(a) Plot of the calculated energy difference between magnetic and nonmagnetic states along with total magnetic moment of these two concentrations. The possibility of magnetic order has been checked by considering different spin configurations for 12.5% and 15.625% F concentration. The unpaired electron in the system mainly originates from N atoms and, therefore, three different spin configurations of N atoms in FBN structure have considered as shown in the lower panels (b–d). The calculated energy difference between ferromagnetic and non-magnetic states is negative indicating that the magnetic states are preferable. The energy difference increases with increasing F-concentration. Although ferromagnetic states are the preferable magnetic ordering, the differences of three different magnetic states are much less than the ferromagnetic state. This indicates the possibility of coexistence of magnetic and ferromagnetic states. Moreover, the spins on N atoms in the FBN sheet are arranged in a triangular lattice, as shown in three configurations, which causes frustrated magnetization due to conflicting inter-atomic forces. These theoretical results explain the experimental observation of the coexistence of different magnetic states.
(a) Plot of the calculated energy difference between magnetic and nonmagnetic states along with total magnetic moment of these two concentrations. The possibility of magnetic order has been checked by considering different spin configurations for 12.5% and 15.625% F concentration. The unpaired electron in the system mainly originates from N atoms and, therefore, three different spin configurations of N atoms in FBN structure have considered as shown in the lower panels (b–d). The calculated energy difference between ferromagnetic and non-magnetic states is negative indicating that the magnetic states are preferable. The energy difference increases with increasing F-concentration. Although ferromagnetic states are the preferable magnetic ordering, the differences of three different magnetic states are much less than the ferromagnetic state. This indicates the possibility of coexistence of magnetic and ferromagnetic states. Moreover, the spins on N atoms in the FBN sheet are arranged in a triangular lattice, as shown in three configurations, which causes frustrated magnetization due to conflicting inter-atomic forces. These theoretical results explain the experimental observation of the coexistence of different magnetic states.

Fluorine transforms the two-dimensional, ceramic insulator hexagonal boron nitride (h-BN) into a wide-bandgap semiconductor with magnetic properties, a team of researchers have discovered [Radhakrishnan et al., Science Advances 3 (2017) e1700842].

Two-dimensional materials like h-BN – also know as ‘white graphene’ – have attracted great interest in recent years as novel electronic materials. But while functionalization of these materials has become an indispensible tool for tailoring their physical and chemical properties, fluorinating two-dimensional materials had required specialized instruments.

Now researchers from Rice University, together with colleagues from the Indian Institute of Science, University of Houston, Louisiana State University, Baker Hughes’ Center for Technology Innovation, University of Toronto, and Air Force Research Laboratories at Wright-Patterson, have developed an easy and straightforward way of fluorinating h-BN.

“The simple solvo-thermal method involves Nafion, a fluoropolymer,” says researcher Chandra Sekhar Tiwary. “Nafion acts as the fluorinating agent by degrading at the synthesis temperature to produce fluorine free radicals, which break the B-N bonds to form B-F and N-F bonds.”

In its normal state, h-BN is a chemically inert, thermally conductive, layered ceramic made up of B and N atoms arranged in alternating positions in a hexagonal lattice. But the addition of F, and creation of B-F and N-F bonds, changes the bandgap of h-BN and introduces defect states. Moreover, the F atoms alter the spin of electrons in the N atoms and their magnetic moments. The randomly angled spins create pockets of magnetism.

“Magnetic centers introduced by fluorination give ferromagnetic behavior at room temperature, while the low temperature measurements reveal signatures of unconventional magnetism,” explains Tiwary.

This unconventional or ‘frustrated’ magnetism arises from the change in charge density on the N atoms produced by the introduction of the F atoms.

“There has been a lot of effort to try to modify the electronic structure [of h-BN], but we didn’t think it could become both a semiconductor and a magnetic material,” says Pulickel M. Ajayan. "This is something quite different, nobody has seen this kind of behavior in h-BN before.”

The researchers believe fluorinated-BN (FBN) could be useful for spintronic applications, where the material’s high thermal conductivity should be a boon for high power electronic devices. FBN could also represent an attractive replacement for GaN in compact lasers, since the level of fluorination could be used to tune the emission wavelength.

“The versatility of the method lies in its ability to fluorinate other two-dimensional materials, as well, which is now being pursued,” adds Tiwary. “Moving forward, the work is branching out in all different directions, looking at a variety of applications.”

Hexagonal-BN is currently widely studied and increasingly useful because of its insulating properties and two-dimensional nature, points out Chris Howard of University College London.

“The authors show that adding fluorine atoms significantly affects the net electronic structure of the h-BN framework, narrowing the band gap and, unusually, giving rise to room temperature ferromagnetism. Upon cooling,the magnetic ground state becomes frustrated – the first observation of such a phenomena in a truly two-dimensional material, which makes this system really interesting,” he comments.

This article was originally published in Nano Today (2017), doi: 10.1016/j.nantod.2017.08.004.