  Alkali antimonide semiconductor photocathodes provide a promising platform
for the generation of high brightness electron beams, which are necessary for
the development of cutting-edge probes including x-ray free electron lasers and
ultrafast electron diffraction. However, to harness the intrinsic brightness
limits in these compounds, extrinsic degrading factors, including surface
roughness and contamination, must be overcome. By exploring the growth of CsxSb
thin films monitored by in situ electron diffraction, the conditions to
reproducibly synthesize atomically smooth films of CsSb on 3C-SiC (100) and
graphene coated TiO2 (110) substrates are identified, and detailed structural,
morphological, and electronic characterization is presented. These films
combine high quantum efficiency in the visible (up to 1.2% at 400 nm), an
easily accessible photoemission threshold of 550 nm, low surface roughness
(down to 600 pm on a 1 um scale), and a robustness against oxidation up to 15
times greater then Cs3Sb. These properties suggest that CsSb has the potential
to operate as an alternative to Cs$_3$Sb in electron source applications where
the demands of the vacuum environment might otherwise preclude the use of
traditional alkali antimonides.
