The inefficient luminescence performance
of Ce3+ activated
glasses is primarily responsible for their commercial failure compared
to the Ce3+ activated crystalline materials that are widely
used as phosphors and scintillators. We observed that this behavior
is explicitly related to the intrinsic characteristics of the host
material. Here, we present a systematic study on Ce3+ luminescence
in amorphous borate glass and make a comparison with the well-known
polycrystalline Y3Al5O12:Ce3+ (YAG:Ce) phosphor. In borate glass, Ce3+ exhibits blue
luminescence with quantum yield (QY) of about 42%, whereas the QY
is more than 85% in YAG:Ce ceramic that exhibits yellow luminescence.
This typical behavior has been discussed in terms of the site rigidity
of dopant ions in the glassy and crystalline hosts, and its influence
on the Ce3+ 5d
j
states’
crystal field splitting, Stokes shift, and the centroid shift, as
well as the probability of thermal ionization, host’s intrinsic
absorption, and the influence of Ce4+ impurity presence
in the respective host materials. This study gives a quantitative
understanding of host’s contribution on dopant’s luminescence
properties and thereby provides an optimization guideline, which is
highly demanding for the design of novel luminescent materials.
Recent research on white light LED (w-LED) phosphors has focused on narrow-band green and red luminescent materials to improve the efficacy of w-LEDs and to widen the color gamut of w-LED-based displays. Mn 2+ is a promising emitter capable of narrow-band emission, either green or red, depending on the local coordination. However, the extremely low absorption coefficients for the spin-and parity-forbidden d−d transitions in Mn 2+ form a serious drawback and require addition of a sensitizer ion such as Ce 3+ or Eu 2+ , with strong absorption in the blue. The performance of the codoped phosphor then critically depends on efficient energy transfer. Despite extensive research, a clear understanding of the Eu 2+ → Mn 2+ and Ce 3+ → Mn 2+ transfer mechanism is lacking. Typically, Dexter exchange interaction or electric dipole−quadrupole coupling are considered. Here we investigate Eu 2+ → Mn 2+ energy transfer in Ba 2 MgSi 2 O 7 and show that the most probable mechanism is exchange interaction with fast (nanoseconds) energy transfer from Eu 2+ to nearest-neighbor Mn 2+ and much slower (>100 ns) transfer to next-nearest neighbors, as expected for exchange interaction. We critically evaluate previous studies where the assignment of dipole−quadrupole interaction was erroneously based on C Mn 8/3 concentration dependence of energy transfer efficiencies. Preferential Eu 2+ −Mn 2+ pair formation is suggested as a mechanism that enhances energy transfer efficiencies.
We developed a persistent phosphor of Y3Al2Ga3O12 doped with Nd3+, Ce3+, Cr3+ ions (YAGG:Nd-Ce-Cr) exhibiting long (>10 h) persistent luminescence at multi-wavelengths of around 880, 1064, and 1335 nm due to f-f transitions of Nd3+ and at 505 nm due to Ce3+:5d1→4f transition. The intense near-infrared (NIR) persistent luminescence bands from Nd3+ match well with the first (650–950 nm) and second (1000–1400 nm) bio-imaging windows. The NIR persistent radiance of the YAGG:Nd-Ce-Cr phosphor (0.33 × 10−1 mW/Sr/m2) at 60 min after ceasing blue light illumination was over 2 times higher than that of the widely used ZnGa2O4:Cr3+ red persistent phosphor (0.15 × 10−1 mW/Sr/m2).
Europium doped glass-ceramics containing BaF(2) nano-crystals have been prepared by using the controlled crystallization of melt-quenched glasses. X-ray diffraction and transmission electron microscopy have confirmed the presence of cubic BaF(2) nano-crystalline phase in glass matrix in the ceramized samples. Incorporation of rare earth ions into the formed crystalline phase having low phonon energy of 346 cm(-1) has been demonstrated from the emission spectra of Eu(3+) ions showing the transitions from upper excitation states (5)D(J) (J = 1, 2, and 3) to ground states for the glass-ceramics samples. The presence of divalent europium ions in glass and glass-ceramics samples is confirmed from the dominant blue emission corresponding to its 5d-4f transition under an excitation of 300 nm. Increase in the reduction of trivalent europium (Eu(3+)) ions to divalent (Eu(2+)) with the extent of ceramization is explained by charge compensation model based on substitution defect mechanisms. Further, the phenomenon of energy transfer from Eu(2+) to Eu(3+) ion by radiative trapping or re-absorption is evidenced which increases with the degree of ceramization. For the first time, the reduction of Eu(3+) to Eu(2+) under normal air atmospheric condition has been observed in a BaF(2) containing oxyfluoride glass-ceramics system.
Transparent glass–ceramics containing Pr3+:BaF2 nanocrystals in the chemical composition of SiO2–BaF2–K2CO3–La2O3–Sb2O3 oxyfluoride glass systems have been prepared from melt quenching and with a subsequent heat‐treatment method. The luminescence and structural properties of these materials have been evaluated and the results are reported. Rietveld analysis of X‐ray diffraction patterns and investigation of transmission electron microscopy confirmed the presence of BaF2 nanocrystals dispersed in the heat‐treated glass matrices. Measured UV‐Vis‐NIR absorption spectra exhibited nine bands of the transitions 3H4→3P2, (1I6, 3P1), 3P0, 1D2, 1G4, 3F3, 3F2, 3H6, and 3H5 from all the samples with nondegenerated 1I6 and 3P1 levels in the glass–ceramics. The photoluminescence spectra show an enhancement in the intensities upon ceramization, indicating the incorporation of Pr3+ ions into BaF2 nanocrystals that possess a low phonon energy (346 cm−1). This has further been corroborated from the observation of a significant threefold increase in the relative intensity ratio of blue (3P0→3H4) to red (1D2→3H4, 3P0→3H6) emissions from glass–ceramics compared with the glass. This is due to a significant decrease of multiphonon nonradiative relaxation from the 3P0 to the 1D2 level of Pr3+ in glass–ceramics. Time‐resolved spectra exhibit 3P0‐level decays faster than the 1D2 level.
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