Local Structure Modulation Induced Highly Efficient Far-Red

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Local Structure Modulation Induced Highly Efficient Far-Red Luminescence of La1−xLuxAlO3:Mn4+ for Plant Cultivation Jinquan Chen,† Conghua Yang,† Yibo Chen,*,† Jin He,† Zhao-Qing Liu,† Jing Wang,*,‡ and Jilin Zhang§

Inorg. Chem. Downloaded from pubs.acs.org by BOSTON COLG on 05/07/19. For personal use only.

†

School of Chemistry and Chemical Engineering, Institute of Clean Energy and Materials, Guangzhou University, Guangzhou 510006, P. R. China ‡ Ministry of Education Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry, School of Materials Science and Engineering, Sun Yat-Sen University, Guangzhou 510275, P. R. China § Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research and Key Laboratory of Sustainable Resources Processing and Advanced Materials of Hunan Province College, Hunan Normal University, Changsha 410081, P. R. China S Supporting Information *

ABSTRACT: Modulating the local environment around the emitting ions with component screening to increase the quantum yield and thermal stability is an effective and promising strategy for the design of high-performance fluorescence materials. In this work, smaller Lu3+ was introduced into the La3+ site in a Mn4+-activated LaAlO3 phosphor with the expectation of improving the luminescence properties via lattice contraction induced by cation substitution. Finally, a La1−xLuxAlO3:Mn4+ (x = 0−0.04) perovskite phosphor with a high quantum yield of 86.0% and satisfactory thermal stability was achieved, and the emission peak at 729 nm well matches with the strongest absorption peak of the Phytochrome PFR. The favorable performances could be attributed to the suppressed cell volume and superior lattice rigidity after the substitution of Lu3+. This work not only obtains a highly efficient La1−xLuxAlO3:Mn4+ (x = 0.02) phosphor, which holds great potential for application in plant-cultivation light-emitting diodes, but also provides an applicable strategy for further investigation of far-red-emitting phosphors.

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INTRODUCTION

As a new type of light source, phosphor-converted lightemitting diodes (pc-LEDs) have shown overwhelming advantages for plant cultivation, such as diversified light output and flexible light composition as well as high efficiency and durability.6,7 Although the design and manufacture of highly efficient phosphors has attracted worldwide attention,8−12 research on inorganic phosphors with emission wavelengths above 700 nm is quite inadequate, which greatly hinders the development of far-red pc-LEDs in artificial lighting systems. Recently, Mn4+-activated inorganic phosphors have opened a new avenue for the development of far-red-emitting materials. Zhou et al. reported a far-red phosphor La(MgTi)1/2O3:Mn4+ with satisfying thermal stability,13 and Huang et al. reported NaLaMgWO6:Mn4+ and Ca3La2W2O12:Mn4+ phosphors with quantum yields (QYs) of 60% and 47.9% as far-red conversion agents for plant-growth LEDs. 1 4 , 1 5 Besides, the Ca3Al4ZnO10:Bi3+,Mn4+ peaking at 715 nm with a QY of

A hot issue in modern agriculture is managing the light intensity, quality, and cycles flexibly and smartly via an artificial lighting system, while natural light is insufficient for further increasing the production yield of crops and tuning the plant growth.1,2 Phytochrome is one of the most critical signal-transducing photoreceptors, which has great significance for plant photomorphogenesis including the germination of seeds, the growth of the stems and leaves, and the cycle of blooming.3 It has two states: PR is its biologically inactive state, while PFR is a biologically active state. The PR state has the strongest absorption at 660 nm, and the PFR state has the strongest absorption at 730 nm. Under different light stimulation, PR can convert into PFR and vice versa, hence purposefully controlling the rhythms of photosynthesis, phototropism, and photomorphogenesis of the plants.4,5 Therefore, rational design and in-depth investigations on far-red-emitting luminescent materials and devices in order to match well with the absorption spectra of the Phytochrome are in high demand. © XXXX American Chemical Society

Received: February 15, 2019

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DOI: 10.1021/acs.inorgchem.9b00457 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Lattice structure of the LaAlO3 matrix and design scheme of La1−xLuxAlO3:0.001Mn4+. MgF2 was added as a charge-compensation agent.

30.79% and Ca2LaTaO6:Mn4+ with a QY of 34.6% were also reported.16,17 However, the QYs of previously reported far-red emitting phosphors are still too low to satisfy the practical application that requires a QY of ∼90%. Therefore, obtaining a highly efficient and thermally stable far-red-emitting phosphor is still a bottleneck. It is well-known that doping Mn4+ into highly symmetrical AlO6 octahedra can increase the “Mn4+-ligand” bonding covalence and generate deep-red emission because of lowering of the 2Eg → 4A2g emission transition energy.18−20 Very recently, a far-red-emitting Mn4+-activated perovskite aluminate phosphor, LaAlO3:Mn4+, has revealed its encouraging potential both on an agricultural artificial lighting system as a photoluminescence (PL) converter and in medical imaging as a persistent luminescence phosphor.21−24 Especially for the agricultural artificial lighting system, it can convert ultraviolet (UV) light to far-red light, matching well with the absorbance of Phytochrome in the PFR state, and the raw materials are relatively cheap and elementally abundant. Therefore, it is necessary to deeply understand the relationship between the structure and luminescence of the as-reported phosphors, which had not been systematically done and may allow them to modulate the local structure and finally improve the luminescence performance.25,26 It was reported that the luminescence of Mn4+ could be vigorously affected by the local site and crystal structure based on the nephelauxetic effect of Mn4+.27 Besides, the lattice rigidity has a critical effect on the performance of the phosphor. Enhancement of the lattice rigidity means a reduction of the nonradiative energy loss caused by phonon−electron interaction, which helps to improve the QY and thermal stability.28,29 Herein, we conducted a cation substitution strategy on Mn4+activated perovskite aluminate phosphor LaAlO3:Mn4+ by replacing La3+ with smaller and heavier Lu3+ (Figure 1), which can theoretically modify the local crystal environment and make the lattice contract, contributing to a higher PL QY and favorable thermal stability. Fortunately, far-red-emitting La1−xLuxAlO3:Mn4+ (x = 0.02) phosphors with high QY and excellent thermal stability are successfully obtained. Meanwhile, the corresponding mechanism is investigated in detail.

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(Guangzhou, China). All of the reagents were used directly without further purification. Synthesis. The sample was synthesized by a high-temperature solidstate method. First, La2O3, Al2O3, Lu2O3, MnCO3, and MgF2 were weighed by stoichiometry and ground in a mortar for 30 min. Next, the mixture was put into a corundum crucible and sintered at 1500 °C for 5 h under an air atmosphere. Finally, the series of powder samples La1−xLuxAl0.998Mg0.001O3:0.001Mn4+ (LLAO:Mn4+, x = 0−0.04) were collected. Characterization. Powder X-ray diffraction (XRD) was measured by a PANalytical PW3040/60 powder X-ray diffractometer, using a Cu Kα irradiation source (λ = 0.15406 nm) operated at 40 kV. The crystal structure of LLAO:Mn4+ (x = 0−0.04) was refined by applying Materials Studio 7.0 software. Utilizing scanning electron microscopy (SEM; JEOL JSM-7001F) and transmission electron microscopy (TEM; JEM 2100F), the morphology and microstructure of the samples were obtained. UV−vis absorption spectra were obtained by a UV−vis−near-IR spectrophotometer (Lambda 950), with BaSO4 as a standard reference. The PL and PL excitation (PLE) spectra, QY, and temperature-dependent PL spectra of the samples were recorded by a spectrophotometer (FL-7000) equipped with a 450 W xenon lamp as the light source and a piece of self-made heating equipment. QY could be calculated by the following eqs 1−3, where LS, ES, and ER are the emission intensity and the intensity of excitation light with and without samples: ηint =

∫ LS Iem = Iabs ∫ ER − ∫ ES

ηext = ηintξabs

ξabs =

∫ ER − ∫ ES ∫ ER

(1) (2)

(3)

The decay curves were recorded by a spectrophotometer (Edinburgh, FLS920) at room temperature. The Raman spectra were acquired by a laser Raman spectrometer (Thermo Scientific, USA) under a 532 nm laser.

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RESULTS AND DISCUSSION Microstructure and Morphology. It is strongly expected for Lu3+ to replace La3+ rather than Al3+ in the matrix because the radius of Lu3+ (0.0861 nm) is smaller than that of La3+ (0.1032 nm) and bigger than that of Al3+ (0.0535 nm). When the concentration of Lu3+ is lower than 4%, the XRD patterns of the as-prepared La1−xLuxAl1.998Mg0.001O3:0.001Mn4+ (LLAO:Mn4+) are almost consistent with that of LaAlO3

EXPERIMENTAL SECTION

Raw Materials. Rare-earth oxides La2O3 (99.99%) and Lu2O3 (99.99%) were purchased from Aladdin Industrial Inc. (Shanghai, China). Al2O3 (analytical reagent, A.R.), MnCO3 (A.R.), and MgF2 (A.R.) were purchased from Guangzhou Chemical Reagent Co., Ltd. B

DOI: 10.1021/acs.inorgchem.9b00457 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. (a) XRD patterns of LLAO:Mn4+ (x = 0−0.04). (b) Rietveld refinement profile of LLAO:Mn4+ (x = 0.02). (c) SEM image of LLAO:Mn4+ (x = 0.02). (d) Lattice structure schematic diagram of the LaAlO3 matrix vertically along the b axis.

Figure 3. (1) TEM, (2 and 3) HRTEM and (3 inset) the derived FFT SAED images of LLAO:Mn4+: (a) x = 0; (b) x = 0.02; (c) x = 0.04.

LaAlO 3 (ICSD 74494), the diffraction pattern of the LLAO:Mn4+ (x = 0.02) is basically in accordance with the calculated results, and the phase is confirmed to be pure (Figure 2b). The obtained particle shows a typical morphology for powders synthesized by the solid-state method. The smooth surface, clear edges and corners, and rhombohedral-shaped outline similar to the shape of the crystal lattice indicate that a well-crystallized LaAlO3 phase is obtained (Figure 2c,d). Intriguingly, according to the Rietveld refinement results

(LAO; PDF 82-0478) without any visible impurity (Figure 2a). However, the impurity phase of Lu2O3 appears more and more noticeable when the concentration of Lu3+ is higher than 4% (Figure S1). Generally speaking, a continuous solid−solution can form if the radius difference between the doped ions and host cations is less than 15%.30 The difference of the radius is about 16% between Lu3+ and La3+, suggesting that the tolerance to Lu3+ in the lattice structure of LaAlO3 is small, only 4%. According to the Rietveld refinement based on the structure of C

DOI: 10.1021/acs.inorgchem.9b00457 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. (a) PLE (λem = 729 nm) and PL (λex = 337 nm) spectra of LLAO:Mn4+ (x = 0.02) and absorption of the Phytochromes PR and PFR. (b) Integrated intensity and QYs of LLAO:Mn4+ (x = 0−0.04).

Both the PLE and PL intensities of the series of LLAO:Mn4+ (x = 0−0.04) samples first increase and then decrease with an increase of x (Figure S4). The critical substitution concentration of Lu3+ is x = 0.02. Besides, variation of the UV absorption intensity of the LLAO:Mn4+ (x = 0−0.04) samples corresponds well with that of the PLE intensity (Figure S5), which makes it clear that absorption of the samples originates mainly from the electronic transitions of Mn4+. Most importantly, the QYs, one of the most important criteria for fluorescent materials, have been estimated to be 86.0% (internal QY) and 62.1% (external QY) for LLAO:Mn4+ (x = 0.02), showing a 10% and 13% enhancement compared to LAO:Mn4+, respectively (Figures 4b and S6). To our best knowledge, the values are higher than those for most of the reported far-red-emitting Mn4+-activated oxide phosphors (Table 1), and the enhancement could be deemed as

(shown in Table S1 and Figure S2), it can be seen that the lattice parameters and volume of the LLAO:Mn4+ (x = 0−0.04) samples decrease gradually with an increase of the Lu3+ doping concentration (Figure S3). The result strongly suggests that smaller Lu3+ has been successfully incorporated into the LaAlO3 matrix and a contracted crystal lattice has been obtained as expected. The TEM, high-resolution TEM (HRTEM), and corresponding fast Fourier transform (FFT) selected-area electron diffraction (SAED) images of the samples (x = 0, 0.02, and 0.04) were further analyzed (Figure 3). The distinct interplanar space fringes indicate high crystallinity of the samples, and the d values corresponding to the (110) face are determined to be 0.270, 0.266, and 0.264 nm for x = 0, 0.02, and 0.04, respectively. With an increase of the Lu3+ doping concentration, a decrease of the interplanar spacing of the (110) plane also indicates the volume decline of the crystal cell, which is consistent with the Rietveld refinement results mentioned above. These demonstrate that the incorporation of Lu3+ has led to a slight shrinkage of the lattice structure. PL Properties. The PL and PLE spectra of the optimized sample LLAO:Mn4+ (x = 0.02) and the absorption spectra of the Phytochromes PR and PFR are shown in Figure 4a. The PLE spectra show two wide excitation bands in the range of 16666− 40000 cm−1 (250−600 nm) that can be deconvoluted into four peaks by Gaussian fitting at 31188 cm−1 (320 nm), 28117 cm−1 (356 nm), 23537 cm−1 (425 nm), and 19986 cm−1 (500 nm), corresponding to the Mn−O charge-transfer band and the 4A2g → 4T1g, 4A2g → 2T2g, and 4A2g → 4T2g electron transitions of Mn4+, respectively.31,32 The PL spectrum recorded under the strongest excitation peak of 337 nm consists of two sharp peaks at 701 and 729 nm, and its full width at half-maximum (fwhm) is ∼50 nm. It can be clearly clarified that the maximum emission peak of the sample matches well with the absorption peak of the PFR state of the Phytochrome but rarely overlaps with the absorption peak of the PR state. The unique luminescence property makes it possible for the as-synthesized phosphor to selectively activate the PFR state of the Phytochrome in order to promote the corresponding functions of PFR. In this sense, it is strongly expected that the sample can be used as UV-to-far-red photoconverters in far-red-emitting LEDs for plant-growth tuning.

Table 1. Comparison of the Internal QY of the Reported Mn4+-Activated Phosphors (from Higher to Lower QY) compound

excitation (nm)

emission (nm)

QY (%)

ref

La0.98Lu0.02AlO3:Mn4+ Sr2MgGe2O7:Mn4+ Lu3Al5O12:Mn4+ Ca14Zn6Ga10O35:Mn4+ Y3Al5O12:Mn4+ Mg2TiO4:Mn4+, Nb5+ Gd2ZnTiO6:Mn4+ Ba2GdSbO6:Mn4+

337 308 326 310 352 325 365 350

729 659 670 713 673 658 705 687

86 82.5 72.41 64.4 48.6 44.8 39.7 27.7

this work 33 34 35 36 37 38 39

an impressive step toward the practical requirement value of 90%.33−39 However, given the lower price and higher efficiency of the blue-emitting LED chips, we are trying to shift the main excitation band of the material to a blue light band, which may be achieved by introducing fluorine into the matrix. Further research is underway. The thermal quenching behavior of LLAO:Mn4+ (x = 0− 0.04) samples was evaluated in the temperature range of 303− 423 K. Obviously, LLAO:Mn4+ (x = 0.02) has a higher thermal quenching temperature, indicating that it shows better PL efficiency at high temperature compared to others (Figure 5a). The pseudocolor map derived from the PL spectra at different temperatures confirms the superiority of the LLAO:Mn4+ (x = 0.02) phosphor, especially below 373 K (Figure 5b). In the D

DOI: 10.1021/acs.inorgchem.9b00457 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. (a) Relative intensity of LLAO:Mn4+ (x = 0−0.04) versus temperature. (b) HTPL spectra and the pseudocolor maps. (c) Normalized HTPL intensity. (d) Temperature-dependent intensity in the cyclic heating and cooling processes of LLAO:Mn4+ (x = 0.02).

Figure 6. (a) Decay curves and (b) Raman spectra of LLAO:Mn4+ (x = 0−0.04).

normalized temperature-dependent PL spectra of LLAO:Mn4+ (x = 0.02), the relative intensity of the anti-Stokes emission peaks increases significantly (Figure 5c). This phenomenon is similar to many previous reports and can be explained as follows:40,34 when the temperature rises, the vibration of the lattice is enhanced, which would enhance the anti-Stokes radiative transition probability Wa and suppress the radiative transition probability Ws, hence increasing the proportion of the anti-Stokes peaks to Stokes. During the heating and cooling cycles of the sample, the relative intensity−temperature curves basically coincide with each other (Figure 5d). When the temperature reduces from 433 to 303 K, the luminescence intensity of the sample can recover to the same level as before. The results indicate that the increase in the temperature shows no permanent damage to the crystal structure of the sample or induction of any thermally related defect. It is worth mentioning that the emission intensity of LLAO:Mn4+(x = 0.02) at ∼374 K can still maintain 80% of the original value at room temperature,

catering to the requirement for practical application. According to the Arrhenius formula,41,42 the activation energy of thermal quenching (ΔE) can be obtained as 0.70 eV (Figure S7), consistent with its higher thermal quenching temperature. In summary, considering the high QY (86%) and favorable thermal PL stability, LLAO:Mn4+ (x = 0.02) presents a promising application in the fabrication of plant-cultivation LEDs. Local Structure Analysis and the Nephelauxetic Effect. To deeply understand the relationship between the luminescence properties and lattice structure, the decay curves of the samples with different cation-substituted concentrations (x = 0−0.04) were measured. According to eq 4),43,44 ty i It = I0 expjjj − zzz k τ{

(4)

the PL lifetime τ can be calculated as 3.18−3.42 ms. It can be found that the fluorescence lifetime first increases with increasing concentration of Lu3+ and then decreases when the E

DOI: 10.1021/acs.inorgchem.9b00457 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. (a) Tanabe−Sugano energy-level diagram of Mn4+ in an octahedral crystal field. (b) Configurational coordinate diagram for the Mn4+ ions in LLAO:Mn4+.

concentration of Lu3+ ≥ 2%, consistent with the tendency of the QYs (Figure 6a). The crystal lattice with high rigidity could suppress nonradiative relaxation and result in long PL lifetime and large QY.45,46 In order to figure it out, the Debye temperature is calculated because it has been reported to be proportional to the rigidity of the crystal. According to eq 5,47,48 ΘD, i =

3h2TNA Ai kBUiso, i

the addition of an optimized amount of Lu3+ (≤2%), the unit cell volume shrinks because of the replacement of La3+ with smaller Lu3+. The contracted cell could make the [AlO6] octahedra compactly stacked, the crystal rigidity enhanced, and the crystal defects reduced. Therefore, the probability of radiation transition increases and, consequently, the PL properties enhance. In order to analyze the positions of the Mn4+ energy levels in the Tanabe−Sugano energy-level curves, the local crystal-field strength Dq and nephelauxetic ratio β1 can be roughly estimated based on the data of the PLE and PL spectra of LLAO:Mn4+ (x = 0.02) according to eqs 6−10, where B0 is 1160 cm−1 and C0 is 4303 cm−1.53,54

(5)

where Ai is the atomic weight of the atom, Uiso,i is the atomic average displacement parameter, and the Debye temperature (ΘD,i) is inversely proportional to the value of Uiso. The Uiso value of the LLAO:Mn4+ (x = 0−0.04) samples derived by Rietveld refinement first decreases and then increases with the increasing concentration of Lu3+ (Table S2). Accordingly, the Debye temperature of the LLAO:Mn4+ (x = 0−0.04) samples shows a tendency of first increasing and then dragging down. In this way, the increase of the QYs of the series of samples can be explained, which is consistent with enhancement of the lattice rigidity, just as expected. However, when the amount of Lu3+ substitution increases, the lattice rigidity and QY decrease. In order to further explore the influence of the substitution amount on the change of the lattice rigidity, Raman spectra were recorded. There are three distinct peaks at ∼126, 154, and 473 cm−1, respectively (Figure 6b): the peak at 126 cm−1 originates from rotation of the oxygen octahedron along the [001]h crystal direction, the peak at 154 cm−1 is caused by the vibration of La3+ ions in the (001)h crystal plane, and the one at 473 cm−1 originates from the bending vibration of pure oxygen.49−51 The spectrum width of the Raman peak at ∼126 cm−1 first reduces and then reaches the narrowest fwhw value of ∼18.4 cm−1 when x = 0.02. After that, the spectrum further widens with an increase of the Lu3+ concentration. Because defects in the crystal can make the phonon lifetime shorten and contribute to broadening of the vibrational Raman bands,52 the narrowed Raman spectra in this system declare that defects diminish to a certain degree in the range of x = 0−0.02. However, for samples with Lu3+ of 3% and 4%, an uneven local distribution of Lu3+ may bring more defects and, consequently, induce the decreased crystal rigidity, hence the shortened lifetimes and declined QYs. Combined with a discussion on the structure and PL properties, we can rationally explain the origin of the enhanced luminescence: after

E(4 T2g → 4 A 2g)

Dq =

x= Dq B

(6)

10 E(4 A 2g → 4 T1g ) − E(4 A 2g → 4 T2g )

=

Dq

(7)

15(x − 8) x 2 − 10x

(8)

E(2 E 2g → 4 A 2g)/B = 3.05C /B + 7.9 − 1.8B /Dq

ij B yz i y jj zz + jjj C zzz jj B zz jj C0 zz k 0{ k { 2

β1 =

(9)

2

(10)

From the PL and PLE spectra, the values of E( A2g → T2g), E(4A2g → 4T1g), and E(2Eg → 4A2g) are evaluated to be roughly 20184, 28354, and 13991 cm−1. Consequently, Dq, Racah parameters B and C, and β1 are calculated to be 2018 cm−1, 820 cm−1, 2659 cm−1, and 0.94, respectively. From the above results, it can be extrapolated that Dq/B is about 2.46 when x = 0.02. In the Tanabe−Sugano diagram, the corresponding position of Dq/ B and the energy-level splitting of Mn4+ for the sample LLAO:Mn4+ (x = 0.02) in the octahedral field can be seen (Figure 7a). The electron transition process can be described as follows: the ground-state electrons of Mn4+ in LLAO:Mn4+ are initially pumped to the excited states 4T1g, 4T2g, and 2T2g and then transferred to the lowest excited state 2Eg by nonradiative relaxation. Finally, the electrons return to the ground state 4A2g 4

F

4

DOI: 10.1021/acs.inorgchem.9b00457 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry in a spin-forbidden way, giving far-red light. However, if sufficient phonon energy is given, electrons in the lower vibration level of the excited-state level 4T2g of Mn4+ would occupy its higher vibration level. Therefore, it is possible for electrons in the excited-state level 4T2g to come back to the ground-state level 4A2g through the intersection of 4T2g and 4A2g, where the Mn4+ ion releases no photons in a nonradiative pathway. This kind of phenomenon is thermal quenching, and the energy required is the so-called activation energy Ea of thermal quenching, as we already discussed previously (Figure 7b). Fabrication of a Prototype Far-Red-Emitting LED. A prototype far-red LED was prepared by combining a UV chip (365 nm) and the LLAO:Mn4+ (x = 0.02) phosphor in order to explore the application potential of the as-prepared fluorescence material. The assembled LED emits bright far-red light peaking at 729 nm, corresponding well with the absorption position of the Phytochrome PFR (Figure S8a). Moreover, the normalized electroluminenscence (EL) spectral profiles at different currents are basically the same (Figure S8b), which could guarantee the stability of the emission composition under different operating powers. In a word, the fabricated far-red LED prototype has a strong tolerance to different currents, and the fluorescent material LLAO:Mn4+ (x = 0.02) has been confirmed to possess important potential in the domain of far-red LED for plant growth.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.C.). *E-mail: [email protected] (J.W.). Tel: 86-20-39366908. ORCID

Yibo Chen: 0000-0002-0178-9715 Zhao-Qing Liu: 0000-0002-0727-7809 Jing Wang: 0000-0002-1246-991X Jilin Zhang: 0000-0001-7235-341X Author Contributions

J.C. and C.Y. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Guangdong Natural Science Foundation (Grant 2017A030313255), Science and Technology Research Project of Guangzhou (Grant 201804010047), and Featured Innovation Project of Guangdong Universities (Grant 2017KTSCX146).

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CONCLUSIONS By replacement of the La3+ ion with the smaller Lu3+ to rationally tune the crystal structure, a series of La1−xLuxAlO3:Mn4+ (x = 0− 0.04) far-red-emitting phosphors were synthesized by a facile high-temperature solid-state method. The main excitation wavelength of the sample is around 337 nm in the UV region, and the maximum emission wavelength is 729 nm. The QY of the optimum sample LLAO:Mn4+ (x = 0.02) is highly encouraging (86%), and the luminescence intensity can maintain 80% of the room temperature value at 374 K. By means of Raman spectroscopy and Rietveld refinement, we can find that doping with the appropriate concentration of Lu3+ can enhance the lattice rigidity, reduce defects, and then improve the luminescent properties of the phosphors. Furthermore, the asfabricated prototype far-red pc-LED demonstrates that the phosphor can convert UV light to narrow-band far-red light at 729 nm, matching well with absorption of the Phytochrome PFR. All of the results claim that the LLAO:Mn4+ (x = 0.02) phosphor has important application potential in far-red LED lamps used for plant cultivation, and this kind of design tactic is verified to be advanced and applicable.

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LED at 100 mA fabricated with an UV LED chip and a LLAO:Mn4+ (x = 0.02) phosphor, pictures of the fabricated LED, and normalized EL spectra of the LED under different currents, lattice parameters and volume of LLAO:Mn4+ (x = 0−0.04), and equivalent atomic displacement parameters of LLAO:Mn4+ (x = 0−0.04) derived from Rietveld refinement (PDF)

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00457. XRD patterns of LLAO:Mn4+ (x = 0.06, 0.08), XRD Rietveld refinement profiles of LLAO:Mn4+ (x = 0−0.04), variation tendency of the lattice parameters and volume of LLAO:Mn4+ (x = 0−0.04) versus x, PLE and PL spectra of LLAO:Mn4+ (x = 0−0.04), UV−vis absorption spectra of LLAO:Mn4+ (x = 0−0.04), QY of LLAO:Mn4+ (x = 0− 0.04), plot of QY versus x, plot of ln(I0/IT − 1) versus 10000/T for LLAO:Mn4+ (x = 0.02), EL spectrum of the G

DOI: 10.1021/acs.inorgchem.9b00457 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.9b00457 Inorg. Chem. XXXX, XXX, XXX−XXX