Effect of the replacement of Zn2+ with Mg2+ in Ca14Zn6Ga10O35:Mn4+

Zhicong Liu; Guangzhan Shao; Wei Chen; Guangcai Hu; Linli Shen; Yinzi Cheng; Xiaojuan Liang; Weidong Xiang

doi:10.1364/OME.8.002532

1. Introduction

With the improvement of human living standard and quality, more and more people have access to fruits and vegetables of all seasons at any time. The approach to achieve this availability is plant lighting. It is reported that 410–500nm blue light affects chlorophyll synthesis, leaf shape, and flower bud formation; 610–700nm red light affects germination and plant growth; and 700–740nm far-red light is indispensable for plant photosynthesis and growth [1–3]. Unfortunately, commercial white light-emitting diode (LED) phosphors lack the far-red component, which makes them unsuitable for plant lighting. Therefore, developing appropriate far-red phosphors for plant lighting represents the general trend.

To date, Eu ion (Eu²⁺ and Eu³⁺)-activated red phosphors, such as M₂Si₅N₈:Eu²⁺(M = Ca, Sr, Ba), (Sr, Ca)S:Eu³⁺, and Y₂O₂S:Eu³⁺, have been widely investigated because of their outstanding luminescence properties and high quantum yields [4–6]. However, their emission wavelengths are below 700nm instead of in the far-red region. Recently, transition element Mn⁴⁺-activated red phosphors have attracted great attention. It is well known that Mn⁴⁺ with a 3d³ electron configuration will exhibit 620–750nm emission bands under 200–600nm excitation when in an octahedral coordination environment [7]. Mn⁴⁺-doped germanate, titanate, aluminate, and fluoride red phosphors have been reported largely because the ionic radius of Ge⁴⁺, Ti⁴⁺, Al³⁺, or Si⁴⁺ in the crystal structure are quite similar to that of Mn⁴⁺, and these ions can therefore be displaced by Mn⁴⁺. However, most of their emission wavelengths are beyond the far-red region [8–13].

Recently, Ca₁₄Zn₆Al₁₀O₃₅(CZA):Mn⁴⁺ phosphor with 650–750nm far-red emission has been reported to be suitable for plant lighting [14–17]. There are several reports about improving its luminescence properties, such as by determining the appropriate Mn⁴⁺ doping concentration, synthesis temperature, and Mg²⁺ doping concentration, as well as by employing the oxygen-pressure method [17–19]. Despite a crystal structure similar to CZA:Mn⁴⁺, Ca₁₄Zn₆Ga₁₀O₃₅(CZG):Mn⁴⁺ possesses superior luminescence properties and better thermal stability [18,20,21]. Therefore, there is great potential to improve the luminescence properties of CZG:Mn⁴⁺ by an appropriate method, which has not yet been reported.

Herein, a series of Ca₁₄Zn_6-xMg_xGa_9.86O₃₅:0.14Mn⁴⁺(x = 0.00, 0.20, 0.40, 0.60, 0.80, 1.00) phosphors have been prepared by a high-temperature solid-state method to explore the effect of replacing Zn²⁺ with Mg²⁺ and therefore determine the optimal Mg²⁺ doping concentration .

2. Experimental

2.1. Synthesis

A series of Ca₁₄Zn_6-xMg_xGa_9.86O₃₅:0.14Mn⁴⁺(CZMG:Mn⁴⁺) (x = 0.00, 0.20, 0.40, 0.60, 0.80, 1.00) phosphors doping concentrations have been prepared through a simple solid-state reaction. First, the raw materials, including CaCO₃ (99.99%), ZnO (99.99%), Ga₂O₃ (99.99%), MgO(99.99%) and MnCO₃ (99.99%), were weighed based on the stoichiometric ratio and ground in an agate mortar for 30 min. Then, the mixture was inserted in a muffle furnace with 1473K calcination temperature and 6h calcination time. Finally, the samples were removed from the muffle furnace and ground again to pulverulent form.

2.2. Characterizations

The crystal structure of the samples was determined by an X-ray diffractometer (XRD, D8 Advance, Bruker, Germany) with Cu Kα radiation at 40 kV and 40 mA. The particle morphology and size were observed with a JMF-6700F scanning electron microscope (SEM). The excitation spectra, emission spectra, quantum efficiency, and temperature-dependent emission spectra, decay curves were recorded with a FluoroMax-4 spectrophotometer. Luminous efficiency are evaluated using an integrating sphere (PMS-50; Everfine Photo-E-Info Co. Ltd, Hangzhou, China) at an operating current of 20 mA.

3. Results and discussion

3.1 XRD and SEM analysis

Figure 1(a) displays the XRD patterns of CZMG:Mn⁴⁺ phosphor. It should be pointed out that all the diffraction peaks are in agreement with the standard ICSD card No.245649 when x is less than 0.60, which indicates that a small amount of Mg²⁺ substitution has no impact on the CZG host crystal structure, while the impurity peaks around 17°, 22° and 32° appear when x is greater than 0.60. Figure 1(b) is refined results for the XRD profile of the CZMG:Mn⁴⁺. The residual values of Rwp = 10.00%, Rp = 7.35% illustrates refined results are dependable and atom coordinates, thermal vibration parameters and fraction factors are basically in the same line with experimental values.

Fig. 1 (a) XRD patterns of CZMG:Mn⁴⁺(x = 0.00, 0.20, 0.40, 0.60, 0.80, 1. 00); (b) Rietveld refinement of the XRD profile CZMG:Mn⁴⁺(x = 0.40).

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As we know, CZG belongs to cubic system with space group F23(196) and the standard cell parameters of it are a = 15.0794Å, Z = 4, V = 3428.88Å³. In order to corroborate the successful incorporation of Mg²⁺ in CZG host, the cell parameters are obtained by XRD Rietveld refinement as listed in Table 1. With the increase of Mg²⁺ concentration, the diminishing Unit-cell edge length and volume reveal that the substitution of Zn by Mg²⁺ is successful because the ionic radius of Mg²⁺ (0.72Å for CN = 6) is smaller than that of Zn²⁺ (0.74Å for CN = 6). In addition, the cell Volume difference of the phosphor with x less than 0.40 is far less than that of the phosphor with x more than 0.40, implying the substitution of Mg²⁺ ions is almost to a limit.

Table 1. Cell parameters of Ca₁₄Zn_6-xMg_xGa_9.86O₃₅:0.14Mn⁴⁺

View Table

Figure 2(a) and (b) is the SEM image of CZMG:Mn⁴⁺ (x = 0.00) and CZMG:Mn⁴⁺(x = 0.40) respectively. Two images show that the fine particles with few micrometers in size have an irregular morphology because of aggregation. Moreover, it is worth mentioning that the aggregation extent of CZMG (x = 0.40):Mn⁴⁺ is more serious than that of CZMG:Mn⁴⁺(x = 0.00).

Fig. 2 SEM image of (a) CZMG:Mn⁴⁺ (x = 0.00) and (b) CZMG:Mn⁴⁺(x = 0.40).

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3.2 Luminescence properties of CZMG:Mn⁴⁺

The excitation spectra of the CZMG:Mn⁴⁺ phosphor monitored at 711nm were acquired in Fig. 3(a). Two intense excitation bands at 250–420nm and 420–550nm are observed. The former excitation band corresponds to the overlap of the Mn⁴⁺–O^2– charge transfer and the ⁴A₂ → ⁴T₁ transition of Mn⁴⁺, and the latter region, in which the excitation peak is located at 460nm, corresponds to the ⁴A₂ → ⁴T₂ transition of Mn⁴⁺ [22–25]. It can also be known that the excitation intensity increases with the increase of Mg²⁺ concentration and then decreases sharply.

Fig. 3 Normalized excitation (a) and emission (b) spectra of CZMG:Mn⁴⁺(x = 0.00, 0.20, 0.40, 0.60, 0.80, 1.00).

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The emission spectra of CZMG:Mn⁴⁺ excited at 460nm in Fig. 3(b). The emission band at 650nm-750nm produced by spin-forbidden ²E → ⁴A₂ transition of Mn⁴⁺ is detected [22]. The emission intensity of the phosphor continuously increases with the increase in Mg²⁺ doping c oncentration until x = 0.40, then decreases. Moreover, there is a slight shift of the emission peaks toward longer wavelengths with the increase in Mg²⁺ .

Here, two factors account for the phenomenon that the Mg²⁺ substitution has a positive influence in luminous intensity. In reality, Mg²⁺ occupies the Zn²⁺ position at the center of the octahedron, the original Mn⁴⁺-Mn⁴⁺-O^2- pairs tend to form Mn⁴⁺-Mg²⁺ pairs, which increases the transition probability from the ground state to the excited state and inhibits the energy transfer between Mn⁴⁺ and Mn⁴⁺, thus enhancing the luminescence [26,27], The other factor is that a spot of incorporated Mg²⁺ as a flux made the particle size and shape more uniform in Fig. 2(a) and Fig. 2(b), which leads to greater luminescence. With regard to reducing emission intensity subsequently, on the one hand, it is most likely that impurity phase(s) forms and increases when Mg²⁺ concentration more than 0.60 because impurity peaks increase in Fig. 1(a) and cell volume difference decreases in Table 1; on the other, substitutional defects lead to lattice distortion, and then damage the rigidity of crystal structure, increasing nonradiative transition rate. Additionally, the phenomenon of red shift of emission peak can be explained by the reason that the incorporation induced lattice shrinking and therefore crystal field weakening because Mg²⁺ radius less than that of Zn²⁺.

3.3 Thermal resistance performance of CZMG:Mn⁴⁺ and CZG:Mn⁴⁺

To evaluate the application of the phosphor in LED, the temperature-dependent emission spectra of CZMG:Mn⁴⁺(x = 0.00) and CZMG:Mn⁴⁺(x = 0.40) are presented in Fig. 4(a) and Fig. 4(b) respectively. It can be seen that their emission intensity gradually decreases with the increases in temperature from 318 K to 518 K due to thermal quenching behavior. The intensity decreases to only ~50% of the initial emission intensity when the temperature increases by 200°C, implying that both of them have low thermal quenching and outstanding thermal resistance performance. In order to study the effect of Mg²⁺ substitution on phosphor in detail, the activation energy ∆E is used as a specific evaluation indicator of temperature dependence, which can be obtained by the Arrhenius equation as follows:

I T = I 0 [1 + c \exp (- Δ E / k T)]

where I₀ represents the initial emission intensity, I_T is the emission intensity at a given temperature, c and T are both constants, and k is the Boltzmann constant (8.617 × 10⁻⁵ eV/K). Based on the equation, the fitted linear relationship between ln[(I₀/I_T) – 1] and 1/kT is given in Fig. 4(c), and the activation energy ∆E, the abs of the slope was obtained as 0.2065 and 0.1868 for CZMG:Mn⁴⁺(x = 0.00) and CZMG:Mn⁴⁺(x = 0.40), which manifests that the thermal resistance performance of the phosphor is slightly reduced by the addition of Mg²⁺. The mechanism of this phenomenon is that the defects caused by Mg²⁺ substitution lead to poor rigidity of crystal structure, which makes the vibration relaxation or nonradiative transition stronger when the temperature goes up.

Fig. 4 Temperature-dependent emission spectra of (a) CZMG:Mn⁴⁺(x = 0.00) and (b) CZMG:Mn⁴⁺(x = 0.40) excited at 460 nm, (c) Dependence of ln(I₀/I_T–1) on 1/kT.

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3.4 Luminescence decay properties of CZMG:Mn⁴⁺

Figure 5 shows the decay curves of the samples excited at 460 nm and monitored at 711 nm. All the sample curves are well fitted with the following single exponential formula:

I = I 0 \exp (- t / τ) + A

where I and I₀ are the emission intensity of sample at a given time and the initial emission intensity, respectively, and τ is the lifetime. According to the equation, the decay time decreases from 2.72 ms to 2.25 ms with the increase in Mg²⁺ concentration. Note also that the decay time is 2.57ms when x = 0.40. As a matter of fact, lifetime of the phosphor depends on the following formula:

τ = \frac{1}{W R + W N R}

In the formula, W_R and W_NR represent radiation transition rate constant and nonradiative transition rate constant respectively. In consequence, the phosphor lifetime decreasing most likely derives from increase in radiation transition rate and nonradiative transition rate caused by Mn⁴⁺-Mg²⁺ pairs formation and substitutional defects respectively.

Fig. 5 Decay curves of CZMG:Mn⁴⁺(x = 0.00, 0.20, 0.40, 0.60, 0.80, 1.00) excited at 460 nm.

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3.5 Internal quantum efficiency and luminous efficiency of CZMG:Mn⁴⁺

The variation of internal quantum efficiency and luminous efficiency of the phosphor with different Mg²⁺ incorporation is depicted in Fig. 6(a). The internal quantum efficiency was calculated by following formula:

η Q E = \frac{\int L S}{\int E R - \int E S}

Where L_s is the emission spectrum of the sample, E_R and E_S are the excitation light with and without the sample in the integrating sphere. The quantum efficiency first increases from 61.7% to 64.3% and then decrease from 64.3% to 56.8% with the increasing concentration of Mg²⁺ from 0.00 to 1.00, which is positively correlated with the variation of emission intensity. To investigate effect of Mg²⁺ substitution on luminous efficiency, the red LED devices are fabricated by covering mixture on 460nm chip. The mixture is made up of 10% CZMG:Mn⁴⁺(x = 0.00, 0.20, 0.40, 0.60, 0.80, 1.00) and 90% glue respectively. The luminous efficiency in Fig. 6(a) from 14.67lm/w to 16.58lm/w corresponding the increase in Mg²⁺ concentration from 0.00 to 0.40, and then it decreases from maximum to 11.86lm/w as the concentration of Mg²⁺ continues to increase. The change in trends of internal quantum efficiency and luminous efficiency is synchronous on the whole. Figure 6(b) displays electroluminescence of device with 10% CZMG:Mn⁴⁺(x = 0.40), implying that the phosphor has potential application in plant lighting.

Fig. 6 (a). The internal quantum efficiency and luminous efficiency of CZMG:Mn⁴⁺ as a function of Mg²⁺ concentration under 460nm excitation. (b) The electroluminescence spectra of CZMG:Mn⁴⁺(x = 0.40) based on 460nm chip under 20 mA current excitation The inset shows the photo of the phosphor coated LED.

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4. Conclusion

A series of red-emitting CZMG:Mn⁴⁺ phosphors were synthesized by a conventional high-temperature solid-state method, and their properties were investigated in detail. As the degree of Mg²⁺ replacing Zn²⁺ increases, the luminescence of the phosphor first increases and then decreases. The reason for the enhancement is that the addition of Mg²⁺ ions can make the phosphor particles more uniform and form Mn⁴⁺-Mg²⁺ pairs, while the decrease is due to the formation and increase of impurities with the increase of Mg²⁺ concentration. The redshift of the emission peak is caused by the weakening of the crystal field. Besides, the phosphor has good thermal resistance and Mg²⁺ substitution results in poor thermal resistance of the phosphor due to defects formation. The decrease in decay time from 2.72ms to 2.25ms is attributed to the increase in radiation transition rate and nonradiative transition rate. The internal quantum efficiency and luminous efficiency reach the maximum at 64.3% and 16.58lm/w respectively when the doping concentration of Mg²⁺ is 0.40. These results indicate that the optimum doping concentration of Mg²⁺ is determined to be 0.40 and CZMG:Mn⁴⁺ (x = 0.40) is an applicable phosphor for plant lighting.

Funding

National Natural Science Foundation of China (Nos. 51372172 and 51672192).

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X	a(Å)	b(Å)	c(Å)	α(°)	β(°)	γ(°)	Volume(Å³)
0.00	15.0789	15.0789	15.0789	90	90	90	3428.56
0.20	15.0534	15.0534	15.0534	90	90	90	3411.15
0.40	15.0279	15.0279	15.0279	90	90	90	3393.85
0.60	15.0259	15.0259	15.0259	90	90	90	3392.49
0.80	15.0248	15.0248	15.0248	90	90	90	3388.85
1.00	15.0168	15.0168	15.0168	90	90	90	3386.33

Effect of the replacement of Zn²⁺ with Mg²⁺ in Ca₁₄Zn₆Ga₁₀O_35:Mn⁴⁺

Abstract

1. Introduction