Synthesis and photoluminescence properties of novel far-red-emitting BaLaMgNbO6:Mn4+ phosphors for plant growth LEDs.

A series of far-red-emitting BaLaMgNbO6:Mn4+ (BLMN:Mn4+) phosphors were successfully synthesized by a high-temperature solid-state reaction method. Crystal structure and luminescence properties of the obtained samples were systematically investigated. The emission spectra exhibited a strong narrow far-red emission band peaking at 700 nm with a full width at half-maximum (FWHM) of ∼36 nm under 360 nm excitation. The optimal Mn4+ concentration was about 0.4 mol%. The internal quantum efficiency and CIE chromaticity coordinates of the BLMN:0.4% Mn4+ phosphor were 52% and (0.7222, 0.2777), respectively. In addition, the luminescence mechanism has been analyzed using a Tanabe-Sugano energy level diagram. Finally, by using a 365 nm near-ultraviolet InGaN chip combined with BLMN:0.4% Mn4+ phosphors, a far-red LED device was fabricated. This journal is © The Royal Society of Chemistry.

Introduction

Light plays a key role in indoor plant cultivation, especially for plants that have a high demand for light.[1,2] It is known that light controls the process of plant growth, because blue light around 450 nm (410–500 nm) promotes leaf growth, stomatal opening, and photosynthesis, and red light around 660 nm (610–700 nm) contributes to the growth of plant stems and phototropism, while far-red light around 730 nm (700–740 nm) modulates the process of plant germination and plant flowering time.[3-6] Phytochrome regulates plant growth and development, which have two types – PR and PFR, PR absorbs red light peaked at 660 nm and PFR absorbs far-red light centered at about 730 nm.[7,8] Compared to traditional incandescent and fluorescent lamps, light emitting diodes (LEDs) have become the promising candidates for artificial light sources for plant growth due to their extraordinary advantages such as the high luminescence efficiency, energy saving, and good stability.[9-18] Especially, LEDs can adjust the spectral composition by using various phosphors to match with the spectrum of plant photosynthesis and photomorphogenesis. So, nowadays LEDs have been widely applied in indoor plant growth.[19]

In recent years, non-rare-earth Mn4+ doped red-emitting phosphors for LEDs for plant growth have attracted tremendous attention, because the price for the rare-earth materials has been increasing sharply due to the shortage in the supply of rare-earth materials.[20,21] It is well known that Mn4+ ion with the d[3] electron configuration can emit red or far red luminescence with emission wavelengths ranging from 600 to 800 nm under ultraviolet (UV)/blue excitation, resulting from its 2Eg → 4A2g transition in a strong crystal field.[22-25] Recently, Mn4+-activated red phosphors have been extensively studied.[26] Mn4+ doped fluorides show intense excitation band located at ∼460 nm and sharp red emission centered at ∼630 nm.[27-30] Unfortunately, the toxic HF solution used in preparation process is harmful to the environment and humans, and these fluorides have poor stability in moisture environment. As an alternative, Mn4+ doped oxides, such as Li2MgTiO4:Mn4+, Li2MgZrO4:Mn4+, and Ba2YNbO6:Mn4+ give rise to far-red emissions and have eco-friendly preparation process, which thus can be considered as potential red-emitting phosphors for LEDs.[5,23,31,32] Thus, searching for appropriate hosts providing abundant octahedral sites for Mn4+ ions is urgent.

Compounds with perovskite structure are regarded as potential hosts owing to their good chemical stability and superior optical properties.[33-40] Among the oxides, many niobate-based perovskite-structured phosphors, such as Ca2GdNbO6:Mn4+, Ba2YNbO6:Mn4+, Ba2GdNbO6:Mn4+, and Sr2LaNbO6:Mn4+, have attracted considerable attention.[31,41-43] BaLaMgNbO6 (BLMN) has many [NbO6] octahedrons with a cubic crystal structure that are suitable for Mn4+ doping. In this present paper, we reported novel double-perovskite BLMN:Mn4+ far-red-emitting phosphors, which have been synthesized through high-temperature solid-state reaction in air. It was found that under 360 nm excitation, BLMN:Mn4+ phosphors showed an intense emission band centered at 700 nm between 650 and 800 nm, which was matched well with the absorption band of phytochrome PFR. The optimal doping concentration of Mn4+ was about 0.4 mol%. The CIE chromaticity coordinates and internal quantum efficiency (IQE) of BLMN:0.4% Mn4+ sample were (0.7222, 0.2777) and 52%, respectively. Moreover, the concentration quenching mechanism was discussed. In addition, the luminescence mechanism was explained by the Tanabe–Sugano energy level diagram. Thermal quenching characteristics were also analyzed. All these investigations demonstrated that BLMN:Mn4+ phosphors are promising far-red-emitting materials for far-red LEDs for plant growth.

Experimental

A series of BaLaMgNb1−O6:xMn4+ (BLMN:xMn4+; x = 0.05%, 0.1%, 0.2%, 0.4%, 0.8%, and 1.0%) phosphors were successfully synthesized through a high-temperature solid-state reaction method. BaCO3 (analytical reagent, AR), La2O3 (99.99%), MgO (AR), Nb2O5 (AR), and MnCO3 (AR) were used as the raw materials. According to the stoichiometric ratio, these raw materials were weighted and ground in an agate mortar to achieve uniformity. The obtained mixtures were transferred to the alumina crucibles and pre-calcined at 500 °C for 3 h, then sintered at 1500 °C for 6 h in air. Finally, the samples were cooled to room temperature naturally in the furnace, and they were reground again to obtained final samples for further characterizations.

The X-ray diffraction (XRD) patterns of the samples were recorded on a Bruker D8 X-ray diffractometer using Cu Kα radiation. The morphology properties of the samples were analyzed by using a field-emission scanning electron microscope (FE-SEM; TESCAN MAIA3). The photoluminescence (PL) and PL excitation (PLE) spectra and decay lifetimes were measured by an Edinburgh FS5 spectrometer equipped with a 150 W continued-wavelength xenon lamp and a pulsed xenon lamp, respectively. The IQE was measured by an Edinburgh FS5 spectrometer equipped with an integrating sphere coated with BaSO4.

Results and discussion

Fig. 1 shows the XRD patterns of the as-prepared BLMN:xMn4+ (x = 0.05%, 0.1%, 0.2%, 0.4%, 0.8%, and 1.0%) samples. It can be seen that all the samples exhibited similar diffraction patterns, indicating that increasing Mn4+ ions concentration does not significantly influence the crystal structure.

Fig. 1

XRD patterns of BLMN:xMn4+ (x = 0.05%, 0.1%, 0.2%, 0.4%, 0.8%, and 1.0%) phosphors.

To further analyze the crystal structure details of the as-prepared samples, Rietveld refinement of BLMN:0.4% Mn4+ was carried out. The refined results for the XRD profile of BLMN:0.4% Mn4+ are shown in Fig. 2, including the observed (dot), calculated patterns (black line), the difference between the experimental and calculated data (blue line), and the positions of Bragg reflection (short vertical green line). The final refinement data Rwp and Rp were determined to be 10.84% and 8.93%, respectively, indicating the refined results were reliable. The final refined crystallographic data were listed in Table 1.

Fig. 2

Rietveld refinement XRD patterns of BLMN:0.4% Mn4+ phosphors.

Refined structural data of BLMN:0.4% Mn4+ sample

Formula	BLMN:0.4% Mn4+
Crystal system	Cubic
Space group	Fm3̄m
a = b = c	8.0620 Å
α = β = γ	90°
V	523.99 Å3
N	4
R wp	10.84%
R p	8.93%

Fig. 3 presents the crystal structure of BLMN:0.4% Mn4+. The BLMN:0.4% Mn4+ belongs to double-perovskite oxide and has a cubic system with space group Fm3̄m. The lattice parameters are a = b = c = 8.0620 Å, V = 523.99 Å3, N = 4 and α = β = γ = 90°. It can be found that Nb5+ and Mg2+ ions occupy octahedral sites with six oxygen atoms surrounded. BLMN has many [NbO6] octahedral sites, which are suitable for Mn4+ ions doping. Mn4+ ions prefer to substitute for the Nb5+ ions over Mg2+, Ba2+ and La3+ ions owing to the similar ionic radii (0.53 Å for Mn4+, 0.64 Å for Nb5+, 0.72 Å for Mg2+, 1.35 Å for Ba2+ and 1.32 Å for La3+).[44] As well-known, the radius percentage difference between the doped ions and the substituted ions should be less than 30%. The radius percentage difference between Mn4+ ions and Nb5+ ions can be estimated using the following equation:[34]where Dr is the difference in radius percentage, Rs(CN) and Rd(CN) are the radius of the host ions and doped ions with different coordination number CN, respectively. The calculated Dr was determined to be around 17%, which was much smaller than 30%. Therefore, the Mn4+ ions were substitute for Nb5+ ions in the BLMN host lattice.

Fig. 3

The crystal structure of BLMN:0.4% Mn4+.

Fig. 4(a and b) shows the SEM images of the as-prepared BLMN:0.4% Mn4+ sample. It can be clearly seen that the obtain sample was agglomerated and the particle size ranged within 1–4 μm. Moreover, the corresponding element mapping results were shown in Fig. 4(c–h). The components of Ba, La, Mg, Nb, O, Mn elements were uniformly distributed on the surface of particles. The results further proved that the Mn4+ ions were well-doped in the BLMN host.[45]

Fig. 4

FE-SEM images (a and b) and elements mapping images (c–h).

Fig. 5(a) shows the typical PLE and PL spectra of BLMN:0.4% Mn4+ sample. When monitored at 700 nm, the PLE spectrum exhibited two broad excitation bands in the region of 250 to 600 nm, which could be fitted by four Gaussian curves. The four deconvoluted Gaussian peaks centered at 327, 375, 472, and 520 nm could be ascribed to Mn–O charge transfer band (CTB), and the Mn4+ spin-allowed transitions of 4A2g → 4T1g, 4A2g → 2T2g, and 4A2g → 4T2g, respectively.[46] The PLE spectrum indicated that the BLMN:Mn4+ phosphors can be efficiently excited by near-UV or blue LED chips. When excited at 360 nm and 469 nm, the BLMN:Mn4+ phosphors showed an intense far red emission peaking at 700 nm in the wavelength range of 650–800 nm, which was attributed to 2Eg → 4A2g transition of Mn4+ ions in the [MnO6]8− octahedral environment.[47-50] Moreover, the full-width at half maximum (FWHM) was as narrow as ∼36 nm. Fig. 5(b) shows PL spectrum of BLMN:0.4% Mn4+ phosphors and the absorption spectrum of phytochrome PFR. An obvious overlap between the absorption spectrum of phytochrome PFR and the emissions spectrum of BLMN:0.4% Mn4+ phosphors was observed, which suggested the BLMN:Mn4+ phosphors could be used as far-red emitting materials for plant growth LEDs. Fig. 5(c) shows the PL spectra of BLMN:Mn4+ phosphors with different Mn4+ doping concentrations under 360 nm excitation. There was no change in peak shape and position with increasing Mn4+ concentration except for the emission intensity. It can be clearly seen that the PL emission intensity increased gradually with increasing the doping concentration until it reached a maximum at x = 0.4%, then began to decrease with further increasing Mn4+ content owing to the concentration quenching effect.[51-53] With increasing the doping concentration, the distance between Mn4+–Mn4+ ions decreased, the possibility of energy transfer between the nearest Mn4+ ions increased, leading to lower the luminescence efficiency. In the BLMN:Mn4+ phosphors the energy transfer mechanism can not be the radiation reabsorption, because there is no overlap between PLE and PL spectra of Mn4+ ions.[22] It might be related to exchange interaction or electric multipolar interaction. The critical distance (Rc) was roughly estimated by the following formula:[54]where V is the volume of the unit cell, C is the critical doped concentration of Mn4+, and N is the number of dopant sites available per unit cell. In the BLMN:0.4% Mn4+ case, the C = 0.4%; V = 523.99 Å3; N = 4. Thus, Rc was calculated to be about 39.69 Å. Because the exchange interaction is possible only when the Rc is smaller than 5 Å, so the electric multipolar interaction was the major mechanism of the energy transfer between Mn4+ ions in BLMN host. Furthermore, the detailed concentration quenching mechanism can be analyzed by the following equation:[55]where I is the emission intensity, x is the activator ion concentration, A is concentration and the values of θ = 6, 8 and 10 correspond to electric dipole–dipole, dipole–quadrupole and quadrupole–quadrupole interaction, respectively.[56]Fig. 5(d) shows the dependence of log(I/x) on log(x) of the BLMN:xMn4+ phosphors. The slope of the fitting line was −1.4270. Thus θ = 4.281, which was close to 3, which inferred that the concentration quenching mechanism of BLMN:Mn4+ was the non-radiative energy transfer among the adjacent Mn4+ ions.[57]

Fig. 5

(a) PLE and PL spectra of BLMN:0.4% Mn4+ sample. (b) PL spectrum of BLMN:0.4% Mn4+ phosphors and the absorption spectra of phytochrome PFR. (c) PL spectra of BLMN:xMn4+ (x = 0.05%, 0.1%, 0.2%, 0.4%, 0.8%, and 1.0%) phosphors under 360 nm excitation. The inset shows the integrated emission intensity of BLMN:xMn4+ as a function of Mn4+ concentrations. (d) The dependence of log(I/x) on log(x) of the BLMN:xMn4+ phosphors.

The Commission International de I'Eclairage (CIE) coordinates of the BLMN:0.4% Mn4+ sample were calculated on the basis of its PL emission spectrum and the corresponding results are displayed in Fig. 6(a). The CIE chromaticity coordinates of the BLMN:0.4% Mn4+ sample were determined to be (0.7222, 0.2777) under the excitation of 360 nm. The inset shows the digital images of the as-synthesized BLMN:0.4% Mn4+ sample under daylight and 365 nm UV light. The BLMN:0.4% Mn4+ sample showed a bright far-red light. These results revealed that the BLMN:Mn4+ phosphors may be promising candidates as far-red-emitting materials for plant growth LEDs. Under 360 nm excitation, the IQE of BLMN:0.4% Mn4+ sample was measured and illustrated in Fig. 6(b). The IQE value of the BLMN:0.4% Mn4+ sample was calculated by using the following equation:[58]where η is IQE, LS is the emission spectrum of the sample, ES and ER are the spectra of excitation light with sample and only with BaSO4 reference, respectively. Hence, the IQE value of as-prepared BLMN:0.4% Mn4+ sample reached as high as 52%, which was higher than many recently reported Mn4+-activated red-emitting phosphors, such as SrLaScO4:Mn4+ (IQE: 12.2%), Li2Mg3SnO6:Mn4+ (IQE: 36.3%), Li2MgZrO4:Mn4+ (IQE: 32.3%), Ba2YNbO6:Mn4+ (IQE: 29.2%).[5,31,59,60]

Fig. 6

(a) CIE chromaticity coordinates of BLMN:0.4% Mn4+ phosphors (λex = 360 nm). Inset shows the digital images of BLMN:0.4% Mn4+ in daylight and under a 365 nm UV lamp. (b) Excitation line of BaSO4 and the PL spectrum of BLMN:0.4% Mn4+ phosphor collected using an integrating sphere.

Fig. 7 shows the decay curves of BLMN:xMn4+ (x = 0.05%, 0.1%, 0.2%, 0.4%, 0.8%, 1.0%) phosphors monitored at 700 nm with an excitation wavelength of 360 nm. All of the decay curves were fitted with a single-exponential equation:[61]where I and I0 are the luminescence intensities at time t and t = 0, respectively; τ represents the decay time for the exponential components; and A is a constant. Based on the above equation, the lifetimes of BLMN:xMn4+ (x = 0.05%, 0.1%, 0.2%, 0.4%, 0.8%, and 1.0%) phosphors were determined to be 0.783, 0.760, 0.753, 0.733, 0.677, and 0.652 ms, respectively. As can be seen, with increasing Mn4+ doping concentrations form 0.05% to 1.0%, the lifetimes declined monotonously. This was because that the non-radiative energy migration among the Mn4+–Mn4+ pairs became more faster.[62-64]

Fig. 7

Decay curves of BLMN:xMn4+ (x = 0.05%, 0.1%, 0.2%, 0.4%, 0.8%, and 1.0%) phosphors monitored at 700 nm with an excitation wavelength of 360 nm.

The luminescence mechanism has been analyzed by Tanabe–Sugano energy level diagram of Mn4+ ion in the octahedral site, as shown in Fig. 8. Mn4+ ion with a 3d3 electronic configuration belongs to transition metal ion. When excited at 375, 472, and 520 nm, the electrons at ground state 4A2g were pumped to the excited states 4T1g, 2T2g, and 4T2g, after that the excited electrons can relax to the lowest excited state 2Eg by non-radiative transition process. Finally, the electrons from the lowest excited state 2Eg back to ground state may occur, thus resulting in far-red light centered at 700 nm. Crystal field intensity (Dq) can be roughly calculated using the peak energy (19 231 cm−1) of the 4A2g → 4T2g transition by the following equation:[65-67]thus the value of Dq was 1923 cm−1. The Racah parameter B can be estimated according to the equation:[68]where x is defined as follow:[69]by using the peak energy (26 667 cm−1) of 4A2g → 4T1g transition and 4A2g → 4T2g (19 231 cm−1), the value of B was then determined to be 736 cm−1. Thus, Dq/B value can be calculated to be 2.61, which was beyond 2.2, indicating Mn4+ ions located in a strong crystal field. In additional, another Racah parameter C can be evaluated using following equation:[70]according to the peak energy (14 286 cm−1) of 2Eg → 4A2g transition, the value for C was evaluated to be 2944 cm−1.

Fig. 8

Tanabe–Sugano energy-level diagram of Mn4+ in the octahedral site of BLMN host.

According to the Tanabe–Sugano energy-level diagram, the wide variation in the energy of the 2Eg → 4A2g transition was barely dependent on crystal field strength. It should be determined mainly by the nephelauxetic effect, which was related to the variation in the Racah parameters B and C. Brik et al. introduced a new parameter that described the nephelauxetic effect β1.[70,71]where the free ion values B0 and C0 for the Mn4+ are 1160 cm−1 and 4303 cm−1, respectively. B and C are the Racah parameters of Mn4+ in BLMN host. In the BLMN host the β1 for Mn4+ was determined to be 0.93. Brik et al. fitted a liner relationship between the energy of the emission (2Eg → 4A2g transition) and β1, the equation is E(2Eg → 4A2g) = −880.49 + 16 261.92β1 ± σ, which the σ (332 cm−1) represents the root-mean square (rms) deviation of the data points from the fit line.[72] The calculated 2Eg energy level was 13 911–14 575 cm−1. The experimental energy value for the 2Eg → 4A2g transition of Mn4+ in the BLMN host was 14 286 cm−1. Obviously, the result was in accord with the linear line and thus it was reliable.

Fig. 9(a) shows the temperature-dependent emission spectra of BLMN:0.4% Mn4+ phosphors in the temperature range of 303–463 K under the excitation of 360 nm. It could be observed that with increasing the temperature, the peaks shape and position were almost similar. Moreover, the normalized intensity trend of the BLMN:0.4% Mn4+ was also shown in Fig. 9(b). With increasing temperature, the PL emission intensity decreased gradually due to the temperature quenching effect. To better understand the thermal quenching characteristics, the activation energy Ea was evaluated using the following equation:[21]where I0 is the initial emission intensity, I is the intensity at different temperatures, k is the Boltzmann coefficient, A is the constant, Ea is activation energy. Linear relationship between ln(I0/I − 1) and 1/kT is depicted in Fig. 9(c). The experimental data could be linear fitted with a slope of −0.411, so the value of Ea was obtained to be 0.411 eV.

Fig. 9

(a) Temperature-dependent PL spectra of BLMN:0.4% Mn4+ phosphors under the excitation at 360 nm. (b) The dependence of the normalized emission intensity as a function of temperature. (c) Linear relationship between ln(I0/I − 1) and 1/kT and the calculated activation energy (Ea) for the phosphors.

In order to further demonstrate the potential application value of the BLMN:Mn4+ phosphors in LEDs for indoor plant growth, a far-red LED device was fabricated by using as-obtained BLMN:0.4% Mn4+ phosphors coated onto a 365 nm near-UV InGaN chip. Fig. 10 shows the electroluminescent (EL) spectrum of the as-prepared LED device under the driven current of 60 mA. As shown in the inset of Fig. 10, it could be seen that the fabricated LED device clearly exhibited bright far-red light, which was attributed to a narrow emission band peaking at about 700 nm due to the 2Eg → 4A2g transition of Mn4+ ions in BLMN host. The CIE chromaticity coordinates of the LED was (0.7109, 0.2890). In this work, the device has low luminous efficacy of 0.02 lm W−1. Compared with plants, the ability of human eye to perceive of far red light is weaker, resulting in low luminous efficacy. These results indicated that the BLMN:Mn4+ could be considered as far-red-emitting phosphors for plant growth LEDs.

Fig. 10

EL spectrum of the fabricated far-red LED device by using BLMN:0.4% Mn4+ phosphors and a 365 nm near-UV LED chip under a current of 60 mA. Inset shows the fabricated LED device and corresponding luminescent image.

Conclusions

In summary, novel far-red-emitting BLMN:Mn4+ phosphors were successfully synthesized by the conventional high-temperature solid-state reaction method. The Rietveld refinement results indicated that the structure of sample was a cubic system in space group Fm3̄m. Monitored at 700 nm, two broad absorption bands were shown between 250 and 600 nm. With excitation of 360 and 469 nm, a far-red emission located at 700 nm within the range of 650–800 nm attributed to the 2Eg → 4A2g transition of Mn4+ ions, which was matched well with the absorption spectra of phytochrome PFR. The optimal Mn4+ doping concentration was about 0.4 mol%, and the concentration quenching mechanism of BLMN:Mn4+ was the non-radiative energy transfer among the adjacent Mn4+ ions. The CIE chromaticity coordinates and IQE of BLMN:0.4% Mn4+ sample were (0.7222, 0.2777) and 52%, respectively. In addition, the crystal field strength Dq, the Racah parameters B and C were calculated. The value of activation energy Ea was 0.411 eV. Finally, a far-red LED was fabricated by using BLMN:0.4% Mn4+ phosphors and a 365 nm near-UV LED chip. The obtained results demonstrate the BLMN:Mn4+ phosphors could be red-emitting phosphors for far-red LEDs for plant cultivation field.

Conflicts of interest

There are no conflicts to declare.