  The hexagonal structure of graphene gives rise to the property of gas
impermeability, motivating its investigation for a new application: protection
of semiconductor photocathodes in electron accelerators. These materials are
extremely susceptible to degradation in efficiency through multiple mechanisms
related to contamination from the local imperfect vacuum environment of the
host photoinjector. Few-layer graphene has been predicted to permit a modified
photoemission response of protected photocathode surfaces, and recent
experiments of single-layer graphene on copper have begun to confirm these
predictions for single crystal metallic photocathodes. Unlike metallic
photoemitters, the integration of an ultra-thin graphene barrier film with
conventional semiconductor photocathode growth processes is not
straightforward. A first step toward addressing this challenge is the growth
and characterization of technologically relevant, high quantum efficiency
bialkali photocathodes grown on ultra-thin free-standing graphene substrates.
Photocathode growth on free-standing graphene provides the opportunity to
integrate these two materials and study their interaction. Specifically,
spectral response features and photoemission stability of cathodes grown on
graphene substrates are compared to those deposited on established substrates.
In addition we observed an increase of work function for the graphene
encapsulated bialkali photocathode surfaces, which is predicted by our
calculations. The results provide a unique demonstration of bialkali
photocathodes on free-standing substrates, and indicate promise towards our
goal of fabricating high-performance graphene encapsulated photocathodes with
enhanced lifetime for accelerator applications.
