Effect of annealing on the structural, optical and emissive properties of SrWO4:Ln(3+) (Dy(3+), Eu(3+) and Sm(3+)) nanoparticles.

Lanthanide ions, Ln(3+) (Dy(3+), Eu(3+) and Sm(3+)) doped SrWO4 nanoparticles were synthesized using ethylene glycol (EG) as a capping agent as well as reaction medium. The X-ray diffraction (XRD) study reveals that all the Ln(3+) (Dy(3+), Eu(3+) and Sm(3+)) doped samples are well crystalline in nature with a tetragonal scheelite structure of SrWO4 phase. TG study reveals that the nanophosphors are thermally stable. Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy techniques were used to obtain the information about internal and external vibrational modes present in the SrWO4 structure. Optical properties were investigated using UV-vis and photoluminescence (PL) spectroscopy. The average crystallite size was calculated using Debye-Scherrer's for as-prepared and 800°C annealed samples and is found to be in the range of ∼35-70nm. The luminescence intensity of Eu(3+) doped SrWO4 nanoparticles under 364nm excitation wavelength reveals that (5)D0→(7)F2 transition at ∼613nm (red) is more prominent than that of (5)D0→(7)F1 transition at ∼590nm (orange). Also upon excitation by UV radiation, the SrWO4:Dy(3+) phosphor shows the yellow and blue transition lines appearing at ∼572 and 484nm which are the characteristic electronic transitions of (4)F9/2-(6)H13/2 and (4)F9/2-(6)H15/2 emission line of Dy(3+), respectively. Also Sm(3+) doped SrWO4 nanophosphor shows its characteristic emission lines in the range of 550-720nm, corresponding to (4)G5/2→(6)HJ (J=5/2, 7/2, 9/2 and 11/2) transitions of Sm(3+) ions. The predominant orange red color can be attributed to (4)G5/2→(6)H9/2 located at ∼642nm. This is related to the polarizing effect due to the energy transfer from WO4(2-) to the Eu(3+), Dy(3+) and Sm(3+) sites, respectively. Effect of annealing on the photoluminescence properties of samples has been studied and it was found that luminescence intensity increases up to ∼3 times on heating the samples at 800°C. This may be due to reduction in non-radiative decay channels pathways and reduced surface defects associated with the samples. These studies show that these Ln(3+) (Dy(3+), Eu(3+) and Sm(3+)) doped SrWO4 nanophosphors may be used as potential candidates for the advancement in LEDs.