Ab Initio Site Occupancy and Far-Red Emission of Mn4+ in Cubic

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Ab Initio Site Occupancy and Far-red Emission of Mn4+ in Cubic-phased 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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15866 • Publication Date (Web): 24 Jan 2017 Downloaded from http://pubs.acs.org on January 26, 2017

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ACS Applied Materials & Interfaces

Ab Initio Site Occupancy and Far-red Emission of Mn4+ in Cubic-phased La(MgTi)1/2O3 for Plant Cultivation Ziwei Zhoua, Jiming Zhenga, Rui Shib, Niumiao Zhanga, Jiayu Chena, Ruoyu Zhanga, Hao Suoa, Ewa M. Goldysc and Chongfeng Guoa, c* a

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; b

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; c

ARC Centre of Excellence for Nanoscale Biophotonics (CNBP), Macquarie

University, North Ryde 2109, Australia. KEYWORDS: Far-red emission; Phosphor; Phytochrome; Plant growth; LEDs. 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

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properties of Mn4+ doped samples were investigated, which 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 the LMT: Mn4+ phosphor has great potential applications in light-emitting diodes (LEDs) for modulating plant growth.

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 (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. In order to meet different levels of human needs, greenhouse industry has been extensively developed in agriculture and horticulture, where 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 power-economical, environmental friendly, long lifetime and low radiant heat


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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 Phosphor-converted LEDs may be the most appropriate for canopy because of their mature fabrication technology and easily tunable spectral composition and intensity through using ideal phosphor. Red and blue light play the dominant role in making carbohydrates, and the wavelengths are roughly similar for different plants; while the far-red (FR) light absorbed by the phytochrome leads to the changes in gene expression of plant structure and response to growth.6 Phytochrome dominates plant growth, development, differentiation process, including PR and PFR two photo-interconvertible 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 PR state by capturing far-red light centered at about 730 nm.7-8 Generally, the amount of red light exceeds that of far-red light in the daytime, and vice versa. The ratio difference between red light and far red light (or no light) generally results in the physiology change of plant from vegetative to floral growth. When PFR concentration is low and PR is high, short-day plants which need longer dark time flower and long-day plants which need longer daytime do not, and vice versa, which indicates that the flowering time of 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



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short-day plants. Nowadays, the increasing intensity and time of the city nightscape lighting leads that phytochrome (PFR) cannot back to PR, which inhibits the blooming and growth of the short-day plants. For the sun light, there is a large proportion of red light and a little of far-red. When the sun is shining, the plant can absorb the red light and reflect the far-red light because of chlorophyll; if a plant exposes to far-red light, which means that the plant may be covered or lower than other plants in radial height and 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 true to control flower time and radial height.11 Blue and red emitting phosphor for LEDs towards plant growth have been investigated extensively; however, the far-red emitting phosphors used for plant growth LED hardly have been reported.12-14 The present work is aimed at developing a far-red emitting phosphor for LED towards plant growth. Mn4+ ions exhibit strong absorption around the near ultraviolet and blue region in octahedral coordination environment from the 4A2→ 4T1 and 4T 2 spin-allowed transitions, and give off red to far red light ranging from 617 nm (16207 cm-1) in Na2SiF615 to 723 nm (13827 cm-1) in SrTiO316 due to the transition of 2Eg → 4A2g. The emission wavelength of Mn4+ strongly influenced by the covalence of the “Mn4+-ligand” bonding,17 so it is possible to obtain the far red emission in a host with strong covalent effects in order to match with the absorption of phytochrome PFR. Compound La(MgTi)1/2O3 with an


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octahedral coordination environmental [Ti/MgO6] group and excellent physical and chemical stability was selected as host, in which the site occupy 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. 2. EXPERIMENTAL Synthesis of samples. Samples 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, A. R.) was first weighted respectively and dissolved in HNO3 and the redundant HNO3 was removed by volatilization, 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 (A. R.) with an appreciate ratio. Final transparent solution was got through dropping solution B to A solution under vigorously stirring, which 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 pre-sintering 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 was recorded on a Bruker D8 Advance powder diffractometer with Cu Kα radiation (λ = 1.5406 Å). Rietveld refinement analysis of sample crystal structure was performed using the Total Pattern



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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., UK) equipped with 450 W Xe lamp and microsecond flash-lamp µF900 (Edinburgh Instruments Ltd.) as excitation source. An oxford OptistatDN2 nitrogen cryogenics temperature controlling system was combined with spectrophotometer to measure the temperature-dependent PL spectra and duration of stay is fixed at 20 minutes at the measured temperature. Computation characterization. All calculations were performed within the framework of density functional theory (DFT) using the PAW (projector augmented wave method) potentials and a plane-wave basis set, which was implemented in the VASP (Vienna

Ab initio

Simulation

Package).

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. Then we construct 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 Supporting Information, and the detailed information of the energy band for pristine and doped system are listed in Table S1-S7. 3. RESULTS AND DISCUSSION 3.1 Rietveld refinement of structure and the site occupancy


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Figure 1. Rietveld refinement XRD patterns of sample La(MgTi)1/2O3: 0.1%Mn4+ by TOPAS program. The solid black crosses, red lines and the blue line express the observed and calculated XRD patterns of sample as well as their differences. The short vertical green lines display the positions of Bragg reflection. For compound La(MgTi)1/2O3, there are cubic and monoclinic two crystal structures.18-19 For confirming the structure of the prepared samples, XRD Rietveld refinements of La(MgTi)1/2O3: 0.1% Mn4+ was performed using TOPAS program, in which the cubic structure La(MgTi)1/2O3 (ICSD No. 43793) with Pm-3m space group was 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 refinement is convergent well due to the low residual factors Rp = 5.277 %, 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



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cubic structure. Table 1. Rietveld Refinement and Crystallographic data of La(MgTi)1/2O3: 0.1%Mn4+ sample Formula

La(MgTi)1/2O3: 0.1%Mn4+

Space group

Pm-3m

a (Å)

3.930

b (Å)

3.930

c (Å)

3.930

β (degree)

90.000

V (Å3)

60.707

Z

1

Rwp (%)

5.277

Rp (%)

2.616

In compound La(MgTi)1/2O3 with cubic structure, there are La3+, Mg2+ and Ti4+ three cationic sites, but it is impossible for Mn4+ 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 more tend to take the place of Ti4+ (r = 0.605 Å) 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 E form = ET (doped ) − ET ( point defect ) − EMn

(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, EMn is the energy of single Mn atom. According to the calculated result shown in Table 2, the formation energy is -8.658(1)


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eV and -13.941(7) eV 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. In order to comprehensive understanding the La(MgTi)1/2O3 compound, the energy band and density of state (DOS) of pristine and doped system also be described in Figure S1 and S2, the La(MgTi)1/2O3 is a direct band gap compound with 2.92 eV band gap. Table 2. The calculation of formation energy (Eform) Occupied site

ET (doped) (eV)

ET (point defect) (eV)

Mn→Mg

-326.066(1)

-311.572(1)

EMn (eV)

Eform (eV) -8.658(1)

-5.835(8) Mn→Ti

-318.091(3)

-298.313(9)

-13.941(7)

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



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Ti, the bond length Mn/ Ti/ Mg-O will changed, leading to the change of center atoms (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 electro-negativity decreasing in order 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 enter the sites of Mg (or Ti). The calculation results illuminate that Mn (Ti)-O bond length decreases by 0.05157 Å and Mg-O bond length increases by 0.05157 Å if Mn occupies Ti sites; but Mn(Mg)-O bond length decreases by 0.06385 Å and Ti-O bond length increases by 0.03159 Å if Mn occupies Mg site. 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 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.


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Figure 3. The 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+ doped in Ti site or Mg site. In present system, Mg and Ti locates in the center of octahedral environment with Oh symmetry, as shown in Figure 3a, the original five degenerate 3d-orbital (dx2-y2, dz2, dxy, dyz and dxz) of free Mn will split into two groups, one is two degenerate, dx2-y2, dz2 and the other is three degenerate dxy, dyz, dxz orbitals, as Mn enter the sites of Mg or Ti. In octahedral crystal field, the energy of orbitals dx2-y2, dz2 is higher than those of another three orbitals dxy, dxz, dyz. The projected density of states (PDOS) on five degenerated 3d orbitals of Mn at Ti or Mg sites were displayed in Figure 3b and c, respectively. It is found that five 3d orbitals can be divided into two groups whether Mn occupied Mg or Ti site: dx2-y2, dz2 with higher energy and another dxz, dyz, 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



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bonding state below Fermi-level as Mn occupied Ti site. Whereas, the dxz, dyz and dxy orbitals form a non-bonding state if Mn doped in Mg site due to the dominant energy distribution at Fermi-level. Furthermore, the PDOS of Mn is significant different when it enters site of Mg or Ti, as shown in Figure 3d and e, respectively. A significant overlap between PDOS peaks of Mn and O atom implies the formation of strong bonding state if Mn enters Ti site, but as Mn enters Mg site, the energy of Mn distributes near the Fermi-surface forms a non-bonding state and have a weak overlap between the orbitals of Mn and O in PDOS. And the energy of bonding state is lower than that of non-bonding state, so the system energy is lower and more stability when Mn substitutes for Ti. In addition, almost no charge was found around 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 hypothesis of Mn enters the site of Mg or Ti, all results indicate that the crystal is more stable when Mn take places Ti site. Thus Mn was expected to enter Ti sites. 3.2 The phase purity and crystal structure


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Figure 4. XRD patterns of LMT: xMn4+ (x = 0.6%-2.0%) (a), the enlarged (220) peak (b) and the cell parameter as function of Mn4+ contents (c) as well as the standard data of LMT (ICSD No. 43793) and the crystal structure of LMT (d). According to above results, Mn prefers to enter the site of Ti instead of Mg. In order to identify above conclusion 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 enter the lattice without significantly influence on crystal structure. However, the strongest diffraction peak (220) shows a small shift towards 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) comparing with that of Ti4+ (r = 0.605 Å). The corresponding lattice constants of La(MgTi)1/2O3: xMn4+ declined mono-directional with the increase of Mn4+ concentration, as shown in Figure 4c, from a = b = c = 3.934 Å for blank sample (x = 0) through a = b = c = 3.926, 3.915 Å for x = 0.6, 0.8% to a = b = c = 3.914 and 3.910



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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+

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. 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 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 four Gaussian peaks centered at 324, 360, 414 and 487 nm corresponding to


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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 chip, which implies that present samples could be used as a potential far-red emitting phosphor with the efficiently excitation of commercial LED chip. Note that sharp lines ranging in 450-500 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 consists of six distinguishable Stokes/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 Comparing with the absorption spectra of phytochrome PR and PFR, peaking at 660 nm and 730 nm, a significant overlap was found between the emissions spectrum of phosphor LMT: Mn4+ and absorption of phytochrome PFR, which indicate that the present phosphor could be used to modulate plant growth.25

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).



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Figure 6a and b displays the PL spectra and corresponding integrated intensity with variable Mn4+ concentration under the 345 nm excitation. The shape and the position of all emission spectra are identical except for their relative intensities (I). With increasing 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 increasing the Mn4+ content on account of 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 other Mn4+ doped host like Gd2ZnTiO6 (26%)26 and CaMg2Al16O27 (16%)24. Generally, the non-radiative 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 non-radiative energy transfer and radiative emission is equal, could be estimated by eq (2): 27 1

 3V  3 Rc ≈ 2   4π xcN 

(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, 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 present system while 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


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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) interaction, respectively. Figure 6c illuminates the relationship between ln(x) vs. 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 7. Decay curves LMT: xMn4+ (x = 0.5%-2.0%) under excitation at 345 nm and monitoring at 708 nm. 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,



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which could be obtained by eq (4):30 ∞

∫ τ= ∫

0 ∞

I (t )tdt

0

I (t )dt

(4)

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+ions 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 afterwards the first gradually increases 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

Figure 8. Tanabe-Sugano energy-level diagram for Mn4+ (d3) electron configuration in


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the octahedral site of LMT host and the schematic diagram of Mn4+ energy level. The schematic diagram of Mn4+ energy level transition is usually dependent on the crystal field environment due to 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 illuminated 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, then relax to the lowest excited level 2Eg through nonradiative transitions pathway 4T1g → 2T2g → 4T2g → 2Eg and 4T2g → 2Eg. Then far-red 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 ( 4T2 g - 4 A2 g ) /10

(5)

on the basis of above PL and PLE spectra, the peak energy difference between the 4

A2g → 4T2g (20528cm-1) and 4A2g → 4T1g (27778cm-1) is about 7250 cm-1, thus the

Racah parameter B can be gotten according to the equation: Dq 15( x − 8) = B ( x 2 − 10 x)

(6)

in which x is expressed as: x=

E ( 4 A2 g →4 T1 g ) − E ( 4A2 g → 4T2 g ) Dq

(7)

According to the emission spectra of samples LMT: xMn4+, the peak energy of 2Eg → 4

A2g transition is about 14124 cm-1, and the Racah parameter C is also evaluated



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according to following equation: E( 2Eg → 4 A2g ) / B = 3.05C / B + 7.9 −1.8B / Dq

(8)

The crystal field parameters Dq, B, C were 2053, 700 and 2959 cm-1 in light of eq (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 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 clearly 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 4

A2g → 4T2g and 4A2g → 4T1g spin-allowed transitions. The lowest excited state of

Mn4+ is 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 2Eg 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 neighbors ligand anions will enhanced with the strongly 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 And a new non-dimensional parameter β1 that quantitatively describes the nephelauxetic effect in the spectroscopy of the Mn4+ ion


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was introduced by Brik: B B0

C C0

β1 = ( )2 + ( )2

(9)

in which B, C and B0, C0 are the Racah parameters of Mn4+ in a crystal or free state, B0 = 1160 cm-1 and C0 = 4303 cm-1 for Mn4+. The calculated values of β1 that reflecting 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 2Eg 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, 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 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 since the operating temperature of the LED chip could reach about 423 K (150 ℃), which will exert great effect on the light output, lifetime, chromaticity and



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color rendering index (Ra).34 Thus, the thermal stability of phosphors is evaluated by temperature-dependent PL spectra. Figure 9a and b shows the normalized integrated intensity of LMT: 0.8%Mn4+ as 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. Comparing heating and cooling two processes, 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 and b), which means that the as-prepared phosphor LMT: Mn4+ offers a better thermal stability and reversible thermal behavior.


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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 10000/T for sample LMT: 0.8%Mn4+ with a thermal quenching mechanism configuration coordinate diagram (c). Generally, the emission intensity thermal quenching behavior of samples is attributed to non-radiative relaxation, which can be explained by a configuration coordinate diagram. The ground state 4A2 and excited states 2Eg, 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 higher excited state 4T2 of Mn4+ and the excited electrons non-radiatively transit to 2Eg, then the excited ions go back to the ground state and emit 708 nm far-red light. At high temperature, some excited electrons absorb additional vibration energy ∆Ea to arrive the cross point between the ground state 4A2 and the excited state 4T2; then the electrons return to the ground state through nonradiative relaxation, the energy released in the form of heating rather than lighting and leading to the decrease of intensity. In this process, the thermal activation energy ∆Ea plays a critical role and can be estimated according the following equation:36 I(T) =

I0 1 + cexp( −∆Ea / KT)

(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, ∆Ea is thermal activation energy, K is the Boltzmann constant (K = 8.62×10-5 eV). The thermal activation energy ∆Ea can be calculated by plotting ln[(I0/IT)-1] against 10000/T, where the slope of the



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straight line is -0.415, as shown in Figure 9c. ∆Ea 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 to occur thermal quenching.

4. CONCLUSIONS In summary, a series of far red 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 site in present compound. Mn4+ ions are 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 far red emission of 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 activation energy ∆Ea = 0.36 eV. Results demonstrate that LMT: Mn4+ is a potential far-red emitting phosphor for plant growth LEDs.


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ASSOCIATED CONTENT Supporting Information The energy band and the DOS of pristine and doped system (Fig. S1); The DOS of Mn and O atoms (Fig. S2); The energy band No.128 (Tab. S1) and No.129 (Tab. S2) of pristine system; The energy band No.128 (Tab. S3), No.129 (Tab. S4) and No.132 (Tab. S5) of Mn doped in Ti site system; The energy band No.128 (Tab. S6) and No.132 (Tab. S7) of Mn doped in Mg site system; The crystal field parameter of LMT and SrTiO3 (Tab.S8).These materials are available from http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (No. 11274251, 51672215), Research Fund for the Doctoral Program of Higher Education of China (RFDP) (No.20136101110017) and Natural Science Foundation of Shaanxi Province (No.2014JM1004).

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TABLE OF CONTENTS


Ab Initio Site Occupancy and Far-Red Emission of Mn4+ in Cubic

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