Scientists ‘grow’ graphene nanoribbons

LONDON - Scientists from Swiss Federal Laboratories for Materials Testing and Research (Empa) and the Max Planck Institute for Polymer Research have, for the first time, successfully grown graphene ribbons that are just a few nanometres wide using a simple surface-based chemical method.

Graphene ribbons are considered to be “hot candidates” for future electronics applications as their properties can be adjusted through width and edge shape.

Transistors on the basis of graphene are considered to be potential successors for the silicon components currently in use. Graphene consists of two-dimensional carbon layers and possesses a number of outstanding properties: it is not only harder than diamond, extremely tear-resistant and impermeable to gases, but it is also an excellent electrical and thermal conductor.

However, as graphene is a semi-metal it lacks, in contrast to silicon, an electronic band gap and therefore has no switching capability which is essential for electronics applications.

Scientists from Empa, the Max Planck Institute for Polymer Research in Mainz (Germany), ETH Zrich and the Universities of Zrich und Bern have now developed a new method for creating graphene ribbons with band gaps.

To date, graphene ribbons have been “cut” from larger graphene sheets, akin to tagliatelle being cut from pasta dough. Or carbon nanotubes were slit open lengthwise and unfurled. This gives rise to a band gap via a quantum mechanical effect - the gap being an energy range that cannot be occupied by electrons and which determines the physical properties, such as the switching capability. The width (and edge shape) of the graphene ribbon determines the size of the band gap and thereby influences the properties of components constructed from the ribbon.

If extremely narrow graphene ribbons (well under 10 nanometres wide) that also have well-defined edges could be manufactured, so the reasoning, then they might allow for components exhibiting specific optical and electronic properties: depending on requirements, adjustment of the band gap could be used to fine-tune the switching characteristics of a transistor. This is no mean feat, as the lithographic methods that have been used until now, for example for cutting graphene layers, come up against fundamental barriers; they yield ribbons that are too wide and have diffuse edges.

In the issue of Nature published on 22 July 2010, scientists led by Roman Fasel, Senior Scientist at Empa and Professor for Chemistry and Biochemistry at the University of Bern, and Klaus Mllen, Director at the Max Planck Institute for Polymer Research, describe a simple surface-based chemical method for creating such narrow ribbons without the need for cutting, in a bottom-up approach, i.e. from the basic building blocks. To achieve this, they spread specifically designed halogen-substituted monomers on gold and silver surfaces under ultrahigh vacuum conditions. These are linked to form polyphenylene chains in a first reaction step.

In a second reaction step, initiated by slightly higher heating, hydrogen atoms are removed and the chains interconnected to form a planar, aromatic graphene system. This results in graphene ribbons of the thickness of a single atom that are one nanometre wide and up to 50 nm in length.

The graphene ribbons are thus so narrow that they exhibit an electronic band gap and therefore, as is the case with silicon, possess switching properties - a first and important step for the shift from silicon microelectronics to graphene nanoelectronics.

And if this wasn’t enough, graphene ribbons with different spatial structures (either straight lines or with zig-zag shapes) are created, depending on which molecular monomers the scientists used. (ANI)