Galdi Alice, DeBenedetti William J I, Balajka Jan, Cultrera Luca, Bazarov Ivan V, Maxson Jared M, Hines Melissa A
Cornell Laboratory for Accelerator-Based Sciences and Education and Department of Physics, Cornell University, Ithaca New York 14853, USA.
Department of Chemistry and Chemical Biology, Cornell University, Ithaca New York 14853, USA.
J Chem Phys. 2020 Oct 14;153(14):144705. doi: 10.1063/5.0024020.
High-performance photocathodes for many prominent particle accelerator applications, such as x-ray free-electron lasers, cannot be grown in situ. These highly reactive materials must be grown and then transported to the electron gun in an ultrahigh-vacuum (UHV) suitcase, during which time monolayer-level oxidation is unavoidable. Thin film CsSb photocathodes were grown on a variety of substrates. Their performance and chemical state were measured by x-ray photoelectron spectroscopy after transport in a UHV suitcase as well as after O-induced oxidation. The unusual chemistry of cesium oxides enabled trace amounts of oxygen to drive structural reorganization at the photocathode surface. This reorganization pulled cesium from the bulk photocathode, leading to the development of a structurally complex and O-exposure-dependent cesium oxide layer. This oxidation-induced phase segregation led to downward band bending of at least 0.36 eV as measured from shifts in the Cs 3d binding energy. At low O exposures, the surface developed a low work function cesium suboxide overlayer that had little effect on quantum efficiency (QE). At somewhat higher O exposures, the overlayer transformed to CsO; no antimony or antimony oxides were observed in the near-surface region. The development of this overlayer was accompanied by a 1000-fold decrease in QE, which effectively destroyed the photocathode via the formation of a tunnel barrier. The O exposures necessary for degradation were quantified. As little as 100 L of O irreversibly damaged the photocathode. These observations are discussed in the context of the rich chemistry of alkali oxides, along with potential material strategies for photocathode improvement.
许多重要的粒子加速器应用，如X射线自由电子激光器，所使用的高性能光电阴极无法原位生长。这些高活性材料必须先生长，然后在超高真空（UHV）箱中运输到电子枪，在此期间，单层水平的氧化是不可避免的。在各种衬底上生长了薄膜CsSb光电阴极。在超高真空箱中运输后以及经氧诱导氧化后，通过X射线光电子能谱测量了它们的性能和化学状态。氧化铯的特殊化学性质使得痕量的氧能够驱动光电阴极表面的结构重组。这种重组将铯从体相光电阴极中拉出，导致形成结构复杂且依赖于氧暴露的氧化铯层。这种氧化诱导的相分离导致从Cs 3d结合能的变化测量出至少0.36 eV的向下能带弯曲。在低氧暴露下，表面形成了低功函数的一氧化铯覆盖层，对量子效率（QE）影响很小。在稍高的氧暴露下，覆盖层转变为CsO；在近表面区域未观察到锑或氧化锑。这种覆盖层的形成伴随着量子效率下降1000倍，这通过形成隧道势垒有效地破坏了光电阴极。量化了降解所需的氧暴露量。低至100 L的氧就会不可逆地损坏光电阴极。结合碱金属氧化物丰富的化学性质对这些观察结果进行了讨论，同时还探讨了改善光电阴极的潜在材料策略。
