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Abstract
Nanoscaled Ce3+, Yb3+ co-doped yttrium aluminum garnets (YAG: Ce3+, Yb3+) with Yb3+ concentration of 0–5 mol% were synthesized by the co-precipitation method. The microstructure and surface morphology of the samples were analyzed using X-ray diffraction and scanning electron microscope. The energy transfer (ET) from Ce3+ to Yb3+ in YAG: Ce3+, Yb3+ was verified by excitation spectra, emission spectra, and the luminescence decay time. The maximum ET efficiency (ηET) and the theoretical total quantum efficiency (ηQE) reach 91.2% and 191.2%, respectively. The near infrared cathodoluminescence (CL) of Yb3+ was also investigated. Compared to the single-doped YAG: Ce3+, co-doped YAG: Ce3+, Yb3+ shows a 2.3 times higher emission intensity, demonstrating the possibility of the ET process in CL. The YAG: Ce3+, Yb3+ prepared by the co-precipitation method can be used as a potential scintillator for high energy radiation applications.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The luminescence properties and applications of rare-earth (RE) materials have been widely explored over the last few decades. Owing to their specific characteristics, RE materials have been widely used as scintillators in high energy physics and real-time dose monitors for radiotherapy [1–4], especially for treatments requiring small beam size and steep dose gradients, such as stereotactic radiosurgery and stereotactic body radiation therapy [5–7]. Various RE ions can be doped as activators into different matrices, including glass, polymer, and crystal, to form radiation sensors [8–10].
The preparation and optical properties of Ce3+-doped silicate glasses have been investigated to fulfill the needs of dosimetric measurements in radiotherapy. The radioluminescence and the clinical dosimetric properties of optical fibers based on Ce3+-doped silica have been preliminarily tested by Vedda et al. [9–16]. The Ce3+ ion was regarded as a suitable activator medium owing to its relatively high luminescence efficiency caused by the 5d→4f transition, which covers a broad spectral ultraviolet–visible (UV-vis) range. However, the emission characteristics of the Ce3+ ion is affected by the type of hosts, as the energy broadband of the 5d→4f transition of Ce3+ can be tuned in different host materials with varying crystal field strength [17,18].
In comparison to the silicate glasses, the yttrium aluminum garnet (YAG) crystal matrix is superior in the above application owing to its high mechanical strength, temperature resistance, and radiation resistance [19,20]. Therefore, YAG was employed as the luminescent host in our study. Although the Ce3+ ion was regarded as a suitable activator medium, the Cerenkov emission generated in the passive light guiding fiber and irradiated in the radiation fields may disturb the signals for beam energy well above the Cerenkov threshold (approximately 190 keV for silica) [10,21]. The spurious signals can be scarcely distinguished from the real radioluminescence response of the Ce3+-doped sensor because of their spectral superposition in the UV-vis range. Various methods have been implemented to remove the Cerenkov light for achieving accurate measurements. An effective solution based on the simultaneous use of an additional reference passive fiber for the spurious signal subtraction was proposed [14,22]. However, the addition of the second fiber made the detector system more ponderous. Another method is to exploit the difference in wavelength between the sensor emission and the Cerenkov light. The spectra of the sensor can be changed by improving the scintillator material properties [23].
As a potential solution to the aforementioned problem, the Yb3+ was co-doped with the Ce3+ into the YAG crystal. The Ce3+–Yb3+ ion pair exhibited down-conversion (DC) in the YAG host. The DC process can convert the UV-vis emitted photons of the Ce3+ to the near infrared (NIR) emission of the Yb3+ [17,20,24–26]. The sharp peak of the Yb3+, which was located in the NIR range, can be clearly differentiated from the Cerenkov light. Moreover, the energy conversion from UV-vis to NIR will enhance the luminescence intensity, as the 5d→4f transition energy of the Ce3+ can nearly match twice the 2F5/2→2F7/2 transition energy of the Yb3+ in the YAG host. Therefore, YAG co-doped with the Ce3+– Yb3+ ion pair (YAG: Ce3+, Yb3+), which has been extensively used in the photovoltaic field, has been considered as a suitable material for radiation detection [20,25]. It was expected that the energy transfer between Ce3+ and the Yb3+ would enhance the detection efficiency.
In this study, we prepared the co-doped YAG: Ce3+, Yb3+ powder with different Yb3+ concentrations by the co-precipitation method. The X-ray diffraction (XRD) and scanning electron microscope (SEM) images confirm the nanoscale crystal microstructure of the samples. We investigated the luminescence properties of the co-doped samples to validate the energy transfer (ET) from the Ce3+ to the Yb3+. The NIR cathodoluminescence (CL) property of the Yb3+ in single and co-doped samples has also been examined to verify the highly efficient CL intensity of YAG: Ce3+, Yb3+. It is expected that, in future, this type of material can be combined with optical fibers for remote radiation sensing applications in radiotherapy.
2. Experiment
2.1 Materials synthesis
The effect of the preparation conditions, such as precipitants, sintering temperature, doping concentration, and sintering atmosphere, of the YAG: Ce3+ powder synthesized by the co-precipitation method have been investigated to obtain higher luminescence intensity in previous studies [27]. On the basis of this study, the co-precipitation method was employed to prepare the YAG: Ce3+, Yb3+ powder using the analytical reagents Al(NO3)3·9H2O, Y(NO3)3·6H2O, Ce(NO3)3·6H2O, and Yb(NO3)3·5H2O as starting materials. These materials were separately made into solutions by dissolving them in deionized water at 0.15 mol/L, 0.09 mol/L, 0.15 mol/L, and 0.15 mol/L concentrations, respectively. Appropriate volumes of these solutions were mixed to obtain multi-cation solutions with various concentrations. To avoid the quenching effect of the YAG: Ce3+ (0.5 mol% Ce3+) [27], the compositions of the mixtures are set to be Y(2.96 - x)Ce3+0.04Yb3 + xAl5O12 (x = 0.04, 0.06, 0.08, 0.1, 0.2, 0.4), which can meet the YAG stoichiometry ratio. The 1 mol/L ammonium hydrogen carbonate solution was prepared as the precipitant solution.
The whole precipitation process was performed using a reverse-strike technique (adding mixed salt solution to the precipitant solution) under magnetic stirring at room temperature. For multi-cation solutions, the use of reverse-strike technique can achieve higher cation homogeneity in the precursor. The mixed multi-cation solution was dropped into the 50 ml ammonium hydrogen carbonate solution at a speed of 0.8–0.9 ml/min. Magnetic stirring was performed for an hour more, after the completion of the titration. The resultant suspensions, after aging for 20 h in a fume hood, were centrifuged and washed with deionized water repeatedly (four times) to remove the residual ammonia and nitric ions. The gelatinous products were dried in an oven at 60 °C for approximately 24 h. The dried bulk precursors were crushed in an agate mortar and sieved through a 200-mesh screen. The precursor powder was sintered in a tube furnace in the inert atmosphere (nitrogen) up to 1400 °C. In the first sintering step, the sample was heated up to 700 °C at a 10 °C/min heating rate and maintained for 2 h. In the second step, the same heating rate was used to reach the final temperature of 1400 °C followed by another 2 h of heat preservation.
2.2 Powder characterization
The phase identification of the sintered samples were performed by the XRD method on a D8 Advance X-ray diffractometer (Bruker-AXS, Germany, CuKα radiation, λ = 1.54056 Ǻ) in the range of 2θ = 5–90° with a scanning speed of 2° 2θ/min and a step size of 0.02°. The morphological characteristics of the sample powder were observed by a SEM (JSM-7500F, JEOL, Japan). The photoluminescence (PL) spectra of the YAG: Ce3+, Yb3+ powder as well as the luminescence time decay curve of the Yb3+, were studied with a fluorescent spectrometer FLS980 (Edinburgh, UK), using Xe lamp excitation. The CL spectra of the powder sample were measured using a luminescence spectrometer MonoCL3 (Gatan, UK). All measurements were carried out at room temperature.
3 Results and discussion
3.1 Structural and surface morphology
The XRD patterns of the sintered Y(2.96 - x) Ce3+0.04Yb3 + xAl5O12 (x = 0.04, 0.1, 0.4) powder are shown in Fig. 1. The patterns of all the samples have the same peak position and similar peak intensity. The diffraction peaks of samples with different Yb3+ concentrations are consistent with the standard pattern of YAG (JCPDS 33–0040). The results indicate that all the sample powders prepared by the co-precipitation method completely formed a single-phase crystal structure without other obvious impurities. The average crystallite size of these three samples can be estimated by the Scherrer’s formula: [28]
where D is the average crystallite thickness perpendicular to the crystal plane, β is full-width at half maximum of the diffraction peak (in radians), θ is the Bragg diffraction angle (in degrees), K is the Scherrer constant (taken as 0.89 when β is full-width at half maximum), and λ is the incident x-ray wavelength (λ = 0.154 nm). The average crystalline size calculated by Scherrer’s formula are found to be approximately 50, 46, and 46 nm for Y(2.96 - x) Ce3+0.04Yb3 + xAl5O12 (x = 0.04, 0.2, 0.4).
Fig. 1. XRD pattern of the Y(2.96 - x)Ce3+0.04Yb3 + xAl5O12 (x = 0.04, 0.2, 0.4) powder. (a) x = 0.04, (b) x = 0.2, and (c) x = 0.4
The XRD pattern of the samples indicated that the lattice structure of the YAG crystal was not significantly changed by the introduced dopants. These doped cations have similar ionic radii to that of yttrium (Y3+ : 0.9 Å, Yb3+ : 0.868 Å, Ce3+ : 1.02 Å) [20]. Therefore, the Y3+ in the YAG lattice can be substituted by the Yb3+ and the Ce3+.
The SEM morphologies of the sintered Y(2.96 - x)Ce3+0.04Yb3 + xAl5O12 (x = 0.04, 0.2, 0.4) powder are shown in Fig. 2. It can be clearly observed that spherical particles were obtained by the co-precipitation method. These particles with homogeneous grain size and good dispersibility are loosely agglomerated. The mean grain size is less than 100 nm in all the three SEM micrographs. This result is consistent with the calculated value from Scherrer’s formula.
Fig. 2. SEM micrographs of the Y(2.96 - x)Ce3+0.04Yb3 + xAl5O12 (x = 0.04, 0.2, 0.4) powder (a) x = 0.04, (b) x = 0.2, and (c) x = 0.4.
The co-doped YAG: Ce3+, Yb3+ powders synthesized by the co-precipitation method continue to maintain the pure YAG phase. Moreover, these nanoscale particles appear to have a uniform size and good dispersion. Consequently, the XRD and SEM results reveal that the co-precipitation method is suitable for the YAG crystal preparation.
3.2 Photoluminescence properties and energy transfer
Figure 3 presents the excitation spectra of YAG: Yb3+, YAG: Ce3+, and YAG: Ce3+, Yb3+ crystals. The excitation spectrum of YAG: Yb3+ is monitored at 1030 nm of the Yb3+ emission. The peaks due to the 2F7/2→2F5/2 transition of the Yb3+ can be observed in the range 900–1000 nm (curve (a) in Fig. 3). There are two conspicuous broad bands around 340 and 458 nm in the excitation spectra of the YAG: Ce3+, monitored by the emission spectrum of the Ce3+ at 550 nm (curve (b) in Fig. 3). The two excitation bands are an outcome of the transitions from the 4f ground state to the 5d excited state of the Ce3+. In comparison to the aforementioned stated curves, the excitation spectrum of the co-doped YAG: Ce3+, Yb3+ crystal monitored by the 1030 nm emission of the Yb3+ has two parts (curve (c) in Fig. 3). Besides the characteristic excitation spectra of the Yb3+, similar broad bands were also observed nearly at 340 and 458 nm, which verifies the ET process from Ce3+ to Yb3+.
The visible and NIR emission spectra of the single-doped YAG: Ce3+ and the co-doped YAG: Ce3+, Yb3+ samples with different Yb3+ concentrations are shown in Fig. 4. The emission spectra of the Ce3+ and the Yb3+ are both excited by the Ce3+ excitation at 458 nm. The broad band centered at 550 nm in the visible region is attributed to the parity-allowed transitions of the lowest 5d level to the 4f level of the Ce3+. The emission curve can be fitted into two components centering at approximately 2.37 eV (approximately 523 nm) and 2.17 eV (approximately 571 nm) by the Gaussian fitting, owing to the Stark splitting of the 4f state (Fig. 5). The energy difference between the two sub-levels is approximately 0.2 eV (approximately 1613 cm−1), which is consistent with the theoretical value of the energy difference between the 2F5/2 and 2F7/2 splitting levels of the Ce3+ (approximately 1500–2000 cm−1) [29]. Moreover, the sharp peak located in the NIR range corresponding to the 2F5/2→2F7/2 transition of the Yb3+ is also observed for the co-doped YAG: Ce3+, Yb3+ samples. However, no emission peaks in the NIR region are observed in the single-doped YAG: Ce3+ sample.
Fig. 4. Visible and NIR normalized emission spectra of the Y(2.96 - x)Ce3+0.04Yb3 + xAl5O12 samples with different Yb3+ concentrations under the Ce3+ excitation at 458 nm. The inset shows the relative intensities of 550 and 1030 nm emissions as a function of Yb3+ concentration in Y(2.96 - x)Ce3+0.04Yb3 + xAl5O12.
With the increase in the doped Yb3+ concentration from 0 to 5 mol%, the intensity of the visible emission of the Ce3+ keeps decreasing monotonically. Conversely, the intensity of the NIR emission of the Yb3+ increases rapidly to a maximum value (2.5 mol% Yb3+ concentration) initially, and then decreases with a further increase in the Yb3+ concentration due to concentration quenching. Both the visible and NIR relative emission intensities as a function of the Yb3+ concentration have been depicted in the inset of Fig. 4.
The NIR normalized emissions of the co-doped YAG: Ce3+, Yb3+ samples excited by 458 and 930 nm, respectively, have also been compared in Fig. 6. A distinct sharp peak centered at 1030 nm, a lower peak located at 969 nm, and some small peaks can be observed in the co-doped samples. These different peaks originate from the energy transmission between the stark levels of the 2F5/2 and 2F7/2 multiplets. The 1030 nm emission with significant intensity is beneficial for radiation sensing. It is evident that both the groups show approximately the same peak position and shape. Furthermore, the dependence of emission intensity on the Yb3+ concentration under separate excitation energies shows a similar trend (as shown in the inset of Fig. 6). The existence of ET process from Ce3+ to Yb3+ has been further verified by these PL properties.
Fig. 6. NIR normalized emission spectra of the Y(2.96 - x)Ce3+0.04Yb3 + xAl5O12 samples with different Yb3+ concentrations under the excitation at 458 and 930 nm, respectively. The inset shows the relative emission intensities excited by 458 and 930 nm as a function of Yb3+ concentration in Y(2.96 - x)Ce3+0.04Yb3 + xAl5O12.
As the 5d→4f energy level difference of the Ce3+ matches nearly twice the 2F5/2→2F7/2 transition of the Yb3+, the cooperative energy transfer (CET) may be the theoretically feasible relaxation route to achieve the energy transfer from Ce3+ to Yb3+. The schematic of the CET process from Ce3+ to Yb3+ in YAG: Ce3+, Yb3+ has been portrayed in Fig. 7. As described in Fig. 4, the Ce3+ excited by 458 nm can relax back to the ground state ascribed to the 5d→4f transition. Meanwhile, if the two electric-dipole moments of Yb3+ are coupled into one virtual electric-dipole [30], the energy of one excited Ce3+ can be transferred into two Yb3+ leading to a NIR luminescence owing to the 2F5/2→2F7/2 transition of the Yb3+ (the transition electric-dipole moment of the virtual electric-dipole is approximately equal to that of the Ce3+).
Fig. 7. Schematic of energy level diagram of Ce3+ and Yb3+ in YAG: Ce3+, Yb3+ and the CET process from Ce3+ to Yb3+.
To demonstrate the ET process from Ce3+ to Yb3+ in YAG: Ce3+, Yb3+, the luminescence decay curves of the Ce3+ emission at 550 nm in YAG: Ce3+, Yb3+ with different Yb3+ concentrations is illustrated in Fig. 8. The decay curve of the single-doped YAG: Ce3+ sample can be fitted into a single exponential decay curve. With the increase of Yb3+ concentration in the co-doped YAG: Ce3+, Yb3+ samples, the lifetime decay becomes faster and the curve is no longer a single exponential decay. The change in the lifetime decay pattern implies the introduction of new decay pathways. Therefore, this phenomenon verifies that energy can be transferred from Ce3+ to Yb3+ in YAG: Ce3+, Yb3+.
Fig. 8. Decay curves of 550 nm emission of the Ce3+ excited by 458 nm in Y(2.96 - x)Ce3+0.04Yb3 + xAl5O12 samples with different Yb3+ concentrations.
The ET efficiency ηET and the theoretical total quantum efficiency ηQE can be estimated by the following formulas [31–35]:
where τCe and τCe,Yb denote the lifetime of the Ce3+ in the single-doped YAG: Ce3+ and the co-doped YAG: Ce3+,Yb3+ respectively. ηCe and ηYb denote the quantum efficiency of the Ce3+ and the Yb3+, respectively. Owing to the simple energy level structure and low probability of multi-phonons relaxation of the Ce3+ and Yb3+ ions, all the excited Yb3+ and residual excited Ce3+ are assumed to undergo complete radiative decay (ηCe = ηYb = 1). Based on this assumption, the theoretical upper limit of ηQE can be obtained by the following equation:The calculated average lifetime of Ce3+, the ET efficiency ηET, and the theoretical total quantum efficiency ηQE of the samples with various Yb3+ concentrations are listed in Table 1. The ηET and ηQE increase with increase in doped Yb3+ concentration. However, a part of the absorbed energy of Yb3+ will be released by the non-radiative transition, such as concentration quenching, following the increase in the Yb3+ concentration. Consequently, the assumption of Eq. (4) is not always tenable, and hence, the real radiative emission intensity cannot increase monotonically. The influence of the doped Yb3+ concentration on the calculated average lifetime of Ce3+ and the energy transfer efficiency in YAG: Ce3+, Yb3+ samples are displayed in Fig. 9.
Fig. 9. Average lifetimes of Ce3+ and the energy transfer efficiencies in Y(2.96 - x)Ce3+0.04Yb3 + xAl5O12 samples with different Yb3+ concentrations.
Table 1. Calculated average lifetimes of Ce3+, ET efficiencies ηET and theoretical total quantum efficiencies ηQE for Y(2.96 - x)Ce3+0.04Yb3 + xAl5O12 samples with various Yb3+ concentrations
3.3 Cathodoluminescence property
Considering high energy physics and clinical radiotherapy applications, the NIR CL emission of the Yb3+ in co-doped YAG: Ce3+, Yb3+ and single-doped YAG: Yb3+ samples are depicted in Fig. 10. As the quenching concentration of the Yb3+ in YAG: Ce3+, Yb3+ is approximately 2.5 mol%, the single-doped YAG: Yb3+ with 2.5 mol% Yb3+ ion concentration is assigned as the reference. It can be observed that the NIR CL emission spectra of the two samples peak at approximately the same position. However, the emission peak in the co-doped YAG: Ce3+, Yb3+ is distinctly higher (nearly 2.3 times) than that in YAG: Yb3+. We can infer that the ET process also exists in the NIR CL emission of Yb3+ in the co-doped YAG: Ce3+, Yb3+ sample to enhance the luminescence intensity excited by the cathode ray. This property indicates that the co-doped YAG: Ce3+, Yb3+ is a novel candidate that can be used for dose detection in radiotherapy.
Fig. 10. NIR CL emission of Yb3+ in co-doped Y(2.96 - x)Ce3+0.04Yb3 + xAl5O12 (x = 0.2) and single-doped YAG:Yb3+ samples.
4 Conclusion
In this study, co-doped YAG: Ce3+, Yb3+ crystals with various Yb3+ concentrations were prepared by the co-precipitation method. The microstructural characterization and surface morphology of the samples by XRD and SEM methods proved them to have the YAG nanocrystal structure. The excitation spectra, emission spectra, and luminescence decay time of the samples were investigated. The ET process from Ce3+ to Yb3+ in YAG: Ce3+, Yb3+ was verified by these luminescence properties. The highest theoretically calculated ηET and ηQE can reach 91.2% and 191.2%, respectively, for the sample with the Yb3+ concentration of x = 0.2. The energy of one excited Ce3+ transferred into two Yb3+ by CET was considered to be the most feasible mechanism of the ET process. The NIR CL emission of the Yb3+ was also detected and it was observed that the luminous peak of the co-doped sample was approximately 2.3 times higher. Therefore, the YAG: Ce3+, Yb3+ crystal was demonstrated to be an efficient system to increase the CL intensity. In conclusion, the co-doped YAG: Ce3+, Yb3+ is a promising candidate for clinical dose measurement, and the nanoscale co-doped YAG: Ce3+, Yb3+ powder can be obtained by the co-precipitation method used in our research. As our future research focus, an experimental radiation detection with the co-doped samples will be carried out on a medical linear accelerator.
Funding
National Natural Science Foundation of China (61520106014, 61635006, 61705126, 61935002, 61975113).
Acknowledgments
We thank Prof. Hairul Azhar Bin Abdul Rashid from Fiber Optics Research Center, Faculty of Engineering, Multimedia University, Malaysia for his valuable communication and discussion which have helped to improve the quality of this manuscript.
Disclosures
The authors declare no conflicts of interest.
Data availability
No data were generated or analyzed in the presented research.
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