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Researchers at the National University of Singapore (NUS) have developed an exfoliation method for the two-dimensional (2D) material molybdenum disulfide that leads to crystals of the substance becoming high quality monolayer flakes. These flakes can made into a solution that could be used for printable photonics and electronics.

NUS researchers have been on a bit of a run lately in developing novel manufacturing techniques for 2D materials. Last month, researchers there developed a one-step method for producing graphene for wafer scale films. This latest work also presents improved manufacturing methods for 2D materials, but this time the material of choice is molybdenum disulfide (MoS2), which is itself gaining some favor over graphene in electronics applications. However, the exfoliation technique developed by the NUS team can be applied to other 2D materials such as such as tungsten diselenide and titanium disulfide.

These materials represent a class of chalcogenide compounds. When chalcogens, like sulfur or selenium, are combined with transition metals, like molybdenum or tungsten, they form transition metal dichalcogenides. So far only a few of these transition metal dichalcogenides have been investigated for their electronic properties, but early indications have shown them to be promising for optoelectronic devices such as thin film solar, photodetectors and flexible logic circuits.

However, the process for turning them into a single, printable layer takes a long time and the yield is quite poor. To address this issue, the NUS researchers explored the use of metal adducts (a compound made from two or more substances) of naphthalene. The researchers created naphthalenide adducts of lithium, sodium and potassium and compared the exfoliation efficiency and quality of molybdenum disulfide produced from each. The research appears today's edition of Nature Communications.

The researchers were able to produce high quality single-layer molybdenum disulfide sheets with large flake sizes, and also demonstrated that exfoliated molybdenum disulfide flakes can be made into a printable solution. With this solution, the researchers were able to show that the ink could produce wafer-size films.

 “At present, there is a bottleneck in the development of solution-processed two dimensional chalcogenides,” said Professor Loh Kian Ping, who heads the Department of Chemistry at the NUS, in a press release. “Our team has developed an alternative exfoliating agent using the organic salts of naphthalene and this new method is more efficient than previous solution-based methods. It can also be applied to other classes of two-dimensional chalcogenides. Considering the versatility of this method, it may be adopted as the new benchmark in exfoliation chemistry of two-dimensional chalcogenides.”

In future research, the NUS team will be looking at creating inks from different 2D chalcogenides using its novel method.

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