Enhanced photoluminescence and energy transfer performance of Y₃Al₄GaO₁₂:Mn⁴⁺,Dy³⁺ phosphors for plant growth LED lights

Abstract

Plant growth LEDs have attracted broad attention in modern society, desperate for specific phosphors with a characteristic emission band. A novel Mn⁴⁺ and Dy³⁺ co-doped Y₃Al₄GaO₁₂ phosphor were successfully prepared through a conventional high-temperature solid-state reaction. A three band emission including red (625–700 nm), orange (550–607 nm) and blue (462–490 nm) is observed in these phosphors when excited under a near-UV lamp, which is ascribed to ²E → ⁴A₂ of Mn⁴⁺, ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 of Dy³⁺, respectively. The three emissions match the absorption spectra of chlorophyll A and chlorophyll B well. Meanwhile, the energy transfer from Dy³⁺ to Mn⁴⁺ was confirmed by the luminescence spectra and lifetime analysis. Finally, an LED device was fabricated that consisted of a 365 nm ultraviolet chip and the Y₃Al₄GaO₁₂:Mn⁴⁺,Dy³⁺ phosphor. The excellent properties indicate that the synthesized phosphor has a promising application in the optical agricultural industry.

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1. Introduction

Optical agriculture has been brought to the forefront in the recent years, for providing plants with necessary light. As we know, plant growth needs four main conditions including light, atmosphere, water and nutrients, and light plays crucial roles among the four factors during all the process of plant growth.¹ According to the previous experiments of plant cultivation, the effective emission ranges are orange-red (550–700 nm) and blue (400–500 nm) bands. Plants absorb orange-red and blue light by the photosynthesis pigments of chlorophyll and phytochrome.^2,3 The latest way to promote plant growth is using LEDs to supply light in optical agricultural. Generally speaking, there are two main methods to obtain plant growth LEDs: “blue LED chip + red LED chip” and “phosphor + LED chip”. For the first technology, many shortcomings such as high cost, complex control circuit, high light failure and color drift limit its further application. The main problem is that two different monochromatic semiconductor chips are used, which require different drive circuits to control the emitting light ratio of blue and red dynamically. The “phosphor + LED chip” technology can be divided into two forms for different types of chip. The first method is combined a blue-emitting InGaN chip with red emission phosphors such as (Sr,Ca)AlSiN₃:Eu^2+ 4 and K₂TiF₆:Mn⁴⁺.⁵ This technology has the advantages of simple operate and low cost. However, the inconsistency of light wane and color drift exist between blue chip and red phosphor. On the one hand, chromaticity shifts in light-emitting diode (LED) devices arise from multiple mechanisms that produce chemical changes in the materials used to construct the LED devices. The chromaticity shifts in LED devices usually start with a fast-acting component which quickly reaches its maximum value, followed by one or more slower acting component. The fast-acting and slower acting component can be modeled with a bounded exponential function and a generalized logistic function.⁶ On the other hand, light wane because of the luminescence decay of phosphors in severe conditions such as high temperature, and LED device always reaches high temperature in long time work. What's more, it is difficult to guarantee the combination between the chip and phosphors, which weaken the light without effective excitation. The second approach is combined near-ultraviolet (n-UV) chips with red and blue phosphors or single-matrix red and blue dual emission phosphor, but this strategy depends more on the characteristics of phosphors and put forward higher requirements on phosphors. However, most traditional phosphors cannot match the aimed emission band, thus more accurate light sources are needed to provide the special light in the plant growth process.^7–9

The current primary red-emitting phosphors are Eu²⁺-doped nitrides, such as M₂Si₅N₈:Eu^2+ 10,11 and MAlSiN₃:Eu²⁺ (M = Ca, Sr, Ba).^4,12,13 They usually are synthesized in critical requirements (0.9 MPa N₂ pressure and 1800 °C), the high cost limits their utility in agricultural industry. As an alternative, Mn⁴⁺-doped oxides^14–17 catch the public's attention due to its attractive luminescence properties of high-efficacy, low cost, high stability and eco-friendliness. Many strategies have been tried out to enhance the photoluminescence emission of Mn⁴⁺-doped phosphors, such as matrix solid solution modification,^18–20 co-doped charge compensator^21,22 or fluxing agent,^23,24 and energy transfer from sensitizer to activator.^25–27

Solid solution treatment is a common method in phosphor modification with many advantages such as increase lattice distortion and lattice rigidity. Some materials change phase accompanied by a large volume contraction at high temperature reaction which have damage to their luminescence properties. This phenomenon can be reduced by the addition of solid solution ions. There is no crystal transformation in the formation of solid solution, but volume effect was reduced which can enhance the resistance of phosphors to high temperature. After the formation of solid solution, there are some distortions on the lattice structure which is in a high energy activated state, and it is conducive to chemical reaction. Therefore, solid solution treatment can improve the luminescence performance and thermal stability.^28–30 There are many conditions for solid solution formation, such as similar ion radius size, alike crystal structure or chemical formula, similarity of ion electronegativity, and the same of ion value. For example, Ca₁₄Al₁₀Zn₆O₃₅ ³¹ and Ca₁₄Ga₁₀Zn₆O₃₅ ³² has the similar structure, Mn⁴⁺-doped Ca₁₄Al₁₀Zn₆O₃₅ phosphors show far-red emitting, and the luminescence properties can be improved by substituting Ga³⁺ for Al³⁺ ion.³³ The similar result was found in Mn⁴⁺ or Ce³⁺ doped (Y,Lu)₃(Al,Ga)₅O₁₂ system.^22,34,35

Another important way to enhance luminescence properties is co-doped charge compensator in same matrix. For instance, the luminescence performance of Y₃Al₅O₁₂:Mn⁴⁺ red phosphor can be enhanced by co-doping Ge⁴⁺, Sc³⁺, Ca²⁺, Mg²⁺, Sr²⁺, Li⁺, Na⁺ ions.^21,33,36 In addition, aluminate phosphors are mainly compounded via high-temperature solid-state reaction in traditional production, which requires high sintering temperature. In order to reduce the reaction temperature and surface defects of the product, and increase the crystallinity of the product, fluxing agent often needs to add in the raw material such as BaF₂, NaF, NH₄Cl and H₃BO₃.³⁵ The third method to improve luminescence performances is to use energy transfer effect, such as Ca₁₄Al₁₀Zn₆O₃₅:Ti⁴⁺,Mn⁴⁺,³¹ Ca₃Al₄ZnO₁₀:Bi³⁺,Mn⁴⁺,²⁶ Gd₂ZnTiO₆:Mn⁴⁺,Yb³⁺,³⁷ and so on.

As we know, the Mn⁴⁺ doped Y₃(Al,Ga)₅O₁₂ system show a wide range of 225–575 nm excitation spectrum. The Dy³⁺ can exhibit blue 480 nm (⁴F_9/2–⁶H_15/2) sharp band and orange 585 nm (⁴F_9/2–⁶H_13/2) luminescence performance in Y₃(Al,Ga)₅O₁₂ matrix for n-UV excitation.^38–41 Based on the resonance-type energy transfer theory, we assume that Dy³⁺ could improve the emission intensity of Mn⁴⁺ in Y₃(Al,Ga)₅O₁₂ matrix. Energy transfer from Dy³⁺ to Mn⁴⁺ has been reported in several works. Chen et al.⁴¹ and Zhou et al.³² verified energy transfer from Dy³⁺ to Mn⁴⁺ in Ca₁₄Al₁₀Zn₆O₃₅ and Ca₁₄Zn₆Ga₁₀O₃₅ phosphors respectively, which indicate that it is possible for the occurrence of similar energy transfer in Y₃(Al,Ga)₅O₁₂ matrix. Herein, we intend to synthesize a novel Dy³⁺ and Mn⁴⁺ co-doped Y₃(Al,Ga)₅O₁₂ phosphor which show both blue and red luminescence for plant growth LEDs.

In this study, a novel Y₃Al₄GaO₁₂:Mn⁴⁺,Dy³⁺ (YAGO:Mn⁴⁺,Dy³⁺) phosphor was prepared via conventional solid-state method at high-temperature in air. Samples as-obtained show bright red, orange and blue light emission, which match the absorption of plant pigments in a large extent. The crystal structure, optical properties and luminescence lifetime are researched in detail. All measurements indicated that the YAGO:Mn⁴⁺,Dy³⁺ phosphors is promising to applicate in plant growth LEDs.

2. Experimental work

2.1 Preparation of samples and device

A series of Y₃Al₄GaO₁₂:Mn⁴⁺ (YAGO:Mn⁴⁺), Y₃Al₄GaO₁₂:Dy³⁺ (YAGO:Dy³⁺) and Y₃Al₄GaO₁₂:Mn⁴⁺,Dy³⁺ (YAGO:Mn⁴⁺,Dy³⁺) phosphors were synthesized via high-temperature solid-state method. The chemical reagents were Y₂O₃ (99.999%), Ga₂O₃ (99.999%), Al₂O₃ (99.999%), Dy₂O₃ (99.999%), Mn(NO₃)₂ (AR), MgO (AR), H₃BO₃ (AR) and LiF (99.99%), all raw materials bought from Aladdin without further purification. Simultaneously, 5 w% H₃BO₃ (AR) and LiF acted as a flux. First of all, stoichiometric amounts of the raw material were mixed up adequately with a moderate amount of alcohol and ground thoroughly for 30 min. Then put the powder into alumina crucibles, fired at 1500 °C for heating rate at 5 °C min⁻¹ in ambient atmosphere and last for 4 hours. Finally, the products were cooled to room temperature naturally and then ground for further used. The LED devices were fabricated by using the Y₃Al₄GaO₁₂:Mn⁴⁺,Dy³⁺ phosphors and 365 nm near-UV chips.

2.2 Characterization

X-ray powder diffraction (XRD) analysis was tested through a Rigaku D/SHIMADZU-6000 X-ray diffractometer with Cu-Kα radiation (λ = 1.5406 Å), operating at 40 kV and 40 mA, setting the 2θ from 10° to 80°. The photoluminescence excitation (PLE) and emission (PL) spectra were measured by F-7000 Spectrophotometer (Hitachi, Japan) equipped with a 150 W Xe lamp. For the measurement of UV-vis absorption spectra, U-3310 spectrophotometer (Hitachi, Japan) was used. The decay curves and photoluminescence quantum efficiency were measured under excitation at 366 nm via FLS980 fluorescence spectrometer (Edinburgh, UK) using a millisecond flash lamp. All the characterizations obtained at room temperature except the temperature-dependent PL spectra.

3. Result and discussion

3.1 Structure characterization and phase identification

The phase composition characterized by X-ray diffraction (XRD). Fig. 1(a) presents the XRD patterns of the Y₃Al_5−mGa_mO₁₂:0.004Mn⁴⁺ (m = 0–5). With the doping concentration of Ga³⁺ increased, the curve changed from Y₃Al₅O₁₂ (PDF#72-1315) to Y₃Ga₅O₁₂ (PDF#43-0512) gradually. Fig. 1(b) shows a broad peak ranging from 32° to 34°, indicating the Al site in the matrix was replaced by Ga significantly. This substitution induced the diffraction peak of these samples shifting towards low-angel direction, because Ga³⁺ has large ionic radius (r = 0.47 Å, CN = 4; r = 0.62 Å, CN = 6) than Al³⁺ (r = 0.39 Å, CN = 4; r = 0.54 Å, CN = 6).²⁶ The situation was confirmed by the following analysis of PL spectra excited at different wavelengths. The XRD patterns of Y₃Al₄GaO₁₂:Mn⁴⁺,yDy³⁺ (y = 0–0.010) phosphors are shown in Fig. 1(c), which well indexed to standard card PDF#75-0554 of Y₃Al_3.97Ga_1.03O₁₂ phase. This result indicates Y₃Al₄GaO₁₂:Mn⁴⁺,yDy³⁺ phosphors keep its original crystal structure without significant variation. The main peak between 32.6 and 33.8° was found to be slightly offset to low-angle in Fig. 1(d), for Dy³⁺ has larger ionic radius (r = 1.03 Å, CN = 8) than Y³⁺ ion (r = 1.02 Å, CN = 8) and Mn⁴⁺ ion has similar ionic radius (r = 0.39 Å, CN = 4; r = 0.53 Å, CN = 6) compared to Al³⁺ ion (r = 0.39 Å, CN = 4; r = 0.53 Å, CN = 6).⁴² This result is owing to the Dy³⁺ ion replaced Y³⁺ and Mn⁴⁺ replaced Al³⁺ ion in the [YO₈], [AlO₄] and [AlO₆] site of the matrix effectively which is similar to the literature review reported by Liudmyla M. et al.⁴³ and Chen et al.³² Fig. S1(a)† presents the XRD patterns of Dy³⁺-doped Y₃Al₄GaO₁₂ phosphors, and they match the standard card of Y₃Al_3.97Ga_1.03O₁₂ well and have the same variation tendency like Fig. 1(c).

Fig. 1 (a) The XRD patterns of Y₃Al_5−mGa_mO₁₂:0.004Mn⁴⁺ (m = 0–5); (b) peak position change between 32.0 and 34.0°; (c) XRD curves of Y₃Al₄GaO₁₂:0.004Mn⁴⁺,yDy³⁺ (y = 0–0.01); (d) the enlarge main peak between 32.6 and 33.8°.

Fig. 2 shows the unit-cell structure of Y₃Al_3.97Ga_1.03O₁₂ crystal (abbreviated as YAGO) and details about atomic in the cubic octahedral structure. The unit-cell is composed of three main sites. They are tetrahedral [AlO₄] octahedral [AlO₆], and dodecahedral [YO₈]. Generally, Mn⁴⁺ activator likes to replace the Al³⁺ in [AlO₆] site because Mn⁴⁺ has the similar ionic radii to Al³⁺ and octahedron are unstable compared to tetrahedron. Dy³⁺ should occupy Y³⁺ ion site in the host according to the principle of ionic radii matching,⁴⁴ which has been elucidated in the previous section. The substitution mechanism demonstrates that Dy³⁺ replaced Y³⁺ on dodecahedral site [YO₈] and Mn⁴⁺ prefer to substitute Al³⁺ on octahedral [AlO₆] compared to [AlO₄] in the unit-cell structure of YAGO crystal.

Fig. 2 Unit-cell structure of Y₃Al_3.97Ga_1.03O₁₂ crystal and substitution mechanism among different ion in the phosphors.

3.2 Photoluminescence properties of Y₃Al₅O₁₂:xMn⁴⁺ (x = 0.002–0.010) and Y₃Al_5−mGa_mO₁₂:0.004Mn⁴⁺ (m = 0–5) phosphors

Photoluminescence property is an important part to evaluate phosphors, which should be investigated in details. Fig. 3(a) and (b) show the luminescence excitation and emission spectra of Y₃Al₅O₁₂:0.004Mn⁴⁺ phosphors. When monitoring at 674 nm, there were two broad bands of each excitation spectrum that ranged from 220 to 430 nm (labeled A) and 430 to 600 nm (labeled B), and their peak located at 366 nm and 472 nm respectively. The PLE spectrum can be fitted into five Gaussian peaks, of which the 327 nm and 370 nm peaks are contributed to the ⁴A₂ → ⁴T₁ transitions of Mn⁴⁺, other peaks located at 407 nm, 469 nm and 479 nm all originated from the ⁴A₂ → ⁴T₂ transitions of Mn⁴⁺.²⁷ Upon excitation at 366 and 472 nm, the as-synthesized phosphors show red emission ranging from 600 to 720 nm with two peaks at 648 nm and 674 nm which can be ascribed to the spin-forbidden ²E → ⁴A₂ transition of Mn⁴⁺. The emission intensity came to the maximum when the concentration of Mn⁴⁺ was 0.004 no matter excited at 366 nm or 472 nm, as presented in the inset of Fig. 3(b). The details about the emission spectra of Y₃Al₅O₁₂:xMn⁴⁺ (x = 0.002–0.010) are shown in Fig. S3.†

Fig. 3 (a) PLE spectrum of Y₃Al₅O₁₂:0.004Mn⁴⁺ and fitting into five Gaussian curves, inset is the dependence of log(I/x) versus log(x); (b) PL spectra of these samples under excitation at 366 nm and 472 nm respectively, inset is the relationship between intensity and concentration of Mn⁴⁺; (c) excitation and (d) emission spectra of Y₃Al_5−mGa_mO₁₂:0.004Mn⁴⁺ (m = 0–5) phosphors.

Single ion doped phosphor concentration quenching model is shown in the inset of Fig. 3(a). The type of multipolar–multipolar interaction can be reflected by the formula⁴⁵ based on the Dexter's theory:

where k, β and θ refer to the constants for the same excitation condition, host crystal and indication of multipolar character, I and x stand for luminescence intensity and the activator concentration, respectively. Three types of dipole–dipole (d–d), dipole–quadrupole (d–q), and quadrupole–quadrupole (q–q) interactions can be speculated when θ value to 6, 8 and 10 respectively. The fitting slope is −1.4229 and the θ was calculated to be 4.2687 which is close to 6, indicating that the main quenching mechanism of YAGO:Mn⁴⁺ phosphor is d–d interactions.^22,29,30,32

As shown in Fig. 3(c), it is obvious that the substitution of Ga³⁺ enhances the intensity of excitation spectra, which indicates these phosphors can be excited easily. With concentration of Ga³⁺ increasing, the intensity of photoluminescence excitation and emission spectra reached maximum when m = 1 then decreased. So we chose Y₃Al₄GaO₁₂ (abbreviated as YAGO) as matrix in the follow-up experiment.

3.3 Energy transfer in YAGO:Mn⁴⁺,Dy³⁺phosphors

The emission of Mn⁴⁺ ion can be further improved via the addition of Dy³⁺ ion, as shown in Fig. 4(a). Two characteristic emission bands can be detected at 462–490 nm and 554–607 nm in YAGO:Mn⁴⁺,Dy³⁺ phosphor under excitation at 366 nm, which corresponding to ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions in the 4f levels of Dy³⁺ ions, respectively.⁴⁶ And red emission ranging from 600 to 720 nm should be ascribed to the ²E → ⁴A₂ transition of Mn⁴⁺. What's more, Fig. S2(a) and (b)† show the photoluminescence excitation and emission spectra of YAGO:Dy³⁺, the optimal Dy³⁺ doping concentration is 0.05. The excitation peaks located at 326.5, 352.5, 366.5 and 387.0 nm attribute to the intrinsic f–f transitions of Dy³⁺ from the ground state ⁶H_15/2 to the excited state ⁴L1_9/2, ⁶P_7/2, ⁶P_5/2 and ⁴I_13/2, respectively.⁴⁶

Fig. 4 (a) Photoluminescence spectra of YAGO:0.004Mn⁴⁺,yDy³⁺ (y = 0, 0.002, 0.004, 0.006, 0.008, 0.010), inset is the change of emission intensity of two ions with different Dy³⁺ concentration; (b) the excitation spectrum of YAGO:Mn⁴⁺ (black line), the PLE (green line) and PL spectra (blue line) of YAGO:Dy³⁺; (c) PL spectra of YAGO:0.05Dy³⁺,zMn⁴⁺ (z = 0.001–0.006) under excitation at 366 nm, inset is the change of emission intensity of two ions with different Mn⁴⁺ concentration; (d) decay curves of YAGO:0.05Dy³⁺,zMn⁴⁺ (z = 0.001–0.006) monitored at 483 nm under excitation at 366 nm.

Fig. 4(b) presents a large overlapping part between the PLE spectrum of YAGO:Mn⁴⁺ and the PL spectrum of YAGO:Dy³⁺. It is one of the indispensable factors for effective energy transfer. Therefore, it is possible for the occurrence of energy transfer from Dy³⁺ to Mn⁴⁺ ions in YAGO matrix. In order to confirm the energy transfer further, the content of Dy³⁺ ion was fixed at 0.05 and the concentration of Mn⁴⁺ was increased from 0.001 to 0.006. According to Fig. 4(c), with the concentration of Mn⁴⁺ increasing, the emission of Dy³⁺ decreased gradually while the emission of Mn⁴⁺ increased until Mn⁴⁺ concentration exceed 0.003 and then decreased due to the concentration quenching effect. Such experimental results are consistent with the prediction that there is energy transfer from Dy³⁺ to Mn⁴⁺ in YAGO host.

As mentioned in Fig. 4(c), the emission intensity of Mn⁴⁺ decrease when the content of Mn⁴⁺ exceeds the critical concentration because of the concentration quenching phenomenon which is related to energy transfer. In generally, the critical distance (R_c) is a key parameter to evaluate the performance of sensitizer and activator, which is usually calculated by the following formula:^47,48

where V, N and x_c refer to the volume of the unit cell, the critical concentration of Mn⁴⁺ and the number of lattice sites in the unit cell that can be occupied by activators, respectively. Herein, V is 1754.05 Å, x_c is 0.0025 and N is 8 for YAGO:Mn⁴⁺,Dy³⁺ phosphors, then the critical distance R_c was calculated to be 55.13 Å.

The PL decay curves were investigated to calculate the lifetime and then to verify the energy transfer from Dy³⁺ to Mn⁴⁺. Monitor at 483 nm for YAGO:Mn⁴⁺,Dy³⁺ phosphors under the excitation of 366 nm, the decay curves are illustrated in Fig. 4(d). These data fitted a typical single-exponential function well:²⁵

where t refer to the time and I is the luminescence intensity at that moment, I₀ and τ stand for the initial emission intensity at time 0 and the luminescence lifetime, respectively. For YAGO:0.05Dy³⁺,zMn⁴⁺ (z = 0.001–0.006) phosphors, the lifetime of Dy³⁺ decreased distinctly with the content of Mn⁴⁺ increasing. They were 0.795, 0.741, 0.678, 0.624, 0.608 and 0.594 ms when the content of Mn⁴⁺ changed from 0.001 to 0.006, respectively. This result prove that energy transfer from Dy³⁺ to Mn⁴⁺ exist in these samples, similar conclusion in Ca₁₄(Al,Ga)₁₀Zn₆O₃₅ matrix was reported by Zhou et al.³¹

Fig. 5(a) presents the UV-vis absorption spectra of YAGO, YAGO:Mn⁴⁺, YAGO:Dy³⁺ and YAGO:Mn⁴⁺,Dy³⁺. Two characteristic absorption bands were found in all four samples that name Band A (220–260 nm) and Band B (260 to 430 nm). Band C (430–550 nm) existed only in Mn⁴⁺-doped phosphor, which should be attributed to the ⁴A₂ → ⁴T₁ transition of Mn⁴⁺. The absorption intensity enhanced by adding Dy³⁺ ion, and further enhanced by co-doping Mn⁴⁺ and Dy³⁺ ions.

Fig. 5 (a) Comparison the UV-vis absorption spectra of YAGO phosphors with different dopants; (b) the plot of (αhν)² versus hν based on the direct gap of different samples.

To understand the effect of the addition of Mn⁴⁺ and Dy³⁺ ions on YAGO matrix, the band gap of four materials were researched, which can be calculated by the following equation according to the UV-vis absorption plots:⁴⁹

(αhν)² = A(hν − E_g) (4)

where α, h, ν and A are the absorption coefficient, the Planck constant, the frequency and constant; and E_g is the band gap energy. According to the equation and absorption dates, the relationship between the plot of (αhν)² and hν based on the direct gap are presented in Fig. 5(b). The value of the band gap E_g for YAGO, YAGO:Mn⁴⁺, YAGO:Dy³⁺ and YAGO:Mn⁴⁺,Dy³⁺ were calculated to be 4.12 eV, 4.10 eV, 3.15 eV and 3.06 eV, respectively. There is a great change of band-gap after Mn⁴⁺-doping (from 4.12 to 3.15 eV), which can be explained by the electronegativity of ions. In the one hand, it requires less energy for electrons to get rid of the limiting valance bonds when Mn⁴⁺ ion replace Al³⁺ and Ga³⁺ ion because Mn⁴⁺ ion has lower electronegativity than Al³⁺ and Ga³⁺ ion, which resulting in a reduce in energy band gap.²⁶ Meanwhile, Dy³⁺ dopant could also reduce the band gap slightly for its small electronegativity. In the other hand, the intimate addiction of E_g on Mn⁴⁺ ions concentration indicates that the Mn⁴⁺ orbitals are probably involved in the conduction band besides of the band gap of YAGO.

According to Reisfeld's approximation and Dexter's energy transfer formula of multipolar interaction, the equation can evaluate interaction mechanism:^50,51

herein, C is the sum of the concentration of Mn⁴⁺ and Dy³⁺, η and η₀ stand for the luminescence quantum efficiency of Dy³⁺ in the presence and absence of Mn⁴⁺, respectively. The η₀/η is usually calculated through the ratio of initial luminescence intensity of YAGO:Dy³⁺ and YAGO:Dy³⁺,Mn⁴⁺, and the equation can be written as I_so/I_s ∝ C^n/3. Based on the formula, three types of interaction named dipole–dipole (d–d), dipole–quadrupole (d–p) and quadrupole–quadrupole (p–p) can be expected while n = 6, 8 and 10 respectively. Fig. 6 presents the relationship between I_so/I_s and C. When n = 6, linear fitting come to optimum with R² = 0.998, implying that it is dipole–dipole interaction mechanism for the energy transfer from Dy³⁺ to Mn⁴⁺ in YAGO phosphor.

Fig. 6 Linear relationship between I_so/I_s and C^n/3 (a) n = 6, (b) n = 8, (c) n = 10.

We tested the temperature-dependent emission spectra of Y₃Al₄GaO₁₂:0.004Mn⁴⁺,0.008Dy³⁺ and Y₃Al₅O₁₂:0.004Mn⁴⁺ from 298 K to 473 K in details, as shown in Fig. 7. The integral intensity of the YAGO:Mn⁴⁺,Dy³⁺ sample kept 59.61% when the temperature came to 423 K while YAG:Mn⁴⁺ maintained 90.10%. Though it seems decreased in the value of thermal stability, other effective emission bands for plant growth lighting were induced and keep its thermal stability well. In addition, it is possible that its thermal stability can be improved through a series of modifications in the future.

Fig. 7 Temperature-dependent emission spectra of (a) Y₃Al₄GaO₁₂:Mn⁴⁺,Dy³⁺ and (b) Y₃Al₅O₁₂:Mn⁴⁺ from 298 K to 473 K under excitation at 366 nm.

The quantum yield of Y₃Al₄GaO₁₂:0.004Mn⁴⁺,0.008Dy³⁺ phosphor was measured under 366 nm excitation which as shown in Fig. 8, the value of IQE was calculated to be 18.0% through the following equation:⁵²

where η and L_S are IQE and the emission spectrum of the sample, E_S and E_R are the spectra of excitation light with sample and only with BaSO₄ reference, respectively. This IQE is at a low level likes SrLaScO₄:Mn⁴⁺ (IQE: 12.2%)⁵³ and Ca₁₄Al₁₀Zn₆O₃₅:Mn⁴⁺ (IQE: 19.4%)⁵⁴ which have been reported, and the property would be increased further.

Fig. 8 Excitation line of BaSO₄ and the PL spectrum of Y₃Al₄GaO₁₂:0.004Mn⁴⁺,0.008Dy³⁺ phosphor collected using an integrating sphere.

For clearly describing the energy change in YAGO:Mn⁴⁺,Dy³⁺ phosphor, the excitation, emission and energy transfer processes of the sample were shown in Fig. 9. Dy³⁺ ions were excited from ⁶H_15/2 energy level to high excited state level such as ⁶P_7/2, ⁴I_13/2, ⁴F_9/2 or even the conduction under irradiation of the n-UV light, then relaxed to ⁶H_13/2 and ⁶H_15/2 from ⁴F_9/2 with blue and orange emission. Meanwhile, Mn⁴⁺ ion can be excited to ⁴T₁, ⁴T₂ level and conduction band from ⁴A₂ under n-UV radiation, the excited Mn⁴⁺ dropped to the ²E energy level then relaxed to ⁴A₂ with red light emission. In this process, the energy transfer occurs from excited state level ⁴F_9/2 of Dy³⁺ ions to the excited state level ⁴T₂ of Mn⁴⁺ ions, then relaxed to the lowest excited state level ²E of Mn⁴⁺ ions through non-radiative relaxation, finally came back to the ground state in the form of red emission.

Fig. 9 Energy level, electron transitions and energy transfer schematic diagram of Dy³⁺ and Mn⁴⁺ in YAGO matrix.

3.4 LED device fabricated with YAGO:Mn⁴⁺,Dy³⁺ and its electro-luminescent property

In order to investigate the application of the synthetic YAGO:Mn⁴⁺,Dy³⁺ in indoor plant growth, LED devices were fabricated with YAGO:Mn⁴⁺,Dy³⁺ phosphor and 365 nm near-UV chip. As shown in Fig. 10(a), the phosphor powder was white in daylight and presented purplish red under the ultraviolet lamp; it appeared milky white in the LED device and sent bright purplish red emission under constant current of 20 mA. The electroluminescence spectrum was mainly consisted of red, blue and little orange emission from YAGO:Mn⁴⁺,Dy³⁺ phosphor and near ultraviolet emission from the chip, which consistent with luminescence spectrum of the sample. Fig. 10(b) shows the resultant CIE coordinate value of this LED device was (0.4813, 0.2576). Other CIE coordinate values of devices that combined with different concentration of Mn⁴⁺ and Dy³⁺ co-doped YAGO phosphor are exhibited in Fig. S4,† they changed from purplish red to white light gradually with the increasing content of Dy³⁺. The result suggests that the novel YAGO Mn⁴⁺,Dy³⁺ phosphor have potential application in plant growth lighting and white LED lighting for their unique optical property.

Fig. 10 (a) Electroluminescence spectrum of the as-fabricated LED device by using YAGO:0.004Mn⁴⁺,0.008Dy³⁺ phosphors and a 365 nm near-UV LED chip under a current of constant current of 20 mA. Insets are the images of LED device and phosphor powder in daylight and under a 365 nm UV lamp. (b) CIE chromaticity coordinates of the sample.

4. Conclusions

To sum up, Mn⁴⁺ and Dy³⁺ co-doped YAGO phosphors were successfully synthesized via conventional high-temperature solid-state method. These phosphors are derived from the crystal structure of Y₃Al₅O₁₂ and changed to Y₃Al₄GaO₁₂ host, activators Mn⁴⁺ replacing Al³⁺ site while Dy³⁺ occupying Y³⁺ site in the matrix. The Y₃Al₄GaO₁₂:Mn⁴⁺,Dy³⁺ phosphors show blue, orange and red emission located at 462–490 nm, 554–607 nm and 625–700 nm respectively, which match the absorption spectrum of plant pigment well. And, these three emission bands can be ascribed to the ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transition of Dy³⁺ and the ²E → ⁴A₂ transition of Mn⁴⁺, respectively. When the doping concentration of Mn⁴⁺ was 0.4% and Dy³⁺ was 0.8%, optimal property of these phosphors was obtained. There are many evidences for energy transfer from Dy³⁺ to Mn⁴⁺ in the as-obtained Y₃Al₄GaO₁₂:Mn⁴⁺,Dy³⁺ phosphors. Moreover, LED devices made from the phosphor have good properties. It is indicate that the as-obtained phosphors have potential applications both in optical agriculture and white LED lighting.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors would like to gratefully acknowledge funds from National Natural Science Foundation of China (Grant No. 21706060, 51703061), Natural Sciences Foundation of Hunan Province, China (Grant No. 2017JJ3103), Scientific Research Foundation of Hunan Provincial Education Department (Grant No. 17B118) and Hunan Provincial Engineering Technology Research Center for Optical Agriculture (Grant No. 2018TP2003).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra00700h

‡ These authors contributed equally to this work.

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