Portlandite crystal: Bulk, bilayer, and monolayer structures

Abstract

$\text{Ca}{(\text{OH})}_{2}$ crystals, well known as portlandite, are grown in layered form, and we found that they can be exfoliated on different substrates. We performed first principles calculations to investigate the structural, electronic, vibrational, and mechanical properties of bulk, bilayer, and monolayer structures of this material. Different from other lamellar structures such as graphite and transition-metal dichalcogenides, intralayer bonding in $\text{Ca}{(\text{OH})}_{2}$ is mainly ionic, while the interlayer interaction remains a weak dispersion-type force. Unlike well-known transition-metal dichalcogenides that exhibit an indirect-to-direct band gap crossover when going from bulk to a single layer, $\text{Ca}{(\text{OH})}_{2}$ is a direct band gap semiconductor independent of the number layers. The in-plane Young's modulus and the in-plane shear modulus of monolayer $\text{Ca}{(\text{OH})}_{2}$ are predicted to be quite low while the in-plane Poisson ratio is larger in comparison to those in the monolayer of ionic crystal BN. We measured the Raman spectrum of bulk $\text{Ca}{(\text{OH})}_{2}$ and identified the high-frequency OH stretching mode ${A}_{1g}$ at $3620\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. In this study, bilayer and monolayer portlandite [$\text{Ca}{(\text{OH})}_{2}$] are predicted to be stable and their characteristics are analyzed in detail. Our results can guide further research on ultrathin hydroxites.