Synthesis of Eu3+-doped core and core/shell nanoparticles and direct spectroscopic identification of dopant sites at the surface and in the interior of the particles

A variety of redispersible Eu3+-doped LaPO4 nanoparticles have been prepared in a high-boiling coordinating solvent mixture, and the Eu3+ lattice sites of these materials have been investigated by luminescence line-narrowing measurements. In this spectroscopic method, Eu3+ ions occupying different lattice sites are selectively excited with a tunable narrow-bandwidth laser system and distinguished by their luminescence spectra ("site-selective spectroscopy"). Depending on the concentration of the dopant, up to three different lattice sites could be identified in the interior of the LaPO4 nanoparticles. These sites correspond to those known from bulk LaPO4. In addition, a variety of surface sites is observed, which could be converted completely into bulk sites by overgrowing the nanoparticles with a shell of pure LaPO4. The surface sites are identical to those obtained by reacting Eu3+ with the surface of pure LaPO4 nanoparticles. The spectroscopic properties of Eu3+-doped LaPO4 nanoparticles differ from those of pure EuPO4 nanoparticles, which were also investigated. Remarkably, the core/shell synthesis investigated in this paper allows one to prepare doped nanoparticles that contain no other dopant sites than those known from the corresponding bulk material.