Strongly enhanced luminescence of Sr₄Al₁₄O₂₅:Mn⁴⁺ phosphor by co-doping B³⁺ and Na⁺ ions with red emission for plant growth LEDs

Abstract

Development of a more cost-effective radiation source for use in plant-growing facilities would be of significant benefit for commercial crop production applications. A series of co-doped B³⁺ and Na⁺ ions Sr₄Al₁₄O₂₅:Mn⁴⁺ inorganic luminescence materials which can be used for plant growth were successfully synthesized through a conventional high-temperature solid-state reaction. Powder X-ray diffraction was used to confirm the crystal structure and phase purity of the obtained samples. Then scanning electron microscopy elemental mapping was undertaken to characterize the distribution of the doped ions. Detail investigations on the photoluminescence emission and excitation spectra revealed that emission intensity of tetravalent manganese ions can be well enhanced by monovalent sodium ions and trivalent boron ions under near-ultraviolet and blue excitation. Additionally, crystal field parameters and energies of states are calculated and discussed in detail. Particularly we achieve a photoluminescence internal quantum yield as high as 60.8% under 450 nm blue light excitation for Sr₄Al₁₄O₂₅:Mn⁴⁺, Na⁺, B³⁺. Therefore, satisfactory luminescence properties make these phosphors available to LEDs for plant growth.

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Introduction

It is known that green plants can be grown only using red and blue monochromatic light, because chlorophyll has its second distinct absorption peak in the vicinity of 450 nm (the blue light region) and its first peak in the vicinity of 660 nm (red light region). The blue light is indispensable to the morphologically healthy plant growth, and the red light contributes to the plant leaf photosynthesis.^1,2 The first successful plant growth experiment using only blue and red LEDs were achieved in June 1994 by Okamoto and Yanagi.³

With LEDs price reduced, LEDs are gradually used as plant lighting source, because of its high light efficiency, energy saving and other characteristics. As the driving currents of the blue chip and the red chip are inconsistent, the design of the driver circuit of LEDs for planet growth would be very complex, resulting in increased cost. Therefore, the plant-grown red lamp that is currently in widespread use in the marketplace is composed of a red phosphor rather than a red chip. The most commonly used red phosphors are nitride phosphor (e.g., Sr₂Si₅N₈:Eu²⁺, CaAlSiN₃:Eu²⁺) because of high luminous efficiency. But their high temperature (∼1600 °C), high pressure (1–10 MPa) preparation conditions and expensive raw materials containing rare earth elements lead to high cost.⁴

In recent years, Mn⁴⁺-activated fluoride compounds, as an alternative to commercial (oxy)nitride phosphors, are emerging as a new class of non-rare-earth red phosphors for high-efficacy warm white LEDs⁵ compared to Mn⁴⁺-activated fluoride phosphor, the emission wavelength of Mn⁴⁺-activated oxide phosphors, such as Sr₄Al₁₄O₂₅:Mn⁴⁺, Y₃Al₅O₁₂:Mn⁴⁺, etc., is much longer which is very suitable for plant lighting, because the absorption peak of plant chlorophyll is near 660 nm. The following diagram shows spectroscopic range of Mn⁴⁺ ions in various crystals.⁶ We can see from Fig. 1 that the emission peak of the Mn⁴⁺-activated fluoride phosphor and the absorption peak of chlorophyll rarely overlap, while the emission peak of the Mn⁴⁺-activated oxide phosphors, such as Sr₄Al₁₄O₂₅:Mn⁴⁺, overlap with the absorption peaks of chlorophyll-a and chlorophyll-b.

Fig. 1 Spectroscopic range of Mn⁴⁺ ions in various crystals. Green and blue color indicate the absorption spectrum region of chlorophyll-a and chlorophyll-b, respectively. Red color indicates the emission spectrum region of various phosphors with Mn⁴⁺ ions doped.

As shown in Fig. 2, the emission spectrum of Sr₄Al₁₄O₂₅:Mn⁴⁺ red phosphor has overlap more efficiently with the absorption spectrum of chlorophyll compared with Sr₂Si₅N₈:Eu²⁺ red commercial phosphors. However, most Mn⁴⁺-activated oxide phosphors cannot be excited effectively by blue light, which limited its application on blue chip-based LEDs. There are some methods to improve the luminous efficiency and luminous intensity of Mn⁴⁺-activated oxide phosphors, such as impurity doping with Mg²⁺ ions,^8,9 impurity doping with Bi³⁺ ions, and impurity doping with Na⁺ ions.^10–12 The objective of this work is to develop a red emitting phosphor which can match with blue chip for possible application in plant growth. In this article, the red phosphors Sr₄Al₁₄O₂₅:Mn⁴⁺, Na⁺, B³⁺ with strong absorption at blue region were synthesized. The difference with Sr₄Al₁₄O₂₅:Mn⁴⁺, Na⁺ phosphors reported by Lili Meng¹² was that the excitation intensity of the phosphor we prepared was significantly improved, especially in the blue light region. We also found that the doping amount of B³⁺ ions was very crucial. Specific doping ratio of B³⁺ ions and Na⁺ ions make the luminous performance of the phosphor significantly improved. The advancements of current work include significant improvement of luminescent efficiency of Sr₄Al₁₄O₂₅:Mn⁴⁺ by doping B³⁺ and Na⁺ ions. Notably, doping of B³⁺ and Na⁺ ions can improve its visible light excitation efficiency in the spectral range of 400–500 nm so that it can be incorporated as a red component into blue chip-based LED applications for plant growth.

Fig. 2 (a) Emission spectrum of LEDs fabricated with 445 nm blue chip and Sr₂Si₅N₈:Eu²⁺ red phosphor. (b) Emission spectrum of LEDs fabricated with 445 nm blue chip and Sr₄Al₁₄O₂₅:Mn⁴⁺, red phosphor. (c) Absorption spectrum of chlorophyll-b. (d) Absorption spectrum of chlorophyll-a (redrawn from Zscheile and Comar's (1941) original data⁷).

Experimental section

Materials and synthesis

Polycrystalline phosphors with composition of Sr₄Al₁₄O₂₅:Mn⁴⁺, Na⁺, B³⁺ were prepared with a high-temperature solid-state reaction. Briefly, the constituent raw materials SrCO₃ (A. R., 99.9%), Al₂O₃ (A. R., 99.99%), Na₂CO₃ (A. R., 99%), H₃BO₃ (A. R., 99%) and MnO₂ (A. R., 99.99%) were weighed according to the stoichiometric ratio. Individual batches of 10 g were weighted according to the designed stoichiometry and mixed homogeneously with the same mass of absolute ethyl alcohol as the dispersant. After planetary ball-milling process, the obtained homogeneous slurry was placed in a Petri dish and dried in an oven. Then, the dried mixtures were put into a crucible with a lid and heated in a tubular furnace at 1400 °C for 6 hours in the air. When cooled down to room temperature, the prepared phosphors were crushed and ground for subsequent measurements.

Characterization

All crystal structure compositions were checked for phase formation by using powder X-ray diffraction (XRD) analysis with a Rigaku X-ray diffractometer (Tokyo, Japan) with a graphite monochromator using Cu Kα radiation (λ = 1.54056 Å), over the angular range 10° < 2θ < 80°, operating at 40 kV and 40 mA. The schematic crystal structure of Sr₄Al₁₄O₂₅ was drawn in VESTA.¹³ The photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the samples were analyzed by using a Hitachi F-7000 spectrophotometer (Tokyo, Japan) with a 150 W Xe lamp.

Results and discussion

Microstructure

The compositions of the typical Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, xB³⁺, 0Na⁺, (0 ≤ x ≤ 1.6) samples were displayed in Fig. 3a. When no flux is added, the compound Sr₄Al₁₄O₂₅ is not formed, and instead, the two phases SrAl₂O₄ (JCPDS-No. 34-0379) and SrAl₁₂O₁₉ (JCPDS-No. 80-1195) appear. When boric acid is added, for instance at x = 0.2, the sample turns into a single phase of Sr₄Al₁₄O₂₅. With increase of x from 0.2 to 1.2, XRD patterns of phosphors agree well with that of standard Sr₄Al₁₄O₂₅ (JCPDS-No. 52-1876). Further increment of boric acid content will induce an extra impurity phase of SrAl₂O₄. Because boric acid has lower melting temperature, it (after being homogeneously dispersed throughout the sample) will be the first component in the mixture to melt at high temperature. This can promote the immigration of Sr and Al ions, for instance by diffusion, and increase the possibility that the ions encounter, and thus accelerate the crystallization process. Excess boron will dilute the content of Sr and Al ions in the sample and it is therefore not beneficial for the formation of the phase Sr₄Al₁₄O₂₅. According to the emission spectrum (Fig. 7b), it is concluded that the optimum x of B³⁺ is 0.8. Comparison of B³⁺ dopant content in Sr₄Al₁₄O₂₅:Mn⁴⁺, xB³⁺ is shown in Table 1. The compositions of the typical Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, 0.8B³⁺, yNa⁺, (0 ≤ y ≤ 3) samples were displayed in Fig. 3b. With increase of y from 0 to 2, XRD patterns of phosphors agree well with that of standard Sr₄Al₁₄O₂₅ (JCPDS-No. 52-1876). Further increment of Na⁺ content will induce an extra impurity phase of SrAl₂O₄.

Fig. 3 (a) XRD patterns of Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, xB³⁺, 0Na⁺ (0 ≤ x ≤ 1.6) phosphors. (b) XRD patterns of Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, 0.8B³⁺, yNa⁺ (0 ≤ y ≤ 3). The reference spectra of Sr₄Al₁₄O₂₅ (JCPDS-No. 52-1876), SrAl₁₂O₁₉ (JCPDS-No. 80-1195), SrAl₂O₄ (JCPDS-No. 34-0379) standard patterns were shown at the bottom.

Table 1 Comparison of B³⁺ dopant content in Sr₄Al₁₄O₂₅:Mn⁴⁺, xB³⁺

Range of x	Optimum value of x	Ref.
—	0.4	12
—	0.54	14
0–3.5	0.7	15
0–1.6	0.8	This work

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Fig. 4 shows a schematic of the Sr₄Al₁₄O₂₅ crystal structure. The space group of Sr₄Al₁₄O₂₅ is Pmma and orthorhombic. The networks of orthorhombic Sr₄Al₁₄O₂₅ are built by one layer of the octahedral anion groups (AlO₆) and several layers of tetrahedral anion groups (AlO₄) alternatively. There are three types of AlO₆ octahedrons (Al(4)O₆, Al(5)O₆ and Al(6)O₆) and three types of AlO₄ tetrahedron (Al(1)O₆, Al(2)O₆ and Al(3)O₆). Compared with the strong covalence effect of the AlO₄ tetrahedron, a little weak polarization field of the AlO₆ octahedron is more suitable for Mn⁴⁺ incorporating. In addition, the Mn⁴⁺ ion always experiences a strong CF due to its high effective positive charge with the result that the emission spectrum is always dominated by the sharp emission line corresponding with the spin-forbidden ²E_g → ⁴A_2g transition.¹⁶

Fig. 4 Schematic illustration of crystal structure of Sr₄Al₁₄O₂₅. Black ball: Mn; Green ball: Sr; red ball: O; and brown ball: Al. Brown polyhedron: AlO₆-octahedron/AlO₄-tetrahedron; green polyhedron: SrO₇/SrO₈.

Luminescence property

Fig. 5a show the PLE spectrum of Sr₄Al₁₄O₂₅:0.8B³⁺, 2Na⁺, zMn⁴⁺ (0.05% ≤ z ≤ 0.5%) phosphor monitored at 652 nm. Fig. 5b show the PL spectrum of Sr₄Al₁₄O₂₅:0.8B³⁺, 2Na⁺, zMn⁴⁺ (0.05% ≤ z ≤ 0.5%) phosphor excited at 450 nm. Fig. 5c and d visually show the excitation and emission intensity of Mn⁴⁺ on dopant concentration for Sr₄Al₁₄O₂₅:0.8B³⁺, 2Na⁺, zMn⁴⁺ phosphor, respectively. As we can see, the optimum molar concentration of Mn⁴⁺ in Sr₄Al₁₄O₂₅:0.8B³⁺, 2Na⁺, zMn⁴⁺ in this work was 0.1 mol%. The photoluminescence emission spectrum of the phosphor presented a double-peak structure between 600 and 700 nm with two strong bands at about 654 and 664 nm, which were attributed to the ²E → ⁴A₂ transition of Mn⁴⁺ ions and a phonon sideband transition, respectively.¹³

Fig. 5 (a) PLE spectrum of Sr₄Al₁₄O₂₅:0.8B³⁺, 2Na⁺, zMn⁴⁺ (0.05% ≤ z ≤ 0.5%) phosphor monitored at 652 nm. (b) PL spectrum of Sr₄Al₁₄O₂₅:0.8B³⁺, 2Na⁺, zMn⁴⁺ phosphor excited by 450 nm blue light. (c) Dependence of Mn⁴⁺ PLE intensity on dopant concentration for Sr₄Al₁₄O₂₅:0.8B³⁺, 2Na⁺, zMn⁴⁺ phosphor. (d) Dependence of Mn⁴⁺ PL intensity on dopant concentration for Sr₄Al₁₄O₂₅:0.8B³⁺, 2Na⁺, zMn⁴⁺ phosphor.

After obtained optimum Mn⁴⁺ doping concentration, we adjusted the amount of B³⁺ and Na⁺. Fig. 6a and b show the photographs of Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, xB³⁺, yNa⁺ (0 ≤ x ≤ 1.6, 0 ≤ y ≤ 3) under natural sunlight and 365 nm UV light, respectively. Along with Na⁺ ions and B³⁺ doping concentrations increasing, the color of Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, xB³⁺, yNa⁺ phosphors under natural sunlight change from light yellow to bright yellow. This might be ascribed to the Na⁺ and B³⁺ ions enhanced absorption band (220–500 nm), especially in the region of 400–500 nm, as illustrated in Fig. 7a.

Fig. 6 (a) Photographs of Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, xB³⁺, yNa⁺ (0 ≤ x ≤ 1.6, 0 ≤ y ≤ 3) under natural sunlight. (b) Photographs of Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, xB³⁺, yNa⁺ under 365 nm UV light.

Fig. 7 (a) PLE spectrum of Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, xB³⁺, yNa⁺ phosphor monitored at 652 nm. (b) PL spectrum of Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, xB³⁺, yNa⁺ phosphor excited by 450 nm blue light.

In detail, Fig. 7a shows PLE spectrum of Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, xB³⁺, yNa⁺ phosphor monitored at 652 nm. In order to compare the relative changes of the excitation bands, the PLE intensity is normalized. When no Na⁺ is added, with x of B³⁺ increases from 0.2 to 1.6, the relative magnitude of excitation bands varies little. Furthermore, the optimum x of B³⁺ is 0.4, not 0.8, according to Fig. 7b. When Na⁺ is added, for instance at y = 1, the intensity of the blue excitation band (400–500 nm) has been dramatically increased, as shown in Fig. 7a. In Fig. 7b, we can see that the optimum x of B³⁺ turns to 0.8 from 0.4. When x of B³⁺ is fixed at 0.8, the emission intensity increased first, reached the maximum (y = 2), and then decreased with the increase of Na⁺ content. Cross experiment can clearly found that the optimal combination is x = 0.8, y = 2. To illustrate the difference between our experimental results and Meng's,¹² the detail differences are listed in Table 2. As we can see the amount of doping Na⁺ and B³⁺ is observably different. As Fig. 7a shown, when x = 0.4, the addition of Na⁺ does not significantly affect the relative intensity between the two excitation bands. This is why Meng didn't report the asynchronous increase of excitation bands. This asynchronous increase of excitation bands, however, allows the phosphor to be more easily excited by blue light. As Fig. 7b shows, the PL emission intensity excited by 450 nm blue light sharply increased 3 times when x of B³⁺ turns to 0.8 from 0.4 (when y = 2). Therefore, addition of B³⁺ and Na⁺ ions in Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, xB³⁺, yNa⁺ can significantly improve its visible light excitation efficiency in the spectral range of 400–500 nm so that it can be incorporated as a red component into blue chip-based LED applications for plant growth.

Table 2 Comparison of dopant content in Sr₄Al₁₄O₂₅:xB³⁺, yNa⁺, zMn⁴⁺

Range of x	Optimum value of x	Range of y	Optimum value of y	Optimum value of z	Excitation bands	Ref.
0.4	—	0.05	0–0.09	0.01	Synchronous increase	11
0.8	0–1.6	2	0–3	0.014	Asynchronous increase	This work

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Fig. 8a and b show the photoluminescent excitation and emission spectra of the Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, 0.8B³⁺ based phosphors with or without co-incorporating Na⁺. The fluorescent intensities of the phosphors excited at 450 nm reached a maximum at x = 0.8, y = 2 and z = 0.014, and the strongest emission intensity of Sr₄Al₁₄O₂₅:0.8B³⁺, 2Na⁺, 0.014Mn⁴⁺ sample were increased by 700% compared with Sr₄Al₁₄O₂₅:0.8B³⁺, 0.014Mn⁴⁺ without Na⁺ co-doping. The photographs of the Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, 0.8B³⁺, 2Na⁺ sample and Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, 0.8B³⁺ sample exposed to 450 nm blue light and 365 nm UV light are shown in the insert of the Fig. 8a and b. After the incorporating of sodium ions, the brightness of phosphor becomes larger. We can find that the excitation spectra are consists of three conjoint bands by multi-peaks fitting, which located from near UV region to visible blue region. These three bands are corresponding to ⁴A₂ → ⁴T₂, ⁴A₂ → ²T₂ and ⁴A₂ → ⁴T₁ transition, respectively. Though according to the spin selection rule of ΔS = 0, the transitions between ⁴T₂, ⁴T₁ and ground ⁴A₂ levels are spin-allowed, the spin-forbidden transition ⁴A₂ → ²T₂ is still be found in our result and other Mn-incorporated phosphors, such as CaAl₁₂O₁₉:Mn⁴⁺,¹⁷ SrMgAl₁₀O₁₇:Mn⁴⁺,¹⁸ Mg₂TiO₄:Mn⁴⁺,¹⁰ Li₂Mg₃SnO₆:Mn⁴⁺,¹⁹ Ba₂TiGe₂O₈:Mn⁴⁺,²⁰ and Na₂MgAl₁₀O₁₇:Mn⁴⁺.²¹ By fitting the peaks of the excitation spectra, we found that all the excitation peaks have a red shift as the sodium incorporated.

Fig. 8 (a) PL (EX = 450 nm) and PLE (EM = 652 nm) spectra of Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, 2Na⁺, 0.8B³⁺ and (b) Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, 0.8B³⁺ respectively; (c) Tanabe–Sugano energy diagram of a 3d³ system in an octahedral crystal field; (d) configurational coordinate diagram for Mn⁴⁺ ions in Sr₄Al₁₄O₂₅ hosts.

Crystal filed strength calculation

The values of D_q, B and C can be calculated based on experimentally determined energy levels using the following equations:⁵

D_q = E(⁴T_2g − ⁴A_2g)/10 (1)

where D_q represents the crystal field strength and the parameter x is defined as

E(²E_g − ⁴A_2g)/B = 3C/B + 9 − 9B/D_q (4)

From Fig. 8a, the energy levels of ⁴T₂, ⁴T₁ and ²E_g in the Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, 0.8B³⁺, 2Na⁺ host were determined at 22 124, 28 986 and 15 314 cm⁻¹, respectively. From Fig. 8b, the energy levels of ⁴T₂, ⁴T₁ and ²E_g in the Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, 0.8B³⁺ host were determined at 22 321, 30 769 and 26 667 cm⁻¹, respectively. From eqn (1)–(4), the crystal field parameters of D_q, B, C in the Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, 0.8B³⁺, 2Na⁺ were calculated to be 2212, 644 and 3735 cm⁻¹, respectively. The crystal field parameters of D_q, B, C in the Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, 0.8B³⁺ were calculated to be 2232, 830 and 3540 cm⁻¹, respectively. Once these parameters have been determined, the energies of all other states such as ²T₁, ²A₁ and ⁴T₁ can be theoretically predicted by:

E(²T₁ − ²E_g) = 66B²/(10D_q) (5)

E(²A₁ − ⁴A₂) = 10D_q + 4B + 3C (6)

E(⁴T₁(P) − ⁴A₂) = 15D_q + 7.5B − 0.5[100D_q² − 180D_qB + 225B²]^1/2 (7)

The crystal field parameters and the energies of states in Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, 0.8B³⁺, 2Na⁺ and Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, 0.8B³⁺ crystal lattices are summarized in the Table 3. As shown in Fig. 8c, the dependence of energy levels of Mn⁴⁺ on crystal field strength can be illustrated by Tanabe–Sugano energy diagram. The ²E_g levels are almost parallel to the ground state ⁴A₂, which results that the location of the emission peak is difficult to be influenced by crystal field strength. While, the energy gap between ⁴T₁ (or ⁴T₂) levels and ground state ⁴A₂ can be changed by variation of the crystal field strength. The electron transition schematic diagrams are shown in the Fig. 8c with blue and green dot lines. The value of D_q/B increased to 3.43 from 2.69 as the Na⁺ ions addition. There is a difference in the asynchronous increases of the near UV and visible absorption bands. The increase of the excitation intensity at visible blue region is much larger than at near UV region, even both excitation intensity at these two regions are almost equal. Na⁺ compounds are well known fluxes in solid state synthesis. However, the shapes of the excitation bands cannot be changed by fluxes and meanwhile the redshift of the excitation is a change on the luminous mechanism instead of fluxes effect.

Table 3 Crystal field parameters and energies of states in Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, 0.8B³⁺, 2Na⁺ and Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, 0.8B³⁺ crystal lattices

	Sr4Al14O25:0.014Mn^4+, 0.8B^3+, 2Na^+ (cm^−1)	Sr4Al14O25:0.014Mn^4+, 0.8B^3+ (cm^−1)
Dq	2212	2232
B	644	830
C	3735	3540
Dq/B	3.43	2.69
^4A2 → ^4T2	22 124	22 321
^4A2 → ^2T2	26 455	26 667
^4A2 → ^4T1	28 985	30 769
^2Eg → ^4A2	15 314	15 314
^2Eg → ^2T1	1238	2039
^4A2 → ^2A1	35 905	36 264
^4A2 → ^4T1	28 986	30 769

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Fig. 8d shows a schematic diagram of a process of photoluminescence. The ²E_g, ²T₁, ²T₂ and ⁴A₂ levels are derived from the t³₂ electronic orbital, whereas the ⁴T₁ and ⁴T₂ levels are formed from another t²₂e orbital, resulting a displacement between the parabolas of ground state ⁴A₂ and ⁴T₁ (or ⁴T₂). The electronics are excited from to ground state ⁴A₂ to ⁴T₁, ⁴T₂ or ²T₂ levels by radiation. Then, the excited electronics usually relax non-radiatively to ²E_g followed by the spin-forbidden ²E_g → ⁴A₂ transition characterized by wide emission bands.

Conclusions

A series of Sr₄Al₁₄O₂₅:xB³⁺, yNa⁺, zMn⁴⁺ red phosphors were synthesized by a high-temperature solid-state reaction method at 1400 °C for 6 hours in the air. The fluorescent intensities of the phosphors excited at 450 nm reached a maximum at x = 0.8, y = 2 and z = 0.014, and the strongest emission intensity of Sr₄Al₁₄O₂₅:0.8B³⁺, 2Na⁺, 0.014Mn⁴⁺ sample were increased by 700% compared with Sr₄Al₁₄O₂₅:0.8B³⁺, 0.014Mn⁴⁺ without Na⁺ co-doping. In comparison with Mn⁴⁺ single incorporated phosphor, Sr₄Al₁₄O₂₅:0.014Mn⁴⁺, 0.8B³⁺, 2Na⁺ shows greater advantage of promising application incorporated as a red component into blue chip-based LED for plant growth because of much stronger absorption at blue light region and enhanced red emission. The prepared phosphors could be efficiently excited by both near-UV light and the commercially available blue light of LED chips at 450 nm.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the programs of the National Key Research and Development Program of China (2017YFB0403200).

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