Lanthanide-Doped Nanoparticles with Excellent Luminescent Properties in Organic Media

Surface-coated nanoparticles of LaF3 and LaPO4 doped with the luminescent trivalent lanthanide ions Eu3+, Nd3+, Er3+, Pr3+, Ho3+, and Yb3+ have been prepared. These ions emit in the visible and in the near-infrared part of the electromagnetic spectrum. The ions Nd3+, Er3+, Pr3+, and Ho3+ are the main focus in this research because they show emissions in telecommunication windows. The Yb3+ ion is of interest because it can be used as a sensitizer for Er3+. The Eu3+ ion has been used as a probe for the structural environment of the luminescent ion. It is shown that these lanthanide ions are incorporated in the inorganic host of the particles and that the particles are dispersable in organic solvents. The luminescent lifetimes of the ions are increased by orders of magnitude compared to organic complexes, with values ranging from several microseconds for Pr3+ and Ho3+, up to about 200 μs for Nd3+, and 1 ms for Er3+. This increase in the luminescence lifetime is indicative of an effective shielding of the lanthanide ions from nonradiative decay of the excited state by the high-energy vibrations of the solvents and the coordinated organic ligands. A model is proposed to describe the nonexponential behavior of the luminescence decay by quenching from outside the particle. It uses two fit parameters, kR, a parameter describing the luminescence decay rate in the absence of surface quenching, and C, a parameter describing the sensitivity toward quenching. Using this model, the luminescence decays can be fitted very well, and factors influencing the luminescence lifetime like concentration quenching and solvent effects can be described accurately.