The new surface coating technology increases the material’s electron emission sevenfold

May 12, 2023

(Nanowerk News) An international research group has developed a new surface coating technology capable of significantly increasing the emission of electrons in materials (Applied Physics Letters, “Decreased work function of LaB6 with monolayer hexagonal boron nitride coating for better photocathode and thermionics”). Their breakthrough is expected to increase the production of high-efficiency electron sources, and lead to improved performance in electron microscopy, electron beam lithography systems, and synchrotron radiation facilities.

Free electrons are electrons that are not bound to certain atoms or molecules, move freely in a material. They play important roles in a wide range of applications, from photoreactors and microscopes to accelerators. Photoemission electron microscopy (PEEM) and thermal electron emission microscopy (TEEM) images of LaB6 the surface is coated with graphene (Gr) and hBN. The bright areas in the image show a large number of electrons being emitted. (Image: Tohoku University)

One property that measures the performance of free electrons is the work function: the minimum energy required for an electron to escape from the surface of a material into a vacuum. Materials with a low work function require less energy to remove electrons and make them move freely; whereas materials with high work function require more energy to remove electrons.

Lower work functions are critical to improving the performance of electron sources and contributing to the development of advanced materials and technologies that can have practical applications in fields such as electron microscopy, accelerator science and semiconductor manufacturing.

Currently, hexaboride lanthanum (LaB6) is widely used as an electron source because of its high stability and durability. To improve Lab6efficiency, the research group turned to hexagonal boron nitride (hBN), a versatile chemical compound that is thermally stable, has a high melting point, and is particularly useful in harsh environments,

“We found the LaB layer6 with hBN lowering the work function from 2.2 eV to 1.9 eV and increasing electron emission,” said Shuichi Ogawa, a co-author of the study and current associate professor at Nihon University (formerly at Tohoku University’s Multidisciplinary Research Institute for Advanced Materials). Schematic diagram of work function modulation mechanism with graphene and hBN layers Schematic diagram of work function modulation mechanism with graphene and hBN layers. When LaB6 and the coating material comes into contact with the coating, their Fermi level (EF) becomes the same. In terms of LaB coating6 with graphene ((a), (b)), the work function W after graphene coating is greater than the original work function LaB6WLaB6. On the other hand, in the case of hBN coating ((d), (e)), the work function W after hBN coating is lower than WLaB6. Figures (c) and (f) show the redistribution of costs by first principle calculations. (Image: Tohoku University

Photoemission electron microscopy and thermionic emission electron microscopy performed by the groups confirmed the lower work function compared to the uncoated and graphene-coated regions.

In the future, Ogawa and his friends hope to hone their coating technique. “We still need to develop the hBN-to-LaB coating technique6non-oxidized surface, as well as how to coat LaB6 electron source with a pointed triangular shape.”

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