A team of scientists from the National University of Singapore (NUS) has successfully developed a method to chemically exfoliate molybdenum disulfide crystals, a class of chalcogenide compounds, into high quality monolayer flakes, with higher yield and larger flake size than current methods. The exfoliated flakes can be made into a printable solution, which can be applied in printable photonics and electronics.
Transition metal dichalcogenides, formed by a combination of chalcogens, such as sulphur or selenium, and transition metals, such as molybdenum or tungsten, have recently attracted great attention as the next generation of two-dimensional materials due to their unique electronic and optical properties, for applications in optoelectronic devices such as thin film solars, photodetectors flexible logic circuits and sensors.
However, current processes of producing printable single layer chalcogenides take a long time and the yield is poor. The flakes produced are of submicron sizes, which make it challenging to isolate a single sheet for making electronic devices.
As most applications require clean and large-sized flakes, this pinpoints a clear need to explore new ways to make high quality single-layer transition metal dichalcogenides with high yield.
To address the production bottleneck, the NUS team explored the metal adducts of naphthalene. They prepared naphthalenide adducts of lithium, sodium and potassium, and compared the exfoliation efficiency and quality of molybdenum disulfide generated. The processing steps are detailed in the Annex.
Using a two-step expansion and intercalation method, the researchers were able to produce high quality single-layer molybdenum disulfide sheets with unprecedentedly large flake size.
“Considering the versatility of this method, it may be adopted as the new benchmark in exfoliation chemistry of two-dimensional chalcogenides”Professor Loh Kian Ping, Department of Chemistry at the NUS Faculty of Science.
The researchers also demonstrated that the exfoliated molybdenum disulfide flakes can be made into a printable solution, and wafer-size films can be printed, as the good dispersion and high viscosity of the flakes render it highly suitable for inkjet printing.
In a comparative analysis, Dr Zheng Jian, the first author of the paper, who is also a Research Fellow with the Department of Chemistry at NUS Faculty of Science, found that the alkali metal naphthalenide intercalation method applied possesses significant advantages in comparison to the conventional method.
Commenting on the significance of the findings, Prof Loh said, “At present, there is a bottleneck in the development of solution-processed two dimensional chalcogenides. 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.”
This story is reprinted from material from National University of Singapore, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.