CHEM01200604004 Shri Sanyasinaidu Boddu - Homi Bhabha ...

charge transfer transition and sharp peaks to f-f transitions of Eu 3+ ion present in the lattice.
Eu 3+ doped CaWO 4 nanoparticles showed highest luminescence intensity as compared to
Eu 3+ doped SrWO 4 and BaWO 4 . Hence, detailed studies were carried out on luminescence
properties of undoped and different lanthanide doped CaWO 4 nanoparticles.
6.4. Luminescence studies on un-doped CaWO 4 nanoparticles: Emission spectrum
observed from undoped CaWO 4 nanoparticles after exciting with 253 nm light is shown in
Fig.109 (a). The spectrum is characterized by a broad band extended over a region of 325 to
600 nm with maximum ~425 nm. A photograph of the emission (blue) observed from the
sample dispersed in methanol after shining with 253 nm light is shown in Fig.109 (b).
Excitation spectrum from the sample corresponding to 425 nm emission is shown in Fig.109
(c). The spectrum consists of a broad peak centered at 253 nm and arises from the charge
transfer transitions within the WO 4
2-
tetrahedra of CaWO 4 [69]. From the view point of
molecular orbital theory, the excitation and emission from CaWO 4 nanoparticles can be
attributed to transitions involving 1 A 1 ground state and 1 B ( 1 T 2 ) excited state of WO 2- 4 ion.
Excited state [ 1 B ( 1 T 2 )] decay profile of the WO 2- 4 ion monitored at 425 nm emission is
shown in Fig.109 (d). The decay is found to be bi-exponential with lifetime components 0.5
μs (25%) and 2.6 µs (75%). The shorter component has been assigned to the emission
originating from the surface tungstate groups and longer component to the emission
originating from bulk tungstate groups. Stabilizing ligands on the surface quenches the
excited state of WO 4
2-
structural units and thereby leading to reduced lifetime of the
corresponding excited state. In order to check whether the emission due to WO 2- 4 group is
sensitive to the particle size of CaWO 4 , as prepared samples were heated at different
temperatures and their structural and luminescent properties were investigated. The results
are described below.
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