Study of Concentration Quenching and Energy Transfer Mechanism in Ce Doped Y2O3 Nanomaterials.

We study concentration quenching and energy transfer mechanisms of yttrium oxide (Y2O3) nanomaterials doped with different concentrations (0-5 mol%) of cerium (Ce). Photoluminescence (PL) spectra recorded under an excitation wavelength of 350 nm show a broad emission band at ∼ 406 nm and a feeble emission band at ∼ 463 nm in the undoped Y2O3 sample. The doping of Ce in Y2O3 induced multiple PL peaks within the blue-green region of the spectrum in all the doped samples with the peak at ∼ 466 nm being notably the prominent one. This prominent emission band exhibits a decrease in intensity with increasing Ce concentration due to concentration quenching. Analysis of Time-resolved photoluminescence (TRPL) spectra reveal that the average emission lifetime of Ce-doped Y2O3 is shorter than that of the undoped Y2O3 sample. The concentration quenching effect and the decrease of average emission lifetime of the dominant emission band are explained on the basis of energy transfer from the host Y2O3 to the Ce3+ ion centres. The critical quenching concentration of Ce3+ ion in Y2O3:Ce phosphor was identified to be 1 mol% and the critical transfer distance was estimated to be 23.74 Å. Analysis reveal that the concentration quenching mechanism involves nearest-neighbour interaction.

Optical properties of blue-light-emitting Y2 O3 :Ce nanophosphor for solid-state lighting application.

The structural, surface morphological, optical absorption and emission features of Y2 O3 :Ce (0%-5%) were studied. The samples had a body-centred cubic crystal structure. The undoped sample had a crystallite size of 29.03 nm, and it varied after doping with Ce. The grain size of the samples varied from 23.00 to 50.78 nm. All the samples exhibited a strong absorption band at 206 nm due to F-centre absorption and absorption involving the delocalised bands. In addition, the doped samples exhibited a secondary band at ~250 nm due to 4f → 5d transitions of Ce3+ ions. The optical bandgap of the undoped sample was found to be ~5.37 eV, and it decreased to 5.20 eV with an increase in Ce concentration to 5%. The undoped sample under 350-nm excitation exhibited a broad photoluminescence (PL) emission band with the maxima at 406 nm and a secondary band at 463 nm. In contrast, multiple PL peaks were centred at ~397, 436, 466, 488 and 563 nm in all the doped samples. The average lifetime of the emission band at 406 nm was 1.05 ns and that of the emission band at ~466 nm was 1.63 ns. The material has potential for solid-state lighting applications.

Temperature assisted radiative and non-radiative recombination mechanisms in sillimanite (Al2SiO5) mineral.

Temperature assisted luminescence in sillimanite (Al2SiO5) mineral was studied using thermoluminescence (TL). TL characteristics were studied in un-annealed and different annealed samples. Analysis showed that in the un-annealed sample, there was four electron trapping sites at depths ~0.56, 0.87, 1.08, 1.32eV and a hole trapping site at depth ~3.63eV from the conduction band acting as a recombination center. Further analysis on the annealed samples showed that the 0.56eV trapping site was a pressure induced surface trap and it disappeared after annealing. However, the other trapping and recombination sites were found to be stable under thermal treatment. Due to this trap distribution, three partially overlapping glow peaks were observed. The glow peaks were found to be affected by thermal quenching. The thermal quenching parameters were evaluated from the composite glow curves by using Computerized Resolved Peak (CRP) technique. The activation energies for thermal quenching (W) estimated from the three peaks were found to be ~0.69±0.05, 0.92±0.06 and 1.15±0.03eV respectively and the pre-exponential factors (C) were ~1.12×108, 2.65×1010 and 9.23×1011 respectively. Based on the analysis, a band model was proposed and the whole radiative and non-radiative recombination mechanisms were discussed.

Dosimetric feature of natural biotite mineral.

A thermoluminescence (TL) study relevant to radiation dosimetry has been carried out for X-ray irradiated biotite mineral under un-annealed and different annealed (473, 573, 673 and 773 K) conditions. Some significant variations in dosimetric characteristics have been observed with annealing treatment. Due to generation of an additional shallow trap level at depth 0.78 eV in 673 and 773 K annealed samples, the dose response is found to improve. For the 773 K annealed sample, a linear dose response has been observed from 10 to 1100 mGy. The fading is ∼13% within 5 d after irradiation and onward it reduces to 7% up to 60 d. Reproducibility of this (773 K) sample is excellent. After 10 recycles the coefficient of variations in the results for the 60, 180 and 1000 mGy dose-irradiated samples are found to be 0.97, 1.31 and 1.03%, respectively. The potential use of biotite as a natural X-ray dosemeter is discussed.

Thermoluminescence study of X-ray and UV irradiated natural calcite and analysis of its trap and recombination level.

Thermoluminescence (TL) of natural light-orange color calcite (CaCO3) mineral in micro-grain powder form was studied at room temperature X-ray and UV irradiation under various irradiation times. TL was recorded in linear heating rate (2 K/s) from room temperature (300 K) to 523 K. Trapping parameters such as activation energy, order of kinetics, frequency factor have been evaluated by Computerized Glow Curve Deconvolution technique. Three electron trap centers had been estimated at depth 0.70, 1.30 and 1.49 eV from the conduction band. Investigation of emission spectra recorded at various temperatures showed single recombination center at depth 2.74 eV from the conduction band. Due to thermally assisted tunneling of electron and subsequent center-to-center recombination, a distinct peak of lower activation energy (0.60 eV) was observed at relatively higher temperature (~360 K) for X-ray irradiated sample. In UV excitation, there was an indication of photo-transfer phenomenon, where low TL intensity might have been observed; but due to simultaneous excitation of electrons from valence band to the trap level, TL intensity was found to increase with UV irradiation time. The results obtained within temperature range 300-523 K were explained by considering a band diagram.