Eiichiro Nakazawa

Traps in SrAl2O4:Eu2+ phosphor with rare-earth ion doping

Abstract The depths and relative densities of the traps in a series of SrAl2O4:Eu2+, Ln (14 rare-earths) phosphors are measured with a transient luminescence method. The observed hole activation energies (trap depths) are quite different across the series and related to the ionization potential and 4fn-4fn−15d transition energies of the divalent ions of this series, reflecting the 4f-electron interaction energy of each rare earth ion.

Mechanism of the persistent phosphorescence in Sr4Al14O25:Eu and SrAl2O4:Eu codoped with rare earth ions

Luminescence and phosphorescence properties are investigated in two series of strontium aluminates, Sr4Al14O25:Eu2+ and SrAl2O4:Eu2+, codoped with rare earth (lanthanoid, Ln) ions. Persistent phosphorescence is observed for the phosphors codoped with Pr, Nd, Dy, Ho, and Er. In the phosphorescence process of these phosphors, the excited electron in the Eu2+ ion is transferred between the Eu and the codopant Ln ions through the 5d states of these ions. When the trivalent codopant ion Ln3+ captures the excited electron in the 4f shell, the ion is reduced to the divalent state and, therefore, the energy necessary to thermally relieve the electron from the ion is determined by the energy difference between the 4fn and the 4fn−15d configurations of the divalent Ln ion. The trapping depths (activation energies) of the Ln ions, estimated theoretically on the basis of this mechanism, coincide with the observed depths (0.8±0.2eV) for the persistently phosphorescent phosphors, and those of the nonpersistent Ln ions ...

Lowest 4f-to-5d and Charge-Transfer Transitions of Rare-Earth Ions in LaPO4 and Related Host-Lattices

In order to study the high energy optical transitions i.e., 4f-to-5d (f-d) and charge-transfer (CT) spectra of trivalent rare earth (4fn-series, lanthanide) ions in crystal lattices, the luminescence excitation spectra of the series ions in LaPO4 host lattices are measured in the vacuum ultraviolet (VUV)-ultraviolet (UV) spectral region of 120–350 nm, and the CT-excitation spectra of Y2O2S and La2O2S are measured in a UV region. The f-d and CT bands are observed for the most 4fn-series ions in which the predicted transition energies are lower than the host lattice absorption edge, that is, the f-d bands for Ce3+, Pr3+, Sm3+, Tb3+ and Dy3+, and the CT band for Nd3+, Sm3+, Eu3+, Dy3+, Ho3+, Er3+, Tm3+ and Yb3+. The energies of these observed peaks of f-d and CT bands agree fairly well with the values calculated by the method proposed in our previous report. The relationship between the energies of the CT transition and the absorption edge in various host lattices is discussed.

Charge transfer type luminescence of Yb3+ ions in RPO4 and R2O2S (R=Y, La, and Lu)

Abstract Charge transfer (ct) type luminescence, i.e. the reverse process of the ct absorption, of the lanthanide ions is observed for the first time with Yb3+ ions in several inorganic hosts. From the dependence of the spectra and intensity of the ct luminescence on temperature, a configurational coordinate model is proposed for the ct state of Yb3+ in Y2O2S.

Luminescence of YPO4 : Zr and YPO4 : Zr , Mn under Vacuum Ultraviolet Excitation

Novel UV and blue phosphors, YPO 4 :Zr and YPO 4 :Zr,Mn, respectively, are synthesized and investigated. YPO 4 :Zr shows the emission about 290 nm assigned to the charge transfer transition of O-Zr. The excitation spectrum of the emission has main band peaked at 175 nm. The codoped phosphor YPO 4 :Zr,Mn shows the emission peaked at 477 nm assigned to the d-d transition of Mn 2 + within 3d 5 configuration. The excitation bands of the blue emission are around 152 and 193 nm. YPO 4 :Mn does not show any emission and the 290 nm emission observed for YPO 4 :Zr is diminished for YPO 4 :Zr,Mn. It is suggested that Zr plays an important role in the excitation of the Mn blue emission of YPO 4 :Zr,Mn. The emission wavelength of YPO 4 :Zr,Mn is remarkably short compared with the usual Mn 2 + activated phosphors.

A New Method for the Characterization of Traps in Luminescent Materials

It is shown that measurements of the temperature dependence of the transient intensity of luminescence afterglow sampled at several points in time after the sudden stop of excitation serve as a reproducible and accurate experimental method to investigate traps in luminescent materials which can be superior to the prevailing thermo-luminescence method. An experimental result on Y2O2S activated by Eu3+ is presented as a practical example of this method.

Improvement of Luminescence Degradation of PDP Blue Phosphor with New UV Phosphor

BaMgAl 10 O 17 :Eu 2+ phosphor (BAM:Eu 2+ ) is a useful blue phosphor for plasma display panel (PDP) and fluorescent lamps, but is liable to be degraded by vacuum ultraviolet (VUV) irradiation. We found that YAl 3 (BO 3 ) 4 :Gd 3+ phosphor (YAB:Gd 3+ ) has a high efficiency of emission at 313 nm under VUV excitation and strong resistance to luminescence degradation caused by heat and VUV irradiation. To prevent this degradation, YAB:Gd 3+ is mixed with BAM:Eu 2+ . In this mixed phosphor, the VUV energy absorbed by the former is efficiently transformed to the blue emission of the latter. Consequently, the luminescence degradation inherent in the blue phosphor is decreased with a small sacrifice of the emission intensity.

Vacuum Ultraviolet Excitation Processes of YAl3(BO3)4:R (R = Eu3+, Gd3+ and Tb3+)

The luminescence characteristics of Gd3+, Tb3+ and Eu3+ activated YAl3(BO3)4 phosphors in the vacuum ultraviolet (VUV) and ultraviolet (UV) regions of 130–300 nm are investigated. It is shown that the non activated YAl3(BO3)4 phosphor emits UV light under the VUV excitation, and the emission is related to BO3 atomic-groups. The rate of the energy transfer from the host to the activators is in the order of Eu3+ > Tb3+ > Gd3+.

Trapping states in CaS and SrS single crystals, films and powders as revealed using the technique of transient thermoluminescence

Trapping centres of doped and undoped CaS and SrS phosphors, in single crystal, thin film and powder forms, have been investigated by the transient thermoluminescence technique. Comparison between the forms are made, and conclusions, regarding the possible sources of the trapping centres, are postulated. The thin films specimens are found to have dramatically different trap spectra, to those observed for single crystal and powder forms. Analysis of the trapping centres, that is the determination of the trap depths, was also carried out, using the aforementioned technique, and the subsequent results compared with those reported in the literature employing the technique of thermoluminescence. The result was a favourable match. Most of the dominant trapping states in these compounds seem related to natural defects and not, directly, to the added dopants.

Cooperative optical transitions of Yb3+−Yb3+ and Gd3+−Yb3+ ion pairs in YbPO4 hosts

Abstract Spectra of three kinds of cooperative optical transitions, i.e. cooperative absorption, cooperative luminescence, and Raman luminescence are studied for Yb3+−Yb3+ and Gd3+−Yb3+ pairs in YbPO4 hosts. It is shown that the structure observed in the low temperature spectra is well understood by referring to the crystal field-split levels of the ions in a pair, indicating that the coupling interaction in the pair does not change the energy levels of the ions.