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Penn researchers develop new technique for making MoS2

"˜Seeding' with a precursor allows growth control using CVD

University of Pennsylvania researchers have made an advance in manufacturing the 2D material, MoS2. By growing flakes of the material around "˜seeds' of MoO2, they have made it easier to control the size, thickness and location of the material. The study was published in the journal Nature Communications.

"Everything we do with regular electronics we'd like to be able to do with 2D materials," said A. T. Charlie Johnson, professor in the department of Physics and Astronomy in Penn's School of Arts and Sciences, who led the study. "Graphene has one set of properties that make it very attractive for electronics, but it lacks this critical property, being able to turn on and off. MoS2 gives you that."

"MoS2 is not as conductive as graphene but it has a very high on/off ratio. We need 1's and 0's to do computation; graphene can only give us 1's and .5's," said Carl Naylor, another member of the research team.

Some groups have been able to make small flakes of MoS2 by exfoliating it, or peeling off atomically thin layers from the bulk material. More recently, other researchers have used CVD, where the molybdenum and sulphur 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.

"Between hunting down the flakes," said Nicholas Kybert, who also worked on the study "and making sure they're the right size and thickness, it would take days to make a single measurement of their properties"

The Penn team's advance was in developing a way to control where the flakes form in the CVD method, by "˜seeding' the substrate with a precursor. "We start by placing down a small amount of MoO2 in the locations we want," Naylor said, "then we flow in sulphur gas. Under the right conditions, those seeds react with sulphur and flakes of MoS2 being to grow."

"There's finesse involved in optimising the growth conditions," Johnson said, "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."

Being able to match up the location of the MoS2 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.

Future work on these MoS2 devices will complement the research team's research on graphene-based biosensors; rather than outputting the detection of some molecule to a computer, MoS2-based sensors could directly report a binding event through a change in the light they emit.

 This research also represents first steps that can be applied toward fabricating a new family of 2D materials.

"We can replace the molybdenum with tungsten and the sulphur with selenium," Naylor said, "and just go down the periodic table from there. We can imagine growing all of these different materials in the places we choose and taking advantages of all of their different properties."

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