Nowadays, an intense research effort has recently been devoted to developing red-emitting phosphors for phosphor-converted light-emitting diodes (pc-LEDs), due to the fact that pc-LEDs, which show many merits including high durability, low energy consumption, short response time, and environmental protection, can be used in indoor plant cultivation, decoration, and medical applications.[1-9] For the indoor plant growth, the growth status can be controlled by adjusting the ratio of the phytochrome PR and PFR, because the PFR can convert to PR by absorbing far-red light centered at 730 nm.[10,11] But in the agricultural industry, especially indoor plant cultivation, there is lack of far-red light to meet the requirement of mutual transformation between the phytochrome PFR and PR due to small percentage of far-red light in the sunlight compared to red light.[10,12] Meanwhile, the traditional lamp cannot match well with the absorption spectrum of the phytochrome PFR. So pc-LEDs play an important role in indoor plant growth, and many research on the phosphors used in indoor plant growth lighting have been implemented such as Ca1−YAl12−MgO19:Mn4+,[13] La(MgTi)1/2O3:Mn4+,[10] Ca14Al10Zn6O35:Bi3+,Mn4+,[14] SrMg2La2W2O12:Mn4+,[15] and Ca3Al4ZnO10:Bi3+,Mn4+.[16] Since the far-red emitting phosphors are vital to the pc-LEDs and relatively little research has been conducted on that kind of phosphors, thus it is meaningful to study the far-red emitting phosphors for applications in indoor plant cultivation.It is known that Mn4+ ions with a 3d3 electronic configuration in a strong crystal fields of octahedral coordination (coordination number (CN) = 6) can give rise to red emission in the wavelength region of 620–800 nm owing to the spin-forbidden 2Eg → 4A2g transition.[17-24] Mn4+-doped phosphors contain two categories: (1) Mn4+-activated fluorides, such as Li3Na3Ga2F12:Mn4+ (λem = 627 nm),[25] Na3TaF8:Mn4+ (λem = 627 nm),[26] and K3ScF6:Mn4+ (λem = 631 nm);[27] and (2) Mn4+-activated oxides, such as Ca3La2W2O12:Mn4+ (λem = 711 nm),[28] Li2MgZrO4:Mn4+ (λem = 670 nm),[29] and Ba2GdSbO6:Mn4+ (λem = 687 nm).[30] Mn4+-activated fluorides generally exhibit red emissions in the 600–650 nm wavelength range and they are widely used in white LEDs application.[13,30-33] In sharp contrast, Mn4+-activated oxides showed far-red emissions at around 700 nm, which are vital to plant growth in photomorphogenesis process. So Mn4+-activated oxides have potential applications as spectral converters for pc-LEDs towards indoor plant growth lighting.[14,34,35]In recent years, double-perovskite oxides with octahedral crystal sites are considered as good host materials for Mn4+ activator.[35-37] In a previous paper by Liu and co-workers, the Sr1−CaLaMgSbO6:Eu3+ phosphors have been researched, and the structural analysis shows that the host compound has two kind of octahedral structures, [MgO6] and [SbO6].[38] Thus, in this present paper, Mn4+-doped SrLaMgSbO6 (SLMS) phosphors have been synthesized and their crystal structure and optical properties were studied. The SLMS host contains two octahedral structures of [MgO6] and [SbO6], and the Mn4+ ions prefer to occupy the Sb5+ sites to form the phase of SLMS:Mn4+. Under the excitation of 333 nm, the SLMS:Mn4+ phosphors emitted far-red light peaking at 705 nm in the spectral range from 625 to 800 nm, which was attributed to the 2Eg → 4A2g transition of Mn4+ ions. More importantly, the emission spectrum of the SLMS:Mn4+ phosphors could match well with the absorption curve of phytochrome PFR, indicating the far-red light from the SLMS:Mn4+ phosphors could be efficiently absorbed by phytochrome PFR. The CIE chromaticity coordinates, internal quantum efficiency (IQE), and thermal stability of SLMS:Mn4+ phosphors were also investigated in detail. The results demonstrated that the Mn4+-activated SLMS phosphors have much potential to serve as far-red emitting phosphors for indoor plant growth lighting.
Experimental section
SrLaMgSb1−O6:xMn4+ (abbreviated as: SLMS:xMn4+; x = 0.05–1.2%) phosphors were synthesized by a traditional solid-state reaction method. The starting materials of SrCO3 (analytical reagent, AR), La2O3 (99.99%), MgO (AR), Sb2O5 (99%), and MnCO3 (AR), were weighed according to the stoichiometric ratio. Using an agate mortar, the mixed starting materials were ground and then transferred to Al2O3 crucible to preheat at 500 °C for 3 h in air. After cooling to room temperature, the preheated mixtures were reground again into powders for further calcination at 1500 °C for 6 h in air to get the target SLMS:xMn4+ phosphors.The X-ray diffraction (XRD) patterns and morphology properties of SLMS:xMn4+ phosphors were measured on a X-ray diffractometer (Cu-Kα radiation; Bruker D8 Advance) and a field-emission scanning electron microscope (FE-SEM; MAIA3 TES-CAN), respectively. Structure refinements were made using Fullprof program. The room-temperature photoluminescence (PL) spectra and the PL excitation (PLE) spectra were measured by an Edinburgh FS5 spectrofluorometer with a 150 W continued-wavelength xenon lamp. The luminescence decay lifetimes, the temperature-dependent spectra, and IQE were recorded by using the same spectrofluorometer, equipping with a pulsed xenon lamp, a temperature controller, and an integrating sphere, respectively. The prototype far-red light-emitting device was fabricated by coating the SLMS:0.8%Mn4+ phosphors onto a 365 nm near-UV InGaN LED chip, and the corresponding electroluminescent (EL) properties were measured by using a spectroradiometer system (HAAS-2000, Everfine).
Results and discussion
Fig. 1 shows the XRD patterns of the SLMS:xMn4+ (x = 0.05%, 0.8%, and 1.2%) phosphors together with the standard card of the CaLaMgMoO6 (JCPDS #43-0043). Compared to the standard card of the CaLaMgMoO6 compound, no other crystalline phase was formed in the SLMS:xMn4+ phosphors. Fig. 2(a) and Fig. 2(b) represent the refinement result and the corresponding crystal structure of the SLMS:0.8%Mn4+ phosphors, respectively. Table 1 gives the main parameters of the crystal cell for SLMS:0.8%Mn4+ phosphors, in which a = 5.63586(16) Å, b = 5.62543(13) Å, c = 7.95407(19) Å, and V = 252.177(6) Å3. The SLMS:0.8%Mn4+ phosphors with double perovskite structure was belong to monoclinic crystal system and a P21/n space group. The host was composed by two octahedral structures of [MgO6] and [SbO6]. Considering the ion radii and CN of the Sb5+ ions (r = 0.6 Å, CN = 6), Mg2+ ions (r = 0.72 Å, CN = 6), and Mn4+ ions (r = 0.53 Å, CN = 6), the Mn4+ ions were more inclined to occupy the Sb5+ sites in SLMS host to form the SLMS:Mn4+ compound.[39-41] Another parameter to determine which ions were substituted by Mn4+ ions was the radii percentage difference (Dr) between the doping ions (here was Mn4+ ions) and the possible substituted ions (here were Sb5+ and Mg2+ ions). The ratio can be estimated by the following expression:[42,43]where Rm(CN) and Rd(CN) are the ionic radii of the host cations (Mg2+ and Sb5+ ions) and the doped Mn4+ ions, respectively. Herein, Rd(CN) = 0.53 Å; the value of Rm(CN) is 0.72 Å and 0.6 Å for Mg2+ and Sb5+ ions, respectively. In this host, CN = 6 for Mn4+, Mg2+, and Sb5+ ions. Thus, the values of Dr were 26% and 12% for Mg2+ host cation and Sb5+ host cation, respectively. Because when Mn4+ ions substituted for Mg2+ ions, the Dr ratio was higher than Mn4+ ions substituted for Sb5+ ions, so the Mn4+ ions were prefer to substitute for Sb5+ than Mg2+ ions. On the other hand, the valence of the Sb5+ ions was closer to Mn4+ ions than Mg2+ ions. Therefore, considering the radii and the valence, the Mn4+ ions would substitute for Sb5+ sites to form the SLMS:Mn4+ phosphors. Fig. 3 shows the typical FE-SEM image of the SLMS:0.8%Mn4+ phosphors. The sizes of the as-prepared sample was found to be about 1–6 μm, and the crystal-surfaces was smooth which was beneficial to get uniform luminescence intensity.
Fig. 1
XRD patterns of the SLMS:xMn4+ (x = 0.05%, 0.8%, and 1.2%) phosphors together with the stand card of CaLaMgMoO6 (JCPDS #43-0043).
Fig. 2
(a) Rietveld refinement of the XRD pattern for the SLMS:0.8%Mn4+ phosphors. (b) The crystal structure of the SLMS:0.8%Mn4+ phosphors.
Refined crystallographic parameters of SLMS:0.8%Mn4+ phosphors
Formula
SLMS:0.8%Mn4+
Crystal system
Monoclinic
Space group
P21/n
a
5.63586(16) Å
b
5.62543(13) Å
c
7.95407(19) Å
α
90°
β
90.054(4)°
γ
90°
V
252.177(6) Å3
RP
10.53%
RWP
13.86%
Fig. 3
The typical FE-SEM of SLMS:0.8%Mn4+ phosphors.
The PLE spectrum (λem = 705 nm) of the SLMS:0.8%Mn4+ phosphors with four Gaussian fitting peaks was shown in Fig. 4(a). Four PLE peaks at 298 nm (33 557 cm−1; Mn4+–O2− charge-transfer transition), 338 nm (29 586 cm−1; 4A2g → 4T1g transition), 423 nm (23 641 cm−1; 4A2g → 2T2g transition), and 495 nm (20 202 cm−1; 4A2g → 4T2g transition) were observed, and the PLE peaks at 333 nm and 499 nm dominated the PLE spectrum.[44-46] Under excitations at 333 nm and 499 nm, the SLMS:0.8%Mn4+ phosphors exhibited a far-red emission band peaking at 705 nm (2Eg → 4A2g transition) in the range of 625–800 nm,[47-49] as shown in Fig. 4(b). Fig. 4(c) displays the PL spectra of the SLMS:xMn4+ (x = 0.05%, 0.6%, 0.8%, 1.0%, and 1.2%) phosphors with different Mn4+ doping concentrations under 333 nm excitation. As the Mn4+ concentration increased, the PL intensity firstly showed an increasing trend, and then decreased when the concentration was higher than 0.8 mol%, indicating the occurrence of the concentration quenching effect. Fig. 4(d) shows the luminescence decay curves of the SLMS:xMn4+ (x = 0.05%, 0.6%, 0.8%, 1.0%, and 1.2%) phosphors. The decay lifetimes of the as-prepared samples were obtained by using the following double exponential expression:[50-53]herein, I refers to the luminescent emission intensity of SLMS:xMn4+ phosphors at time t. τ1 and τ2 are the lifetimes for the exponential component; A1 and A2 are constants. Accordingly, the decay lifetimes of the SLMS:xMn4+ phosphors were found to be 1.257, 1.130, 1.068, 1.034, and 0.950 ms when x = 0.05%, 0.6%, 0.8%, 1.0%, and 1.2%, respectively. Obviously, the decay lifetimes decreased with the increasing Mn4+ concentration, due to the non-radiative energy transfer in the SLMS:Mn4+ phosphors.[54]
Fig. 4
(a) The PLE spectrum of SLMS:0.8%Mn4+ phosphors monitored at 705 nm. The four Gaussian fitting curves also displayed in the figure. (b) The PL spectra of SLMS:0.8%Mn4+ phosphors excited at 333 nm and 499 nm. (c) The PL spectra of SLMS:xMn4+ (x = 0.05–1.2%) phosphors excited at 333 nm. (d) The luminescence decay curves of SLMS:xMn4+ (x = 0.05–1.2%) phosphors (λex = 333 nm; λem = 705 nm).
The critical distance (Rc) is usually used to determine which kind of interaction contribute to the concentration quenching in phosphors. And the Rc for SLMS:xMn4+ phosphors can be estimated by the following expression:[55-58]where V is the volume of the unit cell and xc is the optimal doping concentration of Mn4+ ions in SLMS host; N is the number of host cations which is available for Mn4+ ions to be substituted. In SLMS host, N = 2, V = 252.177(6) Å3, and xc = 0.8%. Thus, the value of xc was calculated to be 31.1 Å. The Rc for SLMS:Mn4+ phosphors was much larger than 5 Å, and thus the electric multipole interaction was the mechanism responsible for the concentration quenching effect in SLMS:Mn4+ phosphors.[59]Fig. 5(a) shows the CIE chromaticity diagram of SLMS:0.8%Mn4+ phosphors. The corresponding CIE coordinates were determined to be (0.733, 0.268), which were located in the far-red region. Fig. 5(b) compares the PL spectrum of the SLMS:0.8%Mn4+ phosphors with the absorption spectrum of phytochrome PFR in the range of 500–800 nm, in which the phytochrome PFR was taken from ref. 10. Apparently, the far-red light emission of the SLMS:0.8%Mn4+ phosphors was matched well with the absorption spectrum of the phytochrome PFR, indicating that the phytochrome PFR was sensitive to the far-red light from the SLMS:0.8%Mn4+ phosphors. Thus, the luminescent properties of the SLMS:0.8%Mn4+ phosphors demonstrated that they had bright prospects to be used in far-red LEDs for indoor plant growth.
Fig. 5
(a) The CIE chromaticity diagram of the SLMS:0.8%Mn4+ phosphors. Insets represent the digital photographs under daylight (i) and a 365 nm UV lamp (ii). (b) The PL spectrum of the SLMS:0.8%Mn4+ phosphors excited at 333 nm and the absorption spectrum of the phytochrome PFR.
The IQE of SLMS:0.8%Mn4+ phosphors was measured to be 35%, as shown in Fig. 6(a). Moreover, the thermal stability of the SLMS:0.8%Mn4+ phosphors was also investigated. Fig. 6(b) shows the temperature-dependent PL spectra of the SLMS:0.8%Mn4+ phosphors under 333 nm excitation. The PL intensity was gradually decreased when the temperature increased, and the emission intensity at 363 K remained about 55% of that at 303 K, which could be seen clearly in Fig. 6(b) and (c). Although the thermal stability was not good as the Ca2YSbO6:Mn4+ (remained 63% at 373 K),[60] Ca2LaNbO6:Mn4+ (remained about 50% at 400 K),[40] and CaLaMgNbO6 (remained about 45% at 423 K),[41] in the next work, we may improve the thermal behavior and luminescence intensity by co-doping another ions such as Li+, Na+, Mg2+, and Ba2+.[49,60-62] Based on the temperature-dependent PL spectra of the SLMS:0.8%Mn4+ phosphors, the corresponding relationship between 1/kT and ln(I0/I − 1) was shown in Fig. 6(d), and the activation energy can be estimated by using the following expression:[38,63,64]where I(T) and I0 are the PL intensity at temperature T and the initial intensity at 303 K, respectively; c is a constant; k is the Boltzmann constant. The slope was found to be −0.3, indicating the activation energy was 0.3 eV. The possible luminescence thermal quenching process could be expressed by the inset in Fig. 6(d). Generally, under the excitation of 333 nm or 499 nm, the electrons in the 4A2g state were firstly excited to the higher 4T1g or 4T2g level, then relax to the lowest 2Eg excited state by non-radiative process.[65] After that, the electrons in the 2Eg level may return to the ground state in two ways. The one way was taken place through 2Eg → 4A2g radiation transition (path: 1 → 2) with 705 nm far-red emission, and such process was occurred at room temperature. The other way was that the electrons in the 2Eg level may absorb enough energy to reach the intersection point at higher temperature, then return to the ground state (path: 1 → 3 → 4) by non-radiative relaxation and resulted in the decrease of the emission intensity of the SLMS:0.8%Mn4+ phosphors.[66]
Fig. 6
(a) Excitation profile (λex = 333 nm) of the reference BaSO4 and the emission spectrum of the SLMS:0.8%Mn4+ phosphors obtained by using an integrating sphere. (b) The temperature-dependent spectra of the SLMS:0.8%Mn4+ phosphors in the temperature range of 303–443 K. (c) The normalized intensity of the PL intensity as a function of the sample temperature. (d) The plot of 1/kT vs. ln(I0/I − 1) for the SLMS:0.8%Mn4+ phosphors. Inset shows the simple configuration coordinate diagram of the 2Eg and 4A2g levels of Mn4+.
The crystal-filed strength (Dq) and two Racah parameters (B and C) of the SLMS host can be calculated by the following expressions (5)–(8):where E(4A2g → 4T1g), E(4A2g → 4T2g), and E(2Eg → 4A2g) was obtained from the emission peaks of 338 nm, 495 nm, and 705 nm, respectively. Thus, according to the exp. (5)–(8), the crystal-filed strength Dq and Racah parameters B and C were obtained to be 2020, 999, 2356 cm−1, respectively. Correspondingly, the Dq/B was calculated to be 2.02, which was higher than that of the Lu3−YAl5O12:Mn4+ (Dq/B = 1.45; x = 0),[49] Ba2YSbO6:Mn4+ (Dq/B = 1.88),[61] and SrGe4O9:Mn4+ (Dq/B = 1.20).[67]Fig. 7 shows the Tanabe–Sugano energy level diagram of d3 electron configuration for Mn4+ in the octahedral crystal filed. Apparently, in an octahedral environment, the ground state 4A2g and excited state 4T1g and 4T2g were split by the 4F term, and the excited state 2T2g and 2Eg were derived from the 2G term.[39,68] The levels of 4T1g, 4T2g, 2T2g, and 2Eg made up the important energy levels for Mn4+ ions to emit far-red light in an octahedral environment.
Fig. 7
Tanabe–Sugano energy level diagram of d3 electron configuration for Mn4+ in the octahedral crystal filed.
Fig. 8 shows the EL spectrum of the as-prepared far-red LED device. A far-red emission band in the 600–780 nm wavelength range was observed, attributing to the 2Eg → 4A2g transition. The external quantum efficiency of the as-prepared far-red LED device was found to be 0.34%. The far-red light from the as-prepared prototype LED device could be easily seen in the inset in Fig. 8. All the results indicated the as-prepared SLMS:Mn4+ phosphors have a promising prospect to be used in plant growth LEDs.
Fig. 8
EL spectrum of the fabricated far-red-emitting LED device by using SLMS:0.8%Mn4+ phosphors and a 365 nm near-UV LED chip under a bias current of 240 mA. Insets show the fabricated LED device without (i) and with (ii) a 240 mA bias current.
Conclusions
In summary, double perovskite SLMS:Mn4+ phosphors were synthesized by using the high-temperature solid-state reaction. The excitation band of the SLMS:Mn4+ phosphors could be fitted into four Gaussian peaks at 298, 338, 423, and 495 nm. The PL spectrum of the SLMS:Mn4+ phosphors showed a far-red emission band peaking at 705 nm, which could match well with the absorption spectrum of the phytochrome PFR. The optimal doping concentration of Mn4+ ions in SLMS:xMn4+ phosphors was x = 0.8%. It was worth noting that the emission intensity of the SLMS:0.8%Mn4+ phosphors was remained about 55% at 363 K compared with that at 303 K, indicating that the as-prepared phosphors showed good thermal-stability behavior. Furthermore, the IQE of the SLMS:0.8%Mn4+ phosphors reached as high as 35%. All the above results suggest that the SLMS:xMn4+ phosphors with far-red emission possess an outstanding potential to be used as far-red emitting phosphors in the application of far-red LEDs for indoor plant growth.
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