Silicene

Silicene is a two-dimensional allotrope of silicon, with a hexagonal honeycomb structure similar to that of graphene. Contrary to graphene, silicene is not flat, but has a periodically buckled topology; the coupling between layers in silicene is much stronger than in multilayered graphene; and the oxidized form of silicene, 2D silica, has a very different chemical structure from graphene oxide.

History Although theorists had speculated about the existence and possible properties of free-standing silicene, researchers first observed silicon structures that were suggestive of silicene in 2010. Using a scanning tunneling microscope they studied self-assembled silicene nanoribbons and silicene sheets deposited onto a silver crystal, Ag(110) and Ag(111), with atomic resolution. The images revealed hexagons in a honeycomb structure similar to that of graphene, which, however, were shown to originate from the silver surface mimicking the hexagons. Density functional theory (DFT) calculations showed that silicon atoms tend to form such honeycomb structures on silver, and adopt a slight curvature that makes the graphene-like configuration more likely. However, such a model has been invalidated for Si/Ag(110): the Ag surface displays a missing-row reconstruction upon Si adsorption and the honeycomb structures observed are tip artifacts. This was followed in 2013 by the discovery of dumbbell reconstruction in silicene that explains the formation mechanisms of layered silicene and silicene on Ag. In 2015, a silicene field-effect transistor was tested. that opens up opportunities for two-dimensional silicon for fundamental science studies and electronic applications. In 2022, it was found that silicene/Ag(111) growth on top of a Si/Ag(111) surface alloy, functions as a foundation and scaffold for the two-dimensional layer. This, however, raises questions of whether silicene can be truly regarded as two-dimensional material at all, due to its strong chemical bonds to the surface alloy.

Similarities and differences with graphene Silicon and carbon are similar atoms. They lie above and below each other in the same group on the periodic table, and both have an s2 p2 electronic structure. The 2D structures of silicene and graphene also are quite similar, but they have important differences. While both form hexagonal structures, graphene is completely flat, while silicene forms a buckled hexagonal shape. Its buckled structure gives silicene a tuneable band gap by applying an external electric field. Silicene's hydrogenation reaction is more exothermic than graphene's. Another difference is that since silicon's covalent bonds do not have pi-stacking, silicene does not cluster into a graphite-like form. The formation of a buckled structure in silicene unlike planar structure of graphene has been attributed to strong Pseudo Jahn–Teller distortions arising due to vibronic coupling between closely spaced filled and empty electronic states. Silicene and graphene have similar electronic structures. Both have a Dirac cone and linear electronic dispersion around the Dirac points. Both also have a quantum spin Hall effect. Both are expected to have the characteristics of massless Dirac fermions that carry charge, but this is only predicted for silicene and has not been observed, likely because it is expected to only occur with free-standing silicene which has not been synthesized. It is believed that the substrate on which the silicene is made has a substantial effect on its electronic properties. Unlike carbon atoms in graphene, silicon atoms tend to adopt sp3 hybridization over sp2 in silicene, which makes it highly chemically active on the surface and allows its electronic states to be easily tuned by chemical functionalization. Compared with graphene, silicene has several prominent advantages: (1) a much stronger spin–orbit coupling, which may lead to a realization of quantum spin Hall effect in the experimentally accessible temperature, (2) a better tunability of the band gap, which is necessary for an effective field effect transistor (FET) operating at room temperature, (3) an easier valley polarization and more suitability for valleytronics study. Unlike graphene, it has been shown that, at least silicene supported by Ag(111) grows on a surface alloy. Hence, decoupling silicene is much less trivial, if possible at all, than decoupling graphene.

Surface alloying Silicene on Ag(111) grows on top of a Si/Ag(111) surface alloy, which has been shown by a combination of different measurement techniques. The surface alloy precedes the growth of silicene, acting both as foundation and as scaffold for the two-dimensional layer. Upon further increase of silicon coverage, the alloy is covered by silicene, yet pervasively exists for all coverages. This implies that the properties of the layer are strongly influenced by its alloy.

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