Identifying the Correct Host-Guest Combination To Sensitize Trivalent Lanthanide (Guest) Luminescence: Titanium Dioxide Nanoparticles as a Model Host System

This work develops a rationale for effective sensitization of trivalent lanthanide cation (Ln(3+)) luminescence in a semiconductor nanoparticle by examining the luminescence characteristics of Ln(3+) dopants in titanium dioxide nanoparticles [Ti(Ln)O-2] [Ln = praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), or ytterbium (Yb)] as a representative model system. For excitation of the TiO2 host at 350 nm the intraconfigurational 4f-4f sharp luminescence bands are observed for the Nd, Sm, Eu, Ho, Er, Tm, and Yb incorporated (doped) nanoparticles, and no such luminescence is observed for the Pr, Gd, Tb, and Dy containing nanoparticles. While host sensitized luminescence of lanthanide ions dominate the emission in the Nd and Sm incorporated nanoparticles, the host sensitization effect is less pronounced for the Eu and Yb containing systems, and for the Ho, Er, and Tm doped nanocrystals only a subset of the dopant ions' luminescence bands is sensitized. The experimental observations of the host sensitized Ln(3+) luminescence properties in the [Ti(Ln)O-2] nanoparticles can be rationalized by considering that the dopant ions act as charge traps in the host lattice and associated environment induced luminescence quenching, effects. Using these results, an energy offset between the trap site and the nanoparticle's band edge that will generate an optimal host sensitized dopant emission is proposed. The approach presented necessarily improves over a combinatorial approach to select the host and dopant moieties, with the benefit of providing physicochemical insight regarding the nature of photophysical processes in a given host (semiconductor nanoparticle) guest (Ln(3+)) composite system.