“Silicon” by Mollie Cowger

If you’re reading this, you have silicon to thank. The versatile element, closely related to carbon, forms the basis for a huge portion of the computing power of the modern world—silicon microchips are a primary reason that our computers and phones can do the things they do. So it should come as no surprise that when researchers want to improve computing power, silicon is often the first place they look.

Silicon’s widespread use is largely due to its properties as a semiconductor. Because computation requires the transfer of huge amounts of data via electrical currents, materials that fully conduct electricity aren’t nearly as useful as semiconductors, which can be manipulated to have varying degrees of conductance in different circumstances. A wide variety of silicon compounds have been used in this capacity. Because silicon has proven itself so useful as a semiconductor, a substantial amount of research has gone into finding ways to find silicon compounds that will allow computation with greater speed and efficiency. One promising compound is silicene.


Figure 1: Two allotropic forms of carbon. On the left is diamond, in which each carbon atom is bonded to four others. On the right is graphite, in which multiple sheets, where each carbon atom is bonded to three others, stack on top of each other. Source: https://commons.wikimedia.org/wiki/File:Diamond_and_graphite.jpg

Silicene is an allotrope of silicon—that is, it refers to a specific arrangement of silicon atoms in relation to one another. Because silicon has the same number of outer electrons as carbon, the allotropes it’s able to form mirror those of carbon. The two most well-known carbon allotropes diamond, where each carbon atom is bonded to four others (as on the right), or graphite, formed by multiple sheets in which each carbon atom is bonded to three others (as on the left). While graphite refers to multiple stacked sheets of carbon atoms, a single sheet is called graphene.

The silicon analogue of diamond is readily formed and is fairly stable at normal conditions. The silicon analogues of graphite, and graphene, are much more elusive. Silicene refers to the silicon analogue of graphene: a single sheet of silicon atoms, each bonded to three others, arranged hexagonally. While graphene has been studied fairly extensively (the 2010 Nobel Prize in Physics was awarded for some of the most important research), research on silicene is still in its early stages of development. Graphene is promising because of, among other things, its properties of electrical conductance —a quality researchers are exploring in its analog, silicene.

Just 20 years ago, silicene existed only in theory.[1] Or, rather, an “infinite 2D Si AS system” existed only in theory—the name silicene hadn’t yet been used. But the initial theoretical studies on silicene suggested that it would have distinct properties from graphene. Notably, while graphene is most stable as a flat plane, computations for silicene suggested that it would be more stable as a “buckled” or “puckered” plane (Figure 2). That is, if you synthesized a plane of silicon with the same hexagonal structure as graphene, it would naturally fold in on itself a little bit rather than remaining flat.


Figure 2: a) In a sheet of graphene, all carbon atoms occupy the same plane. b) A sheet of silicene is “buckled,” and is not strictly planar. While this quality allows for silicene to be manipulated in unique ways, it also contributes to silicene’s instability. Source: adapted from reference 2.

This buckled property is due to differences in the orbitals occupied by electrons in graphene and those occupied by electrons in silicene. The precise mechanics are outside the scope of this post, but the essence of what occurs is that there are orbitals in flat silicene that are so close in energy as to be unstable, so the silicene buckles to regain some stability.[2] That initial instability isn’t present in graphene, so it stays flat.
The buckling also makes silicene very promising as an efficient semiconductor. Because of its structure, a sheet of silicene can be manipulate with other elements to conduct electricity at a wide variety of levels[3]—it’s this kind of manipulability that makes semiconductors so valuable. And because silicene is so thin, finding a way to use it could drastically alter the scale at which we can make electronics.

But silicene’s instability is a problem. And it’s given researchers a lot of trouble synthesizing it. Unlike graphene, which is stable in air at room temperature, silicene is oxidized in air. And it’s oxidized quickly in air: designing a process that lets silicene stick around long enough that it can be characterized has been an obstacle for researchers. Some researchers got around this by growing silicene on the surface of gold.[4][5]

Earlier this year, though, there was a breakthrough. In the March 2015 issue of Nature Nanotechonology, researchers published a landmark paper documenting the first use of silicene as a transistor.[6] Unlike any previous team, they managed to contain silicene in an atmosphere that allowed it to exist long enough to be studied—and that’s new and important. About 20 years ago, the idea for silicene didn’t yet exist in a meaningful sense. Now, researchers have succeeded in creating and characterizing it, and the future directions for research are incredibly exciting.


[1] Takeda, K.; Shiraishi, K. Phys. Rev. B 1994, 50, 14916-14922.
[2] Jose, D.; Datta, A. Acc. Chem. Res. 2014, 47, 593–602.
[3] Ni, Z.; Zhong, H.; Jiang, X.; Quhe, R.; Luo, G.; Wang, Y.; Ye, M.; Yang, J.; Shi, J.; Lu, J. Nanoscale 2014, 6, 7609–7618.
[4] Vogt, P.; Padova, P. De; Quaresima, C.; Avila, J.; Frantzeskakis, E.; Asensio, M. C.; Resta, A.; Ealet, B.; Lay, G. Le Phys. Rev. Lett. 2012, 108, 155501.
[5] Lalmi, B.; Oughaddou, H.; Enriquez, H.; Kara, A.; Vizzini, S.; Ealet, B.; Aufray, B. Applied Physics Letters 2010, 97, 223109.
[6] Tao, L.; Cinquanta, E.; Chiappe, D.; Grazianetti, C.; Fanciulli, M.; Dubey, M.; Molle, A.; Akinwande, D. Nature Nanotechnology 2015, 10, 227–231.

Media References:

Figure 1: https://commons.wikimedia.org/wiki/File:Diamond_and_graphite.jpg

Figure 2: Created by the author, adapted from reference 2 (Jose, D.; Datta, A. Acc. Chem. Res. 2014, 47, 593–602.)