Quantum efficiency enhancement of bialkali photocathodes by an atomically thin layer on substrates

TL;DR

This study demonstrates that placing atomically thin 2D crystal layers like graphene or hexagonal boron nitride between photocathodes and reflective substrates significantly enhances their quantum efficiency, especially at longer wavelengths.

Contribution

It introduces a novel method of QE enhancement using 2D crystal layers on reflective substrates, without altering the photocathode's physical properties.

Findings

QE increases by up to 80% at 633 nm with graphene coatings.

Enhancement occurs only on reflective substrates, not transparent ones.

Optical interactions, not structural changes, cause QE improvements.

Abstract

We report quantum efficiency (QE) enhancements in accelerator technology relevant antimonide photocathodes (K2CsSb) by interfacing them with atomically thin two-dimensional (2D) crystal layers. The enhancement occurs in a reflection mode, when a 2D crystal is placed in between the photocathodes and optically reflective substrates. Specifically, the peak QE at 405 nm (3.1 eV) increases by a relative 10 percent, while the long wavelength response at 633 nm (2.0 eV) increases by a relative 36 percent on average and up to 80 percent at localized hot spot regions when photocathodes are deposited onto graphene coated stainless steel. There is a similar effect for photocathodes deposited on hexagonal boron nitride monolayer coatings using nickel substrates. The enhancement does not occur when reflective substrates are replaced with optically transparent sapphire. Optical transmission, X-ray diffraction (XRD) and X-ray fluorescence (XRF) revealed that thickness, crystal orientation, quality and elemental stoichiometry of photocathodes do not appreciably change due to 2D crystal coatings. These results suggest optical interactions are responsible for the QE enhancements when 2D crystal sublayers are present on reflective substrates, and provide a pathway toward a simple method of QE enhancement in semiconductor photocathodes by an atomically thin 2D crystal on substrates.