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The aim of this research is to study photoluminescent properties and particularly energy transfer between Sm 3+ ions in LaF 3 nanocrystals because the energy transfer process has a significant effect on the luminescence efficiency and lifetime. Sm 3+-doped LaF 3 nanocrystals with 0.1, 0.2, 0.3, 1.0, 2.0, 3.0, 4.0 and 5.0 mol% Sm 3+ have been prepared by hydrothermal method. The obtained nanocrystals were characterized by X-ray diffraction, transmission electron microscopy, photoluminescence and luminescence decay measurement. The results showed that the LaF 3 :Sm 3+ nanocrystals possess hexagonal structure with 3 1 space group. The room temperature photoluminescence and photoluminescence excitation spectra of LaF 3 :Sm 3+ were investigated in detail and interpreted by optical intra-configurational f-f transitions within Sm 3+ ions. When Sm 3+ ion concentration in the nanocrystals is increased, the excitation energy is transferred from the "bulk" Sm 3+ ions to the surface Sm 3+ ions followed by non-radiative recombination at centers at the surface of the nanocrystals. The photoluminescence decay curves of 593 nm peak in the LaF 3 nanocrystals doped with 1.0-5.0 mol% Sm 3+ were best fitted to the Inokuti-Hirayama model with the dominant dipole-quadrupole interaction (S = 8). The values of fitting parameters for the energy transfer process were determined.

The colloidal ZnS:Mn nanoparticles were obtained by using thermochemical method with the reaction duration varied from 1 h to 7 h. The bandgap of the ZnS:Mn nanocrystals increases from 3.75 eV to 4.06 eV with decreasing particle size from 5.8 nm to 2.4 nm, respectively. At room temperature, the ZnS:Mn nanoparticles exhibit orangered emission due to the 4 T1(4 G) → 6 A1(6 S) transition within the 3d 5 configuration of Mn 2+ ions inside the ZnS host. The orange-red photoluminescence can be excited at energies corresponding to the Mn 2+ ion own excited states. The 6 A1(6 S)→ 4 E2(4 P); 6 A1(6 S)→ 4 T2(4 P); 6 A1(6 S)→ 4 E(4 G), 4 A1(4 G); 6 A1(6 S)→ 4 T2(4 G), and 6 A1(6 S)→ 4 T1(4 G) absorption transitions have been observed in the photoluminescence excitation spectra. Photocatalytic properties of the samples have been investigated via absorption spectra of methyl orange (MO) in present of ZnS:Mn nanoparticles. The photo-degradation of MO obeys the pseudo-first-order kinetics law. The reaction rate constant k for MO degradation was 0.0065 min −1 .

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