Enhanced Photoluminescence of Gd3Al4GaO12: Cr3+ by Energy Transfers from Co-Doped Dy3+

LEDs for plant lighting have attracted wide attention and phosphors with good stability and deep-red emission are urgently needed. Novel Cr3+ and Dy3+ co-doped Gd3Al4GaO12 garnet (GAGG) phosphors were successfully prepared through a conventional solid-state reaction. Using blue LEDs, a broadband deep-red emission at 650–850 nm was obtained due to the Cr3+ 4T2 → 4A2 transition. When the Cr3+ concentration was fixed to 0.1 mol, the crystal structure did not change with an increase in the Dy3+ doping concentration. The luminous intensity of the optimized GAGG:0.1Cr3+, 0.01Dy3+ was 1.4 times that of the single-doped GAGG:0.1Cr3+. Due to the energy transfer from Dy3+ to Cr3+, the internal quantum efficiency reached 86.7%. The energy transfer from Dy3+ to Cr3+ can be demonstrated through luminescence spectra and fluorescence decay. The excellent properties of the synthesized phosphor indicate promising applications in the agricultural industry.


Introduction
Lighting is one important factors affecting plant growth. Photopigment P R and P FR mainly absorb deep-red light at 660-730 nm. P R and P FR play vital roles at all stages of plant growth and development, such as promoting seed germination, desiccating, stem growth, leaf expansion, shading and inducing effects, etc. [1][2][3][4]. However, traditional light sources, such as incandescent lamps, metal halide lamps, fluorescent lamps, and high-pressure sodium lamps, have the disadvantages of high costs and short lives. At present, the white LEDs existing in the market mainly cover the yellow-green wavelength range [5,6], and the near-infrared LEDs do not match well with the chlorophyll absorption band of plants because of their narrow luminous wavelength and low luminous intensity [7][8][9]. Therefore, broad, deep-red lighting devices suitable for plant growth have become the focus [10]. At present, phosphor-converted light-emitting diodes (pcLEDs) based on blue chips, are among the most effective lighting means [11][12][13][14].
They are energy efficient, ensure environmental protection, and a have long service life, a small size, and low costs. Red emission phosphors, such as (Sr, Ca)AlSiN 3 :Eu 2+ [15], and K 2 TiF 6 :Mn 4+ [16], have become commercially available to improve the color quality of white LEDs. However, their emission wavelengths cannot be tuned to the deep-red band and the strongest emission cannot be effectively absorbed by plants. In addition, the nitride synthesis conditions are harsh, rare-earth materials are expensive, and the mining/purification process of Eu 2+ is harmful to the environment, making it expensive and less stable [17]. Further K 2 TiF 6 :Mn 4+ phosphor cannot provide an effective absorption band of red light covered plants with broad spectra. Thus, how to achieve broad, deep-red phosphors that can be efficiently excited by blue light is a more important challenge [7].
Recently, materials doped with the transition-metal ion Cr 3+ have been considered to be ideal red phosphors. Its 3d electrons are located in the outer layer and are very sensitive to the crystal environment [18][19][20][21][22]. Therefore, the selection of different matrices allows for tunable emission for Cr 3+ from deep-red to NIR light by adjusting the surrounding crystal field environment. The Gd 3 Al 4 GaO 12 garnet (GAGG) is a stable material for scintillators and phosphors that have a lower synthesis temperature compared to the commonly used garnets [23]. Karolina Elzbieciak and Lukasz Marciniak reported the strategy for modulating the relative sensitivity of Cr 3+ -based luminescent thermometers through substituting Al 3+ ions with Ga 3+ in Gd 3 Al 5−x Ga x O 12 :Cr 3+ , Nd 3+ and caused the gradual decline of the crystal field strength from Dq/B = 2.69 to Dq/B = 2.18, respectively, for Gd 3 Al 5 O 12 :Cr 3+ , Nd 3+ and Gd 3 Ga 5 O 12 :Cr 3+ , Nd 3+ [24]. Zhang et al. reported on broad-band near-infrared Ca 2 LuZr 2 Al 3 O 12 :Cr 3+ garnet phosphor, which was used in combination with a 460 nm LED chip to fabricate pc-LED devices. Its photoelectric efficiency was 4.1%, which was better than that of tungsten lamps (2.9%) in the 750-820 nm spectrum range [25]. Cr 3+ had the advantage of high efficiency and matching with blue LED chips compared to other NIR phosphors. However, the luminous efficiency needs further improvement.
Cr 3+ luminescence can suffer from impurities and oxidation into Cr 4+ when the materials are sintered in the air. As a result, the luminous efficiency of Cr 3+ doped substrates reported so far has not been very high because of impurities. For instance, the external quantum efficiency (EQE) of Ca 3 Sc 2 Si 3 O 12 (CSSG) was 12.8% [26]. The external quantum efficiency was increased to 21.5% by adding flux and sintering in a CO-reducing atmosphere [7]. In addition, rare-earth/Cr 3+ co-doping appears to be a very promising method of improving luminous efficiency. For instance, a Ca 2 LuHf 2 Al 3 O 12 :Ce 3+ , Cr 3+ sample synthesized using a conventional high-temperature solid-phase method is three times brighter than a single-doped Cr 3+ . Therefore, rare-earth/Cr 3+ co-doping appears to be a very promising method of improving luminous efficiency [27].
In this paper, GAGG:Cr 3+ , Dy 3+ samples were synthesized using a conventional hightemperature solid-phase method to obtain phosphors with high brightness and deep-red luminescence. The synthesis method is environment-friendly, simple, and cheap and leads to a pure Gd 3 Al 4 GaO 12 phase. In GAGG:Cr 3+ , the absorption of Cr 3+ comes from the d-d forbidden transition, its excitation efficiency is low. In order to obtain higher luminescence intensity, the sensitized ion Dy 3+ was introduced into the GAGG:Cr 3+ material. The energy transfer process between Cr 3+ and Dy 3+ in the GAGG is addressed. To the best of our knowledge, this is the first report detailing an energy transfer and the luminescent properties of a Cr 3+ -Dy 3+ co-doped GAGG host. Moreover, this work represents an advance in the development and application of plant growth lighting.
Powder X-ray diffraction was measured at room temperature using a PANalytical heaven II diffractometer employing CuKα radiation. The scanning step was 0.02 • in the range of 10-90 • with 4 s per step integration. A scanning electron microscope (SEM, Hitachi S-3400-N, Homewood, AL, USA) was used to observe the product morphology. A fluorescence spectrophotometer (FS5, Edinburgh, Livingston, UK) equipped with a 450 W xenon lamp, was used to record the excitation and emission spectra of the samples and explore their luminescence performances. The thermal quenching test was completed using the FLS980 steady state transient fluorescence/phosphorescence spectrometer. The corresponding temperature-dependent emission properties of as-synthesized phosphors were measured on the FS5 fluorescence spectrometer. The temperature of the samples was controlled through an externally connected temperature controller (Orient KOJI, Hongkong, China). The samples were heated from 25 to 200 • C at a constant rate of 5 • C/min. The (EVERFINE) analysis system was used to test the packaged sample device.

Phase Identification and Crystal Structure
The XRD patterns of GAGG, GAGG:0.1Cr 3+ , and GAGG:0.1Cr 3+ , 0.01Dy 3+ are shown in Figure 1a. These samples were basically consistent with the standard card (PDF # 46-0447). After Dy 3+ doping, the XRD patterns shown in Figure 1b shifted toward smaller angles, which proved the successful doping of Dy 3+ . The shift was less pronounced after co-doping with Cr 3+ , indicating that the single Cr 3+ doping or Cr 3+ and Dy 3+ co-doping had little effect on the matrix lattice parameters. The ionic radius of Cr 3+ was 0.615 Å (CN = 6), Al 3+ was 0.540 Å (CN = 6), Dy 3+ was 0.912 Å (CN = 8) and Gd 3+ was 0.938 Å (CN = 8); therefore, it is most likely that Dy 3+ fits in the Gd 3+ sites. Cr 3+ is expected to emit near-infrared light in an octahedral rather than a tetrahedral environment, so Cr 3+ prefers to replace Al 3+ in an octahedral environment rather than Ga 3+ in a tetrahedral environment [28][29][30]. All diffraction peaks were well indexed to GAGG, as shown in Figure 1a,c, and calculated using the Rietveld structure refinement method. Additionally, no extra peak appeared in the patterns, indicating that pure phase GAGG:Cr 3+ , Dy 3+ phosphors had been achieved. Figure 1d shows the dodecahedral, octahedral, and tetrahedral positions. In the GAGG structure, the dodecahedral lattice (24c lattice) was occupied by Gd 3+ , and Al 3+ and Ga 3+ occupied octahedral and tetrahedral positions. However, when Cr 3+ ions were doped into the system, they gave preference to octahedral coordination and then entered the tetrahedra. Figure 2a,b presents SEM images of GAGG:0.1Cr 3+ and GAGG:0.1Cr 3+ , 0.01Dy 3+ . EDX scanning was performed at 15 keV and 10 k magnification. Since the samples were ground, they showed an almost identical irregular morphology. Figure 2c,d shows the EDS energy spectra of GAGG: 0.1Cr 3+ and GAGG:0.1Cr 3+ ,0.01Dy 3+ . The EDS analysis confirmed that the samples contained Gd 3+ , Ga 3+ , O 2-, Al 3+ , Cr 3+ and Dy 3+ as trace elements. XRD and SEM/EDS results, therefore, confirmed that Cr 3+ and Dy 3+ ions were successfully doped into the GAGG matrix.

Luminescence Properties
The excitation and emission spectra of GAGG:Cr 3+ are shown in Figure 3a. There are two excitation bands at 350-500 nm and 500-650 nm, which belong to the 4 A 2 → 4 T 1 and the 4 A 2 → 4 T 2 transitions of Cr 3+ , respectively. Under the 450 nm excitation, the characteristic Cr 3+ emission composed of narrow peaks at 693 and 713 nm in the range of 650-850 nm, was observed. The emission from 650 to 850 nm originates from the 4 T 2 → 4 A 2 transition. The peak at 693 nm originates from the zero phonon line of the 2 E → 4 A 2 spin forbidden transition (the R line) [31][32][33].
Typically, the Cr 3+ sample shows a sharp line normally attributed to the spin-forbidden leap 2 E → 4 A 2 [29]. The excitation spectra of GAGG:Cr 3+ were mainly located in the blue light region, indicating that the GAGG:Cr 3+ phosphor matches well with the emission of blue LED chips. The emission of GAGG:Cr 3+ is located in the deep-red region and has a good overlap with the absorption spectrum of photosensitive pigments P R and P FR . LEDs constructed using the GAGG:Cr 3+ phosphor and blue chips could be ideal lighting devices for plant lighting.   Figure 2a,b presents SEM images of GAGG:0.1Cr 3+ and GAGG:0.1Cr 3+ , 0.01Dy 3+ . EDX scanning was performed at 15 keV and 10 k magnification. Since the samples were ground, they showed an almost identical irregular morphology. Figure 2c,d shows the EDS energy spectra of GAGG: 0.1Cr 3+ and GAGG:0.1Cr 3+ ,0.01Dy 3+ . The EDS analysis confirmed that the samples contained Gd 3+ , Ga 3+ , O 2-, Al 3+ , Cr 3+ and Dy 3+ as trace elements. XRD and SEM/EDS results, therefore, confirmed that Cr 3+ and Dy 3+ ions were successfully doped into the GAGG matrix. wavelength was used to excite the sample. The excitation peaks at 352, 366, 387, 427, 452, and 476 nm were attributed to the Dy 3+ transition from 6 H15/2 to 6 p7/2, 6 p5/2, 4 p7/2, 4 G11/2, 4 I15/2, and 4 F9/2, respectively [34,35]. The emission peaks at 479 and 575 nm were attributed to the 4 F9/2 → 6 H15/2 and 4 F9/2 → 6 H13/2 transitions, respectively [36,37]. The PLE spectra show that both Cr 3+ and Dy 3+ can be excited by blue light at 450 nm. It is also found that the emission peak of Dy 3+ overlapped with the excitation peak of Cr 3+ ; thus, energy transfers in the Cr 3+ -Dy 3+ co-doped sample were possible. The luminescence properties of Cr 3+ and Dy 3+ co-doped GAGG were further investigated, as shown in Figure 4. The content of Cr 3+ was fixed at 0.1 mol, and the Dy 3+ ion doping concentration changed from 0.002 to 0.18. For GAGG:0.1 Cr 3+ ,0.01Dy 3+ , the luminescence intensity was the highest and the luminescence intensity was 1.4 times that of the Cr 3+ single doped sample. At the same time, the internal quantum efficiency of GAGG:0.1 Cr 3+ ,0.01Dy 3+ reached the maximum of 86.65%. Even more interestingly, only The PLE (blue line) and PL (red line) spectra of GAGG:0.01Dy 3+ are shown in Figure 3b. A wavelength of 575 nm was selected to detect the PLE spectrum, and a 450 nm wavelength was used to excite the sample. The excitation peaks at 352, 366, 387, 427, 452, and 476 nm were attributed to the Dy 3+ transition from 6 H 15/2 to 6 p 7/2 , 6 p 5/2 , 4 p 7/2 , 4 G 11/2 , 4 I 15/2 , and 4 F 9/2 , respectively [34,35]. The emission peaks at 479 and 575 nm were attributed to the 4 F 9/2 → 6 H 15/2 and 4 F 9/2 → 6 H 13/2 transitions, respectively [36,37]. The PLE spectra show that both Cr 3+ and Dy 3+ can be excited by blue light at 450 nm. It is also found that the emission peak of Dy 3+ overlapped with the excitation peak of Cr 3+ ; thus, energy transfers in the Cr 3+ -Dy 3+ co-doped sample were possible.
The luminescence properties of Cr 3+ and Dy 3+ co-doped GAGG were further investigated, as shown in Figure 4. The content of Cr 3+ was fixed at 0.1 mol, and the Dy 3+ ion doping concentration changed from 0.002 to 0.18. For GAGG:0.1 Cr 3+ ,0.01Dy 3+ , the luminescence intensity was the highest and the luminescence intensity was 1.4 times that of the Cr 3+ single doped sample. At the same time, the internal quantum efficiency of GAGG:0.1 Cr 3+ ,0.01Dy 3+ reached the maximum of 86.65%. Even more interestingly, only the emission of Cr 3+ was produced in GAGG:Cr 3+ ,0.01Dy 3+ . Concentration quenching started to occur when the concentration of Dy 3+ was greater than 0.01. This was probably caused by a total energy transfer from 4 F 7/2 toward 4 T 2 levels and to the E g level that led to deep-red emission [25]. There was an energy transfer between Dy 3+ and Cr 3+ . In addition, it was found that the addition of Dy 3+ did not affect the luminescence peak position and waveform of Cr 3+ . Since the radius of Dy 3+ (0.912 Å, CN = 8) was almost equal to that of Gd 3+ (0.938 Å CN = 8), Dy 3+ entered the lattice and only occupied the position of Gd 3+ , and the formed REO 8 (RE = Gd, Dy) hardly affected the crystal field of the neighboring of CrO 6 (GaO 6 ) octahedrons [38].

Energy Transfer in GAGG: Cr 3+ , Dy 3+
In order to further study the energy transfer between Dy 3+ and Cr 3+ , the fluorescent decays of Dy 3+ were measured, as shown in Figure 5a. The fluorescence attenuation curves of GAGG: xCr 3+ , 0.01Dy 3+ (x = 0, 0.08, 0.1, 0.15, 0.2) were measured under the 450 nm excitation and the 575 nm detection. The fluorescence attenuation curve was fitted using a second-order exponential attenuation model, with the formula as follows [39].
caused by a total energy transfer from F7/2 toward T2 levels and to the Eg level that led to deep-red emission [25]. There was an energy transfer between Dy 3+ and Cr 3+ . In addition, it was found that the addition of Dy 3+ did not affect the luminescence peak position and waveform of Cr 3+ . Since the radius of Dy 3+ (0.912 Å, CN = 8) was almost equal to that of Gd 3+ (0.938 Å CN = 8), Dy 3+ entered the lattice and only occupied the position of Gd 3+ , and the formed REO8 (RE = Gd, Dy) hardly affected the crystal field of the neighboring of CrO6 (GaO6) octahedrons [38].

Energy Transfer in GAGG: Cr 3+ , Dy 3+
In order to further study the energy transfer between Dy 3+ and Cr 3+ , the fluorescent decays of Dy 3+ were measured, as shown in Figure 5a. The fluorescence attenuation curves of GAGG: xCr 3+ , 0.01Dy 3+ (x = 0, 0.08, 0.1, 0.15, 0.2) were measured under the 450 nm excitation and the 575 nm detection. The fluorescence attenuation curve was fitted using a second-order exponential attenuation model, with the formula as follows [39].
A1 and A2 are the fitting constants, I is the intensity of fluorescence at time t, τ1 and τ2 are the fluorescence lifetime; The average attenuation-times are also fitted with a second-order index, as shown in Formula (2) [40].

τ = (A τ + A τ ) / (A τ + A τ )
(2) The fluorescence lifetime decreased from 0.84 to 0.458 ms with increasing Cr 3+ concentration from 0 to 0.2 respectively. This result proves that energy transfers from Dy 3+ to Cr 3+ existed in these samples. We note a somewhat similar phenomenon in the Ca14(Al, Ga)10Zn6O35 matrix to what was reported by Zhou et al. [41].
The following equation can be used to calculate the energy transfer efficiency (ηET) [42].
where τ and τ0 are the lifetimes of Dy 3+ with and without Cr 3+ . On the basis of the formula, the energy transfer efficiency increased from 23.9% to 45.48%.

Temperature-Dependent Emission Spectra
The normalized emission intensity, as a function of temperature is shown in Figure  6a. When the temperature is 440 K, the light intensities at 693 and 712 nm are 73.4% and 81.6%, respectively, of those at 300 K. Figure 6b illustrates the emission spectra of GAGG:0.1Cr 3+ , 0.01Dy 3+ excited at 450 nm in the temperature range of 300-470 K. The profiles of the PL spectra do not experience major changes at different temperatures, while the intensity decreases with increasing temperatures owing to the thermal quenching effect [32]. Figure 6c shows the projection of the emission spectrum with increasing temperature, which shows the change of luminous intensity with increasing temperature. The increase in temperature leads to the intensification of lattice vibrations and an increase in the probability of non-radiative relaxations. The particles of each metastable state relax back to the ground state without radiation, and finally, the excitation energy is dissipated in the matrix lattice in the form of thermal energy. Compared with the spin-allowed transition from 4 T2 (4F) to 4 A2, the lattice vibration has a greater influence on the spin-forbidden 2 E → 4 A2 transition of Cr 3+ [25,42.  A 1 and A 2 are the fitting constants, I is the intensity of fluorescence at time t, τ 1 and τ 2 are the fluorescence lifetime; The average attenuation-times are also fitted with a second-order index, as shown in Formula (2) [40].
The fluorescence lifetime decreased from 0.84 to 0.458 ms with increasing Cr 3+ concentration from 0 to 0.2 respectively. This result proves that energy transfers from Dy 3+ to Cr 3+ existed in these samples. We note a somewhat similar phenomenon in the Ca 14 (Al, Ga) 10 Zn 6 O 35 matrix to what was reported by Zhou et al. [41].
The following equation can be used to calculate the energy transfer efficiency (η ET ) [42].
where τ and τ 0 are the lifetimes of Dy 3+ with and without Cr 3+ . On the basis of the formula, the energy transfer efficiency increased from 23.9% to 45.48%.
To describe the energy change in GAGG:0.1Cr 3+ ,0.01Dy 3+ phosphor, the excitation, emission and energy transfer processes are shown in Figure 5b. Under the irradiation of 450 nm light, electrons are excited from the Dy 3+ 6 H 15/2 energy level to the excited state, such as 6 p 7/2 , 6 p 5/2 , 4 p 7/2 , 4 G 11/2 , 4 I 15/2 , and 4 F 9/2 , and then relax to 6 H 13/2 and 6 H 15/2 from 4 F 9/2 with blue and orange emission. Meanwhile, electrons can also be excited into the Cr 3+ 4 T 1 and then relax to the 4 T 2 and 2 E energy levels, thus providing deep-red emission when relaxing to the 4 A 2 state. In this process, the energy transfer occurs from the excited state 4 F 9/2 of Dy 3+ to the excited states 4 T 2 and 2 E of Cr 3+ .

Temperature-Dependent Emission Spectra
The normalized emission intensity, as a function of temperature is shown in Figure 6a. When the temperature is 440 K, the light intensities at 693 and 712 nm are 73.4% and 81.6%, respectively, of those at 300 K. Figure 6b illustrates the emission spectra of GAGG:0.1Cr 3+ , 0.01Dy 3+ excited at 450 nm in the temperature range of 300-470 K. The profiles of the PL spectra do not experience major changes at different temperatures, while the intensity decreases with increasing temperatures owing to the thermal quenching effect [32]. Figure 6c shows the projection of the emission spectrum with increasing temperature, which shows the change of luminous intensity with increasing temperature. The increase in temperature leads to the intensification of lattice vibrations and an increase in the probability of nonradiative relaxations. The particles of each metastable state relax back to the ground state without radiation, and finally, the excitation energy is dissipated in the matrix lattice in the form of thermal energy. Compared with the spin-allowed transition from 4 T 2 (4F) to 4 A 2 , the lattice vibration has a greater influence on the spin-forbidden 2 E → 4 A 2 transition of Cr 3+ [25,42].

Temperature-Dependent Emission Spectra
The normalized emission intensity, as a function of temperature is shown in Figure  6a. When the temperature is 440 K, the light intensities at 693 and 712 nm are 73.4% and 81.6%, respectively, of those at 300 K. Figure 6b illustrates the emission spectra of GAGG:0.1Cr 3+ , 0.01Dy 3+ excited at 450 nm in the temperature range of 300-470 K. The profiles of the PL spectra do not experience major changes at different temperatures, while the intensity decreases with increasing temperatures owing to the thermal quenching effect [32]. Figure 6c shows the projection of the emission spectrum with increasing temperature, which shows the change of luminous intensity with increasing temperature. The increase in temperature leads to the intensification of lattice vibrations and an increase in the probability of non-radiative relaxations. The particles of each metastable state relax back to the ground state without radiation, and finally, the excitation energy is dissipated in the matrix lattice in the form of thermal energy. Compared with the spin-allowed transition from 4 T2 (4F) to 4 A2, the lattice vibration has a greater influence on the spin-forbidden 2 E → 4 A2 transition of Cr 3+ [25,42.

LED Packages
In order to demonstrate the applicability of the synthetic GAGG:0.1Cr 3+ , 0.01Dy 3+ for indoor plant growth, LED devices were fabricated with the GAGG:0.1Cr 3+ , 0.01Dy 3+ phosphor and a 450 nm blue chip. Figure 7 shows the resultant CIE coordinates of this LED device, which were found at (0.6387, 0.2873). It appears as a milky white light in the LED device and provides a bright purplish-red emission driven by a current of 20 mA. It gives

LED Packages
In order to demonstrate the applicability of the synthetic GAGG:0.1Cr 3+ , 0.01Dy 3+ for indoor plant growth, LED devices were fabricated with the GAGG:0.1Cr 3+ , 0.01Dy 3+ phosphor and a 450 nm blue chip. Figure 7 shows the resultant CIE coordinates of this LED device, which were found at (0.6387, 0.2873). It appears as a milky white light in the LED device and provides a bright purplish-red emission driven by a current of 20 mA. It gives a strong red emission and yields a luminous efficacy of 27.8 lmW −1 . The results show that the new GAGG:0.1Cr 3+ ,0.01Dy 3+ phosphor can be excited by 450 nm of blue light and its red emission has a good overlap with the red light absorption of chlorophyll [8], demonstrating its potential for plant growth lighting and white LED lighting.

Conclusions
To sum up, Dy 3+ and Cr 3+ co-doped GAGG phosphors were successfully synthesized using a conventional high-temperature solid-state method. Dy 3+ ions fit into Gd 3+ sites and played the role of sensitizing the luminescence center for Cr 3+ . The luminescence intensity in deep-red light (650-850 nm) was enhanced by Dy 3+ /Cr 3+ co-doping. The luminous in-

Conclusions
To sum up, Dy 3+ and Cr 3+ co-doped GAGG phosphors were successfully synthesized using a conventional high-temperature solid-state method. Dy 3+ ions fit into Gd 3+ sites and played the role of sensitizing the luminescence center for Cr 3+ . The luminescence intensity in deep-red light (650-850 nm) was enhanced by Dy 3+ /Cr 3+ co-doping. The luminous intensity of optimized GAGG:Cr 3+ ,0.01Dy 3+ was 1.4 times that of the Cr 3+ single-doped sample and its quantum efficiency was up to 86.65%. Many results point toward an energy transfer from Dy 3+ to Cr 3+ in GAGG:0.1Cr 3+ , 0.01Dy 3+ phosphors. Finally, LED devices made from GAGG:0.1Cr 3+ , 0.01Dy 3+ phosphors have good properties. This indicates potential applications of the phosphor in agriculture.
Author Contributions: Y.Z. contributed to study design, data collection, data analysis and writing; X.L. contributed to data collection, data interpretation and figures; D.H. contributed to literature search; Q.S. contributed to data interpretation; X.W. contributed to figures; F.W. contributed testing; K.W. contributed to testing; X.Z. contributed data collection; Z.S. contributed SEM testing; Y.L. contributed manuscript modification; K.C. contributed to study design, literature search, data interpretation. All authors have read and agreed to the published version of the manuscript.