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Jan 24, 2017 - Mn4+ (d3 electronic configuration) in host La(MgTi)1/2O3 ... samples emit far-red light centered at 708 nm with ultraviolet light (345 ...
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Ab Initio Site Occupancy and Far-Red Emission of Mn4+ in Cubic-Phase La(MgTi)1/2O3 for Plant Cultivation Ziwei Zhou,† Jiming Zheng,† Rui Shi,‡ Niumiao Zhang,† Jiayu Chen,† Ruoyu Zhang,† Hao Suo,† Ewa M. Goldys,§ and Chongfeng Guo*,†,§

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†

National Key Laboratory of Photoelectric Technology and Functional Materials (Culture Base) in Shaanxi Province, National Photoelectric Technology and Functional Materials & Application of Science and Technology International Cooperation Base, Institute of Photonics & Photon-Technology, Northwest University, Xi’an 710069, China ‡ MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China § ARC Centre of Excellence for Nanoscale Biophotonics (CNBP), Macquarie University, North Ryde 2109, Australia S Supporting Information *

ABSTRACT: Mn4+-activated oxide phosphors La(MgTi)1/2O3 (LMT) with far-red emitting were prepared via a sol−gel route. The structures of samples were determined by X-ray diffraction (XRD) and Reitveld refinement. The occupied sites of Mn4+ (d3 electronic configuration) in host La(MgTi)1/2O3 were confirmed by ab initio calculations in which the system has the lower formation energy, stable lattice structure, and strong bonding state as Mn4+ enters into Ti site. The luminescent properties of Mn4+-doped samples were investigated; the samples emit far-red light centered at 708 nm with ultraviolet light (345 nm) or blue light (487 nm) excitation. According to the photoluminescence (PL) and excitation (PLE) spectra, the crystal field strength of the Mn4+-occupied environment was estimated. The thermal stability of phosphor was also evaluated through temperature-dependent PL intensity in a heating and cooling cycle process. The emission band is well-matched with the absorption band of phytochrome PFR under the excitation of light in near-ultraviolet to blue, which suggests that the LMT: Mn4+ phosphor has great potential applications in light-emitting diodes (LEDs) for modulating plant growth. KEYWORDS: far-red emission, phosphor, phytochrome, plant growth, LEDs

1. INTRODUCTION Light not only offers the energy source for plant growth but also control some of the thousands of processes occurred in plant cell because blue (400−500 nm), red (620−690 nm), and far-red (FR; 700−740 nm) light are generally responsible for photosynthesis, phototropism, and photomorphogenesis, respectively.1,2 Thus, the natural rhythms of the plant could be controlled by light. To meet different levels of human needs, the greenhouse industry has been extensively developed in agriculture and horticulture, in which the light environment, including the intensity and spectral composition, was controlled by using different illuminants. The advent of light-emitting diodes (LEDs) provide several unique advantages over existing light systems gas-discharge lamps used in controlled-environment crop production system, such as being power-economical and environmental friendly and having a long lifetime and low radiant heat output.3 Thus, LEDs are recognized as the first light source with capability of controlling spectral composition, allowing emission light matching with plant photoreceptors to offer ideal production and to affect plant morphology.4,5 © 2017 American Chemical Society

Phosphor-converted LEDs may be the most appropriate for canopy because of their mature fabrication technology and easily tunable spectral composition and intensity through the use of ideal phosphor. Red and blue light play the dominant role in making carbohydrates, and the wavelengths are roughly similar for different plants; the FR light absorbed by the phytochrome leads to the changes in gene expression of plant structure and response to growth.6 Phytochrome dominates the plant growth, development, and differentiation processes, including PR and PFR, two photointerconvertible states with different absorption spectra. PR is its biologically inactive state, switching to its PFR isoform by strongly absorbing red light peaked at 660 nm, whereas PFR is biologically active state, reverting to the PR state by capturing far-red light centered at about 730 nm.7,8 Generally, the amount of red light exceeds that of FR light in the daytime, and vice versa. Received: December 12, 2016 Accepted: January 24, 2017 Published: January 24, 2017 6177

DOI: 10.1021/acsami.6b15866 ACS Appl. Mater. Interfaces 2017, 9, 6177−6185

Research Article

ACS Applied Materials & Interfaces

sample crystal structure was performed using Total Pattern Solution (TOPAS Academic) software. All photoluminescence (PL) and excitation (PLE) spectra together with lifetime were measured on an Edinburgh FLS920 fluorescence spectrophotometer (Edinburgh Instruments Ltd.) equipped with a 450 W Xe lamp and microsecond flash-lamp μF900 (Edinburgh Instruments Ltd.) as the excitation source. An Oxford OptistatDN2 nitrogen cryogenics temperature controlling system was combined with a spectrophotometer to measure the temperature-dependent PL spectra, and the duration of the stay is fixed at 20 min at the measured temperature. Computation Characterization. All calculations were performed within the framework of density functional theory (DFT) using the projector augmented wave (PAW) method potentials and a plane-wave basis set, which was implemented in the Vienna Ab initio Simulation Package (VASP). The generalized gradient approximation (GGA) was used for describing the exchange-correlation interaction. The program uses periodic boundary conditions and a plane-wave basis with a cutoff energy of 500 eV. The sampling of the Brillouin zone was done with an 8 × 8 × 8 Mockhorst−Pack (MP) grid. Next, we constructed a supercell to calculate the formation energy, energy band, and density of state; the calculation processes are given in the part of ab initio calculation details in the Supporting Information, and the detailed information on the energy band for pristine and doped system are listed in Tables S1−S7.

The ratio difference between red light and FR light (or no light) generally results in the physiology change of plant from vegetative to floral growth. When the PFR concentration is low and PR is high, short-day plants, which need longer dark time, will flower, and long-day plants, which need longer day time, do not, and vice versa; this indicates that the flowering time of the plant could be controlled through changing the ratio of red to far-red.9,10 That is to say, red light could make flower time ahead of the schedule for the long-day plants but postpone the flower time for the short-day plants. Nowadays, the increasing intensity and time of the city nightscape lighting means that phytochrome (PFR) cannot revert back to PR, which inhibits the blooming and growth of the short-day plants. In sunlight, there is a large proportion of red light and a little FR. When the sun is shining, the plant can absorb the red light and reflect the FR light because of chlorophyll; if a plant is exposed to FR light because the plant may be covered or lower than other plants in radial height, the plant will have fast elongation rate to get more sunlight. This phenomenon is called shade-avoidance responses. According to the above-mentioned, the designed plant-growth LEDs with different spectral composition could be come useful for controlling flower time and radial height.11 Blue- and red-emitting phosphor for LEDs for plant growth have been investigated extensively; however, the far-red emitting phosphors used for plant growth LEDs have hardly been reported.12−14 The present work is aimed at developing a FR emitting phosphor for LED toward plant growth. Mn4+ ions exhibit strong absorption around the near-ultraviolet and blue regions in octahedral coordination environment from the 4 A2→ 4T1 and 4T 2 spin-allowed transitions, and give off red to FR light ranging from 617 nm (16207 cm−1) in Na2SiF615 to 723 nm (13827 cm−1) in SrTiO316 due to the transition of 2 Eg → 4A2g. The emission wavelength of Mn4+ strongly influenced by the covalence of the “Mn4+ ligand” bonding,17 so it is possible to obtain the FR emission in a host with strong covalent effects to match with the absorption of phytochrome PFR. Compound La(MgTi)1/2O3 with an octahedral coordination environmental [Ti/MgO6] group and excellent physical and chemical stability was selected as the host, in which the site occupancy of Mn4+ was determined by the theory of ab initio global optimization algorithms. The luminescent properties and crystal field of the sample were also investigated on the basis of experimental results.

3. RESULTS AND DISCUSSION 3.1. Rietveld Refinement of Structure and the Site Occupancy. For compound La(MgTi)1/2O3, there are cubic and monoclinic two crystal structures.18,19 For the confirmation of the structure of the prepared samples, XRD Rietveld refinements of La(MgTi)1/2O3: 0.1% Mn4+ was performed using the TOPAS program, in which the cubic structure La(MgTi)1/2O3 (ICSD no. 43793) with the Pm-3m space group used as the reference model. The experimental (crosses) and calculated (red) XRD profiles together with their differences (blue) and Bragg reflection positions for the Rietveld refinement of La(MgTi)1/2O3: 0.1%Mn4+ were presented in Figure 1. The

2. EXPERIMENTAL SECTION Synthesis of Samples. Samples of La(MgTi)1/2O3: xMn4+ (x = 0.6−2.0%) were prepared through a modified sol−gel route. The stoichiometric amount of La2O3 (99.99%), MgO (99.99%), and MnCO3 (analytical reagent, AR) was first weighted, respectively, and dissolved in HNO3, and the redundant HNO3 was removed by volatilization, and a subsequently appropriate volume of distilled water was introduced to get transparent solution A. Transparent solution B was formed by dissolving hydrolysis inhibitor ethanol and titanium source Ti(OC4H9)4 into the acetic acid solvent (AR) with an appropriate ratio. Final transparent solution was got through dropping solution B to A solution under vigorous stirring; the solution was kept in an oven at 80 °C for 24 h and then at 120 °C for 24 h to get white dried gel. The dried gel was presintered at 500 °C for 3 h and then at 1300 °C for 8 h to get phosphor samples. Characterization and Calculation. The phase purity and structure of samples were identified using X-ray diffraction (XRD) patterns, which were recorded on a Bruker D8 Advance powder diffractometer with Cu Kα radiation (λ = 1.5406 Å). Rietveld refinement analysis of

Figure 1. Rietveld refinement XRD patterns of sample La(MgTi)1/2O3: 0.1%Mn4+ by the TOPAS program. The solid black crosses, red lines, and blue lines express the observed and calculated XRD patterns of the sample as well as their differences. The short vertical green lines display the positions of Bragg reflection.

refinement is well-convergent due to the low residual factors Rp = 5.277% and Rwp = 2.616%, and the final refined crystallographic data are listed in Table 1. The refinement results demonstrate that all experimental peaks satisfy the reflection conditions, confirming the sample La(MgTi)1/2O3: 0.1%Mn4+ crystallized in a pure cubic structure. In compound La(MgTi)1/2O3 with cubic structure, there are La3+, Mg2+, and Ti4+ cationic sites, but it is impossible for Mn4+ 6178

DOI: 10.1021/acsami.6b15866 ACS Appl. Mater. Interfaces 2017, 9, 6177−6185

Research Article

ACS Applied Materials & Interfaces Table 1. Rietveld Refinement and Crystallographic Sata of La(MgTi)1/2O3: 0.1%Mn4+ Sample formula space group a (Å) b (Å) c (Å) β (degree) V (Å3) Z Rwp (%) Rp (%)

La(MgTi)1/2O3: 0.1% Mn4+ Pm-3m 3.930 3.930 3.930 90.000 60.707 1 5.277 2.616

activator ions to substitute for La3+ due to the big difference between the radii of Mn4+ (r = 0.530 Å) and La3+ (r = 1.032 Å). It seems that Mn4+ ions tend to take the place of Ti4+ (r = 0.605 Å) moreso than that of Mg2+ (r = 0.720 Å) because of closer radius and identical valence according Pauling’s first and second rules.20 To further prove our deduction, the formation energy Eform was calculated using eq 1 as the Mn enters the site of Ti or Mg, respectively.21 Eform = E T(doped) − E T(point defect) − EMn

Figure 2. Structure of [MgO6]−[TiO6] in cubic LMT (a) and the change of the Mn/Ti/Mg−O bond length as Mn atoms enter the Mg or Ti site (b).

The calculation results illuminate that the Mn(Ti)−O bond length decreases by 0.05157 Å and the Mg−O bond length increases by 0.05157 Å if Mn occupies Ti sites; however, the Mn(Mg)−O bond length decreases by 0.06385 Å and the Ti−O bond length increases by 0.03159 Å if Mn occupies Mg sites. Obviously, the value of decrease in Mn(Ti)−O distance is equal to that of increase in Mg−O distance for the substitution of Ti by Mn; on the contrary, the difference is big between the decrease of the Mn(Mg)−O bond length and the increase of Ti−O bond length if Mn substitutes for Mg. Thus, the big difference in bond length change will cause the distortion of lattice and make the whole lattice instable, which illuminates that the whole lattice structure is relatively stable as Mn atoms occupy the sites of Ti. This is coincident with the result of formation energy. In the present system, Mg and Ti are located in the center of octahedral environment with Oh symmetry, as shown in Figure 3a. The original five degenerate 3d orbitals (dx2-y2, dz2, dxy, dyz, and dxz) of free Mn will split into two groups; one is two degenerate, dx2-y2, and dz2, and the other is three degenerate, dxy, dyz, and dxz, orbitals, as Mn enters the sites of Mg or Ti. In the octahedral crystal field, the energy of orbitals dx2-y2, and dz2 is higher than those of another three orbitals, dxy, dxz, and dyz. The projected density of states (PDOS) on five degenerated 3d orbitals of Mn at Ti or Mg sites were displayed in panels b and c of Figure 3, respectively. It is found that five 3d orbitals can be divided into two groups whether Mn occupied the Mg or Ti site: dx2-y2, and dz2 with higher energy and another dxz, dyz, and dxy with lower energy, which proves that the doped Mn locates in an octahedral crystal field. Comparing Figure 3b and c, it is found that the dxz, dyz and dxy orbitals form a strong bonding state below Fermi level as Mn occupied the Ti site, whereas the dxz, dyz, and dxy orbitals form a nonbonding state if Mn doped in Mg site due to the dominant energy distribution at Fermi level. Furthermore, the PDOS of Mn is significantly different when it enters the site of Mg or Ti, as shown in panels d and e of Figure 3, respectively. A significant overlap between PDOS peaks of Mn and O atom implies the formation of strong bonding state if Mn enters the Ti site, but as Mn enters the Mg site, the energy of Mn distributes near the Fermi surface forms a nonbonding state, and there is a weak overlap between the orbitals of Mn and O in PDOS. The energy of bonding state is lower than that of nonbonding state, so the system energy is lower and has more stability when Mn substitutes for Ti. In addition, almost no

(1)

where ET (point defect) is the total energy of the crystal with a point defect introduced by removing one Mg or Ti atom from the cell, ET (doped) is the total energy of the crystal with a single Mn atom in the point defect, and EMn is the energy of single Mn atom. According to the calculated results shown in Table 2, the formation energy is −8.658(1) and −13.941(7) eV Table 2. Calculation of Formation Energy (Eform) occupied site

ET (doped) (eV)

ET (point defect) (eV)

Mn→Mg Mn→Ti

−326.066(1) −318.091(3)

−311.572(1) −298.313(9)

EMn (eV)

Eform (eV)

−5.835(8)

−8.658(1) −13.941(7)

as Mn substitute for Mg or Ti in compound La(MgTi)1/2O3, which indicates that the formation energy is lower if Mn enters into Ti site than that of sample with substitution of Mg by Mn. This result illuminates that the system is more-stable when Mn occupied the Ti site, which verified our deduction that Mn tends to occupy the Ti site instead of the Mg site in the same condition. For the comprehensive understanding of the La(MgTi)1/2O3 compound, the energy band and density of state (DOS) of pristine and doped system are also described in Figures S1 and S2; the La(MgTi)1/2O3 is a direct band gap compound with 2.92 eV band gap. Figure 2a shows the schematic illustration of the [MgO6]− [TiO6] structure in cubic La(MgTi)1/2O3, in which Mg or Ti atoms are coordinated by six O atoms belonging to octahedral Oh symmetry. As Mn enters the lattice site through substitution for Mg or Ti, the bond length Mn/Ti/Mg−O will be changed, leading to the change of center-atom (Mn/Ti/Mg) crystal field strength. Figure 2b displays the changes of the bond length with the substitution of Mg or Ti by Mn. It is observed that bond length changes with the difference of electronegativity decreasing in the order of Mn4+ > Ti4+ > Mg2+.22 Because the electronegativity of Mn is stronger than that of Ti and Mg, the attraction between surrounding O atoms and Mn is stronger than that of O atoms and Ti (Mg), resulting in the shorter distance (bond length) between O atom and Mn and longer distance of Ti(Mg)−O when Mn enters the sites of Mg (or Ti). 6179

DOI: 10.1021/acsami.6b15866 ACS Appl. Mater. Interfaces 2017, 9, 6177−6185

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ACS Applied Materials & Interfaces

Figure 3. Schematic diagram of the d-orbital splitting in octahedral crystal field (a), the projected density of states (PDOS) of the d-orbital splitting energy level of Mn4+ doped in Ti (b) or Mg site (c), the PDOS (d,e) and the charge density (f) of LMT when Mn4+ was doped in the Ti or Mg site.

Figure 4. XRD patterns of LMT: xMn4+ (x = 0.6%−2.0%) (a), the enlarged (220) peak (b), and the cell parameter as a function of Mn4+ contents (c) as well as the standard data of LMT (ICSD no. 43793) and the crystal structure of LMT (d).

lattice without significant influence on crystal structure. However, the strongest diffraction peak (220) shows a small shift toward the larger 2θ angles with the increase of Mn4+ concentration, as shown in Figure 4b, due to the smaller ionic radii of Mn4+ (r = 0.530 Å) compared to that of Ti4+ (r = 0.605 Å). The corresponding lattice constants of La(MgTi)1/2O3: xMn4+ declined monodirectionally with the increase of Mn4+ concentration, as shown in Figure 4c, from a = b = c = 3.934 Å for the blank sample (x = 0) through a = b = c = 3.926, 3.915 Å for x = 0.6 and 0.8% to a = b = c = 3.914 and 3.910 Å for x = 1.0 and 2.0%, respectively. The schematic illustration of LMT structure with cubic structure belonging to the space group Pm-3m was depicted in Figure 4d, in which Mg and Ti sites are both located in the center of the regular octahedron. This result directly demonstrates above theoretical conclusion that Mn4+ entered the site of Ti4+. 3.3. Luminescent Properties of La(MgTi)1/2O3: Mn4+. The PLE and PL spectra of the LMT: 0.8% Mn4+ sample with the brightest emission together with the absorption spectra of phytochrome PR and PFR are shown in Figure 5. The PLE

charge was found around the Mg atom because of the weak interaction between Mg and O atom, and the localized electrons around Mn atom indicate that the strong interaction between Mn and O atom resulting in the decrease of Mn−O bond length when Mn substitutes for Ti, as displayed in the charge density of LMT in Figure 3f. Based on above formation energy, bond length changes, and the DOS analysis through the hypothesis of Mn entering the site of Mg or Ti, it was determined that all results indicate that the crystal is more stable when Mn occupies the Ti site. Thus, Mn was expected to enter Ti sites. 3.2. Phase Purity and Crystal Structure. According to the above results, Mn prefers to enter the site of Ti instead of Mg. For the identification of the above conclusions from experiments, Figure 4a displays the XRD patterns of samples with the nominal formula La(MgTi1−xMnx)1/2O3 (x = 0.6%, 0.8%, 1.0%, and 2.0%). All XRD patterns of these samples match well with the profile of pure La(MgTi)1/2O3 (ICSD no. 43793) with cubic double perovskite structure, and no any impurity peaks appeared, indicating that dopant Mn enters the 6180

DOI: 10.1021/acsami.6b15866 ACS Appl. Mater. Interfaces 2017, 9, 6177−6185

Research Article

ACS Applied Materials & Interfaces

345 nm excitation. The shape and the position of all emission spectra are identical except for their relative intensities (I). With an increase in the contents of Mn4+ from 0.5 to 2.0%, the PL intensity first grows gradually before reaching the peak value at x = 0.8 and declines with further increase of the Mn4+ content on account of the concentration quenching effect. The quantum efficiency with optimal composition was measured to be 27.2% under 365 nm excitation, which is higher than those of another Mn4+-doped host like Gd2ZnTiO6 (26%)26 and CaMg2Al16O27 (16%).24 Generally, the nonradiative energy transfer among activators triggers concentration quenching, which is determined by the distance among the dopant Mn4+. The critical distance (Rc) at which the possibility of nonradiative energy transfer and radiative emission is equal, could be estimated by eq 2:27 Figure 5. Room temperature PLE (λem = 708 nm) and PL (λex = 345 nm) spectra of LMT: 0.8% Mn4+ and the absorption spectra of phytochrome PR and PFR.

⎡ 3V ⎤1/3 R c ≈ 2⎢ ⎥ ⎣ 4πxcN ⎦

spectrum monitored at 708 nm from 2Eg → 4A2g transition of Mn4+ includes two broad absorption bands in the region of 250 to 550 nm, which could be well-fitted by our Gaussian peaks centered at 324, 360, 414, and 487 nm corresponding to Mn4+−O2− charge-transfer band (CTB) and the transitions from ground state of 4A2g to the excited states 4T1g, 2T2g, and 4T2g of Mn4+, respectively.23 The two dominant excitation bands are well-matched with the emission of n-UV or blue chips, which implies that present samples could be used as a potential FRemitting phosphor with the efficient excitation of commercial LED chips. Note that the sharp lines ranging from 450 to 500 nm were attributed to the Xe lamp excitation source. Under the excitation of 345 or 487 nm light, an intense asymmetric broad far red band consisting of six distinguishable Stokes and anti-Stokes side bands at 675, 680, 685, 690, 695, and 708 nm was observed, which corresponds to the different vibrational modes of 2Eg → 4A2g transitions for the 3d3 electrons in the [MnO6]8− octahedral environment.24 Compared to the absorption spectra of phytochrome PR and PFR, peaking at 660 and 730 nm, a significant overlap was found between the emissions spectrum of phosphor LMT: Mn4+ and the absorption of phytochrome PFR, which indicate that the present phosphor could be used to modulate plant growth.25 Figure 6a,b displays the PL spectra and corresponding integrated intensity with variable Mn4+ concentration under the

(2)

where V is the volume of the unit cell, xc is the critical concentration of the activator ions, and N is the number of available Ti4+ sites in one unit cell. As for the present LMT: Mn4+ phosphors, V = 60.0 Å3, xc = 0.008, and N = 1; thus, Rc is calculated to be 24.29 Å. This value is far beyond 5 Å, implying that exchange interaction mechanism is impossible for the energy transfer among Mn4+ in the present system, and it is only working for the sample with significant overlap between the excitation and emission spectra.28 In consequence, the multipolar interaction energy transfer will dominate the concentration quenching mechanism in Mn4+-doped LMT, and the type of interaction could be decided eq 3:29 I /x = K[1 + β(x)θ /3 ]−1

(3)

where I is the PL intensity, x is the activator concentration over the critical concentration, K and β are constants for the same excitation condition, and host lattice, θ = 6, 8, and 10 represents the dipole−dipole (d−d), dipole−quadrupole (d−q), and quadrupole−quadrupole (q−q) interactions, respectively. Figure 6c illuminates the relationship between ln(x) versus ln(I/x), which can be well-fitted by a straight line with a slope of −1.898 (−θ/3). The value of θ is calculated as 5.694 and close to 6, which indicates that the d−d interaction dominates the concentration quenching mechanism in LMT: Mn4+.

Figure 6. PL spectra of LMT: xMn4+ (x = 0.5−2.0%) (a) and their integrated intensities with Mn4+ concentration (b) as well as dependence of ln(I/x) on ln(x) (c). 6181

DOI: 10.1021/acsami.6b15866 ACS Appl. Mater. Interfaces 2017, 9, 6177−6185

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Figure 7. Decay curves LMT: xMn4+ (x = 0.5%−2.0%) under excitation at 345 nm and monitoring at 708 nm.

Figure 8. Tanabe−Sugano energy-level diagram for Mn4+ (d3) electron configuration in the octahedral site of LMT host and the schematic diagram of Mn4+ energy level.

The decay curves of LMT: xMn4+ samples monitored at 708 nm under the excitation of 345 nm near-ultraviolet light at room temperature were presented in Figure 7. The average lifetime was used to characterize the effective decay process, which could be obtained by eq 4:30 τ=

4 A2g → 4T1g (27778 cm−1) is about 7250 cm−1; thus, the Racah parameter B can be acquired according to the equation:

∞

Dq 15(x − 8) = 2 B (x − 10x)

∞

in which x is expressed as

∫0 I(t )t dt ∫0 I(t ) dt

(4)

x=

where I(t) represents the luminescence intensity at a time t. In light of eq 4, the mean lifetimes were approximately 2.122, 2.135, 2.169, 1.938, and 1.836 ms for the concentration of Mn4+ x = 0.5, 0.6, 0.8, 1.0, and 2.0%. As published in previous publications,31 the lifetime of Mn4+ is at a microsecond level due to the forbidden transition character of Mn4+ ion intra-d-shell transitions. In addition, it is found that the decay curves gradually deviate from a single-exponential function and the average lifetime and then rapidly decrease after the first gradual increase before arriving at the maximum value 2.169 ms as x = 0.8% because of the high energy-transfer probability among the Mn4+ ions. 3.4. Crystal-Field Analysis of Mn4+ Occupied Sites. The schematic diagram of Mn4+ energy level transition is usually dependent on the crystal-field environment due to the outside shell of d3 electron configuration, which can be interpreted the influence from the octahedral filed in LMT host by the Tanabe−Sugano energy-level diagram together with the possible transitions between different energy levels, as illustrated in Figure 8. The electrons of Mn4+ at ground state 4A2g were pumped to the excited levels 4T1g and 4T2g with the excitation of 345 nm n-UV light or 487 nm blue light and then relaxed to the lowest excited level 2Eg through the nonradiative transitions pathway 4T1g → 2T2g → 4T2g → 2Eg and 4T2g → 2Eg. Next, FR light centered at 708 nm emitted from the electrons transition from excited level 2Eg back to the ground state. For Mn4+ in the LMT host, the mean energy gap of the 4A2g → 4T2g transition (20528 cm−1) can be used to roughly estimate the local crystal-field strength, Dq, by the following equation:32 Dq = E( 4T 2g − 4A 2g )/10

(6)

E(4 A 2g → 4T 1g) − E(4A 2g → 4T 2g) Dq

(7)

According to the emission spectra of samples LMT: xMn4+, the peak energy of 2Eg → 4A2g transition is about 14124 cm−1, and the Racah parameter C is also evaluated according to following equation: E(2E g → 4A 2g )/B = 3.05C /B + 7.9 − 1.8B /Dq

(8)

The crystal field parameters Dq, B, and C were 2053, 700, and 2959 cm−1 in light of eqs 5−8, respectively. The relationship between the energy of crystal-field splitting and orbital energy level curves for Mn4+ in host LMT were clearly illuminated in the Tanabe−Sugano energy-level diagram presented in the left of Figure 8. The crystal field is regarded as a strong crystal field when Dq/B ≥ 2.2; it is clear that the present crystal-field in LMT is strong one because of Dq/B = 2.93. Thus, the emission spectrum of Mn4+ is dominated by the spin-forbidden transition 2Eg → 4A2g (sharp line), whereas the excitation spectra are consisted of two broad-bands attributed to the 4A2g → 4T2g and 4A2g → 4T1g spin-allowed transitions. The lowest excited state of Mn4+ is the 2Eg state, and the wide variation of the 2Eg → 4A2g emission wavelength is not due to the variation of crystal field strength because the energy of the 2 Eg level is independent of crystal field. The emission energy of Mn4+ is mainly dependent on the nephelauxetic effect, which is dependent on the two Racah parameters B and C.33 M. G. Brik et al. put forward a rule that in more-covalent hosts (such as oxides), the covalent interaction between the Mn4+ ions and nearest neighboring ligand anions will enhanced with the strong decrease of Racah parameters B and C; therefore, the Mn4+2Eg level is shifted to lower-energy in oxides relative to fluorides, which means the enhanced covalency with the result that higher nephelauxetic effect and further “red shift” of the Mn4+ red emission.17 A new nondimensional parameter

(5)

on the basis of above PL and PLE spectra, the peak energy difference between the 4A2g → 4T2g (20528 cm−1) and 6182

DOI: 10.1021/acsami.6b15866 ACS Appl. Mater. Interfaces 2017, 9, 6177−6185

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Figure 9. Temperature-dependent normalized integrated intensity and emission spectra in heating (a) and cooling (b) process as well as the plot of ln(I0/I − 1) vs 10 000/T for sample LMT: 0.8%Mn4+ with a thermal quenching mechanism configuration coordinate diagram (c).

β1 that quantitatively describes the nephelauxetic effect in the spectroscopy of the Mn4+ ion was introduced by Brik: β1 =

⎛ B ⎞2 ⎛ C ⎞2 ⎜ ⎟ +⎜ ⎟ ⎝ B0 ⎠ ⎝ C0 ⎠

color rendering index (Ra).34 Thus, the thermal stability of phosphors is evaluated by temperature-dependent PL spectra. Figure 9a,b shows the normalized integrated intensity of LMT: 0.8% Mn4+ as a function of heating and cooling temperature ranging from 293 to 493 K. It is found that the PL intensity decreases with increasing temperature during the course of heating, whereas the PL intensity increases with the declining of temperature in the process of cooling. After the heating and cooling processes are compared, it is clear that the corresponding PL intensity curves with different temperature are almost completely symmetric at the same temperature; furthermore, the PL intensity at 423 K is 53.0% of the initial intensities at room temperature, in comparison with 27.2% and 50% of the phosphor Gd2ZnTiO6: Mn4+ and Sr4Al14O25: Mn4+.26,35 Moreover, no significant shift was found in their spectra (the inset in Figure 9a,b), which means that the as-prepared phosphor LMT/Mn4+ offers better thermal stability and reversible thermal behavior. Generally, the emission intensity thermal quenching behavior of samples is attributed to nonradiative relaxation, which can be explained by a configuration coordinate diagram. The ground state 4A2 and excited states 2Eg and 4T2 of Mn4+ were described with three parabolas, as shown in the inset of Figure 9c; the electrons at ground state 4A2 can be excited to the higher excited state 4T2 of Mn4+, and the excited electrons nonradiatively transit to 2Eg, then the excited ions go back to the ground state and emit 708 nm far-red light. At high temperatures, some excited electrons absorb additional vibration energy ΔEa to arrive the cross-point between the ground state 4A2 and the excited state 4T2; next, the electrons return to the ground state through nonradiative relaxation, and the energy is released in the form of heating rather than lighting and leading to the decrease of intensity. In this process,

(9) 4+

in which B, C and B0, C0 are the Racah parameters of Mn in a crystal or free state, respectively, such that B0 = 1160 cm−1 and C0 = 4303 cm−1 for Mn4+. The calculated value of β1 that reflects the rejection between the electronic pairs in LMT: Mn4+ is 0.915; this value of nephelauxetic effect is higher than those of some reported titanate, and the energy of 2 Eg state grows with the increase of β1 (as shown in Table S8). The Mn4+ emission energy 2Eg → 4A2g in double perovskite LMT: Mn4+ (14124 cm−1) is higher than that in the perovskite SrTiO3: Mn4+ (13827 cm−1),16 and the nephelauxetic parameter β1 of LMT: Mn4+ is higher than that of STO: Mn4+ because of the difference in the neighboring coordination environment surrounding of Mn4+: [−Mn4+−O2−−Mg2+−] in LMT and [−Mn4+−O2−−Ti4+−] in STO. The electronegativity of Mn4+ (1.912) is larger than that of Ti4+ (1.730) and Mg2+ (1.234), so the attractive interaction between ions in STO: Mn4+ is stronger than that in LMT: Mn4+, and the covalent bond with less polarity tends to form. In consequence, the Mn4+−O2− bond in STO has stronger covalence, resulting in a lower β1 and 2Eg, which is well-matched with experiment data in Table S8. The stronger covalence results in the lower emitting energy for Mn4+, which is in agreement with M. G. Brik’s conclusion. 3.5. Thermal Stability of Samples. For the phosphor used in high-power LEDs, the thermal stability plays an important role because the operating temperature of the LED chip could reach about 423 K (150 °C), which will exert great effect on the light output, lifetime, chromaticity, and 6183

DOI: 10.1021/acsami.6b15866 ACS Appl. Mater. Interfaces 2017, 9, 6177−6185

Research Article

ACS Applied Materials & Interfaces the thermal activation energy ΔEa plays a critical role and can be estimated according the following equation:36 I (T ) =

I0 1 + c exp( −ΔEa /KT )

of China (RFDP) (grant no. 20136101110017), and the Natural Science Foundation of Shaanxi Province (grant no. 2014JM1004).

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(10)

where I0 is the integrated intensity at initial temperature, I(T) expresses the PL intensity of sample at temperature T, c is a constant, ΔEa is thermal activation energy, and K is the Boltzmann constant (K = 8.62 × 10−5 eV). The thermal activation energy ΔEa can be calculated by plotting ln[(I0/IT)-1] against 10 000/T, where the slope of the straight line is −0.415, as shown in Figure 9c. ΔEa was found to be 0.36 eV, which is bigger than that of the Sr4Al14O25: Mn4+ (Ea = 0.097 eV) phosphor,35 indicating that the LMT: Mn4+ phosphors have excellent thermal stability and less possibility of thermal quenching occurring.

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4. CONCLUSIONS In summary, a series of FR-emission La(MgTi)1/2O3 (LMT): Mn4+ phosphors peaking at 708 nm (Mn4+: 2Eg → 4A2g) were synthesized successfully by a sol−gel route, which can be excited by commercial n-UV and blue LED chips. The structure of sample was refined as a cubic phase with the Pm-3m space group and [MgO6]/[TiO6] octahedral sites in the present compound. Mn4+ ions tend to occupied the sites of Ti4+ in LMT on the basis of theoretical calculation of lower formation energy, longer bond length, and stronger bonding state. The optimal composition of phosphor was determined as LMT: 0.8%Mn4+, and the FR emission of the sample can significantly overlap with the absorption spectrum of phytochrome PFR, implying that the present sample could be applied to moderate plant growth. The crystal-field strength Dq, the Racah parameters B and C, and the nephelauxetic ratio β1, of LMT: Mn4+ is calculated according to their luminescent spectra. Moreover, the thermal stability and repeatability of LMT: Mn4+ phosphor was evaluated by temperature-dependent emission spectra, the thermal quenching temperature T0.5 is 428 K, and the activation energy ΔEa is 0.36 eV. Results demonstrate that LMT: Mn4+ is a potential FR-emitting phosphor for plant-growth LEDs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15866. Figures showing the energy band and the DOS of pristine and doped systems and the DOS of Mn and O atoms. Tables showing energy bands and crystal field parameters of LMT and SrTiO3.(PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Chongfeng Guo: 0000-0003-0177-4369 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (grant nos. 11274251 and 51672215), Research Fund for the Doctoral Program of Higher Education 6184

DOI: 10.1021/acsami.6b15866 ACS Appl. Mater. Interfaces 2017, 9, 6177−6185

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DOI: 10.1021/acsami.6b15866 ACS Appl. Mater. Interfaces 2017, 9, 6177−6185