Nanotribological properties of organic molecular crystals

Crystalline surfaces show unique properties when at the interface with other solids or fluids. The interaction of the crystal surface with another medium promotes a structural re-arrangement of the surface atoms. Structural surface reconstruction then tunes the physical and chemical properties of the crystal surface. The overall process plays a fundamental role in both science and technology for example modulating the band gap in inorganic semiconductors or tuning the lubricating properties of organic crystal when used as solid lubricants. While extensively studied in inorganic solids, surface reconstruction has not yet been fully elucidated in organic molecular compounds. According to current models, surface properties of organic molecular crystals can be described assuming a bulk-terminated structure, possibly with a slight relaxation. Within this framework, organic crystals surfaces show marked corrugations, in particular when compared to inorganic surfaces, with deep furrows developing often along more than one crystallographic direction (Fig. 1). Still, our knowledge is limited by only a few experimental studies validating current models’ assumptions. A better understanding of surface reconstruction in organic compounds would however have a significant impact in technology with applications ranging from optoelectronic design and thin film synthesis to lubrication. Here, using atomic force microscopy high-resolution imaging and shear spectroscopy, we explore model organic surfaces in air and at the interface with fluid lubricants. Our results elucidate the key role played by their well-defined corrugations in modulating their tribological properties, giving rise to a strong anisotropy and non-linear behaviour of dry and lubricated friction probed at the nano and microscale.