Abstract
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 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.
Highlights
The evolving needs of advanced electron accelerator-based X-ray light sources, such as energy recovery linacs and free electron lasers have motivated continual development in photocathode (PC) materials with a particular emphasis on high brightness and long lifetime.[1,2,3,4,5,6] Semiconductor materials, those of the alkali-antimonide family are of keen interest as their relatively high quantum efficiencies, and low thermal emittances can yield a high brightness beam
Cu foils with size of 25 × 75 mm were placed in a quartz tube furnace and pre-cleaned under hydrogen gas flow at 1000 °C for 30 min
Emission from the PC-side of graphene shows no difference in quantum efficiency (QE) and work function between the 5 and 8L cases, suggesting that for (Fig. 2)
Summary
The evolving needs of advanced electron accelerator-based X-ray light sources, such as energy recovery linacs and free electron lasers have motivated continual development in photocathode (PC) materials with a particular emphasis on high brightness and long lifetime.[1,2,3,4,5,6] Semiconductor materials, those of the alkali-antimonide family are of keen interest as their relatively high quantum efficiencies, and low thermal emittances can yield a high brightness beam. After being cooled down to room temperature under vacuum, graphene was transferred onto SiO2/Si substrates (using the aforementioned wet-transfer technique) for material characterization by Raman spectroscopy and AFM This trend of increased work function with increased layer thickness should be expected to continue for thicker graphene substrates.
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