Structural and optical characterization of Eu and Dy doped CaWO4 nanoparticles for white light emission.

The structural and light emission properties of calcium tungstate (CaWO 4) nanoparticles containing Eu3+ and Dy3+ are studied. CaWO 4 nanoparticles were prepared by chemical routes followed by drying at 80°C, doping with 1, 3, 5, 7 and 10 mol% of Eu 2 O 3 and/or Dy 2 O 3 and final annealing at 800oC. The samples were characterized by field emission-scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), synchrotron X-ray diffraction (XRD), Raman and photoluminescence (PL) spectroscopy. FE-SEM studies show that CaWO 4 sample dried at 80°C contains spherical particles of diameter ∼5 μm, and upon annealing at 800°C, dense and irregular shaped structures are formed. EDS mapping found a uniform distribution of Eu and Dy ions and only a small segregation in the CaWO 4 matrix is observed. XRD studies revealed the co-existence of the tetragonal CaWO 4 and cubic Dy 2 O 3 and/or Eu 2 O 3 phases in the doped samples and rule out the replacement of Ca2+ by rare-earth ions. The structural parameters: crystallite size, lattice strain, unit cell dimensions, atomic position coordinates, bond-lengths, bond-angles and cation-oxygen coordination numbers were determined. The short-range structure of CaWO 4 consists of interconnected CaO 8 and WO 4 units. All the W–O bonds in WO 4 units have the same length whereas two types of Ca–O bond-lengths exist in CaO 8 units. The undoped CaWO 4 sample annealed at 800°C, has the smallest Ca–O bond-lengths and unit cell dimensions due to the compressive macrostrain induced by heat-treatment. On incorporating Eu3+ and Dy3+, the W–O and Ca–O bond-lengths increase slightly. Raman spectra of all the samples are similar and show W–O vibrational modes. PL studies found that the undoped CaWO 4 , Eu and Dy-doped CaWO 4 samples emit blue, red and yellow light respectively. The co-doped sample: 2 mol% Eu 2 O 3 –5 mol% Dy 2 O 3 –CaWO 4 was closest to the ideal white light emission properties. • Structure-property correlations in Eu 2 O 3 /Dy 2 O 3 and CaWO 4 composite samples. • Eu and Dy ions expand CaWO 4 lattice. • 3-Mol% Eu 2 O 3 gives maximum luminescence in CaWO 4. • Luminescence intensity decreases with increase in Dy 2 O 3 from 1 to 10 mol%. • Eu and Dy co-doping in CaWO 4 produces white light emission. [ABSTRACT FROM AUTHOR]