Two-dimensional black phosphorus has sparked enormous research interest due to its high carrier mobility, layer-dependent direct bandgap and outstanding in-plane anisotropic property. It is one of the few 2D materials where it is possible to tune the bandgap over a wide energy range from the visible to the IR spectrum.
When a few atomic layers of BP are exposed to an electric field, a physical phenomenon known as the Stark effect is observed. In atomic spectra, the Stark effect is the shifting and splitting of atomic energy levels under the influence of an externally applied electric field. Similarly, this Stark effect causes the conduction and valence band to shift towards each other, resulting in the reduction of the bandgap of few-layer BP.
A team led by Assistant professor Jiong LU from the Department of Chemistry has demonstrated this effect in electrostatically-gated few-layer BP by using a low-temperature scanning tunnelling microscopic. This imaging technique allowed them to “see” individual atomic positions in the BP material and probe energy states where electrons reside in. Prof Lu and his team employed this state-of-the-art instrument and gained knowledge on the behaviour of the continuously tunable band gaps of a few-layers of BP material. This experimental demonstration is further verified by DFT calculation conducted by Dr Alexandra CARVALHO and Prof A.H. CASTRO NETO from the CA2DM and Department of Physics, NUS.
The research findings also point out that for thin BP layers, the band gap can be controlled over a large spectrum, from visible to far infra-red (IR) regime. This suggests the possibility of using BP for broadband optical application.
This work has recently been published in Nano Letters (DOI:10.1021/acs.nanolett.6b05381) and highlighted in Nature Photonics (doi:10.1038/nphoton.2017.102).