Opening a semiconducting band gap in graphene is one of the most intense areas of research on this remarkable material. These two-dimensional carbon sheets are well known for their exceptional electronic properties, but in its pristine state, graphene has no semiconducting band gap, which in practical terms means that it is has limited use in electronic devices such as transistors.

The band gap is the minimum energy required for an electron to hop from the ‘valence’ band to the ‘conduction’ band in a material's electronic energy structure. In metals, these two bands are touching, resulting in a zero band gap and efficient electron conduction. The profound utility of semiconductors such as silicon, on the other hand, derives from a non-zero band gap, which causes them to become conducting only when subject to an applied voltage. Graphene generally does not have a band gap, but it is known that geometric confinement can cause a band gap to open in narrow graphene ‘nanoribbons’. Kausik Majumdar from the Indian Institute of Science and colleagues1 have now demonstrated that both the size of the gap and the shape of the band structure in graphene nanoribbons can be controlled by applying an electric field laterally across the ribbon width. In particular, they showed that it is possible to induce a transition from a conventional direct band gap to an ‘indirect’ gap, providing a range of additional functionality that could prove useful in future advanced electronics applications.

Fig. 1: Diagram showing the transition from a direct band gap (black) to an indirect gap (white) in a graphene nanoribbon depending on the voltage applied to the left (VL) and right (VR) edges of the ribbon.From Ref. 1. Reproduced with permission. © 2010 ACS

Using a theoretical technique known as ‘self-consistent tight binding calculations’, the researchers showed that at certain voltages applied to opposite edges of the nanoribbon, the usual direct band gap of the nanoribbon transitioned to an indirect gap (Fig. 1). Closer inspection revealed that the transition is related to the strong asymmetry of electric potential across the width of the ribbon under specific voltage conditions.

“The external bias-dependent band-gap transition, coupled with a change in the magnitude of the gap, can significantly affect physical phenomena like band-to-band tunnelling, electron–phonon interaction and optical properties,” says Majumdar. “Such tailoring of the electronic structure can provide us with the possibility of a wide variety of fascinating electronic and optoelectronic device applications.”