White Light-Emitting Diode Coated with K3Lu(PO4)2: Tb3+, Eu3+ Nanocrystal Films Enveloped by SiO2

A series of novel adjustable-illumination including K3Lu(PO4)2: Tb3+, and Eu3+ (KLPO:Tb,Eu) phosphors with improved energy-transfer efficiency were made using strong-heat solid-state (SS) reactivity. X-ray diffractivity (XRD), luminescence spectroscopy, chromatic coordination, and fluorescence decomposition analyses were used to describe the materials. The first-principles method during the analysis was used to investigate power band structure as well as denseness for statuses in K3Lu(PO4)2. The result showed that the doping dose ratio for Tb3+/Eu3+ was found to be the main factor controlling the energy transmission (ET) between Tb3+ and Eu3+ elements. The ET between Tb-Eu was governed by an electric dipole-dipole interaction which produced tunable emissions from green to orange-red regions. During the study, a 365 nm chip was incorporated with BaMgAl10O17: Eu2+ blue phosphor, SiAlON:Eu2+ green phosphor, and K3Lu(PO4)2:0.1Tb3+,0.06Eu3+ (KLPO:0.1Tb,0.06Eu) orange-red phosphor to create a triple-phosphor-based white light-emitting diode (WLED). The device generated white light featuring a great chroma rendition indicator reaching 91.4 as well as an acceptable associated chroma temperature reaching 3678 K. These properties showed that the developing phosphor was suitable for warm WLED applications, as the performance was comparable to those of perfect white light.

Eu3+; Multi-color tuning; Power transition; SiO2; Tb3+; Warm WLED

White light-emitting diode units (WLEDs) are semiconductors that may become transformed into phosphorus and have several exceptional qualities, including high energy converting effectiveness, color tunability, long lifespan, small size, environmental friendliness, as well as dependability (Tung et al., 2024). These qualities enable the devices to effectively replace traditional incandescent and fluorescent lamps. Following the discussion, LED has two different types of parts. The first is near-ultraviolet (UV) or blue LED chip, where other consists of phosphors made of rare-earth replacement substances (Anh et al., 2024). The total light-emitting effectiveness, color rendition, and heat stability of the resulting white illumination are therefore all determined by phosphor, allowing it to be a crucial element (Tung et al., 2024). Up until recently, adding Y3Al5O12:Ce3+ (YAG:Ce) to cover LED chips in blue was the easiest and most popular way to form commercial WLED (Anh et al., 2024; Le et al., 2024). The white illumination generated by this device has color flaws in green and red zones, detrimentally impeding practicality as well as leads to a poor color rendering intent lower than 80 as well as elevated correlated color temperature (CCT) greater than 4500 K (Tung et al., 2024). Increasing the proportion of red emissions is necessary to improve the efficiency of WLED (Huu and Thi, 2022). Additionally, the supply of phosphors in the market has an inadequate red bandwidth, which dramatically lowers CCT and color rendering intent (CRI). The development of the red luminescent phosphor suitable to be coupled for making LED with outstanding CRI and acceptable CCT is an extensively investigated topic in this direction. Under such states, there is a necessity for examining three-color illuminating phosphor samples proficiently roused by near-UV radioactivity and satisfy the specifications of white illumination industrial LED devices (My et al., 2022).

Rare-earth (RE) ions are crucial in current screen lighting, photodetection, optic amplification, and further relevant applications due to the exceptional luminous qualities as well as distinct emitting bands (Le et al., 2022). Based on common knowledge, illumination of centric RE elements results from the proficient ET between the trinity status for ligand and the crystalline field statuses (Tran et al., 2020). Therefore, ET has a significant impact on both theory and practice when it comes to the color tuning of phosphors (Tran et al., 2020). Relating to the discussion, Tb3+ element would be the activator with highest utilization frequency within phosphor samples. Based on the dopant amount, the emission results from 5D3–7FJ transition within blue zones or 5D4–7FJ mechanism (J = 6–2) within green zones (Thai et al., 2023). The interactivity among Tb3+ element shows greater potency under surging Tb3+ presence, leading to a cross-alleviation amid 5D3 as well as 5D4 levels and the induction of 5D47FJ shift featuring primarily green radioactivity (Zhang et al., 2022; Lui, et al., 2019). Moreover, the red component in disparate Tb3+ and Eu3+ joint-integrated phosphor samples can be compensated for by magnetism dual-polar shift between 5D0 and 7F1 as well as electrical dual-polar shift between 5D0 and 7F2 in Eu3+ element, respectively (Mohamed et al., 2020). The interactivity from phosphate categories (PO4)3- in orthophosphate variants comprising functioning elements, including Eu3+ as well as Tb3+, leads to extremely high power-converting effectiveness in addition to effective UV photon energy absorption (Wang et al., 2017). This qualifies phosphate as the main component in near-UV white-illumination LED.

According to this study, a novel high-temperature solid-state KLPO:Tb,Eu phosphor with controllable illumination emission was created. The latticework configuration as well as electron characteristics for the base KLPO were modeled using Density functional theory (DFT) method in Vienna Ab-initio simulation Package (VASP) program because of the dearth of data on the luminous properties of KLPO phosphors. In addition, several analytical methods, including luminescence spectroscopy, X-ray diffractivity (XRD), and photoelectron spectroscopic assessment (XPS) were used for clarifying the luminous characteristics of the KLPO:Tb,Eu. The outcomes showed the promise of KLPO:Tb,Eu for UV WLED color tunability.

2.1. Substances and combination

K3Lu1-x-y(PO4)2 series: xTb3+, yEu3+ Phosphors were formed using traditional high-temperature solid-state process (x = 0, 0.07-0.13; y = 0-0.26). The raw ingredients were stoichiometric high-purity, including Lu2O3 (99.99%), NH4H2PO4 (99%), Tb4O7 (99.99%), and Eu2O3 (99.99%), which made it possible to prevent future cleansing before use. During the process, the granules were well combined and afterward mixed in an agate mortar. Mixtures were then heated again for 4 hours at 800 °C, and sintered in a muffle for 4 hours at 1150 °C. The finished specimens were crushed into fine powders after being cooled to room temperature for further characterization (Kumar and Nishchal, 2019).

2.2. Manufacturing a model of LED

The commercial SiAlON:Eu2+ green powder and BaMgAl10O17: Eu2+ blue powder BAM were combined with KLPO:0.1Tb,0.06Eu orange powder in a proportion of 10: 1: 2. The powdered compound was mixed unto UV chip (1 W, ex = 365 nm) as well as OE6550 silica gel as a fixing agent to create a WLED. Figure 1 showed the technical details of WLED during the process.

2.3. Characterization 

At room temperature, XRD was used to analyze the phase element and clarity of numerous typical powder specimens. The X-ray diffractivity profile was obtained in the range of 5 2 80 applying a diffractometric apparatus operating at a voltage of up to 40 kilovolts and a current reaching 30 milliamperes, using one Cu K pipe in form of luminous means ( = 0.15406 nm) (Thi et al., 2023a). During the process, the Rietveld rectification was conducted via GSAS program. The chemical composition as well as elemental valent property were ascertained via XPS utilizing one ESCALAB Xi+ electron spectrometer. The fluorescence spectrophotometer PL system Hitachi F-4700 was used to collect the luminescence stimulation/emission (PLE/PL) spectra, and the system was fitted with a 400 V, 150 W Xe lamp. Additionally, a spectrofluorometer with a 150 W xenon illumination supply was used to calculate the quantum yield (Thi et al., 2023b). An FS5 spectrofluorometer fitted via one 150-W CW ozoneless Xe arc light captured the normal-temperature fluorescence decay curves (Reyes-Alberto et al., 2023; Gong et al., 2021).

Figure 1 Picture showing utilized WLED: (a) WLED used in the research herein, (b) Tethering setting, (c) Graphic depiction for WLED, (d) WLED recreation using LightTools program

The analysis created spherical, almost mono-disperse silica particles using Stober method (Wang and Sun, 2020). In previous studies, water and TEOS were used as a couple of the reactants, methanol or ethanol as the solvent, and ammonium hydroxide as the catalyst to produce silica nanoparticles with diameters ranging from 10 to 450 nm (Mohan and Negi, 2024; Shaban et al., 2020; Chan et al., 2010). As the dosages for TEOS as well as NH4OH decreased and that of water increased, the particle size declined. Nearly monodispersed silica nanoparticles with diameters ranging from 50 to 450 nm could be generated as ethanol was employed in the form of the solvating agent. Consequently, when methanol was used, silica nanoparticles with dimensions ranging from 10 to 50 nm could be produced. Due to variations in nucleus size generated in each solvent, the particle size varied depending on the solvent used. Virtually monodispersing and globular nanogranules were formed when particles exceeded 25 nm in diameter. On the other hand, main silica nanoparticle aggregates with a network topology smaller than 25 nm developed. The reaction temperature and method were adjusted, as shown in Figure 2, to lessen the size of the silica nanoparticles as well as prevent aggregate formation. Figures 2(a)-(e) corresponds to disparate silica nanoparticle sizes, including 5%, 10%, 15%, 20%, and 25%, respectively.

The power gap was estimated using Kubelka-Munk function to further distinguish when KLPO was an indirect or direct band gap substance.

       (1)

Where h was the photoenergy, R represented reflectivity, and Eg signified optic band gap power. The combination was considered one straightforward band gap agent with n equal to 2. Following the discussion, the non-direct band gap was signified via n = ½.

The emitting color was adjusted from Tb3+-integrated samples using an effective CR procedure among identical Tb3+ ions in extra to ET among various Tb3+ as well as Eu3+ ionic granules. The procedure used for Tb3+ during the analysis was explained as follows (Yu et al., 2021).

      (2)

The process was performed since the energy gaps between 5D3-5D4 (5725 cm^-1) and 7F6-7F0 (6000 cm^-1) stated were comparable. When the positions of Tb3+ ionic granules were close enough, a resonation ET was probably to occur, in which the transfer of electrons from 7F6-7F0 was followed by the relaxation of electrons from 5D3 to 5D4 (Zhou et al., 2021).

Figure 2 Number density of K3Lu(PO4)2:Tb3+,Eu3+ phosphor particles: 5%-25%

Based on the process, a dual-exponential decay function matched the deterioration arch well (Huang et al., 2019):

      (3)

Where I(t) represented the Tb3+ emitting strengths at the delay period of t,1, and2 were the exponential component's quick and slow fluorescence durations, and t signified the delay period. Additionally, A1 and A2 represented the particular constant values Calculations for the mean duration (*) were as follows (Francois et al., 2021):

      (4)

Figure 3 showed YGA:Ce phosphor dose based on SiO2 particle sizes. When SiO2 particle size was 25 wt.%, YGA:Ce dosage hit the lowest point at 24.5%. Consequently, YGA:Ce dosage reached the highest points at 27.5 - 28.5% when the particle sizes were approximately 5 wt.%. During the process, YGA:Ce dosage increased when the particle sizes were reduced. Figure 4 showed the color variations (CCT) depending on SiO2 particle sizes. CCT value reached the peak at over 3150 K when the particle size was 5 µm and achieved the lowest point at ~2950 K when the particle size was 15 µm. Moreover, CCT value stayed quite steadily at ~3000 K with particle sizes of 25 µm. The value would be greatest with the appropriate particle sizes of 5 µm.

Figure 3 The dosage of YGA:Ce phosphor according to SiO2 concentration

Figure 4 CCT variations according to SiO2 concentration

Equation (3) and (4) were used to find the precise values of A1, A2, 1, 2, and the mean decay period. When Eu3+ content increased, the emitting lifespan of Tb3+ reduced monotonically, supporting the existence of an active additional decomposition channel and ET of phosphor from Tb3+ to Eu3+. In Tb/Eu ET procedure, the transmission probability (PTb-Eu) was represented as follows (Liu et al., 2020).

      (5)

Where x and 0 were the lives of Tb3+ both being presented and not of Eu3+ for the equal sensitizer doses, accordingly. The process showed a plot of the computed degradation periods and PTb-Eu values against Eu3+ doses. Moreover, the following formula was used to compute the power transition effectiveness (T) from Tb3+ to Eu3+ ions (Bui et al., 2020). 

      (6)

Where x sgnified the lifespan for Tb3+ within the joint-doped Tb, Eu samples, 0 was     the radioactive deterioration lifespan for Tb3+, while T was the energy transition efficient. The transference performance increased from 26.93% towards nearly 100% as Eu presence surged, which surpassed the bulk of Tb/Eu co-doped value of phosphors.

Figure 5 showed color deviation variations (D-CCT) according to SiO2 particle sizes. D-CCT values reached the peaks at 180–220 K when the particle sizes were up to 5 wt.%. The value hit the smallest point at 40 K when the particle size was 25 wt.%. Accordingly, D-CCT value was greatest with the suitable particle sizes at ~5 wt.%. Figure 6 showed the light-emitting beam by LED during the analysis. When the particle sizes were 0 – 10 wt.%, there were maximum lumen values at nearly 73.5. On the other hand, lumen value was around 71 when the particle size was 15 wt.%. The particle sizes 0 – 10 wt.% were the greatest for lumen output during the process.

Figure 5 Investigation of the color deviation changes according to SiO2 concentration

Figure 6 The luminous flux emitted by LED according to the concentration change of SiO2

Blasse estimated that the crucial separation Rc for ET from Tb3+ to Eu3+ was as follows (Toslak et al., 2020).

      (7)

Where xc was the combined dosage of Tb3+ and Eu3+, N signified the quantity for formulaic elements within a unit cellule, V represented the volume for said cellule. Consequently, since the associated Rc reached 10.274 Å, well surpassing 5 Å, the multipolar interactivity led to ET activity.

The multipole interaction governed the resonance power transition from Tb3+ to Eu3+ in KLPO:Tb,Eu phosphors. Reisfeld's theory and Dexter's formula allowed the study to examine ET process using the following equation (Yang et al., 2019).

      (8)

Is as well as Iso were the illumination strengths for Tb3+ having and not having Eu3+. In line with this process, C represented Eu3+-doping concentration, and n was the interaction types. When n equaled 6, 8, and 10, the interactions were assigned to dipole-dipole, dipole-quadrupole, as well as quadrupole-quadrupole types, respectively. Following this analysis, 0 and were the light-emitting quantum effectiveness (QEs).

The emitting strengths of Tb3+ and Eu3+ were lessened via heat abatement to 68.1% and 84.0% of those at room temperature. During the process, Arrhenius expression was employed for determining activation energy (E) as well as examining the heating consistency (Beldi et al., 2019; Feng et al., 2019).

      (9)

Where I0 and IT were, luminous intensity of KLPO:0.1Tb,0.06Eu phosphor at normal heat as well as a specific heat.

Figure 7 showed the changes of CRI as well as CQS values, respectively. Both CRI and CQS values tended to decrease as the particle sizes increased. CRI value decreased from approximately 56.2 to 55.7 when the particle sizes increased from 0 to 25 wt.%. Similarly, CQS value reduced from about 42.5 to 39.5 as the particle sizes increased from 0 to 25 wt.%. Figure 8 showed the emission spectra of WLED during the analysis. At various phosphor concentrations, a visible emission range with a maximum point around 600 nm was observed. The outcome signified that the luminescence at this wavelength in WLED could improve color properties of the device, including color rendering index (CRI), variation of the angular CCT, and color quality scale (CQS).

Figure 7 CRI (a) and CQS (b) variations according to SiO2 concentration

Figure 8 Spectrum color range according to SiO2 concentration

    In conclusion, a sequence of K3Lu(PO4)2:xTb3+,yEu3+ phosphors with a sole composition and color-adjustable radiation were made via the strong-heat SS reactivity. KLPO host passed through crystallization in a trilateral setting featuring P-3 (147) spatial category, as confirmed by Rietveld refinement data. This host was identified as a direct bandgap substance based on the band structure analysis of KLPO. The band obtained via the structural graph (5.3537 eV) highly resembled its counterpart acquired via the dispersal reflection arch (5.59 eV). Relating to the discussion, luminescence investigations showed that Tb3+ could proficiently sensitize the emission for Eu3+ subject to UV stimulation. The associated ET became subject to regulation via the dipole-dipole process during the analysis. Furthermore, increasing Eu3+ composition allowed ET effectiveness to reach a value of up to 98.36%. As a result, the discharge chroma for KLPO:Tb,Eu samples, ranging between yellow-green and orange-red could be tuned by adjusting Tb/Eu dosage ratio. Finally, the white illumination generated by white LED using near-UV LED apparatuses presented great CRIs (Ra = 91.4) and a suitable CCT when applying KLPO:0.1Tb,0.06Eu phosphor (3678 K). These results showed that KLPO:Tb,Eu was a promising phosphor component for near-UV LED applications.

    The authors would like to thank Professor Hsiao-Yi Lee, National Kaohsiung University of Science and Technology, for providing software and experiment support.

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