Materials-by-design approach reveals lowered work function for lanthanum hexaboride

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Experimentalist Hisato Yamaguchi, left, and Theoretical division theorist Gaoxue Wang, right, hold an atom structure model.

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In work recently published in Applied Physics Letters and selected as an Editor’s Pick, a research team led by Los Alamos National Laboratory scientists demonstrated the first ever lowering of work function for lanthanum hexaboride, a widely used electron emissive material, by coating its surface with a two-dimensional, atomically thin layer of hexagonal boron nitride. Work function is the minimum energy required for electrons to escape from a material surface into a vacuum, thus an important property associated with the cathode technology in devices from photosensors and microscopes to accelerators. For photocathodes, work function determines quantum efficiency, the rate of free electrons generated per incident photons. Exemplifying a “materials by design” approach that combines theoretical and experimental research, the work demonstrates an efficient way to tune materials properties.

The team used photoemission electron microscopy (PEEM) and thermionic emission electron microscopy (TEEM) to discover that when coated with hexagonal boron nitride, a single crystal of lanthanum hexaboride has lower work function compared to non-coated (bare) and graphene-coated regions. A 0.4 electronvolt decrease in work function quantitatively supported the broad and uniformly brighter image of the hexagonal boron nitride region revealed by PEEM. The team also found, through TEEM, that unlike the bare and non-coated regions, the hexagonal boron nitride region exhibited thermionic emission at 905 degrees Celsius. Density functional theory calculations qualitatively supported the work function decrease in the hexagonal boron nitride-coated lanthanum hexaboride.

By adding an oxide layer in the calculations, the team was able to improve consistency between the theoretical inputs and experimental results. Synchrotron-radiation X-ray photoelectron spectroscopy confirmed the presence of an oxide layer on the lanthanum hexaboride. The degree of work function modification on hexagonal boron nitride and graphene-coated oxidized surfaces proved to be smaller than on the clean surfaces. That result indicates that the oxide layer reduces the change transfers between the lanthanum hexaboride and the two-dimensional materials.

Theoretical inputs helped steer the experimental research, predicting that hexagonal boron nitride would lower the work function, whereas a similarly atomically thin material, graphene, would not. Without the theoretical inputs, experimentalists in the project did not foresee a possibility that an atomically thin material coating would lower the work function of electron emissive materials like lanthanum hexaboride. Even if experimentalists have thought of this concept, finding the right atomically thin materials out of hundreds to thousands of material choice is unrealistic, so the ability of theoretical calculations to pinpoint a promising candidate material with a desired effect was important.

Funding and mission

The work was supported by the U.S. Department of Energy Office of Science, U.S.-Japan Science and Technology Cooperation Program in High Energy Physics. The work was also supported, in part, by the Laboratory Directed Research and Development program at Los Alamos. Studies were performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility, operated for the U.S. Department of Energy Office of Science. The work supports the Global Security mission area and the Materials for the Future capability pillar.

Reference

“Work function lowering of LaB₆ by monolayer hexagonal boron nitride coating for improved photo- and thermionic-cathodes,” Applied Physics Letters, 122, 14 (2023); DOI: 10.1063/5.0142591. Authors: Hisato Yamaguchi, Gaoxue Wang, Michael T. Pettes and Nathan A. Moody (Los Alamos National Laboratory); Ryunosuke Yusa and Tadashi Abukawa (Tohoku University); Shuichi Ogawa (Nihon University); Fangze Liu (Beijing Institute of Technology); and Yasutaka Tsuda and Akitaka Yoshigoe (Japan Atomic Energy Agency).

Technical contact: Hisato Yamaguchi (AOT-AE)