Abstract
Single crystal of La2Si2O7:Ce (LaPS:Ce) was successfully grown by floating zone method for the first time. The structure of as grown sample is confirmed to be monoclinic with space group P21/c by x-ray diffraction and energy dispersive spectra. Its photoluminescence (PL) and scintillation properties of LaPS:Ce crystal were characterized through vacuum ultra-violet excitation and emission spectra, PL decay curve, x-ray excitation luminescence spectrum, γ-ray multi-channel spectrum and afterglow spectrum. Emission peaks of this material are around 3.5 eV and 3.3 eV, corresponding to the typical transition 5d → 4f of Ce3+. The PL decay time of LaPS:Ce is 29.3 ns. The light yield of the sample is determined to be about 5400 ph MeV−1 with a low afterglow level. The trap states in LaPS:Ce are also studied through the thermally stimulated luminescence measurement.
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Introduction
Scintillator is the material that can convert x-ray, γ-ray and other high-energy particles into visual or ultraviolet light. Scintillators can be applied in various fields such as medical, imaging astronomy [1].
Ce3+ doped rare-earth pyrosilicate scintillators RE2Si2O7:Ce (RE = Lu, Gd, Y, Sc, La) have been investigated. Lu2Si2O7:Ce shows relatively high light output of 26300 photons/MeV and good energy resolution at 662 keV. It remains high scintillation efficiency even in the condition of high temperature up to 400 K [2]. Ce-doped gadolinium pyrosilicate (GPS:Ce) also has drawn extensive concern as an outstanding inorganic scintillator with high light yield, fast decay time, and high luminescence efficiency even at high temperatures up to 450 K [3–5]. Thus it owns good application prospect in the field of medical positron emission tomography, geophysical explorations and well logging applications. In addition, photoluminescence (PL) and scintillation properties of Sc2Si2O7:Ce and Y2Si2O7:Ce single crystal scintillators have also been reported in our previous work [6, 7].
The La2Si2O7 (LaPS) is one end composition in the Gd2Si2O7-La2Si2O7 mixing system which is designed to improve the crystallization behavior of GPS:Ce, avoiding cerium concentration quenching problem [8–12]. Similar to Y2Si2O7 and Gd2Si2O7, La2Si2O7 also melts incongruently [13–15]. LaPS:Ce polycrystalline sample was prepared and some basic PL and radioluminescence properties are reported [16]. So far, there is no report about the scintillation properties of LaPS:Ce single crystal.
It is the aim of this paper to obtain LaPS:Ce single crystal. And its related PL, scintillation and trap states were measured and studied. LaPS:Ce single crystal sample was obtained by floating zone(FZ) method in this work. Its phase and structure were measured through x-ray diffraction (XRD) and Energy dispersive spectrum(EDS)elemental analysis. The PL and scintillation properties were investigated through vacuum ultra-violet (VUV) excitation and emission, PL decay, x-ray excited luminescence (XEL), γ-ray multi-channel and afterglow spectra. The trap states in the sample were studied via thermally stimulated luminescence (TSL) measurement.
Experimental
Considering that the FZ method has been successfully applied in preparation of incongruently melting compounds Y2Si2O7:Ce [7], FZ method was used to prepare LaPS:Ce single crystal in this work. Starting materials were powders of La2O3, CeO2 and SiO2 with 4N purity. They were weighted and mixed at a stoichiometric ratio of the desired component (La0.99Ce0.01)2Si2O7 formula. The SiO2 was slightly excessive about 2at% in favor of avoiding the nucleation of 2La2O3·3SiO2 (The same method has been adopted in the crystal preparation of incongruent compound Y2Si2O7:Ce [7]). The resulting powder was filled into strip balloon and molded into cylinder with about Φ10 mm under 100 MPa isostatic pressure. These shaped rods were then calcined at 1400 °C for 6 h under air atmosphere.
Single crystal was grown in a vertical double-ellipsoid mirror optical FZ furnace under N2 atmosphere, equipped with one 6500 W Xe arc lamp as heating source (SciDre HKZ). The growth rate was 5 mm h−1; rotation rates of the upper and lower shaft were both 20 rpm in opposite directions.
Structure of LaPS:Ce powder sample was carried out by using an x-ray diffractometer (Rigaku D/MAX 2500 V+/PC with a Cu target, tube voltage 18 kV, and tube current 20 mA). The EDS elemental analysis was studied using a scanning electronic microscope (SEM, FEI Magellan 300). The VUV excitation and emission spectra of 10 K and room temperature (RT) were recorded by at 4B8 beam line, Beijing Synchrotron Radiation Facility. The photon flux was 1.0 × 109 photons/s/0.5%bw/mA. The PL decay curve under RT was recorded by using the Edinburg FLS 980 fluorescence spectrometer. The excitation source for PL decay curve was μF2 lamp. The XEL curve under RT was recorded by using the XEL accessory in Edinburg FLS 980 fluorescence spectrometer. X-ray source is a tungsten x-ray tube operated at 40 kV and 40 μA. PMT Hamamatsu R928P was used to detect the scintillation luminescence and the curves were corrected according to the quantum efficiency of PMT. The γ-ray multi-channel spectrum was measured under a 137Cs source and a photomultiplier tube PMT R2059 to detect the luminescence with 3000 ns shaping time. After glow curve was measured after 2 s continuous x-ray irradiation, which provided by a tungsten x-ray tube operated at 15 kV and 15 μA. The afterglow was collected by Hamamatsu R2059 PMT operated under 1200 V. The TSL spectrum (TSL intensity as the function of temperature) of sample was recorded in the 300–600 K range with a heating rate of 1 K s−1. Before measurement, the sample was exposed to x-ray (W target, 50 kV and 0.5 mA) for 300 s and kept in a black box.
Results and discussion
Structure determination
The obtained boule was cracked severely due to the large temperature gradient in the crystal growth and cooling process. The LaPS:Ce single crystal sample, which is shown in the insert of figure 1, is transparent without visible inclusions. The XRD pattern of powdered LaPS:Ce sample is represented in figure 1. Referencing to the PDF cards database, the structure of this sample can be well assigned to the monoclinic structure with a space group P21/c (JCPDS 82-0792).
Figure 1. The XRD pattern of the LaPS:Ce single crystal and the photograph of sample.
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Standard image High-resolution imageThe phase of the as grown LaPS:Ce single crystal sample was confirmed by the EDS elemental analysis as shown in figure 2. The corresponding result of the EDS was listed in table 1. It is shown the atomic ratio of n(Si): n(RE) in the final sample was about 1.14:1. In addition, element mapping also indicates that the La and Si distribute uniform in the selected area. Based on the above XRD and SEM results, it is reasonable to conclude that the major phase of the as grown single crystal was LaPS [17]. However, it is also noticed that the atomic ratio of n(Si): n(RE) has a relative large deviation from the stoichiometric ratio. The secondary phase in the as grown sample can't be excluded totally from this point. Further detailed investigation is needed to make clear the crystallization behavior of LaPS.
Figure 2. SEM image (a) and EDS elemental analysis (b) of as grown LaPS:Ce single crystal sample.
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Standard image High-resolution imageTable 1. EDS result of the LaPS:Ce single crystal sample.
| Element | Atomic(%) |
|---|---|
| O | 67.55 |
| Si | 17.26 |
| La | 15.03 |
| Ce | 0.16 |
| Total | 100.00 |
PL and scintillation properties
Figure 3 shows the normalized VUV excitation and emission spectra of as grown LaPS:Ce samples at 10 K and RT. The excitation spectra were measured under emission of 360 nm and the emission spectra were measured under excitation of 274 nm. The excitation peak around 7.4 eV is corresponding to the absorption of LaPS host. Splitting information of 5d level in the crystal field can be obtained through the excitation curve. There are four excitation peaks locating at 3.8, 4, 4.3 and 5.2 eV, which are corresponding to the electron transition from 4 f ground level to 5d1, 5d2, 5d3 and 5d4 of Ce3+, respectively. The emission peaks can be decomposed into two peaks through Gaussian fitting, locating at 3.5 eV and 3.3 eV, corresponding to two 5d1 → 4f transition of Ce3+(5d1 → 2F5/2 and 5d1 → 2F7/2) respectively [18]. Figure 3 presents the comparison of the excitation and emission curves under different temperatures. The positions of peaks are almost unchanged. Only the excitation curves under RT mainly present a broadening effect due to the temperature increase. The full widths at half maximum (FWHM) of the two emission peaks under RT are larger compared to those of peaks at 10 K. This phenomenon is probably caused by stronger electron thermal vibration due to the increasing temperature. Similar phenomenon also can be observed in other cerium doped scintillators, such as Lu2Si2O7:Ce [19] and Lu2SiO5:Ce [20].
Figure 3. Normalized VUV excitation and emission spectra of LaPS:Ce at 10 K and RT, the XEL curve of LaPS:Ce sample at RT is also shown in figure 3(c).
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Standard image High-resolution imageFigure 4 depicts PL decay curve of LaPS:Ce single crystal sample under 330 nm excitation and 377 nm emission at RT. The PL decay time was estimated by fitted with single exponential function, I = I0*exp(–t/τ) + A0, where I0 is the initial spectral intensity and τ is the decay time constant. The fitted result time is 29.3 ns, which is typical cerium decay time and also similar with the value of La2Si2O7:1% Ce phosphor [16].
Figure 4. PL decay curve under 330 nm excitation and 377 nm emission at RT.
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Standard image High-resolution imageXEL spectrum of LaPS:Ce single crystal at RT is also depicted in figure 3(c). It is noticed that two peaks around 3.5 and 3.3 eV are presented in the curve. The positions of XEL peaks are almost the same with the VUV emission peaks, which are corresponding to the 5d → 4f transition of Ce3+.
Figure 5 exhibits the pulse-height spectra of LaPS:Ce single crystal sample and BGO standard sample with similar dimension under 662 keV γ-ray from a 137Cs source at RT. The LY in units of ph/MeV was calculated with the following equation [21].
Figure 5. Pulse height spectra of LaPS:Ce single crystal sample and BGO standard sample irradiated under gamma ray.
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Standard image High-resolution imageThe light yield of the BGO standard sample is 8600 ph MeV−1. EWQE is the emission weighted quantum efficiency of the PMT R2059. At 500 nm and 360 nm, the EWQE values were 9% for BGO and 16% for LaPS:Ce [22]. The channel numbers of the full energy peak positions were 555 and 446 for LaPS:Ce and BGO, respectively. Thus, the light yield is about 5400 ph MeV−1 for LaPS:Ce.
Figure 6 shows the afterglow profile for LaPS:Ce after x-ray irradiation for 2 s. The afterglow intensities were determined as 3.62‰, 3.14‰, and 2.52‰ after 20, 50, and 100 ms, respectively. The afterglow of LaPS:Ce has a low intensity. The afterglow in the sample can be explained by delayed electron recombination in the Ce centers coming from the electron trap centers in LaPS:Ce.
Figure 6. Afterglow profile obtained for LaPS:Ce after continuous x-ray irradiation.
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Standard image High-resolution imageTrap states
The trap states in LaPS:Ce are studied through the TSL measurement. From the temperature dependence curve of La2Si2O7:Ce in previous report [16], it is found that obvious thermal quenching occurs above 473 K. So the TSL curve has be corrected according to the temperature dependency curve. The analysis of TSL curve was simulated through the TSL general kinetics order mode: the TSL intensity I as a function of temperature T in general order is as followed equation (1) [23]:
n0 is the concentration of trapped charges at t = 0, Et the energy level of the trap, κB the Boltzmann constant, l the kinetics order, s the frequency factor and β is the heating rate,1 K s−1 in this measurement. The integral part in expression (1) was disposed by Fourier expansion approximately presented in equation (2) [23]:
Equation (2) was substitute into equation (1) and the experimental TSL curve was fitted in Origin. The fitted line was depicted in figure 7 and the fitted result was listed in table 2.
Figure 7. The TSL curve of LaPS:Ce single crystal.
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Standard image High-resolution image
Table 2. TSL parameters of as grown LaPS:Ce single crystal.
| Peak temperature(K) | n0 | E(eV) | l | s(s−1) |
|---|---|---|---|---|
| 368 | 7.63 × 105 | 0.68 | 0.76 | 3.29 × 108 |
| 386 | 6.37 × 106 | 1.84 | 3.08 | 1.71 × 1023 |
| 446 | 4.00 × 106 | 1.62 | 1.84 | 1.96 × 1017 |
| 495 | 5.97 × 106 | 2.64 | 10.00 | 8.19 × 1025 |
| 534 | 8.71 × 105 | 3.07 | 1.66 | 2.00 × 1027 |
According to the TSL fitting result, the experimental curve can be decomposed to five peaks around 368 K, 386 K, 446 K, 495 K, 534 K, whose corresponding trap depth are about 0.68 eV, 1.84 eV, 1.62 eV, 2.64 eV, 3.07 eV respectively. As for the kind of charge trap in LaPS:Ce, it is known the noble gas configuration of Ce4+ is assumed to be more stable than Ce2+ [24]. So it is probably that the charge traps in LaPS:Ce single crystal are electron traps. In addition, the 'trap to center' type is adopted to describe the recombination mechanism of the TSL process [25]. When LaPS:Ce single crystal is heated, the electrons escape from the traps and reach the 5d excitation state of Ce4+ directly, then return to the 4 f ground state to recombine with the holes and emit the fluorescence.
Conclusion
In summary, single crystal of LaPS:Ce was firstly obtained by FZ method. The crystal structure is monoclinic with a space group P21/c. Four splitted Ce3+ 5d sublevels were observed on the VUV excitation spectrum. Its emission peaks locate at 3.5 eV and 3.3 eV assigned to 5d → 4f emission of Ce3+. The cerium luminescence in LaPS host presents single exponential decay of 29.3 ns. The LY of LaPS:Ce is 5400 photons/MeV. It shows a low intensity of LaPS:Ce. Through the TSL analysis, it is found that the energy depth of the main charge traps defect in LaPS:Ce are about 0.68 eV, 1.84 eV, 1.62 eV, 2.64 eV, 3.07 eV. The charge traps in LaPS:Ce single crystal are electron traps.
Acknowledgments
The authors gratefully acknowledge Doctor Yan Huang for the help in VUV measurements. This work is supported by National Nature Science Foundation of China (grant numbers 51402184, 51772185, 11575276, and 11275249) and 111 Project (grant number D16002).