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Luminescent properties of Yb-doped LaSc3(BO3)4 under VUV excitation
Radiation Measurements 42 (2007) 874 – 877 www.elsevier.com/locate/radmeas
Luminescent properties ofYb-doped LaSc3(BO3)4 under VUV excitation N. Guerassimova a , I. Kamenskikh a , D. Krasikov a,∗ , V. Mikhailin a , A. Zagumennyi b , S. Koutovoi b , Yu. Zavartsev b , C. Pedrini c a Physics Department, M.V. Lomonosov Moscow State University, Leninskie gory, 119992 Moscow, Russia b Laser Crystals Department, General Physics Institute of RAS, Vavilova str. 38, 119991 Moscow, Russia c Laboratoire de Physico-Chimie des Materiaux Luminescents, Universite Claude Bernard Lyon-1, UMR 5620 CNRS, 69622 Villeurbanne, France
Received 20 December 2006; accepted 1 February 2007
Abstract Ytterbium doped borate crystals are promising laser media, e.g. in LaSc3 (BO3 )4 (LSB) matrices large distance between ytterbium ions results in reduced concentration quenching of the ytterbium f–f luminescence [Petermann, K., Fagundes-Peters, D., Johansen, O., Mond, M., Peters, V., Romero, J.J., Kutovoi, S., Speiser, J., Giesen, A., 2005. Highly Yb-doped oxides for thin-disc lasers. J. Crystal Growth 275, 135-140]. Yb3+ ions in complex oxides in addition to the 4f → 4f transitions often manifest fast charge transfer luminescence (CTL) in the UV-visible range. In some borates it was not observed at all, like in orthoborates of Sc, Y and La [Van Pieterson, L., Heeroma, M., de Heer, E., Meijerink, A., 2000. Charge transfer luminescence of Yb3+ . J. Lumin. 91, 177–193]; in haloborates Sr 2 B5 O9 X, where X = Cl, Br, the UV/visible luminescence was attributed to ytterbium CTL though it looked substantially different from other matrices [Dotsenko, V.P., Berezovskaya, I.V., Pyrogenko, P.V., Efryushina, N.P., Rodniy, P.A., Eijk van, C.W.E., Sidorenko, A.V., 2002. Valence states and luminescence properties of ytterbium ions in strontium haloborates. J. Solid State Chem. 166, 271–276]; while in oxyborate Li2 Lu5 O4 (BO3 )3 “classical” CTL was observed [Jubera, V., Garcia, A., Chaminade, J.P., Guillen, F., Sablayrolles, Jean, Fouassier, C., 2007. Yb3+ and Yb3+ -Eu3+ luminescent properties of the Li2 Lu5 O4 (BO3 )3 phase. J. Lumin. 124(1), 10–14]. In this work the luminescence properties of another borate, namely LSB doped by Yb are presented. © 2007 Elsevier Ltd. All rights reserved. Keywords: Charge transfer luminescence; Borates; Ytterbium
1. Introduction Yb3+ -doped crystals are known to demonstrate two types of luminescence: f–f luminescence in the IR spectral range and charge transfer luminescence (CTL) in the UV-visible range. While the f–f luminescence of Yb3+ is of interest for laser applications, the CTL has recently been shown to be promising for scintillator applications (Guerassimova et al., 2002). CTL of rare-earth doped matrices is much less understood compared to the f–f luminescence of trivalent rare-earth elements studied for several decades by now. The first paper on CTL goes back to the late 1970s (Nakazawa, 1978), extensive study of CTL began only several years ago when the possibility of using
∗ Corresponding author. Tel.: +7 495 939 31 69; fax: +7 495 939 29 91.
E-mail address: [email protected] (D. Krasikov). 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.02.027
Yb-containing scintillators for neutrino detection was demonstrated (Bressi et al., 2001). Now a number of Yb-doped matrices are known to manifest efficient CTL (Van Pieterson et al., 2000). At the same time, single crystals demonstrating CTL with most attractive properties for application as scintillators (YAG-Yb, YAP-Yb, Lu2 O3 .Yb) manifest strong temperature quenching resulting in intensity decrease by the factor of 2 at ∼ 130 K and by two orders of magnitude at room temperature (Kamenskikh et al., 2005; Krasikov et al., 2006). At present, the objective is to find matrices with efficient CTL at room temperature. Among investigated matrices the highest CTL quenching temperature was observed for phosphate powders (Van Pieterson et al., 2000; Voloshinovskii et al., 2003). We looked at another family of crystals with complex oxianion and large bandgap, namely borate crystals. Experimental data on CTL of borates are inconsistent: in some borates CTL was not observed at all, like in orthoborates of Sc, Y and La
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(Van Pieterson et al., 2000); in haloborates Sr 2 B5 O9 X, where X = Cl, Br, UV/visible luminescence was attributed to ytterbium CTL though it looked substantially different from other matrices (Dotsenko et al., 2002); while in oxyborate Li2 Lu5 O4 (BO3 )3 “classical” CTL was observed (Jubera et al., 2007). In contrast to phosphate compounds, the growth methods for a number of borate single crystals of high optical quality are well developed (namely for non-linear optics applications). Yb-doped borates are of interest as laser media as well. Yb-doped LaSc3 (BO3 )4 (LSB) single crystals attract attention due to a large distance between the nearest Yb ions resulting in reduced concentration quenching of the Yb3+ f–f luminescence (Petermann et al., 2005). Here we present first results on the VUV and X-ray excited UV-visible-IR luminescence of Yb-doped LSB single crystals. 2. Experimental LSB crystals with Yb concentration of 0.5%, 3% and 25% were grown in the General Physics Institute using Czochralsky technique. The measurements were performed at the SUPERLUMI station of DORIS III positron storage ring at DESY (Hamburg, Germany) (Zimmerer, 1991). Time-resolved spectra of luminescence and luminescence excitation as well as luminescence decay kinetics were measured at room temperature and 10 K. During luminescence data acquisition two types of time windows were used: fast (2–10 ns from the peak of the excitation pulse) and slow (60–180 ns from the excitation pulse). Luminescence spectra excited by X-rays were measured using an X-ray source XRG3000 INEL with tungsten anode operating at 35 kV.
Fig. 1. Luminescence (1, 1 ) and excitation (2, 2 ) spectra of LSB–Yb 25% measured at 9 K (a) and LSB–Yb 0.5% measured at 300 K (b); dots—integrated luminescence, open dots—slow luminescence component, line—fast luminescence component. Luminescence spectra measured under excitation in the slow peak: 6.7 eV (a) and 6.5 eV (b), excitation spectra measured for 3.2 eV luminescence (a), and 3.65 eV luminescence (b).
3. Results and discussion Luminescence spectrum of LSB-25%Yb measured at 9 K is presented in Fig. 1a. Two broad bands are observed in the range 1.5–4.5 eV. The spectrum looks as “classical” CTL spectra Ybdoped matrices. CTL is an allowed transitions from the charge transfer state (formed by transfer of an electron from the ligands to the rare-earth ion) to the two Yb3+ levels 2 F 5/2 and 2 F 7/2 . So two broad luminescence bands separated by ∼ 1.25 eV (corresponding to separation between 2 F 5/2 and 2 F 7/2 levels) with large Stokes shift and nanosecond kinetics are observed in case of CTL. Spectrum presented in Fig. 1a meets all these conditions: it can be approximated by two Gaussians with separation in energy close to 1.25 eV, the Stokes shift is 2.7 eV, which is typical for the CT transitions and has nanosecond kinetics, which can be fitted by two exponentials with characteristic decay times of 3 and 10 ns (Fig. 2, curve 1). Excitation spectrum of 3.2 eV luminescence in LSB-25%Yb at 9 K (Fig. 1a) looks similar to the excitation spectra usually observed for CTL: a broad fast band (characterized by nanosecond kinetics) just below the fundamental absorption edge. In the case of LSB there is no experimental data on band gap width. In our measurements of reflectivity no features that could assist in its determination (like exciton peaks) were observed. We can evaluate the band gap energy from the excitation spectra
Fig. 2. Decay profiles of 3.2 eV luminescence in LSB–Yb 25% (1), LSB–Yb 3% (2) and LSB–Yb 0.5% (3) excited by 6.05 eV photons measured at 9 K.
(Fig. 1a): the onset of slow luminescence component begins at 6.5 eV with the maximum at 6.7 eV, so the band gap energy in LSB is higher than 6.5 eV. The band in the range 5.5–6.5 eV of excitation spectrum is fast and situated probably just below fundamental absorption edge. In other words all mentioned above characteristics of luminescence in the range 1.5–4.5 eV in LSBYb 25% at 9 K are suitable for attribution of this luminescence to the CTL. Moreover f–f luminescence of Yb3+ in LSB–Yb 25% (observed in IR spectral range with maximum near 1.25 eV) at 9 K is excited with photons of energies higher than 5.5 eV (Fig. 3, open dots), e.g. in the same range as 3.2 eV luminescence. Observation of similar excitation spectra of the CTL
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N. Guerassimova et al. / Radiation Measurements 42 (2007) 874 – 877
Fig. 3. Luminescence spectrum excited with 5.5 eV photons (1), excitation spectrum of 2.6 eV luminescence (2) and excitation spectrum of IR luminescence (open dots; obtained by integration of the luminescence spectra in the range 960–1040 nm) in LSB–Yb 25% measured at 9 K.
and f–f luminescence in the range of the CT absorption and fundamental absorption edge is typical for Yb-doped matrices (Kamenskikh et al., 2003). In this case the CTL and f–f luminescence are consecutive processes: 2 F 5/2 state of Yb3+ ions is populated via CT transitions and followed by f–f luminescence. The profile of the luminescence spectra in LSB–Yb 25% at 9 K depends on the excitation energy. While under excitation in the range 6–7 eV the high-energy luminescence band in the range 3–4.5 eV is dominating (Fig. 1a), the low-energy luminescence band in the region 1.5–3 eV is most pronounced for the excitation energies below 5.5 eV (Fig. 3). In the excitation spectrum of 2.6 eV luminescence in addition to the bands in the range 5.5–7 eV (similar to the bands in the excitation spectrum of 3.2 eV luminescence) a low-energy excitation band in the range 4–5 eV appears. So one can conclude that in LSB-Yb at least two types of luminescence are observed in the UV-visible range: the first one in the range 1.5–4.5 eV is excited with photons of energies higher than 5.5 eV, and another one in the range 1.5–3 eV is excited with photons of the same energies and also with photons of lower energies. The first type of luminescence is tentatively attributed to the CTL, the second one is probably of defect type. It should be noted that situation when in undoped crystals and in doped crystals under low-energy excitation (energies lower than CT absorption band) intrinsic and defect luminescence are predominant just in the region of CTL is typical (Guerassimova et al., 2005; Voloshinovskii et al., 2003). At room temperature we did not succeed in detection of fast luminescence characterized by nanosecond kinetics. Only slow luminescence with characteristic decay time longer than microsecond was observed. Typical luminescence spectrum measured at room temperature is presented in Fig. 1b. The only luminescence band is observed in the range 2.5–4.5 eV. Spectra obtained under X-ray excitation are similar to those measured under VUV excitation (Fig. 4). Since this luminescence is slow and no luminescence is observed in the range 1.5–2.5 eV,
Fig. 4. Luminescence spectra of LSB–Yb 25% (1, 1 ), LSB–Yb 3% (2, 2 ) and LSB–Yb 0.5% (3, 3 ) measured at 300 K under X-ray excitation.
one can conclude that both types of luminescence observed at 9 K and discussed above are quenched at 300 K. Instead of these types of luminescence another luminescence is observed at 300 K. This luminescence is excited with photons of energies higher than 6 eV. The yield of this luminescence increases with the decrease of Yb concentration, whereas concentration dependence of Yb3+ f–f luminescence yield is inverse (Fig. 4). These tendencies suggest that this luminescence is not related to Yb but seems to be of intrinsic origin. To our knowledge, there are no data on the intrinsic luminescence of LSB. To understand the origin of the observed luminescence the study of undoped LSB crystals is planned. Since the spectral range of the slow luminescence observed at RT and the spectral range of the fast luminescence observed at 9 K overlap, slow luminescence can contribute to the luminescence spectra at low temperature as well. The increase of slow luminescent component contribution with decrease of Yb concentration (Fig. 2) can imply that for low Yb concentration slow luminescence contribute to luminescence spectra at 9 K while for concentrated sample only fast luminescence is observed at 9 K in the range 3–4.5 eV. Summarizing, at least three types of luminescence were observed for LSB-Yb single crystals in the UV-visible range. The first one observed in the spectral range 1.5–4.5 eV consists of two broad luminescence bands and is characterized by nanosecond kinetics. The properties of this luminescence allow to attribute it to the charge transfer luminescence. The second type of luminescence is observed in the range 1.5–3 eV and is probably of defect origin. Both types of luminescence are quenched at room temperature. Finally, at room temperature and low Yb concentration slow luminescence in the range 2.5–4.5 eV is dominating. To understand unambiguously the origin of observed UVvisible luminescence in LSB-Yb single crystals further investigation of LSB-Yb as well as of undoped LSB crystals is planned.
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Acknowledgments Authors are grateful to Prof. G. Zimmerer for providing access to the SUPERLUMI facility. Financial support of RFBR Grant no. 06-02-26-911 is gratefully acknowledged. References Bressi, G., Carugno, G., Conti, E., Del Noce, C., Iannuzzi, D., 2001. New prospects in scintillating crystals. Nucl. Instrum. Methods A 461, 361. Dotsenko, V.P., Berezovskaya, I.V., Pyrogenko, P.V., Efryushina, N.P., Rodniy, P.A., Eijk van, C.W.E, Sidorenko, A.V., 2002. Valence states and luminescence properties of ytterbium ions in strontium haloborates. J. Solid State Chem. 166, 271–276. Guerassimova, N., Dujardin, C., Garnier, N., Pedrini, C., Petrosyan, A.G., Kamenskikh, I.A., Mikhailin, V.V., Ovanesyan, K.L., Shirinyan, G.O., Chipaux, R., Crebier, M., Mallet, J., Meyer, J.-P., 2002. Charge transfer luminescence and spectroscopic properties of Yb3+ in aluminum and gallium garnets. Nucl. Instrum. Methods A 486, 278–282. Guerassimova, N.V., Kamenskikh, I.A., Krasikov, D.N., Mikhailin, V.V., Petermann, K., de Sousa, D.F., Zimmerer, G., 2005. Charge-transfer luminescence of ytterbium-containing sesquioxides: Lu2 O3 , Y2 O3 , Sc2 O3 and Yb2 O3 . Proceedings of International Conference on Inorganic Scintillators and their Industrial Applications, SCINT, 2005. Jubera, V., Garcia, A., Chaminade, J.P., Guillen, F., Sablayrolles, Jean, Fouassier, C., 2007. Yb3+ and Yb3+ -Eu3+ luminescent properties of the Li2 Lu5 O4 (BO3 )3 phase. J. Lumin., 124 (1), 10–14.
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Kamenskikh, I., Guerassimova, N., Dujardin, C., Garnier, N., Ledoux, G., Pedrini, C., Kirm, M., Petrosyan, A., Spassky, D., 2003. Charge transfer fluorescence and f–f luminescence in ytterbium compounds. Opt. Mater. 24, 267–274. Kamenskikh, I., Dujardin, C., Garnier, N., Guerassimova, N., Ledoux, G., Mikhailin, V., Pedrini, C., Petrosyan, A., Vasil’ev, A., 2005. Temperature dependence of the charge transfer and f–f luminescence of Yb3+ in garnets and YAP. J. Phys. Conden. Matter 17, 5587–5594. Krasikov, D.N., Guerassimova, N.V., Kamenskikh, I.A., Mikhailin, V.V., Scherbinin, S.N., Petermann, K., de Sousa, D.F., Pedrini C., Zimmerer, G., 2006. Centers of charge-transfer luminescence in ytterbium-doped sesquioxides. Izvestia Vuzov. Fizika 49 (4), 81–84. Nakazawa, E., 1978. Charge-transfer type luminescence of Yb3+ ions in LuPO4 and YPO4 . Chem. Phys. Lett. 56, 161. Petermann, K., Fagundes-Peters, D., Johansen, O., Mond, M., Peters, V., Romero, J.J., Kutovoi, S., Speiser, J., Giesen, A., 2005. Highly Yb-doped oxides for thin-disc lasers. J. Crystal Growth 275, 135–140. Van Pieterson, L., Heeroma, M., de Heer, E., Meijerink, A., 2000. Charge transfer luminescence of Yb3+ . J. Lumin. 91, 177–193. Voloshinovskii, A., Sol’skii, I., Stryganyuk, G., Romanishin, Yu., Khapko, Z., Antonyak, O., 2003. Charge transfer luminescence of Yb3+ ions in LiY0.1 Yb0.9 P4 O12 phosphor. HASYLAB Annual Report, 2003. Zimmerer, G., 1991. Status report on luminescence investigations with synchrotron radiation at HASYLAB. Nucl. Instrum. Methods A308, 178–186.
Luminescent properties ofYb-doped LaSc3(BO3)4 under VUV excitation N. Guerassimova a , I. Kamenskikh a , D. Krasikov a,∗ , V. Mikhailin a , A. Zagumennyi b , S. Koutovoi b , Yu. Zavartsev b , C. Pedrini c a Physics Department, M.V. Lomonosov Moscow State University, Leninskie gory, 119992 Moscow, Russia b Laser Crystals Department, General Physics Institute of RAS, Vavilova str. 38, 119991 Moscow, Russia c Laboratoire de Physico-Chimie des Materiaux Luminescents, Universite Claude Bernard Lyon-1, UMR 5620 CNRS, 69622 Villeurbanne, France
Received 20 December 2006; accepted 1 February 2007
Abstract Ytterbium doped borate crystals are promising laser media, e.g. in LaSc3 (BO3 )4 (LSB) matrices large distance between ytterbium ions results in reduced concentration quenching of the ytterbium f–f luminescence [Petermann, K., Fagundes-Peters, D., Johansen, O., Mond, M., Peters, V., Romero, J.J., Kutovoi, S., Speiser, J., Giesen, A., 2005. Highly Yb-doped oxides for thin-disc lasers. J. Crystal Growth 275, 135-140]. Yb3+ ions in complex oxides in addition to the 4f → 4f transitions often manifest fast charge transfer luminescence (CTL) in the UV-visible range. In some borates it was not observed at all, like in orthoborates of Sc, Y and La [Van Pieterson, L., Heeroma, M., de Heer, E., Meijerink, A., 2000. Charge transfer luminescence of Yb3+ . J. Lumin. 91, 177–193]; in haloborates Sr 2 B5 O9 X, where X = Cl, Br, the UV/visible luminescence was attributed to ytterbium CTL though it looked substantially different from other matrices [Dotsenko, V.P., Berezovskaya, I.V., Pyrogenko, P.V., Efryushina, N.P., Rodniy, P.A., Eijk van, C.W.E., Sidorenko, A.V., 2002. Valence states and luminescence properties of ytterbium ions in strontium haloborates. J. Solid State Chem. 166, 271–276]; while in oxyborate Li2 Lu5 O4 (BO3 )3 “classical” CTL was observed [Jubera, V., Garcia, A., Chaminade, J.P., Guillen, F., Sablayrolles, Jean, Fouassier, C., 2007. Yb3+ and Yb3+ -Eu3+ luminescent properties of the Li2 Lu5 O4 (BO3 )3 phase. J. Lumin. 124(1), 10–14]. In this work the luminescence properties of another borate, namely LSB doped by Yb are presented. © 2007 Elsevier Ltd. All rights reserved. Keywords: Charge transfer luminescence; Borates; Ytterbium
1. Introduction Yb3+ -doped crystals are known to demonstrate two types of luminescence: f–f luminescence in the IR spectral range and charge transfer luminescence (CTL) in the UV-visible range. While the f–f luminescence of Yb3+ is of interest for laser applications, the CTL has recently been shown to be promising for scintillator applications (Guerassimova et al., 2002). CTL of rare-earth doped matrices is much less understood compared to the f–f luminescence of trivalent rare-earth elements studied for several decades by now. The first paper on CTL goes back to the late 1970s (Nakazawa, 1978), extensive study of CTL began only several years ago when the possibility of using
∗ Corresponding author. Tel.: +7 495 939 31 69; fax: +7 495 939 29 91.
E-mail address: [email protected] (D. Krasikov). 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.02.027
Yb-containing scintillators for neutrino detection was demonstrated (Bressi et al., 2001). Now a number of Yb-doped matrices are known to manifest efficient CTL (Van Pieterson et al., 2000). At the same time, single crystals demonstrating CTL with most attractive properties for application as scintillators (YAG-Yb, YAP-Yb, Lu2 O3 .Yb) manifest strong temperature quenching resulting in intensity decrease by the factor of 2 at ∼ 130 K and by two orders of magnitude at room temperature (Kamenskikh et al., 2005; Krasikov et al., 2006). At present, the objective is to find matrices with efficient CTL at room temperature. Among investigated matrices the highest CTL quenching temperature was observed for phosphate powders (Van Pieterson et al., 2000; Voloshinovskii et al., 2003). We looked at another family of crystals with complex oxianion and large bandgap, namely borate crystals. Experimental data on CTL of borates are inconsistent: in some borates CTL was not observed at all, like in orthoborates of Sc, Y and La
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(Van Pieterson et al., 2000); in haloborates Sr 2 B5 O9 X, where X = Cl, Br, UV/visible luminescence was attributed to ytterbium CTL though it looked substantially different from other matrices (Dotsenko et al., 2002); while in oxyborate Li2 Lu5 O4 (BO3 )3 “classical” CTL was observed (Jubera et al., 2007). In contrast to phosphate compounds, the growth methods for a number of borate single crystals of high optical quality are well developed (namely for non-linear optics applications). Yb-doped borates are of interest as laser media as well. Yb-doped LaSc3 (BO3 )4 (LSB) single crystals attract attention due to a large distance between the nearest Yb ions resulting in reduced concentration quenching of the Yb3+ f–f luminescence (Petermann et al., 2005). Here we present first results on the VUV and X-ray excited UV-visible-IR luminescence of Yb-doped LSB single crystals. 2. Experimental LSB crystals with Yb concentration of 0.5%, 3% and 25% were grown in the General Physics Institute using Czochralsky technique. The measurements were performed at the SUPERLUMI station of DORIS III positron storage ring at DESY (Hamburg, Germany) (Zimmerer, 1991). Time-resolved spectra of luminescence and luminescence excitation as well as luminescence decay kinetics were measured at room temperature and 10 K. During luminescence data acquisition two types of time windows were used: fast (2–10 ns from the peak of the excitation pulse) and slow (60–180 ns from the excitation pulse). Luminescence spectra excited by X-rays were measured using an X-ray source XRG3000 INEL with tungsten anode operating at 35 kV.
Fig. 1. Luminescence (1, 1 ) and excitation (2, 2 ) spectra of LSB–Yb 25% measured at 9 K (a) and LSB–Yb 0.5% measured at 300 K (b); dots—integrated luminescence, open dots—slow luminescence component, line—fast luminescence component. Luminescence spectra measured under excitation in the slow peak: 6.7 eV (a) and 6.5 eV (b), excitation spectra measured for 3.2 eV luminescence (a), and 3.65 eV luminescence (b).
3. Results and discussion Luminescence spectrum of LSB-25%Yb measured at 9 K is presented in Fig. 1a. Two broad bands are observed in the range 1.5–4.5 eV. The spectrum looks as “classical” CTL spectra Ybdoped matrices. CTL is an allowed transitions from the charge transfer state (formed by transfer of an electron from the ligands to the rare-earth ion) to the two Yb3+ levels 2 F 5/2 and 2 F 7/2 . So two broad luminescence bands separated by ∼ 1.25 eV (corresponding to separation between 2 F 5/2 and 2 F 7/2 levels) with large Stokes shift and nanosecond kinetics are observed in case of CTL. Spectrum presented in Fig. 1a meets all these conditions: it can be approximated by two Gaussians with separation in energy close to 1.25 eV, the Stokes shift is 2.7 eV, which is typical for the CT transitions and has nanosecond kinetics, which can be fitted by two exponentials with characteristic decay times of 3 and 10 ns (Fig. 2, curve 1). Excitation spectrum of 3.2 eV luminescence in LSB-25%Yb at 9 K (Fig. 1a) looks similar to the excitation spectra usually observed for CTL: a broad fast band (characterized by nanosecond kinetics) just below the fundamental absorption edge. In the case of LSB there is no experimental data on band gap width. In our measurements of reflectivity no features that could assist in its determination (like exciton peaks) were observed. We can evaluate the band gap energy from the excitation spectra
Fig. 2. Decay profiles of 3.2 eV luminescence in LSB–Yb 25% (1), LSB–Yb 3% (2) and LSB–Yb 0.5% (3) excited by 6.05 eV photons measured at 9 K.
(Fig. 1a): the onset of slow luminescence component begins at 6.5 eV with the maximum at 6.7 eV, so the band gap energy in LSB is higher than 6.5 eV. The band in the range 5.5–6.5 eV of excitation spectrum is fast and situated probably just below fundamental absorption edge. In other words all mentioned above characteristics of luminescence in the range 1.5–4.5 eV in LSBYb 25% at 9 K are suitable for attribution of this luminescence to the CTL. Moreover f–f luminescence of Yb3+ in LSB–Yb 25% (observed in IR spectral range with maximum near 1.25 eV) at 9 K is excited with photons of energies higher than 5.5 eV (Fig. 3, open dots), e.g. in the same range as 3.2 eV luminescence. Observation of similar excitation spectra of the CTL
876
N. Guerassimova et al. / Radiation Measurements 42 (2007) 874 – 877
Fig. 3. Luminescence spectrum excited with 5.5 eV photons (1), excitation spectrum of 2.6 eV luminescence (2) and excitation spectrum of IR luminescence (open dots; obtained by integration of the luminescence spectra in the range 960–1040 nm) in LSB–Yb 25% measured at 9 K.
and f–f luminescence in the range of the CT absorption and fundamental absorption edge is typical for Yb-doped matrices (Kamenskikh et al., 2003). In this case the CTL and f–f luminescence are consecutive processes: 2 F 5/2 state of Yb3+ ions is populated via CT transitions and followed by f–f luminescence. The profile of the luminescence spectra in LSB–Yb 25% at 9 K depends on the excitation energy. While under excitation in the range 6–7 eV the high-energy luminescence band in the range 3–4.5 eV is dominating (Fig. 1a), the low-energy luminescence band in the region 1.5–3 eV is most pronounced for the excitation energies below 5.5 eV (Fig. 3). In the excitation spectrum of 2.6 eV luminescence in addition to the bands in the range 5.5–7 eV (similar to the bands in the excitation spectrum of 3.2 eV luminescence) a low-energy excitation band in the range 4–5 eV appears. So one can conclude that in LSB-Yb at least two types of luminescence are observed in the UV-visible range: the first one in the range 1.5–4.5 eV is excited with photons of energies higher than 5.5 eV, and another one in the range 1.5–3 eV is excited with photons of the same energies and also with photons of lower energies. The first type of luminescence is tentatively attributed to the CTL, the second one is probably of defect type. It should be noted that situation when in undoped crystals and in doped crystals under low-energy excitation (energies lower than CT absorption band) intrinsic and defect luminescence are predominant just in the region of CTL is typical (Guerassimova et al., 2005; Voloshinovskii et al., 2003). At room temperature we did not succeed in detection of fast luminescence characterized by nanosecond kinetics. Only slow luminescence with characteristic decay time longer than microsecond was observed. Typical luminescence spectrum measured at room temperature is presented in Fig. 1b. The only luminescence band is observed in the range 2.5–4.5 eV. Spectra obtained under X-ray excitation are similar to those measured under VUV excitation (Fig. 4). Since this luminescence is slow and no luminescence is observed in the range 1.5–2.5 eV,
Fig. 4. Luminescence spectra of LSB–Yb 25% (1, 1 ), LSB–Yb 3% (2, 2 ) and LSB–Yb 0.5% (3, 3 ) measured at 300 K under X-ray excitation.
one can conclude that both types of luminescence observed at 9 K and discussed above are quenched at 300 K. Instead of these types of luminescence another luminescence is observed at 300 K. This luminescence is excited with photons of energies higher than 6 eV. The yield of this luminescence increases with the decrease of Yb concentration, whereas concentration dependence of Yb3+ f–f luminescence yield is inverse (Fig. 4). These tendencies suggest that this luminescence is not related to Yb but seems to be of intrinsic origin. To our knowledge, there are no data on the intrinsic luminescence of LSB. To understand the origin of the observed luminescence the study of undoped LSB crystals is planned. Since the spectral range of the slow luminescence observed at RT and the spectral range of the fast luminescence observed at 9 K overlap, slow luminescence can contribute to the luminescence spectra at low temperature as well. The increase of slow luminescent component contribution with decrease of Yb concentration (Fig. 2) can imply that for low Yb concentration slow luminescence contribute to luminescence spectra at 9 K while for concentrated sample only fast luminescence is observed at 9 K in the range 3–4.5 eV. Summarizing, at least three types of luminescence were observed for LSB-Yb single crystals in the UV-visible range. The first one observed in the spectral range 1.5–4.5 eV consists of two broad luminescence bands and is characterized by nanosecond kinetics. The properties of this luminescence allow to attribute it to the charge transfer luminescence. The second type of luminescence is observed in the range 1.5–3 eV and is probably of defect origin. Both types of luminescence are quenched at room temperature. Finally, at room temperature and low Yb concentration slow luminescence in the range 2.5–4.5 eV is dominating. To understand unambiguously the origin of observed UVvisible luminescence in LSB-Yb single crystals further investigation of LSB-Yb as well as of undoped LSB crystals is planned.
N. Guerassimova et al. / Radiation Measurements 42 (2007) 874 – 877
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