Atomic Armor for accelerators enables discoveries
Advancement in single-atom layer graphene coatings improves accelerator electron source lifespans

Date:
January 25, 2022
Source:
DOE/Los Alamos National Laboratory
Summary:
Protective coatings are common for many things in daily life
that see a lot of use: we coat wood floors with finish; apply
Teflon to the paint on cars; even use diamond coatings on medical
devices. Protective coatings are also essential in many demanding
research and industrial applications.



FULL STORY ========================================================================== Protective coatings are common for many things in daily life that see a
lot of use: we coat wood floors with finish; apply Teflon to the paint on
cars; even use diamond coatings on medical devices. Protective coatings
are also essential in many demanding research and industrial applications.


==========================================================================
Now, researchers at Los Alamos National Laboratory have developed
and tested an atomically thin graphene coating for next-generation, electron-beam accelerator equipment -- perhaps the most challenging
technical application of the technology, the success of which bears out
the potential for "Atomic Armor" in a range of applications.

"Accelerators are important tools for addressing some of the grand
challenges faced by humanity," said Hisato Yamaguchi, member of the
Sigma-2 group at the Laboratory. "Those challenges include the quest
for sustainable energy, continued scaling of computational power,
detection and mitigation of pathogens, and study of the structure
and dynamics of the building blocks of life. And those challenges all
require the ability to access, observe and control matter on the frontier timescale of electronic motion and the spatial scale of atomic bonds."
The challenge of photocathodes Current electron-beam accelerators
generally use thermionic emission -- the heating of material to release electrons. The next generation of accelerators will generate electron
sources from photons, using photocathodes -- materials that can convert
photons to free electrons and thus electron beams. The nature of that
process produces corrosive gases that add significant wear and tear on
the photocathodes, interrupting research for service and adding time
and cost to projects.

"Accelerators of the future demand increasingly high-performance
electron beams," said Yamaguchi. "But those performance requirements dramatically outstrip the capabilities of present state-of-the-art
electron sources." For photocathodes to work in next-generation
accelerators, a suitable protective coating needed to be found. That's
because the reaction from photons striking the photocathodes to emit
electrons also produces corrosive gas that can quickly degrade the
bialkali thin-film photocathodes, made of antimony, potassium and cesium.



========================================================================== Cesium is the ideal material for accelerators because it has a low work function. Work function is the amount of energy needed to remove an
electron from the material and place it in a vacuum, a necessary step in electron-beam production. That low work function comes at a cost, though,
in the form of increased damage from chemical reactions and sensitivity
to ion back- bombardment. Thin film photocathode lifetimes are limited
even in ultrahigh vacuum states.

Graphene provides promising results Researchers sought a material that
could protect the photocathode while also allowing electrons to be
emitted. They found their answer in graphene.

"As far as I know, there is no other material which can both transmit
electrons and at the same time protect the material," said Yamaguchi. "A
very porous material will allow electrons to transmit, but then you
can't protect the material from corrosive gas. The uniqueness of
graphene is that it's atomically thin enough to transmit electrons,
but the atomic structure is also packed just enough so that no corrosive
gas can permeate it." Coating the bialkali photocathodes presented an ambitious technical challenge.

Distributed on the photocathode in a layer just one atom thick, graphene possesses high gas impermeability, which protects the photocathode from
the damage of gases created by the photon-to-free-electron conversion. At
the same time, graphene's high quantum efficiency (the measure of how
well a material converts photons to electrons) means that electrons can
still pass through the coating -- essential for creating and accelerating
the electron beam for research. Researchers found that the transmission efficiency of the photoelectrons was 5%, which in theory has room
to improve up to approximately 50%, a promising rate that indicates
the material is protected while still allowing an electron beam to
be produced.

"These results demonstrate important progress toward fully encapsulated bialkali photocathodes having both high QEs and long lifetimes using
atomically thin protection layers," said Yamaguchi.

The photocathode coating builds on "Atomic Armor" technology, which was selected for the R&D 100 in 2019. Previous research with the graphene technology has explored its usefulness as a corrosion barrier, potentially applied to cars, ships, aircraft and other goods.

========================================================================== Story Source: Materials provided by
DOE/Los_Alamos_National_Laboratory. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Fangze Liu, Lei Guo, Jeffrey DeFazio, Vitaly Pavlenko, Masahiro
Yamamoto,
Nathan A. Moody, Hisato Yamaguchi. Photoemission from Bialkali
Photocathodes through an Atomically Thin Protection Layer. ACS
Applied Materials & Interfaces, 2021; 14 (1): 1710 DOI:
10.1021/acsami.1c19393 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220125151001.htm

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