Photoluminescence of Pb2+-doped SrHfO3

Photoluminescence of Pb2+-doped SrHfO3

Radiation Measurements 45 (2010) 406–408 Contents lists available at ScienceDirect Radiation Measurements journal homepage: www.elsevier.com/locate/...

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Radiation Measurements 45 (2010) 406–408

Contents lists available at ScienceDirect

Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas

Photoluminescence of Pb2þ-doped SrHfO3 V. Jary a, c, *, M. Nikl a, E. Mihokova a, b, P. Bohacek a, B. Trunda a, K. Polak a, V. Studnicka a, V. Mucka c a

Institute of Physics AS CR, Cukrovarnicka 10, 162 53 Prague, Czech Republic Department of Materials Science, University of Milano-Bicocca, 20125 Milan, Italy c Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University, Brehova 7, 115 19 Prague, Czech Republic b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 August 2009 Received in revised form 7 December 2009 Accepted 6 January 2010

Pb2þ excitation and emission spectra and decay kinetics in SrHfO3 host are measured within 10–350 K temperature interval. Temperature dependence of decay times is fit by the phenomenological model describing the dynamics of the lowest excited state of Pb2þ ion. The best fit provided corresponding quantitative parameters of the model. Suitability of the Pb-doped SrHfO3 for phosphor and scintillator application is discussed. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: SrHfO3:Pb2þ Photoluminescence Decay kinetics

1. Introduction Ce-doped SrHfO3 (SHO) is known phosphor material attractive because of the high density, low intrinsic radioactivity and fast response due to Ce3þ 5d–4f emission at 408 nm with the photoluminescence decay time of about 35 ns (Villanueva-Ibanez et al., 2004; Retot et al., 2008; Van Loef et al., 2007). However, doping Ce3þ at the Sr2þ site introduces a coulombic disbalance requiring a compensation by either some point defects, or deviations from stoichiometry or codoping the host lattice (Loureiro et al., 2005; Ji et al., 2005). Searching for another fast emission center to be introduced in the SHO host, the Pb2þ ion seems perfectly suitable candidate to substitute for Sr2þ thanks to similar ionic radius and equivalent charge state. Due to high degree of spin–orbit interaction the 6s2– 6s6p electronic transition becomes partly allowed and some of the 6s2 ions (Tlþ, Pb2þ, Bi3þ) have been successfully applied as emitting centers in phosphors and scintillators (Jacobs, 1991; Weber, 2002). Recently the Pb2þ -doped SHO was prepared in a powder form by acetate and citrate combustion method and mainly its radio- and thermo- luminescence were studied in broad temperature region 10–300 K (Mihokova et al., submitted for publication). Emission band at 340 nm and the decay time of about 190 ns at room temperature (RT) were assigned to the Pb2þ center.

* Corresponding author. Institute of Physics AS CR, Cukrovarnicka 10, 162 53 Prague, Czech Republic. Tel.: þ420 724539175. E-mail address: [email protected] (V. Jary). 1350-4487/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2010.01.013

In this work we study the SHO:Pb2þ powder prepared by the multi-step solid state reaction within 500–1200  C. Photoluminescence spectra and decay kinetics of the Pb2þ center were measured in the 10–350 K temperature range. We set up a phenomenological model to explain a temperature dependence of both the decay times and emission intensities. 2. Experimental details Powder samples of SHO:Pb2þ were prepared by the solid state reaction, multi-step annealing technique and were fixed in the microcuvette for the optical measurements. Photoluminescence spectra and decay curves were measured by the modified Spectrofluorometer 199S, Edinburgh Instrument using the steady state hydrogen lamp (spectra) and microsecond xenon and nanosecond hydrogen pulsed flashlamps (decays) as the excitation sources. Single grating monochromators and photon counting photomultiplierbased detectors were used in the optical part of the set-up. Spectra were corrected for instrumental effects and a convolution procedure was applied to the decay curves to determine true decay times (SpectraSolve software package, Ames Photonics). 3. Results and discussion Normalized RT excitation and emission spectra as well as an Xray excited radioluminescence spectrum of SHO:Pb (0.3 mol%) are presented in Fig. 1. For all Pb2þ concentrations and temperatures studied, the excitation spectra consists of two bands. These two bands, peaking at 215 nm (less intense) and 270 nm, can be

V. Jary et al. / Radiation Measurements 45 (2010) 406–408

Fig. 1. RT excitation (a), emission (b) and radioluminescence (c) spectra of SHO:Pb.

assigned to the transitions from 1S0 (ground state) of Pb2þ luminescence center to the excited states 1P1 and 3P1, respectively. As expected (Jacobs, 1991), the emission spectra feature one broad band. Its maximum is high-energy shifted by 95 meV from 346 nm at 10 K to 337 nm at RT. The shape of the band is the same under both UV and X-ray excitations and only slightly depends on Pb2þ concentration. From excitation and emission maxima, Fig. 1, one can determine the Stokes shift of 960 meV. Decay kinetics of the 340 nm band was studied in the temperature range from 10 to 350 K. As an example, the decay curve at 10 K is displayed in Fig. 2. There is one dominant component with the decay time 279 ms, originating from the strongly forbidden transition 3P0 / 1S0 of the Pb2þ luminescence center. At higher temperatures this decay time gets faster due to phonon induced thermal population of 3P1 (Hlinka et al., 1991), see Fig. 3. The very beginning of the decay curve seems to be much faster than the rest of the decay. It can be due to the residue of the fast component resulting from non-zero population of 3P1 level at low temperature. This hypothesis though needs a separate investigation.

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Fig. 3. Temperature dependence of decay times of the slow component (ex ¼ 269 nm, em ¼ 340 nm) of SHO:Pb. Empty circles are experimental data, solid and dashed lines are the best fit to the data using the model described in the text with or without consideration of the quenching channel, respectively. The parameters of the best fit are listed. In the inset, the sketch of the model is displayed.

The temperature dependence of the photoluminescence decay times of the 340 nm emission of SHO: Pb (0.3 mol%) excited at 269 nm is presented in Fig. 3. Experimental data were fit by the phenomenological model, similar to (Nikl et al., 2005; Hlinka et al., 1991) sketched in the inset. A pair of excited levels 3P0 (level 1), 3P1 (level 2) with an energy splitting D is considered. The time evolution of the populations N1, N2 of the excited levels 1,2 can be described by the set of ordinary differential equations: dN 1 =dt [ Lk1 N 1 L k12 N1 D k21 N2 L k1x N1

(1)

dN 2 =dt [ Lk2 N 2 L k21 N2 D k12 N1

(2)

where: k1, k2, k12, k21 and k1x are radiative transition rates from levels 1 and 2 to the ground state (1S0), non-radiative rates of phonon assisted transitions between the levels 1, 2 and from the quenching channel from the level 1, respectively. Non-radiative transitions between levels 1 and 2 can be written as k21 [ Kðn D 1Þ; k12 [ Kn; n [ ½expðD=kB TÞ L 1L1 Here K, n, kB and D are the zero-temperature transition rate between the levels 1 and 2, the Bose–Einstein factor, Boltzmann constant and the energy separation between the levels, respectively. The non-radiative quenching channel is considered in the usual barrier from: k1x [ K 1x expðLE1x =kB TÞ;

Fig. 2. Decay curve of SHO:Pb at 10 K. Empty squares are experimental data, dashed line is the excitation pulse and the solid line is the fit to the data by the function I(t).

with K1x being a frequency factor and E1x the height of the barrier. The model parameters corresponding to the best fit, in particular the energy separation 37 meV of the levels 1 and 2, are listed in Fig. 3. The figure also demonstrates that taking into account the quenching channel from the level 1 provides a better fit to the data (cf. solid line vs dashed line in Fig. 3). Temperature dependence of the emission intensity for SHO: Pb (0.3 mol%) under excitation 269 nm is shown in Fig. 4. To fit these data we did not apply the described three-level model. The reason is the lack of experimental data, in particular, missing fast component, that would help to more reliably determine extensive

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shorter decay time at 10 K with respect to the values on the order of milliseconds usually observed for heavy ns2 ions (Jacobs, 1991; Hlinka et al., 1991; Nikl et al., 2005). Though the RT quantum efficiency of Pb2þ center in SHO host is somewhat lower due to the presence of non-radiative quenching, the radioluminescence efficiency of Pb-doped SHO was found higher with respect to the well-known SHO:Ce (Mihokova et al., submitted for publication). Photoluminescence decay time of Pb2þ center of about 190 ns at RT is still acceptable for application of such a phosphor in most of current imaging applications based on 2D phosphor screen. Therefore, its further practical testing is worth pursuing. 4. Conclusions

Fig. 4. Temperature dependence of emission intensity (ex ¼ 269 nm) of SHO:Pb. The emission spectra were integrated in the region 290–400 nm. The empty circles are experimental data and the solid line is the fit (see the text).

Pb2þ photoluminescence emission in SHO host is peaking at about 340 nm, related excitation spectrum shows maxima at 215 nm and 270 nm and the RT decay time is about 190 ns. These are all consistent with the heavy ns2 ion emission center. Phenomenological model based on the closely spaced radiative and metastable levels of the split lowest excited state of Pb2þ center was successfully used to fit the temperature dependence of the Pb2þ decay times. The best fit provides the energy separation of the radiative and metastable levels of about 37 meV. A mild nonradiative quenching is governed by the 100 meV energy barrier. Due to high density and effective atomic number, easy doping, higher radioluminescence efficiency with respect to well-known Ce-doped SHO and yet reasonably short decay time, the SHO:Pb phosphor may find suitable application in the field of X-ray or highenergy particles imaging. Acknowledgments This research was supported by Czech GAAV project No KAN300100802, Czech Science Foundation project No 202/08/ 0893, Ministry of Education, Youth and Sports of the Czech Republic project No MSM 6840770040 and by the Italian CARIPLO Foundation project ‘‘Energy transfer and trapping phenomena in nanostructured scintillator materials’’ (2008 – 2011). References

Fig. 5. Shift of the emission maximum with temperature (ex ¼ 269 nm) of SHO: Pb

number of model parameters. Therefore we show the fit of the data considering a decay from the single level, mainly to demonstrate that the high temperature data are fit considering the quenching with the same energy barrier 100 meV as was used to fit the decay time data. Fig. 5 displays a temperature dependence of the emission maximum position. We mark the energy distance of 37 meV corresponding to the radiative and metastable level separation obtained from the model. However, as demonstrated in the figure, much larger high-energy shift of emission maximum, namely about 95 meV, is observed. This discrepancy is not yet understood and requires further investigation. As pointed out in (Mihokova et al., in press), a possible interplay with an exciton state localized nearby Pb2þ centers is not excluded. This hypothesis is supported by much

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