The applications of 2D materials is one that is growing. Being used as a semiconducting material is one of the most common ways in which 2D materials are exploited. This is a result of the excellent direct band gap and general electronic properties that these materials possess. In this article we look at how both phosphorene and molybdenite are well-suited as semi-conducting materials.
There are not many monoatomic 2D materials outside of graphene which are stable. However, phosphorene is one such example. Phosphorene is composed of only phosphorous atoms and can be seen as a single layer of black phosphorous -a thermodynamically stable bulk material of phosphorous. Black phosphorous is the most stable allotrope of bulk phosphorous (there are other colours such as red, white and yellow), meaning that its 2D monolayers are stable compared to other monoatomic 2D materials (other than graphene).
Phosphorene as a 2D Semiconductor
Phosphorene is a p-type semiconductor material. Whilst naturally occurring phosphorene does possess a small band gap, this can be tuned to create direct band semi-conductors - this is where there is no gap between the minima of the conduction band and the maxima of the valence band, and the electrons and holes can recombine and emit a photon. This is often done by stacking phosphorene layers on top of each other.
Phosphorene is a relatively new 2D material, and its properties aren’t as widely explored as others. However, it does possess excellent electrical, stability and charge transport properties (in the case of phosphorene, it has a high hole mobility) similar to those in graphene. Whilst the properties aren’t quite at the level of graphene, they are much better than many other 2D materials out there today. All these properties, alongside an induced direct band gap, make phosphorene ideal for semi-conductor applications.
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Molybdenite is a 2D allotrope of the naturally occurring mineral molybdenum disulphide (MoS2). By appearance, molybdenum disulphide is very similar to the graphite (and the two are often confused), and even possesses similar interlayer lubrication properties as seen with graphite.
Unlike graphene, molybdenite exists as a tri-layer, where a layer of molybdenum atoms is bonded to two layers of sulphur atoms. Because it is bonded, it is still a monolayer molecule. Many people think that 2D materials are defined by just a single layer of atoms, but this is not the case. 2D materials can be a few atomic layers thick, so long as the electrons are confined to two dimensions (in the case of molybdenite, that is within the S-Mo-S trilayer). It is a structure more reminiscent of transition metal dichalcogenides (TMDCs) than monoatomic 2D materials such as phosphorene and graphene.
When you look at the bonded structure in more depth, you find that each molybdenum centre adopts a +4-valence state and occupies a trigonal prismatic coordination centre. Each molybdenum centre is also bound to six sulphide ligands. By contrast each sulphur centre adopts a pyramidal conformation and is bound to three molybdenum ions. This orientation of atoms is what causes the sandwiched tri-layer.
Molybdenite as a 2D Semiconductor
Like many of the stable 2D materials found today, molybdenite has properties that makes it a useful material for semiconductor applications.
Bulk molybdenum disulphide is known to be an indirect semiconductor, and this is where the electrons and holes don’t recombine because there is a gap between the valence and conduction bands. Instead, indirect semiconductors require the absorption or emission or a phonon with a momentum that equals the difference in the electron-hole momentum. On the other hand, molybdenite is a direct band semiconductor.
In terms of its individual properties, molybdenite has a high conductivity, Hall coefficient, a thermal electric power that is present across a wide temperature range, and a high stability. Molybdenite has been known to adopt both a p-type and n-type role, with p-type being the most common.
Molybdenite also possesses a high charge carrier mobility which varies depending on the temperature. The charge carrier fluctuation arises from the thermal vibrations within the lattice when a charge carrier is scattered. This means that at higher temperatures, such as those found in electronic devices, the amount of charge carriers is higher than at lower temperatures. It also means that the conductivity of molybdenite can change depending on the temperature environment that it is exposed to.
“Phosphorene: An Unexplored 2D Semiconductor with a High Hole Mobility”- Liu H., et al, ACS Nano, 2014, DOI: 10.1021/nn501226z
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“Electroluminescence in Single Layer MoS2”- Sundaram R. S., et al, Nano Letters, 2013, DOI:10.1021/nl400516a
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“Electrical Properties of Molybdenite”- Mansfield R. and Salam S. A., Proceedings of the Physical Society. Section B,
“Electrical properties of molybdenite single crystals”- Agarwal M. K., et al, Pramana Journal of Physics, 1979, DOI:10.1007/BF02846137
Graphene Supermarket: https://graphene-supermarket.com/MoS2-Pristine-Flakes-in-Solution.html
2D Semiconductors: http://www.2dsemiconductors.com/phosphorene/