On-Surfaces Synthesis on Insulating Substrates

Abstract

On-surface synthesis has attracted great attention in recent years due to its promising potential for creating functional structures on surfaces. An important aspect of on-surface synthesis is the capability to arrive at covalently linked thermally stable structures that offer the possibility for application even in harsh environments outside ultra-high vacuum conditions. Additionally, covalent linking allows for fabricating conjugated structures with superior electron transport properties. Especially, the latter is of tremendous interest when considering future applications in the field of molecular electronics. Having molecular electronics applications in mind explains the need for decoupling of the electronic structure of the molecular network from the underlying support surface. Thus, it is highly interesting to transfer on-surface synthesis strategies from metallic to insulating surfaces. Albeit, insulating surfaces pose several challenges for on-surface synthesis. First, many prototypical insulating support materials interact only weakly with organic molecules. This weak binding frequently results in molecule desorption rather than reaction activation when thermally initiating the reaction. Second, it is known that metals act as catalyst for several reactions that have been performed successfully on metallic surfaces. A simple transfer of these reactions to insulating surfaces in the absence of metal atoms is, therefore, questionable and requires different reaction pathways to be considered. In this chapter, we review the current state-of-the-art in on-surface synthesis on electrically insulating substrates carried out in ultra-high vacuum. Proof-of-principle reactions are discussed with an emphasis on strategies to overcome challenges related to the weak molecule-surface binding often present on insulating surfaces, e.g., by means of photochemical activation. Site-specific and sequential reactions are presented as a promising way for enhancing control and structural complexity of on-surface synthesis on insulating support materials. Finally, the influence of the substrate is shown to induce directionality in on-surface synthesis by favoring specific surface directions.