Charge Transfer Luminescence of Yb3+

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182L. van Pieterson et al. / Journal of Luminescence 91 (2000) 177–193Table 1Positions of absorption edges and emission bands in the orthophosphatehost lattices REPO 4 (RE=Sc, Lu, Y, La)data obtained for the orthophosphate host latticesare summarized and Table 2 summarizes theresults of the orthophosphates doped with Yb 3+ .In the case of LaPO 4 :Yb 3+ no evidence for Yb 3+charge transfer luminescence could be obtained,indicating that the charge transfer luminescence isquenched already at the lowest temperatures inthis host lattice.3.2. OxidesAbsorption edge (nm)Emission (nm)ScPO 4 168 211, 350, 470LuPO 4 140 Not measuredYPO 4 144 233, 440LaPO 4 153 262, 328Fig. 5 shows the reflection spectrum of LiScO 2and LiScO 2 doped with 3% Yb 3+ . The host latticeabsorption edge is observed at 187 nm. In theYb 3+ -doped sample, an extra absorption isobserved with a maximum at 209 nm. Thisabsorption band is assigned to the charge transferband of Yb 3+ .Fig. 6 shows the emission and excitation spectraof the LiScO 2 host lattice and LiScO 2 doped with3% Yb 3+ . The emission spectrum of the hostlattice shows one broad band at 250 nm whenexciting at 180 nm. In the excitation spectrum theabsorption edge is observed at 186 nm. Whenexciting at longer wavelengths, defect emission isobserved between 350 and 450 nm.In the emission spectrum of LiScO 2 :Yb 3+ twobroad bands are observed at 312 and 448 nmwhich are not present in the spectrum of theundoped lattice. The energy separation betweenthese bands is about 10 000 cm 1 . The FWHM ofthe bands is 6000 cm 1 and the Stokes shift is16 000 cm 1 . At 972 nm the transition between the4f states of Yb 3+ is observed. In the excitationspectrum of LiScO 2 :Yb 3+ the host lattice absorptionedge is observed at 188 nm. At longerwavelengths an extra band is observed with theonset at 225 nm and a maximum at 208 nm, whichcan be assigned to the charge transfer band ofYb 3+ .The decay time of the charge transfer luminescenceis 190 25 ns and the quenching temperatureis about 180 K in LiScO 2 . Table 3 summarizesthe results for the NaREO 2 and LiREO 2 hostlattices. Data on NaYO 2 and LiYO 2 host latticesare lacking due to problems with synthesis. Table 4shows charge transfer luminescence data obtainedfor NaREO 2 and LiREO 2 lattices doped withYb 3+ . In NaLaO 2 and LiLaO 2 no Yb 3+ chargetransfer luminescence was observed, indicating thatFig. 5. Reflection spectrum of LiScO 2 (broken line) and LiScO 2doped with 3% Yb 3+ (solid line) at 10 K.Table 2Positions of the charge transfer emission and absorption bands for Yb 3+ in orthophosphate host lattices. For decay time ðtÞ measurements(at 10 K) and determination of the quenching temperature ðT q Þ, the CT ! 2 F 7/2 emission is monitoredAbsorption max. (nm) Emission (nm) Stokes shift (cm 1 ) tðnsÞ T q ðKÞScPO 4 195 270, 370 14 500 163 225LuPO 4 210 290, 440 15 000 175 250YPO 4 210 304, 445 15 000 120 290LaPO 4 228 } } } }
L. van Pieterson et al. / Journal of Luminescence 91 (2000) 177–193 183Table 3Positions of absorption edges and emission bands in NaREO 2(RE=Sc, La) and LiREO 2 (RE=Sc, La) host latticesAbsorption edge (nm)Emission (nm)NaScO 2 189 400 (very broad)NaLaO 2 242 Not measuredLiScO 2 188 250, 350–450LiLaO 2 229 400Fig. 6. Emission and excitation spectra of the LiScO 2 host lattice(a, b) and LiScO 2 doped with 3% Yb 3+ (c, d), T ¼ 10 K. Thehost lattice emission spectrum was recorded for excitation at180 nm, the excitation spectrum was measured for the 272 nmemission. The emission spectrum of the Yb 3+ -doped sample wasrecorded for excitation at 200 nm, while the excitation spectrumwas measured for the 300 nm emission. In Fig. 6 (c), the brokenline represents measurements performed with a SEYA monochromator,the solid line represents measurements performed witha SPEX monochromator equipped with a Tektronics ccd-array.the CT luminescence is quenched in these hostlattices.The reflection spectra of Y 2 O 3 and Y 2 O 3 dopedwith 3% Yb 3+ are shown in Fig. 7. The hostlattice absorption edge is observed at 208 nm, inagreement with results by Tomiki et al., whoobserved the exciton creation at 5.88 eV (210 nm)[17]. In the Yb 3+ -doped Y 2 O 3 the charge transferabsorption band is observed at 227 nm.Fig. 8 shows the emission and excitation spectraof the Y 2 O 3 host lattice and Y 2 O 3 :Yb 3+ . Hostlattice emission is observed at 370 nm. In theexcitation spectrum, the host lattice excitationedge is observed at 206 nm.The emission spectrum of Y 2 O 3 doped with 3%Yb 3+ shows two broad bands with maxima at 368and 533 nm. The energy separation between thesebands is 8500 cm 1 , which is less than the energyseparation of 10 000 cm 1 between the 2 F 5/2 and2 F 7/2 states of Yb 3+ . This may be caused byunderlying host lattice defect emission and by thefact that the emission spectra are not corrected.The FWHM of the bands is 7000 cm 1 . At 995 nm,the transition between the 2 F 5/2 and 2 F 7/2 state canbe observed. In the excitation spectrum, an extraband is observed with a maximum at 228 nm, ingood agreement with the position of the chargetransfer band in the reflection spectrum. TheStokes shift is 15 000 cm 1 . In Table 5, data onluminescence properties of Yb 3+ -doped and undopedoxide lattices are summarized.In addition to the oxides discussed above, welooked for Yb 3+ CT luminescence in LaYO 3 .Inthis lattice the host lattice absorption edge is observedat 206 nm. No indication for CT luminescencefrom Yb 3+ was found, not even at thelowest temperatures.
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Charge Transfer Luminescence of Yb3+

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