CHEM01200604004 Shri Sanyasinaidu Boddu - Homi Bhabha ...

espectively. Value of this integral depends on the extent of overlap between the donor
emission and acceptor absorption profiles. P DA depends strongly on the critical distance
between donors, as well as on their excited state lifetimes. The critical distance is the
minimum distance between donor and acceptor above which no energy transfer between them
is possible. In the SbPO 4 lattice, each SbO 4 polyhedron is separated by PO4 tetrahedron and
the minimum average distance between Sb 3+ ions is around 5.01Å [226]. As only 2.5 at % of
Ce 3+ and 5 atom % of Tb 3+ are randomly distributed at the Sb 3+ site in the lattice the average
3+ 3+
distance between Ce and Tb ions will be significantly higher than 5.01Å thereby resulting
in an increased distance between the donor and acceptor. Hence, the possibility of energy
transfer either by the exchange mechanism or by the multi-polar interactions is ruled out.
Based on these results, it is inferred that a long-range energy migration from different Ce 3+ to
Tb
3+ 3+ 3+
4
ions is taking place in SbPO :Ce , Tb nanomaterials.
The steady state and time resolved luminescence studies described above establish
that covering the nanoribbons/ nanoparticles with silica matrix is very effective in reducing
the surface contribution to luminescence from nanomaterials. Further, these studies also
3+ 3+
confirmed that the energy transfer takes place from Ce to Tb ions, which are occupying
Sb 3+ site in SbPO lattice, a low symmetric environment with a coordination number of four
4
4.4 Bismuth phosphate nanomaterials: Different forms of BiPO 4 nanomaterials were
prepared by adding ammonium dihydrogen phosphate solution to ethylene glycol containing
bismuth nitrate followed by refluxing at various temperature for two hours.
4.4.1 Effect of reaction temperature on the particle size, shape and crystal structure:
Figure 76 shows the XRD patterns Eu 3+ doped BiPO 4 nanomaterials prepared at different
temperatures namely room temperature, 100, 125 and 185°C. For samples prepared at room
temperature, XRD pattern is characteristic of hydrated hexagonal form of BiPO 4 . With
increase in the reaction temperature, it slowly converted to anhydrous hexagonal BiPO at 4
133