Static and dynamic friction of individual nanostructures

Friction is a ubiquitous process that has an enormous impact on our daily life. Friction-based processes at the macroscopic level were first pioneered few centuries ago. Interestingly, the same physics laws used to describe friction processes in the macroscopic world do not provide a suitable description of friction down to the atomic scale. There are indeed so many fundamental aspects related to the reduced size and chemistry whose interplay complicates a comprehensive understanding of friction at level of single atoms/molecules. The goal of this research is to explore the friction behavior of a system based on an individual molecular structure lying on a crystal surface. Open fundamental questions in this regard concern the force needed to trigger the motion of an individual molecular structure lying on a surface, and how the friction behavior is influenced by the interplay between the chemical and structural aspects of the system. Advanced synthesis methods will be used to grow suitable molecular structures on a crystal surface. These will be molecular wires, i.e., long linear structures made of the repetition of the same molecule. Extended clusters comprised of few molecules as well as individual single- molecules will be investigated. It is of crucial importance to rely on molecular structures with a well-defined chemical structure down to the single atom. The surface has to be exceptionally clean and to possess a regular flat structure. Individual molecular structures will be then identified and imaged on the surface by using state-of-the-art microscopy. The tip of the microscope will be furthermore used to address individual molecules and move them across different surface directions. The force needed to induce this lateral motion and the corresponding mechanical behavior will be characterized by systematically fine-tuning the structure and chemical composition either of the surface or molecular structures. Understanding the interplay between these fundamental chemical and structural aspects is essential to gain control of the friction process. This will be furthermore an important input to develop model theories and new chemical designs to synthesize materials with predefined friction behavior.

In this project, the mechanical properties of individual molecules adsorbed on a surface were investigated, in particular friction at the atomic scale. A key question is how small controlled changes in the molecular structure influence the static and dynamic frictional properties. For a better understanding the dynamics of individual molecules were explored by atomic force microscopy as a function of controlled chemical changes to the molecular structure and the supporting surface. In this respect the perylene molecule and its deuterated counterpart were chosen as a model system to investigate the role of isotopic substitution. The replacement of all hydrogen atoms by deuterium atoms in a molecule merely means a change in the molecular mass, while the chemistry of the system remains unchanged. The key point here is whether this might imply a change of the mechanical response of the molecule. Molecules were adsorbed on two different metallic surfaces: Ag(111), which is atomically smooth and isotropic; and Au(110), which is anisotropic and rougher. In this way, the interaction between molecules and surface can be further tuned. Isolated molecules were individually laterally manipulated in a controlled manner along different surface directions using the tip of the microscope. A quantitative comparison of the molecular manipulations shows no significant difference between the mechanical response of perylene and its deuterated counterpart. In other words, no measurable effects were detected. In addition, metal-free H2-phthalocyanine (H2Pc) and metallic Copper-phthalocyanine (CuPc) molecules were selected as model systems to be studied on different metal surfaces, Ag(111) and Au(110). Individual phthalocyanine molecules were successfully displaced across different surface directions. The lateral manipulation curves of these phthalocyanine molecules reveal a complex shape that is closely related to the complexity of the molecular motion when dragged across the surface. In this regard, phthalocyanines are extended molecules and have many internal degrees of freedom. A higher force is required to pull the CuPc over Ag(111) compared to H2Pc. The CuPc is more strongly bound to the surface compared to the metal-free phthalocyanine. A similar trend is observed when the molecules are dragged across the Au(110) surface. In summary, these results show how the mechanical response of a single molecule can be influenced by well-defined and controlled chemical changes. It proved to be non-trivial to detect the change in the molecular mechanical response triggered by an isotope substitution in a small organic molecule (perylene), while differences in the molecular behaviour of phthalocyanine molecules with different atomic arrangements in their centre were observed during lateral manipulation. These experimental results will contribute further to the understanding of the chemical-physical processes that occur at the nanoscale between molecule and substrate. This further knowledge will be beneficial to larger organic molecules adsorbed at solid-gas or even solid-liquid interfaces.