Importance of Volume Ratio in Photonic Effects of Lanthanide-Doped LaPO₄ Nanocrystals

Experimental variation of the volume ratio (filling factor: i.e., volume of nanoparticles (NPs) compared with that of medium) of nanocomposite materials with doped lanthanide ions demonstrates that it has a significant affect upon local field effects. Lanthanum orthophosphate NPs are doped with Eu³⁺ and/or Tb³⁺ and immersed in organic solvents and lead borate glasses for Tb^{3+ 5}D₄ lifetime measurements. For media with a refractive index (n_med) less than that of LaPO₄ (n_np = 1.79), the ⁵D₄ emission decay rate increases with increasing volume ratio of the NPs, whereas for n_med > 1.79, the decay rate decreases with increasing volume ratio. Fitting with the model of Pukhov provides an estimation of the radiative lifetime of ⁵D₄ and the quantum yield. Energy transfer (ET) from Tb³⁺ to Eu³⁺ occurs in co-doped LaPO₄ NPs with excitation into a Tb³⁺ absorption band. The ET rate is independent on n_med and the energy transfer efficiency decreases with an increase in n_med. The behavior of ET rate with regard to the local field is consistent with the Dexter, but not Förster, equation for ET rate involving the electric dipole–electric dipole mechanism. This has consequences when using the spectroscopic ruler approach to measure distances between donor–acceptor chromophores. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.