Phosphorus comes in a few different forms (or allotropes), including white, red, violet, and black. For a few years scientists have predicted a fifth allotrope should be possible, but it had yet to be confirmed. Now, researchers in Germany have managed to show that this new allotrope, known as blue phosphorus, does exist, and mapped out its different properties.
Phosphorus is an abundant element and plays some pretty important roles in biology and technology. It's one of just six elements on the Must-Have list for life – which makes its rarity outside our solar system troubling for those looking for alien life. Red phosphorus is probably best known for its use in the igniting strip on match boxes, while the volatile white phosphorus was once used in explosives. Black phosphorus, meanwhile, is beginning to look useful in electronics as the "new silicon."
But that's not the whole family, apparently. In 2014 researchers calculated that a new form, dubbed blue phosphorus, should also be stable, and in theory could be an effective semiconductor like black phosphorus and graphene. In 2016, blue phosphorus was successfully created in the lab, but its identity remained uncertain.
Now researchers from Helmholtz Zentrum Berlin (HZB) have studied the stuff and confirmed that it is the predicted new phosphorus allotrope. By evaporating the material onto a gold substrate, the team found that the phosphorus atoms line up in a honeycomb pattern, similar to that of graphene. But rather than lie flat, the lattice "buckles" as the atoms arrange themselves around the gold atoms underneath.
That changes the way electrons move through the material, which gives blue phosphorus different electronic properties to black. Using angle-resolved photoelectron spectroscopy, the team measured the distribution of electrons in the material's valence band, and found that it had a band gap of at least 2 electron volts – more than seven times larger than that of bulk black phosphorus. In electronics, wider band gaps allow semiconductors to operate at higher voltages and temperatures.
"So far, researchers have mainly used bulk black phosphorus to exfoliate atomically thin layers," says Oliver Rader, an author of the study. "These also show a large semiconducting band gap but do not possess the honeycomb structure of blue phosphorus and, above all, cannot be grown directly on a substrate. Our work not only reveals all the material properties of this novel two-dimensional phosphorus allotrope but highlights the impact of the supporting substrate on the behavior of electrons in blue phosphorus, an essential parameter for any optoelectronic application."
The research was published in the journal Nano Letters.
Source: Helmholtz Zentrum Berlin
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