Graphene Intermediate\nLayer for Robust and Spectrum-Extended\nCu Photocathode Activated with Cs and O

Developing an effective method to stably enhance the\nquantum efficiency\n(QE) and extend the photoemission threshold of Cu photocathodes beyond\nthe ultraviolet region could benefit the photoinjector for ultrafast\nelectron source applications. The implementation of a 2D material\nprotective layer is considered a promising approach to extending the\noperating lifetime of photocathodes. We propose that graphene can\nserve as an intermediate layer at the interface between photocathode\nmaterial and low-work-function coating. The role of oxygen in the\nCs/O activation process on the Cu surface is altered by the graphene\ninterlayer. Besides, the few-layer graphene (FLG) surface could be\nmore likely to induce the formation of Cs<sub>2</sub>O. Thus, the\ngraphene–Cu composite photocathode can achieve an ultralow\nsurface work function of down to 0.878 eV through Cs/O activation.\nThe photoemission performance of the composite cathode with a FLG\ninterlayer is significantly enhanced. The photocathode has an extended\nspectral response to the near-infrared region and a higher QE. At\n350 nm, its QE is more than twice that of the cesiated bare Cu, reaching\n0.247%. After degradation, the graphene–Cu cathode can be fully\nrestored by reactivation, with remarkably enhanced stability. In addition,\nthe composite cathode can be operated reliably under a poor vacuum\npressure of over 4 × 10<sup>–6</sup> Pa. This study validates\na new method for incorporating 2D materials into photocathodes, offering\nnovel approaches to explore robust and spectrum-extended photocathodes.