Observations of energy transfer and anisotropic behavior in ZnO nanoparticles surface-modified by liquid-crystalline ligands

Here we report the fabrication of a novel nano-level hybrid of ZnO nanoparticles (NPs) and liquid crystals (LCs) by the attachment of organic LC molecules with a mercapto moiety, namely, 4′-n-(6-mercaptohexyloxy)-4-cyanobiphenyl (6CNBP-SH), to the surface of ZnO nanoparticles. The dispersion of modified ZnO NPs (6CNBP-SH@ZnO) is greatly improved by the surface modification of 6CNBP-SH ligands. The photoluminescence (PL) measurement shows that the ultra-violet emission of ZnO can be enhanced by the surface modification of 6CNBP-SH ligands and annealing at liquid crystal state temperature of 6CNBP-SH@ZnO (110 °C). Meanwhile, defect-related emission of ZnO in 6CNBP-SH@ZnO almost disappears. We attribute this observation to the energy transfer between the ZnO NPs and 6CNBP-SH, surface passivation of the ZnO and formation of ZnO nano-dispersing structure induced by 6CNBP-SH molecules. The anisotropic behavior of 6CNBP-SH@ZnO is also investigated. The results indicated that the 6CNBP-SH liquid-crystalline ligands could endow the 6CNBP-SH@ZnO hybrid obvious mesoscopic behavior. In addition, the increased optical anisotropy of 6CNBP-SH@ZnO is also observed upon thermal treatment at 110 °C.