Protecting high quantum efficiency photocathodes with two dimensional materials for long lifetimes

Since soon after the discovery of the photoelectric effect, the goal of reaching highly efficient photoelectron emission has been a difficult and well sought out goal. Currently, applications such as XFEL and XERL, call for high intensity electron sources that can lead to bright and intense electron and x-ray beams. The intensity of the photoemitted beam, the quantum efficiency of the photocathode, is dominated by the absorption cross section, electron mobility, and surface work function. For practical reasons, the lifetime of the photocathode is important and long-lived photocathodes are very much sought to improve performance. Whereas alkali-based semiconducting photocathodes display much higher QE relative to metal surfaces, their chemical composition makes them very reactive to residual gases even at UHV conditions. We will discuss here approaches for neutralizing such reactivity on the surface not introducing higher barriers to the emitted electrons. A computational study will be presented for screening 2-dimensional coating materials and fundamental principles for the characteristics needed are extracted. We present not only design principles but also candidate materials that should be used for protecting these highly reactive surfaces