Ribbons Nano-Style

Near Zürich, Switzerland, ribbons are being fabricated, but these are not the kind of ribbons that the average consumer would choose to decorate their presents with...

Ribbons Nano-Style

Near Zürich, Switzerland, ribbons are being fabricated, but these are not the kind of ribbons that the average consumer would choose to decorate their presents with.  They are made out of a super material, graphene, and they are also rather small, too small even for your smallest gift box.  Graphene is a form of carbon consisting of hexagonal layers (like chicken wire) one atom thick.

Structure of Graphene, Image ©

At EMPA (EidgenössischeMaterialprüfungs- und Forschungsanstalt = Swiss Federal Laboratories for Materials Science and Technology), Prof Roman Fasel is preparing and investigating these tiny ribbons made out of carbon atoms; they are so small they are called nanoribbons (also known as GNRs[1]), their width below 50 nanometres (nm), several thousand times narrower than the width of a human hair.  In the world of atoms, these nanoribbons are hundreds of atoms wide.  They are too big to be called molecules and they are still smaller than the tiniest speck of dust. We’ve entered the world of nanotechnology.

Nanotechnology can be defined “as the manipulation of matter with at least one dimension sized from 1 to 100 nanometres” [Wikipedia]. This definition reflects the fact that quantum mechanical effects are important at this scale, and this leads to materials with often special properties.  In GNRs, the conductance can be fine-tuned; calculations show that certain ribbon types can be semiconducting, whilst others are metallic, depending on their width and orientation of the hexagonal network.

Device Design

Prof Fasel’s group uses molecular precursors as building blocks for the GNRs, and such a “bottom-up” approach allows for very precise synthesis not only of pure (carbon-based), but also of “doped” devices (“doping” here means that atoms other than carbon are incorporated into the molecular structure of the ribbon) – in silicon-based chips, found in today’s consumer electronic devices, the doping is normally done after the crystal of pure silicon has been formed and cannot be controlled as precisely.  The ribbon precursor molecules resemble small parts of the atom-size chicken wire mesh, which can be fused in well-controlled chemical reactions.

Future Directions I - Ultrafast Transistors

If we take a peek into the near future, astonishing applications await us that come from graphene nanoribbons. Prof Fasel predicts that “due to their intriguing properties, atomically precise graphene nanoribbons (GNRs) bear high potential for applications based on electronic and other advanced functionalities. One particularly exciting aspect of the “bottom-up” fabrication of GNRs is the possibility of nitrogen doping by a simple modification of the precursor molecules, where some of the carbon atoms are replaced with nitrogen. The incorporation of nitrogen alters the GNR’s electronic properties. By assembling “normal” segments next to ones that have been doped with nitrogen, so-called heterojunctions may be produced. Such heterojunctions exhibit similar properties to a classic p-n junction, as found in everyday diodes or transistors”.

Theoretical models had previously predicted that graphene could be made into transistors more than several hundred times faster than today's silicon transistors. Already arrays of hundreds of graphene transistors on a single chip have been fabricated. And very recently new observations of graphene nanoribbons have shown exceptionally good electron transport, which is actually at least a factor of 10 better than theoretical predictions. The reason for this enhanced conductivity isn’t yet clear, but these nanoribbons could form some of the necessary components (e.g. wires) for graphene-based nanoelectronics.

Structural model (left) and 3D picture of a scanning tunnelling microscope view
of a zigzag shaped graphene nanoribbon. (Courtesy: Empa)

Silicon, ubiquitous in today's computer processors easily overheats at high clock speeds, limiting processing throughput. In graphene, electrons move with almost no resistance, generating little heat. Additionally, graphene is a good thermal conductor, allowing heat to dissipate quickly. Hence graphene-based electronics could operate at much higher speeds. Right now silicon-based computers are stuck in the low gigahertz range. But with graphene we can speed up computer clocks to terahertz frequencies, which is a factor of a thousand faster.

Besides making computers faster, graphene electronics could be useful for communications and imaging technologies that require ultrafast transistors. Indeed, graphene is likely to find its first use in high-frequency applications such as terahertz-wave imaging, which can be used to detect hidden weapons. And speed isn't graphene's only advantage. Silicon can't be carved into pieces smaller than about 10 nanometres without losing its attractive electronic properties. But the basic physics of graphene remain the same, and in some ways its electronic properties actually improve, in pieces smaller than a single nanometre.

Future Directions II – Solar Cells

Furthermore, ultra-narrow GNRs are expected to exhibit an unusually high absorption of visible light and should thus be well-suited as absorption layers in organic solar cells, and could help to create future sustainable energy supplies.

Currently these amazing materials are only available in the tiniest of amounts, so production needs to be scaled up many orders of magnitude before we can reap the benefits in real-life products.

Other groups in the NanoTP network are focussing on synthesising molecular precursors of “rolled-up” graphene – carbon-nanotubes, and such precursors could be used to build nanotubes with precisely defined structure, in analogy to the process to build nanoribbons mentioned above.

Nanoribbons are not for wrapping up presents, they are gifts for novel technologies themselves.

Bernd Eggen

NanoTP associate

NanoTP is an EC-funded COST action, bringing together some 320 scientists from well over 100 institutions in 30 countries.  Its aims are “designing novel materials for nanodevices: from theory to practice” through research grouped around synthesis and characterisation of nanomaterials, device design and theoretical modelling.

NanoTP focuses on the understanding and control of interfaces between nanostructures.  The control of materials and their interfaces remains a central goal of modern materials physics and chemistry as atomically controlled interfaces become increasingly important in electronic nanodevices and their integration.

The author is grateful for additional input & advice from Dr Chris Ewels and Prof Roman Fasel.

[1]GNRs = graphene nanoribbons

3 July 2014