Strain-tunable anionic states and work function in 2D electrenes: A DFT analysis

The anionic electrons in two-dimensional (2D) electrenes form loosely bound and delocalized 2D sheets located on the surfaces and in the interstitial gaps between the atomic layers. The spatial separation of the anionic electrons from the lattice results in weakened electron–phonon coupling and low work function. Using density functional theory, we examine how the surface and interlayer states in monolayer and bilayer alkaline earth sub-pnictogenides (Ca2N, Sr2N, Sr2P, Ba2N, Ba2P, Ba2As, and Ba2Sb) vary under tensile and compressive biaxial strain up to 5%. The Ba-based electrenes display strain-induced band shifts near the Fermi level, which occur close to the zone edge where the anionic states are more localized and exhibit greater overlap with the atoms. To assess how strain affects conduction, we analyze three key transport-related metrics: the surface and interlayer state densities, which relate to the number of conducting anionic electrons, and the average distance of the anionic states from the lattice, a proxy for electron–phonon coupling. Strain can increase the state density in most bilayers and some monolayers by as much as 18%, and modulate the electron–lattice separation by up to 10%. Work function decreases with tensile strain, with bilayer Ba2Sb exhibiting the lowest value of 2.2 eV at 5% strain. These findings indicate that strain engineering can enhance conduction by increasing the number of carriers and/or suppressing electron–phonon interactions, and can promote electron donation and emission in 2D electrenes.