Luminescence properties of Tb ~(3+)-doped CaMoO _4 nanoparticles: Annealing effect, polar medium dispersible, polymer film and core-shell formation

摘要

Tb ~(3+)-doped CaMoO _4 (Tb ~(3+) = 1, 3, 5, 7, 10, 15 and 20 atom%) core and core-shell nanoparticles have been prepared by urea hydrolysis in ethylene glycol (EG) as capping agent as well as reaction medium at low temperature ～150 °C. As-prepared samples were annealed at 500 and 900 °C for 4 h to eliminate unwanted hydrocarbons and/or H _2O present in the sample and to improve crystallinity. The synthesised nanophosphors show tetragonal phase structure. The crystallite size of as-prepared sample is found to be ～18 nm. The luminescence intensity of the ~5D _4 → ~7F _5 transition at 547 nm of Tb ~(3+) is much higher than that of the ~5D _4 → ~7F _6 transition at 492 nm. 900 °C annealed samples show the highest luminescence intensity. The intensity ratio R (I[~5D _4 → ~7F _6]/I[~5D _4 → ~7F _5]) lies between 0.3-0.6 for as-prepared, 500 and 900 °C annealed samples. The luminescence decay of ~5D _4 level under 355 nm excitation shows biexponential behaviour indicating availability of Tb ~(3+) ions on surface and core regions of particle; whereas, contribution of Mo-O charge transfer to lifetime is obtained under 250 nm excitation. The CIE coordinates of as-prepared, 500 and 900 °C annealed 5 atom% Tb ~(3+)-doped CaMoO _4 samples under 250 nm excitation are (0.28, 0.32), (0.22, 0.28) and (0.25, 0.52), respectively. The dispersed particles in polar medium and its polymer film show green light emission. The luminescence intensity is improved significantly after core-shell formation due to extent of decrease of non-radiative rates arising from surface dangling bonds and capping agent. Quantum yields of as-prepared samples of 1, 5 and 7 atom% Tb ~(3+)-doped CaMoO _4 samples are found to be 10, 3 and 2, respectively.