(Invited, Digital Presentation) Towards the Use of 2D Materials As Unique Protection Layer for Accelerator Beam Source
Authors: Yamaguchi Hisato 
Published: July 2022

Many grand challenges for humanity, including 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 require the ability to access, observe, and control matter on the frontier timescale of electronic motion and the spatial scale of atomic bonds. The only instruments with such capability are future coherent x-ray sources and advanced colliders, which demand increasingly high-performance electron beams. The performance requirements dramatically outstrip the capabilities of present state-of-the-art electron sources and cathode technologies. The need for breakthrough advances in electron sources has come into sharp focus; recent studies commissioned by the U.S. Department of Energy (DOE) have repeatedly identified electron sources as critical risk area, one of the highest accelerator R&D priorities for the next decade, requiring transformational advance of cold cathode performance in particular.

To address this issue, we propose to build and demonstrate a novel photocathode system that make parallel, transformational advances in the two critical performance areas of an electron source: lifetime and efficiency. Traditional approaches have failed because advances in one of these parameters have come at the expense of the other parameter. In our approach, we coat state-of-the-art high quantum efficiency (QE) bialkali photocathodes with atomically thin 2D materials as protecting layers to enhance their lifetime. Bialkali photocathodes exhibit one of the highest QE achievable, however, they are chemically susceptible and require an operation pressure of 10-10 Torr or lower. 2D materials, on the other hand, is the only material class that is capable of transmitting electrons while protecting material surfaces from undesired gases. Successful outcome of our efforts will decouple the competing mechanisms so that both high efficiency and long
lifetime, addressing a critical national need and technology gap which materially impacts the future of discovery science. Additionally, our effort enables scalability of electron sources that would likely lead to widespread advances in adjacent fields and applications.

We are working as a U.S.-Japan collaboration team to address this challenging and important problem. The U.S. team provides the expertise of atomically thin 2D materials and interfacing them with bialkali photocathodes on both experiments and theory. The Japanese team provides its expertise of high QE bialkali photocathode deposition and demonstration of photocathode performance as photoinjectors under accelerator operation environments. By combining the U.S. and Japanese teams that possess world’s leading expertise in accelerator related technologies, we are in unique position to establish research efforts that make transformational breakthroughs in the photocathode development.

Regarding our achievements so far, our U.S. patent on the concept was granted in 2014. In 2019, our protection coating technology won R&D 100 Award (IT/Electronics category) as well as being selected as gold medalist in R&D Special Recognition: Market Disruptor - Products category (Figure 1(Left)). We initially demonstrated our concept on metal photocathodes in 2017. On copper cathodes, we showed that there is no degradation in QE even with graphene coating, and the operation pressure improved by 8 orders of magnitudes. 10-6 Torr or lower was required for operation of bare non-coated copper cathodes but graphene-coated copper cathodes operated even at 200 Torr. In 2017, we also demonstrated a good material compatibility between graphene and bialkali photocathodes. We improved bialkali photocathode quality on graphene in 2018 and achieved QE of as high as ~17 % (Figure 1(Middle)). Recently, we achieved QE from bialkali photocathodes through graphene protection layer, which is a milestone towards our ultimate goal of protecting bialkali photocathodes with 2D materials.

In my talk, I will also present our future plans along with some of the unexpected results that we observed (Figure 1(Right)).
Figure 1 (Left) R&D 100 Award won in 2019 (Middle) QE of ~17 % achieved from bialkali photocathodes on graphene (Right) Unexpected QE enhancement of bialkali photocathodes when metal substrates are coated with graphene

Figure 1