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Protection of free-electron sources has been technically challenging due to lack of materials that transmit electrons while preventing corrosive gas molecules. Two-dimensional materials uniquely possess both of required properties. Here, we report three orders of magnitude increase in active pressure and factor of two enhancement in the lifetime of high quantum efficiency (QE) bialkali photocathodes (cesium potassium antimonide (CsK2Sb)) by encapsulating them in graphene and thin nickel (Ni) film. The photoelectrons were extracted through the graphene protection layer in a reflection mode, and we achieved QE of ~ 0.17% at ~ 3.4 eV, 1/e lifetime of 188 h with average current of 8.6 nA under continuous illumination, and no decrease of QE at the pressure of as high as ~ 1 × 10–3 Pa. In comparison, the QE decreased drastically at 10–6 Pa for bare, non-protected CsK2Sb photocathodes and their 1/e lifetime under continuous illumination was ~ 48 h. We attributed the improvements to the gas impermeability and photoelectron transparency of graphene. [ABSTRACT FROM AUTHOR]

The subheading "Photoemission measurement", now reads: "Spectral response photoemission measurements." Finally, in the Methods, under the subheading 'Sandwiching bialkali antimonide photocathodes between graphene and nickel thin films', "Graphene consisting of mainly monolayer regions with some multiple layer regions was synthesized via chemical vapor deposition (CVD) SP 43 sp on Cu substrates and transferred onto TEM mesh grids using an established polymer-supported method that involves distilled water exposure SP 44 sp . [Extracted from the article]