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Author: search.filters.author.Devi, L. Lakshmi

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    Photoluminescence characteristics of Ln3+-doped phosphors derived from sustainable resources for solid state lightning applications
    (2022-08-01)
    Reddappa, R.
    ;
    Devi, L. Lakshmi
    ;
    Basavapoornima, Ch
    ;
    Depuru, Shobha Rani
    ;
    Kaewkhao, J.
    Di-calcium silicate (Ca<inf>2</inf>SiO<inf>4</inf>) phosphors doped with series of Ln<sup>3+</sup> ions have been prepared by using high temperature solid-state reaction technique. The required raw chemicals of CaO from egg shell and SiO<inf>2</inf> from rice husk powders have been used instead of chemicals which were derived from non-renewable minerals. The structural, morphological and photoluminescence characteristics were studied through XRD, SEM and FLS 980 spectrometer measurements, respectively. The XRD patterns of all the phosphors are fit well with the established ICDD card No.01–083–0464 with monoclinic crystal structure. Photoluminescence spectra of Ce<sup>3+</sup>, Pr<sup>3+</sup>, Sm<sup>3+</sup>, Eu<sup>3+</sup>, Tb<sup>3+</sup>, Dy<sup>3+</sup> and Er<sup>3+</sup>-doped phosphors exhibit characteristic sharp intense emission peaks at 385 (d-f), 606 (<sup>1</sup>D<inf>2</inf>→<sup>3</sup>H<inf>4</inf>), 600 (<sup>4</sup>G<inf>5/2</inf>→<sup>6</sup>H<inf>7/2</inf>), 703 (<sup>5</sup>D<inf>0</inf>→<sup>7</sup>F<inf>4</inf>), 544 (<sup>5</sup>D<inf>4</inf>→<sup>7</sup>F<inf>5</inf>), 573 (<sup>4</sup>F<inf>9/2</inf>→<sup>6</sup>H<inf>13/2</inf>) and 545 nm (<sup>4</sup>S<inf>3/2</inf>→<sup>4</sup>I<inf>15/2</inf>), respectively. Emission characteristics of developed phosphors such as CIE co-ordinates, correlated colour temperature, colour purity, lifetimes of fluorescent level, etc., of Ln<sup>3+</sup> have been quantified. The decay curves for lower concentrations of Ln<sup>3+</sup> ion exhibit single exponential behaviour and turns into bi-exponential behaviour accompanied with shortening in lifetime for higher concentrations. These results have been discussed with reported values and found to be better than many of the reported phosphors derived from non-renewable minerals. Therefore, Ca<inf>2</inf>SiO<inf>4</inf>:Ln<sup>3+</sup> phosphors could also be useful to design and develop low-cost solid state lightning devices besides effective utilization of renewable agricultural and food waste materials.
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    Luminescence and energy transfer studies of Ce3+/Dy3+ doped fluorophosphate glasses
    (2019-04-01)
    Rao, V. Rajeswara
    ;
    Devi, L. Lakshmi
    ;
    Jayasankar, C. K.
    ;
    Pecharapa, Wisanu
    ;
    Kaewkhao, J.
    The fluorophosphate (PKABfNf:P<inf>2</inf>O<inf>5</inf>+K<inf>2</inf>O+Al<inf>2</inf>O<inf>3</inf>+BaF<inf>2</inf>+NaF<inf>2</inf>) glasses doped with single and co-doped (Ce and/or Dy) were prepared by conventional melt quenching technique and characterized their luminescence properties. The emission spectra of co-doped glass (PKABfNfCeDy) showed a broad blue band of Ce<sup>3+</sup> ions combined with the sharp peaks of Dy<sup>3+</sup> ions. Judd-Ofelt (JO) intensity parameters have been obtained from the optical absorption spectra of 1.0 mol% of Dy<sup>3+</sup> (PKABfNfDy1.0) glass. JO theory has been used to find the radiative properties of PKABfNfDy1.0 glass by using Futchbauer-Ladenbergh formula. The (Ce<sup>3+</sup>/Dy<sup>3+</sup>) co-doped glass exhibit radiative energy transfer (ET) from Ce<sup>3+</sup> to Dy<sup>3+</sup> upon UV excitation and then intensity is increased with increase in Dy<sup>3+</sup> ion concentration. The ET mechanism from Ce<sup>3+</sup> to Dy<sup>3+</sup> ion has been analyzed. The theoretical ET efficiency and quantum yield values were calculated from the emission spectrum of the Ce<sup>3+</sup> and (Ce<sup>3+</sup>/Dy<sup>3+</sup>) co-doped glasses. These results suggest that the co-doped PKABfNfCeDy glasses are promising candidate for commercial white light applications.