Enhancing optical properties of WLEDs with LaOF:Eu&SiO2 application

Received Feb 24, 2021 Revised May 20, 2021 Accepted Dec 1, 2021 After many efforts, the core-shell nanostructure of LaOF:Eu&SiO2 that emits bright red radiation can be fabricated by simple solvothermal application succeeded by heat treatment. The resulted particles from the fabrication process are small in size, able to demonstrate circular form more efficient and prevent stacking. Emissions spectra of photoluminescence (PL) that display strong peaks of 593 nm, 611 nm, and 650 nm are D0 —>FJ (J=0, 1 and 2), respectively, Eu3+ transitions. Judd-Ofelt (J-O) theory estimates spectral strength parameters and Eu-O ligand behavior. It has been noticed that CIE co-ordinates are very similar to regular NTSC values (x=0.63, y=0.36). Besides, the correlated color temperature value can be observed at below 5000 K, which is 3475 K, and thus it is ideal for warmlight diodes. A fluorescent marker for identification of latent fingerprints was used with the tailored core-shelled SiO2 (coat III)@LaOF:Eu3+ (5 mole %) on porous and non-porous surfaces. The results of fingerprint have high sensitivity, selectivity and also has no obstruction caused by the back-ground which supports level-I to level-III fingerprint ridge recognition. The experiments outcomes suggest that the enhancements brought by the coreshell NS structure can be further examined to apply in forensic and solid state lightning applications.


INTRODUCTION
Regarding efforts to increase the quality of lighting devices, researchers have focused on core-shell structures, which have a number of novel features. There are many models that were applied ranging from hard to soft to create the composition. Despite the variation of models, silica is a material that was consistently used to create core-shell structure because of the changeable dimension [1]- [4]. The core-shell structure fabrication is complete when the layers of shell particles wrapped around the silica centers and generate a spherical core-shell structure. The additional adjustments made to the testing environment and subject production can help achieve non-accumulate circular composition particles. These particles will reduce the light scattering while enhance the intensity and quality of light. In certain fields such as entertainment showcase, bio-medical imaging and anti-counterfeiting applications, these qualities can worth great values [5]- [12]. The rare earth (RE) in composition with nanophosphors can demonstrate acute inception and radiation spectra in UV-Vis region with great amount efficiency, extended working duration, stable chemical composition, heat, and light display, environment friendly and minimal waste. In recent studies, the quantum yield of Sr4Al14O25 [13]; Ca14Al10Zn6O35 [14]; CaAl12O19 [15]; Sr4Al14-O25:Mn 4+ , Na + ,  [16] is estimated to be 38%, 50%, 35.5%, 60.8% under 450 nm blue light excitation, and 166% respectively [17];. The trivalent luminous lanthanide (Ln 3+ ) ions, which have a comparable ion radius to composite particles, are reasonably easy to mix to the particle configuration and provide good lighting performance even at ambient temperature [18]- [21]. The most notable ion in RE particles' research is Eu 3+ , which can act as the best material for the creation of red light [22]. Until recently, oxides, phosphates, vanadate, molybdates, borates and fluorides have been proposed as host to add in Eu 3+ ions. From the doping results, it is confirmed that rare-earth fluorides are the most suitable for integration of various optical active Ln 3+ ions. The qualities of these substances are high index of refraction, great ionicity low phonon energy, high durability and ionic conductance, resulting in lower non-radiative decay and higher quantum efficiency [23]. The adjustment of exterior layer has resulted in the most changes to the range among luminous and quenching centers, which contribute to the reduction of surface blemishes and nonradiating channels. Regarding all the effects, the core-shell nanostructure is expected to benefits the lighting performance WLEDs [8], [24]. The RE that is integrated with nanomaterials having particle size smaller than 100 nm have been studied especially in latent fingerprints (LFPs) recognition; however, the low fluorescent imaging is the main issue for the application of these materials [25], [26]. Therefore, the importance of innovating an uncomplication, cost savings, non-detrimental yet precise process to visualize the fluorescent powders of LFPs with features of non-toxicity, highcontrasting chemical and thermal steadiness, super selectivity, minimal interference of background, and high performance in natural environments.
Because of the boundaries pose by the present imaging instrumentation and techniques, LFPs identification is mainly from the primary tool of recognition, analyzing the level II configurations (for example ridge competition, dichotomy, and intersection). The amount of level II properties set must in between 6-17 to ensure the precision of the examination. The problem is the level II configurations on fingertips locate randomly, which demands the size of the finger print images to be large so the result can be more accurate. The LFPs in some cases do not resemble all the properties or missing details after being gathered, which clearly means that factors other than level II of fingerprints must be added to improve the identification process. This need leads to the analyzation of level III structures which has the sweat pore, a persistent trait on the fingertips, unchangeable and distinct, that can improve the result of identification. In current research, researchers developed a core-shell amalgam of LaOF:Eu 3+ &SiO2 phosphors as a distinctive fluorescent tag for displaying level I to level III crest layouts on various outside platforms. Many researches are being conducted to analyze innovative optical properties or improve performance to apply in more advance usage such as anti-counterfeiting and forensic.

PREPARATION AND SIMULATION 2.1. Material preparation
The material preperation process for the composition composes of mixing and firing the components. First, the ingredients are mixing by adding them into water or methanol. Allow the mixture to dry in the air after it has been thoroughly combined, and then crush it into powder. Then prepare a capped quartz tube and fire the mixture at 1000°C for an hour with N2. After that, grinding the material chunk into powder before proceeding with the next firing step. Continue to place the powder on capped quartz tube and fire for another hour, this time at 1200°C. After the second firing step, the product is complete and ready to be collected. The obtained material emits red radiation with emission peak at 1.981-2.145 eV. The chemical composition of the compound is presented in Table 1.

Simulation
The LightTools 8.1.0 simulation program as well as the Mie scattering theory are crucial tools for modeling WLEDs with double layers of phosphors. The Mie-principle also enables the investigation of phosphor particle scattering as well as the investigation of LaOF:Eu 3+ &SiO2 phosphor effects on WLEDs with correlated temperatures ranging from 5600 K to 7000 K. The illustration of the WLED model with specific arrangement of the LaOF:Eu 3+ &SiO2 and YAG:Ce 3+ layers is shown in Figure 1. The WLED phosphor package includes the silicone glue, and the two phosphors of LaOF:Eu 3+ &SiO2 and YAG:Ce 3+ .

Scattering computation
According to Mie scattering theory, the scattered parameter μsca(λ), anisotropic component g(λ), and decreased scattered parameter δsca(λ) are likely written as the following formulas: N(r) denotes the intensity of allocations for spreading phosphor grains (mm3), Csca denotes the dispersing cross sections (mm 2 ), p(θ,λ,r) demonstrates the phase purpose, whereas θ, λ, r means respectively the lighting beam wavelength (nm), the diffusional spheres diameter(µm), the dispersing anglular position (°C), while f(r) here indicates the diffusional particles scale dispersed ability within WLED's phosphor film. Besides, the f(r) calculation is demonstrated: Diffusive and phosphor densities, Ndif(r) and Nphos(r), are included in N(r). The diffusive and phosphor dimension distribution function values are given by fdif(r) and fphos(r), respectively. KN is the amount of diffusor units for a specific diffusional-particle concentration and can be measured as: M(r) denotes the diffusor segment's dispersion volume, which is calculated using the formula: The densities of diffusional ions and phosphorus crystals are represented by ρdiff(r) and ρphos(r), respectively. By applying the Mie theory, Csca computation is performed as: With k=2π/λ, where the calculation of an as well as bn can be shown as: With x=k.r, while the indicator of refraction is denoted by m, ( ) together with ( ) represent the Riccati-Bessel function.
In which ( , , ), the angular scattered magnitudes include S1(θ) with S2(θ) whose computations are shown as the: With kexi=462 nm, the PL spectroscopic radiation density was examined via the similar structure as the item except the concealing sheets. When the covering layer is present in the system, the radiation rate is increased by a factor of three. This effect on the rate of core-shell NS emission contributes to the theory that non-radiative positions on SiO2 core surfaces are removed by the LaOF:Eu 3+ shell. The missing surface -OH groups in core-shell structure is a result of coating layer on the radiative relaxation pathway. The surface -OH groups is still essential to the PL quenching even after the reduction of emission intensity from III coating. Other than the intensities, LaOF:Eu 3+ &SiO2 have the equal peak region as well as emission parameters. This result verifies the boosted intensity of LaOF:Eu 3+ &SiO2 with NS coating effect of offered by the amorphous silica film, which can be presented as Figure 2. Figures 2 and 3 show that when SiO2 is present at 450 nm, the scattering is greater than at 550 nm. This means that the blue light component is dispersed more strongly when SiO2 is introduced. This boosts luminous flux and improves color homogeneity. The scattering enhances in the phosphor layer, the rays are more uniformly combined, and the resultant color becomes more consistent. The yellow ring phenomena, in other words, is greatly decreased. The blue photons are dispersed more forcefully by the SiO2 particle, resulting in more light being emitted. These show that the results obtained in section 3 are correct. By using the McCamy imperial equation, the CCT indicators of the un-coated is estimated to be 1854 K and coated sample is 2332 K, which is inadequate for the average lighting requirement of warm white light, see Figure 3.

RESULTS AND DISCUSSION
Usually, fingerprints (FPs) total ridge patterns can be obtained using a fluorescent powder that was mostly applied when investigating a scene of the crime. The customized LaOF:Eu 3+ &SiO2 NS is then discolored on the LFPs that are created from the typical and 254 nm ultra-violet light on a range of hard surfaces, such as metal sheets, aluminum foils, blades, or staplers, to test the quality of the latent FP (LFP) picture collection. Under standard white radiation, the expression of II ridgeline characteristics is optimal; 147 however, the representation is enhanced by 254 nm ultra-violet lamp. The background interference analyzation is conducted by placing tailored LaOF:Eu 3+ &SiO2 NS upon the porous platforms of outside surroundings such as the newspapers, playing cards, currency note or the covers of several magazines. All of these experimented objects are illuminated with normal white-emitting light and ultra-violet light of 254 nm. FPs resultsvisualized achieved with UV light display have better properties than the other illuminating source. The tailored LaOF:Eu 3+ &SiO2 NS is also stained beneath white-emitting light and ultra-violet light of 254 nm to investigate more on the specific ridge features of single and over-lapped complex fingerprint pictures. The diversity of level II ridge patterns is fully expressed and without any obstruction in both cases. These outcomes and the content of Figure 4 support the idea that LFPs recognition on porous and non-porous outside surroundings can be performed with optimized powders. Fluorescent powders performance and different overlapped compound LFPs on the surface of a aluminum foil are verified smearing the LaOF:Eu 3+ &SiO2 NS illuminated with luminous sources of normal white-emitting light and ultra-violet light of 254 nm. Results from the graph show that viewer can observe ridge details from I-III below usual as well as ultra-violet light of 254 nm. This is the evidence that using phosphor powders with the right adjustment can serve as fluorescent labeling agent to detect fingerprints. In addition, fingerprints from many sources are gathered and applied processed LaOF:Eu 3+ &SiO2 NS by coating the composite dust below white light like UV light of 254 nm. Each of three prints have different ridges of configurations: center coil, pouch coil, with ulnar arched shape. For a better outcome, high-quality FPs photographs are obtained after a few days of rest. Figure 5 depicts the impacts of LFPs staining on the surface of tinfoil using LaOF:Eu 3+ &SiO2 NS during a period of 1-9 months. The FPs stains with LaOF:Eu 3+ &SiO2 NS have excellent consistency for both new and old patterns; the reason for this is due to the high similarity even after ageing latent fingerprints. The growing pace of research on anticounterfeiting technologies has increased recently to replace conventional methods such as watermarks, holograms, and barcodes, which have become obsolete due to the simplicity of the technique and the risk of being replicated. To strengthen security and deter counterfeit, more advanced technology is being deployed, such as plasmon access controls, biocompatible microscopic handprints, and magnetic sensitivities. Yet, the establishment cost for accurate and modern devices for these methods is counter-intuitive to the safety and cost-saving points of view. Therefore, using the illuminating method to aid the security concern and reduce the cost is the more beneficial option, which is currently being researched for anti-counterfeiting field. The most typical subject for showing luminescent graphics in ultraviolet (UV) illumination is bank notes. Other nanomaterials have been utilized to provide lighting, such as organic dyeing chemical, carbon ions, and semiconducting components; however, waste generation during long-term use or extensive radiation bands prevent this approach from being widely used. The lanthanidedoped nanostructured phosphors, on the other hand, provide outstanding spectral fingerprints with many other desirable properties, such as duplicate proof, low waste emission and high working expectancy has led to their opportunity for employment in anti-counterfeiting field, which can be seen from Figure 6.
The ink's luminous foundation makes it suitable with a wide range of printing devices, including dip-fountain pen written work, printmaking, gravure printing, and letterpress publishing. Figure 7 shows the printed text produced by a dip pen using tailores LaOF:Eu 3+ &SiO2 NS ink as digital form 254 nm ultra-violet light. The Figure shows distinct and clear images under UV light, which would normally be quite blurry under visible light. The discussed information has proven that LaOF:Eu 3+ &SiO2 NS are effective and advantageous in many factors if applied to the anti-counterfeiting field.

CONCLUSION
The solvothermal process was applied for simple and efficient fabrication of the LaOF:Eu 3+ &SiO2 NS with core-shell structure. After analyzing LaOF:Eu 3+ with SiO2 using PXRD, it is confirmed that the maximum value of pure tetragonal phase is at 22L. Under the experiments, the influences of Eu 3+ concentration on the energy band discrepancy and refractivity were also discovered, although the exact amount is yet to be determined. In the preparation process of the compound, the oxygen disfigurements on the surface were detected from the EPR conclusion. The TEM outcomes confirmed the density of LaOF:Eu 3+ &SiO2 shell is 8 nm. The NS layer improves the energy transmission of atoms three times by eradicating the captive central area on the surface. The PL enhancement effect is linked to the number of coating layers, with more coating layers means stronger enhancement. The lighting capacity is estimated to increase 56.7% with three layers of LaOF:Eu 3+ &SiO2 NS shell. The LaOF:Eu 3+ &SiO2 NS exhibits advanced lighting performance that is sufficient to be used as label factors for LFPs identification on many permeable and semi-permeable object surfaces through dust brushing method. The improved lighting performance comes from the better sticking feature, which allows less air to pass through and oxidizing the compound. The visualization of LFPs using optimized NS achieves greater resolve, more thorough fingerprint ridges, simple light capture, and less rear interference than the current substance used. The subject of this study matter can also be used as a guide to help in anti-counterfeiting applications. Because of its distinct and beneficial properties, core-shell NS is now used in color shows, forensic, and protection systems.