Black phosphorus (purple balls) doped with potassium (K) atoms (blue balls) – by selectively adjusting the concentration of K atoms (generating strong electric field nearby), it was possible to tune the band gap and electronic properties of black phosphorusScientists from Korea’s Pohang University of Science and Technology have managed to tune the band gap in black phosphorus into a unique state of matter as an improved conductor, a finding that could allow greater flexibility in the design and optimization of electronic and optoelectronic devices such as telecommunication lasers and solar panels.
In the area of 2D materials, graphene has of course been receiving much attention due to its properties as an excellent conductor of heat and electricity. However, the much-touted material has the major drawback of having no band gap, which is crucial to determining its electrical conductivity – the smaller the band gap, the more efficiently current can move across the material and the stronger the current. As graphene has a band gap of zero in its natural state, its semiconductor potential cannot be realized since the conductivity cannot be closed down.
“we tuned BP’s band gap to resemble the natural state of graphene, a unique state of matter that is different from conventional semiconductors”Keun Su Kim
Attempts to open a band gap in graphene have proved difficult without reducing its quality, so the Korean team used black phosphorus, the stable form of white phosphorus, as a 2D semiconductor before inducing the important property of graphene in other 2D semiconductors to get round this problem. As Keun Su Kim points out, “we tuned BP’s band gap to resemble the natural state of graphene, a unique state of matter that is different from conventional semiconductors”.
The study, published in Science [Kim et al. Science (2015) DOI: 10.1126/science.aaa6486], demonstrated how the electronic state of black phosphorus could be tuned from a semiconductor to an efficient conductor depending on the strength of electric field applied. At a zero band gap, its electronic state becomes a ‘Dirac semimetal state’, which is similar to the intrinsic state of graphene.
Electrons were transferred from a potassium dopant to the surface of the black phosphorus, which confined the electrons and allowed the team to manipulate this state. Potassium produces the strong electrical field required to tune the size of the band gap. The doping process induced a large Stark effect that tuned the band gap so that the valence and conductive bands moved closer together, reducing the band gap. The vertical electric field therefore modulates the band gap and tunes the material from a moderate-gap semiconductor to a band-inverted semimetal.
The potential of this unique electronic state of black phosphorus needs to be investigated further as it could find also applications in engineering where the band gap could be adjusted for devices dependent on knowledge of their exact behavior, as well as in the realization of high performance and very small transistors for the semiconductor industry.