GCI-Computational Physics Group (Sapienza Università di Roma) & SeMS – Seminario di Modellazione & Simulazione

Lev Kantorovich
King's College London

Ice skating of organic molecules on the gold surface

Self-assembly of organic molecules has become a favourite research topic in surface science due to promises for nanotechnology in developing structures with desired functional properties useful for developing sensors, in drug delivery, etc. However, a drive to creating assemblies with desired properties is impossible without a proper understanding of thermodynamic and kinetic aspects of the assemblies formation. The Au(111) surface provides a perfect “experimental” platform for such an investigation due to relative ease in prepa- ration of large planar terraces and high mobility of many flat molecules on it.

In this talk I’d like to consider a number of exciting examples, from our own experience, of hydrogen bonded assemblies of various flat organic molecules on this surface, which may form a wide variety of two-dimensional (2D) structures. For some molecules these are polymorphic periodic structures. In some other cases structures, which may appear perfectly ordered in Scanning Tunnelling Microscopy (STM) images, are shown upon careful examination to be in fact disordered. Moreover, some molecules have not been seen at all to form ordered monolayers and instead assemble into a completely disordered structures reminiscent of a “2D glass”. A change in experimental conditions (e.g. annealing followed by fast cooling) may lead some assemblies to adopt a lower free energy periodic structure separated from the ground state structure by a considerable free energy barrier.

Understanding of these exciting phenomena is impossible without a proper theoretical analysis which requires considering self-assembly of molecules on crystal surfaces from three points of view: (i) interplay between directional (often hydrogen bonding) and non-directional (van der Waals) inter-molecular interactions; (ii) role of the surface in stability and mobility of the molecules, and (iii) of the kinetics of the assemblies formation under given experimental conditions. Following these basic ideas, I will present our theoretical approach in studying self-assembly, which consists, firstly, of a combination of a systematic method of building all possible 2D structures in the gas phase (based on a topological analysis of all available “connections” between them), and ab initio density functional calculations of the “best” candidates. Secondly, interaction with the surface is studied including molecules mobility and barriers for de- sorption. Specifically, I’ll discuss the role played by the dispersion interaction in stabilising planar organic molecules on the gold surface without affecting their mobility across the surface. Thirdly, the information of their interaction and mobility is utilised in developing Kinetics Monte Carlo models which could shed light on why and how the observed structures come about and the role played by experimental conditions in forming particular assemblies.