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 Tb3+ 5D4 lifetime measurements. For media with a refractive index (n(med)) less than that of LaPO4 (n(np) = 1.79), the D-5(4) 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 D-5(4) 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 Forster, 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.