Photoluminescence and Thermoluminescence Studies of Beta‐Irradiated Ba₃CdSi₂O₈:Tb³⁺ Phosphor for LED and Dosimetry Applications

ABSTRACT

The present work reports the preparation, characterization, and photoluminescence (PL) and thermoluminescence (TL) responses of Tb³⁺‐doped Ba₃CdSi₂O₈ phosphors. X‐ray diffraction analysis confirmed the consistency of the Tb³⁺‐doped Ba₃CdSi₂O₈ samples with the PDF 00‐028‐0128 card structure. The TL glow curve of the material was examined at different dopant concentrations after irradiation with a ⁹⁰Sr/⁹⁰Y beta source. Among the samples, Ba₃CdSi₂O₈: 5% Tb³⁺ exhibited the highest TL intensity compared with the other concentrations. The glow curve deconvolution method was used to determine the number of peaks, trap structure, and kinetic parameters within the TL glow curve, yielding a figure of merit (FOM) value of 1.11. The PL spectra show that the 2.0%, 3.0%, 4.0%, 5.0%, and 6.0% mole Tb³⁺‐doped Ba₃Cd (SiO₄)₂ phosphors capture excitation energy through the 4f‐5d transitions of Tb³⁺ ions and emit light at 417, 440, 492, 552, 589, and 628 nm, corresponding to the 5D₃–7F₅, 5D₃–7F₄, 5D₄–7F₆, 5D₄–7F₅, 5D₄–7F₄, and 5D₄–7F₃ transitions, respectively.

The study investigates the preparation, characterization, and photoluminescence (PL) and thermoluminescence (TL) behavior of Tb³⁺‐doped Ba₃CdSi₂O₈ phosphors. X‐ray diffraction confirms the crystal structure. TL analysis, with a ⁹⁰Sr/⁹⁰Y beta source, identifies optimal doping at 5% Tb³⁺. PL spectra exhibit characteristic emission peaks corresponding to Tb³⁺ transitions.

1. Introduction

Because of their rich energy levels and intrinsic electrical structure, rare earth ions are frequently employed as luminous centers. The benefits of rare‐earth (RE)–doped luminous materials include long fluorescence lifespan, high quantum efficiency, narrow emission band, a low quenching temperature, and strong optical stability. They are therefore widely employed in a variety of industries, including biological detection, information display, solar cells, light storage, infrared induction, and illness treatment [1, 2, 3, 4]. The use of silicate as a host lattice and RE ions as dopant recognizes their potential applications in the realms of optoelectronics, biotechnology, and medicine. The current approach for the use of luminescence materials in radiation dosimeters involves first irradiating the materials with high‐energy photons and then heating light photons to excite a portion of the excitation energy. These applications often relied on host materials with a broad energy gap activated by RE ions [5, 6, 7, 8].

Thermoluminescence (TL) is a fascinating phenomenon that has found diverse applications in various scientific and technological fields. TL is the emission of light observed when certain materials are heated after being exposed to ionizing radiation. This phenomenon has proven to be a valuable tool in a range of scientific disciplines. The study of TL began in the early 20th century, and since then, it has evolved into a powerful technique for dating purposes and radiation dosimetry. The thermoluminescent process involves the trapping of charge carriers (electrons and holes) in defects or impurities within a crystal lattice when exposed to ionizing radiation. Subsequently, the release of these trapped charges occurs during the heating process, leading to the emission of light. This basic principle forms the foundation of the TL theory. TL, with its rich history and diverse applications, continues to be a vital tool in scientific research and technological advancements [9, 10, 11].

In this work, we synthesized Ba₃CdSi₂O₈:Tb³⁺ (2–6 mol%) phosphors using the high‐temperature solid‐state reaction technique. X‐ray diffraction (XRD) analysis was performed on the synthesized phosphor to assess its crystalline structure. Computerized glow curve deconvolution (CGCD) analysis was employed to identify TL trapping and emission centers, as well as to determine the kinetic parameters. The figure of merit (FOM) value was subsequently calculated for potential dosimetry applications. Photoluminescence (PL) results suggest that the phosphors are promising candidates for light‐emitting diode (LED) applications, marking a significant development in this field.

2. Experimental Section

2.1. Materials

Used as starting material BaCO₃, SiO₂, Cd (CH₃COO)₂.2H₂O, Tb₂O₃, and (NH₄)₂Ce(NO₃)₆ were purchased from Sigma‐Aldrich and were not purified further.

2.2. Synthesis of Ba₃CdSi₂O₈:Tb⁺³

BaCO₃, Cd (Cd(CH₃COO)₂.2H₂O)₂, SiO₂, Tb₂O₃, and (NH₄)₂Ce(NO₃)₆ were weighed and mixed in the nominal compositions of 3BaCO₃ + Cd (CH₃COO)₂.2H + 2SiO₂ + with 2%–6% RE³⁺ (RE³⁺: Tb³⁺) and ground in an agate mortar for 20 min. The syntheses were carried out in two stages using a horizontal tube furnace. In the first stage, the mixtures were calcined at 900°C for 6 h and grounded again. In the second stage, the obtained oxides were sintered at 1200°C for 6 h.

2.3. Instrumentation

XRD powder pattern measurements were conducted using a BRUKER AXS D8 ADVANCE model X‐Ray Powder Diffractometer (XRD) (CuKα = 1.54 Å). Additionally, in the performed studies, it is observed that the peaks overlap with the PDF: 00‐028‐0128 Ba₃ Cd (SiO₄) card number of the synthesized luminescent material.

The PL properties were examined with a fluorescence spectrophotometer (Agilent Cary Eclipse). All PL measurements were performed in an open atmosphere and at room temperature.

The morphological properties of the Tb³⁺‐doped Ba₃CdSi₂O₈ compound were analyzed using the ZEISS‐ Gemini SEM 300 at the Central Laboratory of Gaziantep University. The magnification ranges from 12× to 2,000,000×, and the accelerating voltage ranges from 5 to 30 kV. The resolution is 0.6 nm at 30 kV. Its magnification is from 12× to 2,000,000×, and its acceleration voltage is 30 kV. The resolution is 0.6 nm at 30 kV.

During the TL experiments, the phosphor underwent irradiation at room temperature using a beta source derived from a calibrated emitter, specifically ⁹⁰Sr‐⁹⁰Y. Strontium‐90, along with its daughter products (⁹⁰Sr β‐8.73 × 10⁻¹⁴ J together with ⁹⁰Y β‐3.63 × 10⁻¹³ J), emitted high‐energy beta particles at a rate of 0.038 Gy/s. The irradiated sample was subsequently analyzed using the Harshaw QS 3500 TLD reader system. Subsequent experimentation took place within a nitrogen (N₂) atmosphere. This system, characterized by its manual operation, is connected to a computer interface, facilitating interaction with the hand operator. A clear glass filter has been placed in the reader between the photomultiplier tube and planchet to eliminate the emitted infrared lights from the samples plus the reader. The device contains a sample change drawer for removing and inserting the TLD elements. The TLD reader system operates in conjunction with a program running on Win REMS to process and interpret the acquired data.

3. Results and Discussion

3.1. Characterization

The crystallographic structure of Ba₃CdSi₂O₈:Tb³⁺ was characterized by using the conventional XRD technique. Powder XRD patterns of Ba₃CdSi₂O₈: xTb³⁺ phosphors (x varies from 0.02 to 0.06) are presented in Figure 1. The peaks were found to correspond to the card number of the synthesized host material (PDF: 00‐028‐0128 Ba₃Cd (SiO₄)₂). The sample displayed a crystalline structure with distinct, sharp peaks, confirming the successful synthesis. The most intense and sharp diffraction peak was used to determine the size of the crystalline by using the Scherrer formula, assuming that the particles are stress free:

$$D=\frac{\left(0.9\mathrm{\lambda }\right)}{\mathrm{\beta }\mathit{cos}\left(\mathrm{\theta }\right)}$$

FIGURE 1.

Powder X‐ray diffraction patterns of Ba₃CdSi₂O₈: xTb³⁺ phosphors (x varies from 0.02 to 0.06).

Here, D represents the average particle size of the crystallites, λ is the wavelength of the incident radiation, θ is the Bragg angle, and β is the full width at half maximum (in radians) resulting from crystallization. The average crystalline size determined was approximately 80 nm.

Fourier transform infrared (FT‐IR) spectroscopy was employed as an additional characterization technique. FT‐IR spectra of Tb³⁺‐doped Ba₃CdSi₂O₈ phosphors are given in Figure 2. The provided FT‐IR spectrum illustrates the transmittance (%T) of Tb‐doped Ba₃CdSi₂O₈ for different Tb concentrations. The strong absorption bands in the 1000–1200 cm⁻¹ region typically corresponds to the Si‐O‐Si or Si‐O stretching vibrations. The presence of these peaks indicates the silicate structure in Ba₃CdSi₂O₈. A slight shift or change in intensity with increasing Tb doping could indicate modifications in the local bonding environment due to the incorporation of Tb ions, possibly substituting Ba or influencing the crystal structure. The peaks in the 500–800 cm⁻¹ range are often associated with bending or rocking vibrations of Si‐O bonds or lattice vibrations of the Ba₃CdSi₂O₈ structure. Changes in this region may reflect structural distortions or crystal field effects caused by Tb doping. The absence of additional peaks suggests that Tb doping does not create major structural impurities or secondary phases in the material. If broad and weak features are observed in this region, they might indicate O‐H stretching vibrations, likely due to moisture or surface‐adsorbed water. Tb doping modifies the Ba₃CdSi₂O₈ matrix without altering its silicate framework. The changes in transmittance intensities and peak positions are consistent with structural adjustments due to Tb incorporation. These changes could also impact the material's optical and luminescence properties.

FIGURE 2.

FT‐IR spectra of Tb⁺³‐doped Ba₃CdSi₂O₈ phosphor.

3.2. SEM Analysis

The surface morphology of Ba₃CdSi₂O₈:Tb³⁺ investigated by SEM image analysis is given in Figure 3. The SEM image shows a dense arrangement of particles with a mixture of fine and larger agglomerates. The surface appears rough, which is typical for phosphors synthesized via solid‐state or high‐temperature routes. The texture could influence luminescence properties due to scattering effects.

FIGURE 3.

SEM photograph of Tb⁺³‐doped Ba₃CdSi₂O₈ phosphor.

3.3. PL Studies

In the present study, Ba₃CdSi₂O₈ was doped with Tb³⁺ at five different concentrations (0.02, 0.03, 0.04, 0.05, and 0.06 mol%) to investigate the effect of Tb³⁺concentrations on PL and TL properties of the host compound, Ba₃CdSi₂O₈.

To investigate the optimum doping ratio of Tb ions and the effect of doping ratio on PL intensity of Ba₃Cd(SiO₄)₂, PL measurement has been performed. Figure 4 shows the PL excitation and emission spectra of the 2.0, 3.0, 4.0, 5.0, and 6.0 mol% Tb³⁺ ion–doped Ba₃Cd(SiO₄)₂ phosphors. In Figure 4, the excitation spectra of phosphors showed a broad band in the range of 200–300 nm under 552‐nm emission. Tb³⁺ ions mainly capture excitation energy and the observed broad excitation band with a maximum at ~250 nm originates from the 4f‐5d transitions of Tb³⁺ ions [12, 13]. The excitation band of phosphors at ~250 nm shows that these phosphors are extremely suitable candidates for the design of phosphors excited by blue laser diodes or UV/near‐UV.

FIGURE 4.

The excitation and emission spectra of 2.0, 3.0, 4.0, 5.0, and 6.0 mol% Tb³⁺‐doped Ba₃Cd(SiO₄)₂ at 200–700 nm.

The corresponding emission spectra under 250‐nm excitation are shown in Figure 4. All the phosphors have the typical PL emission spectra of Tb³⁺ions. The emission spectra of the 2.0, 3.0, 4.0, 5.0, and 6.0 mol% Tb³⁺ ion–doped Ba₃Cd(SiO₄)₂ phosphors show the radiative transition of Tb³⁺ ions and consist of a series of sharp lines related to 4F intra‐configurational transitions of Tb³⁺ ions. The emission bands peaking at 417, 440, 492, 552, 589, and 628 nm corresponding to ⁵D₃–⁷F₅, ⁵D₃–⁷F₄, ⁵D₄–⁷F₆, ⁵D₄–⁷F₅, ⁵D₄–⁷F₄, and⁵D₄–⁷F₃ transitions, respectively [14, 15, 16, 17, 18]. The emission spectra of phosphors consist of blue, green, yellow, and red emission region. The most intense emission peak at 552 nm correspond to ⁵D₄ → ⁷F₅ transition is assigned to magnetic dipole transition of Tb³⁺ ions with ΔJ = 1 [19]. In the blue region, the emission peaks at 417 and 440 nm belong to ⁵D₃–⁷F₅ and ⁵D₃–⁷F₄ transitions of Tb³⁺ ions, respectively. However, other ⁵D₃–⁷F_j transitions were not observed. As known, the difference between the ⁵D₃ and ⁵D₄ energy levels of Tb³⁺ ions and the difference between the ⁷F₀ and ⁷F₆ energy levels are approximately similar. For this reason, some excited ions at the ⁵D₃ level can move to the ⁵D₄ level through cross‐relaxation. Due to this cross‐relaxation, the number of excited ions at the ⁵D₃ level decreases, while the number of excited ions at the ⁵D₄ level increases. Therefore, while the number of ⁵D₃–⁷F_j transitions and emission intensity decreases, the number of ⁵D₄–⁷F_j transitions and their emission intensity increases. Moreover, all ⁵D₄–⁷F_j transitions can be obtained, while other ⁵D₃–⁷F_j transitions cannot be observed due to the transition via cross‐relaxation [14]. As can be seen from the emission spectrum in Figure 4, the Ba₃Cd(SiO₄)₂ host crystal is extremely suitable for obtaining all ⁵D₄–⁷F_j transitions and shifting the emission mainly to the green and red regions. When the effect of the Tb³⁺ ions doping ratio on the emission intensity was examined, it was observed that the emission intensity increased as the doping ratio increased up to 5 mol%, and after this ratio, the intensity decreased. Therefore, the optimum Tb³⁺ ions doping ratio for the Ba₃Cd(SiO₄)₂ host crystal is 5 mol%.

Further, the color coordinates on a standard Commission International d'Eclairage (CIE) 1931 chromaticity diagram (Figure 5) were studied to label the emitted color of the (2–6 mol%), Tb³⁺ ion–doped Ba₃CdSi₂O₈ phosphors and the chromaticity coordinate (x, y) values for the prepared Ba₃CdSi₂O₈ phosphors were assessed from PL data. The figure shows the CIE chromaticity diagram with the chromaticity coordinates (CIE X, CIE Y) plotted for Tb‐doped Ba₃CdSi₂O₈. Each coordinate corresponds to different concentrations of Tb doping. As the Tb concentration increases, the chromaticity coordinates shift on the CIE diagram. For low concentrations (e.g., 0.02, 0.03), the coordinates are closer to the cyan‐green region. At higher concentrations (e.g., 0.05, 0.06), the points move toward the greenish‐yellow region. The shift in chromaticity coordinates indicates a change in the dominant emitted color with Tb concentration. This suggests that Tb doping alters the energy levels responsible for luminescence, likely due to changes in Tb³⁺ emission transitions. An optimal selection of terbium (Tb) concentration for desired color emission is crucial for applications in display technologies, LEDs, and other luminescent devices. This precise tuning of Tb concentration can significantly enhance the performance and efficiency of these systems by achieving the required spectral characteristics and improving color accuracy and brightness.

FIGURE 5.

CIE chromaticity diagram of Tb³⁺‐doped Ba₃CdSi₂O₈ phosphor.

3.4. TL Studies

The dose–response behavior of the (Ba₃CdSi₂O₈:Tb³⁺) for different doses was also examined. The purpose of the experiment was to examine the impact of dose dependency at a peak position. To do this, samples were exposed to β‐ray radiation at a dose between ≈2.28 Gy and ≈1.36 kGy. The powdered materials (15 mg) were irradiated at room temperature in each experiment, and the results were read out right away. A selection of the Ba₃CdSi₂O₈:Tb³⁺ glow curve shapes at various dose levels are displayed in Figure 6. Overall, the results of the experiments indicated that the temperature did not significantly alter at the site of peak temperature.

FIGURE 6.

Typical glow curves of Ba₃CdSi₂O₈:Tb³⁺ exposed to β‐rays from ≈2.28 Gy and ≈1.36 kGy at β = 1 °C/s.

Figure 7 shows the normalized TL glow curve area of intensities of the phosphors, which were recorded from different amounts of Tb³⁺‐doped Ba₃CdSi₂O₈ powder samples. It can be seen that TL glow peak areas increased with increasing the Tb³⁺ concentration until 0.05 mol%. However, as can be seen, in that the intensities decreased with increasing concentration due to the quenching process, which caused a decrease in the luminescence intensity.

FIGURE 7.

Normalized thermoluminescence glow curve area of intensities of the phosphors of Ba₃CdSi₂O₈: xTb³⁺ (x varies from 0.02 to 0.06).

In order to form an opinion about the number of glow peaks and kinetic orders (b) of all individual glow peaks in the glow curve structure of Ba₃CdSi₂O₈:Tb³⁺, the additive dose method was utilized in the current study. The samples were irradiated at several doses between ≈2.28 and ≈1.36 kGy, and some of the selected glow curves after variable dose levels can be seen in Figure 6. The results of the additive dose experiments were also utilized to calculate the trapping parameters.

In this study, CGCD method was employed to assess the trapping parameters of the main dosimeter peak of Tb³⁺‐doped Ba₃CdSi₂O₈ (see Figure 8). The CGCD method has become the method of preference in determining the kinetic parameters from the glow curves of TL material. Therefore, the kinetic parameters such as the number of glow peaks, activation energies (E _a ), and kinetic order (b) for the dosimetric peaks of Ba₃CdSi₂O₈:Tb³⁺ were evaluated [20, 21].

FIGURE 8.

Computerized glow curve deconvolutation results for Ba₃CdSi₂O₈:Tb³⁺ measured after 12‐Gy irradiation by beta ray at room temperature (β = 1 °C.s⁻¹).

In this study, the following general order kinetic analytical equation was used by the approximation for b ≈ 1.1 [22]. Based on the results of our analysis, we identified the trapping parameters of Ba₃CdSi₂O₈:Tb³⁺ and results were given in Table 1.

$$\begin{array}{c}I\left(T\right)=I_m·b^{\raisebox{1ex}{$b$}\!\left/ \!\raisebox{-1ex}{$b-1$}\right.}·\exp \left(\frac{E}{\mathit{kT}}·\frac{T-T_m}{T_m}\right)·\hfill \\ {\left[\left(b-1\right)·\left(1-∆\right)·\frac{T^2}{T_m^2}·\exp \left(\frac{E}{\mathit{kT}}·\frac{T-T_m}{T_m}\right)+Z_m\right]}^{\raisebox{1ex}{$-b$}\!\left/ \!\raisebox{-1ex}{$b-1$}\right.}\hfill \end{array}$$

where;

$$∆=\frac{2\mathit{kT}}{E},{∆}_m=\frac{2k{T}_m}{E},Z_m=1+\left(b-1\right)·{∆}_m$$

TABLE 1.

The values of the trapping parameters of Ba₃CdSi₂O₈:Tb³⁺.

	P1	P2	P3
T max (°C)	79	106	128
E a (eV)	0.56	0.76	0.61
b	1.87	1.51	1.26

4. Conclusions

This paper describes preliminary investigations of the preparation, characterization, and PL and TL response of Tb³⁺‐doped Ba₃CdSi₂O₈ phosphor to use it at LED and dosimetric applications. In terms of the experimental results, the following conclusions were obtained:

1. Ba₃CdSi₂O₈ was doped with Tb³⁺ at five different concentrations 0.02, 0.03, 0.04, 0.05, and 0.06. 5 mol%. The normalized TL glow curve area of intensities of the phosphors reveals that TL glow peak areas increased with increasing the Tb³⁺concentration until 0.05 mol%. It is concluded that the optimum doping concentration is 0.05 mol%.

2. The CGCD method was used for the determination of the kinetic parameters: the order of kinetics (b), activation energy (E _a ), and the frequency factor (s). The best fit was obtained using three glow peaks for 5% Tb³⁺‐doped Ba₃CdSi₂O₈ where FOM = 1.11. The TL studies reveal that phosphors may be a potential candidate as dosimetry materials for radiation measurement.

3. The PL studies showed that 2.0, 3.0, 4.0, 5.0, and 6.0 mol% Tb³⁺ ion–doped Ba₃Cd(SiO₄)₂ phosphors capture the excitation energy by the 4f‐5d transitions of Tb³⁺ ions. Therefore, Ba₃Cd(SiO₄)₂ phosphors doped withTb³⁺ ions can be effectively excited with UV, near‐UV, and blue laser diodes. According to the emission spectra, Tb³⁺ ion–doped Ba₃Cd(SiO₄)₂ phosphors have blue, green, yellow, and red emission region. For this reason, Tb³⁺ ion‐doped Ba₃Cd(SiO₄)₂ phosphors are extremely suitable material for the mainly green region opto‐electronic applications that are excited by UV, near‐UV, and blue laser diodes.

Author Contributions

Büşra Yazıcı Başaran: methodology. Vural Emir Kafadar: writing – original draft, supervision, funding acquisition. Fatih Mehmet Emen: conceptualization, methodology. Esra Öztürk: formal analysis, data curation. Ali İhsan Karaçolak: software, data curation.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.