One vision currently driving materials scientists is to combine organic molecules (and their diverse functions) with the technological possibilities offered by extremely complex semiconductor electronics. Thanks to modern micro-nano technology methods, the latter has designed more efficient electronic components for various applications.

However, it is also getting closer and closer to its physical limits: methods using traditional techniques cannot produce smaller structures for functionalized semiconductor materials such as silicon.

Scientists have now proposed a new method in the journal Nature Chemistry ("Controlled growth of ordered monolayers of N-heterocyclic carbenes on silicon"): They show that a stable and ordered monolayer of molecules can be produced on the surface of silicon. -Through self-assembly. For this, they use N-heterocyclic carbene. These are small reactive organic ring molecules whose structure and properties are different in many ways, and can be customized with different "functional groups".

Comparison of the theoretical calculation structure of an ordered NHC monolayer (DFT, right) and an experimental scanning tunneling microscope image (STM, left). N: nitrogen atom, C: carbon atom, Si: silicon atom, B: boron atom. (Picture provided by researcher)

Researchers led by Professor Mario Dähne (Technical University of Berlin, Germany), Professor Norbert Esser (Technical University of Berlin and Leibniz Institute for Analytical Sciences, Germany), Professor Frank Glorius (University of Munster, Germany), and Dr. Norbert Esser. Conor Hogan (Institute of the Structure of Matter of the Italian National Research Council, Rome, Italy) and Prof. Wolf Gero Schmidt (University of Paderborn, Germany) participated in this research.

Technology miniaturization reaches its limit

Chemist Frank Glorius said that instead of constantly trying to artificially make smaller and smaller structures, it is obvious that it is better to learn from molecular structures and processes in nature and combine their functions with semiconductor technology. It can be said that this will form an interface between molecular functions and electronic user interfaces for technical applications.

The prerequisite is that ultra-small molecules with variable structures and functions must be physically combined with semiconductor devices, and they must be reproducible, stable and as simple as possible.

Take advantage of molecular self-organization

The self-organization of surface molecules, as the interface of the device, can accomplish this task well. Molecules with a certain structure can be adsorbed on the surface in large quantities and arrange themselves into a desired structure predetermined by the molecular properties.

For example, this works well on metal surfaces, but unfortunately, it has been completely unsatisfactory for semiconductor materials so far, explains physicist Norbert Esser.

This is because in order to be able to arrange themselves, molecules must move (diffusion) on the surface. But the molecules on the surface of the semiconductor will not do this. On the contrary, their bond to the surface is so strong that they will stick wherever they touch the surface.

N-heterocyclic carbene as a solution

Simultaneous movement and stable adhesion to the surface is the key issue, but also the key to potential applications. It is here that researchers now have a possible solution at hand: N-heterocyclic carbene. In the past decade, their use in surface functionalization has attracted a lot of interest.

For example, on metal surfaces such as gold, silver, and copper, they have proven to be very effective surface ligands and are generally superior to other molecules. However, their interaction with the semiconductor surface has actually not been explored.

Form a regular molecular structure

Certain properties of carbene determine that it is now possible for the first time to produce a molecular monolayer on the surface of silicon: N-heterocyclic carbene, like other molecules, forms a very strong covalent bond with silicon and thus binds stably. However, The side groups of the molecules also keep them "a certain distance" from the surface. Therefore, they can still move on the surface.

Although they do not move very far-only a few atoms apart-it is enough to form almost the same regular molecular structure on the surface of a regular-structure silicon crystal.

Interdisciplinary cooperation

The researchers used complementary multi-methods of organic chemical synthesis, scanning probe microscopy, photoelectron spectroscopy, and comprehensive material simulation to clarify the principles of this new type of chemical interaction in their interdisciplinary collaboration. They also demonstrated the formation of regular molecular structures in several examples.

The first author of the study, physicist Dr. Martin Franz, emphasized that this opens a new chapter in the functionalization of semiconductor materials, such as silicon in this case.