Anisotropy of π–π Stacking as Basis for Superlubricity
Wanhao Cai, Johanna L. Trefs, Thorsten Hugel, Bizan N. Balzer
Abstract
When two surfaces move against each other, often high adhesive strength with low friction is desired. Then, the interfacial interaction requires a strong directional anisotropy, as provided by π–π stacking. However, a detailed understanding of the direction-dependent strength of π–π stacking is lacking because such direction-dependent molecular force measurements are challenging. We use atomic force microscopy-based stereographic single molecule force spectroscopy to perform direction-dependent pulling of a π–π stacking model system in solution, namely, a single pyrene molecule on highly oriented pyrolytic graphite. There, the friction force is at least 3 orders of magnitude lower than the adhesive force, indicating superlubricity at the single molecule level. This is in clear contrast to the thiol–Au coordination bond, where we reveal high friction forces. Our measurements quantify and highlight the importance of the directional anisotropy of π–π stacking, which provides a bottom-up understanding of adhesion and friction at material interfaces based on its molecular interactions.