Scanning tunneling microscopy on ordered self-assemblies of cyclodextrin inclusion complexes formed by substrate-induced two-dimensional crystal growth

Cyclodextrin (CyD) inclusion complexes with water and methanol, both of which had been known to form the cage-type crystal structure in the three dimensional crystal growth mode, were observed to form new self-assembled structures on highly oriented pyrolytic graphite (HOPG) and MoS2 surfaces. By removing the surface CyD molecular layers using the atom manipulation technique of scanning tunneling microscopy (STM), ordered molecular stacking of the inner layers was observed. The surface of the observed CyD layers was markedly hat, indicating a stronger interaction between CyDs in each layer compared to that between the adjacent CyD layers, which is possibly related to the two-dimensional crystal growth mode induced by the anisotropic hydrogen bonding interaction between CyDs. On MoS2, CyD has threefold symmetric lattice matching commensurate with the structure of S atoms, which is considered to affect the ordering of the self-assemblies. In fact, numerous defects are observed in the surface layer.