Chain Length Dependence of the Dielectric Constant and Polarizability in Conjugated Organic Thin Films

Dielectric materials are ubiquitous in optics, electronics and materials science. Recently, there has been new effort to characterize the dielectric performance of thin-films made of polarizable molecule assemblies, at both theoretical and experimental levels. In this context, we present the relation between the polarizability of the constituting molecule and the film dielectric constant, using periodic Density Functional Theory (DFT) calculations, for conjugated and saturated chains.
We particularly explore the implication of the peculiar chain length dependence of the polarizability for conjugated molecules. We show and explain from DFT calculations and a simple Clausius-Mossotti-like depolarization model how this chain-length dependence is affected by the collective polarization. A high sensitivity of the dielectric constant to the thin-film thickness is expected for conjugated molecules. This latter can increase by a factor of 3-4 at a reasonable coverage, just by extending the molecular length. This significantly limits the decline of the thin-film capacitance with the inverse of the film thickness, generally observable for non-conjugated materials.

[1] Van Dyck, Marks, Ratner, ACS Nano, 2017, 10.1021/acsnano.7b01807