Abstract
Photocathodes are key components of electron injectors for X-ray free electron laser and X-ray energy recovery linacs, which generate brilliant, ultrafast, and coherent X-rays for the exploration of matter with ultrahigh resolutions in both space and time. Whereas alkali-based semiconducting photocathodes display a higher quantum efficiency (QE) in the visible light spectrum than their metallic counterparts, their lifetimes are much shorter due to the high reactivity of alkali-based surfaces to the residual gases in the vacuum chamber. Overcoming the tradeoff between QE and lifetimes has been a great challenge in the accelerator community. Herein, based on ab initio density functional calculations, we propose an approach to overcome this tradeoff by coating with atomically thin two-dimensional (2D) nanomaterials. On one hand, the 2D coating layers can enhance the lifetimes of photocathodes by preventing the chemical reactions with the residual gases. On the other hand, the 2D coating layers can effectively engineer the work function of photocathodes, thus controlling their QE. A monolayer of insulating BN reduces the work function, whereas a monolayer of semi-metallic graphene or semiconducting molybdenum disulfide (MoS2) increases the work function. This phenomenon originates from the induced interfacial dipoles. The reduction of work function by BN implies that it is capable of maintaining the high QE of semiconducting photocathodes in addition to enhance their lifetimes. This study advances our understandings on the surface chemistry of coated photocathodes and opens new technological avenues to fabricate photocathodes with high QE and longer lifetimes.
Highlights
X-ray free electron laser (XFEL) and X-ray energy recovery linacs (XERL) are facilities for generating brilliant, ultrafast, and coherent X-rays with angstrom wavelength, which allows the experimental exploration of the structure and dynamics of atomic and molecular systems with ultrahigh resolutions in a broad range of research fields in physics, chemistry, biology, material science, and high energy density physics
We focus on Cs3Sb which is one of the most experimentally characterized semiconducting photocathodes to elucidate the change of the work function and its effects on the quantum efficiency (QE) of photocathodes
Based on ab initio density functional calculations, we demonstrate that besides protecting the surfaces a few layers of BN reduces the work function of alkali-based photocathodes
Summary
X-ray free electron laser (XFEL) and X-ray energy recovery linacs (XERL) are facilities for generating brilliant, ultrafast, and coherent X-rays with angstrom wavelength, which allows the experimental exploration of the structure and dynamics of atomic and molecular systems with ultrahigh resolutions in a broad range of research fields in physics, chemistry, biology, material science, and high energy density physics. Photocathodes are key components in the production of electron beams for XFEL and XERL.[1] Generally, there are two families of photocathodes: metal and semiconductor.[2] Metallic photocathodes such as copper, are very robust to chemical degradation but they have low quantum efficiency (QE) at the level of 10−4–10−3%. The low QE is due to the large work function, significant electron–electron scattering and strong surface reflection of incident light.[2] The poor QE of metallic photocathodes limits their usefulness in the production of high brightness electron beams. Alkali based semiconducting photocathodes, for examples, cesium antimony (Cs3Sb), bialkali antimony (K2CsSb), and negative electron affinity photocathodes such as gallium nitride (GaN-Cs), have lower work function and high QE, at the level of 1–20% in the visible light spectrum.[2,3] The limitation of these materials is that they are extremely reactive and even in the ultra-high vacuum (UHV) conditions inside the accelerators, their surfaces get quickly damaged, manifesting as decay in QE and short operational lifetimes.[3,4,5,6,7] Overcoming this tradeoff between QE and lifetimes has been a great challenge and a high priority in the accelerator community
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