Understanding the adsorption and self-assembly of functional organic molecules on insulating substrates
David Z. Gao, Julian Gaberle, Matthew Watkins, Christian Loppacher, Laurent Nony and Alexander L. Shluger
Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom
Aix-Marseille Université CNRS, IM2NP UMR 7334 13397, Marseille, France
Understanding the adsorption and self-assembly of functional organic molecules on insulating substrates is of critical importance for many applications, ranging from molecular electronics to lubrication. We will discuss the challenges facing these studies by presenting the results of combined non-contact atomic force microscopy (NC-AFM) experiments and theoretical calculations of the effects of molecular structure and flexibility on the adsorption and self-assembly of organic molecules on KCl. A set of variable molecules were synthesized, deposited onto the KCl (100) surface, and imaged using NC-AFM. A 2-dimensional periodic implementation of quantum mechanics/molecular mechanics (QM/MM) combined with genetic algorithm (GA) techniques was used to produce classical force fields to study the dynamic properties and film structures of these molecules. The accuracy of the van der Waals corrected DFT-D3 was benchmarked using Møller−Plesset perturbation theory calculations. These force fields were used to perform potential of mean force (PMF) calculations of adsorption of individual molecules and extract information on the entropic contributions to adsorption energy. The formation of self-assembled monolayers was studied using both static and molecular dynamics simulations. We demonstrate that the functional groups, the length of the molecular backbone, and the flexibility of these molecules all greatly affect their ability to form defect free self-assembled networks and the morphology of these films.