- Email: [email protected]
Nanocrystalline Eu-doped Lu3Al5O12 phosphor prepared by radiation method
Optical Materials xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Nanocrystalline Eu-doped Lu3Al5O12 phosphor prepared by radiation method A. Vondrášková a,b,⇑, A. Beitlerová a, J. Bárta b, V. Cˇuba b, E. Mihóková a, M. Nikl a a b
Institute of Physics Academy of Sciences of the Czech Republic, Cukrovarnická 10, 16253 Prague, Czech Republic Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Brˇehová 7, 11519 Prague, Czech Republic
a r t i c l e
i n f o
Article history: Received 10 October 2014 Received in revised form 28 November 2014 Accepted 2 December 2014 Available online xxxx Keywords: Eu3+ doped LuAG Nanopowder Luminescence PDTX X-ray phosphor
a b s t r a c t Using radiation method, a set of Eu doped LuAG nanopowders was prepared with Eu concentrations varying from 0.1 to 10 at.%. We studied the concentration dependence of Eu3+ emission spectra and decays from the 5D0 level in the red part of the spectra. Photoluminescence measurements were complemented with radioluminescence spectra to investigate the scintillation efficiency of the samples. The results were compared with the earlier published characteristics of Eu-doped LuAG phosphors prepared by other methods. Furthermore, based on obtained results we determined an optimum europium concentration for LuAG based X-ray phosphor. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Lu3Al5O12 (LuAG)-based bulk single crystal scintillators have been intensively investigated during the last decade and the Ce and Pr-doped ones became extensively tested in various applications due to their high density, fast response and high light yield [1]. Regarding the presence of the suitably-sized Lu3+ site, LuAG host can be doped by most rare earth ions. Therefore, its emission properties can be altered to great extent by incorporation of specific dopants due to their characteristic emission transitions. At the same time, these materials were also prepared in powder form by various methods [2–5], see also the review [6]. Recently, the novel radiation method [7] has also been employed to prepare a nanocrystalline Ce-doped LuAG phosphor [8]. This method provides a nanopowder material that consists of well separated single-phase LuAG grains with a diameter of a few tens of nanometers, narrow size distribution and perfect garnet structure containing minimum amount of defects and traps. Such characteristics can ensure high scintillation efficiency. The recent interest in nanoscintillators has mainly been triggered by modern medical therapies, namely by X-ray induced photodynamic therapy (PDT) which is under development for cancer treatment [9]. In a typical PDT treatment, photosensitive
⇑ Corresponding author at: Institute of Physics Academy of Sciences of the Czech Republic, Cukrovarnická 10, 16253 Prague, Czech Republic. Tel.: +420 607784178. E-mail address: [email protected] (A. Vondrášková).
drugs usually based on porphyrin are introduced into tumor cells and irradiated by red light. Irradiation transforms the drug from ground to the excited state, therapeutic effect is achieved through electron spin conversion of the excited singlet state to its triplet state. In the presence of oxygen the energy of the excited state can be transferred directly and creates the singlet oxygen. Tumor cells are then attacked and destroyed by the singlet oxygen generated from the PDT drug. However, the irradiation by light is impossible in deeply located tissues due to small penetration depth. Even in the transparency window (700–1000 nm) the penetration depth is only in the order of a few centimeters. Consequently, the concept of X-ray PDT was defined [10]. In such a therapy, the surface of scintillating nanoparticles is functionalized by porphyrin-based chromophore. Under X-ray irradiation of this nanoparticle-chromophore composite, the singlet oxygen is produced. X-ray irradiation of nanoscintillator can be performed even in a deeply-tissue and scintillation of nanoparticles ensures proper irradiation of their functionalized surface. The efficiency of every element of the process is critical [11] to achieve sufficient cytostatic effect, so called ‘‘Niedre killing dose’’. A model to estimate the figure of merit of a compound for such a purpose has been developed [8] and a number of nanoscintillators has recently been tested in view of such an application [12,13]. The above mentioned radiation-induced preparation of nanocrystalline scintillation materials is simple and robust method with no need for strict control over experimental conditions (pH, temperature or pressure). Using this method we report the
http://dx.doi.org/10.1016/j.optmat.2014.12.002 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: A. Vondrášková et al., Opt. Mater. (2014), http://dx.doi.org/10.1016/j.optmat.2014.12.002
2
A. Vondrášková et al. / Optical Materials xxx (2014) xxx–xxx
preparation of Eu-doped LuAG nanoscintillator in this paper, complemented with the study of its luminescence and scintillation characteristics in a broad concentration range of the Eu3+ dopant. The increase of the Eu3+ decay time due to nanometer size grains and effect of surrounding medium is noted and evaluated following the models reported earlier [14,15]. 2. Experimental 2.1. Sample preparation A set of garnet nanopowders was prepared by irradiation of aqueous solutions of soluble salts according to Lu3Al5O12 stoichiometry. The concentrations used were 3 103 mol dm3 of rare-earth ions (Lu3+, Eu3+ nitrates), 5 103 mol dm3 of aluminium nitrate and 0.1 mol dm3 of ammonium formate HCOONH4. Medium-pressure mercury discharge was used as a source of UV light to achieve the precipitation of finely dispersed particles from the solution. The solid phase is created in the solution by highly reactive products of water or formate ion photolysis (or radiolysis in the case of irradiation by ionizing radiation). It can be summarized by following equations [8]:
HCOO þ hm ! COO ; OH; . . .
ð1Þ
HCOO þ OH ! H2 O þ COO :
ð2Þ
Precipitated solid phase mainly consisting of carbonates or basic carbonates of metals (Al and Lu, Eu) was filtered from the solution using microfiltration method. Dried amorphous powder material needs to be annealed at a temperature exceeding 900 °C in order to create well-defined crystalline structure [8]. Crystallite size increases with temperature [8]. The optimum calcination temperature for achieving the best luminescent properties was found to be around 1200 °C. The set of samples doped with 0.1%, 0.5%, 1%, 2%, 5% and 10% of Eu ions was prepared. All chemicals were purchased from Sigma– Aldrich and were used without further purification. Initial aqueous solutions were prepared using aluminium nitrate nonahydrate (99.997%), lutetium nitrate hydrate (99.999%), europium nitrate pentahydrate (99.9%) and ammonium formate (99.995%). The latter was used as a radiation sensitizer. Initial used concentrations of dissolved salts were calculated according to the given stoichiometry of Lu33xAl5O12:Eu3x.
Irradiation of the solutions was performed in a continuously stirred, water-cooled photochemical reactor (volume of irradiated solution 2.5 dm3). As UV light source, medium-pressure mercury discharge with adjustable power input 140–400 W was used (UV Technik Meyer GmbH). The irradiation time was typically 150 min for 300 W lamp power input. After irradiation the created solid phase was separated from the solution by microfiltration (Milipore HAWP, filter 0.45 lm), dried in air (40 °C) for at least 12 h and then calcined for 2 h at 1200 °C in air (Clasic 0415VAK furnace). Crystal structure of calcined powders was measured by X-ray powder diffraction (XRPD, utilizing Rigaku MiniFlex 600 diffractometer with NaI:Tl scintillation detector, using Ni-filtered Cu Ka = 0.154184 nm X-ray radiation. The diffraction patterns were recorded in the range 10–80° 2h and were compared with records from the ICDD PDF-2 database (version 2013). The elemental composition of the solid phase was determined using X-ray fluorescence (XRF) analyzer Niton Xl3t 900 series with GOLDD technology. 2.2. Luminescence characterization experiments Radioluminescence emission (RL), photoluminescence excitation (PLE) and emission (PL) spectra and decay curves were measured at room temperature using a custom-made 5000 M fluorometer (Horiba Jobin Yvon) under excitation by X-ray tube (Seifert, 40 kV, 15 mA). Photoluminescence spectrum (PLE and PL) was measured under excitation by steady state deuterium lamp (Heraus, D200F model). All spectra were corrected for experimental distortions. The decay curves were measured for a free standing powder in cuvette under excitation by microsecond pulse xenon lamp. Photomultiplier-based fast detector (TBX-04 model) working in the photon counting mode was used for detection. A powdered Bi4Ge3O12 (BGO) was used as standard scintillator for quantitative comparison of radioluminescence intensity. 3. Results and discussion 3.1. Solid phase characterization The measured diffraction patterns of LuAG:Eu samples calcined at 1200 °C match the database record of LuAG (PDF #73-1368) very well and show highly pure crystalline phase (Fig. 1). Closer examination of diffraction patterns reveals slight shift of diffraction lines (Fig. 2). This shift to the lower diffraction angles is more
LuAG:Eu 10,0%
120
800
LuAG:Eu 5,0% 700
LuAG:Eu 2,0%
100
LuAG:Eu 1,0%
600
LuAG:Eu 0,5% 80
Intensity [arb.units]
LuAG:Eu 0,1%
500
ICDD-PDF2 no.73-1368
60
400
200
20 100
0
15
Eu concentration
300
40
0
25
35
45
55
65
75
Diffraction angle 2θ [ ° ] Fig. 1. Diffraction patterns of the prepared samples calcined at 1200 °C and compared to the ICDD PDF-2 database record #73-1368 (indicated vertical bars).
Please cite this article in press as: A. Vondrášková et al., Opt. Mater. (2014), http://dx.doi.org/10.1016/j.optmat.2014.12.002
3
A. Vondrášková et al. / Optical Materials xxx (2014) xxx–xxx
100
90
LuAG:Eu 10,0%
0.300
0.040
1200
LuAG:Eu 5,0% 0.035
LuAG:Eu 2,0%
XRF intensity ratio Eu/Lu [arb.units]
1000
80
Instensity [arb.units]
LuAG:Eu 1,0% 70
60
50
LuAG:Eu 0,5%
800
LuAG:Eu 0,1% ICDD-PDF2 no.73-1368
600
40
400
30
20
200
10
0
0.250
0.030
0.200
0.025
0.150
0.020
R² = 0.9954 R² = 0.9954
0.015
0.100
0.010
0.050
0
0.005
32.8
33.0
33.2
33.4
33.6
33.8
34.0
Diffraction angle 2θ [ ° ]
0.000
0.000
Fig. 2. Diffraction pattern showing the shift of a (4 2 0) diffraction peak with variation of europium concentration.
0.000
0.050
0.100
Doping level [%] Fig. 3. Eu/Lu La XRF intensity ratio as a function of Eu doping level.
Table 1 The determined lattice parameters a and crystallite sizes of all samples (ESD = estimated standard deviation). a, ESD
0.1 0.5 1.0 2.0 5.0 10.0
11.46 11.69 11.72 11.85 11.50 11.42
Crystallite size, ESD 0.0008 0.0008 0.0010 0.0010 0.0010 0.0008
46 44 39 45 43 44
2 2 2 2 2 2
1000000
obvious at higher dopant concentrations and suggests enlargement of crystalline lattice due to the incorporation of bigger cation (Eu3+) into the lattice (to compare, ionic radius (Eu3+)8 = 1.066 Å and (Lu3+)8 = 0.977 Å). The total increase in lattice constant reached ca. 0.16% for the LuAG sample doped with 10% Eu. Measured diffraction data were also used to calculate average crystallite size via the Scherrer equation [8]:
Amplitude [arb.units]
Dopant (at.%)
1200000
800000
600000
5D - 7F 0 1 BGO LuAG: Eu 0,1% LuAG:Eu 0,5% LuAG:Eu 2% LuAG:Eu 5% LuAG:Eu 10%
400000
5D
5D 0
0
- 7F2
- 7F4
200000
0
Kk s¼ ; bhkl coshhkl
ð3Þ
where K represents the shape factor (0.89 for spherical particles), k is the radiation wavelength of the X-ray radiation (Cu Ka = 0.154184 nm), h is the diffraction angle of selected reflection and bhkl (rad) is the measured peak FWHM. According to the measured data and Eq. (3) the calculated average crystallite size in all samples was ca. 44 nm. The determined lattice parameters a and crystallite sizes of all samples are summarized in Table 1. The XRF analysis revealed that in all studied samples the ratio of Eu/Lu La line intensities depended linearly on molar ratio of these atoms in the initial solution (Fig. 3). Therefore, the molar ratio in prepared powder is approximately equal to the molar ratio of Eu/Lu in the solution and shows segregation factor of ca. 1. This claim is also based on quantitative observed yield of the solid phase in all preparations (>95%). XRF measurements can also reveal the real concentration in initial solution (or in solid phase) for samples with unknown doping level. 3.2. Luminescence characteristics The Eu3+ centre in aluminium garnets shows characteristic 4f–4f emission transitions starting from the 5D0 level to the ground
380
480
580
680
λ [nm] Fig. 4. RL spectra of all samples, excitation by X-ray tube, 40 kV, 15 mA.
state multiplet 7Fx, x = 1–4. As these transitions are taking place in the inner 4f shell, which is well shielded from the crystal field of the host lattice, the position of emission lines is almost identical in many hosts. Depending on the site symmetry, however, relative intensity of peaks and emission decay times may change [16]. Typically, these spin- and parity-forbidden transitions have decay times on the order of ms. RL spectra of all samples in Fig. 4 show typical emission lines of Eu3+ 4f–4f transitions in the red spectral region; no host-related emission in UV spectral region was observed. The spectrum of BGO reference scintillator is also included to quantitatively compare the scintillation intensity. The highest emission intensity was obtained for 0.5%, 2% and 5% concentration of Eu3+. It is worth noting that the absolute emission intensity in powder materials is rather sensitive to experimental arrangement so it cannot be the sole indicator of e.g. concentration quenching. In Fig. 5, PLE spectrum of Eu3+ luminescence in the sample doped with 0.5 at.% Eu3+ shows an intense band at 225 nm, which is due to the well-known charge transfer mechanism [16] and also
Please cite this article in press as: A. Vondrášková et al., Opt. Mater. (2014), http://dx.doi.org/10.1016/j.optmat.2014.12.002
4
A. Vondrášková et al. / Optical Materials xxx (2014) xxx–xxx 10000
35000
Amplitude [arb.units]
I(t) = 4061exp(-t/10.5 ms) +3.1
Eu3+
25000
LuAG:Eu 5% 20000
15000
4f-4f Eu3+ transitions
10000
log intensity [arb.units]
CT: O2-
30000
1000
100
10
5000
1
0.00E+00
0
190
240
290
340
390
5.00E+04
λ [nm] Fig. 5. PLE spectrum of the LuAG:0.5% Eu sample, emission wavelength 592 nm.
1.00E+05
1.50E+05
time [μs] Fig. 7. Decay curve of LuAG:1% Eu sample, excitation wavelength 225 nm, emission wavelength 592 nm. Solid line is a convolution of instrumental response (not shown) and function I(t) displayed in the figure.
80000
LuAG:Eu 0,1% LuAG:EU 0,5% LuAG: Eu 1% LuAG:Eu 2% LuAG:Eu 5% LuAG:Eu 10%
5D - 7F 0 1
70000
Amplitude [arb.units]
60000
50000
40000
5D
0
- 7F2
5D
30000
0
- 7F4
Table 2 Calculated decay times for all powder samples, excitation wavelength 225 nm, emission wavelength 592 and 710 nm.
s (ms)
Dopant (at.%)
0.1 0.5 1 2 5 10
kem = 592 nm
kem = 710 nm
10.3 10.7 10.5 9.9 9.9 9.0
10.2 10.2 10.3 9.8 9.8 8.9
20000
10000
0
570
620
670
720
λ [nm]
The bulk single crystal of LuAG doped with 0.1% Eu3+ grown by micro pulling down method [17] was used for comparison. Its decay time under excitation at 242 nm and emission at 593 nm in the same experimental setup was determined as 3.8 ms. The radiative lifetime of transition sr can be described by the following equation [18]:
Fig. 6. PL spectra of all samples, excitation wavelength 225 nm.
sr /
k20 2
f ðn2 þ 2Þ n
;
ð4Þ
3+
several 4f–4f transitions of Eu in the near UV part of spectrum. PL spectra of all samples show typical narrow peaks of Eu3+ 4f–4f transitions (Fig. 6), whose shape is identical to the RL spectra. The line emissions are better resolved in the PL spectra because of using narrower slits in the emission monochromator. PL decay curves were measured for each sample using the same excitation wavelength of 225 nm and emission wavelengths of either 592 nm or 710 nm. Example of the measured decay curve is shown in Fig. 7. The decay time values were calculated from decay curves, which were approximated by the convolution of instrumental response with a single exponential function; results are shown in Table 2. The evaluated decay times for both emission wavelengths are very similar and all the decays are excellent single exponentials. The samples with the highest concentrations of Eu3+ show shorter decay time values which indicate a mild concentration quenching. On the other hand, all the decay curves of nanopowder samples show much longer decay time values compared to the bulk material. This effect is caused by the change of refractive index at the length scale on the order of a few hundreds of nanometers and has been described in several materials [8,14,15]. In this case, the free-standing nanoparticles are surrounded by air.
where f is the oscillator strength for the dipole transition, k0 is the wavelength of the luminescence emission maximum and n represents the refractive index of material. To determine the refractive index of a material one needs to take into account the surrounding material in addition to the refractive index of nanoparticles themselves. An effective refractive index neff can be calculated from the following equation [14]:
neff ¼ x n þ ð1 xÞ nmed ;
ð5Þ
where n is the refractive index of a material, nmed is the refractive index of surrounding medium and x is the filling factor – a volume fraction of space occupied by the nanoparticles. Decay curves of all prepared samples were measured in a free standing powder at room temperature (surrounded by air, nmed = nair = 1). Using the measured decay times (Table 2) and the properties of the bulk single crystal LuAG:Eu mentioned above (sr = 3.8 ms; n = 1.842 [17]) the filling factor was calculated for the samples not affected by concentration quenching, where the decay time is the same as radiative lifetime in Eq. (4). Results showed in Table 3 suggest that a quite large volume of the samples was filled with just air and that the nanograins were relatively well-separated without serious
Please cite this article in press as: A. Vondrášková et al., Opt. Mater. (2014), http://dx.doi.org/10.1016/j.optmat.2014.12.002
A. Vondrášková et al. / Optical Materials xxx (2014) xxx–xxx Table 3 Calculated filling factors for 0.1–5% samples measured for free standing powder samples. Dopant (at.%)
x
0.1 0.5 1 2 5
0.42 0.39 0.40 0.44 0.44
agglomeration. The low tendency to agglomerate is one of important qualities for use in the above-mentioned photodynamic therapy. The radiation-induced method provides nanopowders with the size of nanograins comparable to other preparation techniques at given calcination temperature (according to [19], grains of 31 nm at 1100 °C were prepared and in [20] grains of 50 nm at 1100 °C were prepared). The relative intensities of 4f–4f peaks in emission spectra are also very similar (based on the same symmetry of the dopant site in the LuAG lattice) and show the dominant 5D0–7F1 magnetic dipole transition at ca. 590 nm. The main advantage compared to [19] is a much simpler preparation of amorphous precursors with no need of strict control over experimental conditions (pH adjustment, temperature, solution composition). Preparation of amorphous precursor in [20] was again limited by the need of adjusting the pH and addition of organic agents to achieve the precipitation of solid phase. The main problem in the case of UV light initiation of solid phase precipitation is the need of rigorous stirring and cooling of the solution during irradiation. Calcination of amorphous precursors leads to very similar products as in abovementioned works [19,20] and in the case of radiation induced preparation the calcination temperature is the main way of controlling the resulting particle size [8]. 4. Conclusions Lutetium–aluminium garnet powders doped with Eu3+ in concentrations from 0.1% to 10% were prepared using photochemical method. Based on XRPD measurements all samples show very high crystal quality and garnet phase purity. Calculated crystallite size was determined as 44 nm. Radioluminescence spectra of all samples show typical Eu3+ 4f– 4f transitions (5D0–7Fx), the dominant transition being 5D0–7F1 magnetic dipole transition with maximum at 592 nm. Identical dominant emission line at 592 nm (5D0–7F1) and at 710 nm (5D0–7F4) is also found in photoluminescence spectra. Photoluminescence excitation spectra show the highest intensity at 225 nm (position of charge transfer transition O2–Eu3+).
5
Decay curves were single exponential and decay time values of ca. 9–11 ms were very similar for both dominant emission lines, suggesting simple radiative transition from the excited state to the ground state. Calculated decay times show a mild concentration quenching for the highest Eu concentrations. By comparing the decay time value for analogous bulk material (3.8 ms) the role of effective refractive index in a powder material was found to be significant. Based on the calculated filling factor, prepared powders consist of well-separated crystallites without any noticeable agglomeration. Based on collected data the best figure of merit for PDTX application was obtained in the samples with Eu3+ concentration 0.5–5%, where the concentration quenching is insignificant and radioluminescence intensity was very high. Acknowledgement This research was supported by Czech Science foundation Project No. GA 13-09876S. References [1] M. Nikl, A. Yoshikawa, K. Kamada, K. Nejezchleb, C.R. Stanek, J.A. Mares, K. Blazek, Progr. Cryst. Growth Charact. Mater. 59 (2013) 47–72. [2] S.A. Cicillini, A.M. Pires, O.A. Serra, J. Alloys Compd. 374 (2004) 169–172. [3] H.-L. Li, X.-J. Liu, L.-P. Huang, Opt. Mater. 29 (2007) 1138–1142. [4] L. Xing, L. Peng, M. Gu, G. Tang, J. Alloys Compd. 491 (2010) 599–604. [5] J.M. Ogiegło, A. Zych, T. Justel, A. Meijerink, C.R. Ronda, Opt. Mater. 35 (2013) 322–331. [6] A. Speghini, F. Piccinelli, M. Bettinelli, Opt. Mater. 33 (2011) 247–257. ˇ uba, J. Bárta, V. Jary´, M. Nikl, Radiation induced synthesis of oxide [7] V. C compounds, in: B.I. Kharisov, O.V. Kharissova, U. Otiz-Mendez (Eds.), Radiation synthesis of Materials and Compounds, CRC Press, Boca Raton, USA, 2013, pp. 81–100. [8] J. Bárta, V. Cˇuba, M. Pospíšil, V. Jary´, M. Nikl, J. Mater. Chem. 22 (2012) 16590– 16597. [9] A. De Rosa, D. Naviglio, A. Di Luccia, Curr. Cancer Ther. Rev. 7 (2011) 1. [10] P. Juzenas, W. Chen, Y.-P. Sun, M.A.N. Coelho, R. Generalov, N. Generalova, I.L. Christensen, Adv. Drug Delivery Rev. 60 (2008) 1600. [11] N.Y. Morgan et al., Radiat. Res. 171 (2009) 236. [12] J.Y. Jung, G.A. Hirata, G. Gundiah, S. Derenzo, W. Wrasidlo, S. Kesari, M.T. Makale, J. McKittrick, J. Lumin. 154 (2014) 569–577. [13] A.-L. Bulin, C. Truillet, R. Chouikrat, F. Lux, C. Frochot, D. Amans, G. Ledoux, O. Tillement, P. Perriat, M. Barberi-Heyob, C. Dujardin, J. Phys. Chem. C 117 (2013) 21583–21589. [14] R.S. Meltzer, S.P. Feofilov, B. Tissue, H.B. Yuan, Phys. Rev. B 60 (1999) R14012. [15] V. LeBihan, A. Pillonnet, D. Amans, G. Ledoux, O. Marty, C. Dujardin, Phys. Rev. B 78 (2008) 113405. [16] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994. [17] H. Ogino, A. Yoshikawa, M. Nikl, R. Kucerkova, J.-I. Shimoyama, K. Kishio, Phys. Proc. 2 (2) (2009) 191–205. [18] Di Bartolo, in: Optical Interactions of Solids, first ed., Wiley, New York, 1968, pp. 403–414. [19] Z. Wang, M. Xu, W. Zhang, M. Yin, J. Lumin. 122–123 (2007) 437–439. [20] D. Uhlich, P. Huppertz, D.U. Wiechert, T. Justel, Opt. Mater. 29 (2007) 1505– 1509.
Please cite this article in press as: A. Vondrášková et al., Opt. Mater. (2014), http://dx.doi.org/10.1016/j.optmat.2014.12.002
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Nanocrystalline Eu-doped Lu3Al5O12 phosphor prepared by radiation method A. Vondrášková a,b,⇑, A. Beitlerová a, J. Bárta b, V. Cˇuba b, E. Mihóková a, M. Nikl a a b
Institute of Physics Academy of Sciences of the Czech Republic, Cukrovarnická 10, 16253 Prague, Czech Republic Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Brˇehová 7, 11519 Prague, Czech Republic
a r t i c l e
i n f o
Article history: Received 10 October 2014 Received in revised form 28 November 2014 Accepted 2 December 2014 Available online xxxx Keywords: Eu3+ doped LuAG Nanopowder Luminescence PDTX X-ray phosphor
a b s t r a c t Using radiation method, a set of Eu doped LuAG nanopowders was prepared with Eu concentrations varying from 0.1 to 10 at.%. We studied the concentration dependence of Eu3+ emission spectra and decays from the 5D0 level in the red part of the spectra. Photoluminescence measurements were complemented with radioluminescence spectra to investigate the scintillation efficiency of the samples. The results were compared with the earlier published characteristics of Eu-doped LuAG phosphors prepared by other methods. Furthermore, based on obtained results we determined an optimum europium concentration for LuAG based X-ray phosphor. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Lu3Al5O12 (LuAG)-based bulk single crystal scintillators have been intensively investigated during the last decade and the Ce and Pr-doped ones became extensively tested in various applications due to their high density, fast response and high light yield [1]. Regarding the presence of the suitably-sized Lu3+ site, LuAG host can be doped by most rare earth ions. Therefore, its emission properties can be altered to great extent by incorporation of specific dopants due to their characteristic emission transitions. At the same time, these materials were also prepared in powder form by various methods [2–5], see also the review [6]. Recently, the novel radiation method [7] has also been employed to prepare a nanocrystalline Ce-doped LuAG phosphor [8]. This method provides a nanopowder material that consists of well separated single-phase LuAG grains with a diameter of a few tens of nanometers, narrow size distribution and perfect garnet structure containing minimum amount of defects and traps. Such characteristics can ensure high scintillation efficiency. The recent interest in nanoscintillators has mainly been triggered by modern medical therapies, namely by X-ray induced photodynamic therapy (PDT) which is under development for cancer treatment [9]. In a typical PDT treatment, photosensitive
⇑ Corresponding author at: Institute of Physics Academy of Sciences of the Czech Republic, Cukrovarnická 10, 16253 Prague, Czech Republic. Tel.: +420 607784178. E-mail address: [email protected] (A. Vondrášková).
drugs usually based on porphyrin are introduced into tumor cells and irradiated by red light. Irradiation transforms the drug from ground to the excited state, therapeutic effect is achieved through electron spin conversion of the excited singlet state to its triplet state. In the presence of oxygen the energy of the excited state can be transferred directly and creates the singlet oxygen. Tumor cells are then attacked and destroyed by the singlet oxygen generated from the PDT drug. However, the irradiation by light is impossible in deeply located tissues due to small penetration depth. Even in the transparency window (700–1000 nm) the penetration depth is only in the order of a few centimeters. Consequently, the concept of X-ray PDT was defined [10]. In such a therapy, the surface of scintillating nanoparticles is functionalized by porphyrin-based chromophore. Under X-ray irradiation of this nanoparticle-chromophore composite, the singlet oxygen is produced. X-ray irradiation of nanoscintillator can be performed even in a deeply-tissue and scintillation of nanoparticles ensures proper irradiation of their functionalized surface. The efficiency of every element of the process is critical [11] to achieve sufficient cytostatic effect, so called ‘‘Niedre killing dose’’. A model to estimate the figure of merit of a compound for such a purpose has been developed [8] and a number of nanoscintillators has recently been tested in view of such an application [12,13]. The above mentioned radiation-induced preparation of nanocrystalline scintillation materials is simple and robust method with no need for strict control over experimental conditions (pH, temperature or pressure). Using this method we report the
http://dx.doi.org/10.1016/j.optmat.2014.12.002 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: A. Vondrášková et al., Opt. Mater. (2014), http://dx.doi.org/10.1016/j.optmat.2014.12.002
2
A. Vondrášková et al. / Optical Materials xxx (2014) xxx–xxx
preparation of Eu-doped LuAG nanoscintillator in this paper, complemented with the study of its luminescence and scintillation characteristics in a broad concentration range of the Eu3+ dopant. The increase of the Eu3+ decay time due to nanometer size grains and effect of surrounding medium is noted and evaluated following the models reported earlier [14,15]. 2. Experimental 2.1. Sample preparation A set of garnet nanopowders was prepared by irradiation of aqueous solutions of soluble salts according to Lu3Al5O12 stoichiometry. The concentrations used were 3 103 mol dm3 of rare-earth ions (Lu3+, Eu3+ nitrates), 5 103 mol dm3 of aluminium nitrate and 0.1 mol dm3 of ammonium formate HCOONH4. Medium-pressure mercury discharge was used as a source of UV light to achieve the precipitation of finely dispersed particles from the solution. The solid phase is created in the solution by highly reactive products of water or formate ion photolysis (or radiolysis in the case of irradiation by ionizing radiation). It can be summarized by following equations [8]:
HCOO þ hm ! COO ; OH; . . .
ð1Þ
HCOO þ OH ! H2 O þ COO :
ð2Þ
Precipitated solid phase mainly consisting of carbonates or basic carbonates of metals (Al and Lu, Eu) was filtered from the solution using microfiltration method. Dried amorphous powder material needs to be annealed at a temperature exceeding 900 °C in order to create well-defined crystalline structure [8]. Crystallite size increases with temperature [8]. The optimum calcination temperature for achieving the best luminescent properties was found to be around 1200 °C. The set of samples doped with 0.1%, 0.5%, 1%, 2%, 5% and 10% of Eu ions was prepared. All chemicals were purchased from Sigma– Aldrich and were used without further purification. Initial aqueous solutions were prepared using aluminium nitrate nonahydrate (99.997%), lutetium nitrate hydrate (99.999%), europium nitrate pentahydrate (99.9%) and ammonium formate (99.995%). The latter was used as a radiation sensitizer. Initial used concentrations of dissolved salts were calculated according to the given stoichiometry of Lu33xAl5O12:Eu3x.
Irradiation of the solutions was performed in a continuously stirred, water-cooled photochemical reactor (volume of irradiated solution 2.5 dm3). As UV light source, medium-pressure mercury discharge with adjustable power input 140–400 W was used (UV Technik Meyer GmbH). The irradiation time was typically 150 min for 300 W lamp power input. After irradiation the created solid phase was separated from the solution by microfiltration (Milipore HAWP, filter 0.45 lm), dried in air (40 °C) for at least 12 h and then calcined for 2 h at 1200 °C in air (Clasic 0415VAK furnace). Crystal structure of calcined powders was measured by X-ray powder diffraction (XRPD, utilizing Rigaku MiniFlex 600 diffractometer with NaI:Tl scintillation detector, using Ni-filtered Cu Ka = 0.154184 nm X-ray radiation. The diffraction patterns were recorded in the range 10–80° 2h and were compared with records from the ICDD PDF-2 database (version 2013). The elemental composition of the solid phase was determined using X-ray fluorescence (XRF) analyzer Niton Xl3t 900 series with GOLDD technology. 2.2. Luminescence characterization experiments Radioluminescence emission (RL), photoluminescence excitation (PLE) and emission (PL) spectra and decay curves were measured at room temperature using a custom-made 5000 M fluorometer (Horiba Jobin Yvon) under excitation by X-ray tube (Seifert, 40 kV, 15 mA). Photoluminescence spectrum (PLE and PL) was measured under excitation by steady state deuterium lamp (Heraus, D200F model). All spectra were corrected for experimental distortions. The decay curves were measured for a free standing powder in cuvette under excitation by microsecond pulse xenon lamp. Photomultiplier-based fast detector (TBX-04 model) working in the photon counting mode was used for detection. A powdered Bi4Ge3O12 (BGO) was used as standard scintillator for quantitative comparison of radioluminescence intensity. 3. Results and discussion 3.1. Solid phase characterization The measured diffraction patterns of LuAG:Eu samples calcined at 1200 °C match the database record of LuAG (PDF #73-1368) very well and show highly pure crystalline phase (Fig. 1). Closer examination of diffraction patterns reveals slight shift of diffraction lines (Fig. 2). This shift to the lower diffraction angles is more
LuAG:Eu 10,0%
120
800
LuAG:Eu 5,0% 700
LuAG:Eu 2,0%
100
LuAG:Eu 1,0%
600
LuAG:Eu 0,5% 80
Intensity [arb.units]
LuAG:Eu 0,1%
500
ICDD-PDF2 no.73-1368
60
400
200
20 100
0
15
Eu concentration
300
40
0
25
35
45
55
65
75
Diffraction angle 2θ [ ° ] Fig. 1. Diffraction patterns of the prepared samples calcined at 1200 °C and compared to the ICDD PDF-2 database record #73-1368 (indicated vertical bars).
Please cite this article in press as: A. Vondrášková et al., Opt. Mater. (2014), http://dx.doi.org/10.1016/j.optmat.2014.12.002
3
A. Vondrášková et al. / Optical Materials xxx (2014) xxx–xxx
100
90
LuAG:Eu 10,0%
0.300
0.040
1200
LuAG:Eu 5,0% 0.035
LuAG:Eu 2,0%
XRF intensity ratio Eu/Lu [arb.units]
1000
80
Instensity [arb.units]
LuAG:Eu 1,0% 70
60
50
LuAG:Eu 0,5%
800
LuAG:Eu 0,1% ICDD-PDF2 no.73-1368
600
40
400
30
20
200
10
0
0.250
0.030
0.200
0.025
0.150
0.020
R² = 0.9954 R² = 0.9954
0.015
0.100
0.010
0.050
0
0.005
32.8
33.0
33.2
33.4
33.6
33.8
34.0
Diffraction angle 2θ [ ° ]
0.000
0.000
Fig. 2. Diffraction pattern showing the shift of a (4 2 0) diffraction peak with variation of europium concentration.
0.000
0.050
0.100
Doping level [%] Fig. 3. Eu/Lu La XRF intensity ratio as a function of Eu doping level.
Table 1 The determined lattice parameters a and crystallite sizes of all samples (ESD = estimated standard deviation). a, ESD
0.1 0.5 1.0 2.0 5.0 10.0
11.46 11.69 11.72 11.85 11.50 11.42
Crystallite size, ESD 0.0008 0.0008 0.0010 0.0010 0.0010 0.0008
46 44 39 45 43 44
2 2 2 2 2 2
1000000
obvious at higher dopant concentrations and suggests enlargement of crystalline lattice due to the incorporation of bigger cation (Eu3+) into the lattice (to compare, ionic radius (Eu3+)8 = 1.066 Å and (Lu3+)8 = 0.977 Å). The total increase in lattice constant reached ca. 0.16% for the LuAG sample doped with 10% Eu. Measured diffraction data were also used to calculate average crystallite size via the Scherrer equation [8]:
Amplitude [arb.units]
Dopant (at.%)
1200000
800000
600000
5D - 7F 0 1 BGO LuAG: Eu 0,1% LuAG:Eu 0,5% LuAG:Eu 2% LuAG:Eu 5% LuAG:Eu 10%
400000
5D
5D 0
0
- 7F2
- 7F4
200000
0
Kk s¼ ; bhkl coshhkl
ð3Þ
where K represents the shape factor (0.89 for spherical particles), k is the radiation wavelength of the X-ray radiation (Cu Ka = 0.154184 nm), h is the diffraction angle of selected reflection and bhkl (rad) is the measured peak FWHM. According to the measured data and Eq. (3) the calculated average crystallite size in all samples was ca. 44 nm. The determined lattice parameters a and crystallite sizes of all samples are summarized in Table 1. The XRF analysis revealed that in all studied samples the ratio of Eu/Lu La line intensities depended linearly on molar ratio of these atoms in the initial solution (Fig. 3). Therefore, the molar ratio in prepared powder is approximately equal to the molar ratio of Eu/Lu in the solution and shows segregation factor of ca. 1. This claim is also based on quantitative observed yield of the solid phase in all preparations (>95%). XRF measurements can also reveal the real concentration in initial solution (or in solid phase) for samples with unknown doping level. 3.2. Luminescence characteristics The Eu3+ centre in aluminium garnets shows characteristic 4f–4f emission transitions starting from the 5D0 level to the ground
380
480
580
680
λ [nm] Fig. 4. RL spectra of all samples, excitation by X-ray tube, 40 kV, 15 mA.
state multiplet 7Fx, x = 1–4. As these transitions are taking place in the inner 4f shell, which is well shielded from the crystal field of the host lattice, the position of emission lines is almost identical in many hosts. Depending on the site symmetry, however, relative intensity of peaks and emission decay times may change [16]. Typically, these spin- and parity-forbidden transitions have decay times on the order of ms. RL spectra of all samples in Fig. 4 show typical emission lines of Eu3+ 4f–4f transitions in the red spectral region; no host-related emission in UV spectral region was observed. The spectrum of BGO reference scintillator is also included to quantitatively compare the scintillation intensity. The highest emission intensity was obtained for 0.5%, 2% and 5% concentration of Eu3+. It is worth noting that the absolute emission intensity in powder materials is rather sensitive to experimental arrangement so it cannot be the sole indicator of e.g. concentration quenching. In Fig. 5, PLE spectrum of Eu3+ luminescence in the sample doped with 0.5 at.% Eu3+ shows an intense band at 225 nm, which is due to the well-known charge transfer mechanism [16] and also
Please cite this article in press as: A. Vondrášková et al., Opt. Mater. (2014), http://dx.doi.org/10.1016/j.optmat.2014.12.002
4
A. Vondrášková et al. / Optical Materials xxx (2014) xxx–xxx 10000
35000
Amplitude [arb.units]
I(t) = 4061exp(-t/10.5 ms) +3.1
Eu3+
25000
LuAG:Eu 5% 20000
15000
4f-4f Eu3+ transitions
10000
log intensity [arb.units]
CT: O2-
30000
1000
100
10
5000
1
0.00E+00
0
190
240
290
340
390
5.00E+04
λ [nm] Fig. 5. PLE spectrum of the LuAG:0.5% Eu sample, emission wavelength 592 nm.
1.00E+05
1.50E+05
time [μs] Fig. 7. Decay curve of LuAG:1% Eu sample, excitation wavelength 225 nm, emission wavelength 592 nm. Solid line is a convolution of instrumental response (not shown) and function I(t) displayed in the figure.
80000
LuAG:Eu 0,1% LuAG:EU 0,5% LuAG: Eu 1% LuAG:Eu 2% LuAG:Eu 5% LuAG:Eu 10%
5D - 7F 0 1
70000
Amplitude [arb.units]
60000
50000
40000
5D
0
- 7F2
5D
30000
0
- 7F4
Table 2 Calculated decay times for all powder samples, excitation wavelength 225 nm, emission wavelength 592 and 710 nm.
s (ms)
Dopant (at.%)
0.1 0.5 1 2 5 10
kem = 592 nm
kem = 710 nm
10.3 10.7 10.5 9.9 9.9 9.0
10.2 10.2 10.3 9.8 9.8 8.9
20000
10000
0
570
620
670
720
λ [nm]
The bulk single crystal of LuAG doped with 0.1% Eu3+ grown by micro pulling down method [17] was used for comparison. Its decay time under excitation at 242 nm and emission at 593 nm in the same experimental setup was determined as 3.8 ms. The radiative lifetime of transition sr can be described by the following equation [18]:
Fig. 6. PL spectra of all samples, excitation wavelength 225 nm.
sr /
k20 2
f ðn2 þ 2Þ n
;
ð4Þ
3+
several 4f–4f transitions of Eu in the near UV part of spectrum. PL spectra of all samples show typical narrow peaks of Eu3+ 4f–4f transitions (Fig. 6), whose shape is identical to the RL spectra. The line emissions are better resolved in the PL spectra because of using narrower slits in the emission monochromator. PL decay curves were measured for each sample using the same excitation wavelength of 225 nm and emission wavelengths of either 592 nm or 710 nm. Example of the measured decay curve is shown in Fig. 7. The decay time values were calculated from decay curves, which were approximated by the convolution of instrumental response with a single exponential function; results are shown in Table 2. The evaluated decay times for both emission wavelengths are very similar and all the decays are excellent single exponentials. The samples with the highest concentrations of Eu3+ show shorter decay time values which indicate a mild concentration quenching. On the other hand, all the decay curves of nanopowder samples show much longer decay time values compared to the bulk material. This effect is caused by the change of refractive index at the length scale on the order of a few hundreds of nanometers and has been described in several materials [8,14,15]. In this case, the free-standing nanoparticles are surrounded by air.
where f is the oscillator strength for the dipole transition, k0 is the wavelength of the luminescence emission maximum and n represents the refractive index of material. To determine the refractive index of a material one needs to take into account the surrounding material in addition to the refractive index of nanoparticles themselves. An effective refractive index neff can be calculated from the following equation [14]:
neff ¼ x n þ ð1 xÞ nmed ;
ð5Þ
where n is the refractive index of a material, nmed is the refractive index of surrounding medium and x is the filling factor – a volume fraction of space occupied by the nanoparticles. Decay curves of all prepared samples were measured in a free standing powder at room temperature (surrounded by air, nmed = nair = 1). Using the measured decay times (Table 2) and the properties of the bulk single crystal LuAG:Eu mentioned above (sr = 3.8 ms; n = 1.842 [17]) the filling factor was calculated for the samples not affected by concentration quenching, where the decay time is the same as radiative lifetime in Eq. (4). Results showed in Table 3 suggest that a quite large volume of the samples was filled with just air and that the nanograins were relatively well-separated without serious
Please cite this article in press as: A. Vondrášková et al., Opt. Mater. (2014), http://dx.doi.org/10.1016/j.optmat.2014.12.002
A. Vondrášková et al. / Optical Materials xxx (2014) xxx–xxx Table 3 Calculated filling factors for 0.1–5% samples measured for free standing powder samples. Dopant (at.%)
x
0.1 0.5 1 2 5
0.42 0.39 0.40 0.44 0.44
agglomeration. The low tendency to agglomerate is one of important qualities for use in the above-mentioned photodynamic therapy. The radiation-induced method provides nanopowders with the size of nanograins comparable to other preparation techniques at given calcination temperature (according to [19], grains of 31 nm at 1100 °C were prepared and in [20] grains of 50 nm at 1100 °C were prepared). The relative intensities of 4f–4f peaks in emission spectra are also very similar (based on the same symmetry of the dopant site in the LuAG lattice) and show the dominant 5D0–7F1 magnetic dipole transition at ca. 590 nm. The main advantage compared to [19] is a much simpler preparation of amorphous precursors with no need of strict control over experimental conditions (pH adjustment, temperature, solution composition). Preparation of amorphous precursor in [20] was again limited by the need of adjusting the pH and addition of organic agents to achieve the precipitation of solid phase. The main problem in the case of UV light initiation of solid phase precipitation is the need of rigorous stirring and cooling of the solution during irradiation. Calcination of amorphous precursors leads to very similar products as in abovementioned works [19,20] and in the case of radiation induced preparation the calcination temperature is the main way of controlling the resulting particle size [8]. 4. Conclusions Lutetium–aluminium garnet powders doped with Eu3+ in concentrations from 0.1% to 10% were prepared using photochemical method. Based on XRPD measurements all samples show very high crystal quality and garnet phase purity. Calculated crystallite size was determined as 44 nm. Radioluminescence spectra of all samples show typical Eu3+ 4f– 4f transitions (5D0–7Fx), the dominant transition being 5D0–7F1 magnetic dipole transition with maximum at 592 nm. Identical dominant emission line at 592 nm (5D0–7F1) and at 710 nm (5D0–7F4) is also found in photoluminescence spectra. Photoluminescence excitation spectra show the highest intensity at 225 nm (position of charge transfer transition O2–Eu3+).
5
Decay curves were single exponential and decay time values of ca. 9–11 ms were very similar for both dominant emission lines, suggesting simple radiative transition from the excited state to the ground state. Calculated decay times show a mild concentration quenching for the highest Eu concentrations. By comparing the decay time value for analogous bulk material (3.8 ms) the role of effective refractive index in a powder material was found to be significant. Based on the calculated filling factor, prepared powders consist of well-separated crystallites without any noticeable agglomeration. Based on collected data the best figure of merit for PDTX application was obtained in the samples with Eu3+ concentration 0.5–5%, where the concentration quenching is insignificant and radioluminescence intensity was very high. Acknowledgement This research was supported by Czech Science foundation Project No. GA 13-09876S. References [1] M. Nikl, A. Yoshikawa, K. Kamada, K. Nejezchleb, C.R. Stanek, J.A. Mares, K. Blazek, Progr. Cryst. Growth Charact. Mater. 59 (2013) 47–72. [2] S.A. Cicillini, A.M. Pires, O.A. Serra, J. Alloys Compd. 374 (2004) 169–172. [3] H.-L. Li, X.-J. Liu, L.-P. Huang, Opt. Mater. 29 (2007) 1138–1142. [4] L. Xing, L. Peng, M. Gu, G. Tang, J. Alloys Compd. 491 (2010) 599–604. [5] J.M. Ogiegło, A. Zych, T. Justel, A. Meijerink, C.R. Ronda, Opt. Mater. 35 (2013) 322–331. [6] A. Speghini, F. Piccinelli, M. Bettinelli, Opt. Mater. 33 (2011) 247–257. ˇ uba, J. Bárta, V. Jary´, M. Nikl, Radiation induced synthesis of oxide [7] V. C compounds, in: B.I. Kharisov, O.V. Kharissova, U. Otiz-Mendez (Eds.), Radiation synthesis of Materials and Compounds, CRC Press, Boca Raton, USA, 2013, pp. 81–100. [8] J. Bárta, V. Cˇuba, M. Pospíšil, V. Jary´, M. Nikl, J. Mater. Chem. 22 (2012) 16590– 16597. [9] A. De Rosa, D. Naviglio, A. Di Luccia, Curr. Cancer Ther. Rev. 7 (2011) 1. [10] P. Juzenas, W. Chen, Y.-P. Sun, M.A.N. Coelho, R. Generalov, N. Generalova, I.L. Christensen, Adv. Drug Delivery Rev. 60 (2008) 1600. [11] N.Y. Morgan et al., Radiat. Res. 171 (2009) 236. [12] J.Y. Jung, G.A. Hirata, G. Gundiah, S. Derenzo, W. Wrasidlo, S. Kesari, M.T. Makale, J. McKittrick, J. Lumin. 154 (2014) 569–577. [13] A.-L. Bulin, C. Truillet, R. Chouikrat, F. Lux, C. Frochot, D. Amans, G. Ledoux, O. Tillement, P. Perriat, M. Barberi-Heyob, C. Dujardin, J. Phys. Chem. C 117 (2013) 21583–21589. [14] R.S. Meltzer, S.P. Feofilov, B. Tissue, H.B. Yuan, Phys. Rev. B 60 (1999) R14012. [15] V. LeBihan, A. Pillonnet, D. Amans, G. Ledoux, O. Marty, C. Dujardin, Phys. Rev. B 78 (2008) 113405. [16] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994. [17] H. Ogino, A. Yoshikawa, M. Nikl, R. Kucerkova, J.-I. Shimoyama, K. Kishio, Phys. Proc. 2 (2) (2009) 191–205. [18] Di Bartolo, in: Optical Interactions of Solids, first ed., Wiley, New York, 1968, pp. 403–414. [19] Z. Wang, M. Xu, W. Zhang, M. Yin, J. Lumin. 122–123 (2007) 437–439. [20] D. Uhlich, P. Huppertz, D.U. Wiechert, T. Justel, Opt. Mater. 29 (2007) 1505– 1509.
Please cite this article in press as: A. Vondrášková et al., Opt. Mater. (2014), http://dx.doi.org/10.1016/j.optmat.2014.12.002