Photoemission from Bialkali Photocathodes through an Atomically Thin Protection Layer

Abstract

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.

Rugged bialkali photocathodes encapsulated with graphene and thin metal film

“…Successful CsK 2 Sb deposition on graphene is evident from a typical QE response that quickly increases around 2 eV and signature hump around ~ 2.5 eV. Moreover, the peak QE of ~ 13.6% at ~ 3.4 eV is consistent with our previous studies 30 – 34 .

Figure 3 ( a ) QE spectral response of CsK 2 Sb photocathode deposited on TEM-grid-supported bilayer graphene.

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“…Successful CsK 2 Sb deposition on graphene is evident from a typical QE response that quickly increases around 2 eV and signature hump around ~ 2.5 eV. Moreover, the peak QE of ~ 13.6% at ~ 3.4 eV is consistent with our previous studies [30][31][32][33][34] .…”

“…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.…”

“…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.…”

Quantum Systems for Enhanced High Energy Particle Physics Detectors

“…Ongoing promising studies demonstrate an increased operational lifetime by encapsulation of semiconductive [68,69] as well as metallic [70] (i.e., Cu) photocathodes with few layers of graphene (from two up to eight layers) while lowering the QE. The decreased QE is mainly attributed to issues during the transfer process and graphene quality and achieved experimental values are approximately one order of magnitude below theoretical predictions [68].…”