Photoluminescence Control by Subtle Alterations of Surface Capping Ligands in Samarium-Doped Titanium Dioxide Nanoparticles

Surface of inorganic nanoparticles (NPs) controls an enumerable number of properties in the quantum confinement regime. Surface-localized atoms and an appropriate surface capping ligand construct the surface. This work systematically investigates the effect of surface capping ligand identity on the dopant emission in the trivalent samarium (Sm3+)-doped titanium dioxide NPs, in cases where the electronic structure of the host (NP)–dopant (Sm3+) is nearly invariant. Subtle alterations of chemical structures are considered in the capping ligands. These molecules include (3-aminopropyl)trimethoxysilane (APTMS), (3-aminopropyl)triethoxysilane (APTES), (3-mercaptopropyl)trimethoxysilane (MPTMS), (3-mercaptopropyl)triethoxysilane (MPTES), (3-methylaminopropyl)trimethoxysilane (MAPTMS), (3-phenylaminopropyl)trimethoxysilane (PAPTMS), (3-chloropropyl)trimethoxysilane (CPTMS), and (n-propyl)trimethoxysilane (n-PTMS). A noticeably distinct alteration in the rates of dopant emission depopulation is observed in the Sm3+ emission lifetime measurements. These trends can primarily be well correlated with the vibrational frequency of the local environment, with a clearer trend observed for the more protected core-related dopant moieties. Signatures from the surface-localized dopants additionally compete with the surface coverage by the ligands. Finally, considering the Sm3+ emission lifetimes and dispersibility of the NPs, the amine containing ligands, APTMS and APTES, are identified as the most suitable surface capping ligands that can confer optimum dopant emission from these NPs. The results discussed in this work provide a solid foundation to help guide the choice of surface capping ligands in designing doped inorganic NPs for practical applications.