Quantum Efficiency Enhancement of Bialkali Photocathodes by an Atomically Thin Layer on Substrates

Goto

Yamaguchi

et al. 2023

J. Phys.: Conf. Ser.

A high-performance photocathode is required to advanced accelerators and electron microscopes. In particular, the CsK2Sb photocathode is of interest because it has features such as low emittance, excitability with visible light, and high quantum efficiency (QE). Generally, the CsK2Sb photocathode is produced by depositing a cathode element on a substrate, so that the cathode performance strongly depends on the surface condition of the substrate. We have found graphene as reusable substrate, which has the property of being chemically inactive. In this study, graphene film quality dependence of CsK2Sb photocathode performance was evaluated. Specifically, CsK2Sb photocathode was deposited using different quality graphene film substrates and their QE values and uniformity were compared. The quality of graphene films was analyzed using X-ray Photoelectron Spectroscopy (XPS) and X-ray Absorption Spectroscopy (XAS). We found that the graphene film can be cleaned by heating at 500 deg. The QE of the cathode on a good quality graphene film was higher and more uniform than that on a poor quality graphene film.

Section: Qe Measurementsupporting

confidence: 84%

Section: Introductionmentioning

confidence: 99%

Dependence of CsK₂Sb photocathode performance on the quality of graphene substrate film

Goto

Yamaguchi

et al. 2023

J. Phys.: Conf. Ser.

“…Our previous studies have successfully demonstrated the compatibility of graphene and alkali antimonide (K 2 CsSb) photocathodes by directly depositing high-quality K 2 CsSb on both supported and suspended few-layer graphene. − The K 2 CsSb deposited on graphene had good crystallinity, close to theoretical stoichiometry, and high QEs that were comparable to those deposited on conventional thick metal substrates, with the peak QE exceeding 14% at 3.1 eV on the graphene substrate. However, no photoemission through graphene was observed due to the possible polymer residue from the graphene transfer process and the limited instrument sensitivity.…”

Section: Introductionmentioning

confidence: 99%

Photoemission from Bialkali Photocathodes through an Atomically Thin Protection Layer

Liu

ACS Appl. Mater. Interfaces

DeFazio³

et al. 2021

Self Cite

Photocathodes are essential components for various applications requiring photon-to-free-electron conversion, for example, high-sensitivity photodetectors and electron injectors for free-electron lasers. Alkali antimonide thin films are widely used as photocathode materials owing to their high quantum efficiency (QE) in the visible spectral range; however, their lifetime can be limited even in ultrahigh vacuum due to their high reactivity to residual gases and sensitivity to ion backbombardment in these applications. An ambitious technical challenge is to extend the lifetime of bialkali photocathodes by coating them with suitable materials that can isolate the photocathode films from residual gases while still maintaining their highly emissive properties. We propose the use of graphene, an atomically thin two-dimensional material with gas impermeability, as a promising candidate for this purpose. Here, we report that high-quality bialkali antimonide can be grown on a two-layer (2L) suspended graphene substrate with a peak QE of 15%. More importantly, by comparing the photoemission through varying layers of graphene, we demonstrate that photoelectrons can transmit through few-layer graphene with a maximum QE of over 0.7% at 4.5 eV for 2L graphene, corresponding to a transmission efficiency of 5%. These results demonstrate important progress toward fully encapsulated bialkali photocathodes having both high QEs and long lifetimes using atomically thin protection layers.

“…Our strategy to achieve this was to deposit thin metal films onto CsK 2 Sb photocathodes via thermal deposition from the opposite side of graphene protection layer. We had already established a protocol to fabricate CsK 2 Sb photocathodes on graphene protection layer [30][31][32][33][34] thus the remaining task was to select an appropriate sealing metal for our purpose. It is well known that substrate material has severe effects on the QE of deposited ~ 20 nm think CsK 2 Sb photocathodes 8 , therefore it was crucial for us to identify a metal with minimal degradation.…”

Section: Resultsmentioning

confidence: 99%

“…Au was chosen for its chemical inertness, which could prevent photocathodes from interacting with corrosive gases such as oxygen and moisture. Ni was chosen for its potential material compatibility with CsK 2 Sb based on our prior studies [30][31][32][33][34] . We did not test molybdenum (Mo) despite its known material compatibility with CsK 2 Sb because of a concern on its high evaporation temperature.…”

Section: Resultsmentioning

confidence: 99%

Rugged bialkali photocathodes encapsulated with graphene and thin metal film

Liu

Koyama

et al. 2023

Sci Rep

Self Cite

Protection of free-electron sources has been technically challenging due to lack of materials that transmit electrons while preventing corrosive gas molecules. Two-dimensional materials uniquely possess both of required properties. Here, we report three orders of magnitude increase in active pressure and factor of two enhancement in the lifetime of high quantum efficiency (QE) bialkali photocathodes (cesium potassium antimonide (CsK2Sb)) by encapsulating them in graphene and thin nickel (Ni) film. The photoelectrons were extracted through the graphene protection layer in a reflection mode, and we achieved QE of ~ 0.17% at ~ 3.4 eV, 1/e lifetime of 188 h with average current of 8.6 nA under continuous illumination, and no decrease of QE at the pressure of as high as ~ 1 × 10–3 Pa. In comparison, the QE decreased drastically at 10–6 Pa for bare, non-protected CsK2Sb photocathodes and their 1/e lifetime under continuous illumination was ~ 48 h. We attributed the improvements to the gas impermeability and photoelectron transparency of graphene.