Polarizability, susceptibility, and dielectric constant of nanometer-scale molecular films: A microscopic view

The size-dependence of the polarizability, susceptibility, and dielectric constant of nanometer-scale molecular layers is explored theoretically. First-principles calculations based on density functional theory are compared to phenomenological modeling based on polarizable dipolar arrays for a model system of organized monolayers composed of oligophenyl chains. Size trends for all three quantities are primarily governed by a competition between out-of-plane polarization enhancement and in-plane polarization suppression. Molecular packing density is the single most important factor controlling this competition and it strongly affects the bulk limit of the dielectric constant as well as the rate at which it is approached. Finally, the polarization does not reach its "bulk" limit, as determined from the Clausius-Mossotti model, but the susceptibility and dielectric constant do converge to the correct bulk limit. However, whereas the Clausius-Mossotti model describes the dielectric constant well at low lateral densities, finite size effects of the monomer units cause it to be increasingly inaccurate at high lateral densities.