Highly efficient red emission and multiple energy transfer properties of Dy³⁺/Mn⁴⁺ co-doped Ca₁₄Zn₆Ga₁₀O₃₅ phosphors

Abstract

Novel Dy³⁺/Mn⁴⁺ co-doped Ca₁₄Zn₆Ga₁₀O₃₅ phosphors have been synthesized by a solid state reaction technique. Strong blue emission ranging from 370 nm to 500 nm was observed for the Ca₁₄Zn₆Ga₁₀O₃₅ host, attributed to the recombination of a donor–acceptor pair through a tunneling process. High internal and external quantum efficiencies of 64.4% and 56.2% respectively were obtained under the excitation of 310 nm in Mn⁴⁺ doped Ca₁₄Zn₆Ga₁₀O₃₅. This external quantum efficiency is the highest one reported for Mn⁴⁺ doped oxides. The temperature-dependent quantum efficiency of Ca₁₄Zn₆Ga₁₀O₃₅:Mn⁴⁺ is also measured, indicating the unchanged absorption of the excitation light with temperature. In Ca₁₄Zn₆Ga₁₀O₃₅:Dy³⁺, Mn⁴⁺ phosphors, multiple energy transfer from the host to Dy³⁺ and from Dy³⁺ to Mn⁴⁺ is observed and is confirmed to be a result of the dipole–dipole interaction. The emission changes from deep blue to white to deep red according to the different Dy³⁺/Mn⁴⁺ concentration ratio, and the warm white emission can be realized with the chromaticity coordinate (0.345, 0.275), CCT 3525 K and CRI 87. These results suggest that Ca₁₄Zn₆Ga₁₀O₃₅:Mn⁴⁺ phosphors have potential application as high efficiency red phosphors for solid-state lighting, while Dy³⁺/Mn⁴⁺ co-doped Ca₁₄Zn₆Ga₁₀O₃₅ phosphors can be used as a single-phased white phosphor.

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1 Introduction

The Mn⁴⁺ ion, which is similar to the isoelectronic Cr³⁺ (d³), gives a rather complicated optical spectrum. It shows absorption in the whole ultraviolet region^1,2 and the subsequently emitted phosphorescence is in the deep red (620–720 nm) due to transition from the ²E → ⁴A₂.^3,4 Thus, Mn⁴⁺-doped red phosphors might have potential application in lighting,⁵ holography,⁶ lasers,⁷ and dosimetry.⁸

As a kind of red phosphor with a high quantum efficiency, Mn⁴⁺ activated fluoride phosphors have attracted considerable interest.⁹ Mn⁴⁺ doped K₂TiF₆ red phosphors show internal quantum yields as high as 98%, and high performance white LEDs with 3556 K correlated color temperature, 81 color rendering index (R_a) and luminous efficacy of 116 lm W⁻¹ have been fabricated with these red phosphors.¹⁰ Unfortunately, fluoride host is not stable because of their easy deliquescence.¹¹ Moreover, the toxic HF solution is harmful to the environment in the synthesis process, which restricts its applications. Fluoride phosphors are also not suitable for fluorescent lamps because of their reactivity with the mercury vapor.¹² Unlike fluoride phosphors, the Mn⁴⁺ activated oxide phosphors exhibit good chemical stability.¹³ However, the currently known Mn⁴⁺ activated oxide phosphors cannot meet the needs of general lighting due to their low quantum efficiency. The internal quantum of Sr₄Al₁₄O₂₅:Mn⁴⁺ and 3SrO·5Al₂O₃:Mn⁴⁺ under 380 nm is close to 18%, 27% respectively.¹⁴ The quantum efficiency of La₂LiTaO₆:Mn⁴⁺, Mg²⁺ is 21.4%.¹⁵ The internal and external quantum of CaMg₂Al₁₆O₂₇:Mn⁴⁺ is 35.6% and 16.0% respectively.¹⁶ The highest external quantum reported for Mn⁴⁺ doped phosphors is about 80% for Sr₂MgAl₂₂O₃₆:Mn⁴⁺, but the internal quantum efficiency which is more important for application is still needed identification. Moreover, the synthesis temperature for Sr₂MgAl₂₂O₃₆:Mn⁴⁺ as high as 1500 °C is not benefit for its application.¹⁷ Therefore, Mn⁴⁺ activated oxide phosphors with high internal quantum efficiency under low synthesis temperature focused recently.¹⁸

It is known that Dy³⁺ ions exhibit characteristic emissions of the blue and yellow regions corresponding to ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions under the excitation of NUV light, and thus result the emission of near white-light. Due to the overlap of the emission spectrum of Dy³⁺ ions and the excitation spectrum of Mn⁴⁺ ions, the energy transfer process between Dy³⁺ ions and Mn⁴⁺ ions can be expected based on Dexter's energy transfer theory.¹⁹ Therefore, it is reasonable to believe Dy³⁺/Mn⁴⁺ co-doped samples possess higher color rendering index (CRI) and lower color temperature than the single Dy³⁺-doped samples.

In our work, photoluminescence properties of Mn⁴⁺ doped and Mn⁴⁺/Dy³⁺ co-doped Ca₁₄Zn₆Ga₁₀O₃₅ (CZGO) are studies. The intense blue light emission of CZGO host was observed for first time. High quantum efficiency for Mn⁴⁺ doped CZGO was obtained. The internal and external quantum efficiencies of Ca₁₄Zn₆Ga_9.85O₃₅:0.15Mn⁴⁺ synthesized under 1210 °C reached 64.4% and 56.2%, the highest external quantum efficiency in Mn⁴⁺ activated oxide phosphors reported according to our knowledge. For Dy³⁺/Mn⁴⁺ co-doped CZGO, the CIE chromaticity coordinate (0.345, 0.275), color rendering index (CRI) 85 and color temperature 3525 K are obtained. Moreover, multiply energy transfer processes have been observed and discussed by Inokuti–Hirayama (I–H) model and Dexter's theory. The results show that Mn⁴⁺ doped CZGO is a potential red phosphor for high performance white light LED devices.

2 Experiment

2.1 Material synthesis

Ca_14−xZn₆Ga_10−yO₃₅:xDy³⁺, yMn⁴⁺ (x = 0–0.18, y = 0–0.25) were prepared by the solid-state reaction method. CaCO₃ (99.9% purity), ZnO (99.9% purity), Ga₂O₃ (99.9% purity), Dy₂O₃ (99.99% purity) and MnO₂ (99.99% purity) were used as the starting reactants. According to the stoichiometric composition, the reactants were weighed and mixed thoroughly in an agate mortar, then sintered in a tubular furnace at 1210 °C for 6 h in air. After cooled down to the room temperature, the synthetic products were ground for subsequent analysis.

2.2 Characterizations

The phase compositions of the synthesized samples were studied using an Ultima IV X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å) operated at 36 kV tube voltage and 20 mA tube current. The morphology of the samples were characterized using a S3400N scanning electron microscope (SEM). The PL spectra were obtained using a Hitachi F-7000 spectrophotometer at room temperature with a Xe lamp as source. Time-resolved photoluminescence decays were recorded by a FLS980 (Edinburgh) time-correlated single-photon counting (TCSPC) spectrofluorometer using an interchangeable Nano LED source for excitation. The temperature dependence of luminescence and photoluminescence quantum yields were measured by an Olsuka Electronics QE-2100 intensified multichannel spectrometer.

3 Results and discussion

3.1 XRD and SEM analysis

Fig. 1(a) shows the X-ray diffraction (XRD) patterns of Ca_14−xZn₆Ga_10−yO₃₅:xDy³⁺, yMn⁴⁺ (x, y = 0, 0; 0.1, 0; 0, 0.12; 0.1, 0.12). All patterns of the samples are well in agreement with the standard XRD pattern of CZGO (#245649).²⁰ Fig. 1(b) shows the unit cells viewed from [100] for CZGO crystal which possesses cubic structure with space group F23 (196) and lattice parameters a = 15.0794 Å and V = 3428.88 ų. In the crystal lattice of CZGO, tetrahedral GaO₄ and ZnO₄ (partial disorder) share vertices to form a 3D network with two types of large empties. One type of these empties is filled with octahedral (Ga, Zn)O₆⁻, while the other ones are half occupied by four corner-linked tetrahedral ZnO₄ sharing common oxygen atom, according to Pauling's rules.²¹ On the basis of the effective ionic radii of cation with different coordination numbers (CN),²² Dy³⁺ (6CN, 0.91 Å; 7CN, 0.97 Å) ions are expected to randomly occupy six- and seven-coordinated Ca²⁺ (6CN, 1.00 Å; 7CN, 1.06 Å) sites, and Mn⁴⁺ (6CN, 0.53 Å) ions are preferentially accommodated at the Ga³⁺ (6CN, 0.62 Å) sites with an octahedral coordination in the crystal structure.

Fig. 1 (a) XRD patterns of Ca_14−xZn₆Ga_10−yO₃₅:xDy³⁺, yMn⁴⁺ (x, y = 0, 0; 0.1, 0; 0, 0.15; 0.1, 0.15). (b) A schematic of the CZGO crystal structure viewed in x-direction.

Fig. 2 displays the SEM image of Ca_13.9Zn₆Ga_9.85O₃₅:0.1Dy³⁺, 0.15Mn⁴⁺. The sample exhibits non-identical grains with the particle size approximately sub-micrometer to a few micrometers which tend to aggregate.

Fig. 2 SEM image of Ca_13.9Zn₆Ga_9.85O₃₅:0.1Dy³⁺, 0.15Mn⁴⁺ sample.

3.2 High PL QEs of CZGO:Mn⁴⁺

Fig. 3(a) shows the diffuse reflection spectra of Ca₁₄Zn₆Ga_10−yO₃₅:yMn⁴⁺ (y = 0.00, 0.03, 0.12, and 0.25). There are three dips of reflectivity between 280 and 550 nm. Two dips locating at 332 and 465 nm are assigned to strong spin-allowed transitions in Mn⁴⁺ ions corresponding to ⁴A₂ → ⁴T₂ and ⁴A₂ → ⁴T₁, respectively. The broad dip in the range of 250–350 nm is aroused by both Mn⁴⁺–O²⁻ charge transfer transition and ⁴A₂ → ⁴T₁ transition of Mn⁴⁺ ions, which can be judged from the excitation spectra as below.

Fig. 3 (a) Representative diffuse reflection spectra of Ca₁₄Zn₆Ga_10−yO₃₅:yMn⁴⁺ (y = 0.00, 0.03, 0.12, 0.25). (b) PLE and PL of CZGO (λ_em = 410 nm, λ_ex = 266 nm) and Ca₁₄Zn₆Ga_9.99O₃₅:0.01Mn⁴⁺ (λ_em = 713 nm, λ_ex = 310 nm).

PLE and PL of CZGO (λ_em = 410 nm, λ_ex = 266 nm) and Ca₁₄Zn₆Ga_9.99O₃₅:0.01Mn⁴⁺ (λ_em = 713 nm, λ_ex = 310 nm) are shown in Fig. 3(b). For CZGO, an absorption band from about 200 nm to 300 nm centered at 266 nm (monitored emission wavelength λ_em = 410 nm) and a broad-band PL blue emission centered at 410 nm extend from 360 nm to 500 nm (excitation wavelength λ_ex = 266 nm) of CZGO are observed. The broad-band blue emission has not been reported in CZGO before,^19,20 but similar blue emission was observed in ZnGa₂O₄ crystals owning to the distorted octahedral Ga–O groups serving as the self-activated luminescent centers.^23–27 It is reasonable to ascribe the broad-band blue emission to the recombination of a donor–acceptor pair (DAP) through a tunneling process in CZGO host. For Ca₁₄Zn₆Ga_9.97O₃₅:0.03Mn⁴⁺, the excitation spectrum can be fitted by three Gaussian curves, leading to three distinguished bands peaking at 303 (band I, 33 003 cm⁻¹), 332 (band II, 30 120 cm⁻¹) and 465 (band III, 21 505 cm⁻¹) which are in good agreement with those in the diffuse reflection spectra. The excitation bands located at 332 and 465 nm are assigned to the spin-allowed (⁴A₂ → ⁴T₂) and (⁴A₂ → ⁴T₁) transitions of Mn⁴⁺, respectively. The broad band, which is composed by bands I and II, is due to the overlap between the transitions of Mn⁴⁺–O²⁻ and the spin-allowed transitions of Mn⁴⁺ (⁴A₂ → ⁴T₁). Under excitation at 310 nm, the intense red emission is composed of some distinguishable sharp R lines and Stokes/anti-Stokes side-peaks, located at 676, 684, 695, 704 and 713 nm, due to different vibrational modes for the 3d³ electrons when Mn⁴⁺ is of the octahedral complex.²⁸ Although there is an overlap between the PLE spectrum of CZGO and Mn⁴⁺ ion, our measurement for the average lifetimes of the PL spectrum of CZGO with different Mn⁴⁺ concentration suggesting there is no energy transfer between CZGO and Mn⁴⁺.

The energy splitting of Mn⁴⁺ ion with octahedral coordination on the crystal field strength can be well illustrated by Tanabe–Sugano energy diagram (Fig. 4).²⁹ The value of the local crystal-field parameter D_q can be obtained from the peak energy (21 505 cm⁻¹) of the ⁴A₂ → ⁴T₂ transition³⁰

D_q = E(⁴A_2g → ⁴T_2g)/10 (1)

Fig. 4 Tanabe–Sugano diagram for Mn⁴⁺ in CZGO.

Moreover, based on the obtained energy difference (8615 cm⁻¹) between the ⁴A₂ → ⁴T₂ and ⁴A₂ → ⁴T₁ transitions, Racah parameter B can be evaluated from the expression³¹

here the parameter x is defined as

According to the peak energy (14 025 cm⁻¹) corresponding to ²E → ⁴A₂ transition of Mn⁴⁺ derived from emission spectrum above, Racah parameter C can be calculated by the following eqn³²

E(²E → ²A₂)/B = 3.05C/B + 7.9 − 1.8B/D_q (4)

The values of D_q, B and C in the CZGO:Dy³⁺, Mn⁴⁺ are then determined to be 2150, 860 and 2572 cm⁻¹, respectively. The values of B is higher than those reported in oxides while the values of C is smaller than those reported in oxides, and the values of D_q is similar to those reported in oxides.³³ In fact, the emission peak energy of ²E → ⁴A₂ transition is singularly dependent on the covalence of the “Mn⁴⁺–ligand” bonding (nephelauxetic effect). The nephelauxetic ratio β can be determined by following equation:³⁴

here B₀ and C₀ represent Racah parameters for free ions. For Mn⁴⁺ ions, B₀ and C₀ are equal to 1160 cm⁻¹ and 4303 cm⁻¹, respectively.³⁵ So β for Mn⁴⁺ in CZGO is calculated to be 0.952. This value is similar to those reported in oxides, but higher than those in fluorides, due to the more ionic Mn⁴⁺–F⁻ bonding than Mn⁴⁺–O²⁻ bonding.³⁶

Fig. 5 shows PL spectra (λ_ex = 310 nm) of Ca₁₄Zn₆Ga_10−yO₃₅:yMn⁴⁺ phosphors as a function of y at room temperature. The PL intensity of Mn⁴⁺ ions increases with increased Mn⁴⁺ concentration, and concentration quenching occurs beyond y = 0.15. Photoluminescence internal and external quantum efficiency of Ca₁₄Zn₆Ga_10−yO₃₅:yMn⁴⁺ phosphors is measured in Table 1. The photoluminescence internal an external quantum efficiencies (QEs) reach 64.4% and 56.2% respectively when y = 0.15. Although the internal quantum efficiency of our sample is lower than the phosphor Sr₂MgAl₂₂O₃₆:Mn⁴⁺ reported by Renping Cao,¹⁷ the external quantum efficiency of our sample has been the highest one according to our knowledge.

Fig. 5 Photoluminescence emission spectra of Ca₁₄Zn₆Ga_10−yO₃₅:yMn⁴⁺ (y = 0.03, 0.09, 0.12, 0.15, 0.20, 0.25) under λ_ex = 310 nm.

Table 1 Photoluminescence internal and external QEs at different y in Ca₁₄Zn₆Ga_10−yO₃₅:yMn⁴⁺ phosphors excited at 310 nm

y	0.03	0.09	0.12	0.15	0.20	0.25
Internal QE (%)	41.3	49.5	59.4	64.4	54.6	44.6
External QE (%)	36.2	43.7	52.4	56.2	48.2	38.3

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The temperature-dependent emission spectra is shown in Fig. 6, and two features can be observed: (1) emission intensity decreases with temperature increasing (2) all emission peaks shift to longer wavelength (red shift) region with increasing temperature.

Fig. 6 PL spectra at different temperature for the Ca₁₄Zn₆Ga_9.85O₃₅:0.15Mn⁴⁺ excited at 310 nm. The inset: the activation energy 0.156 eV for Ca₁₄Zn₆Ga_9.85O₃₅:0.15Mn⁴⁺.

The Arrhenius equation can be used to evaluate activation energy ΔE for thermal quenching:³⁷

I_T = I₀/[1 + c exp(−ΔE/kT)] (6)

here I₀ is the initial emission intensity, I_T is the intensity at temperature T, c is a constant, k is the Boltzmann constant, and ΔE is the activation energy for thermal quenching. Based on the PL spectra in Fig. 6, the ΔE of CZGO:Mn⁴⁺ is obtained as 0.156 eV by fitting the curve of ln[(I₀/I_T) − 1] versus 1/kT, as shown in the inset of Fig. 6.

The peak position, the internal and external QEs are shown in Table 2. The peak position shifts from 713.4 to 716.6 nm as temperature from 298 K to 573 K. The red-shift behavior can be explained by the Varshini equation for temperature dependence^38,39

where E(T) is the energy difference between excited states and ground states at a temperature T, E₀ is the energy difference at 0 K, and a and b are fitting parameters. The bond lengths between the luminescent center and its ligand ions increase with increased temperature, which results in the decreased crystal field. Then, it will cause the split of degenerate excited state or ground state, resulting in the decrease of the transition energy. Therefore, the emission peak is red-shifted with the increase of temperature.⁴⁰ The red-shift behavior can also be explained by Tanabe–Sugano energy diagram (Fig. 4), decreased crystal field correspond to less value of D_q/B, resulting in the smaller transition energy between excited state ²E and ground state ⁴A₂.

Table 2 The peak position, internal and external QEs of Ca₁₄Zn₆Ga_9.85O₃₅:0.15Mn⁴⁺ at different temperature

Temperature (K)	298	323	373	423	473	523	573
Peak (nm)	713.4	714.0	714.3	715.0	715.6	715.9	716.6
Internal QE (%)	64.4	63.7	61.4	59.4	56.4	53.0	45.9
External QE (%)	56.2	55.4	53.4	51.6	49.0	46.1	39.9

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Both internal QE and external QE decreases with increased temperature. The internal and external QEs decrease 28.7% and 29.0% of the values at 298 K, respectively. Almost the same decreasing rate indicates that the change of QEs with temperature is not brought by the change of the absorption of the incident light on the phosphors.

3.3 White light of CZGO:Dy³⁺

Fig. 7 displays the PL and PLE spectra of Ca_13.9Zn₆Ga₁₀O₃₅:0.1Dy³⁺ (λ_em = 410 nm and 486 nm, λ_ex = 266 nm and 352 nm). The broad emission blue band centered at 410 nm is obviously from CTB of host. The absorption peak centered at 352 nm and the sharp emission bands centered at 486 nm and 578 nm come from the transition ⁶H_15/2 → ⁶P_7/2, ⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_15/2 for Dy³⁺ ion respectively. It can be seen that the emission spectrum under the excitation 266 nm have the similar profile with that excited at 352 nm except for the appearance of the wide band centered at 410 nm. In addition to, the intensity of emission excited at 266 nm is much times higher than the emission excited at 352 nm, which indicating the high photon energy that host absorbed can be transferred efficiently to Dy³⁺ ion.

Fig. 7 PL and PLE spectra of Ca_13.9Zn₆Ga₁₀O₃₅:0.1Dy³⁺ (λ_em = 410 nm and 486 nm, λ_ex = 266 nm and 352 nm).

The dependence of the emission host intensities of Ca_14−xZn₆Ga₁₀O₃₅:xDy³⁺ phosphors on Dy³⁺ concentration x is shown in Fig. 8. The change of the intensity for ⁴F_9/2 → ⁶H_13/2, ⁶H_15/2 transition shows the feature of concentration quenching with the maximum intensity as x = 0.1. However, the blue intensity for host decreases monotonically with x increase, indicating further the existence of the energy transfer from the host to Dy³⁺ ions.

Fig. 8 Emission spectra of Ca_14−xZn₆Ga₁₀O₃₅:xDy³⁺ under the excitation of 266 nm. The inset shows the dependence of three emission peakshost on Dy³⁺ concentration.

An energy band diagram is proposed in Fig. 9. The broad-band blue emission of host is suggested to be ascribed to the recombination of a donor–acceptor pair (DAP) through a tunneling process, that is, from the transition between electrons trapped by the donor band (DB) (being formed by oxygen vacancies) and holes captured by the acceptor band (AB) (being formed by gallium vacancy) (V_Ga), or pair of gallium vacancy and oxygen vacancy (V_O, V_Ga). It is worth noting that the absorption energy (4.7 eV, 266 nm) of host is located at approximately twice the energy of the Dy³⁺:⁴F_9/2 → ⁶H_13/2 (2.1 eV, 578 nm) or ⁴F_9/2 → ⁶H_15/2 (2.5 eV, 486 nm) transition. Moreover, absorption bands of the host and Dy³⁺ are almost overlapped in Fig. 7, which means the energy transfer process from the host to Dy³⁺ is dominated by the cooperative energy transfer.^41,42 The cooperative energy transfer between the CZGO host and Dy³⁺ ions can be understood as following: in first step, the electrons in the conduction band de-excite to DB (process I in Fig. 9), accompanying the excitation of Dy³⁺ from ⁶H_15/2 to ⁴F_9/2 (process II in Fig. 9) followed by the emission of Dy³⁺ (⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_15/2); In second step, the electrons in DB can either de-excite to AB (process III in Fig. 9) which results the emission of the host, or to valance band (process IV in Fig. 9) which results the excitation of Dy³⁺ from ⁶H_15/2 to ⁴F_9/2 (process IIV in Fig. 9) followed by the emission of Dy³⁺. The concentration quenching of Dy³⁺ emission intensity can be ascribed to the cross relaxation between neighboring Dy³⁺ ions. Cross relaxation (CR) occurs when the energy from excited states promotes the ground state to the metastable levels. For Dy³⁺ ions, the CR mechanisms [⁴F_9/2, ⁶H_15/2] → [⁶H_7/2/⁴F_9/2, ⁶F_3/2], [⁴F_9/2, ⁶H_15/2] → [⁶H_9/2/⁶F_11/2, ⁶F_5/2] and [⁴F_9/2, ⁶H_15/2] → [⁶F_1/2, ⁶H_9/2/⁶F_11/2] denoted by CRC1, CRC2, and CRC3, respectively, are possible responsible to the concentration quenching based on the energy match rule, as illustrated in Fig. 9.

Fig. 9 Energy level diagram showing energy transfer and the CR processes of CZGO:Dy³⁺, Mn⁴⁺.

The fluorescence decay curves of Ca_14−xZn₆Ga₁₀O₃₅:xDy³⁺ (x = 0.00, 0.05, 0.09, 0.10, 0.12, 0.15) monitored at 410 nm with the excitation of 266 nm are shown in Fig. 11. The curve can be nearly fitted by one exponential function, and the average lifetime τ is given by⁴³

where I(t) is the luminescent intensity at time t. The obtained τ is given in the inset of Fig. 10. The lifetime decreased from 32.8 μs to 22.6 μs when the concentration of Dy³⁺ increased from 0.05 to 0.15. The decrease of lifetime with increase of Dy³⁺ concentration confirms the existence of the energy transfer between host and Dy³⁺ ions.

Fig. 10 Luminescence decay curves of Ca_14−xZn₆Ga_9.88O₃₅:xDy³⁺ (x = 0.00, 0.05, 0.09, 0.10, 0.12, 0.15) (monitored at 410 nm excited at 266 nm). The inset: dependence of the lifetime τ and energy transfer efficiency η on Dy³⁺ doping concentration.

In order to figure out the interaction type of energy transfer between host and Dy³⁺ ions, the donor centers of oxygen vacancies marked as “A”, are considered. If “A” and Dy³⁺ ions are randomly distributed in the host and the migration processes among Dy³⁺ ions are negligible, then the temporal evolution of the Dy³⁺ luminescence intensity I(t), following pulsed excitation at 266 nm, can be given by the Inokuti–Hirayama (I–H) model⁴⁴

where I(0) is luminescence intensity when t = 0, τ₀ is the lifetime of the host in the absence of Dy³⁺ ions, S is the multipolar interaction parameter, and the energy transfer parameter γ is defined byin which C_A is Dy³⁺ concentration, Γ(x) is the gamma function, and C_DA^(S) is the “A” → Dy³⁺ energy transfer parameter. The decay curves for the samples doped with 5 mol% and 10 mol% of Dy³⁺ ions are presented in Fig. 11. The best fitting using I–H model is S = 6, indicating the energy transfer between the host and Dy³⁺ ions is dominantly governed by dipole–dipole interaction.

Fig. 11 Decay curves of Ca_14−xZn₆Ga₁₀O₃₅:xDy³⁺, (x = 0.05, 0.10) measured by 266 nm excitation and monitored at 410 nm, together with the fitted curves using I–H model, showing the best fitting when S = 6. All the experimental data of samples.

Considering no overlap between PLE and PL spectra of Dy³⁺, the concentration quenching of Dy³⁺ emission cannot be related to the radiation re-absorption. It might be attributed to multipole–multipole interaction or exchange interaction. In order to figure out this point, the average distance (R_c) between the nearest Dy³⁺ ions can roughly be calculated using the following eqn.⁴⁵

where V is the volume of the unit cell; N is the number of host cations which can be replaced by Dy³⁺ ions in the unit cell; X_c is the critical concentration of Dy³⁺ ion. For this case, V = 3428.88 ų, N = 4, X_c = 0.10, R_c is obtained to be 25 Å. Blasse⁴⁶ has pointed out that multipolar interaction predominates if R_c is larger than 5 Å. Therefore, the multipolar interaction accounts for the concentration quenching in CZGO:Dy³⁺ phosphors.

Based on the model proposed by Dexter,⁴⁷ the interaction type between Dy³⁺ ions can be identified by

I/x = [1 + β(x)^θ/3]⁻¹ (12)

where I is the emission intensity of Dy³⁺ ion under the excitation of 352 nm; x is Dy³⁺ concentration; and is a constant for the same excitation condition for a given host. θ = 6, 8, or 10 for dipole–dipole (d–d), dipole–quadrupole (d–q) and quadrupole–quadrupole (q–q), respectively. For the phosphor CZGO:Dy³⁺, the dependence of log(I/x_Dy³⁺) on log(x_Dy³⁺) shown in Fig. 12 is almost linear and the fitted line slopes are −1.44 and −1.49 for ⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_15/2 transition respectively. The value of θ is 6 determined by the line slope, indicating the electric d–d interaction is responsible for the concentration quenching of Dy³⁺ ions in CZGO.

Fig. 12 The relations of log(I/x_Dy³⁺) and log(x_Dy³⁺) for ⁴F_9/2 → ⁶H_13/2, ⁴F_9/2 → ⁶H_15/2 transitions of Dy³⁺ ions.

3.4 Optimized white light of CAGO:Dy³⁺, Mn⁴⁺

Fig. 13 shows PL spectra of Ca_13.9Zn₆Ga_10−yO₃₅:0.1Dy³⁺, yMn⁴⁺ phosphors (y = 0.03, 0.09, 0.12, 0.15, 0.20, 0.25) at room temperature (λ_ex = 266 nm). The spectra exhibits blue CTB, blue/yellow emissions and a deep red emission, which result from the emission of host, ⁴F_9/2 → ⁶H_15/2/⁴F_9/2 → ⁶H_13/2 transitions of Dy³⁺ ion and ²E → ⁴A₂ transition of Mn⁴⁺ ion, respectively. As Mn⁴⁺ concentration increases, the emissions intensity of both host and Dy³⁺ ions decrease, and the maximum emission of Mn⁴⁺ ions is shown as y = 0.15. As discussed above, energy transfer exists from host to Dy³⁺ ions at the excitation of 266 nm. Moreover, the external QEs of Mn⁴⁺ ion excited at 266 nm in Ca_13.9Zn₆Ga_9.85O₃₅:0.1Dy³⁺, 0.15Mn⁴⁺ and Ca₁₄Zn₆Ga_9.85O₃₅:0.15Mn⁴⁺ are measured as 15.6% and 10.1%, which indicating the energy transfer from Dy³⁺ to Mn⁴⁺ helps enhancing the QEs of Mn⁴⁺ emission. Energy transfer from Dy³⁺ to Mn⁴⁺ ions is also possible due to the overlap between the PLE spectrum of Mn⁴⁺ ions and PL spectrum of Dy³⁺ ions in CAGO:Dy³⁺, Mn⁴⁺. The energy transfer mechanism from Dy³⁺ to Mn⁴⁺ ion can be explained as non-radiative transitions from ⁴F_9/2 level of Dy³⁺ (20 747 cm⁻¹) to ⁴T₂ level of Mn⁴⁺ (14 025 cm⁻¹) by the assistance of phonons⁴⁸ by Fig. 10. Therefore, multiply energy transfer can be happen in CZGO:Dy³⁺, Mn⁴⁺ at the excitation of 266 nm.

Fig. 13 Photoluminescence emission spectra of Ca_13.9Zn₆Ga_10−yO₃₅:0.1Dy³⁺, yMn⁴⁺ as a function of y under λ_ex = 266 nm.

Fig. 14(a) shows the luminescence decay curves (excited at 266 nm and monitored at 486 nm of Dy³⁺ emission) of Ca_13.9Zn₆Ga_10−yO₃₅:0.1Dy³⁺, yMn⁴⁺ samples. The lifetimes were determined by the fitting of single exponential function to be 0.630, 0.468, 0.360, 0.334, 0.316, 0.296 and 0.261 ms for y = 0, 0.03, 0.09, 0.12, 0.15, 0.20, and 0.25 respectively, shown also in Fig. 14(a). The decrease of the lifetime confirms the existence of energy transfer from Dy³⁺ to Mn⁴⁺. The energy-transfer efficiency η_T is calculated using eqn (5) and shown in Fig. 14(b). Although the value of η_T always increase with increasing Mn⁴⁺ dopant concentration in our experiment, the emission intensity of Mn⁴⁺ tends to decrease at higher Mn⁴⁺ concentration (x > 0.15) due to the concentration quenching. The energy transfer efficiency is 59% when y = 0.25 with maximal 713 nm emission intensity.

Fig. 14 (a) Decay curves of Ca_13.9Zn₆Ga_10−yO₃₅:0.1Dy³⁺, yMn⁴⁺ phosphors doped with various Mn⁴⁺ concentrations (excited at 266 nm and monitored at 486 nm). (b) The dependence of lifetime and energy transfer efficiency (corresponding to the Dy^{3+ 4}F_9/2 → ⁶H_15/2 transition) in Ca_13.9Zn₆Ga_10−yO₃₅:0.1Dy³⁺, yMn⁴⁺ (y = 0.05, 0.09, 0.12, 0.15, 0.20, 0.25) on Mn⁴⁺ concentration.

According to Dexter's energy-transfer model of multipolar interaction and Reisfeld's approximation, the following relation can be given ^49,50

where τ_Dy and τ_Dy–Mn are the lifetimes of Dy³⁺ in the absence and presence of Mn⁴⁺, respectively. C is the sum of the concentrations of Dy³⁺ and Mn⁴⁺, and S = 6, 8 and 10 corresponding to d–d, d–q, and q–q interactions, respectively. Fig. 15 shows the linear fitting of the relationship between τ_Dy/τ_Dy-Mn and C^S/3, and the largest values of R² with S = 6, indicating that the energy transfer from Dy³⁺ to Mn⁴⁺ occurs via d–d interaction.

Fig. 15 Dependence of τ_Dy/τ_Dy−Mn on (a) C^6/3, (b) C^8/3 and (c) C^10/3 in Ca_13.9Zn₆Ga_10−yO₃₅:0.1Dy³⁺, yMn⁴⁺ (y = 0.05, 0.09, 0.12, 0.15, 0.20, 0.25). The best linear fitting indicates the energy transfer from Dy³⁺ to Mn⁴⁺ occurs via d–d interaction.

Fig. 16 shows the CIE chromaticity diagram of (a) CZGO, (b) Ca_13.9Zn₆Ga₁₀O₃₅:0.1Dy³⁺, (c) Ca_13.9Zn₆Ga_9.85O₃₅:0.1Dy³⁺, 0.15Mn⁴⁺ excited at 266 nm and (d) Ca₁₄Zn₆Ga_9.85O₃₅:0.15Mn⁴⁺ excited at 310 nm. The phosphor CZGO emits deep blue light, of which the chromaticity coordinate is (0.158, 0.062). The chromaticity coordinates of Ca_13.9Zn₆Ga₁₀O₃₅:0.1Dy³⁺ and Ca_13.9Zn₆Ga_9.85O₃₅:0.1Dy³⁺, 0.15Mn⁴⁺ with CCTs of 5252 K and 3522 K, with CRIs of 72 and 87, are (0.254, 0.288) and (0.345, 0.275), respectively. Obviously, higher color rendering index (CRI) and lower color temperature are obtained by adding Mn⁴⁺ ions. Except for the short exciting wavelength, the high color rendering index (R_a = 87) and low color temperature 3522 K can well meet the generally lighting.

Fig. 16 CIE chromaticity coordinates of (a) CZGO, (b) Ca_13.9Zn₆Ga₁₀O₃₅:0.1Dy³⁺, (c) Ca_13.9Zn₆Ga_9.85O₃₅:0.1Dy³⁺, 0.15Mn⁴⁺ excited at 266 nm and (d) Ca₁₄Zn₆Ga_9.85O₃₅:0.15Mn⁴⁺ excited at 310 nm in wavelength range 380–800 nm.

4 Conclusion

In summary, strong blue emission band ranging from 370 nm to 500 nm was observed for CZGO host, attributed to the recombination of a donor–acceptor pair (DAP) through a tunneling process. The highest internal and external quantum efficiencies were measured to be 64.4% and 56.2% respectively for CZGO:Mn⁴⁺. This external quantum efficiency is the highest one reported for Mn⁴⁺ doped oxides. The energy transfer processes either from the host to Dy³⁺ or from Dy³⁺ to Mn⁴⁺ are confirmed and demonstrated arising from dipole–dipole interaction in Dy³⁺/Mn⁴⁺ co-doped CZGO, and the emission changes from deep blue to white to deep red according to the different Dy³⁺/Mn⁴⁺ concentration ratio, and Furthermore, the warm white emission can be realized with the chromaticity coordinate (0.345, 0.275), CCT 3525 K and CRI 87. The results suggest CZGO:Mn⁴⁺ phosphors have the potential application as high efficiency red phosphors for solid-state lighting, while Dy³⁺/Mn⁴⁺ co-doped CZGO can used as a single-phased white phosphor.

Acknowledgements

Financial supported from the Key Laboratory of Innovation Method and Decision Management System of Guangdong Province, the Research Guangzhou Science (No. 2016201604030035) and Technology Project of Guangdong Province, China (No. 2016201604030027).

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