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

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.


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 rst 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 rst 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., Sr 2 -Si 5 N 8 :Eu 2+ , CaAlSiN 3 :Eu 2+ ) 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, Mn 4+ -activated uoride 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 LEDs 5 compared to Mn 4+ -activated uoride phosphor, the emission wavelength of Mn 4+ -activated oxide phosphors, such as Sr 4 Al 14 O 25 :Mn 4+ , Y 3 Al 5 O 12 :Mn 4+ , 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 Mn 4+ ions in various crystals. 6 We can see from Fig. 1 that the emission peak of the Mn 4+ -activated uoride phosphor and the absorption peak of chlorophyll rarely overlap, while the emission peak of the Mn 4+activated oxide phosphors, such as Sr 4 Al 14 O 25 :Mn 4+ , overlap with the absorption peaks of chlorophyll-a and chlorophyll-b.
As shown in Fig. 2, the emission spectrum of Sr 4 Al 14 O 25 :Mn 4+ red phosphor has overlap more efficiently with the absorption spectrum of chlorophyll compared with Sr 2 Si 5 N 8 :Eu 2+ red commercial phosphors. However, most Mn 4+ -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 Mn 4+ -activated oxide phosphors, such as impurity doping with Mg 2+ ions, 8,9 impurity doping with Bi 3+ ions, and impurity doping with Na + ions. [10][11][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 Sr 4 Al 14 O 25 :Mn 4+ , Na + , B 3+ with strong absorption at blue region were synthesized. The difference with Sr 4 Al 14 O 25 :Mn 4+ , Na + phosphors reported by Lili Meng 12 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 B 3+ ions was very crucial. Specic doping ratio of B 3+ ions and Na + ions make the luminous performance of the phosphor signicantly improved. The advancements of current work include signicant improvement of luminescent efficiency of Sr 4 Al 14 O 25 :Mn 4+ by doping B 3+ and Na + ions. Notably, doping of B 3+ 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.

Materials and synthesis
Polycrystalline phosphors with composition of Sr 4 Al 14 O 25 :Mn 4+ , Na + , B 3+ were prepared with a high-temperature solid-state reaction. Briey, the constituent raw materials SrCO 3 (A. R., 99.9%), Al 2 O 3 (A. R., 99.99%), Na 2 CO 3 (A. R., 99%), H 3 BO 3 (A. R., 99%) and MnO 2 (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. Aer 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 Ka radiation (l ¼ 1.54056Å), over the angular range 10 < 2q < 80 , operating at 40 kV and 40 mA. The schematic crystal structure of Sr 4 Al 14 O 25 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. Because boric acid has lower melting temperature, it (aer being homogeneously dispersed throughout the sample) will be the rst 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 benecial for the formation of the phase Sr 4 Al 14 O 25 . According to the emission spectrum (Fig. 7b), it is concluded that the optimum x of B 3+ is 0. 8  Compared with the strong covalence effect of the AlO 4 tetrahedron, a little weak polarization eld of the AlO 6 octahedron is more suitable for Mn 4+ incorporating. In addition, the Mn 4+ 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 2 E g / 4 A 2g transition. 16 Luminescence property    Aer obtained optimum Mn 4+ doping concentration, we adjusted the amount of B 3+ and Na + . Fig. 6a and b show the photographs of Sr 4 Al 14 O 25 :0.014Mn 4+ , xB 3+ , yNa + (0 # x # 1.6, 0 # y # 3) under natural sunlight and 365 nm UV light, respectively. Along with Na + ions and B 3+ doping concentrations increasing, the color of Sr 4 Al 14 O 25 :0.014Mn 4+ , xB 3+ , yNa + phosphors under natural sunlight change from light yellow to bright yellow. This might be ascribed to the Na + and B 3+ ions enhanced absorption band (220-500 nm), especially in the region of 400-500 nm, as illustrated in Fig. 7a.

Results and discussion
In detail, Fig. 7a shows PLE spectrum of Sr 4 Al 14 O 25 :0.014-Mn 4+ , xB 3+ , 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 B 3+  increases from 0.2 to 1.6, the relative magnitude of excitation bands varies little. Furthermore, the optimum x of B 3+ is 0.4, not 0.8, according to Fig. 7b. When Na + is added, for instance at y ¼ 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 B 3+ turns to 0.8 from 0.4. When x of B 3+ is xed at 0.8, the emission intensity increased rst, reached the maximum (y ¼ 2), and then decreased with the increase of Na + content. Cross experiment can clearly found that the optimal combination is x ¼ 0.8, y ¼ 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 B 3+ is observably different. As Fig. 7a shown, when x ¼ 0.4, the addition of Na + does not signicantly 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 B 3+ turns to 0.8 from 0.4 (when y ¼ 2). Therefore, addition of B 3+ and Na + ions in Sr 4 Al 14 O 25 :0.014Mn 4+ , xB 3+ , yNa + can signicantly 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. 8a and b show the photoluminescent excitation and emission spectra of the Sr 4 Al 14 O 25 :0.014Mn 4+ , 0.8B 3+ based phosphors with or without co-incorporating Na + . The uorescent intensities of the phosphors excited at 450 nm reached a maximum at x ¼ 0.8, y ¼ 2 and z ¼ 0.014, and the strongest emission intensity of Sr 4 Al 14 O 25 :0.8B 3+ , 2Na + , 0.014Mn 4+ sample were increased by 700% compared with Sr 4 Al 14 O 25 :0.8-B 3+ , 0.014Mn 4+ without Na + co-doping. The photographs of the Sr 4 Al 14 O 25 :0.014Mn 4+ , 0.8B 3+ , 2Na + sample and Sr 4 Al 14 -O 25 :0.014Mn 4+ , 0.8B 3+ sample exposed to 450 nm blue light and 365 nm UV light are shown in the insert of the Fig. 8a and b. Aer the incorporating of sodium ions, the brightness of phosphor becomes larger. We can nd that the excitation spectra are consists of three conjoint bands by multi-peaks tting, which located from near UV region to visible blue region. These three bands are corresponding to 4 A 2 / 4 T 2 , 4 A 2 / 2 T 2 and 4 A 2 / 4 T 1 transition, respectively. Though according to the spin selection rule of DS ¼ 0, the transitions between 4 T 2 , 4 T 1 and ground 4 A 2 levels are spin-allowed, the spin-forbidden transition 4 A 2 / 2 T 2 is still be found in our result and other Mn-incorporated phosphors, such as CaAl 12 21 By tting the peaks of the excitation spectra, we found that all the excitation peaks have a red shi as the sodium incorporated.

Crystal led strength calculation
The values of D q , B and C can be calculated based on experimentally determined energy levels using the following equations: 5 where D q represents the crystal eld strength and the parameter x is dened as other states such as 2 T 1 , 2 A 1 and 4 T 1 can be theoretically predicted by: The crystal eld parameters and the energies of states in Sr 4 Al 14 O 25 :0.014Mn 4+ , 0.8B 3+ , 2Na + and Sr 4 Al 14 O 25 :0.014Mn 4+ , 0.8B 3+ crystal lattices are summarized in the Table 3. As shown in Fig. 8c, the dependence of energy levels of Mn 4+ on crystal eld strength can be illustrated by Tanabe-Sugano energy diagram. The 2 E g levels are almost parallel to the ground state 4 A 2 , which results that the location of the emission peak is difficult to be inuenced by crystal eld strength. While, the energy gap between 4 T 1 (or 4 T 2 ) levels and ground state 4 A 2 can be changed by variation of the crystal eld 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 uxes in solid state synthesis. However, the shapes of the excitation bands cannot be changed by uxes and meanwhile the redshi of the excitation is a change on the luminous mechanism instead of uxes effect. Fig. 8d shows a schematic diagram of a process of photoluminescence. The 2 E g , 2 T 1 , 2 T 2 and 4 A 2 levels are derived from the t 3 2 electronic orbital, whereas the 4 T 1 and 4 T 2 levels are formed from another t 2 2 e orbital, resulting a displacement between the parabolas of ground state 4 A 2 and 4 T 1 (or 4 T 2 ). The electronics are excited from to ground state 4 A 2 to 4 T 1 , 4 T 2 or 2 T 2 levels by radiation. Then, the excited electronics usually relax non-radiatively to 2 E g followed by the spin-forbidden 2 E g / 4 A 2 transition characterized by wide emission bands.

Conclusions
A series of Sr 4 Al 14 O 25 :xB 3+ , yNa + , zMn 4+ red phosphors were synthesized by a high-temperature solid-state reaction method at 1400 C for 6 hours in the air. The uorescent intensities of the phosphors excited at 450 nm reached a maximum at x ¼ 0.8, y ¼ 2 and z ¼ 0.014, and the strongest emission intensity of Sr 4 Al 14 O 25 :0.8B 3+ , 2Na + , 0.014Mn 4+ sample were increased by 700% compared with Sr 4 Al 14 O 25 :0.8B 3+ , 0.014Mn 4+ without Na + co-doping. In comparison with Mn 4+ single incorporated phosphor, Sr 4 Al 14 O 25 :0.014Mn 4+ , 0.8B 3+ , 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 conicts to declare.