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Fluorine grants white graphene new powers

Fluorine turns an insulating ceramic known as white graphene from an insulator into a wide-bandgap semiconductor with magnetic properties, say Rice University scientists. They claim that because the atomically thin material is a conductor of heat it may be useful for electronics in high-temperature applications and potentially even as magnetic memory devices.

"Boron nitride (h-BN) is a stable insulator and commercially very useful as a protective coating, even in cosmetics, because it absorbs ultraviolet light," said materials scientist Pulickel Ajayan.

"There has been a lot of effort to try to modify its electronic structure, but we didn't think it could become both a semiconductor and a magnetic material.

The researchers found that adding fluorine to h-BN introduced defects into its atomic matrix that reduced the bandgap enough to make it a semiconductor.

"We saw that the gap decreases at about 5% fluorination," said postdoctoral researcher Chandra Sekhar Tiwary. The gap is said to get smaller with additional fluorination, but only to a point.

The team determined that tension applied by invading fluorine atoms altered the ‘spin’ of electrons in the nitrogen atoms and affected their magnetic moments.

"We see angle-oriented spins, which are very unconventional for 2D materials," said graduate student Sruthi Radhakrishnan.

Rather than aligning to form ferromagnets or cancelling each other out, the spins are randomly angled, giving the flat material random pockets of net magnetism. These ferromagnet or anti-ferromagnet pockets can exist in the same swatch of h-BN, which the researchers claim make them ‘frustrated magnets’ with competing domains.

The researchers said their simple, scalable method can potentially be applied to other 2D materials.

Peggy Lee

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