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Jan 7th, 2014
 
Novel exfoliation method developed by NUS chemists paves the way for two-dimensional materials to be used in printable photonics and electronics
 
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.
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Fig 1: Schematic of pre-exfoliation, intercalation and exfoliation processes.
Fig 1: Schematic of pre-exfoliation, intercalation and exfoliation processes.

The exfoliated flakes can be made into a printable solution, which can be applied in printable photonics and electronics.

  • Versatility of new technique shows promise as new benchmark in exfoliation chemistry of two-dimensional chalcogenides

This breakthrough, led by Professor Loh Kian Ping, who heads the Department of Chemistry at the NUS Faculty of Science, and is also a principal investigator with the Graphene Research Centre at the Faculty, has generic applicability to other two-dimensional chalcogenides, such as tungsten diselenide and titanium disulfide, and results in high yield exfoliation for all of these two-dimensional materials.

The NUS team collaborated with scientists from the Ulsan National Institute of Science and Technology in Korea, and the findings were first published online in prestigious scientific journal Nature Communications on 2 January 2014.

Fig 1: Schematic of pre-exfoliation, intercalation and exfoliation processes

a.Bulk molybdenum disulfide crystal was expanded by decomposition of hydrazine.

b.The expanded molybdenum disulfide reacted with sodium naphthalenide to form an intercalation sample, then exfoliated into single layer sheets by immersing in water.

c.Scanning electron microscopy image of single layer molybdenum disulfide on silicon dioxide.

d.Atomic force microscopy image of single layer molybdenum disulfide on silicon dioxide.

e.Photograph of bulk single crystal molybdenum disulphide.

f.Photograph of pre-exfoliated molybdenum disulphide.

g.Photograph of sodium-exfoliated single layer molybdenum disulfide dispersion in water.

Demand for high efficiency exfoliation method

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.

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.

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.


 
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