Giant Stark effect in black phosphorus

12 Apr 2017. NUS chemists have demonstrated an electrical field-controlled giant Stark effect in black phosphorus for potential applications in advanced electro-optic modulators and photodetector devices.

The bandgap is a material property that determines electrical conductivity. Solids with large bandgaps are generally insulators as they do not conduct electricity. Those with smaller bandgaps are semiconductors. A team of NUS researchers have demonstrated that the bandgap of black phosphorus (BP) changes in the presence of an electric field. By controlling the electric field strength, they have shown that the bandgap of BP can be adjusted to a desired level. This means that by changing the electric field, the material can be made to either conduct or not to conduct electricity. The findings can potentially be used for advanced optoelectronic devices operating beyond the visible light 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 Prof Jiong LU and Prof Kian Ping LOH from the Department of Chemistry, NUS 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 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 applications.

BP is one of the few two-dimensional (2D) materials where it is possible to tune the bandgap over a wide energy range from the visible to the IR spectrum. It is also known that the bandgap of BP can be tuned by introducing surface dopants such as potassium. However, there are drawbacks when using chemical doping methods. These include instability in air and the increased probability of creating charged scattering centres. In addition, it is also difficult to achieve a reversible modulation of the bandgap when using chemical dopants. This is in contrast to electrostatic gating which is a method to modify the electronic and optical properties of materials by application of an electrical voltage. Electrostatic gating is continuously tunable, non-destructive and can be carried out in ambient atmosphere. It has been widely adopted to tune the optoelectronic properties of 2D materials and their heterostructures. Although theoretical computations have predicted that the electrical and optical properties of ultrathin BP can be effectively tuned by electrostatic doping, it has not been physically demonstrated. This is due to difficulties in the preparation of 2D BP devices and its high reactivity when exposed to air which has so far limited experimental investigations.

Prof Lu and his team has achieved a notable bandgap reduction of ~35.5% (from 0.31 eV to 0.2 eV) by applying a vertical electric field of 0.1 V/nm. It is predicted that few-layer BP material can transform from a moderate-gap semiconductor to a band-inverted semimetal under a more intense electric field. This is a future research area in which phenomena such as Dirac Cone physics can be explored in the black phosphorus with a puckered atomic structure distant from that of graphene.

Figure shows a gate-controlled Stark effect in a few-layer BP flake device. (a) Schematic drawing and structural characterisation of a few-layer BP device. (b) STM imaging of atomic lattice in BP. (c) Field tunable bandgap measured by local STM spectroscopy.

Reference

Y Liu; Z Qiu; A Carvalho; Y Bao; H Xu; SJR Tan; W Liu; AHC Neto; KP Loh*; J Lu*, “Gate-tunable giant stark effect in few-layer black phosphorus” NANO LETTERS DOI: 10.1021/acs.nanolett.6b05381 Published: 2017.