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Investigation of the scintillation properties of LuAG:Tb3+ nanocrystalline powders and nanoceramic prepared by SPS method
Accepted Manuscript Investigation of the scintillation properties of LuAG:Tb𝟑+ nanocrystalline powders and nanoceramic prepared by SPS method Amin Aghay Kharieky, Khadijeh Rezaee Ebrahim Saraee, Wieslow Strek
PII: DOI: Reference:
S0168-9002(17)31054-9 https://doi.org/10.1016/j.nima.2017.10.017 NIMA 60163
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Nuclear Inst. and Methods in Physics Research, A
Received date : 24 August 2017 Revised date : 4 October 2017 Accepted date : 9 October 2017 Please cite this article as: A.A. Kharieky, K.R. Ebrahim Saraee, W. Strek, Investigation of the scintillation properties of LuAG:Tb𝟑+ nanocrystalline powders and nanoceramic prepared by SPS method, Nuclear Inst. and Methods in Physics Research, A (2017), https://doi.org/10.1016/j.nima.2017.10.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Investigation of the scintillation properties of L u A G : T b 3+ nanocrystalline powders and nanoceramic prepared by SPS method Amin Aghay Kharieky1, Khadijeh Rezaee Ebrahim Saraee1, *, Wieslow Strek2
1
Faculty of Advance Sciences and Technologies, University of Isfahan, Isfahan 81746-73441, Iran 2
Institute of Low Temperature and Structure Research, PAS, 50-422 Wroclaw, Poland
A bstract 1 mole percent Tb3+ doped lutetium aluminium garnet (LuAG:Tb3+) nanocrystalline powders were prepared by Pechini method. Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) measurements confirmed the atomic binding energy and crystal structure. The XRD results showed that the nanocrystalline powders had cubic structure with grain sizes of 11nm and 34 nm at minimum and maximum annealing temperatures, respectively. The morphology of the nanocrystalline powders with grain sizes 11 and 34 nm was characterized by transmission electron microscopy (TEM) analysis. The UV-Vis-NIR analyses were done to obtain energy absorption values and to investigate the effect of different grain sizes on absorption spectra of nanocrystalline powders. Excitation, emission spectra and decay time were measured for different grain sizes of the nanocrystalline powders. The results showed that increasing grain size was caused an increasing in emission intensity. The different grain sizes and quenching parameter such as an OH group affected the decay time of the LuAG:Tb3+ nanocrystalline powders. Then the LuAG:Tb3+ nanoceramic -plasma sintering (SPS) method for the first time from nanopowders prepared by the . The comparison of photoluminescence spectra and decay time of LuAG:Tb3+ powder and ceramic which were *
Corresponding Author: Khadijeh Rezaee Ebrahim Saraee Email Address: [email protected]; Tel: +989131015519; Fax:+983117934226
measured at room temperature in ultraviolet (VU) region, was done. Moreover, the X-ray excitation luminescence measurements of LuAG:Tb3+ nanoceramic were investigated.
Keyword: Garnet, Nano-crystal, Scintillation, Terbium
1. Introduction Garnet structure oxide materials such as Lu3Al5O12 have good luminescence properties that promising a candidate for scintillation applications [1-5]. The garnets have good chemical and radiation hardness and show efficient luminescence when doped with lanthanide ions [67]. In rare earth ions doped Lu3Al5O12 host lattice, terbium (Tb) as luminescent activator has the strongest emission from 400 to 800 nm which can be matched well with the sensitivity curves of the commercial photomultiplier tubes. This advantage can be applied in modern imaging equipment where there is no special requirement for time resolution [8]. The growth of single crystal of LuAG:Tb3+ with high optical quality is an arduous and high cost process. Synthesizing LuAG:Tb3+ nanocrystalline powders with high density, fine dispersive and less agglomerated can be a good alternative for the single crystals with the same luminescent properties and lower cost [7,8]. Ultimately, these LuAG:Tb3+ nanocrystalline powders can be used to fabricate the transparent LuAG:Tb3+ ceramic. It is needed to mention that the nanometer size effects on some properties of the luminescence materials, because the surface to volume ratio is high for the nano structure material and it in turn affects the available energy levels. It was observed that increasing or decreasing the nano particle size affected the optical properties and scintillation decay time [9-11]. Most of the scintillation materials which were used in ionizing radiation detectors are in a single crystalline form. But, as mentioned already, growing high melting point scintillation crystals are time-consuming and expensive. The single crystals could be replaced by ceramic materials because they are characterized by lower preparation temperature, more uniform
doping and lower cost in comparison with their single crystal analogs [12]. In fact, up to now several scintillation ceramics are successfully used in medical imaging devices [12, 13]. So the aim of this study is an examination of the luminescence properties of the different LuAG:Tb3+ nanocrystalline powders which were annealed and prepared at temperatures 800 fabrication and characterizations of LuAG:Tb3+ powders were studied but also the study on LuAG:Tb3+ ceramic was done. The method which was used for synthesizing LuAG:Tb3+ nanocrystalline powders was based on the Pechini method [14-16]. The details of this method were reported by Stefanski (2015) and Amani (2010) [14-16]. The structure and morphology of the nanocrystalline powder that were synthesized at different temperatures were analysed by an XRD, FTIR and TEM analysis. Also, photoluminescence properties of the annealed powders at different temperatures (900-) in the air atmosphere were presented. Then the effect of the different nanocrystalline sizes of LuAG:Tb3+ on optical and luminescence properties were reported. Moreover, the scintillating performance of LuAG:Tb3+ ceramic was studied and compared with LuAG:Tb3+ nanocrystalline powders.
2. E xperimental 2.1. Method of preparation of nanocrystalline powders Nano-crystalline powders of terbium (1 mole percent) doped lutetium aluminium garnet (LuAG:Tb3+) were prepared by the Pechini method. All raw chemicals were analytical grade and prepared from POCH S. A. and Alfa Aesar companies. First, lutetium and terbium nitrate were prepared. Stoichiometric amount of lutetium oxide (99.99%) and terbium oxide (99.99%) were dissolved in an aqueous nitric acid. Then, the aluminium nitrate nonahydrate solution (99.99%), citric acid and ethylene glycol were added to the mixture aqueous of the nitrate solution of lutetium and terbium with the molar ratio of 3:27:27:1. Next, the final
solution was heated at the 90C for one week to obtain a brownish resin. The dried resin was annealed at different temperatures from 800C to 1200C for 16 hours in a furnace to obtain different grain sizes of nano crystalline powders. 2.2. Method of preparation of nanoceramic The LuAG:Tb3+ nanoceramic body was prepared by the SPS apparatus as follows procedure. Two grams of the nanocrystalline powders that were prepared by the Pechini method at 900C was introduced into a carbon die with an inner hole diameter of 20 mm and closed with two carbon punches. The samples were sintered in vacuum. In the apparatus, a constant mechanical pressure of 50 MPa was applied all times. The temperature was increased to 1000C in 20 min and kept for 5 min. Then the temperature was raised in 10 min to the sintering temperature of 1500C and was kept for 20 min. Next, at final step the power was switched off leaving the apparatus to cool down. No sintering aids were added during this time. 2.3. Characterization The structure and shape of the synthesized powders were examined by X-ray diffractometer (PANalytical, The Netherlands) diffractometer (PANalytical, The Netherlands) with C radiation and TEM (FEI Tecnai G2 20 X-TWIN). The lattice parameters and mean crystallite size of samples (nanocrystalline powders and nanoceramic) were obtained with the Rietveld method. The backscattered electron image was examined for nanoceramic by using a field emission scanning electron microscope (FESEM, MIRAZ3XMU). For confirmation of binding energy and presence of the elements, the FTIR spectrum (Nicolet iS50 FT-IR, Thermo Scientific, spectrometer) was studied on nanocrystalline powders. Also, to investigate and examine of different grain size effect on absorption spectra; the UV-Vis-NIR (Cary Varian 5E) spectra were recorded for different nanocrystalline powders which were prepared at different temperatures. The emission and
excitation spectra of nanocrystalline powders and nanoceramic were measured at room temperature with the FLS980 fluorescence spectrometer from Edinburgh instruments. The luminescence decay time profiles of nanocrystalline powders and nanoceramic were collected using the LeCroy WaveSurfer 400 oscilloscope. The radioluminescence (RL) spectra of the LuAG:Tb3+ nanoceramic under X-ray excitation were measured using the high sensitivity spectrometer (Sunshine model) coupled with an ASENWARE X-ray generator (XJ10-60N model, Cu anode). The scintillation light was measured by feeding into the spectrometer through a 2 m optical fiber to avoid direct X-ray exposure of CCD detectors. The spectra were corrected for experiments using a correction curve. All the measurements were performed at room temperature.
3. Results and discussion Figure 1(a) shows the X-ray diffraction pattern of the LuAG:Tb3+ nanocrystalline powders prepared at 800, 850, 900, 1000, 1100 and 1200C temperatures. As the figure shows, the crystalline shape of lutetium aluminium garnet appears at temperature more than 850C. Also, in order to compare of the prepared nanocrystalline powders with pure garnet structure; the reference pattern is shown in the figure (standard data available, ICSD card No. 01-0731368). The average grain size of nanocrystalline powders was calculated with the Rietveld refinement method. The calculated and obtained results confirmed that the nanocrystalline powders had a cubic structure and the average nanocrystalline sizes were 11, 17, 26 and 34 nm. All corresponding peaks of LuAG:Tb3+ nanocrystalline powders a little shifted to a parameters after thermal treatment. More details were reported in Table 1. The comparison of X-ray diffraction pattern of the nanocrystalline powder and the nanoceramic (which was prepared by the same nanocrystalline powder) was shown in figure
1(b). The nanoceramic had
The calculated and obtained results by the Rietveld refinement method confirmed that the nanoceramic had a cubic structure with the average grain size 1.3 m. Increasing pressure and temperature to fabricate the nanoceramic was caused that the crystallite size of nanoceramic increased and grew. Figure 2 shows the nanocrystalline powders have semi spherical shapes and tend to agglomeration. The TEM images of the single crystallite size confirmed the size of the nanocrystalline calculated by the X-ray diffraction. Also, the TEM images showed that the different temperature effects on the size of the nanocrystalline powders. Increasing the temperature from 900C (figure 2(a)) to 1200C (figure 2(b)) was caused that the dimension of the nanocrystalline powders was increased. Figure 3 shows FESEM photograph of LuAG:Tb3+ nanoceramic sample. The sintered nanoceramic body was slightly translucent and had dark-gray colour. The wide surface of the sample was polished for characterizations. The grain size and shape of crystallites seem to be non-uniform. Also the figure illustrates that the grain boundaries are clean and there is not any secondary phases. From the FESEM photograph, the minimum, maximum and the average grain size on the surface of the sample was estimated about 0.3, 4 and 1.7
m,
respectively. The slightly translucent property of nanoceramic may be appeared because of more carbon atom contamination generated by graphite die that penetrate into the sample during the sintering process. Also, the surface is not smooth completely and scattering of light is more. Another phenomenon can affect the transparency of the ceramic needs further investigation. The FTIR spectra of LuAG:Tb3+ nanocrystalline powders prepared at different temperature were shown in figure 4. There are two main sections from 400 to 630 cm-1 and 630 to 1000 cm-1 which were related to Lu-O and Al-O chemical bonds. The spectra consist of fingerprint
vibration of [AlO4] tetrahedral and [AlO6] octahedral at 512, 570, 702, 736 and 802 cm-1. It indicates that the pure LuAG phase was formed [17]. Also, there is a small broad peak from 3000 to 3600 cm-1 that is related to the contamination of the OH group which is absorbed from the air. As the figure shows, the nanocrystalline powders prepared at 900C have the intensive peak for OH absorption. With increasing the temperature from 900C to 1000C, the amount of OH peak decreases. Also, with the increasing temperature from 1000C to 1200C; the amount and height of the OH peak in the FTIR spectra change and approximately remained constant. This is the main change was observed in the FTIR analysis for different nanocrystalline powders prepared at different temperatures. The energy absorption values of nanocrystalline powders with different sizes, prepared at different temperatures, were calculated by the reflectance spectra and using the Tauc plot and KubelkaMank formula: = A (- ) ,
(1)
where is the photon energy, A is a constant and F (R) is the absorption coefficient obtained from the reflectance data. R is the reflectance data in term of percentage. Absorption coefficient was calculated from the reflectance spectra of the powders (Figure 5). Figure 5 shows, changing of temperature or crystallite size had a strong effect on the energy absorption values. These energy absorption values or energy transitions may be created by impurities. The impurities are not part of the regular lattice and can create an energy position within the normally forbidden gap [17-19]. The change of energy absorption values as a function of grain sizes of the powders was shown in the inset of the figure 5. The result showed that increasing the grain sizes was caused that the number of energy absorption values changed. Also, the energy absorption values between 1.5-2.5eV and 4.5-5.5eV increased with increasing the grain sizes of the nanocrystalline powders while the energy absorption values between 2.5-4eV decreased.
Figure 6 shows a series of normalized UV-Visible absorption spectra of LuAG:Tb3+ nanocrystalline powders. The highest broadband at the wavelength of 213 nm is associated with the 4f8 75d transition. Furthermore, there are four weak narrow bands located at 250, 278 and 370 nm, respectively. These bands are assigned to the transitions from the ground state level (7F6) to 5d and 5DJ energy levels. In order to compare the shape of the absorption spectra of the different nanocrystalline powders, each spectrum was normalized using the first strong peak at shorter wavelengths (200 to 230 nm). As the figure 6 illustrates, increasing the grain size of the nanocrystalline powders not only was caused that the peaks a little shift to red part but also both absorption intensity and peak width increased. Also, in the inset of the figure 6; the ratio of two strong absorption peaks (around of 213 nm and 250 nm) plotted versus the sizes of the different nanocrystalline powders. The emission spectra of the Tb doped lutetium aluminium garnet nanocrystalline powders were displayed in figure 7(a) at room temperature. The normalized intensity was plotted for each grain size prepared at different temperatures. The changing of the grain size of crystallite did not effect on the shape of the emission spectrum. There was a little shift for the main emission peaks from the smallest to the biggest nanocrystalline sizes. The emission spectra had several peaks of 5D3-7FJ and 5D4-7FJ=6,5,4,3,2,1 from 400 nm to 800 nm that show the characteristic properties of Tb3+. In figure 7(b), the excitation spectra of LuAG:Tb3+ nanocrystalline powders were shown. The spectra showed there was one main broad peak in 268 nm which was caused by the f-d transition. Also, there was a f-f transition in 320.4nm. All of the photoluminescence spectra were plotted after correction. As the figure 7(b) shows, with increasing the grain size of the nanocrystalline powders there was a little red shift. In order to the correct comparison of the emission intensity of different nanocrystalline powders, the emission spectrum of the different grain sizes were measured in the same condition and an excitation wavelength ( = 380 nm, characteristic f-f transition of Tb). As
the figure 7(c) shows, increasing the annealing temperature and increasing the grain sizes of the nanocrystalline powders causes that the emission intensity increases because the more annealing temperature remove the impurities and other luminescence quencher parameters on the surface of the nanocrystalline powders. Figures 8(a) and (b) show the photoluminescence emission and excitation spectrum of LuAG:Tb3+ nanoceramic which was measured at the same condition of the nanocrystalline powder. The general shape of the emission spectrum of the nanocrystalline powder and the nanoceramic was the same but the peak intensity of 5D3-7FJ transition increased a little in ceramic. There was a little shift to longer wavelengths for all peaks. Also, the obtained excitation spectrum showed that the f-d transition peak around 250-300nm for the nanoceramic was wider than the nanocrystalline powder and shift to longer wavelengths. The decay time measurements were performed for all nanocrystalline powders from the smallest to largest grain sizes at the same condition (figure 9(a)). For the best approximation and achieving the best fitting, the first order exponential curve was fitted for each obtained decay time curve (equation 1).
I(t)= A1 +I0
(1)
where A1 and I0 are a constant and is called decay time. The calculated decay time was shown in the inset. The behaviour of the decay time showed that increasing the temperature from 900C to 1000C causes increasing the decay time. With increasing the temperature from 1000C to 1200C and increasing the nanocrystalline powder size, the decay time decreases. It is well known that the fluorescence lifetime ( ) depends on
radiative
lifetime
( )
and
non-radiative
de-excitation
probability ).
= +
(2)
processes
(transition
The FTIR analysis showed that the amount of OH concentration at 900C was maximum amount. With increasing the temperature from 900C to 1000C; the intensity of OH group decreases drastically. It confirms that the presence and vibration of OH impurity on the surface of nanocrystalline powders at 900C increases the non-radiative process other than nanocrystalline powders prepared at 1000C and in turn decreases the decay time, too. The effect of OH vibration on decay time was proved in our research previously [18]. Also, the FTIR results showed that increasing the temperature from 1000C to 1200 change in OH concentration obviously. The OH concentration is negligible and constant for all different nanocrystalline powders in this range. Increasing the temperature from 1000C to 1200C causes the size of nanocrystalline powders increases while the non-radiative processes are removed or its effect remained constant. When the nanocrystallite size is much less than the radiation wavelength, the radiation feels an effective refraction index which between the index of refraction of the crystallite and that of the surrounding medium. For an electric-dipole transition, the radiative lifetime is defined by equation (3).
=
(3)
where f(ED) 0 and n are called the oscillator strength, wavelength in vacuum and the refraction index, respectively. The nanocrystalline powders occupy only a small fraction x of the volume, an effective refraction index is (equation (4)): (x)= x + (1-x) (4) where refraction index of LuAG and air are and , respectively. With decreasing the size of the nanocrystalline powders, the effective refraction index decreases. The reduction of the effective refraction index increases the radiative lifetime and the total fluorescence lifetime. So, with increasing the nanocrystalline powder sizes obtained
at temperatures from 1000oC to 1200oC the decay time decreases. It confirms that the refractive index impact is predominant in this region [18-19]. Also, the decay time for the nanoceramic was measured and was comprised with the nanocrystalline powder prepared for the same emission and excitation wavelength (figure 9(b)). The obtained results showed that the nanoceramic had the shortest decay time (3.32 ms) among the all obtained decay times from the nanocrystalline powders. Because, the average grain size of constituent of the nanoceramic was increased and also the effect of OH group was at least. In order to characterize the emitted light; the size dependence of the CIE chromaticity coordinates of the LuAG:Tb3+ nanocrystals which were recorded for nanocrystalline samples from 11nm to 34nm were calculated (see figure 10). It was found that the change of nanocrystalline size from 11nm to 34nm did not cause an important shift in the emitted light from green at the room temperature (from black (11nm) to blue (34nm) square in the figure 10). Figure 10 clearly referred to the luminescence spectra presented in figure 7(a) and showed that f-f energy transition of Tb3+ emission spectrum in 542nm was dominant in the visible region. For this reason the green emitted light was observed. X-ray excitation luminescence spectra of the LuAG:Tb3+ nanoceramic sample is displayed in figure 11for voltages 20, 30, 35 and 40kV. The X-ray excitation luminescence spectrum was measured from 200-1100nm at room temperature. It had six broad peaks in the visible region which were approximately similar to the transitions obtained by UV excitation in figure 7(a). However, the 5D3-7FJ and 5D4-7F6 transitions were appeared in the X-ray excitation luminescence spectra of the LuAG:Tb3+ nanoceramic sample with more intensity
The X-ray excitation luminescence allows detecting all the emissions in the sample because X-ray penetrate through the entire sample and produce charge carriers that can recombine or transfer their energy as emitting light [20].
As the figure showed, with increasing the X-ray tube voltage from 20kV to 40kV the emitted intensity increased because the amounts of X-ray photons that interact in the deep of nanoceramic become more.
4. Conclusion The nanocrystalline LuAG:Tb3+ powders were synthesized by the Pechini method. XRD measurements confirmed purified cubic structure of powders. FTIR measurement showed the main binding energies of the compound. The effect of different temperature on grain sizes, absorption band and luminescence properties was examined. It was found that different nanocrystalline sizes and quencher parameters were effective on scintillation properties of LuAG:Tb3+. The LuAG:Tb3+ nanoceramic using SPS technique was developed and its scintillation properties were investigated. Under X-ray irradiation, the LuAG:Tb3+ nanoceramic indicated emission due to 5D3-7FJ and 5D4-7FJ transitions of Tb3+ which appeared from 350 to 750nm. The scintillation spectra showed similar spectral shapes, but with more intensity as those observed in PL. This nanocramic can be attractive for diagnostic and therapeutic application tools such as X-ray computed tomography technique.
A cknowledgment The authors are very thankful to Mr Mariusz Stefanski and Robert Tomala from Institute of Low Temperature and Structure Research, Poland, Dr. Movahedi and Dr. Dini from the faculty of Advance Sciences and Technologies, Iran, for their help in measurement and advice for this paper.
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Table 1. Cell parameters of Tb doped LuAG nanocrystalline powders annealed at different temperatures prepared by Pechini method. Sample standard 850C 900C 1000C 1100C 1200C
a(Å)
b(Å)
11.906 12.0331 12.04967 11.96163 11.93861 11.93003
11.906 12.0331 12.04967 11.96163 11.93861 11.93003
Cell parameters c(Å) ,, 11.906 12.0331 12.04967 11.96163 11.93861 11.93003
90 o 90 o 90 o 90 o 90 o 90 o
Volume(Å 3) 1687.71 1742.337 1749.548 1711.477 1701.615 1697.948
Grain size (nm) 11 11 17 26 34
C aptions of figures F ig. 1. (a) The Xray diffraction patterns LuAG:Tb3+ nano crystalline powders prepared at different annealing temperatures and (b) the Xray diffraction patterns LuAG:Tb3+ nano crystalline powder and nanoceramic. F ig. 2. TEM images of LuAG:Tb3+ nano crystalline powders at temperatures of (a) 900 oC and (b) 1200 oC. F ig. 3. FESEM photograph of surface of LuAG:Tb3+ nanoceramic prepared by SPS method. F ig. 4. The FTIR analysis of LuAG:Tb3+ nano crystalline powders at different annealing temperatures. F ig. 5. The UV-Vis-NIR absorption energy values of Tb3+ doped LuAG nanocrystalline powders for different annealing temperatures.
F ig. 6. Normalized UV-Visible absorption spectra of the different LuAG:Tb3+ nano crystalline powders prepared at different annealing temperatures. F ig. 7. The luminescence properties of LuAG:Tb3+ nano crystalline powders (a) emission spectra, (b) excitation spectra and (c) emission intensity measured at the same condition. F ig. 8. The luminescence properties of LuAG:Tb3+ nanoceramic and nanocrystalline powder (a) emission and (b) excitation spectra measured at the same condition. F ig. 9. (a) Decay time of different nano crystalline sizes prepared at different annealing temperatures and (b) decay time of nanocreamic measured at the same condition and room temperature. F ig. 10. The CIE chromaticity coordinates of LuAG:Tb3+ nanocrystalline powders. F ig. 11. The X-ray excited luminescence of LuAG:Tb3+ nanoceramic measured for four different X-ray tube voltages.
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Highlights (for review)
Scintillation properties of LuAG:Tb nanocrystalline powders were investigated LuAG:Tb powders can be used for diagnostic and therapeutic application Nanocrystalline sizes were effective on scintillation LuAG:Tb properties quencher parameters were effective on scintillation LuAG:Tb properties
PII: DOI: Reference:
S0168-9002(17)31054-9 https://doi.org/10.1016/j.nima.2017.10.017 NIMA 60163
To appear in:
Nuclear Inst. and Methods in Physics Research, A
Received date : 24 August 2017 Revised date : 4 October 2017 Accepted date : 9 October 2017 Please cite this article as: A.A. Kharieky, K.R. Ebrahim Saraee, W. Strek, Investigation of the scintillation properties of LuAG:Tb𝟑+ nanocrystalline powders and nanoceramic prepared by SPS method, Nuclear Inst. and Methods in Physics Research, A (2017), https://doi.org/10.1016/j.nima.2017.10.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Investigation of the scintillation properties of L u A G : T b 3+ nanocrystalline powders and nanoceramic prepared by SPS method Amin Aghay Kharieky1, Khadijeh Rezaee Ebrahim Saraee1, *, Wieslow Strek2
1
Faculty of Advance Sciences and Technologies, University of Isfahan, Isfahan 81746-73441, Iran 2
Institute of Low Temperature and Structure Research, PAS, 50-422 Wroclaw, Poland
A bstract 1 mole percent Tb3+ doped lutetium aluminium garnet (LuAG:Tb3+) nanocrystalline powders were prepared by Pechini method. Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) measurements confirmed the atomic binding energy and crystal structure. The XRD results showed that the nanocrystalline powders had cubic structure with grain sizes of 11nm and 34 nm at minimum and maximum annealing temperatures, respectively. The morphology of the nanocrystalline powders with grain sizes 11 and 34 nm was characterized by transmission electron microscopy (TEM) analysis. The UV-Vis-NIR analyses were done to obtain energy absorption values and to investigate the effect of different grain sizes on absorption spectra of nanocrystalline powders. Excitation, emission spectra and decay time were measured for different grain sizes of the nanocrystalline powders. The results showed that increasing grain size was caused an increasing in emission intensity. The different grain sizes and quenching parameter such as an OH group affected the decay time of the LuAG:Tb3+ nanocrystalline powders. Then the LuAG:Tb3+ nanoceramic -plasma sintering (SPS) method for the first time from nanopowders prepared by the . The comparison of photoluminescence spectra and decay time of LuAG:Tb3+ powder and ceramic which were *
Corresponding Author: Khadijeh Rezaee Ebrahim Saraee Email Address: [email protected]; Tel: +989131015519; Fax:+983117934226
measured at room temperature in ultraviolet (VU) region, was done. Moreover, the X-ray excitation luminescence measurements of LuAG:Tb3+ nanoceramic were investigated.
Keyword: Garnet, Nano-crystal, Scintillation, Terbium
1. Introduction Garnet structure oxide materials such as Lu3Al5O12 have good luminescence properties that promising a candidate for scintillation applications [1-5]. The garnets have good chemical and radiation hardness and show efficient luminescence when doped with lanthanide ions [67]. In rare earth ions doped Lu3Al5O12 host lattice, terbium (Tb) as luminescent activator has the strongest emission from 400 to 800 nm which can be matched well with the sensitivity curves of the commercial photomultiplier tubes. This advantage can be applied in modern imaging equipment where there is no special requirement for time resolution [8]. The growth of single crystal of LuAG:Tb3+ with high optical quality is an arduous and high cost process. Synthesizing LuAG:Tb3+ nanocrystalline powders with high density, fine dispersive and less agglomerated can be a good alternative for the single crystals with the same luminescent properties and lower cost [7,8]. Ultimately, these LuAG:Tb3+ nanocrystalline powders can be used to fabricate the transparent LuAG:Tb3+ ceramic. It is needed to mention that the nanometer size effects on some properties of the luminescence materials, because the surface to volume ratio is high for the nano structure material and it in turn affects the available energy levels. It was observed that increasing or decreasing the nano particle size affected the optical properties and scintillation decay time [9-11]. Most of the scintillation materials which were used in ionizing radiation detectors are in a single crystalline form. But, as mentioned already, growing high melting point scintillation crystals are time-consuming and expensive. The single crystals could be replaced by ceramic materials because they are characterized by lower preparation temperature, more uniform
doping and lower cost in comparison with their single crystal analogs [12]. In fact, up to now several scintillation ceramics are successfully used in medical imaging devices [12, 13]. So the aim of this study is an examination of the luminescence properties of the different LuAG:Tb3+ nanocrystalline powders which were annealed and prepared at temperatures 800 fabrication and characterizations of LuAG:Tb3+ powders were studied but also the study on LuAG:Tb3+ ceramic was done. The method which was used for synthesizing LuAG:Tb3+ nanocrystalline powders was based on the Pechini method [14-16]. The details of this method were reported by Stefanski (2015) and Amani (2010) [14-16]. The structure and morphology of the nanocrystalline powder that were synthesized at different temperatures were analysed by an XRD, FTIR and TEM analysis. Also, photoluminescence properties of the annealed powders at different temperatures (900-) in the air atmosphere were presented. Then the effect of the different nanocrystalline sizes of LuAG:Tb3+ on optical and luminescence properties were reported. Moreover, the scintillating performance of LuAG:Tb3+ ceramic was studied and compared with LuAG:Tb3+ nanocrystalline powders.
2. E xperimental 2.1. Method of preparation of nanocrystalline powders Nano-crystalline powders of terbium (1 mole percent) doped lutetium aluminium garnet (LuAG:Tb3+) were prepared by the Pechini method. All raw chemicals were analytical grade and prepared from POCH S. A. and Alfa Aesar companies. First, lutetium and terbium nitrate were prepared. Stoichiometric amount of lutetium oxide (99.99%) and terbium oxide (99.99%) were dissolved in an aqueous nitric acid. Then, the aluminium nitrate nonahydrate solution (99.99%), citric acid and ethylene glycol were added to the mixture aqueous of the nitrate solution of lutetium and terbium with the molar ratio of 3:27:27:1. Next, the final
solution was heated at the 90C for one week to obtain a brownish resin. The dried resin was annealed at different temperatures from 800C to 1200C for 16 hours in a furnace to obtain different grain sizes of nano crystalline powders. 2.2. Method of preparation of nanoceramic The LuAG:Tb3+ nanoceramic body was prepared by the SPS apparatus as follows procedure. Two grams of the nanocrystalline powders that were prepared by the Pechini method at 900C was introduced into a carbon die with an inner hole diameter of 20 mm and closed with two carbon punches. The samples were sintered in vacuum. In the apparatus, a constant mechanical pressure of 50 MPa was applied all times. The temperature was increased to 1000C in 20 min and kept for 5 min. Then the temperature was raised in 10 min to the sintering temperature of 1500C and was kept for 20 min. Next, at final step the power was switched off leaving the apparatus to cool down. No sintering aids were added during this time. 2.3. Characterization The structure and shape of the synthesized powders were examined by X-ray diffractometer (PANalytical, The Netherlands) diffractometer (PANalytical, The Netherlands) with C radiation and TEM (FEI Tecnai G2 20 X-TWIN). The lattice parameters and mean crystallite size of samples (nanocrystalline powders and nanoceramic) were obtained with the Rietveld method. The backscattered electron image was examined for nanoceramic by using a field emission scanning electron microscope (FESEM, MIRAZ3XMU). For confirmation of binding energy and presence of the elements, the FTIR spectrum (Nicolet iS50 FT-IR, Thermo Scientific, spectrometer) was studied on nanocrystalline powders. Also, to investigate and examine of different grain size effect on absorption spectra; the UV-Vis-NIR (Cary Varian 5E) spectra were recorded for different nanocrystalline powders which were prepared at different temperatures. The emission and
excitation spectra of nanocrystalline powders and nanoceramic were measured at room temperature with the FLS980 fluorescence spectrometer from Edinburgh instruments. The luminescence decay time profiles of nanocrystalline powders and nanoceramic were collected using the LeCroy WaveSurfer 400 oscilloscope. The radioluminescence (RL) spectra of the LuAG:Tb3+ nanoceramic under X-ray excitation were measured using the high sensitivity spectrometer (Sunshine model) coupled with an ASENWARE X-ray generator (XJ10-60N model, Cu anode). The scintillation light was measured by feeding into the spectrometer through a 2 m optical fiber to avoid direct X-ray exposure of CCD detectors. The spectra were corrected for experiments using a correction curve. All the measurements were performed at room temperature.
3. Results and discussion Figure 1(a) shows the X-ray diffraction pattern of the LuAG:Tb3+ nanocrystalline powders prepared at 800, 850, 900, 1000, 1100 and 1200C temperatures. As the figure shows, the crystalline shape of lutetium aluminium garnet appears at temperature more than 850C. Also, in order to compare of the prepared nanocrystalline powders with pure garnet structure; the reference pattern is shown in the figure (standard data available, ICSD card No. 01-0731368). The average grain size of nanocrystalline powders was calculated with the Rietveld refinement method. The calculated and obtained results confirmed that the nanocrystalline powders had a cubic structure and the average nanocrystalline sizes were 11, 17, 26 and 34 nm. All corresponding peaks of LuAG:Tb3+ nanocrystalline powders a little shifted to a parameters after thermal treatment. More details were reported in Table 1. The comparison of X-ray diffraction pattern of the nanocrystalline powder and the nanoceramic (which was prepared by the same nanocrystalline powder) was shown in figure
1(b). The nanoceramic had
The calculated and obtained results by the Rietveld refinement method confirmed that the nanoceramic had a cubic structure with the average grain size 1.3 m. Increasing pressure and temperature to fabricate the nanoceramic was caused that the crystallite size of nanoceramic increased and grew. Figure 2 shows the nanocrystalline powders have semi spherical shapes and tend to agglomeration. The TEM images of the single crystallite size confirmed the size of the nanocrystalline calculated by the X-ray diffraction. Also, the TEM images showed that the different temperature effects on the size of the nanocrystalline powders. Increasing the temperature from 900C (figure 2(a)) to 1200C (figure 2(b)) was caused that the dimension of the nanocrystalline powders was increased. Figure 3 shows FESEM photograph of LuAG:Tb3+ nanoceramic sample. The sintered nanoceramic body was slightly translucent and had dark-gray colour. The wide surface of the sample was polished for characterizations. The grain size and shape of crystallites seem to be non-uniform. Also the figure illustrates that the grain boundaries are clean and there is not any secondary phases. From the FESEM photograph, the minimum, maximum and the average grain size on the surface of the sample was estimated about 0.3, 4 and 1.7
m,
respectively. The slightly translucent property of nanoceramic may be appeared because of more carbon atom contamination generated by graphite die that penetrate into the sample during the sintering process. Also, the surface is not smooth completely and scattering of light is more. Another phenomenon can affect the transparency of the ceramic needs further investigation. The FTIR spectra of LuAG:Tb3+ nanocrystalline powders prepared at different temperature were shown in figure 4. There are two main sections from 400 to 630 cm-1 and 630 to 1000 cm-1 which were related to Lu-O and Al-O chemical bonds. The spectra consist of fingerprint
vibration of [AlO4] tetrahedral and [AlO6] octahedral at 512, 570, 702, 736 and 802 cm-1. It indicates that the pure LuAG phase was formed [17]. Also, there is a small broad peak from 3000 to 3600 cm-1 that is related to the contamination of the OH group which is absorbed from the air. As the figure shows, the nanocrystalline powders prepared at 900C have the intensive peak for OH absorption. With increasing the temperature from 900C to 1000C, the amount of OH peak decreases. Also, with the increasing temperature from 1000C to 1200C; the amount and height of the OH peak in the FTIR spectra change and approximately remained constant. This is the main change was observed in the FTIR analysis for different nanocrystalline powders prepared at different temperatures. The energy absorption values of nanocrystalline powders with different sizes, prepared at different temperatures, were calculated by the reflectance spectra and using the Tauc plot and KubelkaMank formula: = A (- ) ,
(1)
where is the photon energy, A is a constant and F (R) is the absorption coefficient obtained from the reflectance data. R is the reflectance data in term of percentage. Absorption coefficient was calculated from the reflectance spectra of the powders (Figure 5). Figure 5 shows, changing of temperature or crystallite size had a strong effect on the energy absorption values. These energy absorption values or energy transitions may be created by impurities. The impurities are not part of the regular lattice and can create an energy position within the normally forbidden gap [17-19]. The change of energy absorption values as a function of grain sizes of the powders was shown in the inset of the figure 5. The result showed that increasing the grain sizes was caused that the number of energy absorption values changed. Also, the energy absorption values between 1.5-2.5eV and 4.5-5.5eV increased with increasing the grain sizes of the nanocrystalline powders while the energy absorption values between 2.5-4eV decreased.
Figure 6 shows a series of normalized UV-Visible absorption spectra of LuAG:Tb3+ nanocrystalline powders. The highest broadband at the wavelength of 213 nm is associated with the 4f8 75d transition. Furthermore, there are four weak narrow bands located at 250, 278 and 370 nm, respectively. These bands are assigned to the transitions from the ground state level (7F6) to 5d and 5DJ energy levels. In order to compare the shape of the absorption spectra of the different nanocrystalline powders, each spectrum was normalized using the first strong peak at shorter wavelengths (200 to 230 nm). As the figure 6 illustrates, increasing the grain size of the nanocrystalline powders not only was caused that the peaks a little shift to red part but also both absorption intensity and peak width increased. Also, in the inset of the figure 6; the ratio of two strong absorption peaks (around of 213 nm and 250 nm) plotted versus the sizes of the different nanocrystalline powders. The emission spectra of the Tb doped lutetium aluminium garnet nanocrystalline powders were displayed in figure 7(a) at room temperature. The normalized intensity was plotted for each grain size prepared at different temperatures. The changing of the grain size of crystallite did not effect on the shape of the emission spectrum. There was a little shift for the main emission peaks from the smallest to the biggest nanocrystalline sizes. The emission spectra had several peaks of 5D3-7FJ and 5D4-7FJ=6,5,4,3,2,1 from 400 nm to 800 nm that show the characteristic properties of Tb3+. In figure 7(b), the excitation spectra of LuAG:Tb3+ nanocrystalline powders were shown. The spectra showed there was one main broad peak in 268 nm which was caused by the f-d transition. Also, there was a f-f transition in 320.4nm. All of the photoluminescence spectra were plotted after correction. As the figure 7(b) shows, with increasing the grain size of the nanocrystalline powders there was a little red shift. In order to the correct comparison of the emission intensity of different nanocrystalline powders, the emission spectrum of the different grain sizes were measured in the same condition and an excitation wavelength ( = 380 nm, characteristic f-f transition of Tb). As
the figure 7(c) shows, increasing the annealing temperature and increasing the grain sizes of the nanocrystalline powders causes that the emission intensity increases because the more annealing temperature remove the impurities and other luminescence quencher parameters on the surface of the nanocrystalline powders. Figures 8(a) and (b) show the photoluminescence emission and excitation spectrum of LuAG:Tb3+ nanoceramic which was measured at the same condition of the nanocrystalline powder. The general shape of the emission spectrum of the nanocrystalline powder and the nanoceramic was the same but the peak intensity of 5D3-7FJ transition increased a little in ceramic. There was a little shift to longer wavelengths for all peaks. Also, the obtained excitation spectrum showed that the f-d transition peak around 250-300nm for the nanoceramic was wider than the nanocrystalline powder and shift to longer wavelengths. The decay time measurements were performed for all nanocrystalline powders from the smallest to largest grain sizes at the same condition (figure 9(a)). For the best approximation and achieving the best fitting, the first order exponential curve was fitted for each obtained decay time curve (equation 1).
I(t)= A1 +I0
(1)
where A1 and I0 are a constant and is called decay time. The calculated decay time was shown in the inset. The behaviour of the decay time showed that increasing the temperature from 900C to 1000C causes increasing the decay time. With increasing the temperature from 1000C to 1200C and increasing the nanocrystalline powder size, the decay time decreases. It is well known that the fluorescence lifetime ( ) depends on
radiative
lifetime
( )
and
non-radiative
de-excitation
probability ).
= +
(2)
processes
(transition
The FTIR analysis showed that the amount of OH concentration at 900C was maximum amount. With increasing the temperature from 900C to 1000C; the intensity of OH group decreases drastically. It confirms that the presence and vibration of OH impurity on the surface of nanocrystalline powders at 900C increases the non-radiative process other than nanocrystalline powders prepared at 1000C and in turn decreases the decay time, too. The effect of OH vibration on decay time was proved in our research previously [18]. Also, the FTIR results showed that increasing the temperature from 1000C to 1200 change in OH concentration obviously. The OH concentration is negligible and constant for all different nanocrystalline powders in this range. Increasing the temperature from 1000C to 1200C causes the size of nanocrystalline powders increases while the non-radiative processes are removed or its effect remained constant. When the nanocrystallite size is much less than the radiation wavelength, the radiation feels an effective refraction index which between the index of refraction of the crystallite and that of the surrounding medium. For an electric-dipole transition, the radiative lifetime is defined by equation (3).
=
(3)
where f(ED) 0 and n are called the oscillator strength, wavelength in vacuum and the refraction index, respectively. The nanocrystalline powders occupy only a small fraction x of the volume, an effective refraction index is (equation (4)): (x)= x + (1-x) (4) where refraction index of LuAG and air are and , respectively. With decreasing the size of the nanocrystalline powders, the effective refraction index decreases. The reduction of the effective refraction index increases the radiative lifetime and the total fluorescence lifetime. So, with increasing the nanocrystalline powder sizes obtained
at temperatures from 1000oC to 1200oC the decay time decreases. It confirms that the refractive index impact is predominant in this region [18-19]. Also, the decay time for the nanoceramic was measured and was comprised with the nanocrystalline powder prepared for the same emission and excitation wavelength (figure 9(b)). The obtained results showed that the nanoceramic had the shortest decay time (3.32 ms) among the all obtained decay times from the nanocrystalline powders. Because, the average grain size of constituent of the nanoceramic was increased and also the effect of OH group was at least. In order to characterize the emitted light; the size dependence of the CIE chromaticity coordinates of the LuAG:Tb3+ nanocrystals which were recorded for nanocrystalline samples from 11nm to 34nm were calculated (see figure 10). It was found that the change of nanocrystalline size from 11nm to 34nm did not cause an important shift in the emitted light from green at the room temperature (from black (11nm) to blue (34nm) square in the figure 10). Figure 10 clearly referred to the luminescence spectra presented in figure 7(a) and showed that f-f energy transition of Tb3+ emission spectrum in 542nm was dominant in the visible region. For this reason the green emitted light was observed. X-ray excitation luminescence spectra of the LuAG:Tb3+ nanoceramic sample is displayed in figure 11for voltages 20, 30, 35 and 40kV. The X-ray excitation luminescence spectrum was measured from 200-1100nm at room temperature. It had six broad peaks in the visible region which were approximately similar to the transitions obtained by UV excitation in figure 7(a). However, the 5D3-7FJ and 5D4-7F6 transitions were appeared in the X-ray excitation luminescence spectra of the LuAG:Tb3+ nanoceramic sample with more intensity
The X-ray excitation luminescence allows detecting all the emissions in the sample because X-ray penetrate through the entire sample and produce charge carriers that can recombine or transfer their energy as emitting light [20].
As the figure showed, with increasing the X-ray tube voltage from 20kV to 40kV the emitted intensity increased because the amounts of X-ray photons that interact in the deep of nanoceramic become more.
4. Conclusion The nanocrystalline LuAG:Tb3+ powders were synthesized by the Pechini method. XRD measurements confirmed purified cubic structure of powders. FTIR measurement showed the main binding energies of the compound. The effect of different temperature on grain sizes, absorption band and luminescence properties was examined. It was found that different nanocrystalline sizes and quencher parameters were effective on scintillation properties of LuAG:Tb3+. The LuAG:Tb3+ nanoceramic using SPS technique was developed and its scintillation properties were investigated. Under X-ray irradiation, the LuAG:Tb3+ nanoceramic indicated emission due to 5D3-7FJ and 5D4-7FJ transitions of Tb3+ which appeared from 350 to 750nm. The scintillation spectra showed similar spectral shapes, but with more intensity as those observed in PL. This nanocramic can be attractive for diagnostic and therapeutic application tools such as X-ray computed tomography technique.
A cknowledgment The authors are very thankful to Mr Mariusz Stefanski and Robert Tomala from Institute of Low Temperature and Structure Research, Poland, Dr. Movahedi and Dr. Dini from the faculty of Advance Sciences and Technologies, Iran, for their help in measurement and advice for this paper.
Reference
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Table 1. Cell parameters of Tb doped LuAG nanocrystalline powders annealed at different temperatures prepared by Pechini method. Sample standard 850C 900C 1000C 1100C 1200C
a(Å)
b(Å)
11.906 12.0331 12.04967 11.96163 11.93861 11.93003
11.906 12.0331 12.04967 11.96163 11.93861 11.93003
Cell parameters c(Å) ,, 11.906 12.0331 12.04967 11.96163 11.93861 11.93003
90 o 90 o 90 o 90 o 90 o 90 o
Volume(Å 3) 1687.71 1742.337 1749.548 1711.477 1701.615 1697.948
Grain size (nm) 11 11 17 26 34
C aptions of figures F ig. 1. (a) The Xray diffraction patterns LuAG:Tb3+ nano crystalline powders prepared at different annealing temperatures and (b) the Xray diffraction patterns LuAG:Tb3+ nano crystalline powder and nanoceramic. F ig. 2. TEM images of LuAG:Tb3+ nano crystalline powders at temperatures of (a) 900 oC and (b) 1200 oC. F ig. 3. FESEM photograph of surface of LuAG:Tb3+ nanoceramic prepared by SPS method. F ig. 4. The FTIR analysis of LuAG:Tb3+ nano crystalline powders at different annealing temperatures. F ig. 5. The UV-Vis-NIR absorption energy values of Tb3+ doped LuAG nanocrystalline powders for different annealing temperatures.
F ig. 6. Normalized UV-Visible absorption spectra of the different LuAG:Tb3+ nano crystalline powders prepared at different annealing temperatures. F ig. 7. The luminescence properties of LuAG:Tb3+ nano crystalline powders (a) emission spectra, (b) excitation spectra and (c) emission intensity measured at the same condition. F ig. 8. The luminescence properties of LuAG:Tb3+ nanoceramic and nanocrystalline powder (a) emission and (b) excitation spectra measured at the same condition. F ig. 9. (a) Decay time of different nano crystalline sizes prepared at different annealing temperatures and (b) decay time of nanocreamic measured at the same condition and room temperature. F ig. 10. The CIE chromaticity coordinates of LuAG:Tb3+ nanocrystalline powders. F ig. 11. The X-ray excited luminescence of LuAG:Tb3+ nanoceramic measured for four different X-ray tube voltages.
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Highlights (for review)
Scintillation properties of LuAG:Tb nanocrystalline powders were investigated LuAG:Tb powders can be used for diagnostic and therapeutic application Nanocrystalline sizes were effective on scintillation LuAG:Tb properties quencher parameters were effective on scintillation LuAG:Tb properties