Hopping To Open Bandgap...

(a) Top view of the crystal structure of monolayer phosphorene, and side views of the occupied orbitals, corresponding to (b) bonding orbitals and (c) lone pairs. Courtesy: Phys. Rev. B

Single electrons hopping between individual atomic layers are responsible for opening up a bandgap in multilayer black phosphorus (or phosphorene) – a new technologically important 2D material. This unexpected finding, from researchers at Radboud University Nijmegen in the Netherlands, is very different to what happens in other 2D materials like graphene and the transition-metal dichalcogenides.

Like other 2D materials, such as graphene and the transition-metal dichalcogenides (TMDCs), black phosphorus has dramatically different electronic and mechanical properties from its bulk, 3D, parent and so may find use in a host of novel device applications. And just like graphene (which is a sheet of carbon atoms arranged in a honeycomb lattice), black phosphorus is a layered material containing individual phosphorus atoms that are arranged hexagonally. Each atomic layer is held together by weak van der Waals forces. However, in phosphorene, the surface is puckered, and this seems to make all the difference when it comes to bandgap behaviour.

Bulk phosphorene is a semiconductor with a moderate bandgap of between 0.31 and 0.35 eV, but the monolayer material is predicted to be an insulator with a much larger bandgap that varies with the number of phosphorus layers. Although such predictions have already been confirmed in laboratory experiments, researchers are still unsure as to where this considerable bandgap broadening comes from as the material is scaled down to monolayers.

Nano Tech Web: Hopping to open up a bandgap in phosphorene, Belle Dumé