Electronic structure and optical properties of Na₂KSb and NaK₂Sb from first-principles many-body theory

Abstract

In the search for novel materials for vacuum electron sources, multi-alkali antimonides and in particular sodium-potassium-antimonides have been recently regarded as especially promising due to their favorable electronic and optical properties. In the framework of density-functional theory and many-body perturbation theory, we investigate the electronic structure and the dielectric response of two representative members of this family, namely Na2KSb and NaK2Sb. We find that both materials have a direct gap, which is on the order of 1.5 eV in Na2KSb and 1.0 eV in NaK2Sb. In either system, valence and conduction bands are dominated by Sb states with p- and s-character, respectively. The imaginary part of the dielectric function, computed upon explicit inclusion of electron–hole interactions to characterize the optical response of the materials, exhibits maxima starting from the near-infrared region, extending up to the visible and the ultraviolet band. With our analysis, we clarify that the lowest-energy excitations are non-excitonic in nature and that their binding energy is on the order of 100 meV. Our results confirm the potential of Na2KSb and NaK2Sb as photoemissive materials for vacuum electron sources, photomultipliers, and imaging devices.

Ab Initio Quantum-Mechanical Predictions of Semiconducting Photocathode Materials

“…These values indicate that screening in these materials is not very effective and even in the BSE spectra, the absorption peaks at the onset retain their character of band-to-band transitions [65]. A similar behavior was found also in the optical spectra of Na-based potassium antimonides [123]. Hence, in these materials, excitonic effects manifest themselves only as a redistribution of spectral weight to lower energies.…”

“…The high-throughput analysis presented above shows the potential of this novel computational approach to explore the vast configurational space of photocathode materials. The promising results predicted by earlier theoretical works [58,62,117,119,121,123] suggest favorable properties also for alkali antimonides with lighter species than Cs. Including both binary and ternary phases dramatically enhances the amount of candidate systems, making high-throughput calculations the most efficient and feasible way to explore their properties.…”

“…As shown in Figure 4 for DFT calculations with the SCAN functional, in all three materials, SOC leads to a splitting of the valence bands, which are dominated by Sb p-states [65,70]. Notice that the character of these bands and the corresponding spin-orbit splittings (see below) do not change significantly when Cs is replaced by Na [123]. The SOC-induced splittings up-shift the energy of the VBM by a few hundred meV (see Figure 4).…”

Novel Ultrabright and Air‐Stable Photocathodes Discovered from Machine Learning and Density Functional Theory Driven Screening

“…[10] Similarly, performing detailed ab initio studies of photoemission properties is computationally expensive, even for tens of photocathode materials. [11][12][13] With the recent development of massive materials databases, computationally screening for materials with desirable properties is a powerful new tool for accelerating materials discovery. [14][15][16] In this work, we present two large-scale and comprehensive computational screenings for novel high brightness photocathode materials.…”

Exploring the Cs-Te phase space via high-throughput density-functional theory calculations beyond the generalized-gradient approximation

“…50 DFT can greatly help in this regard, providing unrivaled insight into the structure-property relations of photocathode materials, as demonstrated for Cs 2 Te itself 33,51 as well for (multi)alkali antimonides. 7,[52][53][54][55][56][57][58] A. Initial dataset As a starting point for our analysis, we consider all materials exclusively containing Cs and Te elements stored in the Materials Project (MP) database 12 and in the Open Quantum Materials Database (OQMD).…”