Surface-confined chemistry of halogenated aromatic compounds on Ag(110): self-assembly and polymerization

Abstract

Carbon-based nanostructures have been extensively studied over the years due to the interesting electronic, mechanical, and thermal properties of low dimension C-allotropes. Among this class of molecules, graphene derivatives have attracted particular attention due to the combination of excellent ballistic transport and structure-dependent properties. E.g., confinement in 1D leads to bad gap opening and edge effects. Therefore, it is possible to modify graphene to tune its properties for appropriate applications. Another way to modify the properties of C-based nanostructures is to couple them with either heteroatoms or transition metal (TM) atoms. Introduction of such dopants allows for the tuning of the electronic and chemical properties of the nanostructures. Cyclometallated complexes on surfaces, for example, could lead to the realization of a promising class of C-based nanostructures.

Several methods have been explored to grow these materials. Among them, bottom-up approaches exploiting the surface assisted synthesis of nanostructures from precursor molecules self-assembled and subsequently polymerized by Ullmann coupling was found to produce better ordered and uniform 1D/2D nanostructures.

In this dissertation, I present my work on the synthesis and characterization of different C-based nanostructures and self-assemblies supported on Ag(110). The experiments are performed under UHV conditions and the samples are characterized by LT-STM and XPS. High resolution XPS and NEXAFS data acquired at the Aloisa beamline of ELETTRA synchrotron radiation source and theoretical models based in DFT calculations performed by partners in UNIMIB complement my analysis to provide a complete description of the systems.

I will first describe my investigation of achiral GNRs synthesized from 1,6-dibromopyrene. Specifically, I will report on the stability of these GNRs showing that they react when exposed to oxygen under controlled conditions and that the morphology of the surface changes considerably under exposure. Furthermore, Ag-O complexes appear on the surface and in between the GNRs, effectively enlarging the gap between ribbons.

Then, I will discuss the interaction of the Pd cyclometallated complex [(5-bromo-2-phenylpyridine)Pd(μ-Cl)]2 with Ag(110). I underline that these molecules are of large use in catalysis but their interaction with metal surfaces has not been explored so far. I found an unexpected chemistry induced by the interaction with Ag(110). Indeed, my data suggests the fragmentation of the molecule and, with the right conditions, cross coupling to form a new compound. More information can be retrieved from comparison between the behaviour of the brominated and hydrogenated versions of the same compound.