“We began by first designing and fabricating a new type of molecule that would self-assemble into nanoribbons that are wider than those previously available,” explains Michael Crommie, who co-led the team with Felix Fischer at the University of California in Berkeley. The wider molecular building block is 2,2′-di((1,1′-biphenyl)-2-yl)-10,10′-dibromo- 9,9′-bianthracene and the narrower molecular building block is 10,10′-dibromo-9,9′- bianthracene. (This molecule was previously discovered by Roman Fasel, Klaus Muellen and colleagues).

“We deposited the new, wider nanoribbon building blocks onto a gold surface along with the narrower molecules. When we heated the surface up, the molecules spontaneously joined together and chemically bonded into junctions containing both narrow and wide segments. The new heterostructures are thus built from the bottom up using molecular elements, as opposed to etching small devices from much larger blocks of material, as is normally the case in semiconductor processing”.

Varying the local bandgap

These nanoribbon heterostructures allow the researchers to create extremely small semiconductors in which they can vary the local bandgap (or energy gap) along the length of the nanoribbon semiconductor. “Such ‘bandgap engineering’ as it is called is important,” explains Crommie, “because the bandgap landscape is responsible for controlling the energetics of how electrons move in a semiconductor (it determines what regions electrons accumulate in, and what regions they avoid). Ab intito calculations by team member Steven Louie explain the physical origin of the observed local bandgap variation and being able to control this property along the length of a nanoribbon should enable us to create more complex and useful nanoribbon-based devices in the future, such as diodes and different types of transistors.”

Since the new nanoribbon structures are based on the same material as graphene (a sheet of carbon just one atom thick), we expect them to have the same exceptional properties as graphene itself, he adds. Such properties include extraordinary strength, flexibility and the ability for electrons to move extremely fast within the material. “The nanoribbons are also much smaller than conventional devices and are structurally perfect at the sub-nanometre scale – something that is not possible to achieve by any conventional fabrication technique.”

“These nanoribbon structures are promising candidates for very high quality circuit elements for future electronics,” Crommie tells nanotechweb.org. “They may allow us to create smaller, faster and more energy efficient electrical devices.”

Important step forward

Although there is still much work to do before these nanoribbons can be incorporated into useful technologies (such as efficiently wiring them up into complex circuitry on a chip), the team says that its work is an important step forward.

Roman Fasel, head of the nanotech@surfaces laboratory at Empa, Swiss Federal Laboratories for Materials Science and Technology, who was not involved in this work, says that Crommie and colleagues have “convincingly demonstrated the ability to engineer the bandgap in these materials by rationally designing the precursors, making it possible to craft artificial junctions from the bottom up. The fabrication of gap-modulated graphene nanoribbons is an important first advance towards realizating complex graphene nanoribbon devices.”

With gap-modulated graphene nanoribbon heterojunctions at hand, the next stage would be to fabricate devices and study their electronic transport properties, he adds. “The researchers have already shown that they can fabricate graphene nanoribbon field-effect transistor geometries incorporating nanoribbons of constant width (this work was published in 2013 in Appl. Phys. Lett. 103 253114) and it will be interesting to see further progress in this direction.

ldentical nanoribbons in large quantities

Val Zwiller of the Kavli Institute of Nanoscience at TU Delft in the Netherlands, who was not involved in this work either, comments that the bottom-up approach described in this story is indeed “very interesting” and “should enable the production of large numbers of identical nanoribbons, something that is not possible at this point with techniques such as etching”.

Crommie and colleagues say that they are now busy looking at how to perfect their growth process for creating these nanoribbon junctions and would like to better control the lengths of the nanoribbon segments. “We also want to explore how these nanoribbon structures behave in electrical circuits when they are placed on insulators and connected at their ends with metal leads,” explains Crommie.

“So far, we have grown these new structures on metal substrates and studied their properties using high-resolution scanning tunnelling microscopy, but in the future we hope to directly incorporate them into electrical circuits. We also hope to create entirely new nanoribbon-based heterostructures by making use of new types of molecular building blocks.”

The work is published in Nature Nanotechnology doi:10.1038/nnano.2014.307.