Electronic, Dielectric, and Plasmonic Properties of Two-Dimensional Electride Materials X$_2$N (X=Ca, Sr): A First-Principles Study

TL;DR

This study uses first-principles calculations to explore the electronic, dielectric, and plasmonic properties of 2D electride materials X$_2$N (X=Ca, Sr), revealing their stability, unique dielectric behavior, and potential for nanoscale plasmonic applications.

Contribution

First comprehensive first-principles analysis of 2D X$_2$N electrides' properties, highlighting their stability and plasmonic potential at nanoscale dimensions.

Findings

Ca$_2$N and Sr$_2$N are stable down to monolayers.

Distinct dielectric behavior with signs of dielectric tensor components.

Support for surface and tightly-bound plasmon modes in near-infrared range.

Abstract

Based on first-principles calculations, we systematically study the electronic, dielectric, and plasmonic properties of two-dimensional (2D) electride materials X$_2$N (X=Ca, Sr). We show that both Ca$_2$N and Sr$_2$N are stable down to monolayer thickness. For thicknesses larger than 1-monolayer (1-ML), there are 2D anionic electron layers confined in the regions between the [X$_2$N]$^+$ layers. These electron layers are strongly trapped and have weak coupling between each other. As a result, for the thickness dependence of many properties such as the surface energy, work function, and dielectric function, the most dramatic change occurs when going from 1-ML to 2-ML. For both bulk and few-layer Ca$_2$N and Sr$_2$N, the in-plane and out-of-plane real components of their dielectric functions have different signs in an extended frequency range covering the near infrared, indicating their potential applications as indefinite media. We find that bulk Ca$_2$N and Sr$_2$N could support surface plasmon modes in the near infrared range. Moreover, tightly-bounded plasmon modes could exist in their few-layer structures. These modes have significantly shorter wavelengths (~few tens of nanometers) compared with that of conventional noble metal materials, suggesting their great potential for plasmonic devices with much smaller dimensions.