribbons synthesized on metal surfaces. However, to be able to use these graphene ribbons in electronics, they need to be synthesized on semiconductor surfaces, or methods developed to transfer them from metal surfaces to semiconductor surfaces. And the first results already obtained in this direction make these researchers optimistic.