Molybdenum disulphide possible graphene alternative as photosensor

A group of researchers from the USA has successfully demonstrated the effectiveness of molybdenum disulphide (MoS2) to convert photon energy into electric current. The group has suggested it as either an alternative or used in conjuction with graphene. The device was presented in the first edition of the Institute of Physics journal, 2D materials, which was released 4 April 2014.

Due to its impressive ability to convert light into electricity at an extremely efficient rate, single layers of the semiconducting material have been used to fabricate a photosensor.

In their study, the researchers highlighted the suitability of MoS2 by successfully demonstrating the photosensor’s ability to efficiently convert the energy from photons, delivered to the device in two separate wavelengths by a laser, into an electric current.

An increasing number of researchers are looking away from graphene and towards alternative materials that perform in a similar way. MoS2 is part of a wider group of materials known as transition metal dichalcogenides, and has been put forward by the group as a potential building block for the next generation of low-cost electrical devices.

On the suitability of MoS2 as an alternative to graphene, lead author of the research, Nestor Perea-Lopez, from The Pennsylvania State University, said: ‘Graphene is a semi-metal, which means that electrons can move through the material very fast even with very small voltages. However, this is both an advantage and disadvantage since electronic devices need to have an “on” and “off” state. Graphene devices can therefore be hard to turn off, but MoS2 has a large energy gap that allows it to have very large on/off ratios of hundreds of millions.’

The goal is to be able to synthesise 2D materials using a bottom-up approach, carefully piecing individual components together like building blocks. The researchers did this successfully in their study by growing tiny triangles of single-layered MoS2, around 5µm wide, onto a silica-based substrate using a bottom-up process known as chemical vapour deposition.

Perea-Lopez continued: ‘The thinnest foil of MoS2 has a thickness of three atoms. One can picture this monolayer foil as a sandwich, where sulphur atoms are the bread and molybdenum is the ham. The monolayer is even more interesting than the material in bulk, because in such thin form it can convert photons into electrons very efficiently, making it an ideal material to use in light detectors, such as the ones used in digital cameras.’

A significant challenge that remains for researchers working with 2D materials is how to produce the materials in bulk. Graphene, for example, can only be produced in bulk through a liquid phase or by exfoliating graphite into very thin layers or flakes, which can be very difficult to control.

However, Perea-Lopez stated: ‘Not everything about graphene is wrong though, and the path in this field must be the integration of metals and semi-metals, such as graphene, with insulators such as boron nitride and semiconductors like MoS2 to create the next generation of devices.’

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