Researchers use 'exfoliation' method to produce nanosheets

Their so-called ‘exfoliation’ method for producing nanosheets is quick and inexpensive and could be scaled up, for example to generate electricity from waste heat in power plants.

Graphene, a nanosheet of carbon, shot to prominence recently after its creators received a Nobel physics prize last year.

‘Because of its extraordinary electronic properties, graphene has been getting all the attention… but in fact there are hundreds of other layered materials that could enable us to create powerful new technologies,’ said Oxford University’s Dr Valeria Nicolosi, who worked on the project.

For decades, researchers have tried to create nanosheets from a range of materials. However, all previous methods (aside from that for graphene) were time consuming, laborious and resulted in fragile materials unsuited to most practical applications.

Using mild ultrasonic pulses (such as those generated by jewellery-cleaning devices) and common solvents, the team was able to create nanosheets at very high yield and large throughput.

The researchers are currently investigating nanosheets of various materials, including those of boron nitride, molybdenum disulfide and tungsten disulfide, which have the potential to be insulators, semiconductors, conductors and even superconductors depending on their chemical composition and how their atoms are arranged.

‘Now that the exfoliation method has been established, we are moving onto characterisation and device testing,’ Nicolosi told The Engineer.

While the team has found that some of the nanosheets possess good thermoelectric properties (the capacity to produce voltage from heat), they tend to be poor conductors and cannot carry current away to produce useful work.

However, the versatility of the exfoliation method allows them to combine layered materials with different properties — for example, they can mix thermoelectric nanosheets with conductors such as carbon nanotubes or graphene.

‘This would result in hybrids that combine the best of both worlds: you can retain the thermoelectric properties and also have an overall conductive material,’ said Nicolosi.