Nitrogen produces 2D material when forced to follow rules

In the periodic table of elements, there is one golden rule for carbon, oxygen and other light elements. Under high pressures, they adopt similar structures to those taken by heavier elements in the same group of elements. Only nitrogen seemed unwilling to toe the line, but now high-pressure researchers at the University of Bayreuth in Germany have revoked its special status.

The researchers have created a crystalline structure with nitrogen that under normal conditions occurs in black phosphorus and arsenic. This structure comprises two-dimensional (2D) atomic layers, and is therefore of great interest for high-tech electronics. They report their finding in a paper in Physical Review Letters.

When the chemical elements are arranged in ascending order according to the number of protons, it soon becomes obvious that certain properties recur at large intervals (‘periods’). The periodic table of elements brings these repetitions into focus. Elements with similar properties are placed one below the other in the same column, and thus form a group of elements. At the top of a column is the element with the fewest protons and the lowest weight compared to the other group members.

Nitrogen heads element group 15, but was previously considered the ‘black sheep’ of the group. This is because, in earlier high-pressure experiments, nitrogen had displayed no structures similar to those that heavier elements of this group – especially phosphorus, arsenic and antimony – exhibit under normal conditions. Instead, similar structures could be observed at high pressures in the neighboring groups headed by carbon and oxygen.

But it now turns out that nitrogen is no exception after all. Researchers at the Bavarian Research Institute of Experimental Geochemistry & Geophysics (BGI) and the Laboratory for Crystallography at the University of Bayreuth have been able to prove this with the help of a measuring method they recently developed. Under the leadership of Dominique Laniel, they have discovered that, at very high pressures and temperatures, nitrogen atoms form a crystalline structure that is characteristic of black phosphorus, a particular variant of phosphorus.

This structure, which also occurs in arsenic and antimony, is composed of 2D layers in which the atoms are cross-linked in a uniform zigzag pattern. These 2D layers have similar conductive properties to graphene, which shows great promise as a material for high-tech applications. Black phosphorus is already being studied for its potential as a material for highly efficient transistors, semiconductors and other electronic components.

The Bayreuth researchers are proposing an analogous name for this novel allotrope of nitrogen: black nitrogen. Some technologically attractive properties, in particular its directional dependence (anisotropy), are even more pronounced in black nitrogen than in black phosphorus. However, black nitrogen can only exist under the exceptional pressure and temperature conditions used to produce it in the laboratory. Under normal conditions, it dissolves immediately.

"Because of this instability, industrial applications are currently not feasible. Nevertheless, nitrogen remains a highly interesting element in materials research," says Laniel. "Our study shows by way of example that high pressures and temperatures can produce material structures and properties that researchers previously did not know existed. "

It took truly extreme conditions to produce black nitrogen. The compression pressure was 1.4 million times the pressure of the Earth's atmosphere, and the temperature exceeded 4000°C. To find out how atoms arrange themselves under these conditions, the Bayreuth scientists cooperated with the German Electron Synchrotron (DESY) in Hamburg and the Advanced Photon Source (APS) at the Argonne National Laboratory in the US. Here, X-rays generated by particle acceleration were fired at the compressed samples.

"We were surprised and intrigued by the measurement data suddenly providing us with a structure characteristic of black phosphorus," says Laniel. "Further experiments and calculations have since confirmed this finding. This means there is no doubt about it: nitrogen is, in fact, not an exceptional element, but follows the same golden rule of the periodic table as carbon and oxygen do. "

This story is adapted from material from the University of Bayreuth, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.