Potential single-phased white-emitting phosphor (Ca_0.33Sr_0.67)₇(SiO₃)₆Cl₂: Ce³⁺, Eu²⁺ for ultraviolet light-emitting diode

Abstract

A series of white-emitting and color temperature-tunable phosphors (Ca_0.33Sr_0.67)₇(SiO₃)₆Cl₂: Ce³⁺, Eu²⁺ have been synthesized by high temperature solid-state reaction. The photoluminescence emission and excitation spectra, and the lifetime are investigated in detail. The samples could display varied color emission from violet (0.1696, 0.0693) towards standard white (0.3314, 0.334) and ultimately to warm white (0.3773, 0.396) under excitation by 340 nm light by adjusting the relative proportion of Ce³⁺/Eu²⁺. The energy transfer mechanism from Ce³⁺ to Eu²⁺ in the (Ca_0.33Sr_0.67)₇(SiO₃)₆Cl₂ phosphors is dominated by the electric dipole–dipole interaction. These results indicate that the (Ca_0.33Sr_0.67)₇(SiO₃)₆Cl₂: Ce³⁺, Eu²⁺ phosphor might be promising as a single-phased white-light-emitting phosphor for UV LEDs.

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1. Introduction

The white light emitting diode (LED) has recently attracted more and more attention in solid-state lighting devices because of its excellent characteristics, such as its high brightness, long lifetime, environmental friendliness, small size and low power consumption.^1,2 The major white LEDs presently in the market are phosphor-converted LEDs (pc-LEDs) made of a blue InCaN chip and yellow phosphor, YAG: Ce³⁺. However, this type of LED has several disadvantages, including the halo effect of blue/yellow color separation and poor color-rendering properties due to the lack of a red component. Another method to get white light is to combine RGB (red, green and blue) phosphors with ultraviolet (UV) LED chips. And yet, for this method, the re-absorption of the blue light by the red and green phosphors lead to low luminescent efficiency and the high cost also make it still has challenge ahead.^3,4 Then scholars found a new way to get white light by the single-phased white-emitting phosphors, which can compensate the above drawback. As an activated ion, Eu²⁺ ion has the 5d electron unshielded from the crystal field by the 5s and 5p electrons, and its spectral properties are strongly affected by the surrounding environment such as asymmetry, covalence, coordination, bond length, site size and crystal-field strength,⁵ so the emission spectra of Eu²⁺ usually show broad bands from long-wavelength ultraviolet to red. It is possible to find a Eu²⁺-doped matrix with yellow-orange emission and UV to blue light excitation trough the strong crystal-field splitting and the lower central energy of the 5d level. Eu²⁺ can be sensitized by a blue-emitting Ce³⁺ in some matrix because of the spectra overlap between the Ce³⁺ emission band and Eu²⁺ excitation band.^6,7 Therefore, white light with blue and yellow-orange emission can be produced in this kind of Ce³⁺/Eu²⁺-codoped single-composition matrix.^8,9

The crystal structure and luminescence of (Ca_1−x−y,Sr_x,Eu_y)₇(SiO₃)₆Cl₂ were first reported by Hisayoshi Daicho et al., (Ca_1−x−y,Sr_x,Eu_y)₇(SiO₃)₆Cl₂ has three cationic sites referred to as M(1), M(2), and M(3), and in these there sites, M(2) and M(3) could accommodate for rare earth ions.¹⁰ The M(2) site, occupied by Sr²⁺ only, is in the metal chloride layer and surround by 9 oxygen atoms, the M(3) site, occupied by a Ca²⁺ and Sr²⁺, is at the boundary between the metal silicate layer and metal chloride layer and surround by 7 oxygen atoms. The luminescence spectrum of (Ca_0.37Sr_0.53Eu_0.10)₇(SiO₃)₆Cl₂ obtained with a 400 nm excitation at room temperature exhibits a single band at 580 nm, with a full width at half-maximum of 3.6 × 10³ cm⁻¹. In the present study, we report a single-phased white-emitting phosphor, Ce³⁺, Eu²⁺ co-doped (Ca_0.33Sr_0.67)₇(SiO₃)₆Cl₂ (abbreviated CSSC), which shows intense emission. The energy transfer mechanism between Ce³⁺ and Eu²⁺ ions in the host lattice is investigated in detail.

2. Experimental

All of the samples were synthesized by conventional solid state reaction under the reductive atmosphere. The starting materials CaCO₃, H₂SiO₃, SrCl₂ and Eu₂O₃ with a purity of 99.99% were taken in stoichiometric molar ratio, after grinding fully, the reagents were put into alumina crucibles and sintered in a tube furnace at 900 °C for 6 hours under reductive atmosphere, respectively. Finally, the samples were obtained by grinding fully in an agate mortar after cooling to room temperature naturally.

The crystal structure of samples was identified by a Rigaku D/Max-2400 X-ray diffractometer (XRD) with Cu Kα radiation (λ = 1.54178 Å) operating at 40 kV and 60 mA. The 2θ ranges from 10° to 80° with the step size of 0.02°. The photoluminescence excitation, emission spectra, quantum efficiencies (QEs) and decay curves were measured by using an FLS-920T fluorescence spectrophotometer equipped with a 450 W Xe light source and nF900 nanosecond flashlamp, respectively. All the measurements were performed at room temperature.

3. Results and discussion

3.1 Phase identification

Fig. 1 presents the XRD patterns of CSSC: mCe³⁺, mLi⁺ (m = 0.005, 0.01 and 0.015) and CSSC: nEu²⁺ (n = 0.01, 0.02 and 0.03) samples. All of the diffraction peaks of the samples can be basically indexed to the standard data of (Ca_0.46Sr_0.57)₇(SiO₃)₆Cl₂.⁶ No other phase is detected, indicating that the obtained samples are single phases and the Ce³⁺ and Eu²⁺ ions have been successfully incorporated in the (Ca_0.33Sr_0.67)₇(SiO₃)₆Cl₂ host lattices by replacing Ca²⁺ due to their similar ionic radii and charge.¹¹

Fig. 1 XRD patterns of samples as compared to the standard fate for (Ca_0.33Sr_0.67)₇(SiO₃)₆Cl₂.

3.2 Photoluminescence of (Ca_0.33Sr_0.67)₇(SiO₃)₆Cl₂

Fig. 2 shows the excitation (PLE) and emission (PL) spectra of Ce³⁺ (a) and Eu²⁺ (b) singly doped in CSSC phosphors. Fig. 2(a) exhibits a strong broad emission band between 350–450 nm under the excitation of 340 nm, and the peak locates at 380 nm due to the 4f⁰5d¹ → 4f¹ transition of Ce³⁺ ion.^12,13 Under the monitoring of 390 nm, the excitation spectrum shows a broad band from 250 to 350 nm, and the peaks locate at 278 and 338 nm, respectively, corresponding to the 4f¹ → 4f⁰5d¹ transition of Ce³⁺ ion.¹⁴ Eu²⁺ doped in CSSC phosphor presents a broad emission band from 450 to 650 nm excited at 350 nm shown in Fig. 2(b), and the peaks locate at 465 and 580 nm, which is attributed to the 4f⁶5d¹ → 4f⁷ transition of Eu²⁺ ion located at two different sites.¹⁵ The excitation spectra show two broad band between 250–450 nm, and the peaks locate at 320 and 373 nm, under the monitoring of 465 nm and 580 nm, respectively, assigning to the 4f⁷ → 4f⁶5d¹ transition of Eu²⁺ ion.¹⁶ Besides, a significant spectral overlap is observed (shown in Fig. 2(c)), According to Dexter's theory,^17–19 it is expected that the effective resonant energy transfer from a sensitizer Ce³⁺ to an activator Eu²⁺ can occur in Ce³⁺/Eu²⁺ co-doped (Ca_0.33Sr_0.67)₇(SiO₃)₆Cl₂ phosphors. The inset of (a) shows the variations characteristic of CSSC: xCe³⁺ emission intensities excited at 340 nm with the increasing of Ce³⁺ doping concentration from 0.0025 to 0.02. The emission intensities increase and achieve a maximum at x = 0.01, then decrease with the further increasing of Ce³⁺ content due to the concentration quenching effect among Ce³⁺ ions.²⁰ And the inset of (b) presents the variations characteristic of CSSC: xEu²⁺ emission intensities excited at 350 nm with the increasing of Eu²⁺ doping concentration from 0.01 to 0.05. The emission intensities increase and achieve a maximum at x = 0.03, the decrease with the further increasing of Eu²⁺ content as well due to the concentration quenching effect among Eu²⁺ ions.²¹

Fig. 2 PL and PLE spectra for (a) CSSC: 0.01Ce³⁺ and (b) CSSC: 0.03Eu²⁺. (c) Spectra overlap between the emission bands of CSSC: Ce³⁺ and the excitation bands of CSSC: Eu²⁺. The inset of (a) the emission intensity of CSSC: xCe³⁺ as function of the Ce³⁺ doping content and the inset of (b) the emission intensity of CSSC: xEu²⁺ as function of the Eu²⁺ doping content.

Fig. 3(a) exhibits the emission spectra of (Ca_0.33Sr_0.67)₇(SiO₃)₆Cl₂: 001Ce³⁺, mEu²⁺ phosphors with different Eu²⁺ concentration. Under the excitation of 340 nm (the optimum excitation wavelength of Ce³⁺), the emission spectra of CSSC: 0.01Ce³⁺, mEu²⁺ (m = 0–0.05) have three broad peaks, and the violet emission peak (locating at 380 nm) is ascribed to the 4f⁰5d¹ → 4f¹ transition of Ce³⁺ ion, the remaining two emission peaks (locating at 465 and 580 nm) are attributed to the 4f⁶5d¹ → 4f⁷ transition of Eu²⁺ ion. In addition, under the condition of unchanging Ce³⁺ concentration, the emission intensity of Ce³⁺ decreases with increasing Eu²⁺ concentration, whereas the 580 nm emission intensity of Eu²⁺ increases with increasing Eu²⁺ concentration, and achieves maximum at m = 0.04, and then decreases which can be ascribe to the energy transfer between Ce³⁺ and Eu²⁺. In addition, the energy transfer efficiencies (η_T) can be expressed as follows.²²

η_T = I − (I_S − I_S0) (1)

where I_S0 and I_S are luminescent intensities of sensitizer Ce³⁺ in the absence and presence of activator Eu²⁺. The η_T from Ce³⁺ to Eu²⁺ of CSSC: 0.01Ce³⁺, mEu²⁺ is calculated and shown in the inset of Fig. 3(a), with the increase of Eu²⁺ concentration, the energy transfer efficiency gradually increases and reaches response (about 91%). The above results mean that the energy transfer from Ce³⁺ to Eu²⁺ exists in CSSC: 0.01Ce³⁺, mEu²⁺ phosphors.

Fig. 3 (a) PL spectra of CSSC: 0.01Ce³⁺, mEu²⁺ (m = 0–0.05) excited at 340 nm, the inset: energy transfer efficiencies (η_T) calculated by luminescent intensities. (b) Gaussian fitting of the emission band of CSSC: 0.01Ce³⁺, 0.04Eu²⁺ under 340 nm excitation. Four Gaussian components are peaked at 373 (M1), 405 (M2), 456 (M3) and 575 (M4) nm, respectively.

As described above, there are one Ca²⁺ site (M(3)) and two Sr²⁺ sites (M(2) and M(3)) in CSSC. Because the similar ionic radius between Ce³⁺ (1.07 Å, CN = 7; 1.196 Å, CN = 9) and Ca²⁺ (1.06 Å, CN = 7; 1.18 Å, CN = 9), Eu²⁺ (1.2 Å, CN = 7; 1.3 Å, CN = 9) and Sr²⁺ (1.21 Å, CN = 7; 1.31 Å, CN = 9), respectively, Ce³⁺ tends to occupy the Ca²⁺ site and Eu²⁺ is tending to occupy the Sr²⁺ site. As illustrated in Fig. 3(b), the emission of CSSC: 0.01Ce³⁺, 0.04Eu²⁺ excited at 340 nm can be decomposed into four Gaussian components with the maxima at 373 (M1), 405 (M2), 456 (M3) and 575 (M4) nm. Wherein, M1 and M2 come from one Ce³⁺ sites while M3 and M4 belong to the two different Eu²⁺ sites. Generally, the characteristic emission of Ce³⁺ includes two components of 5d–²F_5/2 and 5d–²F_7/2 with a separation of around 2000 cm⁻¹ in energy.²³ In our case, the energy separation of Ce^{3+ 2}F_J (J = 7/2, 5/2) is estimated to be 2118 cm⁻¹ for the Ce³⁺ site. Meanwhile, according to Uitert,²⁴ the emission position of Eu²⁺ depends strongly on its local environment. Eu²⁺ with different local environments always generates versatile emissions. It suggests an empirical equation to describe the dependence of Eu²⁺ emission energy on the local circumstance in various compounds:

where, E is the emission position of Eu²⁺, Q* is the energy of the lowest d-band edge for the free Eu²⁺ (Q* = 34 000 cm⁻¹), V is the valence state of Eu²⁺ (V = 2), n is the number of anions in the immediate shell of the Eu²⁺, E_a is the anion's electron affinity (eV), and r is the radius (Å) of the cation replaced by the Eu²⁺. Because the E_a is a fixed value in same host, so E gets larger while r and n get larger. Therefore, the band peaked at 456 nm (M3) is ascribed to the Eu²⁺ occupying the Sr²⁺ site with nine-coordination (M(2)), while the longer wavelength band peaked at 575 nm (M4) can be attributed to Eu²⁺ occupying Sr²⁺ site with seven-coordination (M(3)).

In order to investigate the luminescence dynamics, the decay curves of CSSC: 0.01Ce³⁺, mEu²⁺ samples are measured under the excitation of 340 nm and monitoring of 390 nm and well fitted with second-order exponential decay curves (shown in Fig. 4, the black lines represent test data and the colorized lines represent fitting curves). The second-order exponential function is expressed by equation.

I = A₁ exp(−τ/τ₁) + A₂ exp(−τ/τ₂) (3)

where I is luminescence intensity, A₁ and A₂ are constants, τ is average lifetime, and τ₁ and τ₂ are lifetime for rapid and slow decays, respectively. Average lifetimes can be calculated by formula as follow:²⁵

τ = (A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₂) (4)

Fig. 4 Decay curves of CSSC: 0.01Ce³⁺, mEu²⁺ excited at 340 nm and monitored at 390 nm, the inset: dependence of decay time on Eu²⁺ content.

The constants A₁, A₂ and lifetimes τ₁, τ₂ and τ are calculated and presented in Table 1. The average lifetimes of Ce³⁺ ions are calculated to be 35.2173, 31.7578, 28.4294, 27.9756, 25.3708, and 24.9408 ns for CSSC: 0.01Ce³⁺, mEu²⁺ samples with m = 0, 0.01, 0.02, 0.03, 0.04 and 0.05, respectively. The average lifetime decreases monotonically with the increasing of Eu²⁺ concentration, which corresponds to the previous analysis and strongly demonstrates the energy transfer from Ce³⁺ to Eu²⁺.

Table 1 Fitting parameters of different decay curves for different samples and the CIE (x, y) and CCT for CSSC: 0.01Ce³⁺, mEu²⁺ phosphors excited at 340 nm

Sample	x	y	CCT (K)	τ1 (ns)	A1	τ2 (ns)	A2	τ1 (ns)
m = 0	0.1696	0.0693	—	38.7976	2294.92	15.0148	1050.619	35.2173
m = 0.01	0.2583	0.213	—	36.3309	2595.498	12.3013	1801.745	31.7578
m = 0.02	0.2913	0.2705	9618	31.5835	1093.383	8.9313	625.471	28.4294
m = 0.03	0.3314	0.334	5538	30.7232	1100.864	8.6390	556.301	27.9756
m = 0.04	0.363	0.3759	4489	29.5694	1023.592	7.7896	927.61	25.3708
m = 0.05	0.3773	0.396	4195	32.1275	656.794	9.7897	1022.396	24.9408

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In general, the energy transfer from a sensitizer to an activator in a phosphor may take place through exchange interaction and electric multipolar interaction.^26,27 On the basic of Dexter's energy transfer expressions of multipolar interaction and Reisfeld's approximation, the following relation can be given as:^28,29

where η₀ and η_s are the luminescence quantum efficiencies of Ce³⁺ in the absence and presence of Eu²⁺, respectively; the values of η₀/η_s can be estimated approximately by the ratio of relative luminescence intensity ratio (I_S0/I_S); C is the concentration of the sum of Ce³⁺and Eu²⁺. Eqn (4) corresponds to the exchange interaction, and eqn (5) with n = 6, 8, and 10 is ascribed to dipole–dipole (d–d), dipole–quadrupole (d–q), and quadrupole–quadrupole (q–q) interactions, respectively.

The relationship of ln(I_S0/I_S) ∝ C and I_S0/I_S ∝ C^n/3 are illustrated in Fig. 5. The liner relationship reaches the optimal one by comparing the fitting factor of R value. The relationship I_S0/I_S ∝ C^n/3 has the best fitting (Fig. 5(b)), which means that the energy transfer from Ce³⁺ to Eu²⁺ occurs via the dipole–dipole interaction. For an evaluation of the luminescence efficiency, the QEs of our phosphors are estimated. The QE of CSSC: 0.01Ce³⁺, 0.05Eu²⁺ under 340 nm excitation is measured to be 41.2% at room temperature. It is a common value for the rare earth ions co-doped phosphors and half of the YAG: Ce³⁺ (70–90%).^30–33 The experimental conditions and the compositions of the phosphors are expected to be further optimized to enhance the luminescence efficiency.

Fig. 5 Dependence of ln(I_S0/I_S) of Ce³⁺ on C (a) and I_S0/I_S of Ce³⁺ on (b) C^6/3, (c) C^8/3 and (d) C^10/3.

The CIE chromaticity of (Ca_0.33Sr_0.67)₇(SiO₃)₆Cl₂: 001Ce³⁺, mEu²⁺ phosphors excited at 340 nm is shown in Fig. 6, and the corresponding color coordinates and color temperatures are represented in Table 1. With the increasing of Eu²⁺ concentration, the color coordinates (x, y) of CSSC: 001Ce³⁺, mEu²⁺ phosphors shift from violet (0.1696, 0.0693) to standard white (0.3314, 0.334) and ultimately to warm white (0.3773, 0.396), the corresponding color temperatures of the samples change from 9618 K to 4195 K. When m is greater than 0.03, the CCK of CSSC: 0.01Ce³⁺, mEu²⁺ are much lower than that of the w-LEDs fabricated by the InGaN chips and YAG phosphors (CCT ≈ 7750 K),³⁴ which proves that CSSC: 0.01Ce³⁺, mEu²⁺ can provide warm white light emission. Meanwhile, the CSSC: 001Ce³⁺, mEu²⁺ phosphors could emit adjustable white light under the excitation of 340 nm by changing the concentration of Eu²⁺ from 1% to 5%.

Fig. 6 CIE chromaticity diagram for CSSC: 001Ce³⁺, mEu²⁺ excited at 340 nm.

All these results indicate that this single-phased white-emitting phosphor can be excited by UV-LED chips and has the potential application in white LEDs.

4. Conclusions

In summary we have successfully prepared a series white-emitting and color temperature-tunable (Ca_0.33Sr_0.67)₇(SiO₃)₆Cl₂: Ce³⁺, Eu²⁺ phosphors for UV-LEDs. The samples could display varied color emission from violet (0.1696, 0.0693) towards standard white (0.3314, 0.334) and ultimately to warm white (0.3773, 0.396) under the excitation of 340 nm light by adjusting the relative proportion of Ce³⁺/Eu²⁺. The energy transfer mechanism from Ce³⁺ to Eu²⁺ in (Ca_0.33Sr_0.67)₇(SiO₃)₆Cl₂ phosphors is dominant by electric dipole–dipole interaction. These results indicate that the (Ca_0.33Sr_0.67)₇(SiO₃)₆Cl₂: Ce³⁺, Eu²⁺ phosphor might be promising as a single-phased white-light-emitting phosphor for UV LEDs.

Acknowledgements

We acknowledge the financial support from the National Natural Science Funds of China (Grant No. 51372105) and State Key Laboratory on Integrated Optoelectronics (No. IOSKL2013KF15) and the Fundamental Research Funds for the Central Universities of Lanzhou University (No. lzujbky-2015-205) and Gansu Province Development and Reform Commission (No. 2013, 1336).

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