Contributing to the foundations of nanoelectronics and other nanotechnologies by fundamental research in surface science – this is our mission.
Surface science is devoted to the physical and chemical properties of surfaces of solid or liquid materials. Of course, surface properties are closely connected to the properties of the respective material, which may be a certain metal, semiconductor or insulator. But virtually all properties of bulk materials are modified in the surface region where the bulk material is truncated. Even some entirely new phenomena arise at surfaces that cannot exist in bulk materials. The same is true of interfaces between two different materials: The properties of the interface are different from those of either of the bulk materials that meet at the interface.
With the rise of nanotechnology (e.g. nanoelectronics, nanomaterials), surfaces and interfaces are becoming increasingly important, because of the following fundamental principle: The smaller an object, the larger the proportion of its atoms that are located at the surface (or interfaces) in relation to its atoms in the bulk. Accordingly, the impact of surface (or interface) phenomena on the overall properties of nanoscale objects increases with their decreasing size and may even become dominant for very small objects. A well-known example are colloidal gold particles, which -- depending on their sizes -- have different colours, because the so-called plasmon excitations that determine the particles’ colour are affected by the presence of surfaces: The more surface there is, the stronger is this influence.
It is our conviction that a thorough understanding and mastering of surface properties and surface processes are absolutely essential for nanotechnology. Surface science has many methods, concepts and insights on offer that may teach us how to deal with the challenges of nanoscience and nanotechnology.
A good illustration for the role of surface science in nanoelectronics research is provided by the field of molecular electronics. The idea of embedding the functions of an electronic device in the chemical structure of a single molecule is one of the most radical visions for the future of electronics. Formidable problems must be solved to develop this idea into a viable technology, many of them related to surface science. Examples are:
- What are the properties of the contacts between a molecular device and the current leads that are used for input and output? In essence, this is a surface science problem: How do molecules and metals interact at their contact points? Insight into this question can be gained by systematic experiments on molecules that are adsorbed on single-crystalline metal surfaces.
- Can we engineer molecular switches that perform well even if they are adsorbed on a surface and connected to leads? In solution or gas phase, many molecules undergo reversible changes if excited by an external stimulus (e.g. light). Such molecules are good candidates for switches in electric circuits. However, in many cases the switching functionality is lost if the molecule is adsorbed on a surface. Here systematic surface science experiments can help to understand how the switching functionality can be preserved.
- How can molecules be assembled into hierarchical structures in two or three dimensions? It is clear that a purpose-designed structural organisation is an important prerequisite for achieving desired functionalities. Surface science offers many tools to investigate the self-assembly of molecules into hierarchical structures.
Moreover, some of the challenges to experimental methodology that are inherent in molecular electronics can be addressed by techniques which are well-established in surface science: For example, it has been demonstrated recently that scanning tunnelling microscopes allow very controlled measurements of charge transport through individual molecules. In this way the electrical properties of molecules can be studied with high precision. This has led to the development of multi-tip scanning tunnelling microscopes which are even more versatile tools in the context of nanoscale charge transport research.