Electrostatic Field Driven Alignment of Organic Oligomers on ZnO Surfaces

TL;DR

This study demonstrates how electrostatic fields influence the alignment of organic oligomers on ZnO surfaces, revealing a new method for creating highly ordered molecular arrays through first-principles calculations.

Contribution

It uncovers the role of quadrupole-dipole coupling in molecule orientation and introduces a novel electrostatic mechanism for oligomer alignment on ZnO surfaces.

Findings

Molecular orientation is governed by quadrupole-dipole interactions.

Long oligomers align along the electric field with significant stabilization energies.

Results agree with recent experimental observations.

Abstract

We study the physisorption of organic oligomers on the ZnO($10\bar{1}0$) surface using first-principles density-functional theory and non-empirical embedding methods. We find that both in-plane location and orientation of the molecules are completely determined by the coupling of their quadrupole moments to the periodic dipolar electric field present at the semiconductor surface. The adsorption is associated with the formation of a molecular dipole moment perpendicular to the surface, which bears an unexpected linear relation to the molecule-substrate interaction energy. Long oligomers such as sexiphenyl become well-aligned with stabilization energies of several 100 meV along rows of positive electric field, in full agreement with recent experiments. These findings define a new route towards the realization of highly-ordered self-assembled arrays of oligomers/polymers on ZnO($10\bar{1}0$) and similar surfaces.