2D Materials: After Graphene, Researchers Now Find Miracle Molybdenum Disulfide

By Shweta Iyer on April 4, 2014 2:13 PM EDT

graphene
Researchers have found that two-dimensional molybdenum disulfide (MoS2) exhibits properties as good as or even better than graphene. (Photo: Photo courtesy of Shutterstock)

Wonder material graphene is used in anything and everything from solar cells and water purifiers to transistors and sensors that diagnose diseases. But scientists across the world have been trying to find alternatives to graphene and as researchers from The Pennsylvania State University have discovered, the two-dimensional molybdenum disulfide (MoS2) exhibits properties as good as or even better than graphene, according to a press release Thursday.    

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MoS2 is an inorganic compound, similar in appearance to graphene, and is part of a wider group of materials known as transition metal dichalcogenides. MoS2 is widely used in dry lubricants and petroleum refining but in this new research scientists have shown that the semiconducting, two-dimensional MoS2 possesses unique properties that are promising for future low-cost optical and electrical devices.

MoS2 is also known as a phosphorescent material as it is sensitive to the full spectrum of sunlight, and can potentially convert light into electricity at 100% efficiency.  This property of MoS2 is exploited widely in a device known as photosensor, which is found in a range of appliances like solar panels, digital cameras, and remote controls.

The device has been presented today, 4 April, in the very first issue of IOP Publishing's new journal 2D Materials-a multidisciplinary journal with a focus on applications and fundamental science concerning all aspects of graphene and 2D materials.

The researchers showed that the photosensor made from MoS2 could successfully convert energy from absorbed photons into an electric current. The photons were delivered to the device in two separate wavelengths by a laser.

Lead author of the research, Nestor Perea-Lopez, said, "The thinnest foil of MoS2 has a thickness of three atoms. One can picture this monolayer foil as a sandwich, where sulfur 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."

Although 2D materials have tremendous scope in several applications, producing them on an industrial scale is still a challenge for researchers. Graphene, for example, can only be mass produced through a liquid phase or by exfoliating graphite into very thin layers or flakes, which can be very difficult to control.

But with MoS2, the researchers used a bottom-up approach by carefully piecing individual components together like building blocks. They did this by growing tiny triangles of single-layered MoS2, around five micrometers wide, onto a silica-based substrate using a bottom-up process known as chemical vapor deposition.

"The devices we built are very small which means that we could integrate millions in a few millimeter squares," said Perea-Lopez. Presenting MoS2's case as an alternative to graphene, Perea-Lopez 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.

"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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