Ca8MgY(PO4)7:Eu2+,Mn2+: A Promising Phosphor in Near-Ultraviolet WLED Devices

Through employing the solid-state process under significant temperature, we produced a sequence of single-element radiation-adjustable phosphors Ca₈MgR(PO₄)₇:Eu²⁺,Mn²⁺ or CaMg:EM (R = La, Y), and thoroughly analyzed the crystalline arrangement, photoluminescence (PL), PL excitation (PLE), as well as the degrading period. The findings suggest that Ca₈MgLa(PO₄)₇:Eu²⁺,Mn²⁺ could be a green contender to be used within close ultraviolet-chip white light-emitted diode (WLED) devices. It is possible to change the emitting green hue from Ca₈MgLa_1-xY_x(PO₄)₇ treated with Eu²⁺ to cyan by adjusting the La³⁺/Y³⁺ proportion for the purpose of lowering the crystal field potency. Furthermore, power shift among Eu²⁺ and Mn²⁺ would be used to create a conventional one-stage white CaMg:EM having a CIE hue coordinate shown as (0.330, 0.328). The power transmission process was shown in the form of resonant dipole-quadrupole nature. In addition, we also computed the critical range. The white-emitting single-composition CaMg:EM was effectively coated above one close ultraviolet chip under 375 nm, indicating that it may become a strong choice of phosphor within close ultraviolet WLED devices.

Keywords

Color homogeneity; Double-layer phosphor; Luminous flux; Monte Carlo theory; WLED

     Being used for LCD devices as well as solid-state illumination, white light-emitted diode (WLED) devices have gotten significant academic as well as commercial interest (Yigen et al., 2021; Yu et al., 2021; Liu et al., 2021). In the latest days, the commercialized white LED, which is made by mixing a blue chip of LED with YAG:Ce³⁺ phosphor in yellow, has become a significant part of solid-state illumination (Kumar et al., 2019). This mixture, though, has certain drawbacks, including a poor CRI measured at roughly 70 to 80, as well as a significant CCT measured at 7750 K, caused by a shortage of elements in red (Chen et al., 2019a). Because of their great color-rendering characteristics, WLED devices with three hues may prove effective for improving CRIs (Xia et al., 2019). However, the varied aging rates of each phosphor, as well as the blue illumination’s repeating absorptivity via phosphors in green as well as red, would result in small luminous effectiveness in this formation. One-stage phosphors generating illumination in white activated via ultraviolet or close ultraviolet chip may become a decent solution for such difficulties (Adnan et al., 2021; Jen et al., 2021; Chen et al., 2019b). For the study, we produced multiple CaMg:EM phosphors possessing crystal formation of ?-Ca₃(PO₄)₂ via the solid-state technique under a significant temperature below a reducing environment: 15% H₂ along with 85% N₂. We produced the usual white one-element phosphor having CIE hue coordinates shown as (0.330, 0.328) by altering the ratios between La³⁺ and Y³⁺, as well as Eu²⁺ and Mn²⁺ via a hypothesis concerning crystal field potency, as well as power shift in both, whereas past projects concerning associated mixtures concentrated on power shift or achieved illumination in white through combining a pair of phosphor elements (Sun et al., 2022; Huu and Thi, 2022; Hakim et al., 2021).  
     Our study examined the brightness characteristics for CaMg:EM and the power conversion between Eu²⁺ and Mn²⁺ within Ca₈MgY(PO₄)₇ using PL under excitation wavelengths measured at 334 nm as well as 375 nm, along with the lifetime duration as well as concentration. Eventually, the electroluminescence characteristics of a white LED device were revealed after creating a merger between CaMg:EM and one close ultraviolet chip. 

2.1. Materials characterization

Table 1 Phosphor characteristics and tools used for assessment under room temperature

Characteristics	Tools
Phase purity	X-ray diffractometer, model D-max 2200 from Rigaku (Japan), accompanied by Cu-K? radiation with a wavelength of 1.5405, under 30 kilovolts as well as 30 milliamperes
PL, PLE, and degradation arches	Edinburgh Instruments FPS 920 Time Resolved and Steady State Fluorescence Spectrometers outfitted with 450-W xenon as well as 150-W nF900 lights

2.2. LED manufacturing

        We created the WLEDs by coupling one close ultraviolet chip under 375 nm with the as-yet-unsynthesized phosphors. The gadgets’ spectra of EL (short for electroluminescence) were acquired using a spectrophotometer of EVERFINE PMS-80 UV-VIS-IR at room temperature and a direct current of 0.02 A (Dang et al., 2020; Orudzhev, Abdullaeva, and Dzhabbarov, 2019; Lin et al., 2019).

    The cause for this will be explained more within a segment concerning CaMg:EM. It is possible to estimate the critical spacing among ions of E^u2+ by utilizing Blasse's equation (Kaur et al., 2020).

        R_c denotes the cleavage among the closest ions of Eu²⁺ under x_c. V denotes the unit-cell volume. N denotes the amount of accessible dopant locations in a cell. x_c indicates the critical concentration.

It is possible to calculate the intensity of the crystal field surrounding Eu²⁺ via a formula (Kim et al., 2019).

                                             

        D_q indicates the intensity of the crystal field. Z indicates the anion's valence. r indicates the d wavefunction’s radius. e indicates the charge of electron. R indicates the range separating the center ion and ligands. The replacement of lesser Y³⁺ ions for more La³⁺ ions in the latticework of CaMg:EM generates a longer gap extending from Eu²⁺ to O₂ within the latticework of the host, causing a continual rise of the blue-shift for the discharge between green and cyan. Identical phenomena have been observed in (Ca,Mg,Sr)Y(PO4)7:Eu2+,Mn2+. The incorporated strength rises as the Y³⁺ ion level rises, showing that Ca₈MgY(PO₄)₇ could become a superior phosphor’s host (Kang et al., 2020).

        The CaMg:EM has a PLE spectrum consisting of one wide line under the range between 250 and 450 nm, a result of the 4f⁷ (⁸S_7/2) - 4f⁶5d¹ permitted conversion in the ions of Eu²⁺, showing that CaMg:EM becomes activated via the emitting illumination from several ultraviolet or close ultraviolet chips.

        The experiential equation below, based on Van Uitert, gives an effective match to the emitting maximum for Eu²⁺ and Ce³⁺. This equation can be used to analyze the crystal structure of the host lattice (Liu et al., 2020):

                                                                  

        E indicates the energy location for the rim of the d-band in the rare earth ion (measured via cm^-1). Q indicates the energy location for the lower rim of the d-band in free ions (measured via cm^-1). V indicates the trigger's valence. n indicates the anion amount within the trigger’s surrounding exterior. r indicates the host cation’s radius replaced with the trigger (measured via Å). E_a indicates the electron association for the atoms establishing anion (measured via eV). For this situation, we have a Q value of 34000 cm-1, V value of 2, along with Ea value of 2.19 eV. By employing the said numbers for formula 3, the Eu²⁺ discharge peak may be calculated.

        Generally, if the radiative transfer of energy works, the sensitizer decay period stays unchanged with or without a trigger. It is possible to calculate the lifetime duration using the equation as follows (Nabavi, and Yuksel, 2020):

                                                                        

        In our situation, judging the average degradation period of the sensitizer lowered steadily alongside rising activator concentration, from 1247.98 ns to 777.00 ns, indicating non-radioactive power conversion among Eu²⁺ and Mn²⁺ as the ions of Mn²⁺ take in the power from Eu²⁺ prior to exponential damping. The absorptivity grows in severity as the content of the ions of Mn²⁺ increases. It is possible to assess the power shift performance between Eu²⁺ and Mn²⁺ using the given equation (Xu et al., 2019):                

                                                                                      

        T_so and T_s indicate the degradation periods for the ions of Eu²⁺ with or without the ions of Mn²⁺, respectively. It can be shown that the T rises steadily as the Mn²⁺-doping concentration rises, eventually reaching 0.377 at z = 0.21. The shift of energy from Eu²⁺ to Mn²⁺ appears to be relatively effective based on the reducing decay period and the rising value of T noted earlier in this section, the overlap among the PLE spectrum in Mn²⁺, the PL spectrum of Eu²⁺, as well as the reducing strength in Eu²⁺ under rising Mn²⁺ content described.

        If the exchange interaction works, the formula below will be fitted linearly (Zhang and Yang, 2019):        

                                                         

        C represents the full content for the ions of Eu²⁺ as well as Mn²⁺. n_so and n_s represent the luminescent quantum performance for Eu²⁺ with or without Mn²⁺. The corresponding ratio of degradation period (T_so/T_s) may become useful for approximating the value of n_so/n_s.²⁶ As a result, equation 6 can be stated as follows (Sun et al., 2019):

                                                

        The exchange interaction does not produce a linear relationship (R² = 0.947). As a result of the multiple-multiple interaction, the energy of Eu²⁺ is transferred to Mn²⁺. The following relation can be established for multipolar interactions (Zhao et al., 2021):

                                           

        As previously stated, it is possible to express the formula above with another formula:

                                               

        denote dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interactivities, which assume respective values of 6, 8, and 10. It can be seen that a well-fitting linear relationship was found when value is 8. Therefore, the dipole-quadrupole interactivity may be the major characteristic of the power shift between Eu²⁺ and Mn²⁺.

        The spectral overlap approach is used to measure the critical distance of energy conversion:

                                                                 

        f_q represents the oscillator potency for the trigger (which is 10-10 for Mn²⁺). s indicates the wavelength location for the discharge of sensitizers (Eu²⁺) (measured via Å). E indicates the power related to the shift (measured via eV). F_S(E)F_A(E)dE/E⁴ indicates the normalized spectrum overlay among the Eu²⁺ discharge F_S(E). The Mn²⁺ excitation F_A(E) assumed a value of 0.0340 (eV)^-5. As a result, the critical range in the case of dipole-quadrupole power transmission would be 11.17 Å.

        In order to illustrate the possible usage of CaMg:EM phosphor, we created a LED device made of transmuting phosphor via creating a merger between one ultraviolet chip under 375 nm and CaMg:EM in white powered by a forward-bias current of 0.02 A. The white LED's CIE hue coordinate along with the corresponding hue temperature are (0.337, 0.334) and 5278 K, respectively. Prevention of UV light seeping out is vital when it comes to the preservation of our safety as well as packaging resins. In addition, the spectrum of EL exhibits a small strength for the ultraviolet chip, indicating that CaMg:Em possesses strong absorptivity towards ultraviolet illumination as well as fulfilling the criteria of safety due to the particularly small ultraviolet discharge intensity, unlike earlier studies.

        Figure 1 is the illustration showing the influence of CaMg:EM green phosphor on the concentration of YAG:Ce³⁺. As shown, the green phosphor content grew from 2% to 20%, resulting in a decrease in yellow-phosphor content. This opposition means retaining average CCT levels and changing the illuminating scattering and absorbing factors for WLED devices with a pair of phosphor sheets. When the scattering and absorbing activities are influenced, the luminescence and color production of WLEDs can be modified and enhanced.

Figure 1 Altering phosphor content for the task of retaining median CCT values

Figure 2 The discharge spectra in 5000-K WLED device correlating with CaMg:EM doping percentage

        Figure 2 depicts the influence caused by CaMg:EM concentration on the emission power in the WLED device. By judging the production demands, we can pick an appropriate option. The WLED devices demanding good hue quality would slightly decrease illumination. From the line graph in Figure 2, the peak spectral power values are located at ~450 nm and 550-600 nm. Besides, the wide-band wavelength of 500 nm - 640 nm is also presented in this figure. These two findings suggest that the presence of CaMg:EM phosphor helps increase the illumination intensity of the WLED, especially for the blue (450 nm) and green-yellow (550-600 nm) spectral regions. The wide-band of 500 nm - 640 nm additionally indicates the addition of orange spectral energy to the white-illumination wavelength, thus, the color performance can be improved. Moreover, the rise of emission power implies that the blue-illumination dispersion and transmission are better with the addition of green phosphor CaMg:EM. Thus, the lumen output of the WLED when varying CaMg:EM concentration is presented to examine this state, as shown in Figure 3. The graph points out that the light ray emitted rises dramatically when CaMg:EM content rises from 2% to 20%.

        The hue divergence was considerably reduced alongside CaMg:EM content under every CCT value, shown by the results from Figure 4. This can be explained by the green phosphor layer's absorption. When CaMg:EM green phosphor takes in LED-chip blue illumination, it changes the blue lighting into green lighting. The CaMg:EM granules will take in yellow light as well. On the other hand, due to the absorptivity qualities in the material, the blue-illumination absorption would be greater than that of the phosphor-emitted yellow illumination. As a result, the increasing CaMg:EM amount boots the presence of green light energy in the WLED, and as such, the hue consistency is augmented. Color uniformity (lower hue-divergence level) is one of the critical features of today’s WLED devices. Obviously, if hue consistency rises, the cost of WLED will rise as well. The advantage of employing CaMg:EM is its inexpensive cost. CaMg:EM can therefore be widely employed.

Figure 3 The luminous flux in WLED device correlating with CaMg:EM doping percentage

Figure 4 The color deviation in WLED device correlating with CaMg:EM doping percentage

Figure 5 The color rendering indices (CRI) in WLED device correlating with CaMg:EM doping percentage

Figure 6 The color quality scales (CQS) in WLED device correlating with CaMg:EM doping percentage

       In addition to the hue homogeneity, CRI and CQS are other important metrics for the color production of WLED light. Significant CRI alone does not define hue quality. As a result, subsequent studies provide a CRI as well as CQS. If light shines on the CRI, it determines the entity’s real hue. The color imbalance will be generated by a surplus of green lighting hue over other crucial hues of blue and yellow, which notably decreases the WLED light’s color fidelity (Li et al., 2020; Guo and Chi, 2020). Figure 5 shows the CRI performance regarding the varied concentration of CaMg:EM green phosphor, in which the CRI reduction is noticed as CaMg:EM doping percentage increases. On the other hand, such drawbacks would still be acceptable. When comparing CRI to CQS, the latter is much more important. It would be determined via CRI, the chromatic coordinates, and the viewer’s visual preferences; so, CQS proves to be a better metric to access white-light color quality evaluation and enhancement. Figure 6 exhibits a CQS boost with the presence of the remote CaMg:EM layer. Furthermore, when the CaMg:EM concentration is raised, CQS does not alter considerably with CaMg:EM concentrations less than 10% wt. When the CaMg:EM doping percentage surpasses 10% wt., significant degradation in CQS and CRI is observed, which is caused by the mentioned surplus in green illumination. If CaMg:EM is employed, proper concentration selection would be necessary.

    In conclusion, we used the solid-state technique under significant temperatures to create multiple Ca₈MgLa(PO₄)₇:Eu²⁺ as well as Ca₈MgY(PO₄)₇:Eu²⁺,Mn²⁺ phosphors. The luminescence in Ca₈MgLa(PO₄)₇:Eu²⁺ suggests that it possesses one major promising applicability when it comes to WLED devices with three hues along with various applications. Furthermore, we exhibited the process of ions of Eu²⁺ replacing three distinct locations of Ca²⁺ within Ca₈MgY(PO₄)₇ using crystal formation data, the Gaussian Deconvolution technique, Van Uitert's hypothetical computation, along with degradation period. More significantly, adjusting the Mn²⁺ level in the phosphor resulted in color-adjustable CaMg:EM ranging between cyan (0.216, 0.352), white (0.330, 0.328), and finally pale red (0.370, 0.320). In addition, we studied the power transmission among Eu²⁺ as well as Mn²⁺, then concluded that it was resonant through dipole-quadrupole interactivity having a threshold range measured at 11.17 Å. Eventually, one WLED apparatus made using a 375 nm ultraviolet chip as well as CaMg:EM produces illumination in white yielding one CIE shown as (0.337, 0.334) as well as corresponding hue temperature measured at 5278 K. As a result, CaMg:EM may be useful for phosphor-converted white LEDs. According to our findings, more optimization studies may be conducted for the task of acquiring superior photoluminescent features for Ca₈MgLa(PO₄)₇:Eu²⁺ as well as Ca₈MgY(PO₄)₇:Eu²⁺,Mn²⁺ using different processes including hydrothermal as well as sol-gel methods. Additionally, the drop near the wavelength region of 550 nm where our sight possesses the greatest responsiveness would be one frequent flaw from phosphors treated with a pair of Eu²⁺ and Mn²⁺ as well as Ce³⁺ and Mn²⁺.The technique that involves doping ions of Tb³⁺ as well as altering the content for ions of Eu²⁺,Tb³⁺, as well as Mn²⁺ is predicted to overcome this issue.

    This research is supported by Industrial University of Ho Chi Minh City (IUH) under grant number 138/HD-DHCN.

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