Effects of Impurity Doping on the Luminescence Performance ... - MDPI

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Effects of Impurity Doping on the Luminescence Performance of Mn4+-Doped Aluminates with the Magnetoplumbite-Type Structure for Plant Cultivation Xiaoshuang Li 1 , Zikun Chen 1 , Bo Wang 1, * , Ruizhao Liang 1 , Yongting Li 1 , Lei Kang 1 and Pengfei Liu 2 1

2

*

School of Applied Physics and Materials, Wuyi University Jiangmen, Jiangmen 529020, China; [email protected] (X.L.); [email protected] (Z.C.); [email protected] (R.L.); [email protected] (Y.L.); [email protected] (L.K.) Dongguan Neutron Science Center, Dongguan 523803, China; [email protected] Correspondence: [email protected]; Tel.: +86-189-2909-7459

Received: 20 November 2018; Accepted: 24 December 2018; Published: 27 December 2018







Abstract: Mn4+ activated LaMgAl11 O19 (LMA/Mn4+ ) with red emitting phosphor was obtained by sintering under air conditioning. The X-ray diffraction pattern Rietveld refinement results reveal that three six-fold coordinated Al sites are substituted by Mn4+ ions. Furthermore, the chemical valence state of manganese in the LMA host was further confirmed through X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR). Photoluminescence emission (PL) and excitation (PLE) spectra of LMA/Mn4+ as well as the lifetime were measured, and the 663 nm emission is ascribed to the 2 Eg →4 A2g from the 3d3 electrons in the [MnO6 ]8− octahedral complex. The emission spectrum matches well with the absorption of phytochrome. Temperature-dependent PL spectra show that the color changes of the phosphor at 420 K are 0.0110 for ∆x and −0.0109 for ∆y. Moreover, doping Zn2+ and Mg2+ ions in the host enhances the emission intensity of Mn4+ ions. These results highlight the potential of LMA/Mn4+ phosphor for a light-emitting diode (LED) plant lamp. Keywords: LED; phosphor; Mn4+ ; luminescence

1. Introduction Indoor agriculture has attracted considerable attention because of its relatively stable grow environment without outside interference [1,2]. Studies show that light distribution in blue (400–500 nm) and red (600–690 nm) regions has significant implications for plants as it affects the photosynthetic reaction along with the developmental processes of flowering [3–5]. Very recently, phosphor converted light emitting diodes (pc-LEDs) have been recognized as the primary artificial light source for indoor plant growth because of its unique advantages over the traditional gas-discharge lamps, such as simple fabrication technology, being power-economical, low radiant heat output, and the ease of controlling the spectral composition [6–10]. Currently, LED grow light could be produced by the blue-emitting LED, which is prepared though packing of a red phosphor on the GaN chip surface using silicone [11]. Moreover, the plant growth lamps are adjustable according to demand by changing the spectra and luminous efficacy of the phosphor. In particular, the radiation at 650–750 nm wave-bands is necessary for plant growth, thus red phosphor has a major effect on the plant lamps [4,7,12]. It is well known that the transition metal ion Mn4+ (3d3 electronic configuration) presents broad and intense absorption in the near ultraviolet (n-UV) and blue region, resulting from the 4 A2 →(4 T1 , 2 T2 , and 4 T2 ) spin-allowed transitions, and emits Materials 2019, 12, 86; doi:10.3390/ma12010086

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red to near-infrared (NIR) light, arisen by the transition from 2 E→4 A2 in an octahedral coordination environment [13–17]. Furthermore, the Mn4+ ion doped oxide red phosphors perform well as red luminescent materials for LED plant lamps, as demonstrated in Ba2 TiGe2 O8 [5], La(MgTi)1/2 O3 [18], NaLaMgWO6 [7], Li2 MgZrO4 [8], and so on. Besides, LaMgAl11 O19 (LMA) with a magnetoplumbite structure exhibits excellent physical and chemical stability and contains an amount of [AlO6 ] octahedral sites to accept doped Mn4+ ions into the LMA lattice [19]. The LaMgAl11 O19 host doped with Mn2+ ions has been reported in a great deal of research [20–23]. However, the related luminescence property of LaMgAl11 O19 /Mn4+ (LMA/Mn4+ ) phosphor is rarely reported. In this study, a promising red-emitting Mn4+ -activated LMA phosphor for plant growth will be reported. The chemical valence state of manganese in the LMA host was investigated by electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS). As expected, the LMA/Mn4+ phosphor shows a broad excitation band in the UV-blue region, and a narrow red light emission band peaking at 663 nm, which, in accordance with the absorption band of phytochrome, is observed just under 467 nm. The optimal doping concentration of Mn4+ ion is 1 mol %, and the influence of the doping concentration of Mn4+ ions on luminescent property is discussed. By introducing impurities to the LMA host, such as Zn2+ and Mg2+ ions, the luminescent efficiency is significantly improved. All these results demonstrate the great potential of LMA/Mn4+ phosphors for application in the agriculture industry as red-emitting luminescent materials. 2. Experimental Details 2.1. Sample Preparation Sample LaMgAl11 O19 /xMn4+ (LMA/xMn4+ ) phosphors were prepared by the solid-state reaction method. The starting raw materials were La2 O3 , Al2 O3 , MgO, MnCO3 , and R (R = Li2 CO3 , Na2 CO3 , MgO, CaO, ZnO, GeO2 ). LaMgAl11−x−y O19 /xMn4+ /yR (x = 0.02%~5.0%; y = 1% mol) was synthesized by calcination of the mixture of starting materials at 1600 ◦ C for 6 h in an ambient atmosphere. The samples were prepared and then ground in an agate mortar. 2.2. Sample Characterization Structural characterizations were executed by X-ray diffraction (XRD) measurements (X’ Pert PRO, Cu Kα , λ = 1.5418 Å, PANalytical, Holland). The morphology and grain size of the LMA/0.01Mn4+ were investigated using a scanning electron microscope (SEM, Zeiss Sigma500, Jena, Germany). The program Material Studio (MS 5.5, Biovia, San Diego, CA, USA) was used to analyze the crystal structure and atomic position. The photoluminescence (PL) and photoluminescence excitation (PLE) were recorded by Edinburgh Instruments (FLS 980; Livingston, UK) equipped with 450 W xenon lamps as a lighting source. The quantum efficiency (QE) measurements were performed by the spectrophotometer with a barium sulfate coated integrating sphere. The QE, which is defined as the ratio of the total number of photons emitted (Iem ) to the number of photons absorbed (Iabs ), is expressed as R LS Iem R η= = R Iabs ER − ES where LS is the emission spectrum of the sample, and ES and ER are the spectra of the excitation light with and without the sample in the integrating sphere, respectively. The diffuse refection (DR) spectra of the samples were measured by a UV–Vis–NIR spectrophotometer (Lambda 950, Pzserkin Elmer, Canton, MA, USA), using BaSO4 as a standard reference. The electron paramagnetic resonance measurement was carried out using a Bruker A300 (Rheinstetten, Germany) with the microwave frequency fixed at 9.8 GHz. The X-ray photoelectron spectroscopy (Thermo ESCALAB 250XI, Waltham, MA, USA) was conducted by Thermo Fisher Scientific. The first-principles calculations for LMA were performed by CASTEP (MS 5.5, Biovia, San Diego, CA, USA) [24], a plane-wave pseudopotential total energy package based on density functional theory (DFT) [25].

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The Perdew–Burke–Emzerhoff (PBE) function[26] within thefor generalized gradient approximation (GGA) pseudopotentials in the Kleinman–Bylander form all the elements were used to model the form was adopted to describe the exchange-correlation energy. The optimized norm-conserving effective interaction between atom cores and valence electrons. The high kinetic energy cutoff 1000 pseudopotentials Kleinman–Bylander form for allin the elements used to model eV and dense 5 × in 1 ×the 5 Monkhorst–Pack [27] [26] k-point meshes the Brillouinwere zones were chosen the for effective interaction between atom cores and valence electrons. The high kinetic energy cutoff 1000 eV LMA. and dense 5 × 1 × 5 Monkhorst–Pack [27] k-point meshes in the Brillouin zones were chosen for LMA. 3. Results and Discussion 3. Results and Discussion 3.1. Phase Purity and Crystal Structure Analysis 3.1. Phase Purity and Crystal Structure Analysis The XRD patterns of LMA/xMn4+ (x = 0, 0.01, 0.02, and 0.05) were conducted and are shown in The XRD patterns of LMA/xMn4+ (x = 0, 0.01, 0.02, and 0.05) were conducted and are shown in Figure 1a. It is obvious that all the XRD patterns of the as-prepared samples are consistent with the Figure 1a. It is obvious that all the XRD patterns of the as-prepared samples are consistent with the standard data of LaMgAl11O19 (PDF #78-1845), indicating that there is not an observable change of the standard data of LaMgAl114+O19 (PDF #78-1845), indicating that there is not an observable change of the crystal structure with Mn ion doping. Then, the influence of the doping concentration of Mn4+ ion crystal structure with Mn4+ ion doping. Then, the influence of the doping concentration of Mn4+ ion on on the crystal structure was studied by comparing the dominant diffraction peak at 36.16°. the crystal structure was studied by comparing the dominant diffraction peak at 36.16◦ . Apparently, the Apparently, the diffraction peak gradually shifts to the lower angles with the increased doping diffraction peak gradually shifts to the lower angles with the increased doping concentration of Mn4+ concentration of Mn4+ ion, which is the result of the bigger ions Mn4+ occupying the Al3+ ion sites in 4+ 3+ ion, which is the result of the bigger ions Mn occupying the Al ion sites in the LMA host lattice. the LMA host lattice. 4+ was selected as To achieve crystallographic data of the prepared samples, LMA/0.01Mn To achieve crystallographic data of the prepared samples, LMA/0.01Mn4+ was selected as the the representative to carry out XRD Rietveld refinement (Figure 1b). The refining adopted the representative to carry out XRD Rietveld refinement (Figure 1b). The refining adopted the crystallographic data of LaMgAl11 O19 (ICSD #48171) as an initial model and it converged to crystallographic data of LaMgAl11O19 (ICSD #48171) as an initial model and it converged to Rwp = Rwp = 11.5%, demonstrating the high reliability of the refined results [28]. Comparing the different 11.5%, demonstrating the high reliability of the refined results [28]. Comparing the different profiles profiles between the experimental and calculated ones, we can confirm that the sample has a between the experimental and calculated ones, we can confirm that the sample has a hexagonal hexagonal structure belonging to the P63/mmc space group. The lattice parameters of a = 5.5950(6) Å, structure belonging to the P63/mmc space group. The lattice parameters of a = 5.5950(6) Å, c = c = 21.9680(2) Å, α = 90◦ (4), γ = 120◦ (8), and cell volume V = 594.86(3) Å3 3 are consistent with reference 21.9680(2) Å, α = 90°(4), γ = 120°(8), and cell volume V = 594.86(3) Å are consistent with reference data, as listed in Table 1. data, as listed in Table 1.

4+ ; (b) experimental Figure 1. (a) X-ray diffraction (XRD) pattern of LaMgAl11O19 (LMA)/xMn Figure 1. (a) X-ray diffraction (XRD) pattern of LaMgAl11O19 (LMA)/xMn4+; (b) experimental 4+ .(black (black solid line) and calculated (yellow crossed symbol) XRD profiles of LMA/0.01Mn The 4+ solid line) and calculated (yellow crossed symbol) XRD profiles of LMA/0.01Mn . The difference difference profile (blue solid line) and Bragg position (vertical line) are also provided. profile (blue solid line) and Bragg position (vertical line) are also provided.

Many previous studies have proven that Mn4+ ions in octahedral geometry will produce red 1. Crystallographic data ]and refinement parameters of the LMA/0.01Mn4+. emission [11]. Table Hence, the more [Al(O/F) 6 octahedral sites the structure has, the greater the capacity it will offer toAtom adopt Mn4+ xions, which to emit efficient red light. As shown y is beneficial z Occ U Coordination Numberin the LMA structure built based on 0.33333 the refining results 0.75000 (Figure 2),1the coordination environments of Al1−5 atoms La 0.66667 0.00911 MgIt is worth 0.33333 0.00240 4 (Al1 , Al4 , and Al5 ), can be observed. noting0.66667 that there0.02720 are three0.5 six-fold coordinated Al sites 4+ Al1 0.00000 0.00000 0.00000 1 0.00190 6 which are identical to the coordination situation in other Mn ion doped oxide hosts. Correspondingly, 4+ Al2 0.00000 0.00000 0.25000 1 0.02305 5 form the [MnO ]8− the Mn ions could substitute for the Al1 , Al4 , and Al5 sites in the LMA host to 6 Al3 0.33333 0.66667 0.02720 0.5 0.00240 4 octahedral complex. Al4 0.33333 0.66667 0.18950 1 0.00228 6 In order to assess the manganese (IV) center obtained in the phosphor, the EPR spectrum of Al5 0.16740 0.33480 0.89200 1 0.00304 6 LMA/0.01Mn4+O1at 77 K0.00000 was conducted. As shown in1Figure 3a, the typical hyperfine sextet doubly 0.00000 0.15100 0.00278 degeneracy energy are observed, indicating possess a strong crystal field. The center O2 levels0.33333 0.66667 0.94200the Mn 1 ions 0.00329 of the signals corresponding to the0.36000 g value is 2.11 based the following equation: O3 0.18000 0.25000 1 on 0.00709 O4 O5

0.15200 0.50500

0.30400 0.010000

0.05300 0.15100

1 1

0.00455 0.00380

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hv = gβH

(1)

where h is the Planck constant (6.626620 ×10−27 erg/s), ν is the microwave frequency, g is the nondimensional spectral splitting factor (g value), and β is the Bohr magneton (9.27410×10−21 erg/G). The result verifies that the magnetic dipolar transition of Mn4+ ions occupies symmetric octahedral sites. Meanwhile, XPS was further used to confirm the chemical valence state of manganese in LMA/Mn4+ . The XPS spectra of LMA/0.01Mn4+ , LMA/0.05Mn4+ , MnCO3 , and MnO2 are displayed in Figure 3b, in which MnCO3 and MnO2 are used as the reference standards for Mn2+ and Mn4+ ions, respectively. The peaks of the as-synthesized sample could be assigned to the Mn 2p3/2 and are close to the peaks of MnO2 . Meanwhile, the peak intensity increases with the increasing content of Mn as shown in Figure 3b. All these results suggest that the oxidation state of manganese in the LMA host is +4 [29,30]. Table 1. Crystallographic data and refinement parameters of the LMA/0.01Mn4+ . Atom

x

y

z

Occ

U

Coordination Number

La 0.33333 0.66667 0.75000 1 0.00911 Mg 0.33333 0.66667 0.02720 0.5 0.00240 4 Materials 2018, 11, x FOR PEER REVIEW 4 of 11 Al1 0.00000 0.00000 0.00000 1 0.00190 6 Al2 0.00000 0.00000 0.25000 1 0.02305 5 Symmetry: hexagonal Space group: P63/mmc Al3 0.33333 0.66667 0.02720 0.5 0.00240 4 Lattice parameters: a = b = 5.5950 Å; c = 21.9680 Å; α = β = 90°; γ = 120°; V = 594.86 Å3 Al4 0.33333 0.66667 0.18950 1 0.00228 6 Many Al5 previous 0.16740 studies have proven that Mn4+ ions geometry 6will produce red 0.33480 0.89200 1 in octahedral 0.00304 O1 Hence,0.00000 0.00000 6] octahedral 0.15100 1 the 0.00278 emission [11]. the more [Al(O/F) sites structure has, the greater the capacity 0.66667 is beneficial 0.94200 to emit 1 efficient 0.00329red light. As shown in the LMA it will offerO2 to adopt 0.33333 Mn4+ ions, which O3 0.18000 0.36000 0.25000 1 0.00709 structure built based on the refining results (Figure 2), the coordination environments of Al1−5 atoms O4 0.15200 0.30400 0.05300 1 0.00455 can be observed. It is 0.50500 worth noting that there are three six-fold coordinated Al sites (Al1, Al4, and Al5), O5 0.010000 0.15100 1 0.00380 which Symmetry: are identical tohexagonal the coordinationSpace situation Mn4+ ion doped oxide hosts. group: in other P63/mmc ◦ ; γ = 120◦ ; V = 594.86 Å3 4+ Lattice parameters: a = b = 5.5950 Å; c = 21.9680 Å; α = β = 90 Correspondingly, the Mn ions could substitute for the Al1, Al4, and Al5 sites in the LMA host to form

the [MnO6]8− octahedral complex.

Figure 2. The crystal structure of LaMgAl11 O1919and andthe theoctahedral octahedralAlO AlO66ligands. ligands. 11O

Scanning microscope (SEM) energy dispersive X-ray spectroscopy (EDS) were In order toelectron assess the manganese (IV) and center obtained in the phosphor, the EPR spectrum of 4+ performed to further display the detailed morphological feature and elements of LMA/0.01Mn LMA/0.01Mn4+ at 77 K was conducted. As shown in Figure 3a, the typical hyperfine sextet doubly particles. Asenergy shownlevels in Figure 4a, the sizeindicating of the selected particles is about µm, illustrating the degeneracy are observed, the Mn ions possess a 5–15 strong crystal field. The good crystallization of the as-synthesized sample. The EDS analysis of the phosphors is presented center of the signals corresponding to the g value is 2.11 based on the following equation: in Figure 4b. All the targeted peaks of the elements (lanthanum (La), magnesium (Mg), aluminum (1) ℎ =The elemental mapping was carried out to further (Al), and oxygen (O)) could be clearly observed. confirmhthe distribution elements.×10 The−27EDS image shows presence frequency, of La, Al, Mg, and O where is uniform the Planck constantof(6.626620 erg/s), ν is the the microwave g is the in the sample, as well as the homogenous distribution of the corresponding elements in the phosphor nondimensional spectral splitting factor (g value), and β is the Bohr magneton (9.27410×10−21 erg/G). (Figure 4c).verifies that the magnetic dipolar transition of Mn4+ ions occupies symmetric octahedral The result sites. Meanwhile, XPS was further used to confirm the chemical valence state of manganese in LMA/Mn4+. The XPS spectra of LMA/0.01Mn4+, LMA/0.05Mn4+, MnCO3, and MnO2 are displayed in Figure 3b, in which MnCO3 and MnO2 are used as the reference standards for Mn2+ and Mn4+ ions, respectively. The peaks of the as-synthesized sample could be assigned to the Mn 2p3/2 and are close to the peaks of MnO2. Meanwhile, the peak intensity increases with the increasing content of Mn as

sites. Meanwhile, XPS was further used to confirm the chemical valence state of manganese in LMA/Mn4+. The XPS spectra of LMA/0.01Mn4+, LMA/0.05Mn4+, MnCO3, and MnO2 are displayed in Figure 3b, in which MnCO3 and MnO2 are used as the reference standards for Mn2+ and Mn4+ ions, respectively. The peaks of the as-synthesized sample could be assigned to the Mn 2p3/2 and are close to the peaks of MnO2. Meanwhile, the peak intensity increases with the increasing content of Mn as Materials 2019, 12, 86 5 of 11 shown in Figure 3b. All these results suggest that the oxidation state of manganese in the LMA host Materials2018, 2018,11, 11,xxFOR FORPEER PEERREVIEW REVIEW of 11 11 Materials 55 of is +4 [29,30]. Figure 3. 3. (a) (a) Electron Electron paramagnetic paramagnetic resonance resonance (EPR) (EPR) spectrum spectrum of of LMA/0.01Mn LMA/0.01Mn4+4+ at at 77 77 K; K; (b) (b) the the Mn Mn Figure 4+ and 2p X-ray photoelectron spectroscopy (XPS) spectra of MnO 2 , MnCO 3 , and LMA/0.01Mn 4+ 2p X-ray photoelectron spectroscopy (XPS) spectra of MnO2, MnCO3, and LMA/0.01Mn and LMA/0.05Mn4+4+.. LMA/0.05Mn

Scanning electron electron microscope microscope (SEM) (SEM) and and energy energy dispersive dispersive X-ray X-ray spectroscopy spectroscopy (EDS) (EDS) were were Scanning 4+ performed to further display the detailed morphological feature and elements of LMA/0.01Mn performed to further display the detailed morphological feature and elements of LMA/0.01Mn4+ particles. As As shown shown in in Figure Figure 4a, 4a, the the size size of of the the selected selected particles particles is is about about 5–15 5–15 μm, μm, illustrating illustrating the the particles. good crystallization crystallizationof of the the as-synthesized as-synthesized sample. sample. The The EDS EDS analysis analysisof of the the phosphors phosphorsis ispresented presented in in good Figure 4b. All the targeted peaks of the elements (lanthanum (La), magnesium (Mg), aluminum (Al), Figure 4b. All the targeted peaks of the elements (lanthanum (La), magnesium (Mg), aluminum (Al), andoxygen oxygen(O)) (O))could couldbe beclearly clearlyobserved. observed.The Theelemental elementalmapping mappingwas wascarried carriedout outto tofurther furtherconfirm confirm and 4+ the uniform uniform distribution of elements. elements. The The EDS image image shows theof presence of La, La, Al, Al, Mg, and O in in the the Figure 3.distribution (a) Electron of paramagnetic resonance (EPR)shows spectrum LMA/0.01Mn atMg, 77 K;and (b) O the the EDS the presence of 4+phosphor Mn 2p X-ray photoelectron spectroscopy (XPS) spectra of MnO , MnCO , and LMA/0.01Mn and sample, as well as the homogenous distribution of the corresponding elements in the 2 3 elements in the phosphor sample, as well as the homogenous distribution of the corresponding 4+ . LMA/0.05Mn (Figure 4c). (Figure 4c).

4+ (b) EDS spectra of Figure (a) Scanning Figure 4. 4. (a) (a) Scanning Scanning electron electron microscope microscope (SEM) (SEM) image imageof ofLMA/0.01Mn LMA/0.01Mn4+4+;;; (b) (b) EDS EDS spectra spectra of of Figure 4. electron microscope (SEM) image of LMA/0.01Mn 4+ (c) SEM and element distribution mapping of the corresponding sample. LMA/0.01Mn 4+; ;(c) LMA/0.01Mn SEM and element distribution mapping of the corresponding sample. 4+ LMA/0.01Mn ; (c) SEM and element distribution mapping of the corresponding sample.

3.2. Electronic Structure Calculations of LMA 3.2. Electronic Electronic Structure Structure Calculations Calculations of of LMA LMA 3.2. The electronic structure of LaMgAl11 O19 is presented in Figure 5a. The top of the valence band The electronic electronic structure structure of of LaMgAl LaMgAl1111O O1919 is is presented presented in in Figure 5a. 5a. The top top of of the the valence valence band band (VB) The maximum and the bottom of the conduction band (CB)Figure minimumThe locate at different k-points, (VB) maximum and the bottom of the conduction band (CB) minimum locate at different k-points, (VB) maximum and the bottom of the conduction band (CB) minimum locate at different k-points, revealing that LaMgAl 11 O19 is an indirect semiconductor with a band gap of 4.05 eV. The wide band revealing that that LaMgAl LaMgAl1111O O1919 is is an an indirect indirect semiconductor semiconductor with with aa band band gap gap of of 4.05 4.05 eV. eV. The The wide wide band band 4 revealing gap demonstrates that the LaMgAl 11 O19 is a good luminescent material for accommodating the A 2g gap 2demonstrates demonstrates that that the LaMgAl LaMgAl1111O O1919 is is aa good good luminescent luminescent material material for for accommodating accommodating the the 44A A2g2g 4+ gap the and Eg states of Mn ion [16]. Additionally, Figure 5b shows the partial density of states (DOS) of La, and 22EEgg states states of Mn Mn4+4+ ion ion [16]. [16]. Additionally, Additionally, Figure Figure 5b 5b shows shows the the partial partial density density of of states states (DOS) (DOS) of of and Mg, Al, and O.of Corresponding with the total DOS for LaMgAl 11 O19 , the VB top is mainly composed of La, Mg, Al, and O. Corresponding with the total DOS for LaMgAl 11 O 19 , the VB top is mainly La, Mg, Al,La-4f and states O. Corresponding with contribution the total DOS forCB LaMgAl O-2p, while make a significant to the top. 11O19, the VB top is mainly composed of of O-2p, O-2p, while while La-4f La-4f states states make make aa significant significant contribution contribution to to the the CB CB top. top. composed

Figure 5. (a) Calculated energy band structure of LMA; (b) total and partial (La, Al, Mg, and O atoms) Figure5. 5.(a) (a)Calculated Calculatedenergy energyband bandstructure structureof ofLMA; LMA;(b) (b)total totaland andpartial partial(La, (La,Al, Al,Mg, Mg,and andO Oatoms) atoms) Figure density of states (DOS) for LMA. PDOS—partial DOS. density of states (DOS) for LMA. PDOS—partial DOS. density of states (DOS) for LMA. PDOS—partial DOS.

3.3. Components Components Luminescent Luminescent Properties Properties of of LMA/Mn LMA/Mn4+4+ 3.3. Figure 6a 6a depicts depicts the the diffuse diffuse reflection reflection (DR) (DR) spectra spectra of of LMA LMA with with different different doping doping Figure 4+ concentrations of Mn ion. Similar to the results reported previously, there is a characteristic intense concentrations of Mn4+ ion. Similar to the results reported previously, there is a characteristic intense

LMA/Mn4+ sample in the Commission Internationale de L'Eclairage (CIE) 1931 color spaces is calculated to be (0.725, 0.274), which is beneficial to the long-day plants. The PLE spectrum of the sample comprises two ultraviolet peaks at 345 nm and 390 nm, and one distinguishable blue band centered around 470 nm derived from the inner d–d transitions of Mn4+ ions. The peak locations are the same as the absorption peaks in the DR spectra and the peaks originate from the Mn4+–O2− chargeMaterials 2019, 12, 86 6 of 11 transfer band (CTB) 4A2g→4T1g and spin-allowed transitions 4A2g→4T2g of Mn4+ ions, respectively. The internal quantum efficiency of LMA/Mn4+ is measured to be 38.5% for the excitation wavelength of 4+ 3.3. Components Luminescent Properties LMA/Mn 465 nm, as demonstrated in Figure S1. of The value is comparable to the one reported by Peng et al. [13]. 4+-doped aluminates, TheseFigure spectral features with (DR) otherspectra previous studies Mndoping 6a depicts the agree diffusewell reflection of LMA withabout different concentrations 4+ 4+ suggesting the Mn had reported been successfully incorporated into the LMA host. spin-allowed The Tanabe– of Mn ion.that Similar to theions results previously, there is a characteristic intense 3 on the parameters 4 4+ 4 Sugano energy-level diagram illustrates the dependence of energy levels of 3d Mn / A2g → T2g transition peaking at ~470 nm and a weak recognizable spin-forbidden 4 A2g →2 Tof 1g Dq, B, and C,atin390 which is a parameter theband strength of the octahedral crystal field, one located nm Dq [11,16]. However,that thecharacterizes charge transfer is too weak to separate from the while B and C are Racah parameters. From the supplementary equations (1)–(4), the values of Dq, B, DR spectra. and C in the LMA/Mn4+ are then determined to be 2150 cm−1, 321 cm−1, and 4076 cm−1, respectively.

4+ (x = 0.005, 0.01, and 0.05); 4+ (x = Figure (a)The Thediffuse diffusereflection reflection(DR) (DR)spectra spectraofofLMA/xMn LMA/xMn Figure 6. 6. (a) 0.005, 0.01, and 0.05); (b) 4+ at room temperature (b) photoluminescence excitation (PLE) spectra of LMA/0.01Mn 4+ at room photoluminescence (PL)(PL) andand PL PL excitation (PLE) spectra of LMA/0.01Mn temperature and and the absorption spectrum of phytochrome is defined thered redlight light absorbed absorbed by by the RR(P the absorption spectrum of phytochrome PR P(P isR defined as asthe the phytochrome). phytochrome).

Figure 6b shows the normalized andintegrated PL excitation (PLE)is spectra The doping concentration of Mn4+photoluminescence ion dependence of (PL) the PL intensity shown of in 4+ at room temperature. Apparently, one prominent red emission band at 663 nm the LMA/0.01Mn Figure 7a. Typically, the luminescence intensity increases gradually at first with the increase of the 2 E , 2 T →4 A of the 3d3 electrons from Mn4+ ions caused the anti-stokes and stokes transition g a maximum 2g 2g value at 0.01, and finally decreases 4+ ion, doping by concentration of Mn then approaches 8− octahedra is observed under excitation of 467 nm [11]. The chromaticity coordinate in the [MnO ] 6 4+ when the doping concentration of Mn ion is slightly higher than 0.01. The exchange interaction or of the LMA/Mn4+ sample in the Commission Internationale de L’Eclairage (CIE) 1931concentration color spaces multipole–multipole interaction within the nearest Mn4+ ions are ascribed to the is calculated to be (0.725, 0.274), which is beneficial to the long-day plants. The PLE quenching phenomenon. To clarify this point, it is necessary to calculate the critical transferspectrum distance of sample comprises two ultraviolet peaks at 345 nm 390 nm,via and distinguishable 4+ ions. Upon (Rc)the among the Mn the Blasse mechanism [31], Rc and is evaluated theone following equation: blue band centered around 470 nm derived from the inner d–d transitions of Mn4+ ions. The peak 1 locations are the same as the absorption peaks in the DR 3spectra and the peaks originate from the   3 V (2)4+ Mn4+ –O2− charge-transfer band (CTB) 4 AR2gC → transitions 4 A2g →4 T2g of Mn 42T1g and spin-allowed  4+ Z ions, respectively. The internal quantum efficiency is measured to be 38.5% for the  4ofxc LMA/Mn excitation wavelength of 465 nm, as demonstrated in Figure S1. The value is comparable to the In this work, V = 594.86 Å3, N = 28, xc = 0.01, and the critical Rc of Mn4+ ion in LMA is calculated one reported by Peng et al. [13]. These spectral features agree well with other previous studies to be ~15.9 Å. The estimated Rc value is bigger than 5 Å, hence it is inferred that an exchange about Mn4+ -doped aluminates, suggesting that the Mn4+ ions had been successfully incorporated interaction may not be the main possible approach. into the LMA host. The Tanabe–Sugano energy-level diagram illustrates the dependence of energy levels of 3d3 on the parameters of Dq, B, and C, in which Dq is a parameter that characterizes the strength of the octahedral crystal field, while B and C are Racah parameters. From the supplementary Equations (1)–(4), the values of Dq, B, and C in the LMA/Mn4+ are then determined to be 2150 cm−1 , 321 cm−1 , and 4076 cm−1 , respectively. The doping concentration of Mn4+ ion dependence of the PL integrated intensity is shown in Figure 7a. Typically, the luminescence intensity increases gradually at first with the increase of the doping concentration of Mn4+ ion, then approaches a maximum value at 0.01, and finally decreases when the doping concentration of Mn4+ ion is slightly higher than 0.01. The exchange interaction or multipole–multipole interaction within the nearest Mn4+ ions are ascribed to the concentration quenching phenomenon. To clarify this point, it is necessary to calculate the critical transfer distance (Rc ) among the Mn4+ ions. Upon the Blasse mechanism [31], Rc is evaluated via the following equation:

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3V RC ≈ 2 4πxc Z

1 3

(2)

In this work, V = 594.86 Å3 , N = 28, xc = 0.01, and the critical Rc of Mn4+ ion in LMA is calculated to be ~15.9 Å. The estimated Rc value is bigger than 5 Å, hence it is inferred that an exchange interaction may not2018, be the possible approach. Materials 11, xmain FOR PEER REVIEW 7 of 11

4+ Figure Figure 7. 7. (a) (a) The The PL PL integrated integrated intensities intensities with with Mn Mn4+ concentration, (b) as well as as dependence dependence of of log(I/x) on log(x). log(I/x) on log(x).

Thus, Thus, the the concentration concentration quenching quenching mainly mainly relies relies on on the the electric electric multipolar multipolar interaction, interaction, which which is is 4+ confirmed confirmedby byDexter’s Dexter’s theory theory[32]. [32]. The The type typeof ofinteraction interactionbetween betweenMn Mn4+ ions ions can can be be expressed expressed by by the the following following equation: equation: h i θ −1 I = K 1 + β( x ) 3 1 (3)  xI   3 (1)  K 1    x   and θ is an indication of the electric where x is the activator concentration; Kx and β are constants;   multipolar character in which θ = 6, 8, 10 corresponds to dipole–dipole, dipole–quadrupole and where x is the activator concentration; K and7bβdepicts are constants; and θ is of anlog(x) indication of the electric quadrupole–quadrupole, respectively. Figure the dependence and log(I/x) for the 4+ multipolar character in which θ = slope 6, 8, 10 corresponds to ~2.12 dipole–dipole, dipole–quadrupole and LMA/Mn phosphors. The linear is calculated to be and the value of θ is determined quadrupole–quadrupole, respectively. Figure 7b depicts the dependence of log(x) and log(I/x) for as approximately 6. Consequently, the quenching mechanism is a dipole–dipole interaction the in LMA/Mn4+4+phosphors. The linear slope is calculated to be ~2.12 and the value of θ is determined as LMA/Mn . approximately Consequently, quenching a dipole–dipole interaction in sample LMA/Mnis4+.a Generally,6.as a red color the converter in mechanism blue-chips, isthe thermal stability of the Generally, as a red color converter in blue-chips, the chip thermal stability of the sample is a significant parameter in fundamental research, because the temperature usually undergoes parameter fundamental research, because the chip temperature usually undergoes asignificant high temperature (> in 423K), which would degrade emission intensity and color quality. Figure 8aa 4+ spanning high temperature (> 423K), whichemission would degrade intensity and colorfrom quality. gives the temperature-dependent spectra ofemission LMA/0.01Mn 300 KFigure to 480 8a K 4+ spanning from 300 K to 480 K gives the temperature-dependent emission spectra of LMA/0.01Mn under excitation of 467 nm. Obviously, the emission intensity of the as-prepared sample decreases under excitation of 467 Obviously, the emission intensity of the as-prepared sample decreases monotonically with the nm. increase of temperature. However, there is no distinct peak position shift monotonically with the increase of temperature. However, there is no distinct peak position shift over over 380 K. The integrated emission intensity is presented in Figure 8b. When the sample is heated 380 K. The integrated emission intensity is presented in Figure 8b. When the sample is heated to 420 to 420 K, the integrated intensity retains only 36% of the emission intensity at 300 K. The thermal K, the integrated intensity only by 36%the of the emission intensity at the 300 distributed K. The thermal quenching quenching behavior may retains be affected oscillator strength and killer centers, behavior may be affected by the oscillator strength and the distributed killer centers, which which could strengthen the nonradiative energy relaxation. Despite an obvious drastic emissioncould loss, strengthen the nonradiative energy at relaxation. an −obvious drastic emission loss, the the chromaticity coordinate variations 420 K are Despite 0.0110 and 0.0109, respectively, well satisfying chromaticity coordinate variations at 420Meanwhile, K are 0.0110the and −0.0109, respectively, PL well satisfying the requirement in LED applications [33]. temperature-dependent decay curvesthe of 4+ requirement in LED applications [33]. Meanwhile, the temperature-dependent PL decay curves of LMA/0.01Mn at 663 nm were further measured. As shown in Figure 8c, the decay lifetimes seem to 4+ LMA/0.01Mn at 663 We nm found were further measured. Assix-fold shown in Figure 8c, Al thesites decay lifetimes seem to be multiexponential. that there are three coordinated (Al 1 , Al4 , and Al5 ) 4+ be multiexponential. We found that there are three six-fold coordinated Al sites (Al 1 , Al 4 , and 5) in in the structure of LMA. When Mn ions substitute for the Al1 , Al4 , and Al5 sites in the LMAAlhost, 4+ ions substitute for the Al1, Al4, and Al5 sites in the LMA host, three 8 − the structure of LMA. When Mn three [MnO6 ] octahedral complex luminescence center formed, which would contribute to the decay [MnO 6]8− octahedral complexfunction. luminescence center formed, which would contribute the decay time time with multiexponential Meanwhile, the doping concentration of Mn4+toions also affect 4+ ions also affect the with multiexponential function. Meanwhile, the doping concentration of Mn the decay process via energy transfer. The distance between the activated centers is 2.658 to 5.59Å, decay the process via transfer energy transfer. The distance betweenfrom the activated is 2.658 to 5.59Å, hence hence energy would lead to the deviation linear ofcenters the decay curves. Moreover, the energy transfer would lead to the deviation from linear of the decay curves. Moreover, the decay lifetimes also strongly rely on the temperature. The lifetimes are calculated according to the equation below: 

 I  t  tdt = 0 

(2)

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which is consistent with the change rule of the emission intensity. The decay of Mn4+ in the matrix becomes faster and faster, owing to the increased nonradiative energy migration among Mn4+ ions at the decay lifetimes also strongly rely on the temperature. The lifetimes are calculated according to the a higher temperature. equation below: R∞ The activation energy (ΔE) for thermal quenching can be determined by the following equation I (t)tdt [34]: τ = R0∞ (4) 0 I ( t ) dt

I 0 luminescence / 1  c exp  -intensity E / kT   at time t. Based on Equation(5) where τ is the decay time and I(t)ITisthe (4), the average lifetime of LMA/0.01Mn4+ decreases gradually from 1.35 ms at 300 K to 0.46 ms at 480 K, where I0 and IT stand for the initial emission intensity and the luminescence intensity4+ at temperature which is consistent with the change rule of the emission intensity. The decay of Mn in the matrix T, respectively; c is a constant for a designated host; and k is the Boltzmann constant.4+Thus, the becomes faster and faster, owing to the increased nonradiative energy migration among Mn ions at a activation energy of the LMA/Mn4+ phosphor is calculated to be 0.38 eV. The thermal quenching higher temperature. mechanism can be explained by the configurational coordinate diagram, as shown in Figure 8d.

Figure 8. 8. (a),(a–c) emission spectra, of Figure (b), Temperature-dependent (c) Temperature-dependent emission spectra,integrated integratedintensity, intensity,and and lifetime lifetime of 4+ , respectively; (d) configurational coordinate diagram of Mn4+ 4+ 4+ LMA/0.01Mn in the LMA host for the LMA/0.01Mn , respectively; (d) configurational coordinate diagram of Mn in the LMA host for the possible thermal thermal quenching quenching process. process. possible

Thea further activation energy (∆E) for thermal quenching following 2+, Ca2+, Zn2+,by 4+) ion In experiment, the influence of M (M = Li+, can Na+, be Mgdetermined Gethe on the equation [34]: 4+ emission intensity of Mn /LMA was studied (Figure S2). The ionic radii of these M ions are 0.590 Å = 0.720 I0 /[1Å +for c exp (5) (− )] 1.00 Å for Ca2+ (CN = 6), 0.60 Å for 2+ ∆E/kT for Li+ (CN = 4), 0.99 Å for Na+ (CN =IT4), Mg (CN = 6), 2+ (CN = 4), and 0.530 Å for Ge4+ (CN = 6), respectively [35]. When the impurity ions were co-doped Zn where I0 and IT stand for the initial emission intensity and the luminescence intensity at temperature T, into LMA/Mnc4+is, the characteristic peaks in the emission were notconstant. changed.Thus, The integrated PL respectively; a constant for a designated host; and k isspectra the Boltzmann the activation 4+ intensities of the samples into which the impurity ions were incorporated are presented in Figure 9a. energy of the LMA/Mn phosphor is calculated to be 0.38 eV. The thermal quenching mechanism can 2+ ions have the ability to enhance luminescence of LMA/0.01Mn4+, Na+ and Ca2+ Therein, Mg2+by and Znconfigurational be explained the coordinate diagram, as shown in Figure 8d. 4+ ion luminescence, 4+ + , Mg 2+ , Ge 4+ ) ion on the ions In have no obvious impact on Mn and the 2+Li, +Ca /Ge2+4+, Zn doped LMA/0.01Mn a further experiment, the influence of M (M = Li+ , Na 4+ phosphors show weaker intensities underS2). excitation of radii the same blueM light. emission intensity of Mnluminescence /LMA was studied (Figure The ionic of these ionsSpecifically, are 0.590 Å 4+ phosphors 2+ ion doping are + (CN the ratio of = the of LMA/0.01Mn andÅwithout for Li 4),emission 0.99 Å foryields Na+ (CN = 4), 0.720 Å for Mg2+ (CN with = 6), 1.00 for Ca2+Zn (CN = 6), 0.60 Å for 4+ can be 4+ (CN measured beand 2.04, in favor conclusion that the luminescence intensity LMA/Mn Zn2+ (CN =to4), 0.530 Å for of Gethe = 6), respectively [35]. When the impurityofions were co-doped 4+ 4+ greatly strengthened via impurity peaks doping. Moreover, effectwere of co-dopants forThe improving MnPL into LMA/Mn , the characteristic in the emissionthe spectra not changed. integrated 2+ 2+ + luminescence follows the order of Zn the > Mg > Na ions . As discussed in Figureare 8c, presented the substitution of Mn intensities of the samples into which impurity were incorporated in Figure 9a. for the Al1, Al4, and Al5 sites in the LMA host is attributed to the multiexponential function; the co-

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Therein, Mg2+ and Zn2+ ions have the ability to enhance luminescence of LMA/0.01Mn4+ , Na+ and Ca2+ ions have no obvious impact on Mn4+ ion luminescence, and the Li+ /Ge4+ doped LMA/0.01Mn4+ phosphors show weaker luminescence intensities under excitation of the same blue light. Specifically, the ratio of the emission yields of LMA/0.01Mn4+ phosphors with and without Zn2+ ion doping are measured to be 2.04, in favor of the conclusion that the luminescence intensity of LMA/Mn4+ can be greatly strengthened via impurity doping. Moreover, the effect of co-dopants for improving Mn4+ luminescence follows the order of Zn2+ > Mg2+ > Na+ . As discussed in Figure 8c, the substitution of Mn for the Al1 , Al4 , and Al5 sites in the LMA host is attributed to the multiexponential function; the Materials 2018, 11, x FOR REVIEW 9 of 11 co-doped model alsoPEER presents similar decay curves. Similarly, the decay curves were tested to verify this phenomenon. Figure 9b shows the lifetime of Mn4+ ion in phosphor with co-doping Zn2+ and 2+ ions, 2+ /Zn 2+ doped model also similar decay curves. Similarly, thelonger. decay The curves were tested verify this Mg and thepresents result suggests that the lifetime becomes introduction oftoMg 4+ ion in phosphor with co-doping Zn 2+ and 2+ 4+ 4+ 4+ phenomenon. Figure 9b shows the lifetime of Mn Mg ions increases the PL intensity of Mn ions, which is attributed to the replacement of Mn –Mn pairs 2+ 2+ 2− –Mn 4+ the ions, the4+result suggests that becomes longer. The introduction ions by Mgand and/or Zn2+ –Mn ion lifetime pairs. Hence, the nonradiative depopulationof of Mg the 2 /Zn Eg state is 4+ ions, which is attributed 4+–Mn4+2− 4+ 4+ 4+ increases the PL intensity of Mn to the replacement of Mn pairs by decreased because the energy migration between the Mn –Mn ion pairs is faster than Mg –Mn 4+ 2+ 4+ ion pairs. Hence, the nonradiative depopulation of the 2Eg state is Mg2−–Mn and/or Zn2+and/or –Mn4+ Zn ion –Mn pairs, and thus the probability of the energy terminating at a killer site of the 4+–Mn 4+ 4+ 4+ 2+4+–Mn 4+ pairs decreased because the energy migration the Mn ion4+pairs faster than along Mg2−–Mn Mn –Mn ion pairs is greater. Moreover,between the formation of Mg /Zn2+is–Mn with 2+ 4+ and/or Zn –Mn ion2+pairs, of the energylifetimes terminating atco-doped a killer site of the the impurities of Mg /Zn2+and ionsthus not the onlyprobability lengthen the emission of the samples 4+–Mn4+ ion pairs is greater. Moreover,4+ 2+–Mn4+/Zn2+–Mn4+ pairs along with the Mn the formation of Mg compared with that of the LMA/0.01Mn sample, but also lead to the decrease in the nonradiative impurities of Mg2+/Zn2+ ions not only lengthen the emission lifetimes of the co-doped samples compared rate. [13,16]. with that of the LMA/0.01Mn4+ sample, but also lead to the decrease in the nonradiative rate. [13,16].

4+ /M/LMA (M+ = Li++ , Na2+ 4+/M/LMA Figure 9. (a) Integrated intensity intensity for for the the Mn Mn4+4+(0.01 (0.01mole)/LMA mole)/LMA and Mn and Mn (M = Li , Na , Mg +, 2+ 2+ 2+ 4+ 4+ 2+ 2+ 2+ 2+ 4+ 4+ 2+ 2+ 2+ Mg Ca , Ge , Zn) phosphors; , Ge ) phosphors; (b)curves decay curves Mn Mg with Mg, and , CaZn, and Zn2+ as doping Ca , ,Zn (b) decay of Mn ofwith , Ca doping well as well as without without doping.doping.

4. Conclusions 4. Conclusions In summary, a red emitting phosphor LMA/Mn4+ for an LED plant growth lamp was synthesized In summary, a red emitting phosphor LMA/Mn4+ for an LED plant growth lamp was synthesized by the solid state reaction method and sintering at 1600 ◦ C in the air directly. The crystal structure of by the solid state reaction method and sintering at 1600 °C in the air directly. The crystal structure of 4+ was studied by the XRD Rietveld refinements. XPS and EPR spectra demonstrate that LMA/0.01Mn4+ LMA/0.01Mn was studied by the XRD Rietveld refinements. XPS and EPR spectra demonstrate that the chemical valence of manganese is +4. The LMA/Mn4+ phosphor exhibits a red emission peaking 4+ the chemical valence of manganese is +4. The LMA/Mn phosphor exhibits a red emission peaking at 663 nm, which is attributed to the 2 E2 →4 A transition of Mn4+ ion in the [MnO6 ]8− octahedral at 663 nm, which is attributed to the E→ 4A2 2 transition of Mn4+ ion in the [MnO 6]8− octahedral environment, and the QE is 35.8% under the blue-light excitation. The optimal doping concentration environment, and the QE is 35.8% under the blue-light excitation. The optimal doping concentration of Mn4+ ion is 0.01 mole and the quenching mechanism is a dipole/dipole interaction. Moreover, of Mn4+ ion is 0.01 mole and the quenching mechanism is a dipole/dipole interaction. Moreover, the the chromaticity coordinate variations at 420 K are 0.0110 and −0.0109, respectively. Additionally, chromaticity coordinate variations at 420 K are 0.0110 and −0.0109, respectively. Additionally, the 4+ could be greatly enhanced via impurity doping Zn2+ or Mg2+ ions. the luminescence of LMA/Mn luminescence of LMA/Mn4+ could4+be greatly enhanced via impurity doping Zn2+ or Mg2+ ions. These These properties make LMA/Mn a promising red emitting phosphor for plant growth LEDs. properties make LMA/Mn4+ a promising red emitting phosphor for plant growth LEDs. Supplementary Materials: Supplementary materialsand can designed be found atthe http://www.mdpi.com/1996-1944/12/1/86/s1. Author Contributions: Z.C. and X.L. conceived experiments; R.L. and Y.L. performed the 4+ Figure S1: The quantum the LMA:0.01Mn Figure S2:L.K. PL wrote spectra ofpaper. LMA:0.01Mn4+ , experiments and analyzedefficiency the data; of P.L. contributed to DFTphosphor, analysis; B.W. and the 0.01M+ powder. Funding: This work was supported by the Innovation Projects of Department of Education of Guangdong Province (No. 2017KQNCX207 and 2017KQNCX198).

Conflicts of Interest: The authors declare no conflict of interest.

References

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Author Contributions: Z.C. and X.L. conceived and designed the experiments; R.L. and Y.L. performed the experiments and analyzed the data; P.L. contributed to DFT analysis; B.W. and L.K. wrote the paper. Funding: This work was supported by the Innovation Projects of Department of Education of Guangdong Province (No. 2017KQNCX207 and 2017KQNCX198). Conflicts of Interest: The authors declare no conflict of interest.

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