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Luminescence characteristics of Cu- and Eu-doped Li2B4O7
Radiation Measurements 38 (2004) 567 – 570 www.elsevier.com/locate/radmeas
Luminescence characteristics of Cu- and Eu-doped Li2B4O7 Magdalyna Ignatovycha;∗ , Vadym Holoveyb , Andrea Watterichc , Tam2as Vid2oczyd , P2eter Baranyaid , Andr2as Kelemene , Oleksij Chuikoa a Institute
of Surface Chemistry, NASU, 17 Gen. Naumov str., Kyiv 03164, Ukraine of Electronic Physics, NASU, 21 University str., Uzhgorod 88000, Ukraine c Institute of Solid State Physics and Optics, HAS, P.O. Box 49, Budapest, 1525 Hungary d Photoscience Laboratory, BUTE and CRC HAS, P.O. Box 17, Budapest 1525, Hungary e Institute for Isotope and Surface Chemistry, HAS CRC P.O. Box 77, Budapest 1525, Hungary b Institute
Received 21 November 2003; received in revised form 21 November 2003; accepted 5 January 2004
Abstract Photoluminescence, radioluminescence as well as optical absorption techniques were used to characterize Cu- and Eu-doped lithium tetraborate. The e=ect of host modi>cation (single crystal and glassy state) and a limited range of dopant content were investigated. Time-resolved measurements were helpful to identify the emitting species. c 2004 Published by Elsevier Ltd. Keywords: Photoluminescence; Radioluminescence; Cu- and Eu-doped lithium tetraborate
1. Introduction
2. Experimental
Cu- and Eu-doped Li2 B4 O7 (LTB) exhibit promising characteristics as scintillators, neutron detectors and tissue-equivalent thermoluminescent (TL) materials for clinical and personal dosimetry (Prokic, 2002). Until now most studies have been devoted to TL of doped LTB, while investigations concerning their photoluminescence (PhL) and radioluminescence (RL) are quite limited (Dubovik et al., 2000; Furetta et al., 2001; Senguttuvan et al., 2002). Recently, we have reported for the >rst time results of time-resolved PhL and RL studies of Cu- and Eu-doped LTB (Ignatovych et al., 2003). The present study focuses on the detailed characterization of these samples using steady-state and time-resolved PhL, RL and optical absorption (OA) techniques. The e=ect of LTB host modi>cation—single crystal and glassy—as well as that of the dopant are investigated.
Single crystals of LTB:Eu and LTB:Cu were grown by the Czochralski method from high purity (at least 99.99%) precursor compounds. Glassy samples were prepared by melting doped single crystals to ensure the same dopant content. The doping was performed at the Li2 CO3 –H3 BO3 blend synthesis stage by adding di=erent amounts of CuO or Eu2 O3 . It should be pointed out that the synthesis of both single crystals and corresponding glassy samples were performed under air atmosphere, without adding either oxidizing or reducing compounds. Grown LTB:Cu and LTB:Eu single crystals were generally clear and transparent; glassy samples were also of good optical quality. The dopant content in the samples—estimated by atomic absorption spectroscopy for Cu and by neutron activation analysis for Eu— were found to be of 0.007– 0.05 wt% Cu and of 0.0016 – 0.008 wt% Eu. PhL spectra were recorded either with a Hitachi F4500 spectroKuorimeter under excitation by a xenon lamp or with a Perkin–Elmer LS50B luminescence spectrometer equipped with a pulsed xenon lamp. The latter instrument is
∗ Corresponding author. Tel.: +380-44-444-9698; fax: +38044-452-6735. E-mail address: [email protected] (M. Ignatovych).
c 2004 Published by Elsevier Ltd. 1350-4487/$ - see front matter doi:10.1016/j.radmeas.2004.01.011
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M. Ignatovych et al. / Radiation Measurements 38 (2004) 567 – 570
capable to perform phosphorescence measurements, using selectable time delays between excitation and detection. Time-resolved PhL was excited by the fourth harmonic (266 nm) of a Nd-YAG laser (60mJ/pulse), and the emission was monitored through a f/4.0 monochromator with an EMI 9783R photomultiplier. The signal was stored on a digital oscilloscope (Tektronix TDS 210), the average of 64 measurements has been evaluated with standard spreadsheet program. Time-resolved RL was excited by short, high-energy electron pulses of a LINAC (2:6 s; 4 MeV). The emission was monitored through an Applied Photophysics f/3.4 monochromator with a 1P28 photomultiplier. The signal was stored on a digital oscilloscope (Phillips PM3375), the average of 20 measurements has been evaluated with a scienti>c analysis software. Optical absorption spectra were measured using a JASCO V-550 UV-VIS spectrophotometer. All spectra were taken at room temperature.
3. Experimental results and discussion 3.1. LTB:Cu 3.1.1. Single crystals Fig. 1 displays representative PhL excitation and emission spectra and decay curve for LTB:Cu single crystal. Excitation spectrum was measured in the range of 200 –300 nm for each di=erent Cu content and two strongly overlapping bands at about 240 –260 nm were observed. This observation is in agreement with those reported by Pedrini (1978) on the shape of excitation spectrum of Cu+ at room temperature. Emission spectra published earlier (Ignatovych et al., 2003) exhibited always a single band at about 370 nm. The luminescence intensity was growing with increasing Cu content. From this it is evident that luminescence quenching did not occur within the tested Cu concentration range.
Fig. 1. Excitation and emission spectra of LTB:Cu single crystal. The inset shows the decay of the emission.
Fig. 2. Emission spectra of 1: glassy LTB:Cu measured with the luminescence spectrometer; 2: the same measured by time-resolved method at 250 ns; 3: undoped glassy LTB, measured with the luminescence spectrometer.
The RL spectrum and decay curve at 370 nm for LTB:Cu single crystal reported earlier (Ignatovych et al., 2003) are very similar to that of PhL curves. The comparison of PhL and RL measurements points to similar luminescence centers being involved in both processes. The peak position, the value of the Stokes shift and the long decay time (24 s in PhL and 29 s in RL) permit to ascribe this emission to parity- and spin-forbidden 3d 9 4s → 3d 10 transition from the triplet state of Cu+ . The emission spectra are independent of the excitation wavelength, and excitation spectra do not change upon changing the monitoring wavelength, which, together with the single exponential decay of the emission strongly point to the presence of a single type of Cu+ ions. 3.1.2. Glassy samples PhL emission showed dramatic intensity decrease as compared to the single crystals to the extent that their emission spectra are not easily measurable. The emission is substantially broader and red-shifted as compared to the single crystal, as it can be seen in Fig. 2. (curve 1). A broad envelope with poorly resolved bands in the wavelength range 350 – 650 was observed. Since the undoped LTB glassy sample exhibits a weak emission in the range 300 –500 nm (see Fig. 2, curve 3), we performed time-resolved measurements to distinguish between the two emissions. Indeed, the emission from the undoped LTB glassy sample is very short-lived (under 10ns). At the same time the emission from the Cu doped sample, besides this fast component, has a slower component too (having a lifetime of 22 s), and the spectrum of this slow component (measured at 250 ns after excitation, thus after the complete decay of the fast component) matches the emission spectrum of LTB:Cu, see Fig. 2. Thus, while there is a red shift in the emission spectrum of Cu+ in the glassy sample, the lifetime of the emitting state remained practically the same as compared to the single crystal.
M. Ignatovych et al. / Radiation Measurements 38 (2004) 567 – 570
Fig. 3. Optical absorption of LTB:Cu. Curve 1: single crystal, curve 2: glassy.
It is important to note that very similar emission spectra patterns of Cu+ incorporated in di=erent glassy matrices have been reported (Debnath and Das, 1989; Justus et al., 1999; Verwey et al., 1991). These authors ascribe the observed PhL to metal-centered transition of Cu+ , however, we suspect that the assignation of emission from LTB:Cu glassy samples needs further study. As we pointed out in the experimental part the total Cu content is the same for the single crystal and the corresponding glassy sample prepared from it. So it is reasonable to suppose a considerable decrease of Cu+ species in the glassy sample, caused most probably by their oxidation to Cu2+ . It is known that Cu2+ ions are not emitting, explaining the mentioned drastic emission intensity decrease. The optical absorption data presented in Fig. 3, are in agreement with this assumption. The spectra are di=erent in the UV as well as in the visible range. Curve 1 in Fig. 3, showing the absorption of the single crystal is in good agreement with the excitation spectrum for the single crystal sample. The glassy sample displays an absorption band of high intensity quite typical for charge transfer (CT) of O2− → Cu2+ (curve 2 in Fig. 3). Besides, the crystal->eld transition of Cu2+ was observed in the range of 600 –800 nm (curve 2 in the insert of Fig. 3). 3.2. LTB:Eu It is important to note that steady-state and time-resolved PhL characteristics of Eu-doped single crystals are substantially di=erent from those of the glassy samples. 3.2.1. Single crystals The PhL spectrum displayed a single band emission at about 370nm. Together with the lifetime value (24 s) this points to Eu2+ being the emitting species (Fig. 4). It should be mentioned that changes in the oxidation state of Eu in the grown crystal in comparison with the precur-
569
Fig. 4. Emission spectra of LTB:Eu single crystals of various growth excited at 250 nm.
Fig. 5. Emission spectra of glassy LTB:Eu sample (dashed line) and BGO:Eu single crystal (full line). The inset shows the decay, upper curve: LTB:Eu, lower curve: BGO:Eu.
sor used for doping was reported earlier. Thus (Bacci et al., 1993) observed typical spectral pattern of Eu+2 (single band at 363 nm) in KMgF3 crystal doped with Eu2 O3 while (Danilkin et al., 1995) reported the presence of Eu+2 and Eu+2 pairs in CaS crystals doped with EuBr 3 . The changes in the optical features and valence state of Eu in Li2 B4 O7 glassy samples depending on growth atmosphere was observed too (Kaczmarek, 2002). 3.2.2. Glassy samples Fig. 5 shows the PhL spectrum of glassy LTB:Eu exhibiting >nely structured emission, dominated by 5 D0 → 7 F2 transition (maximum at 615 nm). The high intensity ratio of the electric-dipole transition 5 D0 → 7 F2 and the magnetic dipole transition 5 D0 → 7 F1 (the criterion of (Blasse and Grabmaier, 1994)) and the presence of 5 D0 → 7 F0 transition in the spectrum allow to assume very low site symmetry of Eu3+ in this matrix. The comparison with the PhL spectrum of Eu doped bismuth-germanium oxide (BGO:Eu) single crystal (Fig. 5) leads to an additional proof of this
570
M. Ignatovych et al. / Radiation Measurements 38 (2004) 567 – 570
assumption. The PhL spectrum of BGO:Eu is better resolved, the lines are narrower and the intensities of transitions 5 D0 → 7 F2 (at 601 nm) and 5 D0 → 7 F1 (at 590 nm) are comparable. These features imply a higher site-symmetry of Eu3+ in this lattice and this is in agreement with the assumption in the literature about Eu3+ substitution for Bi3+ in this scintillator. Kinetic parameters of these Eu-doped phosphors—PhL decay curves (Fig. 5) and lifetime data in the ms domain— are quite typical for Eu3+ species. The slightly di=erent lifetime values—1:6 ms for BGO:Eu and 2:1 ms for LTB:Eu— may reKect the host lattice e=ect. 4. Conclusions It has been shown that for: (i) LTB:Cu single crystals PhL features—narrow monoband emission at 370 nm and monoexponential decay with = 24 s strongly point to one type of Cu+ ions as emitting species. (ii) LTB:Cu glassy samples PhL, observed in the range of 350 –600 nm, displayed the remarkable inhomogeneous broadening, red shift and intensity decrease as compared to that for the crystal. Optical absorption data evidenced that, besides Cu+ -ions, Cu2+ -ions are present. (iii) LTB:Eu single crystals—PhL spectrum exhibited blue single band emission at 370 nm with the lifetime 24 s, attributed to Eu2+ . (iv) LTB:Eu glassy samples PhL characteristics—red >ne-structure line emission and lifetime values 2,2 ms unambiguously show that Eu3+ are the emitting species. Thus, it has been revealed that the host-matrix modi>cation crucially determines the status of the dopant—oxidation state, site symmetry—and hence, the luminescence characteristics of the material. Acknowledgements This research was performed in the framework of Collaboration Agreement between Academies of Sciences of Ukraine and Hungary and was also supported by the National Science Research Fund OTKA, Hungary (Grant No.
T-037669) and the Center of Excellence Program (Contract No. ICA1-2000-70029) of the European Union. References Bacci, C., Fioravanti, S., Furetta, C., Missori, M., Ramogida, G., Rossetti, R., Sanipli, C., Scacco, A., 1993. Photoluminescence and thermally stimulated luminescence in KMgF3 : Eu2+ crystals. Radiat. Prot. Dosim. 47, 277–280. Blasse, G., Grabmaier, B.C., 1994. Luminescent Materials. Springer, Berlin. Danilkin, M., Must, M., Pedak, E., Parnoja, E., Ryasnyi, G., Seeman, V., Shpinkov, I., Shpinkova, L., 1995. Di=erent Eu-centres in CaS:Eu,Cl. Radiat. Measur. 24, 351–354. Debnath, R., Das, S.K., 1989. Site-dependent luminescence of Cu+ ions in silica glass. Chem. Phys. Lett. 155, 52–58. Dubovik, M.F., Tolmachev, A.V., Grinyov, B.V., Grin’, L.A., Dolzhenkova, E.F., Dobrotvorskaja, M.V., 2000. Luminescence and radiation-induced defects in Li2 B4 O7 :Eu single crystals. Semicond. Phys. Quant. Electron. Optoelectron. 3, 420–422. Furetta, C., Prokic, M., Salamon, R., Prokic, V., Kitis, G., 2001. Dosimetric characteristics of tissue equivalent thermoluminescent solid TL detectors based on lithium borate. Nucl. Instrum. Meth. A 456, 411–417. Ignatovych, M., Holovey, V., Watterich, A., Vid2oczy, T., Baranyai, P., Kelemen, A., Ogenko, V., Chuiko, O., 2003. UV and electron radiation-induced luminescence of Cu- and Eu-doped lithium tetraborate. Radiat. Phys. Chem. 67, 587–591. Justus, B.L., Pawlovich, K.J., Merritt, C.D., Huston, A.L., 1999. Optically and thermally stimulated luminescence characteristics of Cu+ -doped fused quartz. Radiat. Prot. Dosim. 81, 5–10. Kaczmarek, S.M., 2002. Li2 B4 O7 glasses doped with Cr, Co, Eu and Dy. Opt. Mater. 19, 189–194. Pedrini, C., 1978. Cu+ luminescent centre in LiCl single crystals. Phys. Status Solidi B 87, 273–286. Prokic, M., 2002. Dosimetric characteristics of Li2 B4 O7 :Cu,Ag,P solid TL detectors. Radiat. Prot. Dosim. 100, 265–268. Senguttuvan, N., Ishiii, M., Shimoyama, M., Kobayashi, M., Tsutsui, N., Nikl, M., Dusek, M., Shimizu, H.M., Oku, T., Adachi, T., Sakai, K., Suzuki, J., 2002. Crystal growth and luminescence properties of Li2 B4 O7 single crystals doped with Ce, In, Ni, Cu and Ti ions. Nucl. Instrum. Meth. A 486, 264– 267. Verwey, J.W.M., Coronado, J.M., Blasse, G., 1991. The luminescence of Cu(I) in strontium tetraborate. J. Solid State Chem. 92, 531–536.
Luminescence characteristics of Cu- and Eu-doped Li2B4O7 Magdalyna Ignatovycha;∗ , Vadym Holoveyb , Andrea Watterichc , Tam2as Vid2oczyd , P2eter Baranyaid , Andr2as Kelemene , Oleksij Chuikoa a Institute
of Surface Chemistry, NASU, 17 Gen. Naumov str., Kyiv 03164, Ukraine of Electronic Physics, NASU, 21 University str., Uzhgorod 88000, Ukraine c Institute of Solid State Physics and Optics, HAS, P.O. Box 49, Budapest, 1525 Hungary d Photoscience Laboratory, BUTE and CRC HAS, P.O. Box 17, Budapest 1525, Hungary e Institute for Isotope and Surface Chemistry, HAS CRC P.O. Box 77, Budapest 1525, Hungary b Institute
Received 21 November 2003; received in revised form 21 November 2003; accepted 5 January 2004
Abstract Photoluminescence, radioluminescence as well as optical absorption techniques were used to characterize Cu- and Eu-doped lithium tetraborate. The e=ect of host modi>cation (single crystal and glassy state) and a limited range of dopant content were investigated. Time-resolved measurements were helpful to identify the emitting species. c 2004 Published by Elsevier Ltd. Keywords: Photoluminescence; Radioluminescence; Cu- and Eu-doped lithium tetraborate
1. Introduction
2. Experimental
Cu- and Eu-doped Li2 B4 O7 (LTB) exhibit promising characteristics as scintillators, neutron detectors and tissue-equivalent thermoluminescent (TL) materials for clinical and personal dosimetry (Prokic, 2002). Until now most studies have been devoted to TL of doped LTB, while investigations concerning their photoluminescence (PhL) and radioluminescence (RL) are quite limited (Dubovik et al., 2000; Furetta et al., 2001; Senguttuvan et al., 2002). Recently, we have reported for the >rst time results of time-resolved PhL and RL studies of Cu- and Eu-doped LTB (Ignatovych et al., 2003). The present study focuses on the detailed characterization of these samples using steady-state and time-resolved PhL, RL and optical absorption (OA) techniques. The e=ect of LTB host modi>cation—single crystal and glassy—as well as that of the dopant are investigated.
Single crystals of LTB:Eu and LTB:Cu were grown by the Czochralski method from high purity (at least 99.99%) precursor compounds. Glassy samples were prepared by melting doped single crystals to ensure the same dopant content. The doping was performed at the Li2 CO3 –H3 BO3 blend synthesis stage by adding di=erent amounts of CuO or Eu2 O3 . It should be pointed out that the synthesis of both single crystals and corresponding glassy samples were performed under air atmosphere, without adding either oxidizing or reducing compounds. Grown LTB:Cu and LTB:Eu single crystals were generally clear and transparent; glassy samples were also of good optical quality. The dopant content in the samples—estimated by atomic absorption spectroscopy for Cu and by neutron activation analysis for Eu— were found to be of 0.007– 0.05 wt% Cu and of 0.0016 – 0.008 wt% Eu. PhL spectra were recorded either with a Hitachi F4500 spectroKuorimeter under excitation by a xenon lamp or with a Perkin–Elmer LS50B luminescence spectrometer equipped with a pulsed xenon lamp. The latter instrument is
∗ Corresponding author. Tel.: +380-44-444-9698; fax: +38044-452-6735. E-mail address: [email protected] (M. Ignatovych).
c 2004 Published by Elsevier Ltd. 1350-4487/$ - see front matter doi:10.1016/j.radmeas.2004.01.011
568
M. Ignatovych et al. / Radiation Measurements 38 (2004) 567 – 570
capable to perform phosphorescence measurements, using selectable time delays between excitation and detection. Time-resolved PhL was excited by the fourth harmonic (266 nm) of a Nd-YAG laser (60mJ/pulse), and the emission was monitored through a f/4.0 monochromator with an EMI 9783R photomultiplier. The signal was stored on a digital oscilloscope (Tektronix TDS 210), the average of 64 measurements has been evaluated with standard spreadsheet program. Time-resolved RL was excited by short, high-energy electron pulses of a LINAC (2:6 s; 4 MeV). The emission was monitored through an Applied Photophysics f/3.4 monochromator with a 1P28 photomultiplier. The signal was stored on a digital oscilloscope (Phillips PM3375), the average of 20 measurements has been evaluated with a scienti>c analysis software. Optical absorption spectra were measured using a JASCO V-550 UV-VIS spectrophotometer. All spectra were taken at room temperature.
3. Experimental results and discussion 3.1. LTB:Cu 3.1.1. Single crystals Fig. 1 displays representative PhL excitation and emission spectra and decay curve for LTB:Cu single crystal. Excitation spectrum was measured in the range of 200 –300 nm for each di=erent Cu content and two strongly overlapping bands at about 240 –260 nm were observed. This observation is in agreement with those reported by Pedrini (1978) on the shape of excitation spectrum of Cu+ at room temperature. Emission spectra published earlier (Ignatovych et al., 2003) exhibited always a single band at about 370 nm. The luminescence intensity was growing with increasing Cu content. From this it is evident that luminescence quenching did not occur within the tested Cu concentration range.
Fig. 1. Excitation and emission spectra of LTB:Cu single crystal. The inset shows the decay of the emission.
Fig. 2. Emission spectra of 1: glassy LTB:Cu measured with the luminescence spectrometer; 2: the same measured by time-resolved method at 250 ns; 3: undoped glassy LTB, measured with the luminescence spectrometer.
The RL spectrum and decay curve at 370 nm for LTB:Cu single crystal reported earlier (Ignatovych et al., 2003) are very similar to that of PhL curves. The comparison of PhL and RL measurements points to similar luminescence centers being involved in both processes. The peak position, the value of the Stokes shift and the long decay time (24 s in PhL and 29 s in RL) permit to ascribe this emission to parity- and spin-forbidden 3d 9 4s → 3d 10 transition from the triplet state of Cu+ . The emission spectra are independent of the excitation wavelength, and excitation spectra do not change upon changing the monitoring wavelength, which, together with the single exponential decay of the emission strongly point to the presence of a single type of Cu+ ions. 3.1.2. Glassy samples PhL emission showed dramatic intensity decrease as compared to the single crystals to the extent that their emission spectra are not easily measurable. The emission is substantially broader and red-shifted as compared to the single crystal, as it can be seen in Fig. 2. (curve 1). A broad envelope with poorly resolved bands in the wavelength range 350 – 650 was observed. Since the undoped LTB glassy sample exhibits a weak emission in the range 300 –500 nm (see Fig. 2, curve 3), we performed time-resolved measurements to distinguish between the two emissions. Indeed, the emission from the undoped LTB glassy sample is very short-lived (under 10ns). At the same time the emission from the Cu doped sample, besides this fast component, has a slower component too (having a lifetime of 22 s), and the spectrum of this slow component (measured at 250 ns after excitation, thus after the complete decay of the fast component) matches the emission spectrum of LTB:Cu, see Fig. 2. Thus, while there is a red shift in the emission spectrum of Cu+ in the glassy sample, the lifetime of the emitting state remained practically the same as compared to the single crystal.
M. Ignatovych et al. / Radiation Measurements 38 (2004) 567 – 570
Fig. 3. Optical absorption of LTB:Cu. Curve 1: single crystal, curve 2: glassy.
It is important to note that very similar emission spectra patterns of Cu+ incorporated in di=erent glassy matrices have been reported (Debnath and Das, 1989; Justus et al., 1999; Verwey et al., 1991). These authors ascribe the observed PhL to metal-centered transition of Cu+ , however, we suspect that the assignation of emission from LTB:Cu glassy samples needs further study. As we pointed out in the experimental part the total Cu content is the same for the single crystal and the corresponding glassy sample prepared from it. So it is reasonable to suppose a considerable decrease of Cu+ species in the glassy sample, caused most probably by their oxidation to Cu2+ . It is known that Cu2+ ions are not emitting, explaining the mentioned drastic emission intensity decrease. The optical absorption data presented in Fig. 3, are in agreement with this assumption. The spectra are di=erent in the UV as well as in the visible range. Curve 1 in Fig. 3, showing the absorption of the single crystal is in good agreement with the excitation spectrum for the single crystal sample. The glassy sample displays an absorption band of high intensity quite typical for charge transfer (CT) of O2− → Cu2+ (curve 2 in Fig. 3). Besides, the crystal->eld transition of Cu2+ was observed in the range of 600 –800 nm (curve 2 in the insert of Fig. 3). 3.2. LTB:Eu It is important to note that steady-state and time-resolved PhL characteristics of Eu-doped single crystals are substantially di=erent from those of the glassy samples. 3.2.1. Single crystals The PhL spectrum displayed a single band emission at about 370nm. Together with the lifetime value (24 s) this points to Eu2+ being the emitting species (Fig. 4). It should be mentioned that changes in the oxidation state of Eu in the grown crystal in comparison with the precur-
569
Fig. 4. Emission spectra of LTB:Eu single crystals of various growth excited at 250 nm.
Fig. 5. Emission spectra of glassy LTB:Eu sample (dashed line) and BGO:Eu single crystal (full line). The inset shows the decay, upper curve: LTB:Eu, lower curve: BGO:Eu.
sor used for doping was reported earlier. Thus (Bacci et al., 1993) observed typical spectral pattern of Eu+2 (single band at 363 nm) in KMgF3 crystal doped with Eu2 O3 while (Danilkin et al., 1995) reported the presence of Eu+2 and Eu+2 pairs in CaS crystals doped with EuBr 3 . The changes in the optical features and valence state of Eu in Li2 B4 O7 glassy samples depending on growth atmosphere was observed too (Kaczmarek, 2002). 3.2.2. Glassy samples Fig. 5 shows the PhL spectrum of glassy LTB:Eu exhibiting >nely structured emission, dominated by 5 D0 → 7 F2 transition (maximum at 615 nm). The high intensity ratio of the electric-dipole transition 5 D0 → 7 F2 and the magnetic dipole transition 5 D0 → 7 F1 (the criterion of (Blasse and Grabmaier, 1994)) and the presence of 5 D0 → 7 F0 transition in the spectrum allow to assume very low site symmetry of Eu3+ in this matrix. The comparison with the PhL spectrum of Eu doped bismuth-germanium oxide (BGO:Eu) single crystal (Fig. 5) leads to an additional proof of this
570
M. Ignatovych et al. / Radiation Measurements 38 (2004) 567 – 570
assumption. The PhL spectrum of BGO:Eu is better resolved, the lines are narrower and the intensities of transitions 5 D0 → 7 F2 (at 601 nm) and 5 D0 → 7 F1 (at 590 nm) are comparable. These features imply a higher site-symmetry of Eu3+ in this lattice and this is in agreement with the assumption in the literature about Eu3+ substitution for Bi3+ in this scintillator. Kinetic parameters of these Eu-doped phosphors—PhL decay curves (Fig. 5) and lifetime data in the ms domain— are quite typical for Eu3+ species. The slightly di=erent lifetime values—1:6 ms for BGO:Eu and 2:1 ms for LTB:Eu— may reKect the host lattice e=ect. 4. Conclusions It has been shown that for: (i) LTB:Cu single crystals PhL features—narrow monoband emission at 370 nm and monoexponential decay with = 24 s strongly point to one type of Cu+ ions as emitting species. (ii) LTB:Cu glassy samples PhL, observed in the range of 350 –600 nm, displayed the remarkable inhomogeneous broadening, red shift and intensity decrease as compared to that for the crystal. Optical absorption data evidenced that, besides Cu+ -ions, Cu2+ -ions are present. (iii) LTB:Eu single crystals—PhL spectrum exhibited blue single band emission at 370 nm with the lifetime 24 s, attributed to Eu2+ . (iv) LTB:Eu glassy samples PhL characteristics—red >ne-structure line emission and lifetime values 2,2 ms unambiguously show that Eu3+ are the emitting species. Thus, it has been revealed that the host-matrix modi>cation crucially determines the status of the dopant—oxidation state, site symmetry—and hence, the luminescence characteristics of the material. Acknowledgements This research was performed in the framework of Collaboration Agreement between Academies of Sciences of Ukraine and Hungary and was also supported by the National Science Research Fund OTKA, Hungary (Grant No.
T-037669) and the Center of Excellence Program (Contract No. ICA1-2000-70029) of the European Union. References Bacci, C., Fioravanti, S., Furetta, C., Missori, M., Ramogida, G., Rossetti, R., Sanipli, C., Scacco, A., 1993. Photoluminescence and thermally stimulated luminescence in KMgF3 : Eu2+ crystals. Radiat. Prot. Dosim. 47, 277–280. Blasse, G., Grabmaier, B.C., 1994. Luminescent Materials. Springer, Berlin. Danilkin, M., Must, M., Pedak, E., Parnoja, E., Ryasnyi, G., Seeman, V., Shpinkov, I., Shpinkova, L., 1995. Di=erent Eu-centres in CaS:Eu,Cl. Radiat. Measur. 24, 351–354. Debnath, R., Das, S.K., 1989. Site-dependent luminescence of Cu+ ions in silica glass. Chem. Phys. Lett. 155, 52–58. Dubovik, M.F., Tolmachev, A.V., Grinyov, B.V., Grin’, L.A., Dolzhenkova, E.F., Dobrotvorskaja, M.V., 2000. Luminescence and radiation-induced defects in Li2 B4 O7 :Eu single crystals. Semicond. Phys. Quant. Electron. Optoelectron. 3, 420–422. Furetta, C., Prokic, M., Salamon, R., Prokic, V., Kitis, G., 2001. Dosimetric characteristics of tissue equivalent thermoluminescent solid TL detectors based on lithium borate. Nucl. Instrum. Meth. A 456, 411–417. Ignatovych, M., Holovey, V., Watterich, A., Vid2oczy, T., Baranyai, P., Kelemen, A., Ogenko, V., Chuiko, O., 2003. UV and electron radiation-induced luminescence of Cu- and Eu-doped lithium tetraborate. Radiat. Phys. Chem. 67, 587–591. Justus, B.L., Pawlovich, K.J., Merritt, C.D., Huston, A.L., 1999. Optically and thermally stimulated luminescence characteristics of Cu+ -doped fused quartz. Radiat. Prot. Dosim. 81, 5–10. Kaczmarek, S.M., 2002. Li2 B4 O7 glasses doped with Cr, Co, Eu and Dy. Opt. Mater. 19, 189–194. Pedrini, C., 1978. Cu+ luminescent centre in LiCl single crystals. Phys. Status Solidi B 87, 273–286. Prokic, M., 2002. Dosimetric characteristics of Li2 B4 O7 :Cu,Ag,P solid TL detectors. Radiat. Prot. Dosim. 100, 265–268. Senguttuvan, N., Ishiii, M., Shimoyama, M., Kobayashi, M., Tsutsui, N., Nikl, M., Dusek, M., Shimizu, H.M., Oku, T., Adachi, T., Sakai, K., Suzuki, J., 2002. Crystal growth and luminescence properties of Li2 B4 O7 single crystals doped with Ce, In, Ni, Cu and Ti ions. Nucl. Instrum. Meth. A 486, 264– 267. Verwey, J.W.M., Coronado, J.M., Blasse, G., 1991. The luminescence of Cu(I) in strontium tetraborate. J. Solid State Chem. 92, 531–536.