Optimizing the luminescence efficiency of an europium (Eu³⁺) doped SrY₂O₄ phosphor for flexible display and lighting applications

Abstract

This research paper reports the synthesis and luminescence study of an Eu³⁺ activated SrY₂O₄ phosphor prepared by a modified solid-state reaction method with varying concentrations of Eu³⁺ ions (0.1–2.5 mol%). X-ray diffraction (XRD) revealed the orthorhombic structure and Fourier transform infrared spectroscopy (FTIR) methods were used to analyse the produced phosphors. Photoluminescence emission and excitation spectra were recorded for varying concentrations of Eu³⁺ ions, and an optimum concentration of 2.0 mol% was found to produce the highest intensity. Under 254 nm excitation the emission peaks were found to be at 580 nm, 590 nm, 611 nm and 619 nm, corresponding to transitions at ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₁, and ⁵D₀ → ⁷F₂ respectively. Because of Eu³⁺ inherent luminosity, these emission peaks indicate radiative transitions between excited states of ions, making them useful for developing white light-emitting phosphors for optoelectronic and flexible display applications. The 1931 CIE (x, y) chromaticity coordinates were calculated from the photoluminescence emission spectra and found to be near white light emission, indicating the potential application of the prepared phosphor for light emitting diodes (white component). TL glow curve analysis was also performed for various concentrations of doping ions and UV exposure times, and a single broad peak was observed at 187 °C. Using the computerised glow curve deconvolution (CGCD) method, kinetic parameters were computed.

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1. Introduction

The rosy fantasy of a life that can be controlled with the press of a button and more automation would make the daily lives of humans easier. The current trend of innovation and research has made it easier for people to explore new areas of science and technology. Smart materials are one of the aspects of this new frontier. Smart materials are any substances that are capable of detecting or responding to outside stimuli. There are many other types of stimuli that may be used, including environmental, optical, chemical, electrical, thermal, biological, physical, etc.^1–4 Since the reduction in the size of display systems, luminescent materials have attracted a lot of research attention.^5–7 Applications for smart luminescent materials are widespread, ranging from polymeric optical fibers and LEDs to displays, self-emitting devices, and light sensors.^8–12 Achieving durability and adaptability of the electronics and materials used in flexible electronic devices without compromising performance is a critical challenge for smart materials.^13–18 This obstacle is readily overcome with the use of nanotechnology. The usefulness of several micro- and nanoparticles as smart materials has been reported.^19–22 Because of this, the intrinsic luminescence feature is the main focus of this study. This property has the potential to be employed in an intelligent (smart) material regarding applications of flexible display.

Europium (Eu³⁺) doped SrY₂O₄ is a well-known luminescence material because it exhibits strong and long-lasting red luminescence under ultraviolet or blue light excitation. The luminescence properties of this material are due to the presence of Eu³⁺ ions, which are known for their unique electronic transitions. There are several reasons why Eu³⁺ doped SrY₂O₄ is a good luminescence material:

(1) Quantum yield: the quantum yield refers to the efficiency of a phosphor in converting absorbed energy into emitted light. Eu³⁺-doped SrY₂O₄ phosphor has a relatively high quantum yield, typically ranging from 60% to 80%. However, it's important to note that the quantum yield can vary depending on the specific manufacturing process and conditions.

(2) Color purity:

• Eu³⁺-doped SrY₂O₄: this phosphor typically exhibits a red emission providing good color purity.

• Y₂O₃:Eu: this phosphor is known for its red emission, offering high color purity.

• Y₂O₂S:Eu: this phosphor generally produces a yellowish-red emission, which also has good color purity.

(3) Cost: the cost of phosphor materials can vary based on several factors, including the availability of raw materials and the complexity of the manufacturing process. Eu³⁺-doped SrY₂O₄ phosphor is relatively more expensive compared to some other commonly used phosphors such as cerium-doped yttrium aluminum garnet (YAG:Ce) or various sulfide-based phosphors. The higher cost can be attributed to the specific composition and synthesis methods required to produce SrY₂O₄:Eu³⁺ phosphor.

(4) Stability and lifetime: phosphor stability and lifetime are crucial factors to consider in practical applications. Eu³⁺-doped SrY₂O₄ phosphor exhibits good thermal and chemical stability, allowing it to withstand the harsh conditions typically encountered in solid-state lighting devices. Additionally, it offers a long operational lifetime, ensuring sustained performance over extended periods.

(5) Excitation wavelength: another important aspect is the excitation wavelength required to activate the phosphor material. Eu³⁺-doped SrY₂O₄ phosphor typically requires ultraviolet (UV) or near-UV excitation in the range from 250 to 350 nm. This wavelength range can be achieved using various light sources, including UV LEDs and mercury lamps.

(6) Luminescent properties:

• Eu³⁺-doped SrY₂O₄: this phosphor has a relatively high luminescence efficiency, long decay time, and good thermal stability, making it suitable for various applications such as lighting and displays.

• Y₂O₃:Eu: phosphor is known for its high luminescence efficiency and good thermal stability, making it commonly used in red-emitting devices.

• Y₂O₂S:Eu: phosphor also has high luminescence efficiency and good thermal stability. It is often used in yellowish-red-emitting devices.

(7) Application specifics:

• Eu³⁺-doped SrY₂O₄: due to its blue-green emission, this phosphor is often used in applications requiring a specific color range, such as white LEDs and plasma displays.

• Y₂O₃:Eu: phosphor finds extensive use in red-emitting devices like fluorescent lamps, plasma displays, and cathode ray tubes (CRTs).

• Y₂O₂S:Eu: phosphor is commonly employed in yellowish-red-emitting devices like white LEDs, plasma displays, and CRTs.

Metal oxides with rare earth ion doping have mostly been used for their luminous characteristics. Rare earth ion-doped alkali earth metallic yttrates (MY₂O₄, M = Ba, Ca, and Sr) have been seen to exhibit prominent optical emission. According to reports, SrY₂O₄:RE³⁺ (RE = Dy, Eu, Tb, Gd, Sm, Ce, and Er) may be made using a variety of techniques, including solution combustion, sol–gel, citrate sol–gel, polyol, oil emulsion, aldo-keto gel, and co-precipitation.²³ The red optical emission of europium makes it one of the intriguing and flexible lanthanides, making it a prime candidate for luminescence applications. SrY₂O₄:Eu³⁺ (SYO:Eu) has a structure similar to CaFe₂O₄ and exhibits magnificent down-conversion luminescence. At a wavelength of 611 nm, the very powerful emission line produced by the Eu³⁺ long-lived transition ⁵D₀ → ⁷F₂ may be seen. With the excitation of 254 nm (UV region), europium ions are useful for light emitting phosphor and flexible display applications.^24–36 In this paper, we describe how to easily make new Eu³⁺ doped SrY₂O₄ phosphors, which may be utilized to improve the attractive performance of intelligent (smart) materials for flexible displaying applications. SYO:Eu phosphors emit red light when exposed to UV radiation.

2. Experimental

The preparation of the SrY₂O₄ doped with Eu³⁺ specimen involves a solid-state reaction method. The necessary starting materials, including SrCO₃, Y₂O₃, Eu₂O₃, and H₃BO₃, Here are the steps for synthesis and characterization of Eu³⁺ doped SrY₂O₄ phosphor:

• Preparation of starting materials: SrCO₃, Y₂O₃, and Eu₂O₃ are weighed and mixed in stoichiometric amounts to obtain the desired composition of SrY₂O₄:Eu³⁺.

• Mixing and grinding: the mixture of powders is then ground together in an agate mortar and pestle for 2 hours to ensure a homogenous mixture.

• Calcination: once the mixing was complete, the resulting powder was placed into a furnace and heated to a temperature of 1000 °C for duration of 1 hour.

• Sintering: after calcination the mixture of powders grind for 1 hour in an agate mortar and pestle and the homogenized powder mixture is then calcined in air at a temperature of 1250 °C for 3 hours to form the desired SrY₂O₄:Eu³⁺ phosphor. Higher temperature was needed to ensure that the reaction was complete and that the product was fully formed.

The resulting specimen, SrY₂O₄ doped with Eu³⁺, was then allowed to cool to room temperature. The specimen was characterized using various techniques to confirm the desired properties, such as its crystal structure, morphology, and luminescent properties.

3. Results and discussion

3.1 X-ray analysis

XRD is a widely used technique that involves shining a beam of X-rays onto a sample and measuring the angle and intensity of the diffracted X-rays. The sample's crystal structure and content may be determined from the ensuing XRD pattern. The XRD pattern of Eu³⁺ doped SrY₂O₄ phosphors typically shows characteristic peaks corresponding to the crystalline phases present in the sample. The main phase is SrY₂O₄, which exhibits aorthorhombic crystal structure. The XRD peaks of SrY₂O₄ are sharp and well-defined, indicating a high degree of crystallinity and phase purity. The positions and intensities of the peaks can be used to calculate the lattice parameters and crystal structure of the material.

The XRD pattern of the sample is shown in Fig. 1 where aorthorhombic structure is displayed. These values match those of the International Centre for Diffraction Data (JCPDS) card No. 032-1272. The crystallite size of the phosphor was calculated using the Scherer equation. The average crystallite size of the phosphor was found to be 34.068 nm (Table 1).

Fig. 1 (a) XRD pattern of SrY₂O₄:Eu³⁺ phosphor. (b) XRD pattern of SrY₂O₄:Eu³⁺phosphor for 0.2 mol%.

Table 1 Crystallite size for 0.2 mol% of SrY₂O₄:Eu³⁺ phosphor

hkl	Peak position (2θ)	FWHM	Crystallite size D (nm)	Average size D (nm)
220	23.21705	1.38028	5.876894136	34.06808
*011	27.24806	0.13642	59.93173053
320	30.67829	0.2346	35.12083109
201	31.62791	0.2148	38.44694105
131	35.83333	0.13504	61.83951978
401	44.26357	0.19296	44.45405375
322	63.17829	0.52107	17.90162841
362	79.24419	1.14963	8.973048289

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3.2 FTIR analysis

Fig. 2 displays the SrY₂O₄:Eu³⁺ (2%) phosphor's FTIR spectrum. Strong, sharp peaks may be seen in this spectrum between 445–863 cm⁻¹, which are indicative of Y–O vibrations. An IR peak with a wavelength of 1455–1643 cm⁻¹ is produced when Sr–O is present.³⁷ The production of SrY₂O₄ phosphor is confirmed by all of these observed peaks taken together (Table 2).

Fig. 2 FTIR spectra of SrY₂O₄:Eu³⁺ phosphor (2.0 mol%).

Table 2 Crystallite size for 0.5 mol% of SrY₂O₄:Eu³⁺ phosphor

hkl	Peak position (2θ)	FWHM	Crystallite size D (nm)	Average size D (nm)
220	23.34254	0.9755	8.317365003	33.4503
*011	27.31909	0.13849	59.04481715
320	30.8561	0.29096	28.32989183
201	31.86547	0.26985	30.62172567
131	36.14804	0.15282	54.69350767
401	44.49721	0.18065	47.52278414
322	63.39878	0.3074	30.38083641
362	79.58315	1.1897	8.692154104

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Table 3 Crystallite size for 1.0 mol% of SrY₂O₄:Eu³⁺ phosphor

hkl	Peak position (2θ)	FWHM	Crystallite size D (nm)	Average size D (nm)
220	23.33911	0.6755	12.01116121	34.7837
*011	27.39439	0.1938	42.20033311
320	30.7273	0.30141	27.33922054
201	31.70426	0.28279	29.2088247
131	36.01264	0.17439	47.91012036
401	44.35684	0.17304	49.58794225
322	63.23297	0.58242	16.02063895
362	79.48164	0.19139	53.9915117

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3.3 Scanning electron microscopy (SEM)

Scanning Electron Microscopy (SEM) is a powerful characterization tool that is used to examine the morphology and surface features of europium (Eu³⁺) doped SrY₂O₄ phosphors. The technique involves focusing a beam of electrons onto the surface of the sample, causing it to emit secondary electrons that are then collected and analyzed by detectors (Table 4).

Table 4 Crystallite size for 1.5 mol% of SrY₂O₄:Eu³⁺ phosphor

hkl	Peak position (2θ)	FWHM	Crystallite size D (nm)	Average size D (nm)
220	23.35613	0.38574	21.03434538	32.0282
*011	27.56954	0.1934	42.30342242
320	30.80483	0.28707	28.71023965
201	31.80803	0.25283	32.67844571
131	36.17191	0.17999	46.4405259
401	44.47332	0.19451	44.13273883
322	63.33333	0.42113	22.16839805
362	79.5098	0.551	18.75779935

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When SEM is used to study Eu³⁺ doped SrY₂O₄ phosphors, it provides high-resolution images of the particle size, shape, and distribution (Fig. 3). The technique can also be used to observe any morphological changes that occur as a result of the synthesis method or thermal treatment. SEM imaging can reveal the surface features of the phosphor particles, such as cracks, pores, or other defects, which can affect the luminescence properties of the material (Table 5).

Fig. 3 SEM images of SrY₂O₄:Eu³⁺ (2.0 mol%) (a) ×1.5k 10 μm (b) ×3.5k 5 μm (c) ×7k 2 μm (d) ×10k 1 μm.

Table 5 Average crystallite size for concentration (0.2–1.5) mol% of SrY₂O₄:Eu³⁺ phosphor

S. no	Concentration (mol%)	Average crystallite size D (nm)
1	0.2	34.068
2	0.5	33.4503
3	1	34.7837
4	1.5	32.0282

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3.4 Energy dispersive X-ray spectroscopy (EDX)

Energy Dispersive X-ray Spectroscopy (EDX) analysis is a valuable tool for studying the elemental composition of Eu³⁺ doped SrY₂O₄ phosphors and can provide important insights into the materials' luminescence properties. By optimizing the elemental composition and concentration of the phosphor, using this technique we can enhance the luminescence efficiency and stability of the material, which can have important implications for applications such as lighting and sensing.

It is an analysis of the components of produced phosphor. Fig. 4a displays quantitative representations of the sample's Y, Sr, O, and Eu elements. The data supports the conclusion that SrY₂O₄:Eu³⁺ phosphor can be synthesized. The qualitative analysis of the process of creating the components is shown in Fig. 4b.

Fig. 4 (a) EDX pattern of prepared phosphor Eu³⁺ (2.0 mol%) doped SrY₂O₄. (b) Quantitative EDX of Eu³⁺ doped SrY₂O₄ phosphor (2.0 mol%).

3.5 Photoluminescence (PL) observation

Doping with Eu³⁺ ions improved the emission spectra of the host SrY₂O₄ phosphor, which has no luminescence.²³ The SrY₂O₄ phosphor doped with Eu³⁺ PL excitation spectra are shown in Fig. 5. Excitation spectra were captured at an emission wavelength of 613 nm. It has a wide spectrum between 190 and 254 nanometers. Eu³⁺ doped SrY₂O₄ phosphor's PL emission spectrum, measured at 254 nm, shows the typical emissions at 580, 590, 611, and 619 nm. Fig. 6. The emission spectra of Eu³⁺ are produced by transitions between ⁵D₀ → ⁷F_J (J = 0, 1, 2).³⁸ The brightest emission band can be seen at 611 nm, which is associated with the electric dipole transition ⁵D₀ → ⁷F₂ (ref. 23–25). The ⁵D₀ → ⁷F₀ and ⁵D₀ → ⁷F₁ magnetic dipole transitions⁶⁰ give rise to the 580 nm and 590 nm bands, respectively. Due to similar ionic radii, the emergence of the ⁵D₀ → ⁷F₁ transition supports the substitution of Eu³⁺ ions at Y³⁺ sites.³⁹ The minor separation of the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transition lines in the emission spectra of the SrY₂O₄:Eu³⁺ phosphor amply demonstrates the considerable influence of the host composition, crystal structure, and coordination environment on luminescence qualities.⁴⁰ Fig. 6 shows how the concentration of the dopant affects the intensity of the emission. The intensity continues to rise as the Eu³⁺ ion concentration rises from 0.1 to 2.0 mol%. However, the concentration quenching event causes the intensity to dramatically falldown for 2.5 mol% of the time. Intensity is reduced by quenching, which happens when Eu³⁺ ions move in closer proximity to one another and interact with one another to transfer charge. Eu³⁺ doped SrY₂O₄ phosphors using 2.0 mol% had the highest emission intensity, making it suitable for various light-emitting applications (Table 6).

Fig. 5 PL excitation spectra of SrY₂O₄:Eu³⁺ (2.0 mol%) phosphor.

Fig. 6 PL emission spectra of SrY₂O₄:Eu³⁺ (0.1–2.5 mol%) phosphor.

Table 6 Quantum yield values for different concentrations (mol%) of SrY₂O₄:Eu³⁺

S. no	Concentration (mol%)	Absorption of photons	Emission of photons	Quantum yield
1	0.1	22 988.91862	5870.677682	0.242601398
2	0.2		7114.49341	0.29400116
3	0.5		10 769.65826	0.445048135
4	1		12 347.99237	0.51027162
5	1.5		16 981.28813	0.701739129
6	2		23 090.70313	0.954206169
7	2.5		18 523.81395	0.765482863

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The fluorescence quantum yield (ϕ_F) is a measure of the efficiency of fluorescence, representing the ratio of absorbed photons to emitted photons through fluorescence. It quantifies the likelihood of the excited state being deactivated through fluorescence rather than non-radiative mechanisms. The comparative method developed by Williams et al. is considered the most reliable approach for determining (ϕ_F). This method involves utilizing well-characterized standard samples with known (ϕ_F) values, enabling accurate measurement and comparison of fluorescence efficiencies.⁶²

where the η denote the refractive index of the sample.

The fluorescence quantum yield (ϕ_F) is a parameter that measures the efficiency of fluorescence by comparing the number of absorbed photons to the number of emitted photons. To determine ϕ_F, one common approach involves analyzing the integrated areas of the absorption and emission spectra. The integrated area represents the number of absorbed photons and the number of emitted photons, respectively. By comparing these values, the fluorescence quantum yield can be accurately calculated (Table 7).

Table 7 Table of standard materials and their literature quantum yield values

Compound	Solvent	Literature quantum yield	Emission range/nm	Reference ^63–74
Cresyl violet	Methanol	0.54	600–650	J. Phys. Chem., 1979, 83, 696
Rhodamine 101	Ethanol + 0.01% HCI	1.00	600–650	J. Phys. Chem., 1980, 84, 1871
Quinine sulfate	0.1 M H2SO4	0.54	400–600	J. Phys. Chem., 1961, 65, 229
Fluorescein	0.1 M NaOH	0.79	500–600	J. Am. Chem. Soc., 1945, 1099
Norharmane	0.1 M H2SO4	0.58	400–550	J. Lumin., 1992, 51, 269–74
Harmane	0.1 M H2SO4	0.83	400–550	J. Lumin., 1992, 51, 269–74
Harmine	0.1 M H2SO4	0.45	400–550	J. Lumin., 1992, 51, 269–74
2-Methylharmane	0.1 M H2SO4	0.45	400–550	J. Lumin., 1992, 51, 269–74
Chlorophyll A	Ether	0.32	600–750	Trans. Faraday Soc., 1957, 53, 646–55
Zinc phthalocyanine	1% pyridine in toluene	0.30	660–750	J. Chem. Phys., 1971, 55, 4131
Benzene	Cyclohexane	0.05	270–300	J. Phys. Chem., 1968, 72, 325
Tryptophan	Water, pH 7.2, 25C	0.14	300–380	J. Phys. Chem., 1970, 74, 4480
2-Aminopyridine	0.1 M H2SO4	0.60	315–480	J. Phys. Chem., 1968, 72, 2680
Anthracene	Ethanol	0.27	360–480	J. Phys. Chem., 1961, 65, 229
9,10-Diphenyl anthracene	Cyclohexane	0.90	400–500	J. Phys. Chem., 1983, 87, 83

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3.6 Commission international de I'Eclairage (CIE) coordinates

The Commission International de I'Eclairage (CIE) coordinates are a standard way to describe the colour of light emitted by these phosphors. The CIE colour space is a three-dimensional space that describes all possible colours based on three parameters: X, Y, and Z. The X, Y, and Z coordinates represent the relative amounts of red, green, and blue light needed to create a specific colour. The CIE colour space is a useful tool for describing the colour of light emitted by luminescent materials like Eu³⁺ doped SrY₂O₄ phosphors. The CIE coordinates of Eu³⁺ doped SrY₂O₄ phosphors can be determined using photoluminescence spectroscopy. The emitted light is collected and passed through a monochromator to separate the light into its component wavelengths. The intensity of the light at each wavelength is measured, and the CIE coordinates are calculated based on the spectral distribution of the emitted light. Eu³⁺ doped SrY₂O₄ phosphors typically exhibit a near white region luminescence (Fig. 7), with CIE coordinates of according to Table 8. The exact CIE coordinates can vary depending on the specific composition and synthesis conditions of the phosphor.

Fig. 7 CIE coordinate for SrY₂O₄:Eu³⁺ phosphor (0.1–2.5 mol%).

Table 8 CIE coordinates of Eu³⁺ doped SrY₂O₄ phosphor with different concentration

Concentration	x	y	u′	v′	CCT	CRI
0.1	0.4301	0.3801	0.2568	0.5105	2912	85
0.2	0.43	0.3789	0.2572	0.51	2904	85
0.5	0.4297	0.3797	0.2567	0.5103	2916	85
1	0.4306	0.3802	0.257	0.5106	2906	85
1.5	0.4294	0.3793	0.2566	0.51	2919	85
2	0.4226	0.3692	0.2567	0.5046	2951	85
2.5	0.4295	0.3791	0.2568	0.51	2915	85

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3.7 TL glow curve analysis

Europium (Eu³⁺) doped SrY₂O₄ phosphors exhibit thermoluminescence (TL) properties, which make them useful for radiation dosimetry, medical radiation dosimetry, environmental monitoring, and space radiation dosimetry applications. TL refers to the phenomenon where a material emits light when heated after being exposed to ionizing radiation. The intensity and shape of the TL glow curve, which is the plot of the emitted light intensity as a function of temperature, can provide information about the energy and type of radiation absorbed by the material.

Europium-doped SrY₂O₄ phosphor has a linear response to a 20 minutes UV exposure, as seen by the TL glow curve (Fig. 8). Thermoluminescence (TL) intensity rises with increasing europium concentration (Fig. 8 inset), and its wide peak centers about 187 °C (where the trapped electrons are released from the lattice defects), making it an excellent peak for a thermoluminescence dosimeter. Up to 0.2 mol%, the TL intensity rises; beyond that, it falls as a result of the concentration quenching phenomena (Fig. 9a). The TL glow curve for an optimum concentration of 0.2 mol% is shown in Fig. 10, which displays a linear wide peak with varying UV exposure durations at a fixed rate of heating of 2.5 °C per s. The concentration quenching effect causes TL intensity to drop after the 25 minutes UV treatment for 0.2 mol% (Fig. 11a). Fitting the curve for the optimal concentration of 0.2 mol% using the CGCD method yields an excellent theoretical and experimental fit.

Fig. 8 TL glow curve analysis of SrY₂O₄:Eu³⁺.

Fig. 9 (a) SrY₂O₄:Eu³⁺ concentration vs. intensity. (b) Linear fit with a mean error bar of SrY₂O₄:Eu³⁺ for (0.1–2.5) mol%.

Fig. 10 TL glow curve analysis of SrY₂O₄:Eu³⁺ for 0.2 mol%.

Fig. 11 (a) SrY₂O₄:Eu³⁺ dose vs. intensity. (b) Linear fit with a mean error bar of SrY₂O₄:Eu³⁺ for (5–30 min) UV dose.

Fig. 9b showed the linear fit corresponding to its mean error with respect to concentration response. The obtained TL phosphor exhibits a linear fit for different concentration over from 0.1 to 2.5 mol%. TL intensity was decreased linearly when the dose was increased, obtaining a determination coefficient R² = 0.74214.⁷⁵

Fig. 11b showed the linear fit corresponding to its mean error with respect to dose response. The obtained TL phosphor exhibits a linear fit for different concentration over from 5 to 30 min UV dose exposed. TL intensity was increased linearly when the dose was increased, obtaining a determination coefficient R² = 0.59899.⁷⁵

3.7.1 CGCD analysis. TLD phosphor often shows many peaks upon charge carrier (hole or electron) emission. The kinetic factors/parameters (E, b, and s) affect the dosimetric features of TL materials a lot. These parameters will provide significant information regarding the mechanism of phosphor emission. Understanding the kinetic parameters of a TLD phosphor is crucial for producing effective TLDs. An empirical set of equations^41–51 developed by Chen's allows the estimation of these parameters. Using the Glow fit software, the glow curve is deconvoluted (Fig. 12) in order to implement the peak shape approach. In order to calculate the kinetic parameters of the TL materials' glow peaks, we employed the peak shape approach, also known as Chen's peak method.

Fig. 12 CGCD pattern of UV induced SrY₂O₄:Eu³⁺ doped phosphor for optimized UV dose and 0.2 mol% concentration.

Evidently, there are eight wide peaks in the CGCD pattern (Fig. 12), and kinetic parameters are computed for each of those eight peaks in Table 3. After 20 minutes of UV irradiation at varying concentrations of europium ions, the recorded glow curves of the manufactured phosphors reveal a broad peak, suggesting that they are composite in nature. Therefore, the kinetic parameters were deconvoluted and calculated using the CGCD technique. The glow curves of the greatest intensity peak, 0.2 mol% europium ion irradiated with UV for 20 min, were deconvoluted using the CGCD technique using the Glow fit software. The Halperin and Barner formulas provide the mathematical foundation for this computer software. These equations explain the movement of charges between the different energy levels that occur during the process of trap emptying caused by thermal heating. This software calculated the trap level kinetic parameters for each deconvoluted peak. The experimental glow curves were used to fit the theoretically produced glow curves, and the accuracy of the fit was assessed by computing the figure of merit (FOM) associated with each fitting. When FOM values were less than 5%, it was determined that the fits were satisfactory. Current research puts FOM at 2.04%, confirming excellent agreement between theoretically derived and actually observed glow curves.^52,53 Fig. 12 displays the fitted TL glow curves, and Table 3 provides a summary of the CGCD-method-calculated values for the frequency factors (s) and trap depths (E) of captured charges.

The location of trapping levels inside the forbidden gap is referred to as trap depth or activation energy (E), and it plays a significant role in the loss of dosimetry information that had been preserved in the materials after irradiation. The order of kinetics (b) is the process by which detrapped charge carriers recombine with their equivalents. When the trap is modeled as a potential well, the number of times an electron collides with the wall multiplied by the wall reflection coefficient gives us the frequency factor (s). As a result, the trapping parameters of a thermoluminescent material are the foundation for an accurate dosimetry investigation.^54–61

Here, the information about the trap level obtained from the kinetic parameters (Table 9) shows that almost one electron is trapped there because of the value of (shape factor) 0.435 and it shows the general order kinetics (b) and the needed energy to escape one electron from the trap level is high (0.50–0.86 eV), i.e. activation energy (E), which shows the formation of traps is high and stable. The rate of escaping electrons per second, which falls between 2.12 × 10⁹ and 3.66 × 10⁹ S⁻¹, is likewise fairly high.

Table 9 Calculation of kinetic parameters using CGCD programme for UV induced SrY₂O₄:Eu³⁺ doped phosphor

T1 (K)	Tm (K)	T2 (K)	τ	δ	ω	μ = δ/ω	Activation energy (eV)	Frequency factor s^−1
298	321	338	23	17	40	0.425	0.5	2.12 × 10^9
332	359	380	27	21	48	0.4375	0.5175	2.26 × 10^9
366	398	422	32	24	56	0.4285714	0.5458	2.33 × 10^9
398	432	458	34	26	60	0.4333333	0.6118	2.65 × 10^9
420	467	504	47	37	84	0.4404762	0.5011	2.20 × 10^9
466	522	566	56	44	100	0.44	0.523	2.30 × 10^9
552	600	636	48	36	84	0.4285714	0.8559	3.66 × 10^9
587	666	731	79	65	144	0.4513889	0.5823	2.62 × 10^9

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3.8 Conclusion

The present study provides valuable insights into the properties of SrY₂O₄:Eu³⁺phosphors, as summarized in the following conclusions:

(1) XRD analysis confirmed the existence of a single orthorhombic crystalline phase in the newly synthesized SrY₂O₄:Eu³⁺ phosphor, with nanocrystalline behavior.

(2) The SrY₂O₄:Eu³⁺ phosphors exhibited a fine surface morphology with a spherical shape which is confirmed by SEM imaging.

(3) FTIR spectra revealed specific vibration modes of Y–O, Sr–O, and the production of SrY₂O₄ phosphor is confirmed by all of these observed peaks taken together.

(4) Photoluminescence excitation spectra showed a broad excitation centered at 254 nm, with well-resolved emission peaks 580 nm, 590 nm, 611 nm and 619 nm corresponding to transitions at ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₁, and⁵D₀ → ⁷F₂, respectively.

(5) The emission color of SrY₂O₄:Eu³⁺ phosphors was in the near white light region, as shown in the CIE 1931 graph, and exhibited good color tenability. These can act as a white light-emitting phosphor for optoelectronic and flexible display applications.

(6) The TL glow curve demonstrated that the thermoluminescence intensity of SrY₂O₄:Eu³⁺ phosphors increased with increasing UV exposure time and peaked at 187 °C, indicating a good thermoluminescence dosimeter peak.

(7) The CGCD methodology provided a successful theoretical and experimental fit, with kinetic parameters computed for SrY₂O₄:Eu³⁺ phosphor and itshows general order kinetics (b) (Table 9).

Eu³⁺ doped SrY₂O₄ is a good luminescence material due to its high quantum efficiency, narrow emission bands, long luminescence lifetime, and high stability, making it a versatile material for various applications in optoelectronics, such as solid-state lighting and flat panel displays.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

I am grateful to all of those with whom I have had the pleasure to work during research. My supervisor Dr Jageet Kaur saluja and Dr Vikas Dubey has provided me extensive personal and professional guidance and taught me a great deal about both scientific research and life in general.

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