Active bialkali photocathodes on free-standing graphene substrates

• Published in: NPJ 2D materials and applications Vol. 1; no. 1; pp. 1 - 9
• Main Authors: Yamaguchi, Hisato, Liu, Fangze, DeFazio, Jeffrey, Narvaez Villarrubia, Claudia W., Finkenstadt, Daniel, Shabaev, Andrew, Jensen, Kevin L., Pavlenko, Vitaly, Mehl, Michael, Lambrakos, Sam, Gupta, Gautam, Mohite, Aditya D., Moody, Nathan A.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.06.2017; Nature Publishing Group; Springer Nature; Nature Portfolio
• Subjects: 639/925/918/1053; 639/925/930/1032; Accelerators; Cathodes; Chemistry and Materials Science; design, synthesis and processing; Efficiency; Electron accelerators; Electron sources; Encapsulation; Graphene; material science; MATERIALS SCIENCE; mechanical and structural properties and devices; Nanotechnology; Performance measurement; Permeability; Photocathodes; Photoelectric emission; Quantum efficiency; Resonant tunneling; Single crystals; Spectral sensitivity; Substrates; Surfaces and Interfaces; Thin Films; Work functions
• ISSN: 2397-7132; 2397-7132
• DOI: 10.1038/s41699-017-0014-6

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. Graphene in accelerator technology: A new material for enhanced photocathode performance and lifetime Graphene has shown potential to unlock new capabilities of electron sources and other aspects of accelerator technology. This report focuses on integrating graphene with high performance photocathodes with the goal of extending lifetime by thousands of hours. Scientists at Los Alamos National Laboratory, USA, and colleagues succeeded in growth of chemically susceptible photocathodes on free-standing graphene substrates while maintaining state-of-the-art performance. Successful growth on graphene is a critical step toward a material-centric approach to photocathode design: enhancing lifetime without compromising efficiency or other performance metrics. Graphene, an atomically thin sheet of carbon, is an emerging material that has inspired new cathode design capabilities, including heterostructuring, resonant tunneling, and impermeable gas barriers. Conventional photocathode materials have no performance regimes. The next step is complete graphene encapsulation of photocathode films and demonstration of lifetime enhancement in the operating environment of accelerator facilities.