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Researchers have manufactured molybdenum disulphide, a material with an extremely high conductivity and thinness, that could be a competitor to graphene.

Unlike graphene, molybdenum disulfide has an energy band gap, meaning its conductivity can be turned on and off. Such a trait is critical for semiconductor devices used in computing. Another difference between graphene and molybdenum disulphide is that it emits light meaning it could be used in applications like LEDs, self-reporting sensors and optoelectronics.

Graphene’s ultra-high conductivity means that it can move electrons more quickly than any known material but that is not the only quality that matters for electronics. For the transistors that form the basis for modern computing technology, being able to stop the flow of electrons is also critical.

Although molybdenum disulphide is not as conductive as graphene, it has a very high on/off ratio. Electronic devices need 1s and 0s to do computations and graphene cannot produce that well.

In the past, scientists have been able to make small flakes of molybdenum disulphide the same way graphene was first made, by exfoliating it, or peeling off atomically thin layers from the bulk material.

More recently, researchers have adopted another technique from graphene manufacture, chemical vapor deposition, where the molybdenum and sulfur are heated into gasses and left to settle and crystalize on a substrate. The problem with these methods is that the resulting flakes form in a scattershot way.

In this research, the team developed a way to control where the flakes form in the chemical vapor deposition method, by “seeding” the substrate with a precursor. The scientists placed down a small amount of molybdenum oxide in the locations they wanted and flowed in sulfur gas. The seeds react with sulfur and flakes of molybdenum disulphide being to grow.

Being able to match up the location of the molybdenum disulphide flakes with corresponding electronics allowed the researchers to skip a step they must take when making graphene-based devices. There, graphene is grown in large sheets and then cut down to size, a process that adds to the risk of damaging contamination.

Professor A.T.C Johnson, Professor at the Department of Physics & Astronomy at Penn, said:

“There’s finesse involved in optimizing the growth conditions but we’re exerting more control, moving the material in the direction of being able to make complicated systems. Because we grow it where we want it, we can make devices more easily.”

“We have all of the other parts of the transistors in a separate layer that we snap down on top of the flakes, making dozens and potentially even hundreds, of devices at once. Then we were able to observe that we made transistors that turned on and off like they were supposed to and devices that emit light like they are supposed to.”

The research was led by A. T. Charlie Johnson and includes members of his lab, Gang Hee Han, Nicholas Kybert, Carl Naylor and Jinglei Ping. Also contributing to the study was Professor Ritesh Agarwal, members of his lab, Bumsu Lee and Joohee Park; and Jisoo Kang, a master’s student in Penn’s nanotechnology program. They collaborated with researchers from South Korea’s Sungkyunkwan University, Si Young Lee and Young Hee Lee.

Their study was published in the journal Nature Communications.

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