  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.

Materials Science