Optoelectronic properties of confined water in angstrom-scale slits

The optoelectronic properties of confined water form one of the most active research areas in the past few years. Here we present the multiscale methodology to discern the out-of-plane electronic and dipolar dielectric constants (${\ensuremath{\varepsilon}}_{\ensuremath{\perp}}^{\text{el}}$ and ${\ensuremath{\varepsilon}}_{\ensuremath{\perp}}^{\text{dip}}$) of strongly confined water. We reveal that ${\ensuremath{\varepsilon}}_{\ensuremath{\perp}}^{\text{el}}$ and ${\ensuremath{\varepsilon}}_{\ensuremath{\perp}}^{\text{dip}}$ become comparable for water confined in angstrom-scale channels (with a height of less than $15\phantom{\rule{0.28em}{0ex}}\AA{}$) within graphene (GE) and hexagonal boron nitride (hBN) bilayers. Channel height ($h$) associated with a minimum in both ${\ensuremath{\varepsilon}}_{\ensuremath{\perp}}^{\text{el}}$ and ${\ensuremath{\varepsilon}}_{\ensuremath{\perp}}^{\text{dip}}$ is linked to the formation of the ordered structure of ice for $h\ensuremath{\approx}(7\text{--}7.5)\phantom{\rule{0.28em}{0ex}}\AA{}$. The recently measured total dielectric constant ${\ensuremath{\varepsilon}}_{\ensuremath{\perp}}^T$ of nanoconfined water [L. Fumagalli et al., Science 360, 1339 (2018)] is corroborated by our results. Furthermore, we evaluate the contribution from the encapsulating membranes to the dielectric properties, as a function of the interlayer spacing, i.e., the height of the confining channel for water. Finally, we conduct analysis of the optical properties of both confined water and GE membranes, and show that the electron energy loss function of confined water strongly differs from that of bulk water.