Researchers produce new wonder material: Black Phosphorus

While graphene and its amazing applications are now well known, few would have heard of black phosphorus, boron nitride, or molybdenum disulphide.

What all these materials have in common is that they form two dimensional crystals, and in black phosphorus, the phosphorus atoms join together to form a  two-dimensional puckered sheet that makes it an excellent material for making a field-effect transistor, MIT Technology Review reported.

Not surprisingly therefore, researchers suggest that the material could have a bright future in nano-electronic devices.

However, making black phosphorus in large quantities had posed challenges. But researchers Damien Hanlon at Trinity College Dublin in Ireland and his team have solved the problem, and perfected a way of making large quantities of black phosphorus nanosheets with dimensions they could control. 

They have further tested the black phosphorus, thus produced in several applications, such as a gas sensor, an optical switch, and even to reinforce composite materials to make them stronger.

In bulk form, the material comes in many layers like graphite and one way of separating single sheets was by exfoliation, simply peeling off layers using Scotch tape or other materials. But peeling off the layers is a time-consuming task that severely limited potential applications.

Hanlon and colleagues therefore took a different approach  - they placed the lump of black phosphorus in a liquid solvent and then bombarded it with acoustic waves that shake the material apart.

This split up the bulk into many nanosheets that the team filtered for size with a centrifuge, which left high-quality nanosheets consisting of only a few layers.

But there was another problem that was needed to be overcome - black phosphorus nanosheets degraded rapidly when in contact with water or oxygen. So the team went on to suggest that certain solvents form a solvation shell around the sheet, to keep oxygen or other oxidative species from reaching the phosphorus.

The team used N-cyclohexyl-2-pyrrolidone or CHP as a solvent due to which the nanosheets became surprisingly long-lived.

The big advantage of black phosphorus over graphene was that it had a natural bandgap that physicists could exploit make electronic devices, such as transistors. But Hanlon and co say the new-found availability of black phosphorus nanosheets had allowed them to test a number of other ideas as well.

For instance adding nanosheets to a film of polyvinyl chloride, doubled its strength and increased its tensile toughness sixfold. It was therefore not just carbon allotropes that could increase strength!

They further determined the nonlinear optical response of the nanosheets to a pulsed laser by measuring the amount of light that transmitted and it turned out that the amount of light black phosphorus absorbed decreased with the increasing intensity, a property known as saturable absorption. They found the material to be better at this even than graphene.

Finally, they measured the current through the nanosheets with exposure to ammonia and found that the material's resistance increased when it came into contact with ammonia, probably because ammonia donated electrons that neutralised holes in the black phosphorus sheets.

That immediately made black phosphorous a decent ammonia detector and according to Hanlon and his team, the material could detect ammonia at levels of around 80 parts per billion.

All this could herald a very interesting change in research associated with black phosphorus say experts. Many people would have seen the excitement  associated with the remarkable properties of graphene and according to experts if black phosphorus was half as remarkable there could be an interesting future for material scientists.