Europium-doped LaF3 nanocrystals with organic 9-oxidophenalenone capping ligands that display visible light excitable steady-state blue and time-delayed red emission

Visible light excitable and color tunable similar to 5% Eu3+-doped LaF3 nanocrystals (NCs), containing 9-oxido-phenalenone ligands bound to the surface as visible light sensitizers for Eu3+ dopants, have been synthesized by a facile solution-based method. The crystalline phase structure, size, composition, morphology and luminescence properties of the NCs are characterized using X-ray diffraction, high-resolution transmission electron microscopy, energy dispersive X-ray spectroscopy, and steady-state and time-resolved emission spectroscopy. The results show that these NCs are very small in size (<10 nm), display high degree of crystallinity and have pure tysonite structure of LaF3 with P<(3)over bar>c1 space group. The visible light excitation of the capping ligands triggers an alternate display of steady-state, short-lived blue emission (tau < 1 ns) and time-delayed, long-lived sensitized red Eu3+ emission (tau = 0.41 ms), allowing photoluminescence chromacity tuning as a function of delay time within a specific inorganic composition. The visible light sensitization of the dopant Eu3+ sites proves more efficient than direct excitation of 5% Eu3+-doped LaF3 NCs capped by citrate ligands. The dopant Eu3+ ions are well protected from non-radiative deactivation through high-energy vibrations of the organic capping ligands which is proved by the long lifetime of the sensitized Eu3+ emission. The time-resolved emission spectra collected over a period of several milliseconds reveal that the dopant Eu3+ ions occupy at least three different sites in the NC host. It is further inferred that the sensitized Eu3+ emission primarily comes from surface dopant sites and sites just underneath the surface of the NCs. We propose that some of the interior Eu3+ sites also display sensitized emission, which are indirectly populated via Eu3+-> Eu3+ energy migration from surface-sensitized Eu3+ sites of the NCs.