Charge Transfer Luminescence of Yb3+

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180L. van Pieterson et al. / Journal of Luminescence 91 (2000) 177–193excitation measurements [16]. The spectral resolutionof this setup is about 0.5 nm. Excitationspectra measured on this apparatus were correctedfor the lamp intensity using excitation spectra ofsodium salysilate emission as a reference. Emissionspectra were corrected for the spectral responseof the monochromator and the detector usingcorrection spectra provided by the manufacturer.Temperature-dependent measurements could beperformed between 5 and 850 K. In the lowtemperatureregion a cold finger cryostat was used.For measurements at temperatures higher thanroom temperature a home-made high-temperaturesample holder was used.3. ResultsTo distinguish between Yb 3+ charge transferluminescence and luminescence of the host lattice,it is necessary to have a good knowledge abouthost lattice luminescence. Luminescence bandsobserved in the spectra of Yb 3+ -doped lattices,which are not observed in the spectra of theundoped samples, can be assigned to Yb 3+ . Forthis reason, both undoped samples and Yb 3+ -doped samples were prepared and measured. Acareful analysis of the reflection and luminescencespectra of the doped and undoped samplesprovides information on the CT luminescence ofYb 3+ in the different host lattices.3.1. PhosphatesFig. 2 shows the reflection spectra of ScPO 4 andScPO 4 doped with 3% Yb 3+ . The host latticeabsorption edge is observed at 168 nm. In thispaper the absorption edge is defined as the lowestenergy at which the absorption reaches its maximumvalue (see arrow in Fig. 2). The reflectionspectrum of ScPO 4 doped with 3 mol% of Yb 3+shows the same absorption bands as the undopedsample and in addition a strong absorption bandwith a maximum at 195 nm. This extra absorptioncan be assigned to the 2 F 7/2 ! CT transition of theYb 3+ ion in ScPO 4 . At wavelengths longer than200 nm the signal decreases slowly. The slowFig. 2. Reflection spectrum of the ScPO 4 host lattice (brokenline) and ScPO 4 doped with 3% Yb 3+ (solid line), measuredat 10 K.decrease may be related to the very low intensityof the synchrotron in this wavelength region whichintroduces relatively large errors in the correctedspectrum.Fig. 3 shows the emission and excitation spectraof ScPO 4 and ScPO 4 doped with 1% Yb 3+ . Theemission spectrum of the ScPO 4 host lattice showsseveral broad bands with maxima at 211, 350 and470 nm. The emission at 211 nm can only beexcited with radiation of energies higher than theband gap (l5168 nm). The other emission bandscan also be excited at wavelengths longer than168 nm and are attributed to defect centers in theScPO 4 lattice.If we compare the emission spectrum of ScPO 4doped with Yb 3+ with the spectrum of the hostlattice, two strong extra bands are observed withmaxima at 270 and 370 nm. These bands areassigned to transitions from the charge transferstate to the 2 F 7/2 and 2 F 5/2 state of Yb 3+ . Theenergy separation between the two emission bandsis 10 000 cm 1 , in agreement with the separationbetween the 2 F 5/2 and 2 F 7/2 states of Yb 3+ . Thefull-width at half-maximum (FWHM) of thebands is 6 000 cm 1 . The Stokes shift of the CT! 2 F 7/2 emission is 14 500 cm 1 . The Stokes shiftis the energy difference between the maximum ofthe CT absorption band and the CT ! 2 F 7/2emission band. The CT ! 2 F 7/2 emission band isoften located in a spectral region (250–400 nm)where both SEYA monochromator with detectorand SPEX monochromator with ccd-array have alow sensitivity. For this reason, in this paper theStokes shift is calculated from the position of theCT ! 2 F 5/2 emission minus 10 000 cm 1 , this
L. van Pieterson et al. / Journal of Luminescence 91 (2000) 177–193 181emission is in most cases observed in a spectralregion where the SPEX/ccd sensitivity curve isrelatively flat. Emission due to the transitionbetween the two 4f states can be observed assharp peaks in the infrared at 979 nm.In the excitation spectrum of the Yb 3+ emissionat 270 nm, the host lattice absorption edge isobserved at 168 nm. At 195 nm the 2 F 7/2 ! CTexcitation band is observed. The position of thisexcitation band is in good agreement with thecharge transfer absorption band in the reflectionspectrum.At low temperature, a decay time of 163 25 nshas been measured for the charge transfer luminescence,similar to the decay times measured forYb 3+ charge transfer luminescence in LuPO 4 andYPO 4 [3,4]. To study the temperature quenchingof the charge transfer luminescence, temperaturedependentlifetime measurements and intensitymeasurements were performed. In Fig. 4, theintensity of the 270 nm emission band and thedecay time of this emission are given as a functionof temperature. There is a good agreementbetween the decrease in intensity and decay time.If we define the quenching temperature as thetemperature at which the luminescence intensity orthe decay time have decreased to half of its initialvalue, the quenching temperature of the Yb 3+charge transfer luminescence is estimated to beabout 225 K.Also for Yb 3+ in other orthophosphates luminescence,reflection and lifetime measurementswere performed and compared with the luminescenceof the undoped host lattices. In Table 1, theFig. 3. Emission and excitation spectra of the ScPO 4 hostlattice (a, b) and of ScPO 4 doped with 1% Yb 3+ (c, d) measuredat 10 K. The emission spectra for the ScPO 4 host lattice andYb 3+ -doped sample were both recorded for excitation at140 nm, for the excitation spectrum of the host lattice the emissionwavelength was at 210 nm, for the Yb 3+ -doped sample itwas at 270 nm. In the emission spectra the broken linerepresents measurements performed with a SEYA monochromator,the solid line represents measurements performed with aSPEX monochromator equipped with a Tektronics ccd-array.Fig. 4. Decay time (*), on the left axis, and luminescenceintensity (&), on the right axis, of the charge transferluminescence at 270 nm as a function of temperature forScPO 4 :Yb 3+ (l exc =140 nm).
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Charge Transfer Luminescence of Yb3+

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