There are many layered materials out there which show great promise for many applications, especially within the optoelectronics industry. The most widely used being graphite. However, there are many other naturally occurring materials with similar properties that are currently being investigated. One such example is molybdenite; and in this is article we will look at what molybdenite is, as well as showing why its internal properties are useful for optoelectronic applications.
What is Molybdenite?
Molybdenite is a naturally occurring mineral of molybdenum disulphide (MoS2). Molybdenite is said to have a similar structure to graphite, as it has many lubricating layers of monoatomic sheets. In the case of molybdenite, its atomic layering comprises of a sheet of molybdenum atoms sandwiched between two sheets of sulphur atoms.
The structure of molybdenite does have some planes of clevage weakness, similar to the layer-slippage shown by graphite. If you think of the Mo-S-Mo sheets as a tri-layer, then it is easy to understand. The covalent bonds between the Molybdenum and sulphur within this tri-layer are very strong. However, between tri-layers, the inter-molecular forces are weak and so each tri-layer can be easily removed through these planes of weakness. This creates a thin and useable sheet. It is similar in nature to the removal of layers in graphite to create graphene, but graphite only loses a monolayer (in theory) due to its monoatomic structure.
Another characteristic of molybdenite is that it is very similar in appearance to graphite. In its mineral form, it is soft and exhibits a metallic lustre. By pure observation, it is hard to tell the difference between graphite and molybdenite, and analysis equipment is needed to distinguish the two materials.
What are the Properties of Molybdenite?
Molybdenite has some very beneficial electronic properties. Molybdenite is semi-conducting in nature, and this arises from a direct, but very small, band-gap. Molybdenite possess a high conductivity, Hall coefficient and thermal electric power over a wide temperature range. Most molybdenite material samples have been found to be p-type semi-conductors and the presence of a semi-conducting band-gap is an excellent property for many electronic and optoelectronic applications.
Molybdenite also has a high charge carrier mobility. The mobility has a tendency to vary at different temperatures because the scattering of the charge carriers is due to thermal vibrations within the lattice. Additionally, this means that molybdenite is more current sensitive at a higher temperature, and in turn, this creates a higher amount of charge carriers at higher temperatures. This means that for electronic devices, where the internal temperatures can increase, the conductivity of molybdenite is also increased. It also shows that molybdenite has an inherent thermal stability.
From an optical point of view, molybdenite exhibits a phenomenon known as pleochroism. This means that molybdenite can differ in colour when looked at from different angles. This is particularly prominent under polarized light, as it also displays a high anisotrophism to reflected light. The colour of molybdenite can therefore vary from gray, to yellow, to reddish-brown.
Molybdenite is also known to emit light when excited by an electrical current. The direct semi-conducting nature of the band-gap causes molybdenite to emit light in the visible spectral range. This has been shown through attachment to a transparent substrate. Currently, molybdenite does not emit a lot of light, but this could just be a matter of tuning the experimental parameters.Ho
How Molybdenites Properties Can Be Applied to Optoelectronics
Molybdenum is known to be a scarce element. However, molybdenite is in abundance, so it has the potential to meet with the supply demands of the electronic and optoelectronic industries. The ability of individual sandwiched tri-layers to be cleaved from a bulk material allows for an easy processing compared to many materials. Molybdenite is also thinner and less bulky (internally) than silicon, which is currently used in many optoelectronic applications. It also possesses a more efficient band-gap than silicon.
Once you combine these properties with the excellent electronic and optical properties of molybdenite, you can easily see how it may be used in future electronic and optoelectronic applications. It is also easy to see how silicon may be phased out in the near-future. Areas of potential use have been touted for molybdenite, and it is thought that it will be used as the thin semi-conducting material in future LEDs, transistors, solar cells and photodetectors.
“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
Spectrum Magazine: https://spectrum.ieee.org/nanoclast/semiconductors/nanotechnology/graphene-or-molybdenite-which-replaces-silicon-in-the-transistor-of-the-future
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Web Mineral: http://www.webmineral.com/data/Molybdenite.shtml#.WrGQW-jFLIU