Computational screening of two-dimensional coatings for semiconducting photocathodes

Alkali-based semiconducting photocathodes, due to their high quantum efficiency (QE) in the visible light spectrum, are promising candidates to replace traditional metal photocathodes for high-brightness beam applications such as x-ray free electron lasers (XFELs). However, they suffer from rapid degradation which significantly limits their operational lifetimes. Coating them with two-dimensional (2D) materials has been proposed as a possible avenue to prevent the degradation. Ideally, the 2D coating layer should not increase the work function of semiconducting photocathodes, thus maintaining the high QE of semiconducting photocathodes in visible light. Herein, we report a computational screening of over 4000 2D materials in the Computational 2D Materials Database (C2DB). The assessment of their potential to be good coating layers is based on their effects to the surface electronic properties. We discover several candidate materials that are even capable of decreasing the work function of semiconducting photocathodes. Some of the experimentally synthesized 2D materials, such as hydrogenated graphene (graphane) and several hydroxylated transition metal carbides/nitrides (MXenes), are particularly appealing for this application.

Figure 1

(a) Schematic illustration of passivating photocathodes with a 2D coating layer, (b) band alignment of semiconducting photocathodes and an ideal semiconducting or metallic coating material, and (c) the computaitonal screening strategy in the C2DB. CB (VB) respresents the conduction band (valence band), and $E_{\mathrm{F}}$ is the Fermi level.

Figure 2

Energy levels of semiconducting photocathodes screened from the C2DB. The stars are the energy levels calculated from the PBE functional and bars are from the hybrid functional HSE06.

Figure 3

Energy levels of metallic photocathodes screened from the C2DB.

Figure 4

Representative 2D coating materials for semiconducting photocathodes.

Figure 5

Top views (a) and side views (b) of uncoated $\mathrm{C}{\mathrm{s}}_3\mathrm{Sb}$ (111) surface and $\mathrm{SiC}{\mathrm{H}}_2$ (Si-closer configuration), $\mathrm{M}{\mathrm{n}}_2\mathrm{N}{\mathrm{O}}_2{\mathrm{H}}_2$ coated $\mathrm{C}{\mathrm{s}}_3\mathrm{Sb}$ (111) surfaces, respectively.

Figure 6

Planar averaged electrostatic potentials (a,c) and electronic band structures (b,d) of $\mathrm{SiC}{\mathrm{H}}_2$ and $\mathrm{M}{\mathrm{n}}_2\mathrm{N}{\mathrm{O}}_2{\mathrm{H}}_2$ coated $\mathrm{C}{\mathrm{s}}_3\mathrm{Sb}$ (111) surface.