Centrifuge Process Allows Thickness Sorting Of MoS2
Scientists at Northwestern University, Illinois have developed a scalable way to isolate atomically thin sheets of the transition metal dichalcogenide semiconductor MoS2.
Like graphene, MoS2 can be exfoliated into atomically thin sheets. As it thins to the atomic limit, it becomes fluorescent making it useful for optoelectronics, such as LEDs, or light-absorbing devices, such as solar cells.
For the past six years, Mark Hersam's lab at Northwestern University has developed methods for exfoliating thin layers of graphene from graphite, using solution-based methods."You would think it would be easy to do the same thing for MoS2," he said. "But the problem is that while the exfoliation is similar to graphene, the separation is considerably more challenging."
Hersam's research is described in the paper 'Thickness sorting of two-dimensional transition metal dichalcogenides via copolymer-assisted gradient ultracentrifugation' which was published in the latest issue of Nature Communications.
To sort graphene layers, Hersam used centrifugal force to separate materials by density. To do this, he and his group added the material to a centrifuge tube along with a gradient of water-based solution. Upon centrifugation, the denser species move toward the bottom, creating layers of densities within the centrifuge tube. Graphene sorts into single layer sheets toward the top, then bilayer sheets, trilayer, and so on. Because graphene has a relatively low density, it easily sorts compared to higher density materials.
"If I use the exact same process with MoS2, its higher density will cause it to crash out," Hersam said. "It exceeds the maximum density of the gradient, which required an innovative solution."
Hersam needed to take the inherently dense material and effectively reduce its density without changing the material itself. He realized that this goal could be achieved by tuning the density of the molecules used to disperse MoS2. In particular, the use of bulkier polymer dispersants allowed the effective density of MoS2 to be reduced into the range of the density gradient. In this manner, the sheets of MoS2 floated at layered positions instead of collecting as the bottom of the centrifuge tube. This technique works not just for MoS2, but for other materials in the transition metal dichalcogenides family.
"Now we can isolate single layer, bilayer, or trilayer transition metal dichalcogenides in a scalable manner," Hersam said. "This process will allow us to explore their utility in large-scale applications, such as electronics, optoelectronics, catalysis, and solar cells."
'Thickness sorting of two-dimensional transition metal dichalcogenides via copolymer-assisted gradient ultracentrifugation' by Kang et al appears in Nature Communications 5, Article number: 5478 doi:10.1038/ncomms6478