Graphene, a sheet of carbon just one atom thick, is a promising material for making molecular electronic devices of the future thanks to its unique electronic and mechanical properties, which include extremely high electrical conductivity and exceptional strength. Indeed, many researchers believe that it might even replace silicon as the electronic material of choice in the future. Unlike semiconducting silicon, however, graphene has no gap between its valence and conduction bands. Such a bandgap is crucial for electronics applications because it allows a material to switch the flow of electrons on and off.

One way of introducing a bandgap into graphene is to reduce its width to less than 10 nm by cutting it into nanoribbons, nanomeshes or quantum dots. Another way is to dope it and replace some of the carbon atoms in the lattice with other atoms. Unfortunately, none of these methods are entirely satisfactory because electron scattering introduced during the processes significantly degrades electronic properties like electron mobility in the material.

Band gap opening

A group led by Won Il Park from Hanyang University, Seoul, has now shown that adding 2D islands of silicon to graphene can trigger bandgap opening of between 2 and 3 meV in the carbon material. The researchers also found that field-effect transistors made from the graphene-silicon hybrid have current levels that are three times higher than in graphene sheets alone and that the maximum-to-minimum current ratio in the structure gradually increases with increasing silicon concentration.

The team, which includes researchers from the Korea University Sejong Campus, the Ulsan National Institute of Science and Technology and the University of Illinois at Urbana−Champaign, began by transferring large-surface-area sheets of graphene onto silicon dioxide/silicon substrates. The scientists then formed 2D silicon islands by exposing the graphene to silicon vapour at high temperatures of 1000 °C. “Thanks to graphene’s strong sp2-hybridized C–C bonds, silicon does not readily displace carbon atoms in the lattice,” explained Park. “Instead, physisorped silicon adatoms form very thin islands that are tightly bonded to the graphene via van der Waals interactions. These interactions break the sublattice symmetry in graphene, so opening up a bandgap of 2–3 meV in the material.”

Although 2–3 meV is too small for practical applications (modulation of the band structure only seems to occur in localized regions of the graphene under the silicon islands), the researchers say that they hope to increase this value by spreading the 2D silicon islands over larger areas on the graphene sheets. They will also try introducing islands made of other elements. “Another of our goals is to make different types of devices, other than transistors – such as sensors, for example – from our graphene-silicon sheets,” Park told nanotechweb.org.

The current work is reported in ACS Nano.