Abstract
The development of novel photocathode materials for ultra-bright electron sources demands efficient and cost-effective strategies that provide insight and understanding of the intrinsic material properties given the constraints of growth and operational conditions. To address this question, we propose a viable way to establish correlations between calculated and measured data on core electronic states of Cs-K-Sb materials. To do so, we combine first-principles calculations based on all-electron density-functional theory on the three alkali antimonides Cs3Sb, Cs2KSb, and CsK2Sb with x-ray photoemission spectroscopy (XPS) on Cs-K-Sb photocathode samples. Within the GW approximation of many-body perturbation theory, we obtain quantitative predictions of the band gaps of these materials, which range from 0.57 eV in Cs2KSb to 1.62 eV in CsK2Sb and manifest direct or indirect character depending on the relative potassium content. Our theoretical electronic-structure analysis also reveals that the core states of these systems have binding energies that depend only on the atomic species and their crystallographic sites, with largest shifts of the order of 2 eV and 0.5 eV associated to K 2p and Sb 3d states, respectively. This information can be correlated to the maxima in the XPS survey spectra, where such peaks are clearly visible. In this way, core-level shifts can be used as fingerprints to identify specific compositions of Cs-K-Sb materials and their relation with the measured values of quantum efficiency. Our results represent the first step towards establishing a robust connection between the experimental preparation and characterization of photocathodes, the ab initio prediction of their electronic structure, and the modeling of emission and beam formation processes.
Highlights
The details of the implementation of GW in an all-electron formalism are reported in refs. 38,39 Binding energies of core states are computed from DFT as the the energy of each level with www.nature.com/scientificreports opposite sign
In an XPS experiment, a sample is irradiated with X-ray photons of known energy, and the resulting photoelectrons liberated by this process are collected and their kinetic energy is recorded with an electron spectrometer
The binding energy is dependent on the element from which the electron was emitted, and the spectra provides information of the electronic structure of the sample material from which elemental and chemical composition and can be determined[45]
Summary
On the left-hand side of Eq (1), in addition to the kinetic-energy operator, we find the effective potential per particle vs(r), which consists of the sum of three terms: vs(r) = vext(r) + vH(r) + vxc(r). The GW approximation[34] in the single-shot perturbative approach G0W035 is adopted to estimate the quasi-particle correction to the valence and conduction states in the gap region. Where Zik is the renormalization factor accounting for the energy-dependence of the self-energy and εiKkS are the solutions of the Kohn-Sham equations for the given states.
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