Exfoliating 2-D Materials for Printable Electronics

This story appeared in the Feb issue of Materials Today, but you can read it for free below

Chemists at the National University of Singapore have developed a new chemical exfoliation technique that could herald a step-change in printable electronics.

The field of flexible and printable electronics continues to develop at a rapid pace, with the use of two-dimensional transition metal dichalcogenides attracting particular attention in recent literature. But there has been a bottleneck in their development – current processes for producing high-quality monolayers of a range of chalcogenides are slow, complex and produce very low yields.

But, it seems that a team from Singapore and Korea has smashed that bottleneck, using a new exfoliation method to produce the largest single sheets of molybdenum disulphide yet reported in the literature. The method, published in the January issue of Nature Communications, also applies to other two-dimensional chalcogenides, such as tungsten diselenide and titanium disulfide, and results in high yield exfoliation for all of these materials.

Transition metal dichalcogenides (TMDs) have been heralded as the next generation of 2D materials due to their unique electronic and optoelectronic properties, and their high thermal and mechanical stability. Unlike graphene, TMDs are semiconductors and so, have a bandgap that could be used in a range of nanoscale transistors and optoelectronic devices. This work, led by Loh Kian Ping, is the first to report on a chemical exfoliation route that can produce the large area, high-quality monolayers of TMDs that are needed to make a practical device. The team used a comparative approach, using the metal adducts of naphthalene (compounds of lithium, sodium and potassium) as the basis for their process. Using a two-step expansion and intercalation method, Kian Ping and his team produced single-layer molybdenum disulphide sheets, 400 μm2 in size, greatly surpassing the “sub-micrometre” size more typical of 2D chalcogenides.

The paper also demonstrated that the exfoliated sheets can be made into a printable solution, with the high viscosity of the sheets rendering them highly suitable for inkjet printing. Recent work from that team at the National University of Singapore has focused on making their TMD sheets more suitable for use industrial processing methods, including the development of TMD-inks and the characterisation of their non-linear optical properties. It is hoped that the development of this high-yield process will help to make the next-generation of large area thin film technologies a reality. Potential applications for these materials include flexible logic circuits, photodetectors and solar cells.

Nature Communications (2013) doi:10.1038/ncomms3995