Near-infrared Emission of Redispersible Er 3 + , Nd 3 + , and Ho 3 + Doped LaF 3 Nanoparticles

Nanoparticles of LaF3 doped with Ln3+ (Ln ) Eu, Er, Nd, and Ho) have been prepared that are dispersible in organic solvents. From the spectrum of Eu3+ it has been concluded that the dopant ion occupies a La3+ site. The luminescence decays are fitted biexponentially. A given possible explanation for this is a different probability of nonradiative decay for ions at or near the surface and ions in the core of the particles. The lifetimes of Eu3+ luminescence at a doping concentration of 5% were 7.7 ± 0.2 and 2.9 ± 0.2 ms, indicative of very high quantum yields. The particles doped with Er3+, Nd3+, and Ho3+ are promising materials for polymer-based optical components, because they show luminescence in the telecommunication window (i.e., Er3+ at 1530 nm, Nd3+ at 1330 nm, and Ho3+ at 1450 nm). Luminescent materials based on lanthanide ions attract a great deal of interest as phosphors in lamps and display devices, 1 components in optical telecommunication, 2 and as the active material in lasers. 3 There is a growing interest to use this luminescence in polymer-based materials, because of easy processing of polymers and ease of integrating different components. The luminescence of the trivalent lanthanide ions arises from transitions within the 4f shell of the ions. These transitions are parity forbidden, leading to low absorption cross-sections and long luminescence lifetimes. However, this long-lived excited state can be quenched very efficiently in the presence of the high-energy vibrations of organic solvents, polymers, or ligands, thus hampering the application in polymer-based devices. Therefore, to use lanthanide luminescence in an organic environment it is important to shield the lanthanide ion from the organic surroundings. Shielding of the lanthanide ion 4 can be achieved by doping it in the inorganic part of nanoparticles, which should still be dispersible in organic solvents. Good dispersibility of the particles is generally achieved by having organic groups on the outside of the particles. Nanoparticles with dimensions of a few nanometers are small enough to minimize Rayleigh scattering in polymer films. 5 Especially the lanthanide ions emitting at wavelengths between 1300 and 1600 nm are of interest for optical telecommunication, because at these wavelengths the silicon-based optical fibers have their maximum transparency. 6 Most nanoparticles doped with lanthanide ions are made in high-temperature procedures leading to particles without organic groups on the surface and therefore they have no dispersibility in solvents. 7 Only very few examples of these lanthanide-doped nanoparticles have been reported that have a good dispersibility in organic solvents. 8,9 In particular, the near-infrared emitting ions have received little attention. 10 In this paper we show the synthesis of lanthanide-doped LaF3 nanoparticles that have a good dispersibility in organic solvents. LaF3 was chosen as the host matrix because this material has very low vibrational energies, 11 and therefore the quenching of the excited state of the rare earth ions will be minimal. This is especially important for the rare earth ions emitting in the near-infrared part of the spectrum, because they are very sensitive to quenching by high-energy