Photoemission from Bialkali Photocathodes Through an Atomically Thin Protection Layer

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 in visible spectral range, however, their lifetime can be limited even in ultra-high vacuum due to their high reactivity to residual gases and sensitivity to ion back-bombardment 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 2-layer (2L) suspended graphene substrate with a peak quantum efficiency 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 towards fully encapsulated bialkali photocathodes having both high quantum efficiencies and long lifetimes using atomically thin protection layers.