A scale-bridging modeling approach for anisotropic organic molecules at patterned semiconductor surfaces

TL;DR

This paper introduces a novel coarse-grained model for organic molecules on semiconductor surfaces, capturing their collective behavior and orientational ordering, validated through Monte Carlo simulations that reproduce experimental phenomena.

Contribution

The paper presents a new scale-bridging modeling approach combining intermolecular and molecule-substrate interactions for hybrid organic-inorganic systems.

Findings

Reproduces molecular alignment with surface line charges

Simulates formation of uniaxial herringbone phase

Models lying nematic phase formation

Abstract

Hybrid systems consisting of organic molecules at inorganic semiconductor surfaces are gaining increasing importance as thin film devices for optoelectronics. The efficiency of such devices strongly depends on the collective behavior of the adsorbed molecules. In the present paper we propose a novel, coarse-grained model addressing the condensed phases of a representative hybrid system, that is, para-sexiphenyl (6P) at zinc-oxide (ZnO). Within our model, intermolecular interactions are repre- sented via a Gay-Berne potential (describing steric and van-der-Waals interactions) combined with the electrostatic potential between two linear quadrupoles. Similarly, the molecule-substrate interactions include a coupling between a linear molecular quadrupole to the electric field generated by the line charges characterizing ZnO(10-10). To validate our approach, we perform equilibrium Monte Carlo simulations, where the lateral positions are fixed to a 2D lattice, while the rotational degrees of freedom are continuous. We use these simulations to investigate orientational ordering in the condensed state. We reproduce various experimentally observed features such as the alignment of individual molecules with the line charges on the surface, the formation of a standing uniaxial phase with a herringbone structure, as well as the formation of a lying nematic phase.