Posted in | Graphene

Researchers Convert Common Insulator into Magnetic Semiconductor

Rice University Researchers discovered that a little fluorine converts an insulating ceramic known as white graphene into a wide-bandgap semiconductor possessing magnetic properties. This could make the unique material ideal for electronics in challenging environments.

Rice graduate student Sruthi Radhakrishnan shows samples of pure hexagonal boron nitride and fluorinated hexagonal boron nitride. Fluorination turns the material known as white graphene, a common insulator, into a magnetic semiconductor that may be suitable for electronics and sensors in extreme environments. (Photo by Jeff Fitlow)

A proof-of-concept paper from Rice Researchers shows a way to transform 2D hexagonal boron nitride (h-BN) – aka white graphene – from an insulator to a semiconductor. They state that the magnetism was an unanticipated bonus.

As the atomically thin material is an excellent conductor of heat, the Researchers recommended it may be beneficial for electronics in high-temperature applications, maybe even as magnetic memory devices.

Details of the discovery will be published in Science Advances.

Boron nitride is a stable insulator and commercially very useful as a protective coating, even in cosmetics, because it absorbs ultraviolet light. 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. So this is something quite different; nobody has seen this kind of behavior in boron nitride before.

Pulickel Ajayan, Rice Materials Scientist, whose lab headed the study

The Researchers found that incorporating fluorine to h-BN introduced defects into its atomic matrix that decreased the bandgap sufficiently to make it a semiconductor. The bandgap controls the electrical conductivity of a material.

We saw that the gap decreases at about 5 percent fluorination. Controlling the precise fluorination is something we need to work on. We can get ranges but we don’t have perfect control yet. Because the material is atomically thin, one atom less or more changes quite a bit. In the next set of experiments, we want to learn to tune it precisely, atom by atom.

Chandra Sekhar Tiwary, Rice Postdoctoral Researcher and Co-Author

They established that tension applied by invading fluorine atoms changed the “spin” of electrons in the nitrogen atoms and impacted their magnetic moments, the ghostly quality that defines how an atom will react to a magnetic field like an invisible, nanoscale compass.

We see angle-oriented spins, which are very unconventional for 2-D materials.

Sruthi Radhakrishnan, Rice Graduate Student and Lead Author

Instead of aligning to form ferromagnets or canceling each other out, the spins are haphazardly angled, providing the flat material random pockets of net magnetism. These ferromagnet or anti-ferromagnet pockets can be present in the same swatch of h-BN, which renders them “frustrated magnets” with competing domains.

The team said their simple, scalable technique can potentially be applied to other 2D materials. “Making new materials through nanoengineering is exactly what our group is about,” Ajayan said.

The paper’s Co-Authors are Graduate Students Carlos de los Reyes and Zehua Jin, Chemistry Lecturer Lawrence Alemany, Postdoctoral Researcher Vidya Kochat and Angel Martí, an Associate Professor of Chemistry, of Bioengineering and of Materials Science and Nanoengineering, all of Rice; Valery Khabashesku of Rice and the Baker Hughes Center for Technology Innovation, Houston; Parambath Sudeep of Rice and the University of Toronto; Deya Das, Atanu Samanta and Rice alumnus Abhishek Singh of the Indian Institute of Science, Bangalore; Liangzi Deng and Ching-Wu Chu of the University of Houston; Thomas Weldeghiorghis of Louisiana State University and Ajit Roy of the Air Force Research Laboratories at Wright-Patterson Air Force Base.

Ajayan is chair of Rice’s Department of Materials Science and NanoEngineering, the Benjamin M. and Mary Greenwood Anderson Professor in Engineering and a Professor of Chemistry.

The study was supported by the Department of Defense, the Air Force Office of Scientific Research and its Multidisciplinary University Research Institute, the National Science Foundation and Indian Department of Science and Technology Nano Mission. The Indian Institute of Science provided supercomputer resources.

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