Image credit: Hersam et al, Nature Communications
Image credit: Hersam et al, Nature Communications

Graphene is not the answer to all the world's material problems, according to Mark Hersam of Northwestern University in Evanston, Illinois, USA. He and his colleagues are pursuing the transition metal dichalcogenide, molybdenum disulfide (MoS2) as a frontrunner in the race to find other materials that can form monolayers with interesting and useful optical and electronic properties. Now, in the culmination of six year's work on graphene itself, Hersam's team has developed a large-scale isolation technique for fabricating monolayers of the semiconductor molybdenum disulfide [Hersam et al., Nature Commun (2014) DOI: 10.1038/ncomms6478].

In conventional chemical applications MoS2 has been used as a catalyst for the desulfurization of crude oil. However, when it is exfoliated to atomically thin sheets it becomes fluorescent, a property that has potential in optoelectronics as light-emitting diodes and light-absorbing solar cells. The properties of these materials are very much defined by their thickness.

Hersam had hoped that the exfoliation of MoS2 would be as straightforward as that of graphene and it is, but isolating the monolayers afterwards proved problematic leading to unwanted defects in the materials. To sort graphene layers, Hersam and others have used isopycnic density gradient ultracentrifugation to separate the materials by density across a solvent gradient in the centrifuge tube using ionic small molecule dispersants. The relatively low density of graphene makes it easy to sort in this way. Whereas for the much denser MoS2 "it crashes out," says Hersam, “because it exceeds the maximum density of the gradient, which required an innovative solution."

To overcome this problem, the team reasoned that to disperse the inherently dense material without altering its structure they would need to use bulkier, block copolymer dispersants composed of a central hydrophobic unit flanked by hydrophilic chains to effectively reduce the overall buoyant density in aqueous solution of the materials. This would allow the effective density of the exfoliated molybdenum disulfide to be reduced to within the range of the carrier solution's density gradient.

In this manner, the sheets of MoS2 floated at layered positions instead of collecting at the bottom of the centrifuge tube. Tests on the solution-processed monolayers of MoS2 made this way exhibit strong photoluminescence without further chemical treatment, the team reports.

"Now we can isolate single layer, bilayer, or trilayer transition metal dichalcogenides in a scalable manner," Hersam explains. "This process will allow us to explore their utility in large-scale applications." The team adds that the same technique should also work with other dense members of the transition metal dichalcogenides family.

"We are interested in exploring and realizing applications (e.g., electronics, light-emitting diodes, solar cells, etc.) using our solution-processed transition metal dichalcogenides," Hersam told Materials Today. "We are also expanding our processing methods into other emerging two-dimensional nanomaterials."

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".