Phosphorene holds promise for higher exfoliation yields

Materials World magazine
,
1 Jun 2016

A new process that results in atomically thin, electronically active phosphorene holds promise for higher exfoliation yields. Natalie Daniels reports. 

Exfoliating atomically thin flakes from phosphorous in a quick and effective method has proved difficult. But a team of researchers at Northwestern University, USA, may have overcome this by developing a new method that produces substantially higher exfoliation yield and thinner flakes than previous attempts, and could lead to scalable electronic and optoelectronic applications.  

Mark Hersam, Professor of Chemistry and Materials Science Engineering at Northwestern, said, ‘The key innovation was realising that phosphorene is stable in deoxygenated water, which allowed us to use amphiphilic surfactants as dispersion agents for black phosphorus in water. This exfoliation strategy is more effective than previous methods based on organic solvents.’

Unlike graphene, phosphorene is a semiconductor with a bandgap that varies in thickness. Rather than trying to make graphene semi-conducting, the researchers decided to exploit phosphorene's semiconducting properties. Hersam explained, ‘These thin phosphorene flakes can be used in transistors, photodetectors, and related electronic and optoelectronic applications.’

Graphene’s equivalent 2D material

The team initially set out to use graphene to explore the most scalable and effective method to produce phosphorene. The process described in the paper Stable aqueous dispersions of optically and electronically active phosphorene, published in Proceedings of the National Academy of Sciences uses a solvent that is added to graphite and a surfactant, introducing energy through sonication and exfoliating the graphite down to graphene. When testing this method, they found that when phosphorene is exposed to open air it leads to rapid deterioration. Hersam and the team then exfoliated with organic solvents in a closed, air-free and water-free environment. The results were disappointing – the phosphorene had both a low exfoliation yield and flakes that were relatively thick.

Desperately trying to create thin flakes, and after studying the degradation process for a year, the team discovered phosphorene degraded in water containing oxygen. By bubbling an inert gas through water, the team deoxygenated it to create an aqueous solvent for exfoliating black phosphorous that doesn’t degrade. After sonicating black phosphorous in a mixture of deoxygenated water and surfactants, Hersam found a substantially higher exfoliation yield and much thinner flakes that reached the atomically thin limit.

Unlike other attempts, the researchers' method uses environmentally benign water rather than organic solvents. However, to exploit the 2D material on a full commercial scale in electronic and optoelectronic applications, the team face some hurdles. ‘The largest barrier at the moment is the cost of black phosphorus, although it is conceivable that these costs will decrease significantly due to economies of scale in a real-world manufacturing environment,’ said Hersam. 

‘We are actively working on assembling phosphorene flakes out of aqueous solutions into ordered arrays on substrates, since this will expedite our long-term goal of demonstrating scalable electronic and optoelectronic applications based on this material.’ The researchers hope that this method for isolating phosphorene optically and electronically will lead to the development of phosphorene-based technologies and fruther exploration of other nano materials. 

To read more about phosphorous and its potential as a 2D material, read Simon Frost’s piece Bridging the band gap in our October 2015 issue at bit.ly/1RrWQhQ