Graphene nanoelectronics a step closer

In a recent issue of Nature, scientists from Empa, the Max Planck Institute for Polymer Research in Mainz (Germany), ETH Zürich and the Universities of Zürich und Bern explained how transistors on the basis of graphene are considered to be potential successors for the silicon components currently in use. Graphene consists of 2d carbon layers and is not only harder than diamond, tear-resistant and impermeable to gases, but 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. The researchers now claim to have developed a new method for creating graphene ribbons with band gaps. To date, graphene ribbons have been cut from larger graphene sheets, 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. The researchers believes that if extremely narrow graphene ribbons ( under 10nm wide) that also have well defined edges could be manufactured, 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. Until now, the lithographic methods that have been used yield ribbons that are too wide and have diffuse edges. The team claim to have established a simple surface-based chemical method for creating such narrow ribbons without the need for cutting. 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. According to the team, this results in graphene ribbons of the thickness of a single atom that are 1nm wide and up to 50nm 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. Graphene ribbons with different spatial structures (either straight lines or with zig-zag shapes) are created, depending on which molecular monomers the scientists used. As the scientists claim they can almost produce graphene ribbons at will, they intend to study how the magnetic properties of the graphene ribbons can be influenced by different edge structures. To date, the scientists have focused on graphene ribbons on metal surfaces. However, to be usable in electronics the graphene ribbons need to be created on semiconductor surfaces or methods must be developed to transfer the ribbons from metal to semiconductor surfaces.