A Model for Emission Yield from Planar Photocathodes Based on Photon-Enhanced Thermionic Emission or Negative-Electron-Affinity Photoemission

A general model is presented for electron emission yield from planar photocathodes that accounts for arbitrary cathode thickness and finite recombination velocities at both front and back surfaces. This treatment is applicable to negative electron affinity emitters as well as positive electron affinity cathodes, which have been predicted to be useful for energy conversion. The emission model is based on a simple one-dimensional steady-state diffusion treatment. The resulting relation for electron yield is used to model emission from thin-film cathodes with material parameters similar to GaAs. Cathode thickness and recombination at the emissive surface are found to strongly affect emission yield from cathodes, yet the magnitude of the effect greatly depends upon the emission mechanism. A predictable optimal film thickness is found from a balance between optical absorption, surface recombination, and emission rate

Engineering Ultra-Low Work Function of Graphene

Low work function materials are critical for energy conversion and electron emission applications. Here, we demonstrate for the first time that an ultralow work function graphene is achieved by combining electrostatic gating with a Cs/O surface coating. A simple device is built from large-area monolayer graphene grown by chemical vapor deposition, transferred onto 20 nm HfO₂ on Si, enabling high electric fields capacitive charge accumulation in the graphene. We first observed over 0.7 eV work function change due to electrostatic gating as measured by scanning Kelvin probe force microscopy and confirmed by conductivity measurements. The deposition of Cs/O further reduced the work function, as measured by photoemission in an ultrahigh vacuum environment, which reaches nearly 1 eV, the lowest reported to date for a conductive, nondiamond material