Isolated and self-assembled molecules — Deposition, observation, properties and applications

Since the 2000s, there has been a great deal of interest in the study of molecular systems isolated and self-assembled on a surface. This effervescence is directly linked to the development of near-field microscopes. However, it would be unfair not to also attribute this phenomenon to the coming together of two communities, chemists and physicists, which had remained far apart for too long. Today, these two disciplines intersect at the nanometer scale, to elaborate, functionalize, position and assemble molecular structures whose potential applications seem innumerable. Researchers are imagining these devices in the fields of nanoelectronics, nanobiosensors, nanomotors, photovoltaics...

Molecules have the advantage of being able to be used as elementary building blocks to form self-assembled systems or more complex objects. This concept is known generically as the "bottom-up" approach, or monumentalization. The properties of molecules are modulated by control of their architecture, chosen a priori by the synthetic chemist according to topology (wire, paving) or the physico-chemical properties sought (catalysis, fluorescence, electronic conduction, etc.). In some cases, they can be stable at atmospheric pressure and in a vacuum, and have a very long lifespan (some molecules have a lifespan of several decades), making it possible to imagine long-term applications. By functionalizing them, it is possible to graft them onto surfaces (metal, semiconductor, insulator) via a specific grouping, or to enable them to recognize each other to form large-scale assemblies. The main benefits of grafting or self-assembly on a surface lie in controlling the spatial arrangement and orientation of these nano-objects, thanks to their interactions with the substrate and between molecules.

There is therefore a huge number of molecules that can be used, but to do so it is essential to be able to study them not only in a perfectly controlled environment, but also within structures that are relatively simple to interpret. For this reason, a large number of vacuum studies have been carried out on isolated or self-organized molecules deposited on perfectly crystalline surfaces.

The observation of such objects or structures on the nanometric scale requires the use of particularly high-performance microscopes such as near-field microscopes, tunneling and atomic force microscopes and their derivatives, invented in the 1980s. Since then, it has become possible to observe, analyze, control and manipulate matter with atomic precision, and we are witnessing a veritable revolution in this scientific field. It is this scientific revolution that we wish to present, not from an instrumental technical point of view, but through the main results obtained at international level...