  We report successful deposition of high quantum efficiency (QE) bialkali
antimonide K2CsSb photocathodes on graphene films. The results pave a pathway
towards an ultimate goal of encapsulating technologically-relevant
photocathodes for accelerator technology with an atomically-thin protecting
layer to enhance lifetime while minimizing QE losses. A QE of 17 % at ~3.1 eV
(405 nm) is the highest value reported so far on graphene substrates and is
comparable to that obtained on stainless steel and nickel reference substrates.
The spectral responses of the photocathodes on graphene exhibit signature
features of K2CsSb including the characteristic absorption at ~2.5 eV.
Materials characterization based on X-ray fluorescence (XRF) and X-ray
diffraction (XRD) reveals that the composition and crystal quality of these
photocathodes deposited on graphene is comparable to those deposited on a
reference substrate. Quantitative agreement between optical calculations and QE
measurements for the K2CsSb on free suspended graphene and a graphene coated
metal substrate further confirms the high quality interface between the
photocathodes and graphene. Finally, a correlation between the QE and graphene
quality as characterized by Raman spectroscopy suggests that a lower density of
atomistic defects in the graphene films leads to higher QE of the deposited
K2CsSb photocathodes.
