These images show the graphene nanoribbons lying across the gold substrate. Image: Empa.
These images show the graphene nanoribbons lying across the gold substrate. Image: Empa.

Graphene ribbons that are only a few atoms wide, so-called graphene nanoribbons, have special electrical properties that make them promising candidates for the nanoelectronics of the future. While graphene – a one-atom thick, honeycomb-shaped carbon layer – is normally a conductive material, it can become a semiconductor when in the form of nanoribbons. Graphene nanoribbons have a sufficiently large energy or band gap where no electron states can exist, which means they can be turned on and off – and thus could become a key component of nanotransistors.

Scientists know that the smallest details in the atomic structures of these graphene bands can have massive effects on the size of the energy gap and thus on how well-suited nanoribbons are as components of transistors. The energy gap depends on both the width of the graphene nanoribbons and on the structure of their edges. Since graphene consists of equilateral carbon hexagons, the edges may have a zigzag or a so-called ‘armchair’ shape, depending on the orientation of the ribbons. While bands with a zigzag edge behave like metals and are electrically conductive, bands with an armchair edge are semiconductors.

This poses a major challenge for the production of nanoribbons. If the ribbons are cut from a layer of graphene or made by cutting carbon nanotubes, the edges may be irregular and thus the graphene ribbons may not exhibit the desired electrical properties.

Researchers at Empa in Switzerland, in collaboration with the Max Planck Institute for Polymer Research in Mainz, Germany, and the University of California at Berkeley have now succeeded in growing ribbons exactly nine atoms wide with a regular armchair edge from precursor molecules. As they report in a paper in Nature Communications, the specially prepared molecules are first evaporated in an ultra-high vacuum. After several process steps, they are then combined like puzzle pieces on a gold base to form the desired nanoribbons of about 1nm in width and up to 50nm in length.

These structures, which can only be seen with a scanning tunneling microscope, have an energy gap that is relatively large and precisely defined, allowing the researchers to go one step further and integrate the graphene ribbons into nanotransistors. Initially, however, their first attempts were not very successful: measurements showed that the difference in the current flow between the ‘on’ state (with applied voltage) and the ‘off’ state (without applied voltage) was far too small.

This turned out to be caused by the dielectric layer of silicon oxide, which connects the semiconducting layers to the electrical switch contact. In order to have the desired properties, this layer needed to be 50nm thick, which in turn influenced the behavior of the electrons.

To solve this problem, the researchers massively reduced the thickness of the dielectric layer by replacing the silicon oxide with hafnium oxide (HfO2). As this layer is just 1.5nm thick, the ‘on’-current is orders of magnitudes higher.

Another problem was incorporating the graphene ribbons into the transistor. In the future, the ribbons shouldn’t lie across the transistor substrate, but should instead be aligned along the transistor channel. This would significantly reduce the currently high level of non-functioning nanotransistors.

This story is adapted from material from Empa, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.