A high-performance non-rare-earth deep-red-emitting Ca14-xSrxZn6Al10O35:Mn4+ phosphor for high-power plant growth LEDs

A high-performance non-rare-earth deep-red-emitting Ca14-xSrxZn6Al10O35:Mn4+ phosphor for high-power plant growth LEDs

Journal of Alloys and Compounds 781 (2019) 702e709 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http:...

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Journal of Alloys and Compounds 781 (2019) 702e709

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

A high-performance non-rare-earth deep-red-emitting Ca144þ phosphor for high-power plant growth LEDs xSrxZn6Al10O35:Mn Yibing Wu a, b, Yixi Zhuang b, *, Ying Lv b, Kaibin Ruan a, Rong-Jun Xie b, ** a b

College of Mechanical and Electronic Engineering, Fujian Agriculture and Forestry University, Fuzhou, Fujian, 350002, PR China College of Materials, Xiamen University, Simingnan-Road 422, Xiamen, 361005, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 September 2018 Received in revised form 6 November 2018 Accepted 4 December 2018 Available online 6 December 2018

Red or deep-red-emitting wavelength-conversion materials with high efficiency are highly essential for the applications of high-power plant growth LEDs. Herein, a series of deep-red-emitting Ca144þ phosphors (x ¼ 0e2.4) showing high quantum efficiency (the IQE and EQE values xSrxZn6Al10O35:Mn up to 83% and 53%, respectively), low thermal quenching effect (~98% at 150  C relative to that at room temperature) are successfully synthesized. The X-ray diffraction, scanning electron microscopy, photoluminescence spectra, diffuse reflectance spectra, luminescence decay curves, and temperaturedependent emission spectra are systematically investigated. Furthermore, a plant growth LED lamp is fabricated by using the synthesized Ca14-xSrxZn6Al10O35:Mn4þ phosphors. Considering their excellent wavelength-conversion properties, the non-rare-earth-containing compositions, as well as mild synthesis processes, the Ca14-xSrxZn6Al10O35:Mn4þ phosphors have great potentials in the applications of high-power plant growth LEDs. © 2018 Elsevier B.V. All rights reserved.

Keywords: Phosphors Deep-red emission Mn4þ doping Thermal quenching High luminescent efficiency Plant growth LEDs

1. Introduction The problems of food security caused by droughts, floods, storms, pests, and diseases have become serious threats to agriculture throughout the world [1]. Accordingly, more and more attentions have been paid on plant factory (the so-called green crop production system) due to its outstanding advantages, such as high output, safety, environmental friendliness, pollution-free, no pesticides and chemical fertilizers. However, at present, the major problem that restricts the development of plant factory is the high energy consumption [2]. As the main parts of energy consumption in the plant factory, the artificial lighting devices are not only the essential energy source for photosynthesis of pigments chlorophylls and carotenoids, but also the power source for sprout, blossom, fruits and other morphogenesis of plants. They thus play key roles in the yields and qualities of agricultural products. In order to reduce the energy consumption, the traditional lighting sources (fluorescent lamps, incandescent lamps, or high-pressure sodium lamps, etc.) should be replaced by light-emitting diodes

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Zhuang), [email protected] (R.-J. Xie). https://doi.org/10.1016/j.jallcom.2018.12.056 0925-8388/© 2018 Elsevier B.V. All rights reserved.

(LEDs) due to their superior properties such as energy-saving, longlifetime and environment-friendly features [3e5]. In addition, the LEDs show great tunability of spectral composition by employing various chips and wavelength-conversion materials, which allow for emission light matching with plant photoreceptors to obtain ideal production and plant morphology [6e10]. It has been reported that blue (400e500 nm), red (620e690 nm) and deep-red (700e740 nm) lights are critical in the reactions of photosynthesis, phototropism, and photomorphogenesis [11e13]. Therefore, it is of considerable significance to fabricate suitable LEDs covering the spectral region of blue and red/deep-red light in order to offer an ideal lighting source for the plant factory. Although blue-red dual emission can be easily realized by combining blue (e.g. InGaN) and red (e.g. AlGaInP) chips, these dual-chip LEDs require complex control circuits and high production costs compared with the conventional illuminants. More importantly, the photoelectric properties of these assembled LED chips would vary with different working temperature, drive current, and service time, which leads to a deviation of the spectra and a lowered efficiency of the emission. Alternatively, phosphorconverted LEDs, which are generally fabricated by coating a blue chip with red-emitting phosphors or coating a near ultra-violet (NUV) chip with blue and red phosphors, can greatly simplify the LED lamp structure and reduce the production costs [14e16].

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Obviously, high-performance red or deep-red emitting phosphors with strong absorption in the NUV or blue region and intense emission in the range from 620 nm to 740 nm are urgently needed to fabricate phosphor-converted LED lamps for the plant growth application. At present, red-emitting phosphors based on Eu2þ-doped nitrides, e.g. (Ca,Sr)AlSiN3, (Ca,Sr)2Si5N8, Sr[LiAl3N4], possessing high quantum efficiency and thermal stabilities are commercially available for the warm-white LEDs in general lighting [17e22]. However, the preparation of those Eu2þ-doped nitrides requires air-sensitive starting materials, expensive rare-earth-containing compounds, as well as severe synthesis conditions, making them difficult for large-scale applications. Recently, Mn4þ-doped crystalline luminescent materials, which usually exhibit strong absorptions in the NUV-blue regions and intense emissions between 600 and 740 nm have received great interests [23e27]. In particular, Mn4þ doped fluorides (AMF6:Mn4þ, A ¼ Na, K, Cs, Ba, or Rb; M ¼ Si, Ti, or Ge), showing excellent quantum efficiency and high color purity have been applied to high color rendering index (CRI) white LEDs [28e34]. Nevertheless, toxic HF solutions are generally used in the synthesis process. Also, the synthesized fluoride compounds are unstable in environments with large moisture. As an alternative, Mn4þ doped oxides showing better chemical stability and environmental friendliness have been considered. Recently, the crystalline structure and luminescence properties of LaAlO3:Mn4þ, BaMgAl10O17:Mn4þ, Ca14Zn6Al10O35:Mn4þ, (Lu,Y)3Al5O10:Mn4þ, and (Sr,Ba)Ge4O9:Mn4þ phosphors have been investigated [35e41]. Unfortunately, the luminescent efficiency in the reported Mn4þactivated oxide phosphors are still too low for practical applications. Therefore, in order to meet the requirements in the applications of high-power plant growth LEDs, it is of great importance to improve the luminescent efficiency and the thermal quenching performance in the Mn4þ-activated phosphors. In this paper, a series of Ca14-xSrxZn6Al10O35:Mn4þ deep-redemitting phosphors are successfully synthesized by a traditional solid-state reaction method. Amazingly, the quantum efficiency of Mn4þ emissions can be greatly enhanced by tailoring the host composition, reaching at 83% and 53% for the internal quantum efficiency (IQE) and external quantum efficiency (EQE), respectively. Furthermore, the integrated emission intensity at 150  C remains 98% of that at room temperature (RT). The outstanding optical performance of the reported Ca14-xSrxZn6Al10O35:Mn4þ phosphors suggests their promising applications in high-power plant growth LEDs.

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contained) are abbreviated as CZA:Mn4þ and CSZA:Mn4þ, respectively. In order to estimate the energy of their band gaps, two nonMn-doped samples with the compositions of Ca14Zn6Al10O35 (CZA) and Ca12.4Sr1.6Zn6Al10O35 (CSZA) were also prepared. 2.2. Characterization The phase identification of the as-prepared phosphors were carried out by using an X-ray powder diffractometer (XRD, Bruker, D8 Advance) with Cu Ka radiation (l ¼ 1.5406 Å) operating at 40 kV and 40 mA. The microstructure of the particles was observed in a field-emission scanning electronic microscope (SEM, Hitachi, SU70). Photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the phosphors were measured on a fluorescence spectrophotometer (Hitachi, F-4600). Diffuse reflection (DR) spectra in the wavelength range from 200 to 800 nm were recorded at RT in a UV/visible spectrophotometer (Shimadzu, UV3600 Plus). BaSO4 was used as a reference for 100% reflectance during the measurement. IQE, EQE, and PL decay curves were measured in a fluorescence spectrophotometer (Edinburgh Instruments, FLS980). In order to evaluate the thermal quenching performance, the temperature-dependent PL spectra of the phosphors were carried out by using a charge coupled device detector (Ocean Optics, USB 2000þ) and a heating and freezing stage (Linkam, THMS600). 3. Results and discussion 3.1. Phase formation and structure characteristics The

XRD

patterns of the synthesized Ca14phosphors (x ¼ 0e2.0) are showed in Fig. 1a. The main diffraction peaks in all the samples were consistent with the Joint Committee on Powder Diffraction Standards (JCPDS) card No. 50-0426, indicating that a solid solution of (Ca,Sr)14Zn6Al10O35 was formed without changing the crystal structure when the concentration of Sr was less than 2.0. The enlarged XRD patterns as presented in Fig. 1b showed a clear peak shift towards lower angle with increasing the Sr concentration. The peak shift was majorly attributed to lattice expansion by the substitution of larger Sr2þ cations [1.18 Å, coordination number (CN) ¼ 6] for smaller Ca2þ cations (1.00 Å, CN ¼ 6). According to the standard JCPDS card, the ideal Ca14Zn6Al10O35 phase crystallizes in a cubic structure (space group of F23) with lattice constant a ¼ 14.868 Å [42]. The crystal structure of Ca14Zn6Al10O35 viewed

xSrxZn6Al9.85O35:Mn0.15

2. Experimental section 2.1. Materials and synthesis The samples were prepared through a conventional solid-state reaction. The nominal compositions of the samples were taken on the basis of the following formula: Ca14-xSrxZn6Al10-yO35:Mny, where the values of x and y were varied from 0 to 2.4 and 0 to 0.25, respectively. Chemical reagents CaCO3 (99.9%), SrCO3 (99.9%), MnCO3 (99.99%), ZnO (99.99%) and Al2O3 (99.9%) were used as starting materials. Stoichiometric ratio of the staring materials were weighted and mixed homogeneously in an agate mortar for 30 minutes. The mixture was moved to an alumina crucible with a lip and heated in a box-type furnace in air. The mixed materials were preheated to 900  C for 2 h and heated up to 1220  C with a rate of 3 K/min and held for 4 h. The obtained products were crushed into fine powders after naturally cooling to RT. All the operations were performed in air. For the sake of simplicity, hereinafter the samples with the compositions of Ca14Zn6Al9.85O35:Mn0.15 (Sr-free) and Ca12.4Sr1.6Zn6Al9.85O35:Mn0.15 (Sr-

Fig. 1. (a) XRD patterns of Ca14-xSrxZn6Al9.85O35:Mn0.15 phosphors (x ¼ 0, 0.3, 0.6, 0.9, 1.2, 1.6, and 2.0) together with the standard diffraction pattern of Ca14Zn6Al10O35 (JCPDS: No. 50-0426). (b) Enlarged parts showing the diffraction peaks for the crystal face (0 4 4).

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from a axis is shown in Fig. 2a. In this structure, the CN for Ca2þ ions is 7 and 6. Some Zn2þ ions are coordinated in tetrahedral sites (CN ¼ 4). Meanwhile, one-fifth of lattice positions shared by Al/Zn are in octahedral coordination (CN ¼ 6, Fig. 2b) and the other fourfifth positions are in tetrahedral coordination (CN ¼ 4, Fig. 2c). It is generally accepted that Mn4þ ions with 3d3 electronic configurations are preferentially coordinated into octahedral sites in crystals [43]. Thus, the doped Mn4þ ions are supposed to enter the Al3þ octahedral sites in the (Ca,Sr)14Zn6AlO35 crystals. Additionally, we detected some impurity phases of Ca3Al4ZnO10 (Y) and CaO (A) in the samples with lower Sr concentrations (x ¼ 0 and 0.3). However, these impurity phases disappeared and the diffraction peaks for the major phase of (Ca,Sr)14Zn6Al10O35 became stronger with increasing the Sr concentration. We considered that the substitution of Sr for Ca may improve the crystallinity of the major phase of (Ca,Sr)14Zn6Al10O35. Nevertheless, when the concentration of Sr reached at 2.0, a new unknown impurity phase marked as * in Fig. 1a appeared. Fig. 3 exhibits the SEM images of the Sr-free phosphor CZA:Mn4þ (Fig. 3a and c) as well as Sr-contained sample CSZA:Mn4þ (Fig. 3b and d), which were both sintered at 1220  C. Obviously, the particle size of CSZA:Mn4þ (10-20 m) was larger than that of CZA:Mn4þ (2-10 m). This result confirmed that the adding of Sr enhanced the crystallinity and promoted the crystal growth for the (Ca,Sr)14Zn6Al10O35 phase.

Fig. 3. SEM images of the CZA:Mn4þ (a, c) and CSZA:Mn4þ (b, d) samples.

3.2. Optical properties of Ca14-xSrxZn6Al9.85O35:Mn4þ phosphors The DR spectra of the non-doped samples CZA and CSZA as well as the Mn-doped samples CZA:Mn4þ and CSZA:Mn4þ are presented in Fig. 4a. The CZA and CSZA samples showed intense optical absorption in the spectral region shorter than 400 nm that should be attributed to electronic transitions between the band gaps of the hosts. The band gap can be estimated by the following equation [44]:

  ½FðR∞ Þ$hnn ¼ A$ hn  Eg

(1)

where hv is the photon energy; A is a proportional constant; Eg is the energy of band gap; n is 2 for direct band-gap semiconductor; and FðR∞ Þ is the Kubelka-Munk function defined as follows:

Fig. 4. (a) DR spectra of CZA, CSZA, CZA:Mn4þ and CSZA:Mn4þ measured at RT. Inset: Extrapolation of the band-gap energy for the non-doped CZA and CSZA compounds. (b), (c) and (d) are the photographs of CZA, CZA:Mn4þ and CSZA:Mn4þ under natural light, respectively.

. FðR∞ Þ ¼ S$ð1  R∞ Þ2 ð2$R∞ Þ

Fig. 2. (a) Schematic illustration of the Ca14Zn6Al10O35 crystal structure viewed from a axis. (bec) the coordination environments of [Al/ZnO6] octahedrons and [AlO4] tetrahedrons.

(2)

where R∞ and S represent the diffuse reflectance and scattering coefficient, respectively. As shown in the inset of Fig. 4a, the Eg value for CZA and CSZA are 4.82 and 4.45 eV, respectively. Although the substitution of Sr2þ ions showed a little effect on the band gap of the host, the CSZA compound still had a wide band gap and thus could be a potential host for Mn4þ-doping. The CZA:Mn4þ and CSZA:Mn4þ samples both presented a strong broad absorption band in the range of 420e490 nm. This band was probably attributed to the spin-allowed 4A2/4T2 transition of Mn4þ (also see PLE spectra in Fig. 5a). Other Mn4þ absorption bands including the 4A2/4T1 transition were located in the UV range and overlapped with the band gap absorptions [45,46]. It is worth noting that the absorptions due to Mn4þ in CSZA:Mn4þ were much more intense than those in CZA:Mn4þ. We considered that the more intense absorptions of Mn4þ should be originated from the improved crystallinity as well as less surface defects in the CSZA:Mn4þ compound. Fig. 4bed presents the photographs of CZA, CZA:Mn4þ and CSZA:Mn4þ phosphors under natural light. The Mnfree sample CZA was white; while the Mn-doped samples

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Fig. 6. Tanabe-Sugano energy diagram of a 3d3 system in an octahedral crystal field. Fig. 5. PLE (left) and PL (right) spectra of the CSZA:Mn4þ phosphor. The monitoring wavelength for PLE spectra was 714 nm. The excitation wavelength was 289 nm for PL spectra.

CZA:Mn4þ and CSZA:Mn4þ showed yellow in body color. Due to the stronger Mn4þ absorption in the blue region, the CSZA:Mn4þ sample was deeper colored in yellow. The PLE and PL spectra of the CSZA:Mn4þ sample are depicted in Fig. 5. The PLE spectra monitoring at 714 nm contained multiple bands ranging from 200 to 550 nm. The excitation band centered at 452 nm was attributed to the spin-allowed 4A2/4T2 transition, and the comparatively weak band at 390 nm could be originated from the spin-forbidden 4A2/2T2 transition. The broadband with the maximum wavelength at approximately 320 nm was assigned to the 4A2/4T1 transition, which was overlapped with the charge transfer band (CTB) of Mn4þ-O2- at 289 nm [47e49]. Finally, the band at about 255 nm could be due to the optical absorption of the band gap. Therefore, the synthesized CSZA:Mn4þ phosphors can be excited by various light sources including UV (~320 nm), near-UV (~390 nm) or blue (~460 nm) chips. Under excitation at 289 nm, the CSZA:Mn4þ phosphors exhibited deepred emission with multiple sharp peaks in the range of 660e730 nm due to the spin-forbidden 2Eg /4A2 transitions. The emission wavelength well matched with the absorption spectra of phytochrome [50], suggesting that the CSZA:Mn4þ phosphors were a promising wavelength-conversion material in plant growth LEDs. It should be noted that typical emission of Mn2þ (a single band in the visible region) [51] was not detected in the CSZA:Mn4þ sample under the UV excitation. Thus, Mn ions dominated in þ4 valence state in the (Ca,Sr)14Zn6Al10O35 host after sintering in air atmosphere. The energy levels of Mn4þ coordinated in an octahedral crystal field can be illustrated in the Tanabe-Sugano energy diagram (Fig. 6) [52]. The energy diagram clearly shows that the energy of the excited states is strongly dependent on crystal field strength except for the 2T1 and 2E levels. The crystal field strength (Dq) can be evaluated by the energy of 4A2/4T2 transition according to the following equation [37]:

Dq ¼ Eð4 A2 /4 T2 Þ=10

(3)

Furthermore, the Racah parameters B and C were calculated by the following equations:

 .  Dq B ¼ 15$ðx  8Þ x2  10$x

(4)

 Eð2 E/4 A2 Þ=B ¼ 3:05$C=B þ 7:9  1:8$B Dq

(5)

where the parameter x was defined as:

 x ¼ ½Eð4 A2 /4 T1 Þ  Eð4 A2 /4 T2 Þ Dq

(6) 4

4

According to the PLE spectra, the energy of A2/ T2 and A2/4T1 transitions were 22109 and 31250 cm1, respectively. The values of Dq, B, C and Dq/B for Mn4þ ions in the host of Ca12.4Sr1.6Zn6Al10O35 were calculated to be 2210.9, 924.70, 2550.5 and 2.39. In order to optimize the emission performance, the PL spectra of the Ca14-xSrxZn6Al9.85O35:Mn0.15 phosphors with different Sr concentrations (x ¼ 0e2.4) were systemically investigated (Fig. 7a). The PL spectra had the similar features of multiple peaks in the deep-red region due to the Mn4þ: 2E/4A2 transitions. Interestingly, the wavelength of the maximum peak monotonically blueshifted from 718 to 713 nm with increasing the Sr2þ concentration (see pink curve in Fig. 7b). The blue shift of the emission peaks should be attributed to the increase of Racah parameter B in samples with larger Sr concentrations. Furthermore, the integrated PL intensity of the phosphors grew up with the increase of Sr concentration, reached the maximum at x ¼ 1.6, and dropped with more Sr (blue curve in Fig. 7b). We believe that the PL intensity is highly relative to the crystallinity of (Ca,Sr)14Zn6Al10O35 phase as evidenced in the XRD and SEM examinations. Accordingly, the PL intensity of CSZA:Mn4þ (x ¼ 1.6) was 1.53 times higher than that in CZA:Mn4þ due to the enhanced crystallinity. However, the intensity decreased when the Sr concentration exceeded 2.0, which could be due to the appearance of an unknown impurity phase as shown in Fig. 1. Fig. 7c shows the PL spectra of Ca12.4Sr1.6Zn6Al10-yO35:Mny phosphors with various Mn4þ concentration (y ¼ 0.05e0.25) under excitation at 325 nm. The peak positions of the PL spectra were not changed in different samples. The PL intensity of Mn4þ showed the maximum value at y ¼ 0.15. Further increase of Mn4þ concentration resulted in decline of PL intensity, which is generally known as concentration quenching due to aggravated energy transfer within adjacent activators. Fig. 7d exhibits the corresponding luminescent kinetic study for the Mn4þ emissions. The decays of all the emissions are single-exponential. With the increase of Mn4þ 4

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Fig. 7. (a) PL spectra of the Ca14-xSrxZn6Al9.85O35:Mn0.15 phosphors with various Sr concentration (x ¼ 0e2.4) under excitation at 289 nm. (b) Peak wavelength (pink curve) and integrated PL intensity (blue curve) of the samples as a function of Sr concentration. (c) PL spectra of the Ca12.4Sr1.6Zn6Al10-yO35:Mny phosphors with various Mn4þ concentration (y ¼ 0.05e0.25). The PL intensity as a function of Mn4þ concentration is plotted in the inset. (d) Luminescence decay curves of the Ca12.4Sr1.6Zn6Al10-yO35:Mny phosphors (y ¼ 0.05e0.25). The excitation and monitoring wavelengths were at 460 and 714 nm, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

concentration, the lifetimes almost kept constant and showed little decrease from 3.26 to 3.13 ms. Compared with reported Mn4þactivated compounds (for example 1.600 ms in Ba2TiGe2O8:Mn4þ and 1.044 ms in Mg2Al4Si5O18:Mn4þ) [52,53], the Ca12.4Sr1.6Zn6Al10O35:Mn phosphors show relatively long lifetimes, suggesting low probabilities for non-radiative transitions. We further compared the QE of the as-prepared CZA:Mn4þ and CSZA:Mn4þ phosphors at RT (Fig. 8). The IQE and EQE values for the CZA:Mn4þ phosphors were 78% and 38%, respectively. In the Srcontained phosphors CSZA:Mn4þ, the IQE and EQE were promoted up to 83% and 53%. Therefore, partial substitution of Sr was proved feasible to improve the luminescent efficiency in the Ca14Zn6Al10O35:Mn4þ phosphors. The main reason for the improvement was the great increase of absorption coefficient due to a better crystallinity. Meanwhile, the IQE also showed a ~5%, which could be beneficial from decrease of surface defects and nonradiative transition probability. The PL spectra of the CSZA:Mn4þ phosphor were measured at various temperatures from 80 to 300 K (Fig. 9a). At 80 K, the PL spectra showed several sharp peaks in the range of 690e730 nm. These peaks contained a zero-photon line (ZPL) of 2E/4A2 transition at ~692 nm as well as its Stokes sidebands. The intensity of these peaks showed a continuous decline at a higher temperature. On the other hand, emission peaks in the range of 660e690 nm became stronger with the increase of temperature. Those peaks from 660 to 690 nm should be attributed to the anti-Stokes sidebands. We integrated the intensity of ZPL/Stokes sidebands as Is and

Fig. 8. PL spectra of CZA:Mn4þ and CSZA:Mn4þ samples under excitation of 460 nm at RT. The spectra were recorded in a Teflon integrating sphere. The wavelength response was calibrated by using attached reference data. The inset shows the IQE and EQE values of the CZA:Mn4þ and CSZA:Mn4þ samples in bar charts.

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Fig. 9. (a) PL spectra of the CSZA:Mn4þ phosphor at various temperatures from 80 to 300 K. The inset gives the integrated intensity as a function of temperature. (b) Integrated intensities of NPL/Stokes sidebands (Is), anti-Stokes sidebands (Ia), and the radio of Ia to Is as a function of temperature. (c) PL spectra of the CSZA:Mn4þ phosphor at 300e540 K. The inset plots the PL integrated intensity as a function of temperature. (d) Configurational coordinate diagram for Mn4þ ions. Arrows 1 and 2 are referred to the electronic transitions of excitation (absorption) and emission, respectively. Arrow 3 shows the excited electron crosses over the crossing point by multi-phonon effect. The energy difference from the bottom of excited state to the crossing point is defined as the activated energy (Ea). Arrow 4 is the non-radiative relaxation process. (e) Arrhenius fitting of the emission intensity of the CSZA:Mn4þ phosphor. The calculated Ea for thermal quenching was 0.354 eV.

the intensity of anti-Stokes sidebands as Ia, and plotted them as a function of temperature. As presented in Fig. 9b, the Is and Ia showed opposite dependence on the temperature. Interestingly, the total PL intensity at 300 K was found to increase about 10% relative to that at 80 K (the inset of Fig. 9a). This abnormal temperature-dependent intensity has been reported in Mn4þdoped fluorides, which was attributed to the increase of absorption at a higher temperature [54]. We further investigated the PL properties above RT to evaluate their potentials in high-power LEDs (Fig. 9c). When the temperature increased from 300 to 580 K, the peak intensity gradually dropped and the emission bands were further broadened. The integrated emission intensity as a function of temperature from 300 to 500 K is plotted in the inset of Fig. 9c. The emission intensity was kept at 98% (425 K) and 92% (500 K) of the initial intensity (300 K), respectively. As a reference, the commercial red phosphors (Sr,Ca) AlSiN3:Eu2þ only maintain 93% (425 K) and 81% (500 K) with respect to that at RT. The thermal quenching mechanism of emission intensity for transition metal ions can be explained by using a configurational coordinate diagram as shown in Fig. 9d. The energy difference from the bottom of the excited state to the crossing point was defined as the activation energy (Ea). The activation energy was estimated by the following equation [42]:

IT ¼

I0 1 þ A$expð  Ea =k$TÞ

(7)

where IT is the PL intensity at different temperatures; I0 is the initial

emission intensity at RT; A is a constant; k is the Boltzmann constant. According to the equation, the Ea value can be calculated by plotting lnðI0 =IT  1Þ against 1/kT. Then the slope of a fitting straight line is equal to Ea. As given in Fig. 9e, the activation energy for the CSZA:Mn4þ phosphors was 0.354 eV. This value is larger than those in the reported Mn4þ-doped phosphors, e.g. BaMgAl10O17:Mn4þ,Mg2þ (0.293 eV), Mg3Ga2GeO8:Mn4þ 4þ (0.218 eV), and Sr4Al14O25:Mn (0.097 eV) [37,55,56]. The excellent thermal quenching property indicated that the synthesized Ca144þ phosphors could be applied in high-power xSrxZn6Al10O35:Mn plant growth LEDs. Table 1 shows the main luminescent properties for various red-emitting phosphors. As-prepared phosphor has obvious advantages in IQE and the thermal stability among the deep-emitting phosphors, whose emission is essential for plant growth. 3.3. Application of CSZA:Mn4þ in plant growth LEDs Encouraged by the outstanding PL performance, we fabricated a LED lamp by combining a blue InGaN LED chip with the synthesized Ca14-xSrxZn6Al10O35:Mn4þ deep-red emitting phosphors. Fig. 10a and b are the photographs of a fabricated LED lamp under the natural light and driven by 80 mA current, respectively. The emission spectra of the LED lamps are given in the upper part of Fig. 10c. The spectra contain blue light from the chip and deep-red light from the phosphors. The spectra match well with the absorption wavelength region of Chlorophyll a/b and phytochrome shown in the lower part of Fig. 10c. Furthermore, as shown in Fig. 10d, the

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Table 1 The main luminescent properties for various phosphors. host

Active ion

PL peak (nm)

IEQ

Relative intensity at 150  C relative to RT

Ref.

Sr[LiAl3N4] Sr2Si5N8 ZnTiF6$6H2O Rb2GeF6 Cs2SiF6 k2TiF6 NaKSnF6 BaMgAl102xO17 TiO2-xMgO LiAlO2 Gd2ZnTiO6 La(MgTi)1/2O3 LaNaAlO3 Ca14Al10Zn6O35 Ca14-xSrxZn6Al10O35

Eu2þ Eu2þ Mn4þ Mn4þ Mn4þ Mn4þ Mn4þ Mn4þ Mn4þ Mn4þ Mn4þ Mn4þ Mn4þ Mn4þ Mn4þ

650 618 631 630 632 635 627 660 660 670 705 708 730 700 713

76% ~80% 26.4% 72.9% 89% 98% 84% e 15.5% 48% 39.7% 27.2% 50% 50% 83%

~97% 90% <10% 90% 95% 70% <10% 45% e e 27.2% 53.0% 30% 88% 98%

[20] [18] [30] [31] [32] [57] [33] [37] [41] [24] [45] [16] [40] [35] This work

Fig. 10. (aeb) Photographs of a fabricated LED lamp under the natural light and driven by 80 mA current. (c) Comparison between the emission spectra of the fabricated plant growth LED lamp with the absorption spectra of Chlorophyll a/b and phytochrome. (d) Emission spectra of a series of LEDs with various CSZA:Mn4þ phosphors (ratio: weight ratio of resin to CSZA:Mn4þ phosphors).

emission spectra of CSZA:Mn4þ in the range of 660e730 nm increased with increasing the weight ratio of CSZA:Mn4þ red phosphor. It is possible to adjust the ratio of red light to blue light according to the needs of plant growth and thus realize highly efficient utilization of lighting source [49,58]. 4. Conclusions In

summary,

a series of deep-red-emitting Ca14phosphors were successfully synthesized via a solid-state reaction method. These phosphors could be effectively excited by near-UV/blue LED chips and exhibited deep-red emissions from 650 to 740 nm originated from the spin-forbidden 2 E/4A2 transitions of Mn4þ. With the introduction of Sr, the PL intensity of Ca12.4Sr1.6Zn6AlO35:Mn4þ was increased by 153% relative to that of Ca14Zn6AlO35:Mn4þ, and realized a high quantum efficiency (the IQE and EQE values up to 83% and 53%, respectively). Moreover, the Ca12.4Sr1.6Zn6AlO35:Mn4þ phosphors showed excellent thermal quenching performance (~98% at 425 K relative to that at RT). Finally, LED lamps emitting bright blue and deep-red light were fabricated by combining the synthesized Ca12.4Sr1.6Zn6AlO35:Mn4þ phosphors with blue InGaN chips. The emissions of the LED lamps matched well with the absorption wavelength region of Chlorophyll and phytochrome. All of these results indicate that the reported Ca14-xSrxZn6Al10O35:Mn4þ deep-

xSrxZn6Al10O35:Mn



red emitting phosphors have potential application in high-power plant growth LEDs. Acknowledgements The authors are grateful for the financial support of the National Natural Science Foundation of China (Grant No. 51572232, 51561135015, 51502254, 61575182), the National Key Research and Development Program (MOST, 2017YFB0404301), the Agriculture “Five Innovations” Project of the Development and Reform Commission of Fujian Province (K6015004), and the Natural Science Foundation of Fujian Province (No. 2018J01588, 2018J01080). References [1] S. Liaros, K. Botsis, G. Xydis, Technoeconomic evaluation of urban plant factories: the case of basil (Ocimum basilicum), Sci. Total Environ. 554 (2016) 218e227. [2] N. Yeh, J.P. Chung, High-brightness LEDs-Energy efficient lighting sources and their potential in indoor plant cultivation, Renew. Sustain. Energy Rev. 13 (2009) 2175e2180. [3] T.S. Sreena, P.P. Rao, A.K.V. Raj, T.R. Aju Thara, Narrow-band red-emitting phosphor, Gd3Zn2Nb3O14:Eu3þ with high color purity for phosphorconverted white light emitting diodes, J. Alloys Compd. 751 (2018) 148e158. [4] Z. Xia, A. Meijerink, Ce3þ-doped garnet phosphors: composition modification, luminescence properties and applications, Chem. Soc. Rev. 46 (2017) 275e299. [5] J. Li, J. Yan, D. Wen, W.U. Khan, J. Shi, M.M. Wu, et al., Advanced red phosphors

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