2D materials are gaining a lot attention in the nanotechnology space. Of these, graphene is the most widely known, followed by hexagonal boron nitride (dubbed ‘white graphene). However, there are many other 2D materials out there. Polyatomic 2D materials such as transition metal dichalcogenides (TMDCs) have been widely documented, but more and more, monoatomic 2D materials are gaining interest. One such material with promise is silicene, and in this article we will discuss what it is and why its properties are of interest.
What is Silicene?
Silicene is a 2-dimensional allotrope of silicon, in a similar way that graphene is an allotrope of carbon. Silicene is monoatomic, meaning that it is composed only of silicon atoms. However, unlike graphene and some other 2D materials, silicene is not strictly planar. Instead, it has a buckled honeycomb surface.
Silicene is not as widely produced, nor studied, as other 2D materials and it is a lot harder to create than the likes of graphene. This is because its 3D form is a diamond-like lattice, so currently, silicene can only be produced through epitaxial growth, not exfoliation. However, it does appear to have many beneficial properties. For one, in multi-layered silicene, the interactions between the layers are very strong and are much stronger than the individual layers in multi-layered graphene. In addition to pure silicene, there are also oxygenated forms of silicene named 2D silica; and silicene nanoribbons are known to exist.
One difference to graphene is that it doesn’t have a high dependence on pi-electrons and therefore is not reliant on pi-stacking mechanisms. This has been attributed to filled electronic states lying close to vacant electronic states, giving rise to pseudo Jahn-teller distortions in the 2D lattice.
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Properties of Silicene
Like many 2D materials with a high electrical conductivity, silicene is one of the few materials that possesses Dirac cones. Additionally, silicene exhibits a quantum hall spin effect (QHE) and is thought to possess massless Dirac fermions – much like graphene does. This means that silicene has all the excellent electronic properties that graphene does, but with a greater spin-orbital coupling and the potential to utilize the QHE at ambient temperatures.
Despite its buckled structure, silicene possesses a zero-band gap from the overlap of the conduction and valence bands. However, this band gap is also tuneable. The bonds in a silicene lattice are predominantly sp3 hybridized and this creates a more chemically active surface. Because it is tuneable, silicene can be doped with a wide range of atoms to provide tuneable electronic properties. This allows silicene to be tuned into both a semi-metallic material and a semiconducting material.
The magnetic properties are also affected by doping. Silicene harbours an internal magnetism arising from the high spin-orbit coupling. Under a magnetic field, the electrons move from the conductance to the valence band. When there is no magnetic field, it displays a quantized Hall conductance. Doping can have a wide range of effects on the magnetic properties of silicene, including inducing a local magnetic moment, transforming silicene into a ferromagnetic semiconductor and introducing tuneable magnetic ordering from ferromagnetic to antiferromagnetic.
Silicence is also very stable, particularly thermally, and is theorized to be stable up to 1500 K. The high thermal stability of silicene has been attributed to it possessing a low phonon conductance. This is the opposite of graphene, which exhibits a high thermal conductivity. Instead, silicene is a thermally insulating material.
The tuneability of its electronic band gap provides silicene with versatile electronic and thermal properties that make it a highly desirable material for field effect transistor (FET) and miniature electronic applications. Aside from the electronic aspect, silicene appears to be unreactive to oxygen, suggesting that it would be stable enough to be used in the above electronic applications. Its interchangeable magnetic ordering properties could also lead to the development of new spintronic devices.
“Tuning electronic and magnetic properties of silicene with magnetic superhalogens”- Zhao T., et al, Physical Chemistry Chemical Physics, 2014, DOI:10.1039/C4CP02758B
“A theoretical review on electronic, magnetic and optical properties of silicene”- Chowdhury S., et al, Reports on Progress in Physics, 2016, DOI:10.1088/0034-4885/79/12/126501
“Electronic and magnetic properties of graphene, silicene and germanene with varying vacancy concentration”- Ali M., et al, AIP Advances, 2017, DOI: 10.1063/1.4980836
“Rise of silicene: A competitive 2D material”- Zhao J., et al, Progress in Materials Science, 2016, DOI: 10.1016/j.pmatsci.2016.04.001
“A review on silicene — New candidate for electronics”- Kara A., et al, Surface Science Reports, 2012, DOI: 10.1016/j.surfrep.2011.10.001
“Growth mechanism and modification of electronic and magnetic properties of silicene”- Sheng L. H., et al, Chin. Phys. B., 2015, DOI: 10.1088/1674-1056/24/8/087303
“Silicene: Recent theoretical advances”- Lew Van Yoon L. C., et al, Applied Physics Reviews, 2016, DOI: 10.1063/1.4944631