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Black Phosphorus, the Wonder Material to Rival Graphene

There’s a new glow in town.

Graphene may have met its match. The wonder-material, hai​led as the future of electronics, never had a real monopoly on 2D crystalline semiconductors, but now one of its relatives has a cost-effective manufacturing process: black phosphorus.

Phosphorus is known by its glow. The fifteenth element lends its name to an entire class of compounds classified by their chemiluminescent properties: light without burning, or mostly-heatless light produced via chemical reaction. If anything on the periodic table might be considered an archetype of chemistry itself, it's probably phosphorus.

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More specifically, that archetype is probably white phosphorus. There are different varieties with different physical characteristics. The white variety, a tiny pyramid composed of four chemically bound phosphorus atoms, glows intensely and is quite toxic. It's also prone to self-ignition when exposed to the air; white phosphorus is a component of napalm. But heat the stuff up and expose it to extremely high pressures, and the result is a form of the element that's nearly the polar opposite to the volatile white.

This is black phosphorus, a material that's historically been very difficult to produce yet offers promise in nanoelectronics, where it's more kin to two-dimensional (single-atom thick) wonder materials like graphene. The problem with making it is that, like graphene, it comes in multilayer packages, where in order to get the desired two-dimensions, these layers need to be peeled away one by one, a process called exfoliation.

This is the problem recently solved by materials​ scientists at Trinity College Dublin. Rather than tediously peeling away layers, they found that by submerging the black phosphorus material in a liquid solvent and then bombing it with sound waves, the same effect could be achieved a whole lot easier and a whole lot cheaper. So: the sheets just kind of shake apart, yielding sized-to-order sheets of black phosphorus just a few atoms thick.

The bizarre world of 2D materials has so far has been dominated by graphene, the celebrity material that won a pair of researchers the Nobel Prize in Physics in 2010. The material and its less celebrated kin, when reduced to a single layer of crystalline chicken-wire, take on all sorts of neat properties. It's electrically conductive to an extreme degree, stronger than kevlar—hailed as the strongest ​material on Earth even—and shows promise as a sponge/filter of sorts that might be used to soak up hydrog​en fuel from the air.

There are over 7,000 patents for graphene applications alone, many residing with technology giants like Apple and Sony. It's the new​ silicon, arguably, but graphene isn't the only material with such properties. It might just be the start. For one, as the Trinity team notes, black phosphorus has a ban​dgap while graphene is what's known as a zero-gap semiconductor, which is a bit more like a conventional metal than a proper semiconductor, a fancy version of copper or whatever.

Black phosphorus, as a tunable semiconductor, should then have even more applications in electronics: transistors, sensors, solar cells, switches, battery electrodes, etc. Some of these have already been tested to positive results. Like graphene, black phosphorus is also very strong yet not so easy to produce in large quantities.

"While phosphorene nanosheets have very recently been produced by liquid exfoliation, this method remains problematic, largely because phosphorene is known to be unstable, degrading via reactions with either water or oxygen," the Trinity groupwro​te in a recent paper. "For this method to be useful, ways must be found to stabilise liquid exfoliated nanosheets against oxidation." The solution turned out to be solvent N-cyclohexyl-2-pyrrolidone, a material already used within electronics manufacturing.

Black phosphorus is indeed black, lacking the luminescence of its popular relative, but it does disperse light remarkably well, better even than graphene. Bomb the stuff with increasing intensities of light and the better it gets at absorbing and dispersing it. This is what makes it so well suited to things like optoelectronics. A whole new glow.