Why Silicene is a Competitive 2D Material

By Dr. Liji Thomas, MDMay 14 2018

Silicone is a graphene analog built of silicon, the semiconductor which made modern computing possible. Over the last few years, graphene followed by other 2D materials has become the new focus of interest in the area of nanomaterials.

Back in 1994 the possibility of finding a 2D material like graphite among the analogous forms of silicon has already been studied. It was only in 2009, however, that a sheet-like silicon structure was found to be formed which had a low buckled configuration which was stable under dynamic conditions.

From the year 2012 on, silicene monolayers have been synthesized on substrate surfaces of different kinds, such as silver, iridium, molybdenum disulfide and others.

Synthesis of 2D Silicene

The problem with silicene is that unlike graphene, it cannot be just pulled out of a stack of tight 2-D layers that are held together by loose bonds, but must be synthesized de novo. The tendency of silicon and germanium to form 3D structures means that the 2D form of silicon has a tendency to buckle which leads to altered electronic properties.

Multilayer silicene sheets have also been synthesized on silver surfaces. This has helped to find both intrinsic properties and many potential applications of silicene devices. Epitaxially grown sheets of multilayer silicene interspersed with active metals in stoichiometric proportions can be used to produce 2D layers of silicene.

The fabrication of a silicene FET confirms that it has ambipolar transport of Dirac charge, as expected, and thus many more nanoelectronic devices and components made of silicene may be expected. This is especially so because it is closer to silicone semiconducting devices and may be used without fear of contamination.

Electronic Properties

Despite the buckled structure of silicene, in contrast to the tight honeycomb structure of graphene, the electronic properties which have made graphene so attractive in miniaturized electronics are shared with silicone as well. These include the Dirac cone, the high Fermi velocity and carrier mobility. The presence of Dirac cones is extremely important where electronic properties are concerned, for the following reasons:

• Dirac points correspond to the meeting points of linear bands or rather cones (Dirac cones), given the electronic movement in the x and y directions. They represent given momenta within the electronic band structure of the crystal. In graphene these bands are straight and V-shaped.
• The intersection is important because it allows electrons to transit easily and with the addition of little energy into the higher band. Since most electrons are found within the lower band, they cannot move freely within the graphene, but passing a small current through the material, for example, helps the electrons to pass into the upper band where they can move freely around the graphene.
• Again, the fact that the two bands are straight near the Dirac point means that the electrons act as though they have no mass, and can thus zoom about the graphene, giving it a high electron conductance.
• Finally, the alignment of the two bands results in excellent light absorption despite its thickness being below one nanometer. The absorption of 2-3% of incident light causes electronic energy transitions, which result in changes of energy but not of momentum. The availability of a higher energy band in contact with the first one allows it to receive the transitioned electron which has absorbed light energy, thus facilitating the rapid and efficient absorption of light by graphene.

With many similarities to graphene, silicone has many advantages as well;

• Stronger spin-orbit coupling which can lead to the quantum spin Hall effect at temperatures which can be reached in an experiment
• The energy band gap can be tuned much better which enables a field effect transistor (FET) to operate more effectively at room temperature
• Valley polarization is easier, making it simpler to study valleytronics

Image Credits: Jozef Sivek [CC BY-SA 3.0], from Wikimedia Commons

Magnetic Properties

The study of magnetic properties in 2D materials is a hotspot at present, with research still in the initial stages. This includes looking for the presence of strong ferromagnetism in asymmetrically functionalized silicene sheets, which can be measured using traditional magnetometers, and magnetic anisotropy of various kinds, which can complement the effect of magnetic fields with other properties, especially when these are of a piece with existing semiconductor devices.

Potential Applications

New 2D magnets could be manufactured using such magnetic materials, such as partially hydrogenated and two-layer silicene sheets. These are sturdy and their parent compounds lack ferromagnetic properties, while these sheets themselves are highly sensitive to low magnetic fields. They are also compatible with silicon.

Silicene is a particularly suitable material for ferromagnetism studies because of its buckled honeycomb lattice structure which favors spin-related applications. The ability to tune the 2D Dirac points means it can be used along with current silicon technology because of novel properties, chiefly the various quantum spin effects including the quantum spin Hall effect, quantum anomalous Hall effect, chiral superconductivity, valley polarized quantum Hall effect. Silicene spintronics is thus a rapid and vast field of research, with potential applications such as gas sensor, spin filter, and spin FET.

A primary limitation to such uses is the high reactivity of silicene which makes it difficult to create free-standing silicene sheets. In most cases silicene is part of a substrate layer, or stacked with alternating metal monolayers as with CaSi2.

Sources

1. https://www.sciencedirect.com/science/article/pii/S0079642516300068
2. https://www.researchgate.net/publication/301248802_Rise_of_Silicene_A_Competitive_2D_Material
3. https://spectrum.ieee.org/semiconductors/materials/graphene-gets-some-competition
4. https://www.infona.pl/resource/bwmeta1.element.elsevier-85ed171c-98db-3a70-bbf8-253fc062b9db
5. https://www.nature.com/articles/s41598-017-11360-4
6. https://www.nature.com/articles/s41467-018-04012-2

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