First-Principles Strain Analysis of Atomically-Thin Electrene Materials

Abstract

Electrides are ionically bonded materials that have delocalized electrons serving as anions. When electrides have a layered crystal form, the electrons are confined within the 2D gaps between the atomic layers and operate as high-mobility, low-scattering charge carriers with remarkably low work functions. The atomically-thin form of layered electrides, called electrenes, retain these 2D electron gas-like states at the surface and interlayer regions. In this work, density functional theory is used to explore if the conduction prop- erties and work functions of monolayer and bilayer electrenes can be improved using biaxial strain loading, up to ±5%. The electron transport characteristics are indi- rectly examined by calculating the state density of electrons near the Fermi level, and the average distance of surface states from the atomic lattice, <z>, to approx- imate the electron-phonon coupling. The electrenes of interest are those from the alkaline earth sub-pnictogenide family of inorganic layered electrenes: Ca2N, Sr2N, Sr2P, Ba2N, Ba2P, Ba2As, and Ba2Sb. The manipulation of electronic states in electrenes is found to be highly variable, with some electrenes experiencing minimal changes, while others see a definitive in- crease in state density near the Fermi level, mostly originating from near the edge of the Brillouin zone. The biggest improvement is in monolayer Ba2N, where surface state density increases by 56% under 2% compressive strain. However, state density and <z> do not necessarily correlate, so low electron-phonon coupling of these states is not guaranteed. The bilayer electrenes show the largest <z> and the most variation with strain. Bilayer Ba2Sb is expected to have the lowest electron-phonon coupling with a <z> of 2.2 ˚A at 5% tensile strain. Work function sees a consistent reduction in all cases under tensile strain, with bilayer electrenes having lower values than their monolayer counterparts. The lowest work function value observed in bilayer Ba2Sb, which starts at 2.38 eV when unstrained, and drops to 2.24 eV under 5% tensile strain. These findings indicate that strain can be used to manipulate and enhance the electronic transport properties of electrenes.