Science 349, 723–726 (2015)

Graphene exhibits high electron mobility but is a semimetal — a semiconductor with zero band gap. Ever since this two-dimensional material was first isolated from graphite, inducing a semiconductor gap in it has been a major research goal, as a finite gap between the conduction and valence bands is an essential requirement for use in digital electronic devices. An alternative approach to this problem is to find a different two-dimensional material that exhibits a gap in its natural state, and just over a year ago such a material was found: phosphorene. Keun Su Kim and colleagues have now shown that starting from a sample of black phosphorous, which consists of a few layers of phosphorene, it is possible to use external doping to reduce the gap and create a semimetal similar to graphene.

The researchers — who are based at Pohang University of Science and Technology, Yonsei University, the Institute for Basic Science in Pohang and Lawrence Berkeley National Laboratory — doped the black phosphorous by adsorbing potassium on its top surface. Variations in its electronic band structure were then monitored by angle-resolved photoemission spectroscopy. From an initial value of 0.6 eV, the gap was reduced to zero by increasing levels of doping. Kim and colleagues suggest that the reduction of the gap to zero is a consequence of the electric field generated by excess charge on the top surface created by the potassium dopant atoms, and this was confirmed through density functional theory calculations.