Physics Colloquium, Giovanni Costantini, University of Warwick

Abstract:

Modern Surface Science has developed over the last 60 years achieving stunning results and gaining a deep understanding of the elementary processes that underpin advances across engineering and physical sciences. However, after what many see as its golden period – with the fundamental studies of heterogeneous catalysis, the development of X-ray photoemission spectroscopy, and the invention of scanning probe microscopy (just to cite a few highlights) – Surface Science is nowadays often accused to be excessively inward-looking, mostly capable of studying only model systems, of analysing in the greatest details problems of the lowest relevance, and overall incapable or unwilling to have a practical impact on the “real world”. This attitude has been progressively adopted by impact-obsessed funding agencies around the world, making it increasingly difficult to (openly) support Surface Science research.

In this talk I will try to demonstrate that some of the most recent and powerful Surface Science techniques are able to deliver essential information, that cannot be achieved by any other current analytical method, about “real world” systems with a huge practical and technological relevance. In particular, I will show that by combining vacuum electrospray deposition (ESD) and high-resolution scanning tunnelling microscopy (STM) it is possible to image conjugated polymers used in organic electronics and photovoltaic devices with unprecedented detail. Based on this, it becomes possible to sequence the polymers by visual inspection and to determine their molecular mass distribution by simply counting the repeat units. Moreover, I will demonstrate that we can precisely determine the nature, locate the position, and ascertain the number of defects in the polymer backbone. This unique insight into the structure of conjugated polymers represents a fundamental contribution to the long-discussed issue of defects as a possible source of trap sites.

In the second part of the talk, I will present the self-assembly of a brominated polycyclic aromatic molecule on Au(111), that cannot be fully solved by standard STM because of its inability to conclusively establish the nature of the intermolecular interactions. Conversely, I will show that by performing HR-STM with a CO-functionalised tip, we can clearly identify the location of rings and halogen atoms, unambiguously determining that halogen bonding governs the assembly. Moreover, this analysis reveals the presence of defective molecules whose existence could not be determined by standard chemical analytical methods. I will take this as the starting point for discussing the potential of high resolution molecular imaging as new and powerful analytical technique for chemical structure determination.