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

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.


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][2][3][4] Since the reduction in the size of display systems, luminescent materials have attracted a lot of research attention. [5][6][7] Applications for smart luminescent materials are widespread, ranging from polymeric optical bers and LEDs to displays, self-emitting devices, and light sensors. [8][9][10][11][12] Achieving durability and adaptability of the electronics and materials used in exible electronic devices without compromising performance is a critical challenge for smart materials. [13][14][15][16][17][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][20][21][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 exible display.
Europium (Eu 3+ ) doped SrY 2 O 4 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 Eu 3+ ions, which are known for their unique electronic transitions. There are several reasons why Eu 3+ doped SrY 2 O 4 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. Eu 3+ -doped SrY 2 O 4 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 specic manufacturing process and conditions.
(2) Color purity: Eu 3+ -doped SrY 2 O 4 : this phosphor typically exhibits a red emission providing good color purity. Y 2 O 3 :Eu: this phosphor is known for its red emission, offering high color purity. Y 2 O 2 S:Eu: this phosphor generally produces a yellowishred 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. Eu 3+ -doped SrY 2 O 4 phosphor is relatively more expensive compared to some other commonly used phosphors such as cerium-doped yttrium aluminum garnet (YAG:Ce) or various sulde-based phosphors. The higher cost can be attributed to the specic composition and synthesis methods required to produce SrY 2 O 4 :Eu 3+ phosphor.
(4) Stability and lifetime: phosphor stability and lifetime are crucial factors to consider in practical applications. Eu 3+ -doped SrY 2 O 4 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. Eu 3+ -doped SrY 2 O 4 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: Eu 3+ -doped SrY 2 O 4 : 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. Y 2 O 3 :Eu: phosphor is known for its high luminescence efficiency and good thermal stability, making it commonly used in red-emitting devices. Y 2 O 2 S:Eu: phosphor also has high luminescence efficiency and good thermal stability. It is oen used in yellowish-redemitting devices.
(7) Application specics: Eu 3+ -doped SrY 2 O 4 : due to its blue-green emission, this phosphor is oen used in applications requiring a specic color range, such as white LEDs and plasma displays. Y 2 O 3 :Eu: phosphor nds extensive use in red-emitting devices like uorescent lamps, plasma displays, and cathode ray tubes (CRTs). Y 2 O 2 S:Eu: phosphor is commonly employed in yellowishred-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 (MY 2 O 4 , M = Ba, Ca, and Sr) have been seen to exhibit prominent optical emission. According to reports, SrY 2 O 4 :RE 3+ (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 exible lanthanides, making it a prime candidate for luminescence applications. SrY 2 O 4 :Eu 3+ (SYO:Eu) has a structure similar to CaFe 2 O 4 and exhibits magnicent down-conversion luminescence. At a wavelength of 611 nm, the very powerful emission line produced by the Eu 3+ long-lived transition 5 D 0 / 7 F 2 may be seen. With the excitation of 254 nm (UV region), europium ions are useful for light emitting phosphor and exible display applications. [24][25][26][27][28][29][30][31][32][33][34][35][36] In this paper, we describe how to easily make new Eu 3+ doped SrY 2 O 4 phosphors, which may be utilized to improve the attractive performance of intelligent (smart) materials for exible displaying applications. SYO:Eu phosphors emit red light when exposed to UV radiation.

Experimental
The preparation of the 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: aer 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 SrY 2 O 4 :Eu 3+ phosphor. Higher temperature was needed to ensure that the reaction was complete and that the product was fully formed.
The resulting specimen, SrY 2 O 4 doped with Eu 3+ , was then allowed to cool to room temperature. The specimen was characterized using various techniques to conrm the desired properties, such as its crystal structure, morphology, and luminescent properties.

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 Eu 3+ doped SrY 2 O 4 phosphors typically shows characteristic peaks corresponding to the crystalline phases present in the sample. The main phase is SrY 2 O 4 , which exhibits aorthorhombic crystal structure. The XRD peaks of SrY 2 O 4 are sharp and well-dened, 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). 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 SrY 2 O 4 phosphor is conrmed by all of these observed peaks taken together ( Table 2).

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 (Eu 3+ ) doped SrY 2 O 4 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).
When SEM is used to study Eu 3+ doped SrY 2 O 4 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).

Energy dispersive X-ray spectroscopy (EDX)
Energy Dispersive X-ray Spectroscopy (EDX) analysis is a valuable tool for studying the elemental composition of Eu 3+ doped SrY 2 O 4 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 SrY 2 O 4 :Eu 3+ phosphor can be synthesized. The qualitative analysis of the process of creating the components is shown in Fig. 4b.

Photoluminescence (PL) observation
Doping with Eu 3+ ions improved the emission spectra of the host SrY 2 O 4 phosphor, which has no luminescence. 23 The SrY 2 O 4 phosphor doped with Eu 3+ 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. Eu 3+ doped SrY 2 O 4 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 Eu 3+ are produced by transitions between 5 D 0 / 7 F J (J = 0, 1, 2). 38 The brightest emission band can be seen at 611 nm, which is associated with the electric dipole transition 5 D 0 / 7 F 2 (ref. [23][24][25]. The 5 D 0 / 7 F 0 and 5 D 0 / 7 F 1 magnetic dipole   transitions 60 give rise to the 580 nm and 590 nm bands, respectively. Due to similar ionic radii, the emergence of the 5 D 0 / 7 F 1 transition supports the substitution of Eu 3+ ions at Y 3+ sites. 39 The minor separation of the 5 D 0 / 7 F 1 and 5 D 0 / 7 F 2 transition lines in the emission spectra of the SrY 2 O 4 :Eu 3+ phosphor amply demonstrates the considerable inuence 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 Eu 3+ 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 Eu 3+ ions move in closer proximity to one another and interact with one another to transfer charge. Eu 3+ doped SrY 2 O 4 phosphors using 2.0 mol% had the highest   Table 6). The uorescence quantum yield (f F ) is a measure of the efficiency of uorescence, representing the ratio of absorbed photons to emitted photons through uorescence. It quanties the likelihood of the excited state being deactivated through uorescence rather than non-radiative mechanisms. The comparative method developed by Williams et al. is considered the most reliable approach for determining (f F ). This method involves utilizing well-characterized standard samples with known (f F ) values, enabling accurate measurement and comparison of uorescence efficiencies. 62

Number of emitted photons Number of absorbed photons
where the h denote the refractive index of the sample. The uorescence quantum yield (f F ) is a parameter that measures the efficiency of uorescence by comparing the number of absorbed photons to the number of emitted photons. To determine f 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 uorescence quantum yield can be accurately calculated (Table 7).

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   (Fig. 7), with CIE coordinates of according to Table 8. The exact CIE coordinates can vary depending on the specic composition and synthesis conditions of the phosphor.

TL glow curve analysis
Europium (Eu 3+ ) doped SrY 2 O 4 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 aer 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 SrY 2 O 4 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 xed rate of heating of 2.5°C per s. The concentration quenching effect causes TL intensity to drop aer 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 t. Fig. 9b showed the linear t corresponding to its mean error with respect to concentration response. The obtained TL phosphor exhibits a linear t 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 R 2 = 0.74214. 75 Fig . 11b showed the linear t corresponding to its mean error with respect to dose response. The obtained TL phosphor exhibits a linear t 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 coef-cient R 2 = 0.59899. 75 3.7.1 CGCD analysis. TLD phosphor oen 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 signicant 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 t soware, 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.
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. Aer 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 t soware. The Halperin and Barner formulas provide the mathematical foundation for this computer soware. 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 soware calculated the trap level kinetic parameters for each deconvoluted peak. The experimental glow curves were used to t the theoretically produced glow curves, and the accuracy of the t was assessed by computing the gure of merit (FOM) associated with each tting. When FOM values were less than 5%, it was determined that the ts were satisfactory. Current research puts FOM at 2.04%, conrming excellent agreement between theoretically derived and actually observed glow curves. 52,53 Fig. 12 displays the tted 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 signicant role in the loss of dosimetry information that had been preserved in the materials aer 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 reection 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][55][56][57][58][59][60][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 × 10 9 and 3.66 × 10 9 S −1 , is likewise fairly high.

Conclusion
The present study provides valuable insights into the properties of SrY 2 O 4 :Eu 3+ phosphors, as summarized in the following conclusions: (1) XRD analysis conrmed the existence of a single orthorhombic crystalline phase in the newly synthesized SrY 2 O 4 :Eu 3+ phosphor, with nanocrystalline behavior.
(2) The SrY 2 O 4 :Eu 3+ phosphors exhibited a ne surface morphology with a spherical shape which is conrmed by SEM imaging.
(3) FTIR spectra revealed specic vibration modes of Y-O, Sr-O, and the production of SrY 2 O 4 phosphor is conrmed by all of these observed peaks taken together.
(5) The emission color of SrY 2 O 4 :Eu 3+ 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 lightemitting phosphor for optoelectronic and exible display applications. (6) The TL glow curve demonstrated that the thermoluminescence intensity of SrY 2 O 4 :Eu 3+ 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 t, with kinetic parameters computed for  SrY 2 O 4 :Eu 3+ phosphor and itshows general order kinetics (b) ( Table 9). Eu 3+ doped SrY 2 O 4 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 at panel displays.

Conflicts of interest
There are no conicts to declare.