There are many 2D materials being synthesized today, some of which are more well-known than others. As a general class, 2D materials are ultrathin films composed of a single layer of atomic atoms presented in a regular (and usually honeycomb-shaped) array. Whilst many people know about materials such as graphene and hexagonal boron nitride, bismuthene is a lesser known 2D material. In this article, we look at what bismuthene is, it’s properties, and the potential applications it could be used in.
Bismuth is currently the heaviest element used in the creation of a 2D material. Bismuthene is a single atomically thin layer of bismuth atoms arranged in a hexagonal array. Like many 2D materials, bismuthine has been grown on other substrates. In the case of bismuthine, it is grown onto silicon carbide substrates. Silicon carbide substrates have shown the most growth promise because the structure of the silicon carbide molecules directs the growth into the regular hexagon structure seen by many 2D materials. However, unlike graphene, the deposition process bonds the bismuth atoms to the silicon carbide sheet, giving rise to some beneficial electronic properties.
Unlike graphene and other 2D materials, bismuthine is a room-temperature topological insulator, not a conductor. However, when graphene is layered onto substrates, it can also act in a similar insulating manner. Bismuthene is a novel material because most topological insulators operate at temperatures well-below freezing. As such, bismuthene is considered to have a high stability compared to many other 2D materials.
Depending on the size and stacking of the sheets, the electronic properties can range between narrow band gap semiconducting to semi-metallic and metallic states. Rashba-type spin splitting of the surface states also occurs within bismuthene due to strong spin-orbit coupling. The spin orbit coupling in bismuthene is also responsible for determining the lattice constants, phonon frequencies, band gaps and cohesion within the sheet.
Bismuthene is considered to be a nonmagnetic, narrow band semiconductor. This is for a couple of reasons. Firstly, the individual bismuth atoms are stable to both long wavelength vibrations and thermal excitations at high temperatures, and secondly, the 2D dimensional array of bismuth atoms correspond to the local minima in the Born-Oppenheimer (potential energy) surface.
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Bismuthene’s electronic state is very different to its bulk 3D crystal. Bismuthene is only a narrow-band semiconductor because the 2D lattice is compressed, resulting in a confinement of the electrons in the sheet. By comparison, its 3D bulk allotrope is a semimetal.
There are many ways to modify the properties of bismuthene, making it a versatile material. When bismuthene layers are stacked on top of each other, i.e. to create bilayers, the band gap of bismuthene is decreased. It is thought that stacking more than two layers on top of each would close the band gap completely, changing the topological properties of the sheet. These topological and electronic properties can also be tuned by strain and isoelectronic substitutional alloying.
The electronic structure, magnetic structure, optical properties and topological phase can also be modified by introducing point defects (i.e. atomic vacancies, atomic substitution or interstitial atoms). Even after atomic modification, bismuthene sheets remain stable at high temperatures. How much the properties are changed in this approach depends on both the energies of localized states and the coupling strength between point defects.
So far, the properties of bismuthene (especially the ability to operate at and above room temperature) lends itself to quite a few electronic applications, including in spintronic devices, computing and data transmissions, quantum spin Hall insulating materials and saturable absorbers.
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