# Impact of Polarity on the Anisotropic Diffusion of Conjugated Organic Molecules on (10-10) Zinc Oxide Surface

## TL;DR

This study investigates how polarity influences the binding and anisotropic diffusion of conjugated organic molecules on zinc oxide surfaces, revealing that molecular polarity causes significant directional differences in diffusion rates due to electrostatic and entropic effects.

## Contribution

It provides new insights into the role of molecular polarity and electrostatic mismatch in surface diffusion anisotropy using molecular dynamics simulations.

## Key findings

- Polar molecules diffuse slower in the x-direction and faster in the y-direction compared to apolar molecules.
- Diffusion behavior is governed by electrostatic anisotropic mismatch and entropic contributions.
- Diffusion remains normal and Arrhenius-like across studied conditions.

## Abstract

We study the influence of polarity on the binding and diffusion of single conjugated organic molecules on the inorganic (10-10) zinc oxide surface by means of all-atom molecular dynamics simulations at room temperature and above. In particular, we consider the effects of partial fluorination of the para-sexiphenyl (p-6P) molecule with chemical modifications of one head group (p-6PF2) or both (symmetric) head and tail (p-6PF4). Quantum-mechanical and classical simulations both result in consistent and highly distinct dipole moments and densities of the fluorinated molecules, which interestingly lead to a weaker adhesion to the surface than for p-6P. The diffusion for all molecules is found to be normal and Arrhenius-like for long times. Strikingly, close to room temperature the polar molecules diffuse 1-2 orders of magnitudes slower compared to the p-6P reference in the apolar x-direction of the electrostatically heterogeneous surface, while in the polar y-direction they diffuse 1-2 orders of magnitude faster. We demonstrate that this rather unexpected behavior is governed by a subtle electrostatic anisotropic mismatch between the polar molecules and the chemically specific surface, as well as by increased entropic contributions coming from orientational and internal degrees of freedom.