Graphene as reusable substrate for thin film photocathodes

Photocathodes that are capable of generating high-performance electron beams are one of the most important devices in an advanced accelerator and electron microscopes. In particular, bialkali photocathodes, such as cesium potassium antimonide (CsK₂Sb) is of interest because it can generate a high-brightness electron beam using a high-power green laser. It is known that the quantum efficiency (QE) of these photocathodes is affected severely by their substrates; however, reusability of the substrates is not well known. In this study, we use graphene, silicon (Si), and molybdenum (Mo) substrates to evaluate the effects of substrates on the QE of redeposited CsK₂Sb photocathodes after thermal cleanings. We found the QE of CsK₂Sb photocathodes redeposited on a graphene substrate after a thermal cleaning at 500degC remained largely unchanged. On the other hand, the QE of redeposited photocathodes on Si and Mo substrates after thermal cleaning at the same temperature decreased drastically. We used X-ray photoelectron spectroscopy (XPS) to quantitatively evaluate the residues of photocathodes after thermal cleaning at 400degC and 500degC. We found that the Sb, K, and Cs are removed by thermal cleaning at 500degC for the graphene substrate, but all or the majority of these elements remained on the Si and Mo substrates. The results were consistent with our density functional theory (DFT) calculations for the case of Si, which we investigated. Furthermore, our angle-resolved photoemission spectroscopy (ARPES) on graphene indicated that its intrinsic electronic structure is preserved after photocathode deposition and thermal cleaning at 500degC. Hence, we attributed the difference in amount of photocathode residue to the unique dangling-bond-free surface of inert graphene. Our results provide a foundation for graphene-based reusable substrates for high-QE semiconductor photocathodes. (author)