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Crystal growth and spectroscopic properties of praseodymium and cerium co-doped Y2SiO5
Journal of Luminescence 145 (2014) 547–552
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Crystal growth and spectroscopic properties of praseodymium and cerium co-doped Y2SiO5 Lihe Zheng a,b, Radosław Lisiecki c,n, Witold Ryba-Romanowski c, Gérard Aka b, Juqing Di d, Dongzhen Li d, Xiaodong Xu a,nn, Jun Xu a,nn a
Shanghai Institute of Ceramics, Chinese Academy of Sciences, 588 Heshuo Road, Shanghai 201899, China Laboratoire de Chimie de la Matière Condensée de Paris, ENSCP, 11 Rue P. et M. Curie, 75231 Paris Cedex 5, France c Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Okólna 2, 50-422 Wroclaw, Poland d Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science, 390 Qinghe Road, Shanghai 201800, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 19 December 2012 Received in revised form 18 July 2013 Accepted 2 August 2013 Available online 12 August 2013
Praseodymium and cerium co-doped yttrium silicate (Pr,Ce:YSO) crystal with high optical quality was grown by the Czochralski method. The segregation coefficient of Pr3+ and Ce3+ is measured to be 0.547 and 0.136, respectively. The polarized absorption bands centered at 268 nm, 299 nm and 356 nm were assigned to 4f (2F5/2)-5d inter-configuration transitions of Ce3+, while bands centered at 220 nm and 250 nm were assigned to 4fn–4fn 15d transitions of Pr3+. The VUV–UV excitation spectra for monitoring Pr3+ emissions centered at 610 nm (1D2-3H4) and 489 nm (3P0-3H4), as well as Ce3+ emission centered at 450 nm were recorded at T ¼8 K in the range of 100 nm–320 nm. Polarized excitation spectra for Ce3+ emission centered at 424 nm and Pr3+ emission centered at 611 nm were recorded at T ¼300 K. Polarized emission spectra were obtained upon excitation at 365 nm and 456 nm at T ¼300 K. Lifetimes were estimated to be 170 μs for Pr3+ (1D2), 3 μs for Pr3+ (3P0) and 39 ns for Ce3+ (5d1). & 2013 Elsevier B.V. All rights reserved.
Keywords: Y2SiO5 Praseodymium Cerium Polarized absorption spectra Excitation spectra
1. Introduction Interests in rare-earth ions doped crystals exhibiting broad and n1 n intense 4f 5d-4f emission are stimulated by potential application in environmental sciences, medicine, photolithography, material processing, density optical storage, UV solid-state lasers and quantum computing [1–3]. Yttrium silicate (Y2SiO5, YSO) crystal is well-known host material by accommodating appreciable amounts of rare earth ions. Ce:YSO have been investigated in numerous papers for scintillation applications [4–5]. Compared with Ce3+ ions, the 5d–4f energy transition of Pr3+ in YSO is blueshifted and the excitation energy transfer between Pr3+ and Ce3+ may enhance the luminescence intensity in blue spectral range [6– 8]. On the other hand, Pr:YSO was considered for the initial qubit experiments [9–13]. Yan et. al. reported the incorporation of a low concentration of readout ions Ce3+ serving as a sensor for reading out the state of the nearby qubit of Pr3+ through the interaction between the qubit and readout ion [14].
n Corresponding author at: Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Okólna 2, 50-422 Wroclaw, Poland. nn Corresponding authors at: Shanghai Institute of Ceramics, Chinese Academy of Sciences, 588 Heshuo Road, Shanghai 201899, China. Tel.: +86 (0)21 69987573. E-mail addresses: [email protected] (L. Zheng). [email protected] (R. Lisiecki), [email protected] (X. Xu). [email protected] (J. Xu).
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.08.003
In this paper, we concentrate on VUV-UV-Vis spectroscopic properties of praseodymium and cerium co-doped yttrium silicate (Pr,Ce:YSO) crystal. The intention of this paper is to get insights into the excitation and relaxation process of excited states which are relevant to luminescence phenomena in Pr,Ce:YSO with low concentration of 0.05 at% for Pr3+ and 0.088 at% for Ce3+. Accordingly, the excitation and emission spectra, together with relaxation dynamic were investigated.
2. Experimental 2.1. Crystal growth The Pr,Ce:YSO crystal was grown by the Czochralski method from an inductively heated iridium crucible. The Pr and Ce concentration in the melt was 0.05 at% and 0.088 at% with respect to Y, respectively. The starting materials were SiO2, Y2O3, Pr6O11 and CeO2 powders with purity higher than 99.99%. The powders were weighed, mixed and pressed into tablets before sintered at 1400 1C for 24 h. The total weight of raw materials was 330 g. Nitrogen with purity of 5 N was used as growth atmosphere. The Pr:YSO crystal oriented along (010) with diameter of 4.5 mm and length of 30 mm was used as a seed. The pulling rate was set to 1–5 mm h 1 and the rotation rate of the seed was set to 10–30 rpm. After pulling off from the melting, the crystal was
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cooled to room temperature in 40 h. Fig. 1 shows the obtained Pr,Ce:YSO crystal boule with diameter of 32 mm and length of 65 mm. 2.2. ICP-AES and XRD measurement The segregation coefficient of Pr and Ce in the YSO crystal was calculated according to the measured concentrations of Pr, Ce and Y atoms at the starting position attached to the grown Pr,Ce:YSO crystal using the inductively coupled plasma atomic emission spectrometer (ICP-AES) method. The segregation coefficient km was calculated according to Eq. (1), where C t stands for the concentration of Pr or Ce close to seed crystal, and C 0 stands for the initial concentration of Pr or Ce in the melt. km ¼ C t =C 0
ð1Þ
The segregation coefficient of Pr and Ce ion in Pr,Ce:YSO crystal is accordingly calculated to be 0.547 and 0.136, respectively. The sample for XRD measurement was cut adjacent to the seed crystal position and then ground to fine powder in an agate mortar. The crystal structure was examined by D/MAX 2550V powder X-ray diffractometer (XRD, Cu target, Kα). Fig. 2 shows the powder XRD pattern of as-grown Pr,Ce:YSO crystal which is in good agreement with the YSO structure phase of monoclinic system with a space group C2/m shown in PDF #36-1476 [15].
at the SUPERLUMI station of Hamburger Synchrotron Strahlungslabor (HASYLAB) at Deutsches Elektronen-Synchrotron (DESY). Excitation spectra were scanned within spectral region of 100 nm– 300 nm with the primary 2 m McPherson monochromator when monitoring luminescence intensity with a secondary ARC monochromator coupled to a Hamamatsu R6358P photomultiplier and a PMT detector. The spectra were corrected for the incident photon flux using a reference sodium salicilate (NaSal) sample. The quantum efficiency of NaSal is constant in wide VUV–UV wavelength range. To record low-temperature spectra at 8 K, the sample was placed in a cold-finger liquid-helium cryostat (Cryovac GmbH). The emission spectra excited in the VUV region were monitored with a CCD camera. Polarized absorption spectra in the UV-Vis-NIR region were measured at room temperature with a Varian model 5 spectrophotometer where the resolution was 0.2 nm in the near-infrared region and 0.1 nm in UV-Vis. Luminescence and excitation spectra in UV-Vis were acquired with a Dongwoo Optron system containing a DM 711 emission monochromator and a DM 158i excitation monochromator with 750 mm and 150 mm focal length, respectively. The excitation source was an ozone-free Xenon lamp DL180Xe and the luminescence was detected with a Hamamatsu R-955 photomultiplier. To record luminescence decay curves, a Surelite OPO pumped by a third harmonic of a Nd:YAG laser was used as an excitation source. Luminescence was dispersed by a grating monochromator and detected with a photomultiplier connected to a Tektronix TDS 3052 oscilloscope.
2.3. Spectroscopic measurement Excitation spectra and decay curves of luminescence in vacuumultraviolet VUV were acquired using experimental set-up available
Fig. 1. As-grown Pr, Ce:YSO crystal.
3. Results and discussion Fig. 3 displays survey on polarized absorption spectra of Pr,Ce: YSO crystal recorded at 300 K with electric E vector of incident light parallel to a, b and c crystallographic axes. The absorption spectra are spanned within 200 nm–2500 nm. It can be seen that the absorption bands of active ions appear in the UV region. Owing to small concentration of Pr3+ ions, considerably weaker f–f transition within 4f2 electronic configuration of Pr in Vis-NIR range are hardly observed in this spectrum. The inset shows in detail the absorption spectrum in UV region. The absorption bands peaked at 299 nm and 356 nm can be undoubtedly assigned to 4f (2F5/2)-5d inter-configuration transitions of Ce3+ in YSO. A less apparent absorption in the form of shoulder around 268 nm may be due to Ce3+ transition, too. Since the 5d electrons are located on outer-most orbits, the spectral broadening of absorption bands is quite effective. For comparison, the three absorption bands of Ce3+ within 250 nm–380 nm have also been observed in LYSO [16]. The absorption bands around 220 nm and 250 nm in Fig. 3 can be
Fig. 2. Powder XRD pattern for as-grown Pr,Ce:YSO crystal.
L. Zheng et al. / Journal of Luminescence 145 (2014) 547–552
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YSO:Ce,Pr; T=8K
Luminescence intensity [a.u.]
λ = 610 nm
YSO:Ce,Pr; T=8K
λ = 489 nm
YSO:Ce,Pr; T=8K
λ = 450 nm
100
assigned to transitions from the 3H4 ground state to the lowest 5d1 and 5d2 levels of Pr3+, respectively [17]. It can be seen that the anisotropy of the host crystal weakly affects the relative intensities and shapes of absorption bands in the UV range. Fig. 4 shows excitation spectra recorded between 100 nm and 320 nm at T¼ 8 K when monitoring the luminescence of Ce3+ at 450 nm and Pr3+ at 489 nm and 610 nm. The excitation spectra consisting of several well-defined bands and noticeable differences in the intensity and distribution of the components are clearly seen. The two prominent lines at 200 nm and 219 nm dominate the short-wavelength part of excitation spectra of Pr3+ emission at 610 nm. Besides, the two less-intense components can be discerned at 235 nm and 250 nm indicating that the 1D2-3H4 emission of Pr3+ is mainly activated through 4fn–4fn 15d transitions of Pr. However, the excitation spectrum contains an additional band located at 298 nm associated with 4f–5d transition of Ce3+ implying that the 1D2 level of Pr is also effectively populated by energy transfer from Ce3+ to Pr3+. The excitation spectra acquired when monitoring emission at 489 nm comprise the bands centered at the same spectral positions compared to the spectra pertinent to Pr3+ emission at 610 nm. Nevertheless, two apparent differences can be perceived. First, the bands corresponding to 4f-5d transitions of Pr3+ located at 200 nm and 219 nm as well as 235 nm and 250 nm show a reversed relation of intensity. Second, the occurrence of an additional sharp peak at 186 nm can be discerned. The latter characteristic peculiarity of the spectrum may be due to the absorption edge of the YSO host near 185 nm. The blue emission of Ce3+ at 450 nm is effectively excited through two intense absorption bands peaking at 263 nm and 300 nm and related to the 4f–5d transition of the Ce3+. With reference to spectroscopic properties of Ce:YSO presented in
200 250 Wavelength [nm]
300
Fig. 4. VUV–UV excitation spectra recorded at T ¼ 8 K for Pr3 þ emission at 610 nm (1D2-3H4) and at 489 nm (3P0-3H4) as well as 5d–4f emission of Ce3 þ at 450 nm in Pr,Ce:YSO crystal.
YSO:Ce,Pr ; T=8K (5d-4f) Ce (f-f) Pr (5d4f-4f) Pr
exc. 189 nm
Luminescence intensity [a.u.]
Fig. 3. Polarized absorption spectra of Pr,Ce:YSO crystal measured at T ¼ 300 K.
150
exc. 202 nm
exc. 218 nm exc. 237 nm
exc. 247 nm
exc. 262 nm
exc. 300 nm 300
400
500 600 Wavelength [nm]
700
800
Fig. 5. Comparison of emission spectra of Pr,Ce:YSO recorded at T ¼ 8 K and acquired upon different excitation wavelengths ranging from 189 nm to 300 nm.
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400
Luminescence intensity [a.u.]
Luminescence intensity [a.u.]
550
YSO:Ce, Pr λ =424 nm (4f-5d) Ce E II a
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250
300
350
Luminescence intensity [a.u.]
YSO:Ce, Pr =424 nm (4f-5d) Ce
E II b
200
250
300
350
(4f-5d) Ce E II c
200
250 300 Wavelength [nm]
350
400
Fig. 6. Polarized excitation spectra of Pr,Ce:YSO recorded at T ¼300 K for Ce3 þ emission at 424 nm.
Ref. [2], it may be inferred that excitation wavelengths ranging from 180 nm–300 nm bring about mainly the luminescence of Ce (1) ions entering the local positions with 6-fold coordination number. Dependence of luminescence spectra of Pr,Ce:YSO on excitation wavelengths was examined carefully. Results of measurement are gathered in Fig. 5. It can be seen in Fig. 5 that luminescence spectra measured under excitation at 300 nm and 262 nm contain two bands with maxima centered at 394 nm and 427 nm. Drozdowski et al. reported that the emission in the spectral range of 350 nm– 475 nm is associated mainly with Ce (1) ions. Meanwhile, the intense luminescence band of remaining Ce(2) ions with 7-fold O2 coordination in YSO was distinguished at 500 nm [18]. These findings imply that spectroscopic peculiarities in Pr,Ce:YSO crystal encompassing the excitation and luminescence characteristics are primarily related to Ce(1) ions. In addition to Ce emission, a less-intense Pr3+ luminescence around 610 nm contribute to the spectrum indicating that the red emission of Pr results from energy transfer from Ce3+ to Pr3+. The emission spectra of Pr,Ce:YSO acquired upon excitation within spectral range of 247 nm–202 nm reveal a quite broad band centered at 320 nm, which may be attributed to 5d4f-3F2,3 emission of Pr3+. It may be interesting to notice that the up-conversion luminescence of Pr3+ in YSO was observed in this spectra region utilizing a laser operating at 488 nm as an excitation source [19]. In
500 600 Wavelength [nm]
700
YSO:Ce, Pr; T=300K, Exc. 365nm E II b
(5d-4f) Ce
400
Luminescence intensity [a.u.]
Luminescence intensity [a.u.]
Wavelength [nm] YSO:Ce, Pr λ =424 nm
(5d-4f) Ce
400
Wavelength [nm]
λ
YSO:Ce, Pr; T=300K, Exc. 365nm E II a
500 600 Wavelength [nm]
700
YSO:Ce, Pr; T=300K, Exc. 365nm E II c
(5d-4f) Ce 590 600 610 620
Wavelength [nm]
400
500 600 Wavelength [nm]
700
Fig. 7. Polarized emission spectra of Pr,Ce:YSO obtained upon excitation at 365 nm at 300 K.
addition, the up-conversion phenomena in Pr:YSO leading to luminescence around 300 nm was perceived when exciting with a femtosecond laser generating at λ ¼ 800 nm [20]. Besides, the luminescence of Pr in Pr,Ce:YSO was also recorded in visible spectra range. The narrow emission lines observed around 490 nm, 610 nm, 706 nm and 735 nm were assigned to f–f transitions of Pr3+, namely 3 P0-3H4, 1D2-3H4, 3P0-3F3 and 3P0-3F4, respectively. Among the f-f transitions of Pr3+ in YSO, the emission band appearing around 600–650 nm is the most intense. It may be attributed to luminescence originating in both 3P0 and 1D2 levels. Our study on the luminescence kinetics of Pr3+ in this spectra region implied that emission lines in the short-wavelength part of the band up to 620 nm are related to 1D2-3H4 transition, whereas emission lines in long-wavelength part of the band are associated with 3P0-3H6 transitions. It can be unambiguously revealed from Fig. 5 that the distribution of emission lines in luminescence spectrum excited at 237 nm is different from emission spectra recorded with the remaining excitation wavelengths. Actually, abundant structures of emission lines indicate the contribution of Pr3+ ions accommodated with two different crystallographic sites. The broad-band emission attributed to 5d–4f transition of Ce3+ consists of two maxima when exciting at wavelengths shorter than 262 nm. It is suggested that Ce3+ emission is activated partly through the efficient Pr3+–Ce3+ energy transfer.
Luminescence intensity [a.u.]
YSO:Ce, Pr λ = 611nm (4f-5d) Ce
Luminescence intensity [a.u.]
200
250
300
350 400 Wavelength [nm]
450
500
550
YSO:Ce, Pr λ = 611nm (4f-5d) Ce
E II b
Luminescence intensity [a.u.]
200
250
300
350 400 450 Wavelength [nm]
500
550
200
250
(4f-5d) Ce
300
350 400 450 Wavelength [nm]
E II a
550
500
550
Fig. 8. Polarized excitation spectra of Pr,Ce:YSO for Pr3 þ emission at 611 nm recorded at T ¼ 300 K.
Fig. 6 exemplifies polarized excitation spectra of Pr,Ce:YSO monitoring Ce3+ luminescence at 424 nm. The ozone-free Xe lamp was applied as excitation source. Basically, the excitation spectra comprise two bands around 310 nm and 365 nm attributed to 4f–5d transitions of Ce3+. The anisotropic features of Pr,Ce:YSO crystal can be assessed considering the spectra obtained for E // b. This spectrum is distinctly different from the remaining ones, since the broad band spanning within 320–380 nm contains an additional maxima centered at 350 nm. In relation to previously presented results in Ref. [21], the first component of excitation band centered at 350 nm is assigned to 4f–5d transition of Ce (1) and the second component located at 365 nm corresponds to Ce (2). The emission spectra measured upon excitation at 365 nm are presented in Fig. 7. The broad emission bands correspond to 5d–4f transition of Ce3+. When comparing with excitation spectra in similar case, the spectrum recorded for E // b displays difference in relation to that for E // a and E // c. The top-part of emission band is rather flat and contains two maxima located at 400 nm and 420 nm. Besides, the inset reveals the narrow emission line emerged at 610 nm, which can be assigned to Pr3+ luminescence indicating that the Ce-Pr energy transfer gives rise to luminescence of Pr in YSO.
600
650 700 Wavelength [nm]
750
800
YSO:Ce, Pr Exc. 456 nm
E II b
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YSO:Ce, Pr λ = 611nm E II c
YSO:Ce, Pr Exc. 456 nm
Luminescence intensity [a.u.]
E II a
551
600
650 700 Wavelength [nm]
Luminescence intensity [a.u.]
Luminescence intensity [a.u.]
L. Zheng et al. / Journal of Luminescence 145 (2014) 547–552
750
800
YSO:Ce, Pr Exc. 456 nm
E II c
550
600
650 700 Wavelength [nm]
750
800
Fig. 9. Polarized emission spectra of Pr,Ce:YSO recorded at 300 K obtained upon excitation at 456 nm.
The energy-transfer phenomena of Ce–Pr can be further evidenced taking into account the excitation spectra of Pr3+ emission at 611 nm shown in Fig. 8. In fact, the high-intense excitation bands centered at 455 nm, 478 nm and 490 nm are due to 3H4–3PJ transitions of Pr3+. However, quite intense bands pertinent to Ce3+ 4f–5d transitions at 326 nm and 375 nm contribute to the spectrum. The emission features of Pr,Ce:YSO were examined using excitation at 456 nm. The appropriate luminescence spectra are presented in Fig. 9. These findings corroborate the previously discussed results regarding anisotropic properties of Pr,Ce:YSO crystal. Namely, the emission band acquired for E // b is characterized by specific spectroscopic peculiarities. Emission spectra recorded for E // a and E // c contain predominant emission line centered at 610 nm which is weak in the case of E // b, thereby the remaining components at 605 nm, 608 nm, 620 nm, 703 nm and 727 nm are enhanced. The decay kinetics of Pr3+ and Ce3+ luminescence have been measured at T¼ 300 K. The literature data pointed out that the active ions in YSO are dissimilarly located in two nonequivalent crystallographic positions [22]. However, the decay curves reveal
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e11 9
e
e7
t=39 ns
e5 3
e
e1
Ce
e-1
3+
YSO:Ce, Pr, T=300 K
Luminescence intensity [a.u.]
0
50
100
150
200
250
300
Time [ns]
evidencing the occurrence of Pr–Ce energy transfer suggesting that higher doping level of Pr3+ and Ce3+ is preferred for scintillation application but even lower doping level is favorable for qubit state measurement. On the other hand, the excitation spectra monitoring emission at 610 nm clearly indicates the contribution of Ce–Pr energy transfer in the excitation of the 1D2–3H4 luminescence of Pr3+. Broad-band luminescence of Pr and Ce activators in UV-Vis spectra range in Pr,Ce:YSO is quite intense. Occurrence of energy transfer phenomena involving inter-configuration transitions of Ce3+ and Pr3+ ions combined with strong absorption in the VUV–UV range are beneficial for excitation efficiency in Pr,Ce:YSO luminescent system.
1
e e-1 e-3 e-5 e-7 e-9 e-11
Acknowledgments
t=3 μs 3
P0
YSO:Ce, Pr, T=300 K
0
5
10
15
20
25
30
Time [ μs] e1 e-1 e-3 e-5 e-7 e-9 e-11
We thank Prof. Philippe Goldner, Prof. Stefan Krӧll and Dr. Ying Yan for helpful discussions. Authors are grateful to National Natural Science Foundation of China (Grant no. 60908030, 51272264, 60938001), General Program of Shanghai Municipal Natural Science Foundation (Grant no. 13ZR1446100); European Community′s Seventh Framework Program (FP7/2007–2013) contract II-20090073 EC (DESY_Hasylab) and the Mairie de Paris for financial supports (“Research in Paris” Project). References
t=170 μs 1
D2
0
YSO:Ce, Pr, T=300 K 200
400
600
800
1000
1200
1400
Time [ μs] Fig. 10. Decay curves of cerium and praseodymium luminescence in Pr,Ce:YSO.
single exponential time dependence, implying comparable rates of radiative relaxation of the active ions located in different local positions. The lifetime values were estimated to be 170 μs for Pr3+ (1D2), 3 μs for Pr3+ (3P0) and 39 ns for Ce3+ (5d1). The adequate decay curves are presented in Fig. 10. It is worth noticing that lifetimes of the active ions in single doped Pr:YSO and Ce:YSO are comparable to that observed in Pr,Ce:YSO. For instance, the lifetime of 1D2 in Pr:YSO was found to be of 200 μs at T ¼77 K [23]. 4. Conclusions High optical quality Pr,Ce:YSO crystal with low concentration doping level of 0.05 at% for Pr3+ and 0.088 at% for Ce3+ were grown and its optical properties were determined. The polarized absorption bands for 4f (2F5/2)-5d inter-configuration transitions of Ce3+ and 4fn–4fn 15d transitions of Pr3+ were assigned. The polarized spectra revealed that the anisotropy of absorption and luminescence in Pr,Ce:YSO crystal is rather weak. The excitation spectra measured in UV range indicate that the blue emission of Ce is effectively activated by 4fn–4fn 15d transitions of Pr3+
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Crystal growth and spectroscopic properties of praseodymium and cerium co-doped Y2SiO5 Lihe Zheng a,b, Radosław Lisiecki c,n, Witold Ryba-Romanowski c, Gérard Aka b, Juqing Di d, Dongzhen Li d, Xiaodong Xu a,nn, Jun Xu a,nn a
Shanghai Institute of Ceramics, Chinese Academy of Sciences, 588 Heshuo Road, Shanghai 201899, China Laboratoire de Chimie de la Matière Condensée de Paris, ENSCP, 11 Rue P. et M. Curie, 75231 Paris Cedex 5, France c Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Okólna 2, 50-422 Wroclaw, Poland d Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science, 390 Qinghe Road, Shanghai 201800, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 19 December 2012 Received in revised form 18 July 2013 Accepted 2 August 2013 Available online 12 August 2013
Praseodymium and cerium co-doped yttrium silicate (Pr,Ce:YSO) crystal with high optical quality was grown by the Czochralski method. The segregation coefficient of Pr3+ and Ce3+ is measured to be 0.547 and 0.136, respectively. The polarized absorption bands centered at 268 nm, 299 nm and 356 nm were assigned to 4f (2F5/2)-5d inter-configuration transitions of Ce3+, while bands centered at 220 nm and 250 nm were assigned to 4fn–4fn 15d transitions of Pr3+. The VUV–UV excitation spectra for monitoring Pr3+ emissions centered at 610 nm (1D2-3H4) and 489 nm (3P0-3H4), as well as Ce3+ emission centered at 450 nm were recorded at T ¼8 K in the range of 100 nm–320 nm. Polarized excitation spectra for Ce3+ emission centered at 424 nm and Pr3+ emission centered at 611 nm were recorded at T ¼300 K. Polarized emission spectra were obtained upon excitation at 365 nm and 456 nm at T ¼300 K. Lifetimes were estimated to be 170 μs for Pr3+ (1D2), 3 μs for Pr3+ (3P0) and 39 ns for Ce3+ (5d1). & 2013 Elsevier B.V. All rights reserved.
Keywords: Y2SiO5 Praseodymium Cerium Polarized absorption spectra Excitation spectra
1. Introduction Interests in rare-earth ions doped crystals exhibiting broad and n1 n intense 4f 5d-4f emission are stimulated by potential application in environmental sciences, medicine, photolithography, material processing, density optical storage, UV solid-state lasers and quantum computing [1–3]. Yttrium silicate (Y2SiO5, YSO) crystal is well-known host material by accommodating appreciable amounts of rare earth ions. Ce:YSO have been investigated in numerous papers for scintillation applications [4–5]. Compared with Ce3+ ions, the 5d–4f energy transition of Pr3+ in YSO is blueshifted and the excitation energy transfer between Pr3+ and Ce3+ may enhance the luminescence intensity in blue spectral range [6– 8]. On the other hand, Pr:YSO was considered for the initial qubit experiments [9–13]. Yan et. al. reported the incorporation of a low concentration of readout ions Ce3+ serving as a sensor for reading out the state of the nearby qubit of Pr3+ through the interaction between the qubit and readout ion [14].
n Corresponding author at: Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Okólna 2, 50-422 Wroclaw, Poland. nn Corresponding authors at: Shanghai Institute of Ceramics, Chinese Academy of Sciences, 588 Heshuo Road, Shanghai 201899, China. Tel.: +86 (0)21 69987573. E-mail addresses: [email protected] (L. Zheng). [email protected] (R. Lisiecki), [email protected] (X. Xu). [email protected] (J. Xu).
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.08.003
In this paper, we concentrate on VUV-UV-Vis spectroscopic properties of praseodymium and cerium co-doped yttrium silicate (Pr,Ce:YSO) crystal. The intention of this paper is to get insights into the excitation and relaxation process of excited states which are relevant to luminescence phenomena in Pr,Ce:YSO with low concentration of 0.05 at% for Pr3+ and 0.088 at% for Ce3+. Accordingly, the excitation and emission spectra, together with relaxation dynamic were investigated.
2. Experimental 2.1. Crystal growth The Pr,Ce:YSO crystal was grown by the Czochralski method from an inductively heated iridium crucible. The Pr and Ce concentration in the melt was 0.05 at% and 0.088 at% with respect to Y, respectively. The starting materials were SiO2, Y2O3, Pr6O11 and CeO2 powders with purity higher than 99.99%. The powders were weighed, mixed and pressed into tablets before sintered at 1400 1C for 24 h. The total weight of raw materials was 330 g. Nitrogen with purity of 5 N was used as growth atmosphere. The Pr:YSO crystal oriented along (010) with diameter of 4.5 mm and length of 30 mm was used as a seed. The pulling rate was set to 1–5 mm h 1 and the rotation rate of the seed was set to 10–30 rpm. After pulling off from the melting, the crystal was
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cooled to room temperature in 40 h. Fig. 1 shows the obtained Pr,Ce:YSO crystal boule with diameter of 32 mm and length of 65 mm. 2.2. ICP-AES and XRD measurement The segregation coefficient of Pr and Ce in the YSO crystal was calculated according to the measured concentrations of Pr, Ce and Y atoms at the starting position attached to the grown Pr,Ce:YSO crystal using the inductively coupled plasma atomic emission spectrometer (ICP-AES) method. The segregation coefficient km was calculated according to Eq. (1), where C t stands for the concentration of Pr or Ce close to seed crystal, and C 0 stands for the initial concentration of Pr or Ce in the melt. km ¼ C t =C 0
ð1Þ
The segregation coefficient of Pr and Ce ion in Pr,Ce:YSO crystal is accordingly calculated to be 0.547 and 0.136, respectively. The sample for XRD measurement was cut adjacent to the seed crystal position and then ground to fine powder in an agate mortar. The crystal structure was examined by D/MAX 2550V powder X-ray diffractometer (XRD, Cu target, Kα). Fig. 2 shows the powder XRD pattern of as-grown Pr,Ce:YSO crystal which is in good agreement with the YSO structure phase of monoclinic system with a space group C2/m shown in PDF #36-1476 [15].
at the SUPERLUMI station of Hamburger Synchrotron Strahlungslabor (HASYLAB) at Deutsches Elektronen-Synchrotron (DESY). Excitation spectra were scanned within spectral region of 100 nm– 300 nm with the primary 2 m McPherson monochromator when monitoring luminescence intensity with a secondary ARC monochromator coupled to a Hamamatsu R6358P photomultiplier and a PMT detector. The spectra were corrected for the incident photon flux using a reference sodium salicilate (NaSal) sample. The quantum efficiency of NaSal is constant in wide VUV–UV wavelength range. To record low-temperature spectra at 8 K, the sample was placed in a cold-finger liquid-helium cryostat (Cryovac GmbH). The emission spectra excited in the VUV region were monitored with a CCD camera. Polarized absorption spectra in the UV-Vis-NIR region were measured at room temperature with a Varian model 5 spectrophotometer where the resolution was 0.2 nm in the near-infrared region and 0.1 nm in UV-Vis. Luminescence and excitation spectra in UV-Vis were acquired with a Dongwoo Optron system containing a DM 711 emission monochromator and a DM 158i excitation monochromator with 750 mm and 150 mm focal length, respectively. The excitation source was an ozone-free Xenon lamp DL180Xe and the luminescence was detected with a Hamamatsu R-955 photomultiplier. To record luminescence decay curves, a Surelite OPO pumped by a third harmonic of a Nd:YAG laser was used as an excitation source. Luminescence was dispersed by a grating monochromator and detected with a photomultiplier connected to a Tektronix TDS 3052 oscilloscope.
2.3. Spectroscopic measurement Excitation spectra and decay curves of luminescence in vacuumultraviolet VUV were acquired using experimental set-up available
Fig. 1. As-grown Pr, Ce:YSO crystal.
3. Results and discussion Fig. 3 displays survey on polarized absorption spectra of Pr,Ce: YSO crystal recorded at 300 K with electric E vector of incident light parallel to a, b and c crystallographic axes. The absorption spectra are spanned within 200 nm–2500 nm. It can be seen that the absorption bands of active ions appear in the UV region. Owing to small concentration of Pr3+ ions, considerably weaker f–f transition within 4f2 electronic configuration of Pr in Vis-NIR range are hardly observed in this spectrum. The inset shows in detail the absorption spectrum in UV region. The absorption bands peaked at 299 nm and 356 nm can be undoubtedly assigned to 4f (2F5/2)-5d inter-configuration transitions of Ce3+ in YSO. A less apparent absorption in the form of shoulder around 268 nm may be due to Ce3+ transition, too. Since the 5d electrons are located on outer-most orbits, the spectral broadening of absorption bands is quite effective. For comparison, the three absorption bands of Ce3+ within 250 nm–380 nm have also been observed in LYSO [16]. The absorption bands around 220 nm and 250 nm in Fig. 3 can be
Fig. 2. Powder XRD pattern for as-grown Pr,Ce:YSO crystal.
L. Zheng et al. / Journal of Luminescence 145 (2014) 547–552
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YSO:Ce,Pr; T=8K
Luminescence intensity [a.u.]
λ = 610 nm
YSO:Ce,Pr; T=8K
λ = 489 nm
YSO:Ce,Pr; T=8K
λ = 450 nm
100
assigned to transitions from the 3H4 ground state to the lowest 5d1 and 5d2 levels of Pr3+, respectively [17]. It can be seen that the anisotropy of the host crystal weakly affects the relative intensities and shapes of absorption bands in the UV range. Fig. 4 shows excitation spectra recorded between 100 nm and 320 nm at T¼ 8 K when monitoring the luminescence of Ce3+ at 450 nm and Pr3+ at 489 nm and 610 nm. The excitation spectra consisting of several well-defined bands and noticeable differences in the intensity and distribution of the components are clearly seen. The two prominent lines at 200 nm and 219 nm dominate the short-wavelength part of excitation spectra of Pr3+ emission at 610 nm. Besides, the two less-intense components can be discerned at 235 nm and 250 nm indicating that the 1D2-3H4 emission of Pr3+ is mainly activated through 4fn–4fn 15d transitions of Pr. However, the excitation spectrum contains an additional band located at 298 nm associated with 4f–5d transition of Ce3+ implying that the 1D2 level of Pr is also effectively populated by energy transfer from Ce3+ to Pr3+. The excitation spectra acquired when monitoring emission at 489 nm comprise the bands centered at the same spectral positions compared to the spectra pertinent to Pr3+ emission at 610 nm. Nevertheless, two apparent differences can be perceived. First, the bands corresponding to 4f-5d transitions of Pr3+ located at 200 nm and 219 nm as well as 235 nm and 250 nm show a reversed relation of intensity. Second, the occurrence of an additional sharp peak at 186 nm can be discerned. The latter characteristic peculiarity of the spectrum may be due to the absorption edge of the YSO host near 185 nm. The blue emission of Ce3+ at 450 nm is effectively excited through two intense absorption bands peaking at 263 nm and 300 nm and related to the 4f–5d transition of the Ce3+. With reference to spectroscopic properties of Ce:YSO presented in
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Fig. 4. VUV–UV excitation spectra recorded at T ¼ 8 K for Pr3 þ emission at 610 nm (1D2-3H4) and at 489 nm (3P0-3H4) as well as 5d–4f emission of Ce3 þ at 450 nm in Pr,Ce:YSO crystal.
YSO:Ce,Pr ; T=8K (5d-4f) Ce (f-f) Pr (5d4f-4f) Pr
exc. 189 nm
Luminescence intensity [a.u.]
Fig. 3. Polarized absorption spectra of Pr,Ce:YSO crystal measured at T ¼ 300 K.
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exc. 202 nm
exc. 218 nm exc. 237 nm
exc. 247 nm
exc. 262 nm
exc. 300 nm 300
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Fig. 5. Comparison of emission spectra of Pr,Ce:YSO recorded at T ¼ 8 K and acquired upon different excitation wavelengths ranging from 189 nm to 300 nm.
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YSO:Ce, Pr λ =424 nm (4f-5d) Ce E II a
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YSO:Ce, Pr =424 nm (4f-5d) Ce
E II b
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(4f-5d) Ce E II c
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250 300 Wavelength [nm]
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Fig. 6. Polarized excitation spectra of Pr,Ce:YSO recorded at T ¼300 K for Ce3 þ emission at 424 nm.
Ref. [2], it may be inferred that excitation wavelengths ranging from 180 nm–300 nm bring about mainly the luminescence of Ce (1) ions entering the local positions with 6-fold coordination number. Dependence of luminescence spectra of Pr,Ce:YSO on excitation wavelengths was examined carefully. Results of measurement are gathered in Fig. 5. It can be seen in Fig. 5 that luminescence spectra measured under excitation at 300 nm and 262 nm contain two bands with maxima centered at 394 nm and 427 nm. Drozdowski et al. reported that the emission in the spectral range of 350 nm– 475 nm is associated mainly with Ce (1) ions. Meanwhile, the intense luminescence band of remaining Ce(2) ions with 7-fold O2 coordination in YSO was distinguished at 500 nm [18]. These findings imply that spectroscopic peculiarities in Pr,Ce:YSO crystal encompassing the excitation and luminescence characteristics are primarily related to Ce(1) ions. In addition to Ce emission, a less-intense Pr3+ luminescence around 610 nm contribute to the spectrum indicating that the red emission of Pr results from energy transfer from Ce3+ to Pr3+. The emission spectra of Pr,Ce:YSO acquired upon excitation within spectral range of 247 nm–202 nm reveal a quite broad band centered at 320 nm, which may be attributed to 5d4f-3F2,3 emission of Pr3+. It may be interesting to notice that the up-conversion luminescence of Pr3+ in YSO was observed in this spectra region utilizing a laser operating at 488 nm as an excitation source [19]. In
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YSO:Ce, Pr; T=300K, Exc. 365nm E II b
(5d-4f) Ce
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Luminescence intensity [a.u.]
Wavelength [nm] YSO:Ce, Pr λ =424 nm
(5d-4f) Ce
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λ
YSO:Ce, Pr; T=300K, Exc. 365nm E II a
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YSO:Ce, Pr; T=300K, Exc. 365nm E II c
(5d-4f) Ce 590 600 610 620
Wavelength [nm]
400
500 600 Wavelength [nm]
700
Fig. 7. Polarized emission spectra of Pr,Ce:YSO obtained upon excitation at 365 nm at 300 K.
addition, the up-conversion phenomena in Pr:YSO leading to luminescence around 300 nm was perceived when exciting with a femtosecond laser generating at λ ¼ 800 nm [20]. Besides, the luminescence of Pr in Pr,Ce:YSO was also recorded in visible spectra range. The narrow emission lines observed around 490 nm, 610 nm, 706 nm and 735 nm were assigned to f–f transitions of Pr3+, namely 3 P0-3H4, 1D2-3H4, 3P0-3F3 and 3P0-3F4, respectively. Among the f-f transitions of Pr3+ in YSO, the emission band appearing around 600–650 nm is the most intense. It may be attributed to luminescence originating in both 3P0 and 1D2 levels. Our study on the luminescence kinetics of Pr3+ in this spectra region implied that emission lines in the short-wavelength part of the band up to 620 nm are related to 1D2-3H4 transition, whereas emission lines in long-wavelength part of the band are associated with 3P0-3H6 transitions. It can be unambiguously revealed from Fig. 5 that the distribution of emission lines in luminescence spectrum excited at 237 nm is different from emission spectra recorded with the remaining excitation wavelengths. Actually, abundant structures of emission lines indicate the contribution of Pr3+ ions accommodated with two different crystallographic sites. The broad-band emission attributed to 5d–4f transition of Ce3+ consists of two maxima when exciting at wavelengths shorter than 262 nm. It is suggested that Ce3+ emission is activated partly through the efficient Pr3+–Ce3+ energy transfer.
Luminescence intensity [a.u.]
YSO:Ce, Pr λ = 611nm (4f-5d) Ce
Luminescence intensity [a.u.]
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YSO:Ce, Pr λ = 611nm (4f-5d) Ce
E II b
Luminescence intensity [a.u.]
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350 400 450 Wavelength [nm]
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(4f-5d) Ce
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350 400 450 Wavelength [nm]
E II a
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Fig. 8. Polarized excitation spectra of Pr,Ce:YSO for Pr3 þ emission at 611 nm recorded at T ¼ 300 K.
Fig. 6 exemplifies polarized excitation spectra of Pr,Ce:YSO monitoring Ce3+ luminescence at 424 nm. The ozone-free Xe lamp was applied as excitation source. Basically, the excitation spectra comprise two bands around 310 nm and 365 nm attributed to 4f–5d transitions of Ce3+. The anisotropic features of Pr,Ce:YSO crystal can be assessed considering the spectra obtained for E // b. This spectrum is distinctly different from the remaining ones, since the broad band spanning within 320–380 nm contains an additional maxima centered at 350 nm. In relation to previously presented results in Ref. [21], the first component of excitation band centered at 350 nm is assigned to 4f–5d transition of Ce (1) and the second component located at 365 nm corresponds to Ce (2). The emission spectra measured upon excitation at 365 nm are presented in Fig. 7. The broad emission bands correspond to 5d–4f transition of Ce3+. When comparing with excitation spectra in similar case, the spectrum recorded for E // b displays difference in relation to that for E // a and E // c. The top-part of emission band is rather flat and contains two maxima located at 400 nm and 420 nm. Besides, the inset reveals the narrow emission line emerged at 610 nm, which can be assigned to Pr3+ luminescence indicating that the Ce-Pr energy transfer gives rise to luminescence of Pr in YSO.
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YSO:Ce, Pr Exc. 456 nm
E II b
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YSO:Ce, Pr λ = 611nm E II c
YSO:Ce, Pr Exc. 456 nm
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E II a
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L. Zheng et al. / Journal of Luminescence 145 (2014) 547–552
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YSO:Ce, Pr Exc. 456 nm
E II c
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Fig. 9. Polarized emission spectra of Pr,Ce:YSO recorded at 300 K obtained upon excitation at 456 nm.
The energy-transfer phenomena of Ce–Pr can be further evidenced taking into account the excitation spectra of Pr3+ emission at 611 nm shown in Fig. 8. In fact, the high-intense excitation bands centered at 455 nm, 478 nm and 490 nm are due to 3H4–3PJ transitions of Pr3+. However, quite intense bands pertinent to Ce3+ 4f–5d transitions at 326 nm and 375 nm contribute to the spectrum. The emission features of Pr,Ce:YSO were examined using excitation at 456 nm. The appropriate luminescence spectra are presented in Fig. 9. These findings corroborate the previously discussed results regarding anisotropic properties of Pr,Ce:YSO crystal. Namely, the emission band acquired for E // b is characterized by specific spectroscopic peculiarities. Emission spectra recorded for E // a and E // c contain predominant emission line centered at 610 nm which is weak in the case of E // b, thereby the remaining components at 605 nm, 608 nm, 620 nm, 703 nm and 727 nm are enhanced. The decay kinetics of Pr3+ and Ce3+ luminescence have been measured at T¼ 300 K. The literature data pointed out that the active ions in YSO are dissimilarly located in two nonequivalent crystallographic positions [22]. However, the decay curves reveal
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e11 9
e
e7
t=39 ns
e5 3
e
e1
Ce
e-1
3+
YSO:Ce, Pr, T=300 K
Luminescence intensity [a.u.]
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Time [ns]
evidencing the occurrence of Pr–Ce energy transfer suggesting that higher doping level of Pr3+ and Ce3+ is preferred for scintillation application but even lower doping level is favorable for qubit state measurement. On the other hand, the excitation spectra monitoring emission at 610 nm clearly indicates the contribution of Ce–Pr energy transfer in the excitation of the 1D2–3H4 luminescence of Pr3+. Broad-band luminescence of Pr and Ce activators in UV-Vis spectra range in Pr,Ce:YSO is quite intense. Occurrence of energy transfer phenomena involving inter-configuration transitions of Ce3+ and Pr3+ ions combined with strong absorption in the VUV–UV range are beneficial for excitation efficiency in Pr,Ce:YSO luminescent system.
1
e e-1 e-3 e-5 e-7 e-9 e-11
Acknowledgments
t=3 μs 3
P0
YSO:Ce, Pr, T=300 K
0
5
10
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Time [ μs] e1 e-1 e-3 e-5 e-7 e-9 e-11
We thank Prof. Philippe Goldner, Prof. Stefan Krӧll and Dr. Ying Yan for helpful discussions. Authors are grateful to National Natural Science Foundation of China (Grant no. 60908030, 51272264, 60938001), General Program of Shanghai Municipal Natural Science Foundation (Grant no. 13ZR1446100); European Community′s Seventh Framework Program (FP7/2007–2013) contract II-20090073 EC (DESY_Hasylab) and the Mairie de Paris for financial supports (“Research in Paris” Project). References
t=170 μs 1
D2
0
YSO:Ce, Pr, T=300 K 200
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1400
Time [ μs] Fig. 10. Decay curves of cerium and praseodymium luminescence in Pr,Ce:YSO.
single exponential time dependence, implying comparable rates of radiative relaxation of the active ions located in different local positions. The lifetime values were estimated to be 170 μs for Pr3+ (1D2), 3 μs for Pr3+ (3P0) and 39 ns for Ce3+ (5d1). The adequate decay curves are presented in Fig. 10. It is worth noticing that lifetimes of the active ions in single doped Pr:YSO and Ce:YSO are comparable to that observed in Pr,Ce:YSO. For instance, the lifetime of 1D2 in Pr:YSO was found to be of 200 μs at T ¼77 K [23]. 4. Conclusions High optical quality Pr,Ce:YSO crystal with low concentration doping level of 0.05 at% for Pr3+ and 0.088 at% for Ce3+ were grown and its optical properties were determined. The polarized absorption bands for 4f (2F5/2)-5d inter-configuration transitions of Ce3+ and 4fn–4fn 15d transitions of Pr3+ were assigned. The polarized spectra revealed that the anisotropy of absorption and luminescence in Pr,Ce:YSO crystal is rather weak. The excitation spectra measured in UV range indicate that the blue emission of Ce is effectively activated by 4fn–4fn 15d transitions of Pr3+
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