SrAl4O7:Tm3+/Yb3+ nanocrystalline blue phosphor: structural, thermal and optical properties

Strontium aluminate (SrAl4O7) nanophosphor codoped with Tm3+-Yb3+ has been synthesized through the combustion route using urea as the reducing agent. Structural, thermal and optical characterizations have been carried out. Heat treatment of the samples shows a change in the crystallite phases and the relative luminescence intensities for the different bands. The nanocrystalline particles in the as-synthesized sample seem to arrange in rod like shapes of submicrometer length on annealing. A broad (350-550 nm) emission in the UV-green region is observed when 266 nm radiation is used for excitation. Intense upconversion (UC) emissions in blue, red and infrared are seen with excitation by 976 nm radiation. An emission at 364 nm not observed earlier and attributed to D-1(2)-> H-3(6) transition in Tm3+ is also seen. The blue emission from SrAl4O7:Tm3+/Yb3+ codoped nanophosphor (annealed at 1200A degrees C) exhibits high color purity (89%) and is comparable to phosphors used commercially. The energy transfer mechanisms, responsible for these UC emissions, are proposed and discussed.