Tunable blue-green-emitting Ca₃Si₂O₄N₂:Ce³⁺, Eu²⁺ phosphor with energy transfer for light-emitting diodes

Abstract

A series of Ce³⁺, Eu²⁺ codoped Ca₃Si₂O₄N₂ phosphors have been synthesized by traditional solid state reactions, and the crystal structures and luminescence properties were investigated in detail. Rietveld structure refinement indicates that Ca₃Si₂O₄N₂ crystallizes in a cubic unit cell with space group Pa3̄(205) and lattice constant a = 15.0712 Å. Tunable blue-green emitting Ca₃Si₂O₄N₂:Ce³⁺, Eu²⁺ phosphors have been obtained by codoping Ce³⁺ and Eu²⁺ into the host and varying their relative ratios. Compared with the Eu²⁺ ion singly doped phosphor, the codoped phosphors have wider absorption in the ultraviolet (UV) range and stronger emission of Eu²⁺, which are attributed to the effective energy transfer from Ce³⁺ to Eu²⁺. The energy transfer from Ce³⁺ to Eu²⁺ is demonstrated to be a dipole–dipole mechanism. The Ce³⁺, Eu²⁺ codoped phosphors might be candidates for blue-green components in UV white light-emitting diodes (WLEDs).

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1 Introduction

White light-emitting diodes (WLEDs), the so-called next-generation solid-state lighting, offer benefits in terms of reliability, energy-saving, maintenance and safety, and therefore are gaining much attention.¹ Hence, WLEDs are expected to be promising candidates to replace conventional incandescent and fluorescent lamps. Generally, commercial phosphor converted WLEDs (pc-WLEDs) are mainly fabricated through the combination of a 450–470 nm blue InGaN chip and a yellowish phosphor (typically YAG:Ce³⁺) coating. However, such methods suffer from a poor color-rendering index (CRI) and high correlated color temperature (CCT = 7756 K), owing to the absence of red emissions in the long wavelength region.^2,3 Accordingly, during the past few years, WLEDs fabricated with tricolor phosphors and ultraviolet (UV) LEDs are considered to be very promising for the realization of high-quality light sources because of their higher CRI and homogeneous color distribution in comparison to current commercial WLEDs.⁴ LED phosphors are generally required to have a high conversion efficiency, excellent chemical stability and high thermal quenching temperatures.⁵ In addition, they are also expected to have a tunable emission color,⁶ allowing WLED designers to have greater flexibility in choosing the emission wavelength and gain an improvement either in the CCT or CRI of WLEDs and for phosphor applications in UV LEDs. In certain Eu²⁺ doped matrices, the green to orange emission and UV to blue-light excitation are commonly observed through the strong crystal-field splitting and the lower energy of the centroid of the 5d level.⁷ Hence, Eu²⁺ can be sensitized by a well-known blue-emitting Ce³⁺ in such lattices because of the spectral overlap between the emission band of Ce³⁺ and the excitation band of Eu²⁺. Consequently, tunable emissions can be produced in this kind of Ce³⁺, Eu²⁺ codoped single composition host.^8–10

Nitride/oxynitride hosts are good candidates for host lattices for phosphors due to several advantages, such as high chemical stability and good thermal quenching ability, and exhibit intense luminescence for application in WLEDs when activated with Ce³⁺/Eu²⁺, such as in M₂Si₅N₈,^11,12 MSiN₂,¹³ Ca-α-SiAlON,¹⁴ CaAlSiN₃,^15,16 MSi₂O₂N₂ (M = Ca, Sr, Ba),¹⁷ MSiAl₂O₃N₂ (M = Sr, Ba)¹⁸ and so on. Some of them have already been put to practical use.

Eu²⁺ activated Ca₃Si₂O₄N₂ was first reported by Liu's group.¹⁹ Although the excitation spectrum of Eu²⁺ doped Ca₃Si₂O₄N₂ covers the region from about 290 to 410 nm, the optimal excitation wavelength is about 328 nm, which doesn't match quite well with UV LED chips. Wang et al.²⁰ studied the O/N ordering in the structure of Ca₃Si₂O₄N₂ and simply showed the excitation and emission spectra of Ce³⁺ doped Ca₃Si₂O₄N₂ phosphors, which can be excited at about 350 nm and give a blue emission. Subsequently, C. H. Huang²¹ studied the photoluminescence properties of Ce³⁺ doped Ca₃Si₂O₄N₂ phosphors in detail. A systematic study on the energy transfer between Eu²⁺ and Ce³⁺ in Ca₃Si₂O₄N₂ host, however, was not reported, to the best of our knowledge. In this paper, we report the luminescence properties of Ce³⁺, Eu²⁺ codoped Ca₃Si₂O₄N₂ phosphors. After codoping with Ce³⁺, the emission intensity of Eu²⁺ was enhanced and a series of tunable blue-green colors can be obtained by varying the relative ratio of Ce³⁺/Eu²⁺ under the irradiation of 350 nm. Moreover, the energy transfer mechanism between Ce³⁺ and Eu²⁺ has been investigated systematically.

2 Experimental section

Samples of Ca_3−xSi₂O₄N₂:xCe³⁺ (0.005 ≤ x ≤ 0.03), Ca_2.995Si₂O₄N₂:0.005Eu²⁺ and Ca_2.98−ySi₂O₄N₂:0.02Ce³⁺, yEu²⁺ (0.0025 ≤ y ≤ 0.05) investigated in this work were synthesized through solid state reactions. CaCO₃ (A. R.), Si₃N₄ (Sigma-Aldrich 99.9%), CeO₂ (99.99%) and Eu₂O₃ (99.999%) were employed as raw materials. Briefly, the raw materials were weighed in appropriate proportions and finely ground, and then filled into boron nitride (BN) crucibles. Subsequently, the powder mixtures were preheated at 900 °C (the temperature was increased at a rate of 10 °C min⁻¹) for 2 h to ensure the decomposition of CaCO₃, and then the temperature was increased to 1450 °C (at a rate of 5 °C min⁻¹) and maintained for 8 h. After firing, the samples were cooled to 500 °C (at a rate of 5 °C min⁻¹) and finally quenched to room temperature. In order to control the oxygen content in the reaction, the whole heating processes for both Ce³⁺ doped and Ce³⁺, Eu²⁺ co-doped samples are conducted in a reductive atmosphere of NH₃/N₂.

All measurements were made on finely ground powder. The phase purity of samples was analyzed by X-ray diffraction (XRD) using a D2 PHASER X-ray Diffractometer with Ni-filtered Cu Kα radiation. Structure refinement was performed using the Rietveld method,²² using the Materials Studio (MS) program.²³ Photoluminescence (PL) and PL excitation (PLE) spectra were measured at room temperature using a FLS-920T fluorescence spectrophotometer equipped with a 450 W Xe light source and double excitation monochromators. The PL decay curves were measured by a FLS-920T fluorescence spectrophotometer with a nF900 nanosecond flashlamp as the light source. High temperature luminescence intensity measurements were carried out by using an aluminum plaque with cartridge heaters; the temperature was measured by thermocouples inside the plaque and controlled by using a standard TAP-02 high temperature fluorescence controller. The powder morphology was investigated by using a scanning electron microscope (SEM; S-3400, Hitachi, Japan). The elemental compositions were determined using an energy-dispersive X-ray spectroscope (EDX), which was attached to the transmission electron microscope (TEM, FEI Tecnai F30, operated at 300 kV).

3 Results and discussion

Phase identification and crystal structure

Fig. 1(a) plots the experimental, calculated, background and difference results of the XRD profiles for the Rietveld refinement of Ca₃Si₂O₄N₂ host. The resulting crystallographic data are summarized in Table 1, which indicate that the powder sample crystallizes in a cubic unit cell with space group Pa3̄(205) and lattice constant a = 15.0712 Å and Z = 24. The refinement finally converged to R_wp = 13.08%, R_p = 9.03%. Selected XRD patterns of Ce³⁺ doped and Ce³⁺, Eu²⁺ co-doped phosphors with different doping contents are illustrated in Fig. 1(b), which match well the calculated XRD patterns, indicating that the obtained samples are all of single phase.

Fig. 1 (a) Experimental (crosses) and calculated (red solid line) XRD patterns of the Ca₃Si₂O₄N₂ host. The blue solid lines represent the difference between experimental and calculated data and the pink sticks mark the Bragg reflection positions. The inset shows the corrugated 12-membered ring structure viewed from [111]. (b) XRD patterns of Ca_2.995Si₂O₄N₂:0.005Eu²⁺, Ca₃Si₂O₄N₂:0.02Ce³⁺, and Ca_2.98−ySi₂O₄N₂:0.02Ce³⁺, yEu²⁺ (0.0025 ≤ y ≤ 0.05) samples. (c) Unit cell structure and (d) EDX spectrum of Ca₃Si₂O₄N₂ host. (e) and (f) SEM images of Ca₃Si₂O₄N₂:0.02Ce³⁺, 0.005Eu²⁺.

Table 1 Rietveld refinement and crystal data of Ca₃Si₂O₄N₂

Crystallographic data
Formula	Ca3Si2O4N2
Crystal system	Cubic
Space group	Pa3̄(205)
a (Å)	15.0712
Cell volume (Å^3)	3423.288
Z	24
Rietveld data
Program	Materials studio
Range	5°–110°
Step	0.02
Rwp	13.08%
Rp	9.03%

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Fig. 1(c) shows the refined crystal structure of Ca₃Si₂O₄N₂. In this structure, [SiO₂N₂] tetrahedral construct highly corrugated 12-membered rings rather than the [Si(O,N)_4−m]_n layers. The tetrahedral connect each other by sharing N atoms on bridging positions and the O atoms are located at terminal positions, which can be seen from the inset of Fig. 1(a). In the structure of Ca₃Si₂O₄N₂, the Ca²⁺ ions have seven different coordination environments, and only the Ca(6) ion is eight-coordinated, while the other Ca²⁺ ions are six-coordinated. The ionic radii of the six- and eight-coordinated Ca²⁺ are 1.00 and 1.12 Å, respectively. Similarly, the ionic radii of the six- and eight-coordinated Ce³⁺ are 1.01 and 1.14 Å, while 1.17 and 1.25 for six- and eight-coordinated Eu²⁺, respectively.²⁴ Because of the similarities of their ionic radii, the Ce³⁺ and Eu²⁺ are expected to randomly occupy Ca²⁺ sites in the Ca₃Si₂O₄N₂ crystal structure. The corresponding EDX spectrum analysis which is shown in Fig. 1(d) indicates that the sample has a chemical composition of Ca, Si, N and O and the Cu peak is ascribed to the copper grid supporting the TEM sample.

The SEM images of Ca₃Si₂O₄N₂:0.005Eu²⁺, 0.02Ce³⁺ are shown in Fig. 1(e) and (f). It can be observed that the grains exhibit ellipsoid shapes and they have a particle size ranging from about 2–7 μm.

Photoluminescence properties and energy transfer

The PL and PLE spectra of Ca₃Si₂O₄N₂:0.02Ce³⁺ are shown in Fig. 2. The excitation spectrum consists of three absorption bands that peaking at about 290, 320 and 350 nm, which correspond to the transitions from the ground state of the Ce³⁺ ions to the field splitting levels of the 5d state.²¹ Upon excitation at 350 nm, the emission spectrum shows an asymmetric broad band peaking at 436 nm. This asymmetric emission was attributed to the spin–orbit coupling of the Ce^{3+ 4}f₁ ground state, which splits into two levels (²F_5/2 and ²F_7/2),²⁵ and the occupation of Ce³⁺ ions in independent six- and eight-coordinate Ca²⁺ ion sites.

Fig. 2 PL and PLE spectra of Ca₃Si₂O₄N₂:0.02Ce³⁺.

The dependence of the peak position and emission intensity of Ca₃Si₂O₄N₂:Ce³⁺ on the Ce³⁺ content is shown in Fig. 3. With increasing the content of Ce³⁺, the emission intensity is maximized at x = 0.02, and then decreases due to the concentration quenching effect. In addition, the peak positions of the emission bands shift toward a longer wavelength for about 10 nm as the content of Ce³⁺ dopant increases. This phenomenon can be explained in terms of energy transfer from the Ce³⁺ ions at higher 5d level to those at the lower levels, which decreases the emission energy from the 5d excited state to the 4f ground state and results in a redshift of the emission.²¹

Fig. 3 The dependence of the peak position and the emission intensity on the Ce³⁺ content.

As depicted in Fig. 4(b), the PL spectrum of the Eu²⁺ singly doped phosphor shows a broad green emission band centering at 510 nm attributed to the typical ⁴f₆⁵d₁–⁴f₇ (⁸S_7/2) transition of Eu²⁺, and the PLE spectrum shows a broad absorption from 280 to 420 nm with a maximum at 337 nm. The PL and PLE spectra of Ca₃Si₂O₄N₂:0.02Ce³⁺ are also shown in Fig. 4(a) for comparison. A significant spectral overlap between the PL spectrum of Ca₃Si₂O₄N₂:0.02Ce³⁺ and the PLE spectrum of Ca₃Si₂O₄N₂:0.005Eu²⁺ is observed, which indicates that the energy transfer from the Ce³⁺ to Eu²⁺ ions can be expected in Ce³⁺ to Eu²⁺ codoped host. Fig. 4(c) depicts the PL and PLE spectra of the Ca₃Si₂O₄N₂:0.02Ce³⁺, 0.005Eu²⁺ phosphor. The codoped phosphor shows a blue emission band of Ce³⁺ ions and a green emission band of Eu²⁺ ions at the irradiation of 350 nm. When monitoring at 500 nm and 436 nm, the peak positions of the PLE spectra are all located at about 350 nm, which is more suitable for the UV chip compared to the Eu²⁺ singly doped phosphor.

Fig. 4 PL and PLE spectra of (a) Ca₃Si₂O₄N₂:0.02Ce³⁺, (b) Ca₃Si₂O₄N₂:0.005Eu²⁺, and (c) Ca₃Si₂O₄N₂:0.02Ce³⁺, 0.005Eu²⁺ phosphors.

Fig. 5 displays the PL spectra of Ca₃Si₂O₄N₂:0.02Ce³⁺, yEu²⁺ samples with a fixed Ce³⁺ content of 0.02 and a varying Eu²⁺ content y in the range of 0.0025–0.05, as well as the PL spectrum of Ca₃Si₂O₄N₂:0.005Eu²⁺. One can see that the PL intensity for Ce³⁺ decreases monotonically with an increase in doping Eu²⁺ content. Meanwhile, the emission of Eu²⁺ increases gradually until the Eu²⁺ content is above 0.005 and concentration quenching occurs. Subsequently, the emission of the Eu²⁺ falls with a further increase in y. The observed variations in the emission of the Ce³⁺ and Eu²⁺ ions further support the occurrence of the effective energy transfer from the Ce³⁺ to Eu²⁺ ions. Comparing the PL spectrum of Ca₃Si₂O₄N₂:0.005Eu²⁺ with that of Ca₃Si₂O₄N₂:0.02Ce³⁺, 0.005Eu²⁺, we can find that after codoping with Ce³⁺, the emission of Eu²⁺ in Ca₃Si₂O₄N₂ is obviously enhanced.

Fig. 5 PL spectra of Ca₃Si₂O₄N₂:0.02Ce³⁺, yEu²⁺ (0.0025 ≤ y ≤ 0.05) and Ca₃Si₂O₄N₂:0.005Eu²⁺.

Additionally, it is noticeable that the emission peaks of the Eu²⁺ ions gradually shift to longer wavelength for about 10 nm with an increase of the content of Eu²⁺ from 0.0025 to 0.05. This red-shift behavior is consistent with the phenomenon that reported by Liu,¹⁹ which is ascribed to the change in the crystal-field splitting of Eu²⁺. Consequently, the phenomenon can be explained in terms of energy transfer from Eu²⁺ ions at the higher 5d levels to those at the lower levels and this causes the emission energy from the 5d excited state to the 4f ground state to become lower, and therefore, the emission shifts to longer wavelength.

To further study the energy transfer process, the PL decay curves of the Ce³⁺ ions in Ca₃Si₂O₄N₂:0.02Ce³⁺, yEu²⁺ (0 ≤ y ≤ 0.03) phosphors were measured by monitoring at 436 nm with an irradiation of 350 nm, and depicted in a logarithmic intensity in Fig. 6. The decay curves for Ce³⁺ emission in Ca₃Si₂O₄N₂:0.02Ce³⁺, yEu²⁺ (0 ≤ y ≤ 0.03) phosphors deviate slightly from the single exponential function, which is also consistent with the structure analysis that more than one Ce³⁺ emitting center exists in the phosphor. An increase in Eu²⁺ content will enhance this deviation, which can further verify the energy transfer between Ce³⁺ and Eu²⁺. The decay process of these samples are characterized by an average lifetime, τ, which can be calculated using eqn (1) as follows,^26–28

where I(t) is the luminous intensity at time t. Based on eqn (1), the luminescence lifetimes of Ce³⁺ are determined to be 33.51, 28.79, 27.82, 22.80 and 11.64 ns for Eu²⁺ concentrations of 0, 0.0025, 0.005, 0.01 and 0.03, respectively. The decrease in the lifetimes of Ce³⁺ with increasing Eu²⁺ concentration, strongly demonstrates an energy transfer from Ce³⁺ to Eu²⁺. In addition, the energy transfer efficiency (η) from Ce³⁺ to Eu²⁺ is calculated by the following expression:^27,28

η = 1 − τ/τ₀ (2)

where τ₀ and τ represent the lifetime of Ce³⁺ in the absence and the presence of Eu²⁺, respectively. As shown in Fig. 7 the energy transfer efficiency increases gradually with increasing Eu²⁺ concentration. The energy transfer efficiencies (η) are calculated to be 0%, 14.08%, 16.98%, 31.96% and 65.26% for the Ca₃Si₂O₄N₂:0.02Ce³⁺, yEu²⁺ phosphors with y = 0, 0.0025, 0.005, 0.01 and 0.03, respectively.

Fig. 6 The decay curves of the Ce³⁺ ion in Ca₃Si₂O₄N₂:0.02Ce³⁺, yEu²⁺ (0 ≤ y ≤ 0.03) phosphors demonstrated on a logarithmic intensity scale.

Fig. 7 Dependence of the decay time of Ce³⁺ ions and energy transfer efficiency on the Eu²⁺ concentration in Ca₃Si₂O₄N₂:0.02Ce³⁺, yEu²⁺ (0 ≤ y ≤ 0.05) phosphors.

In general, energy transfer from the sensitizer to activator in a phosphor may take place via a multipolar interaction²⁹ or an exchange interaction at higher concentration. Based on Dexter's energy transfer formula of multipolar interaction and Reisfeld's approximation, the following relation can be obtained:^30,31

where C is the total concentration of Ce³⁺ and Eu²⁺, and n equals 6, 8 or 10 corresponding to dipole–dipole, dipole–quadrupole and quadrupole–quadrupole interactions, respectively. The τ₀/τ ∝C^n/3 plots are further illustrated in Fig. 8. Linear behavior was observed only when n is equal to 6, indicating that energy transfer from Ce³⁺ to Eu²⁺ took place via the dipole–dipole mechanism.

Fig. 8 Dependence of τ₀/τ of Ce³⁺ on (a) C^6/3, (b) C^8/3, and (c) C^10/3.

For electric dipole–dipole interaction, the critical distance R_c between Ce³⁺ and Eu²⁺ can be expressed by³²

where f_osc is the oscillator strength of the donor optical absorption transition, often estimated as 0.01 for Ce³⁺; SO is the spectral overlap of the normalized spectral shapes of the Ce³⁺ emission and the Eu²⁺ excitation, and it is estimated to be about 0.4524 eV⁻¹; E is the energy of maximum energy of spectral overlap. Therefore, R_c is calculated to be 13.5 Å. When Ce³⁺–Eu²⁺ was supposed to form a close pair at a distance of 1.84 Å, which is the nearest distance between Ca and Ca sites, and, hence, R_c was approximated as 7.3 times of lattice dimension in length.

The x and y values of the CIE chromaticity coordinates for the Ca₃Si₂O₄N₂:0.02Ce³⁺, yEu²⁺ (0.0025 ≤ y ≤ 0.05) phosphors are calculated and presented in Fig. 9. It can be seen that the emitting color of the phosphors can be easily modulated from blue (0.245, 0.285) to green (0.283, 0.515) by simply varying the value of y from 0.0025 to 0.05 due to the different emission composition of the Ce³⁺ and Eu²⁺ ions. Thus, blue-green emitting phosphors which can be efficiently excited by the UV chips are obtained via the energy transfer from the Ce³⁺ to Eu²⁺ ions and the Ca₃Si₂O₄N₂:Ce³⁺, Eu²⁺ phosphors could have potential value as blue-green phosphors used for UV WLEDs.

Fig. 9 CIE chromaticity coordinates for the Ca₃Si₂O₄N₂:0.02Ce³⁺, yEu²⁺ (0.0025 ≤ y ≤ 0.05) phosphors.

Thermal quenching properties

For application in high power LEDs, the thermal stability of phosphors is one of the important issues to be considered. The temperature dependence of the emission spectra of Ca₃Si₂O₄N₂:0.02Ce³⁺, 0.005Eu²⁺ and Ca₃Si₂O₄N₂:0.005Eu²⁺ are shown in Fig. 10(a) and (b), respectively, which show a relatively poor thermal stability. At 150 °C, the emission intensity of the Ca₃Si₂O₄N₂:0.02Ce³⁺, 0.005Eu²⁺ phosphor is about 27% of that measured at room temperature while 47% for Ca₃Si₂O₄N₂:0.005Eu²⁺, which can be seen from Fig. 10(c). Compared with Eu²⁺ singly doped Ca₃Si₂O₄N₂, more rare earths ions were doped into Ca₃Si₂O₄N₂:0.02Ce³⁺, 0.005Eu²⁺ and then more deficiencies formed due to the radii mismatch of rare earth ions and Ca²⁺ ions. As a result, the probability of non-radiative process increased and thus leading to the worse thermal stability. The temperature quenching mechanism could be explained by the configuration coordinate diagram as shown in Fig. 10(d). With the increase of the temperature, the phonon vibration is strengthened and more electrons located at the lowest excited state C can absorb the phonon energy and are excited to state E. Electrons at state E can go back to the ground state A by relaxation and give no emission, making the number of electrons which can go back to the ground state through radiation decrease. Thus, the emission intensity of the phosphor will decrease with the increment of temperature.

Fig. 10 The temperature dependence of the emission spectra of (a) Ca₃Si₂O₄N₂:0.02Ce³⁺, 0.005Eu²⁺ and (b) Ca₃Si₂O₄N₂:0.005Eu²⁺, and (c) the dependence of their normalized PL intensity on working temperature. (d) The schematic configuration coordinate diagram for the explanation of temperature quenching.

4 Conclusions

In summary, a series of Ce³⁺, Eu²⁺ codoped Ca₃Si₂O₄N₂ phosphors have been synthesized and the photoluminescence properties have been investigated in detail. Rietveld structure refinement indicates that Ca₃Si₂O₄N₂ crystallizes in a cubic unit cell with space group Pa3̄(205) and lattice constant of a = 15.0712 Å and Z = 24. When Ce³⁺ and Eu²⁺ were codoped in Ca₃Si₂O₄N₂, the photoluminescence spectra displayed tunable blue-green emission by varying their relative ratios. Besides, the emission intensity of Eu²⁺ was enhanced via codoping with Ce³⁺ and the codoped phosphors have wider absorption in the UV range due to the effective energy transfer from the Ce³⁺ to Eu²⁺. The dipole–dipole interaction mechanism should be mainly responsible for the energy transfer from Ce³⁺ to Eu²⁺ in Ca₃Si₂O₄N₂. The experimental results indicate that the Ca₃Si₂O₄N₂:Ce³⁺, Eu²⁺ phosphor might have promising applications in UV WLEDs.

Acknowledgements

This work was supported by the National Natural Science Funds of China (Grant no. 51372105) and Specialized Research Fund for the Doctoral Program of Higher Education (no. 20120211130003).

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