- Email: [email protected]
Charge transfer luminescence in Yb3+-containing compounds
Optical Materials 26 (2004) 545–549 www.elsevier.com/locate/optmat
Charge transfer luminescence in Yb3þ -containing compounds M. Nikl a
a,*
, A. Yoshikawa b, T. Fukuda
b
Institute of Physics Academy of Sciences of the Czech Republic, Cukrovarnicka 10, Prague 162 53, Czech Republic b Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan Available online 19 June 2004
Abstract The characteristics of charge transfer transition of Yb3þ in selected compounds and complexes are reviewed. Systematic comparison of the optical and luminescence spectra as well as the decay kinetics of Yb-doped aluminium perovskites and garnets is provided. Effect of intrinsic defects and possible occurrence of Yb2þ is discussed and possible application areas are briefly mentioned. 2004 Elsevier B.V. All rights reserved. Keywords: Charge transfer luminescence; Yb-doped garnets and perovskites; Decay kinetics
1. Introduction Charge transfer (CT) optical transition in a solidstate structure implies the transfer of the involved electron from the ligand to the central metal ion or (rarely) vice versa. Absorption transitions of the former kind are known, for example, for d10 ions (Zn2þ , Ga3þ ), ns2 ions (Tlþ , Pb2þ ) and also for several rare earth ions, like tetravalent Ce4þ , Pr4þ , Tb4þ and trivalent Eu3þ , Sm3þ , Yb3þ [1,2]. Some of such absorption transitions may be followed by the radiative return of the system to the ground state, i.e., by luminescence. CT luminescence of Yb3þ became of interest due to possible applications of Yb-containing materials for scintillation detectors in neutrino physics [3–5]. For the first time it was observed in the late seventies of the last century mostly in phosphate hosts [6,7]. Recently, the luminescence spectra and decay kinetics were briefly reviewed in many different host crystals [8]. This luminescence is characterized by broad emission band in UV-visible spectral region consisting of two subbands separated by about 10,000 cm1 (equal to the separation of 2 F7=2 and 2 F5=2 levels of Yb3þ ). Large Stokes shift of 7000–17,000 cm1 is typical as well. Fast decay with the
radiative lifetime on the order of 100 ns is observed at sufficiently low temperatures since the Yb3þ CT transition is not inhibited neither by parity nor spin selection rules. An onset of luminescence quenching typically occurs at temperatures well below room temperature. That is why very short decay time of about a few nanoseconds and low emission intensity are observed, for example, in YAG:Yb at 295 K [9]. However, in the YPO4 phosphate host an onset of the Yb3þ CT luminescence quenching only occurs around 290 K [10]. Therefore, an emission center of this kind can, for particular host, provide an efficient luminescence also around the room temperature. In this paper we aim to review the published features of the Yb3þ CT luminescence in various hosts, mainly fluorides and complex oxides. Furthermore, we provide a systematic comparison of the CT luminescence characteristics in the Yb-doped aluminium garnets and perovskites, including parasitic luminescence occurring in these materials. Finally we try to give useful hints for practical applications of these luminescent materials.
2. CT transition of Yb3þ in single crystal compounds or complexes *
Corresponding author. Tel.: +420-2-2031-8445; fax: +420-2-3123184. E-mail address: [email protected] (M. Nikl). 0925-3467/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.05.002
Due to the fact that the CT luminescence center includes both the ligand and central metal ions, it is
546
M. Nikl et al. / Optical Materials 26 (2004) 545–549
Table 1 The position of the lowest energy absorption and highest energy emission bands related to Yb3þ CT transition for selected compounds and Yb-halide complexes Yb-doped compound/ complex
Absorption (nm)
Emission (nm)
TQ (K)
Reference
LiYF4 CaF2 YbCl3 6 YbBr3 6 YbI6 ScPO4 YPO4 LaPO4 YAlO3 LaAlO3 Y3 Al5 O12 Lu3 Al5 O12
159 163 272 342 560 195 210 228 235 244 210 222 (excitation spectra at 10 K, kem ¼ 350 nm) 218 nm (excitation spectra at 9 K, kem ¼ 360 nm)
182 Not reported Not reported Not reported Not reported 270 304 No emission at 10 K 350 No emission at 10 K 334 338
Not Not Not Not Not 225 290 – 115 – 120 95a
[8] [11] [1] [1] [1] [10] [10] [10] [8,12] (TQ ) [8] [8], this work (TQ ) [13], this work (TQ )
375
Not reported
Y3 Ga5 O12
reported reported reported reported reported
[14]
Quenching temperature is defined as temperature of 50% emission intensity compared to the lowest temperature (typically 10 K) at which the spectrum was measured. a Possibly lowered due to energy transfer to a non-radiative trap.
expected that the Yb3þ CT emission characteristics are strongly dependent on the kind of ligand ions and the local symmetry of the central metal and ligand ions. In other words, properties of the second coordination sphere of the central metallic ion become important. Moreover, particular distortion (relaxation) of an excited state of the center, depending on the symmetry of phonon modes and other particular features of the host, will further modify a position of the luminescence bands. The Yb3þ CT luminescence is always identified by the necessary condition of having about 10,000 cm1 energy separation between two emission subbands given by the distance of 2 F7=2 and 2 F5=2 levels of Yb3þ . Table 1 gives an overview of the position of the CT absorption and higher energy (CT fi 2 F7=2 ) emission bands together with the quenching temperature. Several conclusions can be drawn from this table: (i) position of the CT absorption band scales with the kind of anion in the halide compounds/complexes, i.e., the absorption transition energy is dominantly determined by the energy distance between the 2 F7=2 level of Yb3þ and the top of the valence band. CT luminescence of Yb-doped bromides or iodides can be expected in the green or red parts of the spectrum (if not quenched); (ii) in various oxide-based compounds a position of the CT absorption does change to certain extent, the most probably given by particular symmetry of the center mentioned above and the effect of the second coordination sphere of Yb3þ ; (iii) increase of the size of cation, which Yb3þ substitutes for, results in a decrease of the quenching temperature (except Lu3 Al5 O12 , which might be affected by a parasitic energy transfer to traps/defects) as already mentioned in Ref. [8].
3. Charge transfer luminescence of Yb3þ in aluminium perovskites and garnets This section reviews systematic measurements of the luminescence characteristics of the Yb-doped aluminium perovskites (YAlO3 designated as YAP) and garnets (Lu3 Al5 O12 and Y3 Al5 O12 designated as LuAG or YAG, respectively) performed within collaboration of the authors’ laboratories. Crystals of YAP:Yb5% and LuAG:Yb5% were grown by the l-PD technique [12,13], YAP:Yb2% was also grown by the Czochralski method [15] in IMRAM, Tohoku university, Sendai. For further comparison, the high quality single crystal of YAG:Yb10% was purchased from Forschung Institute fur Edelsteine und EdelMaterialen GmBH, Idar-Oberstein, Germany. Absorption and luminescence measurements were performed in the Institute of Physics AS CR, Prague (for details, see Refs. [12,13]). Fig. 1 displays absorption spectra of YAG:Yb10% and YAP:Yb2% at the room temperature. The f–f transitions between 900–1000 nm clearly scale with the concentration of Yb, but an opposite trend is apparent at the edge of the CT absorption transition around 250 nm, which reflects somewhat different position of this absorption band in the YAP and YAG hosts (see Table 1). In Fig. 2 we show the excitation and emission spectra of YAG:Yb10% and LuAG:Yb5% samples at 80 K. Stokes shift of the former sample is evaluated to be about 1.81 eV. For LuAG:Yb5% the Stokes shift is calculated to be about 1.96 eV and different ratio of the 340 and 490 nm subband intensities is obtained, confirming different populations of the 2 F5=2 level in the
M. Nikl et al. / Optical Materials 26 (2004) 545–549 4
onset of the charge transfer transition of Yb3+
3
absorption
Normalised intensity [arb. units]
3.5
2.5 2
2 7/2
F
->2F5/2
absorption transition
1.5
of Yb3+
1
(a) em=350nm
1
YAG:Yb 10% YAP:Yb 2%
0.5
547
Stokes shift 1.50 eV
(b) em=400nm
0.8
(c) x=230nm 9 200
cm-1
0.6 ??? Yb2+ 0.4
0.2
0 200
400
600
800
1000
0 200
wavelength [nm]
500
(a) YAG:Yb, em=350 nm
700
1.2
(b) LuAG:Yb;em=355 nm
LuAG:Yb5%
(c) YAG:Yb, ex=220 nm
0.8
(d) LuAG:Yb, ex=220 nm
1
YAP:Yb5%
9 300 cm-1 0.6
0.4 0.2
0 200
600
Fig. 3. Excitation (a,b) and emission (c,d) spectra of YAP:Yb5% at 9 K.
intensity [arb. units]
Normalised intensity [arb.units]
400
Wavelength [nm]
Fig. 1. Absorption spectra of YAG:Yb10% and YAP:Yb2% at the room temperature.
1
300
250
300
350
400
450
500
550
YAG:Yb10%
0.8
0.6
0.4
600
Wavelength [nm] Fig. 2. Excitation (a,b) and emission (c,d) spectra of aluminium garnets at 80 K.
0.2
0 40
radiative de-excitation process in two hosts. Energy separation of the emission subbands is about 9300 cm1 . Excitation and emission spectra of YAP:Yb5% sample at 9 K are reported in Fig. 3. Somewhat lower Stokes shift of about 1.50 eV is calculated as sketched in the figure. Excitation peak at 245 nm is clearly shifted to lower energies with respect to both Yb-doped garnets in Fig. 2. This behavior is coherent with the positions of the CT absorption edge in Fig. 1. Slight asymmetry of the emission peak around 400 nm and related excitation peak around 280 nm, tentatively ascribed to Yb2þ center [12], is noticed. In Fig. 4 we give the temperature dependence of the integral intensity (integrated emission spectra) for all three samples reported in Figs. 2 and 3. It is interesting to note that the samples of Yb-doped YAP and YAG show very similar characteristics, evidencing the same mechanism of thermal quenching including the height of an energy barrier in both materials, despite somewhat different emission-spectra positions and calculated Stokes shifts. On the contrary, rather different course of
80
120
160
200
240
280
Temperature [K] Fig. 4. Temperature dependence of the integrated emission spectra of YAP:Yb5% (kex ¼ 230 nm) and YAG:Yb10%, LuAG:Yb5% (kex ¼ 220 nm).
the curve in case of LuAG:Yb5% seems to be difficult to explain based only on the data reported in Fig. 2. In Fig. 5 we display the normalized decay curves measured at maxima of emission peaks around 340–350 nm for the same samples. Decay curves of the Yb-doped YAP and YAG show a slight non-exponentiality. The decay times calculated in the curve tails are given in the figure. Decay time of the YAP:Yb2% CT emission was evaluated to be about 100 ± 10 ns in Ref. [8]. Observed non-exponentiality is the most probably due to an onset of concentration quenching and/or a presence of nonequivalent emission centers in the sample (effect of defects close to Yb3þ ions or possible antisite YbAl defect). Faster and considerably non-exponential decay of LuAG:Yb5% may correlate with an observed temperature dependence in Fig. 4 and can be explained as a result
548
M. Nikl et al. / Optical Materials 26 (2004) 545–549 LuAG:Yb5% YAG:Yb10% YAP:Yb5%
calculated decay times in the decay tails: LuAG:Yb5% - 51.4 ns YAG:Yb10% - 75.8 ns YAP:Yb5% - 91 ns
0.1
(a)
Intensity [arb.units]
Intensity [arb.units]
1
0.01
3
10
2
10
1
200
300
400
500
600
700
decay time = 0.8 ns
instrumental response 10
0.001
100
10
0
800
20
Time [ns]
40
50
60
70
Time [ns]
Fig. 5. Normalized photoluminescence decays at 7 K of YAP:Yb5% (kex ¼ 230 nm, kem ¼ 350 nm), YAG:Yb10% (kex ¼ 220 nm, kem ¼ 340 nm) and LuAG:Yb5% (kex ¼ 220 nm, kem ¼ 340 nm).
(b)
Intensity [arb.units]
of an energy transfer to an unknown defect or trap in the LuAG structure. It is worth noting that at the lowest temperatures an additional emission band is located around 480 nm. Its intensity varies for different samples. Possible Yb2þ or a defect emission in this spectral region was mentioned in Ref. [14]. Furthermore, absorption bands around 370 and 625 nm in YAG:Yb were ascribed to a perturbed F center [16], which could serve as an efficient acceptor for energy transfer from the Yb3þ CT luminescence due to enhanced overlap with its emission spectrum. Concentration quenching of the infrared Yb3þ emission in YAG:Yb was in another recent paper [17] ascribed to an existence of color centers and lattice distortions induced by Yb2þ . In Fig. 6 the room temperature decays are displayed for YAG:Yb10% and YAP:Yb5%. Both materials show the single exponential decay course and subnanosecond decay times. Such behavior points to the dominant effect of the thermal quenching, prevailing any non-equivalency of emission centers. Similar decay courses and decay times for both samples at the temperature limits (Figs. 5 and 6) are coherent with rather similar temperature dependences of the emission spectra obtained in Fig. 4. From the characteristics described above it follows that the most probable reason for parasitic luminescence in these materials will be a presence of Yb2þ due to its stable charge state. The luminescence of Yb2þ is known in many materials, see e.g., [18,19]. An overlap of the CT luminescence of Yb3þ with the absorption bands of intrinsic defects (mentioned F center) will inevitably lead to an energy transfer to these defects, which can affect the CT emission characteristics as shown above for LuAG:Yb. Due to allowed CT transition the interactions with such defect states (which usually have also relatively allowed absorption transitions) will have a long-range character and even a low defect concentration will result in considerable changes in the CT emis-
30
10
3
10
2
10
1
10
0
decay time = 0.98 ns
instrumental response 20
30
40
50
60
70
Time [ns] Fig. 6. Photoluminescence decays at room temperature. (a) YAP:Yb5%, kex ¼ 230 nm, kem ¼ 350 nm; (b) YAG:Yb10%, kex ¼ 220 nm, kem ¼ 340 nm. Instrumental response to the excitation pulse is also given in the figure. A solid line is the convolution of the instrumental response and a single exponential with calculated decay time.
sion parameters. It was mentioned e.g., in [17] that annealing in oxygen at 1600 C significantly reduces a concentration of the Yb2þ and related defects and, therefore, can be considered as a feasible way to lower concentrations of such unwanted defect states in the Ybdoped aluminium perovskite and garnet hosts. As mentioned in the introduction, these materials became of interest due to their possible usage in neutrino physics. However, another application may appear in the field of superfast scintillators. In particular, YAP:Yb might be competitive with the cross-luminescence scintillators as BaF2 or other halides (for an overview see e.g., [20]). Studies of this kind are under way in the authors’ laboratories [21].
4. Conclusions Charge transfer luminescence of Yb3þ has already been observed in a number of hosts. Its characteristics
M. Nikl et al. / Optical Materials 26 (2004) 545–549
depend dominantly on the kind of ligand, but also on the local symmetry, second coordination sphere of Yb3þ ion and possibly other particular properties of the host lattice. CT luminescence of the Yb-doped aluminium perovskites and garnets shows emission peaks at about 340–350 nm and around 500 nm with decay times less than 100 ns at low temperatures (around 10 K) and subnanosecond decay times at the room temperature. Decay time shortening is accompanied by the luminescence intensity reduction and both phenomena can be explained by thermal quenching of CT luminescence, the onset of which occurs around 100 K. Applications of these materials as superfast scintillators is not excluded. Acknowledgements Support of Czech GA AV project A1010305 is gratefully acknowledged. This work was partially supported by the Ministry of Education, Culture, Sports, Science and Technology, Japan, Grant-in-Aid for Young Scientists (A), 15686001, 2003 (AY). Thanks are due to E. Mihokova for language corrections.
549
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
Chr.K. Jorgensen, Prog. Inorg. Chem. 12 (1970) 101. H.E. Hoefdraad, J. Inorg. Nucl. Chem. 37 (1975) 1917. R.S. Raghavan, Phys. Rev. Lett. 78 (1997) 3618. P. Antonini et al., Nucl. Instr. Meth. Phys. Res. A 460 (2001) 469. S. Belogurov et al., Nucl. Instr. Meth. Phys. Res. A 496 (2003) 385. E. Nakazawa, J. Lumin. 18–19 (1979) 272. E. Nakazawa, Chem. Phys. Lett. 56 (1978) 161. L. van Pieterson et al., J. Lumin. 91 (2000) 177. N. Guerassimova et al., J. Lumin. 94–95 (2001) 11. L. van Pieterson, A. Meijerink, J. Alloys Compd. 300–301 (2000) 426. T. Szcezurek, M. Schlesinger, in: B. Jezowska-Trzebiatowska, J. Legendziewic, W. Strek (Eds.), Rare Earth Spectroscopy, World Scientific, Singapore, 1985, p. 309. J.B. Shim et al., J. Cryst. Growth 256 (2003) 298. H. Ogino et al., J. Cryst. Growth 253 (2003) 314. I.A. Kamenskikh et al., Opt. Mater. 24 (2003) 267. J.B. Shim et al., J. Appl. Phys. 95 (2004) 3063. H. Yin et al., J. Appl. Phys. 83 (1998) 3825. P. Yang et al., J. Lumin. 97 (2002) 51. G. Blasse et al., Chem. Phys. Lett. 167 (1990) 41. S. Lizzo et al., J. Lumin. 63 (1995) 223. P.A. Rodnyi, Physical Processes in Inorganic Scintillators, CRC Press, New York, 1997. M. Nikl et al., Appl. Phys. Letters 84 (2004) 882.
Charge transfer luminescence in Yb3þ -containing compounds M. Nikl a
a,*
, A. Yoshikawa b, T. Fukuda
b
Institute of Physics Academy of Sciences of the Czech Republic, Cukrovarnicka 10, Prague 162 53, Czech Republic b Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan Available online 19 June 2004
Abstract The characteristics of charge transfer transition of Yb3þ in selected compounds and complexes are reviewed. Systematic comparison of the optical and luminescence spectra as well as the decay kinetics of Yb-doped aluminium perovskites and garnets is provided. Effect of intrinsic defects and possible occurrence of Yb2þ is discussed and possible application areas are briefly mentioned. 2004 Elsevier B.V. All rights reserved. Keywords: Charge transfer luminescence; Yb-doped garnets and perovskites; Decay kinetics
1. Introduction Charge transfer (CT) optical transition in a solidstate structure implies the transfer of the involved electron from the ligand to the central metal ion or (rarely) vice versa. Absorption transitions of the former kind are known, for example, for d10 ions (Zn2þ , Ga3þ ), ns2 ions (Tlþ , Pb2þ ) and also for several rare earth ions, like tetravalent Ce4þ , Pr4þ , Tb4þ and trivalent Eu3þ , Sm3þ , Yb3þ [1,2]. Some of such absorption transitions may be followed by the radiative return of the system to the ground state, i.e., by luminescence. CT luminescence of Yb3þ became of interest due to possible applications of Yb-containing materials for scintillation detectors in neutrino physics [3–5]. For the first time it was observed in the late seventies of the last century mostly in phosphate hosts [6,7]. Recently, the luminescence spectra and decay kinetics were briefly reviewed in many different host crystals [8]. This luminescence is characterized by broad emission band in UV-visible spectral region consisting of two subbands separated by about 10,000 cm1 (equal to the separation of 2 F7=2 and 2 F5=2 levels of Yb3þ ). Large Stokes shift of 7000–17,000 cm1 is typical as well. Fast decay with the
radiative lifetime on the order of 100 ns is observed at sufficiently low temperatures since the Yb3þ CT transition is not inhibited neither by parity nor spin selection rules. An onset of luminescence quenching typically occurs at temperatures well below room temperature. That is why very short decay time of about a few nanoseconds and low emission intensity are observed, for example, in YAG:Yb at 295 K [9]. However, in the YPO4 phosphate host an onset of the Yb3þ CT luminescence quenching only occurs around 290 K [10]. Therefore, an emission center of this kind can, for particular host, provide an efficient luminescence also around the room temperature. In this paper we aim to review the published features of the Yb3þ CT luminescence in various hosts, mainly fluorides and complex oxides. Furthermore, we provide a systematic comparison of the CT luminescence characteristics in the Yb-doped aluminium garnets and perovskites, including parasitic luminescence occurring in these materials. Finally we try to give useful hints for practical applications of these luminescent materials.
2. CT transition of Yb3þ in single crystal compounds or complexes *
Corresponding author. Tel.: +420-2-2031-8445; fax: +420-2-3123184. E-mail address: [email protected] (M. Nikl). 0925-3467/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.05.002
Due to the fact that the CT luminescence center includes both the ligand and central metal ions, it is
546
M. Nikl et al. / Optical Materials 26 (2004) 545–549
Table 1 The position of the lowest energy absorption and highest energy emission bands related to Yb3þ CT transition for selected compounds and Yb-halide complexes Yb-doped compound/ complex
Absorption (nm)
Emission (nm)
TQ (K)
Reference
LiYF4 CaF2 YbCl3 6 YbBr3 6 YbI6 ScPO4 YPO4 LaPO4 YAlO3 LaAlO3 Y3 Al5 O12 Lu3 Al5 O12
159 163 272 342 560 195 210 228 235 244 210 222 (excitation spectra at 10 K, kem ¼ 350 nm) 218 nm (excitation spectra at 9 K, kem ¼ 360 nm)
182 Not reported Not reported Not reported Not reported 270 304 No emission at 10 K 350 No emission at 10 K 334 338
Not Not Not Not Not 225 290 – 115 – 120 95a
[8] [11] [1] [1] [1] [10] [10] [10] [8,12] (TQ ) [8] [8], this work (TQ ) [13], this work (TQ )
375
Not reported
Y3 Ga5 O12
reported reported reported reported reported
[14]
Quenching temperature is defined as temperature of 50% emission intensity compared to the lowest temperature (typically 10 K) at which the spectrum was measured. a Possibly lowered due to energy transfer to a non-radiative trap.
expected that the Yb3þ CT emission characteristics are strongly dependent on the kind of ligand ions and the local symmetry of the central metal and ligand ions. In other words, properties of the second coordination sphere of the central metallic ion become important. Moreover, particular distortion (relaxation) of an excited state of the center, depending on the symmetry of phonon modes and other particular features of the host, will further modify a position of the luminescence bands. The Yb3þ CT luminescence is always identified by the necessary condition of having about 10,000 cm1 energy separation between two emission subbands given by the distance of 2 F7=2 and 2 F5=2 levels of Yb3þ . Table 1 gives an overview of the position of the CT absorption and higher energy (CT fi 2 F7=2 ) emission bands together with the quenching temperature. Several conclusions can be drawn from this table: (i) position of the CT absorption band scales with the kind of anion in the halide compounds/complexes, i.e., the absorption transition energy is dominantly determined by the energy distance between the 2 F7=2 level of Yb3þ and the top of the valence band. CT luminescence of Yb-doped bromides or iodides can be expected in the green or red parts of the spectrum (if not quenched); (ii) in various oxide-based compounds a position of the CT absorption does change to certain extent, the most probably given by particular symmetry of the center mentioned above and the effect of the second coordination sphere of Yb3þ ; (iii) increase of the size of cation, which Yb3þ substitutes for, results in a decrease of the quenching temperature (except Lu3 Al5 O12 , which might be affected by a parasitic energy transfer to traps/defects) as already mentioned in Ref. [8].
3. Charge transfer luminescence of Yb3þ in aluminium perovskites and garnets This section reviews systematic measurements of the luminescence characteristics of the Yb-doped aluminium perovskites (YAlO3 designated as YAP) and garnets (Lu3 Al5 O12 and Y3 Al5 O12 designated as LuAG or YAG, respectively) performed within collaboration of the authors’ laboratories. Crystals of YAP:Yb5% and LuAG:Yb5% were grown by the l-PD technique [12,13], YAP:Yb2% was also grown by the Czochralski method [15] in IMRAM, Tohoku university, Sendai. For further comparison, the high quality single crystal of YAG:Yb10% was purchased from Forschung Institute fur Edelsteine und EdelMaterialen GmBH, Idar-Oberstein, Germany. Absorption and luminescence measurements were performed in the Institute of Physics AS CR, Prague (for details, see Refs. [12,13]). Fig. 1 displays absorption spectra of YAG:Yb10% and YAP:Yb2% at the room temperature. The f–f transitions between 900–1000 nm clearly scale with the concentration of Yb, but an opposite trend is apparent at the edge of the CT absorption transition around 250 nm, which reflects somewhat different position of this absorption band in the YAP and YAG hosts (see Table 1). In Fig. 2 we show the excitation and emission spectra of YAG:Yb10% and LuAG:Yb5% samples at 80 K. Stokes shift of the former sample is evaluated to be about 1.81 eV. For LuAG:Yb5% the Stokes shift is calculated to be about 1.96 eV and different ratio of the 340 and 490 nm subband intensities is obtained, confirming different populations of the 2 F5=2 level in the
M. Nikl et al. / Optical Materials 26 (2004) 545–549 4
onset of the charge transfer transition of Yb3+
3
absorption
Normalised intensity [arb. units]
3.5
2.5 2
2 7/2
F
->2F5/2
absorption transition
1.5
of Yb3+
1
(a) em=350nm
1
YAG:Yb 10% YAP:Yb 2%
0.5
547
Stokes shift 1.50 eV
(b) em=400nm
0.8
(c) x=230nm 9 200
cm-1
0.6 ??? Yb2+ 0.4
0.2
0 200
400
600
800
1000
0 200
wavelength [nm]
500
(a) YAG:Yb, em=350 nm
700
1.2
(b) LuAG:Yb;em=355 nm
LuAG:Yb5%
(c) YAG:Yb, ex=220 nm
0.8
(d) LuAG:Yb, ex=220 nm
1
YAP:Yb5%
9 300 cm-1 0.6
0.4 0.2
0 200
600
Fig. 3. Excitation (a,b) and emission (c,d) spectra of YAP:Yb5% at 9 K.
intensity [arb. units]
Normalised intensity [arb.units]
400
Wavelength [nm]
Fig. 1. Absorption spectra of YAG:Yb10% and YAP:Yb2% at the room temperature.
1
300
250
300
350
400
450
500
550
YAG:Yb10%
0.8
0.6
0.4
600
Wavelength [nm] Fig. 2. Excitation (a,b) and emission (c,d) spectra of aluminium garnets at 80 K.
0.2
0 40
radiative de-excitation process in two hosts. Energy separation of the emission subbands is about 9300 cm1 . Excitation and emission spectra of YAP:Yb5% sample at 9 K are reported in Fig. 3. Somewhat lower Stokes shift of about 1.50 eV is calculated as sketched in the figure. Excitation peak at 245 nm is clearly shifted to lower energies with respect to both Yb-doped garnets in Fig. 2. This behavior is coherent with the positions of the CT absorption edge in Fig. 1. Slight asymmetry of the emission peak around 400 nm and related excitation peak around 280 nm, tentatively ascribed to Yb2þ center [12], is noticed. In Fig. 4 we give the temperature dependence of the integral intensity (integrated emission spectra) for all three samples reported in Figs. 2 and 3. It is interesting to note that the samples of Yb-doped YAP and YAG show very similar characteristics, evidencing the same mechanism of thermal quenching including the height of an energy barrier in both materials, despite somewhat different emission-spectra positions and calculated Stokes shifts. On the contrary, rather different course of
80
120
160
200
240
280
Temperature [K] Fig. 4. Temperature dependence of the integrated emission spectra of YAP:Yb5% (kex ¼ 230 nm) and YAG:Yb10%, LuAG:Yb5% (kex ¼ 220 nm).
the curve in case of LuAG:Yb5% seems to be difficult to explain based only on the data reported in Fig. 2. In Fig. 5 we display the normalized decay curves measured at maxima of emission peaks around 340–350 nm for the same samples. Decay curves of the Yb-doped YAP and YAG show a slight non-exponentiality. The decay times calculated in the curve tails are given in the figure. Decay time of the YAP:Yb2% CT emission was evaluated to be about 100 ± 10 ns in Ref. [8]. Observed non-exponentiality is the most probably due to an onset of concentration quenching and/or a presence of nonequivalent emission centers in the sample (effect of defects close to Yb3þ ions or possible antisite YbAl defect). Faster and considerably non-exponential decay of LuAG:Yb5% may correlate with an observed temperature dependence in Fig. 4 and can be explained as a result
548
M. Nikl et al. / Optical Materials 26 (2004) 545–549 LuAG:Yb5% YAG:Yb10% YAP:Yb5%
calculated decay times in the decay tails: LuAG:Yb5% - 51.4 ns YAG:Yb10% - 75.8 ns YAP:Yb5% - 91 ns
0.1
(a)
Intensity [arb.units]
Intensity [arb.units]
1
0.01
3
10
2
10
1
200
300
400
500
600
700
decay time = 0.8 ns
instrumental response 10
0.001
100
10
0
800
20
Time [ns]
40
50
60
70
Time [ns]
Fig. 5. Normalized photoluminescence decays at 7 K of YAP:Yb5% (kex ¼ 230 nm, kem ¼ 350 nm), YAG:Yb10% (kex ¼ 220 nm, kem ¼ 340 nm) and LuAG:Yb5% (kex ¼ 220 nm, kem ¼ 340 nm).
(b)
Intensity [arb.units]
of an energy transfer to an unknown defect or trap in the LuAG structure. It is worth noting that at the lowest temperatures an additional emission band is located around 480 nm. Its intensity varies for different samples. Possible Yb2þ or a defect emission in this spectral region was mentioned in Ref. [14]. Furthermore, absorption bands around 370 and 625 nm in YAG:Yb were ascribed to a perturbed F center [16], which could serve as an efficient acceptor for energy transfer from the Yb3þ CT luminescence due to enhanced overlap with its emission spectrum. Concentration quenching of the infrared Yb3þ emission in YAG:Yb was in another recent paper [17] ascribed to an existence of color centers and lattice distortions induced by Yb2þ . In Fig. 6 the room temperature decays are displayed for YAG:Yb10% and YAP:Yb5%. Both materials show the single exponential decay course and subnanosecond decay times. Such behavior points to the dominant effect of the thermal quenching, prevailing any non-equivalency of emission centers. Similar decay courses and decay times for both samples at the temperature limits (Figs. 5 and 6) are coherent with rather similar temperature dependences of the emission spectra obtained in Fig. 4. From the characteristics described above it follows that the most probable reason for parasitic luminescence in these materials will be a presence of Yb2þ due to its stable charge state. The luminescence of Yb2þ is known in many materials, see e.g., [18,19]. An overlap of the CT luminescence of Yb3þ with the absorption bands of intrinsic defects (mentioned F center) will inevitably lead to an energy transfer to these defects, which can affect the CT emission characteristics as shown above for LuAG:Yb. Due to allowed CT transition the interactions with such defect states (which usually have also relatively allowed absorption transitions) will have a long-range character and even a low defect concentration will result in considerable changes in the CT emis-
30
10
3
10
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0
decay time = 0.98 ns
instrumental response 20
30
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50
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Time [ns] Fig. 6. Photoluminescence decays at room temperature. (a) YAP:Yb5%, kex ¼ 230 nm, kem ¼ 350 nm; (b) YAG:Yb10%, kex ¼ 220 nm, kem ¼ 340 nm. Instrumental response to the excitation pulse is also given in the figure. A solid line is the convolution of the instrumental response and a single exponential with calculated decay time.
sion parameters. It was mentioned e.g., in [17] that annealing in oxygen at 1600 C significantly reduces a concentration of the Yb2þ and related defects and, therefore, can be considered as a feasible way to lower concentrations of such unwanted defect states in the Ybdoped aluminium perovskite and garnet hosts. As mentioned in the introduction, these materials became of interest due to their possible usage in neutrino physics. However, another application may appear in the field of superfast scintillators. In particular, YAP:Yb might be competitive with the cross-luminescence scintillators as BaF2 or other halides (for an overview see e.g., [20]). Studies of this kind are under way in the authors’ laboratories [21].
4. Conclusions Charge transfer luminescence of Yb3þ has already been observed in a number of hosts. Its characteristics
M. Nikl et al. / Optical Materials 26 (2004) 545–549
depend dominantly on the kind of ligand, but also on the local symmetry, second coordination sphere of Yb3þ ion and possibly other particular properties of the host lattice. CT luminescence of the Yb-doped aluminium perovskites and garnets shows emission peaks at about 340–350 nm and around 500 nm with decay times less than 100 ns at low temperatures (around 10 K) and subnanosecond decay times at the room temperature. Decay time shortening is accompanied by the luminescence intensity reduction and both phenomena can be explained by thermal quenching of CT luminescence, the onset of which occurs around 100 K. Applications of these materials as superfast scintillators is not excluded. Acknowledgements Support of Czech GA AV project A1010305 is gratefully acknowledged. This work was partially supported by the Ministry of Education, Culture, Sports, Science and Technology, Japan, Grant-in-Aid for Young Scientists (A), 15686001, 2003 (AY). Thanks are due to E. Mihokova for language corrections.
549
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