Free‐Standing Bialkali Photocathodes Using Atomically Thin Substrates

Abstract

We report successful deposition of high quantum efficiency (QE) bialkali antimonide K2CsSb photocathodes on graphene films. The results pave a pathway towards an ultimate goal of encapsulating technologically-relevant photocathodes for accelerator technology with an atomically thin protecting layer to enhance lifetime while minimizing QE losses. A QE of 17 % at ~3.1 eV (405 nm) is the highest value reported so far on graphene substrates and is comparable to that obtained on stainless steel and nickel reference substrates. The spectral responses of the photocathodes on graphene exhibit signature features of K2CsSb including the characteristic absorption at ~2.5 eV. Materials characterization based on X-ray fluorescence (XRF) and X-ray diffraction (XRD) reveals that the composition and crystal quality of these photocathodes deposited on graphene is comparable to those deposited on a reference substrate.Quantitative agreement between optical calculations and QE measurements for the K2CsSb on free suspended graphene and a graphene coated metal substrate further confirms the high quality interface between the photocathodes and graphene. Finally, a correlation between the QE and graphene quality as characterized by Raman spectroscopy suggests that a lower density of atomistic defects in the graphene films leads to higher QE of the deposited K2CsSb photocathodes.3

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

“…We previously demonstrated that K 2 CsSb photocathodes grown on graphene‐coated substrates exhibit X‐ray diffraction (XRD) and X‐ray fluorescence (XRF) spectra that are consistent with those obtained when using uncoated silicon substrates . These results indicate that both the crystal quality and elemental stoichiometry of K 2 CsSb photocathodes do not appreciably change due to graphene coating.…”

“…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.…”

Optical Near-Field Electron Microscopy

“…The sample scatters light elastically, leading to nanometric features in the near field. This spatial light distribution is then converted, still in the near field, into a spatially varying electron flux via the photoelectric effect within a thin layer of low-workfunction photocathode (PC) material [27,28]. The emitted photoelectrons are then imaged using aberration-corrected low-energy electron optics [29,30].…”

“…The proposed technique would be intrinsically damage free. First, low-work-function photocathodes can be efficiently excited with green light [27] and thus at photon energies significantly below the absorption band of most proteins. The inset in Fig.…”

Perspectives on Designer Photocathodes for X-ray Free-Electron Lasers: Influencing Emission Properties with Heterostructures and Nanoengineered Electronic States

“…The first examples of successful integration of pristine few-layer graphene coatings with photoemission cathodes has occurred 110 in the form of atomically clean bulk copper (Gr : Cu) applied to graphene photocathodes 111 and the growth of high QE K 2 CsSb on suspended transparent graphene substrates 65,112 . In the copper study, the spectral response of the Gr : Cu system was measured as a function of the copper crystal face and thickness of graphene.…”