Charge Transfer Luminescence of Yb3+

184L. van Pieterson et al. / Journal of Luminescence 91 (2000) 177–193Table 4Positions of the charge transfer emission and absorption bands for Yb 3+ in NaREO 2 and LiREO 2 host lattices. For decay time (t)measurements (at 10 K) and determination of the quenching temperature ðT q Þ, the CT ! 2 F 7/2 emission is monitoredAbsorption (nm) Emission (nm) Stokes shift (cm 1 ) tðnsÞ T q ðKÞNaScO 2 208 322, 431 15 000 130 225NaLaO 2 262 } } } }LiScO 2 206 312, 448 16 000 190 180LiYO 2 214 355, 495 16 500 55 125LiLaO 2 252 } } } }Fig. 7. Reflection spectra of Y 2 O 3 (broken line) and Y 2 O 3doped with 3% Yb 3+ (solid line), measured at 10 K.3.3. AluminatesThe reflection spectra of the Y 3 Al 5 O 12 (YAG)host lattice and YAG doped with 2% Yb 3+ areshown in Fig. 9. At 178 nm the host latticeabsorption edge is observed. When Yb 3+ is incorporatedin the lattice, an extra absorption band isobserved at 209 nm, which is assigned to thetransition from the 2 F 7/2 ground state of Yb 3+ tothe charge transfer state.Fig. 10 shows the emission and excitationspectra of YAG and YAG : Yb 3+ . In the emissionspectrum of undoped YAG, host lattice luminescenceis observed at 256 nm. When exciting atlonger wavelengths defect emission around 450 nmis observed. In the Yb 3+ -doped samples strongemission bands are observed at 334 and 493 nmwhich are not present in the emission spectrum ofthe undoped sample. The energy separationbetween the bands is close to 10 000 cm 1 . Basedon these observations, the bands are assigned toCT luminescence from Yb 3+ . In the excitationspectrum of the 343 nm emission, the CT excitationband is observed at 210 nm, in agreement withthe position in the reflection spectrum. TheStokes shift of the charge transfer transition issome 17 500 cm 1 and the FWHM of both bandsis 6000 cm 1 . The decay time measured for theYb 3+ charge transfer luminescence is 30 5ns at10 K, much shorter than the decay times which aretypically observed for the Yb 3+ CT emission100–200 ns) at low temperatures. Probably, thecharge transfer luminescence is already partlyquenched at very low temperatures. The relativelyhigh intensity of the 2 F 5/2 ! 2 F 7/2 transitioncompared to the charge transfer luminescence alsosuggests that the CT emission is already partlyquenched at 10 K.In Fig. 11, the CT luminescence intensity and itsdecay time are given as a function of temperature.The immediate decrease in intensity in the lowtemperatureregion confirms that the luminescenceTable 5Positions of emission and absorption bands for undoped andYb 3+ -doped oxides RE 2 O 3 (RE=Sc, Y). For the undopedoxides, the absorption edge is given, for the Yb 3+ -doped oxidesthe CT absorption maximum is given. For decay time ðtÞmeasurements and determination of the quenching temperatureðT q Þ, the CT ! 2 F 7/2 emission is monitoredAbsorption(nm)Emission(nm)Stokesshift(cm 1 )tðnsÞ T q (K)Sc 2 O 3 196 Not measuredY 2 O 3 208 370Sc 2 O 3 :Yb 3+ 225 367, 485 14 000 13 530(very weak)Y 2 O 3 :Yb 3+ 227 368, 533 15 500 72 130