Prototypical molecular switches such as azobenzenes exhibit two states, i.e., trans and cis, with different characteristic physical properties. In recent years various derivatives were investigated on metallic surfaces. However, bulk insulators as supporting substrate reveal important advantages since they allow electronic decoupling from the environment, which is key to control the switching properties. Here, we report on the light-induced isomerization of an azobenzene derivative on a bulk insulator surface, in this case calcite (101̅4), studied by atomic force microscopy with submolecular resolution. Surprisingly, cis isomers appear on the surface already directly after preparation, indicating kinetic trapping. The photoisomerization process is reversible, as the use of different light sources results in specific molecular assemblies of each isomer. The process turns out to be very efficient and even comparable to molecules in solution, which we assign to the rather weak molecular interaction with the insulator surface, in contrast to metals.

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.

On-surface synthesis constitutes a rapidly growing field of research due to its promising application for creating stable molecular structures on surfaces. While self-assembled structures rely on reversible interactions, on-surface synthesis provides the potential for creating long-term stable structures with well-controlled properties, for example superior electron transport for future molecular electronic devices. On-surface synthesis holds the promise for preparing insoluble compounds that cannot be produced in solution. Another highly exciting aspect of on-surface synthesis is the chance to discover new reaction pathways due to the two-dimensional confinement of the reaction educts. In this review, we discuss the current state-of-the-art and classify the reactions that have been successfully performed so far. Special emphasis is put on electrically insulating surfaces, as these substrates pose particular challenges for on-surface synthesis while at the same time bearing high potential for future use, for example, in molecular electronics.

AbstractOn‐surface synthesis constitutes a rapidly growing field of research due to its promising application for creating stable molecular structures on surfaces. While self‐assembled structures rely on reversible interactions, on‐surface synthesis provides the potential for creating long‐term stable structures with well‐controlled properties, for example superior electron transport for future molecular electronic devices. On‐surface synthesis holds the promise for preparing insoluble compounds that cannot be produced in solution. Another highly exciting aspect of on‐surface synthesis is the chance to discover new reaction pathways due to the two‐dimensional confinement of the reaction educts. In this review, we discuss the current state‐of‐the‐art and classify the reactions that have been successfully performed so far. Special emphasis is put on electrically insulating surfaces, as these substrates pose particular challenges for on‐surface synthesis while at the same time bearing high potential for future use, for example, in molecular electronics.

AbstractWir zeigen die photochemisch induzierte Reaktion von C60 auf dem Nichtleiter Calcit, die mit Nichtkontakt‐Rasterkraftmikroskopie untersucht wurde. Die Bildung kovalenter Bindungen wird durch die Verringerung intermolekularer Abstände offensichtlich. Diese Interpretation wird zusätzlich durch UV/Vis‐ und Massenspektren an dicken Filmen gestützt. Die Bestrahlung von C60 führt zu wohlgeordneten, kovalent verknüpften Domänen. Darüber hinaus wird die Orientierung der Domänen durch die Gitterdimensionen des Calcitsubstrats vorgegeben. Die Gitterfehlanpassung gezielt zu nutzen, um bewusst die Richtung der Reaktion zu beeinflussen, ist ein neuer Ansatz, um Reaktionen auf Oberflächen maßzuschneidern. Hier stellen wir eine Strategie vor, die es ermöglicht, orientierte, kovalente Netzwerke auf Nichtleitern zu generieren.

A substrate-guided photochemical reaction of C60 fullerenes on calcite, a bulk insulator, investigated by non-contact atomic force microscopy is presented. The success of the covalent linkage is evident from a shortening of the intermolecular distances, which is clearly expressed by the disappearance of the moiré pattern. Furthermore, UV/Vis spectroscopy and mass spectrometry measurements carried out on thick films demonstrate the ability of our setup for initiating the photoinduced reaction. The irradiation of C60 results in well-oriented covalently linked domains. The orientation of these domains is dictated by the lattice dimensions of the underlying calcite substrate. Using the lattice mismatch to deliberately steer the direction of the chemical reaction is expected to constitute a general design principle for on-surface synthesis. This work thus provides a strategy for controlled fabrication of oriented, covalent networks on bulk insulators.

The bottom-up construction of functional devices from molecular building blocks offers great potential in tailoring materials properties and functionality with utmost control. An important step toward exploiting bottom-up construction for real-life applications is the creation of covalently bonded structures that provide sufficient stability as well as superior charge transport properties over reversibly linked self-assembled structures. On-surface synthesis has emerged as a promising strategy for fabricating stable, covalently bound molecular structure on surfaces. So far, a majority of the structures created by this method have been obtained from a rather simple one-step processing approach. But the on-surface preparation of complex structures will require the possibility to carry out various reaction steps in a sequential manner as done in solution chemistry. Only one example exists in literature in which a hierarchical strategy is followed to enhance structural complexity and reliability on a metallic surface. Future molecular electronic application will, however, require transferring these strategies to nonconducting surfaces. Bulk insulating substrates are known to pose significant challenges to on-surface synthesis due to the absence of a metal catalyst and their low surface energy, frequently resulting in molecule desorption rather than reaction activation. By carefully selecting a suitable precursor molecule, we succeeded in performing a two-step linking reaction on a bulk insulating surface. Besides a firm anchoring toward the substrate surface, the reaction sites and sequential order are encoded in the molecular structure, providing so far unmatched reaction control in on-surface synthesis on a bulk insulating substrate.

We report the formation of extended molecular layers of C(60) molecules on a dielectric surface at room temperature. In sharp contrast to previous C(60) adsorption studies on prototypical ionic crystal surfaces, a wetting layer is obtained when choosing the calcite (CaCO(3))(10 ̅14) surface as a substrate. Non-contact atomic force microscopy data reveal an excellent match of the hexagonal lattice of the molecular layer with the unit cell dimension of CaCO(3)(10 ̅14) in the [01 ̅10] direction, while a lattice mismatch along the [ ̅4 ̅261] direction results in a large-scale moiré modulation. Overall, a (2 × 15) wetting layer is obtained. The distinct difference observed microscopically upon C(60) adsorption on CaCO(3)(10 ̅14) compared to other dielectric surfaces is explained by a macroscopic picture based on surface energies. Our example demonstrates that this simple surface-energy based approach can provide a valuable estimate for choosing molecule-insulator systems suitable for molecular self-assembly at room temperature.

We report the formation of extended molecular layers of C60 molecules on a dielectric surface at room temperature. In sharp contrast to previous C60 adsorption studies on prototypical ionic crystal surfaces, a wetting layer is obtained when choosing the calcite (CaCO3)(104) surface as a substrate. Non-contact atomic force microscopy data reveal an excellent match of the hexagonal lattice of the molecular layer with the unit cell dimension of CaCO3(104) in the [010] direction, while a lattice mismatch along the [61] direction results in a large-scale moiré modulation. Overall, a (2 × 15) wetting layer is obtained. The distinct difference observed microscopically upon C60 adsorption on CaCO3(104) compared to other dielectric surfaces is explained by a macroscopic picture based on surface energies. Our example demonstrates that this simple surface-energy based approach can provide a valuable estimate for choosing molecule–insulator systems suitable for molecular self-assembly at room temperature.