Selective excitation of Eu³⁺ in the core of small β-NaGdF₄ nanocrystals

The influence of the nanocrystal matrix on the optical properties of lanthanide dopants is investigated with europium ions used as local crystal field probes. The analysis is performed on small NaYF₄ and NaGdF ₄ nanocrystals obtained by the thermolysis of the corresponding metal trifluoroacetates. An important role in the synthesis is played by trioctylphosphine oxide which induces the crystallization of nanocrystals with small diameters (∼5 to 6 nm). In such small particles, the energy transfer from gadolinium to europium ions is studied with photoluminescence, photoluminescence excitation and time-resolved experiments. We demonstrate that excited gadolinium ions efficiently transfer their energy to europium, and their photoluminescence spectra depend on the nanocrystal size. This is contrary to the direct excitation of Eu³⁺ ions, which produces size-dependent emission corresponding to the surface to volume ratio of europium sites. Finally, we propose that Gd³⁺ ions transfer their energy mainly to the Eu³⁺ in the core of the nanocrystals. These observations provide a base for the optically controlled emission from only the core of the nanocrystals.