A new way of modifying the electronic properties of graphene has been discovered by a team led by Andre Geim and Kostya Novoselov at the University of Manchester. The physicists have shown that when graphene is grown on a hexagonal substrate, a small change in its crystal structure results in a gap opening in the material's electron energy bands. They also found that graphene grown in this way can exist in an alternative structure in which the band gap is much smaller. The result could point to an exciting new way of controlling the electronic properties of graphene-based devices.

Graphene is a honeycomb lattice of carbon just one atom thick that was first isolated in 2004 by Geim and Novoselov. Graphene is blessed with a wealth of fascinating electronic properties, many of which arise from the fact that it is a semiconductor with a zero-energy gap between its valence and conduction bands. One important consequence of how the bands meet is that conduction electrons travel through graphene at extremely high speeds. This means that the material could be used to create extremely fast electronic devices.

But there is an important snag: electronic devices such as transistors rely on the fact that semiconductors such as silicon have a non-zero band gap. Therefore, the challenge for device developers is to create a modified version of graphene that has a band gap. Several schemes have been explored – including applying an electric field, adding chemical impurities or modifying the structure of graphene – but none have proved ideal.

Moiré superlattices

In this latest study, the Manchester team looked at graphene grown on hexagonal boron nitride (hBN), which has a lattice that is very similar to graphene. When the two lattices are overlaid in certain ways, a moiré superlattice is created (see figure). The periodic potential associated with this superlattice causes a number of new and interesting electronic phenomena to occur in graphene, including Hofstadter's butterfly (see: "Hofstadter's butterfly spotted in graphene").

Now the team has added "the commensurate–incommensurate transition" to the list of interesting phenomenon. In the commensurate state, the distance between carbon atoms in the graphene increases by about 1.8%, so that the lattice exactly matches that of hBN. This occurs when the two lattices are more or less aligned in a moiré structure. However, if this alignment is off by as little as one degree, the structure exists in an incommensurate state in which the graphene adopts its natural atomic spacing.

"Although it is extremely difficult to rotate a graphene sheet on a hBN substrate, we have overcome this problem by making many samples at varying angles and testing each one," explains the Manchester Condensed Matter Physics Group.

Solitons and strain

The team, which also includes researchers from China, the Netherlands, Russia and Japan, mapped the locations of commensurate and incommensurate states by measuring the strain across the graphene surface. "In the commensurate state, the strain distribution becomes very abrupt," adds Woods. "This is because there must be a network of domain walls [marked yellow in the figure above], also known as solitons in 1D, between the stretched regions [grey/blue]."

The team then measured the electronic properties of commensurate and incommensurate samples. In the former it found a relatively large band gap, and in the latter a much smaller gap. The team believes that this could explain why previous studies of graphene-on-hBN often resulted in conflicting values for the band gap.

In addition to clearing up the confusion surrounding the value of the band gap, Woods believes that the research has identified a new and exciting way to control and fine-tune the electronic properties of graphene devices.

The research is described in Nature Physics.