Manipulating graphene films

24 Jan 2014. NUS professors from department of Chemistry developed new exfoliation technique may lead the way for two-dimensional materials.

Ultra-thin two-dimensional nanosheets of transition metal dichalcogenides (TMDCs) formed by a combination of chalcogens, such as sulphur or selenium, and transition metals, such as molybdenum or tungsten, have intriguing electronic and mechanical properties. However, the current method of producing a printable single layer of TMDCs is lengthy, suffers from a low yield (of single layer flakes) and disintegration into submicron size flakes. This has limited the use and development of TMDCs for applications which requires large-sized flakes.

A team of scientists, led by Loh Kian Ping from NUS has successfully developed a method to chemically exfoliate molybdenum disulfide (MoS2) crystals, a type of two-dimensional (2D) materials, into high quality monolayer flakes. The exfoliated flakes can be made into a printable solution, which can be used for printable electronics and optoelectronics. This finding opens up exciting prospects for its use in a range of potential applications, such as thin film solar cells, photodetectors, flexible logic circuits and sensors.

Breakthrough in producing MoS2 flakes

The breakthrough makes use of metal adducts of naphthalene and through a two-step expansion and intercalation method, the researchers were able to produce high quality single-layer MoS2 sheets with large flake size. The exfoliated MoS2 flakes can then be easily made into a printable solution. By using the inkjet printing technique, ultra-thin MoS2 films can be printed on silicon substrates. Also, this technique can be applied to other two-dimensional chalcogenides, such as tungsten diselenide and titanium disulfide, with high yields.

Printable ink for optoelectronic devices

Moving forward, Prof Loh and his team will be actively developing printable inks made by exfoliating different types of two-dimensional chalcogenides and explore their suitability for producing printable optoelectronic devices. The NUS team collaborated with scientists from the Ulsan National Institute of Science and Technology in Korea for this research work, and these findings were first published online in the scientific journal Nature Communications in January 2014.

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Zheng Jian, the first author of the paper, demonstrating the printing of Molybdenum Disulfide (MoS2) flakes from a solution of the exfoliated flakes.
[Image source: Office of Corporate Relations, NUS]