Graphene, a sheet of carbon just one atom thick, is promising for making molecular electronic devices of the future thanks to its unique electronic, mechanical and thermal properties that include extremely high electrical conductivity and exceptional strength. Recently, researchers have also turned their attention to multilayer graphene because they expect it to have even more bizarre characteristics thanks to enhanced electronic interactions between the layers making up the structure.

Trilayer graphene comes in two "flavours" with different layer stacking orders: ABA and ABC. The two only differ in that the top layer is shifted by the distance of one carbon atom in the sheet relative to another (see figure). In such multilayer systems, the stacking order dramatically affects the electronic properties of the structures. The effect in graphene is expected to be particularly pronounced as ABA-stacked trilayers are predicted to be semi-metals with tunable band overlaps, and ABC-stacked trilayers are predicted to be semiconductors with tunable bandgaps.

By measuring the transport properties of multilayer graphene Jeanie Lau and colleagues have now come up against some rather unexpected results – that ABA-stacked trilayer graphene is metallic and that ABC-stacked trilayer is insulating. Theoretical calculations predict both types of trilayers to be conducting.

Conductance measurements
The researchers performed conductance measurements on graphene devices that they had fabricated by shadow mask evaporation of electrodes onto graphene sheets that were either supported on substrates or suspended across pre-defined trenches in Si/SiO2 substrates. The field effect mobility (µ) ranges from 210 to 1900 cm2/Vs for non-suspended devices, and 5000 to 280,000 for suspended samples.

The results indicate that ABC trilayer graphene has an intrinsic bandgap, which likely comes from the enhanced electronic interactions in this multilayer system, say the researchers. A bandgap, however small, is important for making electronic devices from graphene (which normally lacks a bandgap).

"Our findings also suggest that graphene's electronic properties can be tuned, in principle, by simply changing the stacking order of the layers," Lau told "The stacking order is thus another 'knob' for controlling this 'wonder' material's electronic characteristics – something that will be important for making real-world devices in the future."

The team is now trying to understand the insulating state in ABC-stacked graphene and is also studying the difference in the transport properties of trilayer graphene with different stacking orders when it is exposed to electric and magnetic fields.

The current work was reported in Nature Physics.