Synthesis, structure, and luminescence characteristics of far-red emitting Mn⁴⁺-activated LaScO₃ perovskite phosphors for plant growth

Abstract

Far-red emitting phosphors LaScO₃:Mn⁴⁺ were successfully synthesized via a high-temperature solid-state reaction method. The X-ray powder diffraction confirmed that the pure-phase LaScO₃:Mn⁴⁺ phosphors had formed. Under 398 nm excitation, the LaScO₃:Mn⁴⁺ phosphors emitted far red light within the range of 650–800 nm peaking at 703 nm (14 225 cm⁻¹) due to the ²E_g → ⁴A_2g transition, which was close to the spectral absorption center of phytochrome P_FR located at around 730 nm. The optimal doping concentration and luminescence concentration quenching mechanism of LaScO₃:Mn⁴⁺ phosphors was found to be 0.001 and electric dipole–dipole interaction, respectively. And the CIE chromaticity coordinates of the LaScO₃:0.001Mn⁴⁺ phosphor were (0.7324, 0.2676). The decay lifetimes of the LaScO₃:Mn⁴⁺ phosphors gradually decreased from 0.149 to 0.126 ms when the Mn⁴⁺ doping concentration increased from 0.05 to 0.9 mol%. Crystal field analysis showed that the Mn⁴⁺ ions experienced a strong crystal field in the LaScO₃ host. The research conducted on the LaScO₃:Mn⁴⁺ phosphors illustrated their potential application in plant lighting to control or regulate plant growth.

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Introduction

Nowadays, to meet the continuous increasing needs of people, the greenhouse industry has been developed. In the agriculture field, when it comes to conditions for plant growth, moderate sunlight, air, and moderate moisture are usually mentioned as the basic conditions.^1–4 The blue (≈400–500 nm), red (≈600–690 nm) and far red (≈700–740 nm) light in sunlight can affect the growth process of natural plants, such as phototropic processes and photomorphogenesis.^5–7 There exist two kinds of phytochrome, P_R and P_FR, and P_R is the biologically inactive state, whereas P_FR is the biologically active state.^4,8 These two phytochromes can be converted into each other by absorbing different wavelengths of light, in which the P_R was sensitive to red light and can turn into P_FR by absorbing light with wavelength peaking at around 660 nm, whereas P_FR should absorb far red light peaking at around 730 nm to switch to P_R.^9,10 It is known that in order to blossom, short-day plants need to stay in the dark for a longer time than long-day plants.^11–13 So plant growth progress can be controlled by changing the spectral composition in artificial light.

The proportion of red light in the natural sunlight is higher than far red light.⁹ Meanwhile, with the development of science and technology in the world, the far red light become even less due to the night lighting, which means the far-red light is insufficient for plant cultivation and even influence the entire life style of the plant.⁴ Thus, finding artificial light to meet the requirement of plant growth is urgent, especially in the greenhouse industry. In the past, the light emitted by traditional gas-discharge lamps cannot match well with the absorption spectrum of phytochrome, especially the P_FR. But the solid state light-emitting diodes (LEDs) device with long lifetime and low power consumption,^14–21 which can exhibit various colors by coating different phosphors onto the blue/near-ultraviolet LED chip,^22–30 can make up for this drawback. That means the light from the specific LEDs can match well with the absorption spectrum of phytochrome. Many research efforts have already been conducted to develop the red phosphors, such as K₂NaAlF₆:Mn⁴⁺ (630 nm),³¹ Na₃MgZr(PO₄)₃:Eu³⁺ (611 nm),³² and Sr₂MgAl₂₂O₃₆:Mn⁴⁺ (658 nm),³³ which emit red light in the range of 610–660 nm. In contrast, relatively less attention has been paid to the far-red phosphors with emission wavelength within 660–730 nm, which can be used for the plant growth. So it is important to find novel phosphors that can emit far-red light to meet the requirement of far-red light.

Mn⁴⁺ ions in particular host materials with octahedral structures can emit red light and even deep red light due to the ²E_g → ⁴A_2g transition,^34–37 such as Sr₄Al₁₄O₂₅:Mn⁴⁺ (652 nm),³⁸ NaMgGdTeO₆:Mn⁴⁺ (697 nm),³⁹ Ca₃La₂W₂O₁₂:Mn⁴⁺ (711 nm),⁴⁰ La₂MTiO₆:Mn⁴⁺ (M = Mg and Zn; 710 nm),⁴¹ and Ba₂GeO₄:Mn⁴⁺ (667 nm).⁴² Considering that the structure of the LaScO₃ compound with [ScO₆] octahedral structure is similar to LaAlO₃ and Mn⁴⁺-doped LaAlO₃ phosphors can emit far-red emission, so the LaScO₃ compound has been chosen as the host for Mn⁴⁺ doping.^43,44 Thus, in this work, LaScO₃ and Mn⁴⁺ ions had been chosen for the host material and activator, respectively, and a series of LaScO₃:xMn⁴⁺ (x = 0.0005–0.0090) phosphors with different Mn⁴⁺ doping concentration had been synthesized. These prepared phosphors can be well excited by 398 nm and emit far-red light peaking at 703 nm with CIE chromaticity coordinates of (0.7324, 0.2676). The crystal structure of the LaScO₃, decay lifetimes and luminescence properties had also been investigated in detail, and the results indicated the LaScO₃:Mn⁴⁺ phosphors could be serve as the far-red emitting phosphor in plant growth LEDs.

Experimental section

LaScO₃:xMn⁴⁺ (x = 0.0005–0.009) phosphors were prepared by a facile solid-state reaction method using Sc₂O₃ (analytical reagent, AR), MnCO₃ (AR), and La₂O₃ (99.99%) as the starting materials. The raw materials were weighed according to the stoichiometric ratio. The mixed starting materials was ground in an agate mortar and transferred to Al₂O₃ crucible to sinter at 1500 °C for 10 h in air. And then, the products were cooled down to room temperature and reground into powders to get the target phosphors LaScO₃:xMn⁴⁺.

The X-ray powder diffraction (XRD) results of LaScO₃:xMn⁴⁺ phosphors were measured on an X-ray diffractometer (Bruker D8 Advance) with Cu-Kα radiation. The room-temperature photoluminescence (PL)/PL excitation (PLE) spectra and luminescence decay lifetimes of LaScO₃:xMn⁴⁺ phosphors were recorded by using the same Edinburgh FS5 spectrofluorometer, equipping with a 150 W continued-wavelength Xenon lamp and a pulsed Xenon lamp, respectively. The internal quantum efficiency (IQE) of the LaScO₃:0.001Mn⁴⁺ phosphors were measured by the Edinburgh FS5 spectrofluorometer with a integrating sphere.

Results and discussion

Fig. 1 showed the XRD patterns of the LaScO₃:xMn⁴⁺ (x = 0, 0.0005, 0.001, and 0.005) phosphors and the standard data of LaScO₃ (JCPDS: 26-1148). It could be seen that all the diffraction peaks of the samples matched well with the standard data of LaScO₃. There was no excess crystal phase formation, which illustrated the Mn⁴⁺ ions could be successfully doped into the LaScO₃ host. Mn⁴⁺ ions can replace specific cationic sites of the specific host, where cationic sites have a coordination number (CN) of six, such as Nb⁵⁺, Zr⁴⁺, Ti⁴⁺, Te⁶⁺, and W⁶⁺ ions.^{39,40,45–48} Considering the cationic radius and the coordination environment of Mn⁴⁺ (r = 0.535 Å, CN = 6) and Sc³⁺ (r = 0.745 Å, CN = 6),^43,49,50 the Mn⁴⁺ ions could replace the Sc³⁺ sites in the LaScO₃ host.

Fig. 1 XRD patterns of LaScO₃:xMn⁴⁺ (x = 0, 0.0005, 0.001, and 0.005) phosphors and the stand pattern of LaScO₃ (JCPDS: 26–1148).

In order to identify the structure of the as-prepared sample, the Rietveld refinement for the LaScO₃ host and LaScO₃:0.001Mn⁴⁺ phosphors were conducted. Fig. 2(a) and (b) depict the Rietveld refinement results of LaScO₃ host and LaScO₃:0.001Mn⁴⁺, respectively. From the values of R_p and R_wp (R_p = 8.13% and R_wp = 7.55% for LaScO₃ host; R_p = 4.86% and R_wp = 3.50% for LaScO₃:0.001Mn⁴⁺ phosphors), we could know that the obtained results were reliable. The refined crystallographic parameters of the LaScO₃ host and LaScO₃:0.001Mn⁴⁺ phosphors were listed in Table 1 and Table 2, respectively. It could be seen that the parameters of LaScO₃ host and LaScO₃:0.001Mn⁴⁺ phosphors changed slightly due to the difference in ions radii between Sc³⁺ and Mn⁴⁺ ions. The crystal systems of the LaScO₃ host and LaScO₃:0.001Mn⁴⁺ phosphors were orthorhombic with a space group of Pbnm. Fig. 2(c) and (d) show the crystal structure of the LaScO₃ host and the [ScO₆] octahedron, respectively. In the unit cell, one Sc³⁺ ion was surrounded by six O²⁻ ions to form [ScO₆] octahedral units and the Mn⁴⁺ ions could substitute for Sc³⁺ ions to form the LaScO₃:Mn⁴⁺ compound. All the results illustrated the formation of LaScO₃:Mn⁴⁺ phosphors.

Fig. 2 Rietveld refinement of LaScO₃ host (a) and LaScO₃:0.001Mn⁴⁺ (b) using the Fullprof suit software. The structure of (c) LaScO₃ host and (d) [ScO₆] octahedron.

Table 1 Refined crystallographic parameters of LaScO₃ host

Formula	LaScO3
Crystal system	Orthorhombic
Space group	pbnm
Lattice parameters	a = 5.68010(17) Å, b = 5.79022(16) Å, c = 8.0952(2) Å, V = 266.244(13) Å^3

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Atom	x	y	z	Occ.	Uaniso (Å^2)
					U11	U22	U33	U12	U13	U23
La1	0.01060	0.95680	0.25	1	0.01218	0.00649	−0.00259	−0.00656	0.00000	0.00000
Sc1	0.00000	0.5	0.00000	1	0.00448	−0.00362	0.00289	−0.00681	−0.00587	−0.00140
O1	0.71276	0.28618	0.01023	1	−0.05698	0.04639	−0.04425	−0.01988	0.00000	0.00000
O2	0.90808	0.52065	−0.01828	1	0.04604	−0.02305	0.00770	0.02371	0.02416	0.03004

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Table 2 Refined crystallographic parameters of LaScO₃:0.001Mn⁴⁺ phosphors

Formula	LaScO3:0.001Mn^4+
Crystal system	Orthorhombic
Space group	pbnm
Lattice parameters	a = 5.67879(18) Å, b = 5.79140(16) Å, c = 8.0956(3), V = 266.251(14) Å^3

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Atom	x	y	z	Occ.	Uaniso (Å^2)
					U11	U22	U33	U12	U13	U23
La1	0.00909	0.95617	0.25	1	0.00781	−0.00411	0.00299	−0.00040	0.00000	0.00000
Sc1	0.00000	0.5	0.00000	0.999	0.01333	−0.00843	−0.00312	0.00057	0.00554	−0.00207
Mn1	0.00000	0.5	0.00000	0.001	0.01333	−0.00843	−0.00312	0.00057	0.00554	−0.00207
O1	0.91109	0.52509	0.25000	1	−0.00405	−0.02970	−0.00531	−0.00072	0.00000	0.00000
O2	0.70010	0.30343	0.01124	1	0.02081	0.03995	−0.02703	−0.02709	−0.00394	0.00625

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The PLE and PL spectra of the LaScO₃:0.001Mn⁴⁺ phosphors were shown in Fig. 3(a). When monitored at 703 nm, the obtained excitation spectrum of the Mn⁴⁺ ions in LaScO₃ was an asymmetric absorption band, which could be fitted to four peaks though Gaussian fitting, namely, peaks at 346 nm (28 902 cm⁻¹; Mn⁴⁺–O²⁻ transition), 392 nm (25 510 cm⁻¹; ⁴A_2g → ⁴T_1g dominant transition), 462 nm (21 645 cm⁻¹; ⁴A_2g → ²T_2g transition), and 543 nm (18 416 cm⁻¹; ⁴A_2g → ⁴T_2g transition).^48,51 Under the excitation of 398 nm or 549 nm, the LaScO₃:0.001Mn⁴⁺ phosphors emitted deep red light peaking at 703 nm (²E_g → ⁴A_2g transition; see Fig. 3(a)) with the CIE chromaticity coordinates of (0.7324, 0.2676) (see Fig. 3(b)). Fig. 3(c) shows the absorption spectra of P_R and P_FR as well as the emission spectrum of the LaScO₃:0.001Mn⁴⁺ phosphor. Obviously, the PL spectrum of the LaScO₃:0.001Mn⁴⁺ phosphor was located within the absorption spectrum of the P_FR. Importantly, the dominant emission peak of LaScO₃:0.001Mn⁴⁺ phosphor at 703 nm was close to the peak wavelength (around 730 nm) of the absorption spectrum of P_FR. So it was meaningful to apply LaScO₃:Mn⁴⁺ phosphors to plant growth LEDs.

Fig. 3 (a) The PLE spectrum of LaScO₃:0.001Mn⁴⁺ phosphors monitored at 703 nm. The four curves represent the Gaussian fitting curves. (b) The CIE chromaticity diagram of LaScO₃:0.001Mn⁴⁺ phosphors. The insets show the phosphor digital pictures under the 365 nm excitation (ii) and daylight (i). (c) The room-temperature PL spectrum of LaScO₃:0.001Mn⁴⁺ sample excited at 398 nm and the absorption spectra of phytochrome P_R and P_FR. (d) The PL spectra of LaScO₃:xMn⁴⁺ (x = 0.0005–0.009) phosphors excited at 398 nm. (e) Plot of log(I/x) vs. log(x) of Mn⁴⁺ ions in LaScO₃:xMn⁴⁺ phosphors. (f) The luminescence decay curves of LaScO₃:xMn⁴⁺ (x = 0.0005–0.009) phosphors under the test condition of λ_em = 703 nm and λ_ex = 398 nm.

To find the optimal doping concentration of Mn⁴⁺ ions in LaScO₃:Mn⁴⁺ phosphors, a series of LaScO₃:xMn⁴⁺ (x = 0.0005–0.009) phosphors were synthesized and the corresponding PL spectra were shown in Fig. 3(d). It could be found that the PL intensity of the LaScO₃:xMn⁴⁺ phosphors firstly increased as Mn⁴⁺ concentration increased and then decreased, and the optimal-doping concentration reached when x = 0.001, which was attributed to the concentration quenching in the LaScO₃:xMn⁴⁺ phosphors. The IQE of the LaScO₃:0.001Mn⁴⁺ phosphor was 15%. The optical doping concentration of Mn⁴⁺ is much lower than rare-earth doped phosphors, because the d-electron wave functions of transition metals (e.g., Mn⁴⁺ ions) extend more widely than 4f electrons of rare earths (e.g., Eu³⁺ and Tb³⁺ ions).^52,53 Due to the energy transfer between the nearest Mn⁴⁺ and ends with energy transfer to traps or killing sites, concentration quenching occurs in LaScO₃:xMn⁴⁺ phosphors.^47,54 Considering the concentration quenching occurred in LaScO₃:xMn⁴⁺ phosphors, the critical distance (R_c) of Mn⁴⁺ ions in the LaScO₃:xMn⁴⁺ phosphors was calculated using the following equation:^55,56

where V and N represents the volume and the number of host cations of the unit cell, respectively; x_c refers to the optimal doping concentration of Mn⁴⁺ ions. Herewith, V = 266.251(14) ų, N = 4, and x_c = 0.001. Hence, the R_c of Mn⁴⁺ ions in LaScO₃:xMn⁴⁺ was calculated to be about 50.29 Å (>5 Å), which was comparable with that in other Mn⁴⁺ doped phosphors, such as KMgLaTeO₆:Mn⁴⁺ (33.83 Å),⁵⁷ Ba₂TiGe₂O₈:Mn⁴⁺ (40.1 Å),⁴⁹ Li₂MgZrO₄:Mn⁴⁺ (33.8 Å),⁴⁸ indicating that the luminescence concentration quenching derived from the electric multipole interaction.²⁵

To determine whether the electric multipole interaction was electric dipole–dipole (d–d), or dipole–quadrupole (d–q), or quadrupole–quadrupole (q–q) interactions, the following equation could be used to get the exact form:^58,59

where I represents the PL intensity; β and k are constants for a given host lattice; θ = 6, 8 and 10 stands for electric d–d, d–q, and q–q interactions, respectively; x is the dopant content of Mn⁴⁺ ions in LaScO₃ host. The relationship between log(I/x) and log(x) was plotted in Fig. 3(e) and the slop of the fitting result was found to be −1.71990 (=−θ/3), revealing that the value of θ was about 5.16. Thus, the luminescence concentration quenching mechanism of Mn⁴⁺ ions in LaScO₃ host was electric d–d interaction.

Fig. 3(f) exhibits the luminescence decay curves of Mn⁴⁺ 703 nm emissions in LaScO₃:xMn⁴⁺ (x = 0.0005–0.009) phosphors. The decay lifetimes of the LaScO₃:xMn⁴⁺ samples were obtained by the following double-exponential model:⁶⁰

where I refers to the luminescent emission intensities at time t; A₁ and A₂ are constants; and τ₁ and τ₂ are the decay time for the exponential component, respectively. The decay lifetimes of the LaScO₃:xMn⁴⁺ phosphors were calculated to be 0.149 ms (x = 0.0005), 0.138 ms (x = 0.001), 0.133 ms (x = 0.005), 0.130 ms (x = 0.007), and 0.126 ms (x = 0.009). Obviously, the decay lifetimes decreased as the concentration x increased, revealing the non-radiative energy-migration of Mn⁴⁺ ions in the LaScO₃:xMn⁴⁺ phosphors.

To explore the influence of the octahedral coordination environment of LaScO₃:xMn⁴⁺ phosphors on the 3d³ energy level of Mn⁴⁺ ions, Fig. 4(b) shows the Tanabe–Sugano energy-level diagram of Mn⁴⁺ ions in the octahedral site of LaScO₃ host together with the simple energy level diagram of Mn⁴⁺ ions. Based on the ⁴A_2g → ⁴T_2g (18 416 cm⁻¹) transition energy gap, the crystal-field strength (Dq) of the LaScO₃:xMn⁴⁺ phosphors can be roughly estimated by the following equation:⁶¹

Fig. 4 (a) Dependence of the ²E_g energy level of Mn⁴⁺ ions on the nephelauxetic ratio β₁ in different hosts including LaScO₃ [E(²E_g) = −880.49 + 16 261.92β₁ ± σ; σ = 332 cm⁻¹]. (b) (Left side) Tanabe–Sugano energy-level diagram of Mn⁴⁺ ions in the octahedral site of LaScO₃ host and (right side) the simple energy level diagram of Mn⁴⁺ ions.

Based on the PLE spectrum of the LaScO₃:0.001Mn⁴⁺ phosphors, the energy difference between the ⁴A_2g → ⁴T_1g (25 510 cm⁻¹) and ⁴A_2g → ⁴T_2g (18 416 cm⁻¹) was about 7094 cm⁻¹. Therefore, the Racah parameter B can be obtained by the following expression:⁶²

in which the x can be calculated by:

From the PL spectrum of the LaScO₃:0.001Mn⁴⁺ phosphors, the energy of the ²E_g → ⁴A_2g (14 225 cm⁻¹) was acquired and the Racah parameter B was evaluated by the following expression:^47,63

According to the eqn (4)–(7), the crystal field parameters Dq, B, and C were 1842, 701, and 3006 cm⁻¹, respectively. Thus the value of the Dq/B, which represented the intensity of the crystal field, was about 2.628 (>2.2), indicating that the Mn⁴⁺ ions experienced strong crystal strength in LaScO₃ host than those in Y₃Al₅O₁₂ (Dq/B = 1.98),⁶⁴ Ba₂YSbO₆ (Dq/B = 1.88),⁶⁵ and Ca₂LaNbO₆ (Dq/B = 2.31).⁶⁶

The emission energy of the ²E_g → ⁴A_2g transition of Mn⁴⁺ ions in specific host could be affected by the nephelauxetic effect.⁶⁷ The nephelauxetic effect can be described as the parameter β₁, which was established by Brik et al.⁶³ And the value of β₁ can be calculated by the following expression:⁶⁸

where B₀ = 1160 cm⁻¹ and C₀ = 4303 cm⁻¹, which was the Racah parameter of the free Mn⁴⁺ ions. Thus, the value of β₁ in the LaScO₃ host was about 0.924. Brik et al. also have presented the linear relationship [E(²E_g) = −880.49 + 16 261.92β₁ ± σ; σ = 332 cm⁻¹)] between the ²E_g energy level of Mn⁴⁺ ions in different hosts.⁶³ Fig. 4(a) depicts the relationship between the ²E_g energy level of Mn⁴⁺ ions and β₁ in different hosts. All the points were located around the line E(²E_g) = −880.49 + 16 261.92β₁, indicating the data were acceptable. The detailed data of these phosphors, which used in Fig. 4(a), were listed in Table 3. It can be seen clearly from Table 3 and Fig. 4(a) that with the increasing of β₁, the value of the energy level of ²E_g increased. And the value of the nephelauxetic effect in LaScO₃:xMn⁴⁺ phosphors was close to the La(MgTi)_1/2O₃:Mn⁴⁺, NaMgGdTeO₆:Mn⁴⁺, and SiTiO₃:Mn⁴⁺, indicating the similar coordination environments of Mn⁴⁺ ions in these phosphors. Based on the Tanabe–Sugano energy-level diagram in the left diagram of Fig. 4(b), the simple schematic diagram of the Mn⁴⁺ energy level transition in the LaScO₃ host was illustrated in the right diagram of Fig. 4(b). The electrons at the ground state ⁴A_2g (from the ⁴F term) absorbed the energy of the excited light (398 nm or 549 nm) and then were pumped to the excited level ⁴T_1g (from the ⁴F term), ²T_2g (from the ²G term), and ⁴T_2g (from the ⁴F term), after which the electrons could relax to the lowest excited state level ²E_g (from the ²G term) through non-radiative transitions process ⁴T_1g → ²T_2g → ⁴T_2g → ²E_g, and finally released the energy by radiation transitions process with a red light emission at 703 nm.⁶¹

Table 3 The spectroscopic parameters of Mn⁴⁺ ions in different host

No.	Hosts	Dq/cm^−1	B cm^−1	C /cm^−1	Dq/B	β1	E(^2Eg)/cm^−1	Ref.
1	SiTiO3	1818	719	2839	2.529	0.905	13 831 (723 nm)	69
2	La(MgTi)1/2O3	2053	700	2959	2.933	0.915	14 124 (708 nm)	9
3	LaScO3	1842	701	3006	2.628	0.924	14 225 (703 nm)	This work
4	NaMgGdTeO6	2083	727	2971	2.865	0.932	14 347 (697 nm)	39
5	Y2Ti2O7	2000	600	3500	3.333	0.964	14 956 (669 nm)	63 and 70
6	CaZrO3	1850	754	3173	2.454	0.983	15 054 (664 nm)	67
7	K2SiF6	2197	599	3750	3.668	1.013	15 873 (630 nm)	63
8	KTeF5	2267	567	3904	3.998	1.031	16 129 (620 nm)	71

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Conclusions

In conclusion, LaScO₃:xMn⁴⁺ (x = 0.0005–0.009) far-red emitting phosphors have been successfully synthesized via high-temperature solid-state reaction process. The crystal structure of the host and LaScO₃:0.001Mn⁴⁺ phosphors had been discussed and the XRD patterns of the LaScO₃:xMn⁴⁺ proved that they all were pure phase. Monitored at 703 nm, the obtained PLE spectrum of the LaScO₃:0.001Mn⁴⁺ phosphors exhibited four Gaussian fitting peaks, centering at 346 nm (28 902 cm⁻¹; Mn⁴⁺–O²⁻ transition), 392 nm (25 510 cm⁻¹; ⁴A_2g → ⁴T_1g dominant transition), 462 nm (21 645 cm⁻¹; ⁴A_2g → ²T_2g transition), and 543 nm (18 416 cm⁻¹; ⁴A_2g → ⁴T_2g transition). Upon 398 nm excitation, the PL spectrum of LaScO₃:0.001Mn⁴⁺ phosphors showed a far-red emission peaking at 703 nm within the 650–800 nm range, which was very close to the central absorption wavelength of the P_FR at around 730 nm. The optimal doping concentration of Mn⁴⁺ ion was 0.001 in LaScO₃ host and the electric d–d interaction contributed to the concentration quenching mechanism. All the results indicated that the LaScO₃:Mn⁴⁺ phosphors are promising far-red emitting materials for plant growth LEDs application.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51502190), the Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi, and the Open Fund of the State Key Laboratory of Luminescent Materials and Devices (South China University of Technology, No. 2017-skllmd-01).

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