Active bialkali photocathodes on free-standing graphene substrates

Abstract

The hexagonal structure of graphene gives rise to the property of gas impermeability, motivating its investigation for a new application: protection of semiconductor photocathodes in electron accelerators. These materials are extremely susceptible to degradation in efficiency through multiple mechanisms related to contamination from the local imperfect vacuum environment of the host photoinjector. Few-layer graphene has been predicted to permit a modified photoemission response of protected photocathode surfaces, and recent experiments of single-layer graphene on copper have begun to confirm these predictions for single crystal metallic photocathodes. Unlike metallic photoemitters, the integration of an ultra-thin graphene barrier film with conventional semiconductor photocathode growth processes is not straightforward. A first step toward addressing this challenge is the growth and characterization of technologically relevant, high quantum efficiency bialkali photocathodes on ultra-thin freestanding graphene substrates. Photocathode growth on free-standing graphene provides the opportunity to integrate these two materials and study their interaction. Specifically, spectral response features and photoemission stability of cathodes grown on graphene substrates are compared to those deposited on established substrates. In addition, we observed an increase of work function for the graphene encapsulated bialkali photocathode surfaces, which is predicted by our calculations. The results provide a unique demonstration of bialkali photocathodes on free-standing substrates, and indicate promise towards our goal of fabricating high-performance graphene encapsulated photocathodes with enhanced lifetime for accelerator applications.

Quantum Efficiency Enhancement of Bialkali Photocathodes by an Atomically Thin Layer on Substrates

“…Optical profilometer measurements of the substrates indicated an average surface roughness of 230 nm, and grooves with an average maximum height of 1.4 μm and pitch of 15–50 μm. We previously demonstrated that free‐standing 2D crystals applied with these transfer techniques will naturally span large voids and still support K 2 CsSb photocathodes; thus, it is most probable that there are effective physical gaps between the photocathodes and the quite rough stainless steel substrates in this case as well.…”

“…The only instruments that are currently capable of generating the high brightness and coherent X‐ray beams required for atomic‐scale material investigations are electron accelerator facilities. An emergent problem, however, is that the performance requirements on the scientific frontier of these investigations dramatically outstrip the capabilities of present state‐of‐the‐art electron sources and cathode technologies . The high demand for increasingly high‐performance electron beams is such that the U.S. Department of Energy (DOE)‐commissioned studies have repeatedly identified electron sources as a critical risk area, forming one of the highest accelerator R&D priorities for the next decade…”

“…There are multiple approaches to this problem. One challenging avenue is to shield high quantum efficiency (QE) bialkali antimonide photocathodes with graphene to meet the required transformational advances in lifetime and efficiency simultaneously 5,6 . This type of approach is important in meeting an ultimate goal, however, the process could take many years and the outcome is not necessarily guaranteed.…”

Overcoming the quantum efficiency-lifetime tradeoff of photocathodes by coating with atomically thin two-dimensional nanomaterials

“…[27][28][29] Yamaguchi et al have successfully coated graphene onto semiconducting K 2 CsSb photocathode recently. 21 Figure 3 shows the calculated results of monolayer graphene on (001) surface of Cs 3 Sb. After structural relaxation, the equilibrium distance (d) between Cs 3 Sb and graphene is found to be 3.2 Å, and the binding energy of the coating graphene layer is −24.3 meV/ Å 2 ( Figure S3 in the Supplementary Materials).…”

“…21,22 It was reported that graphene was an excellent coating layer that increased the stability and QE of metallic Cu photocathode, while graphene was less effective for semiconducting K 2 CsSb photocathode because of the increase of the work function, resulting in a significant reduction of QE. 21 Despite these initial efforts on graphene coated photocathodes, no experimental and theoretical work has been undertaken to explore the possibility of using other 2D materials as coating layers. Herein, we present a theoretical study on 2D materials coated semiconducting photocathodes employing ab initio density functional calculations.…”