Persistent luminescence mechanism of tantalite phosphors
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Date
2014-11
Authors
Noto, Luyanda Lunga
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
Pr3+ ion doped ZnTa2O6, SrTa2O6, CaTa2O6 and ZnTaGaO5 phosphors, which display persistent
luminescence were prepared by solid state chemical reaction at 1200 oC for 4 hours. A
ZnTa2O6:Pr3+ phosphor that resembled an orthorhombic single phase was obtained, as identified
by X-ray diffraction (XRD). ZnTa2O6:Pr3+ displayed both blue and red emission, with the blue
emission spectral line observed at 448 nm from the 3P0 → 3H4 transition, and the red spectral
lines observed at 608, 619 and 639 nm from the 1D2 → 3H4, 3P0 → 3H6 and 3P0 → 3F2 transitions,
respectively. For different concentrations of Pr3+, a concentration of 0.4 mol% Pr3+ proved
suitable to generate a phosphor displaying only red emission with the Commission Internationale
de l'Eclairage (CIE) coordinates matching those of an ideal red color. Enhancement of the
luminescence intensity of ZnTa2O6:Pr3+ phosphor was achieved by preparing it in the presence of
Li2SO4 and Li2CO3, which act as flux agents. The strong absorption by the defect levels due to
the flux was observed from the diffused reflectance spectra. Pr exists in both Pr3+ and Pr4+
oxidation states as revealed by the X-ray photoelectron spectroscopy data. The presence of Pr3+
increased, while Pr4+ decreased in the samples prepared in the presence of a flux. The increased
absorption by the defect levels and the reduction of Pr4+ in the samples prepared using a flux
resulted in the enhancement of the luminescence intensity as observed from the
photoluminescence spectra. The lifetime of the persistent luminescence of ZnTa2O6:Pr3+ prepared
in a flux was calculated using a second order exponential decay curve from the measured
phosphorescence decay curves. This showed an enhancement in the lifetime of the persistent
luminescence of the fluxed sample, which is attributed to the additional electron trapping centres
induced by the flux as observed from the thermoluminescence glow curves. Additional means of
enhancing the lifetime of the persistent luminescence were achieved by co-doping ZnTa2O6:Pr3+
with Li+, Na+, K+ or Cs+ ions, and by also incorporating gallium ions to form a new host
ZnTaGa5:Pr3+. The scanning electron microscopy (SEM) images showed that particles were of
irregular shape and with different sizes. The preparation with the fluxing material showed and
increased particle sizes. The SEM images of ZnTaGa5:Pr3+ showed a surface morphology that is
composed of particles with different shapes, including the irregular, rhombus and rod shapes.
The distribution of the ions in the material was investigated using the Time of Flight Secondary Ion Mass Spectroscopy (ToF SIMS) surface maps, which showed that the ions were uniformly
distributed throughout the matrix. This showed successful incorporation of the ions. Pr3+ exhibits
prominent red emission in most oxide phosphors, which comes from the 1D2 → 3H4 transition,
and greenish-blue emission from 3P0 → 3H4,5 transitions is normally less intense. However, a
greenish-blue emission was observed from the CaTa2O6:Pr3+ oxide phosphor prepared by solid
state reaction at 1200 oC. A combination of emission coming from 1D2 and 3P0 levels was
observed, with the blue emission from the latter much more prominent. Upon investigating the
thermoluminescence properties of the phosphor, the glow curves showed the presence of three
different types of electron trapping centres. Interesting properties of the trapping centres, such as
the competition between the trapping centres, pre-radiation effects and the calculation of the
activation energy were studied. The phosphorescence decay curves showed long lasting
afterglow. Three SrTa2O6:Pr3 + phosphor samples with persistent emission properties were
prepared by solid state reaction at 1200, 1400 and 1500 oC. The crystal structure formation
improved with an increase in temperature as identified by XRD. The scanning electron
microscopy images showed that the particles of the phosphor were agglomerated and co-melting
was induced by increasing the synthesis temperature. The ion distribution in the phosphors was
determined using the time of flight secondary ion mass spectroscopy. The red emission was
obtained from the 1D2 →3H4 and the 3P0 →3H6 transitions at 608 and 619 nm, respectively. The
main absorption occurred at 225 nm (5.5 eV), and the band gap (Eg) calculations confirmed that
it corresponds to band-to-band excitation. The persistent emission time parameters (260 – 296 s)
were calculated from the phosphorescence decay curves using the second order exponential
decay equation. The corresponding electron trapping centres were identified using the
thermoluminescence spectroscopy, and the activation energy was determined using the initial
rise method.
Description
Keywords
Solid State reaction, Persistent luminescence, Flux, Photoluminescence, Electron trapping centres, Thermoluminescence, Tantalite, Phosphors, Luminescence, Thesis (Ph.D. (Physics))--University of the Free State, 2014