Synthesis, structure and photoluminescence properties of a novel color-tunable Si_1.92Al_0.08O_1.08N_1.92:Eu²⁺,Tb³⁺,Sm²⁺phosphor for ultraviolet white light-emitting diodes

Abstract

A novel oxynitride phosphor Si_1.92Al_0.08O_1.08N_1.92:x%Eu²⁺,y%Tb³⁺,z%Sm²⁺ (o-sialon) was successfully synthesized through a high temperature solid-state reaction. The crystal structure, photoluminescence properties and thermal quenching properties have been measured and analyzed. The results indicate that the emission colors can be tuned from blue (0.1716, 0.1508) to green (0.3607, 0.5631) by changing the Eu²⁺/Tb³⁺ ratio and tuned from white (0.2377, 0.2991) to deep red (0.3573, 0.2231) by changing the Sm²⁺ concentration from 0 to 0.03% in Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,1%Tb³⁺,z%Sm²⁺. The energy transfer from Eu²⁺ to Tb³⁺ in Si_1.92Al_0.08O_1.08N_1.92:Eu²⁺,Tb³⁺ and the energy transfer from Eu²⁺/Tb³⁺ to Sm²⁺ in Si_1.92Al_0.08O_1.08N_1.92:Eu²⁺,Tb³⁺,Sm²⁺ have been studied. The energy transfer mechanism of Eu²⁺ to Tb³⁺ is demonstrated to be a dipole–dipole interaction. Their integrated emission intensities at 150 °C all remain at 90% of that measured at room temperature. The results indicate that the Si_2−nAl_nO_1+nN_2−n:x%Eu²⁺,y%Tb³⁺,z%Sm²⁺ phosphor is a very promising candidate for use as an ultraviolet white light-emitting diode phosphor.

---

1. Introduction

In the field of advanced lighting and display technology, white light-emitting diodes (WLEDs) have been attracting intense attention due to their high efficiency, compactness, long operational lifetime and energy savings.^1–4 Until now, the commercial WLEDs were achieved by the combination of an InGaN chip with a Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) phosphor. However, this kind of WLED exhibits a high correlated color temperature (CCT) and a poor color rendering index due to the lack of the red emission band.^5–7 In order to overcome this problem, the method of blending tricolor (blue, green, red) phosphors upon the UV chips has been employed to realize warm-WLEDs. However, this method also suffers from the problem of low luminescence efficiency and color aberration due to emission reabsorption and different degradation rates of the three primary phosphors.^8–10 It is an urgent task to develop novel single-phased multi-color-emitting phosphors with high chemical and thermal stability for UV WLEDs to avoid the abovementioned problems. A single-composition multi-color-emitting phosphor can be produced by co-doping sensitizers and activators into the same crystalline matrix, using the principle of energy transfer from the sensitizers to the activators, such as Ca₆Y₂Na₂(PO₄)₆F₂:Eu²⁺,Mn²⁺,¹¹ La₅Si₂BO₁₃:Ce³⁺,Mn²⁺,¹² CaZr(PO₄)₂:Eu²⁺/Eu³⁺,¹³ Ca₂NaSiO₄F:Ce³⁺,Eu²⁺,Tb³⁺,¹⁴ and so on, but there is not an ideal commercial phosphor for this way, so finding new phosphors and investigating energy transfer between different rare earth ions are important things for single-composition multi-color-emitting phosphor.

As far as the sensitizers and activators, the rare earth ions were used for them in most inorganic phosphors because of their abundant emission color.^15–18 It is well known that the Eu²⁺ ions has broad excitation and emission bands with high efficiency due to their spin and orbit allowed 4f–5d electronic transitions.¹⁹ Moreover, the emission color of Eu²⁺ in different hosts has alterable emission peaks, depending on different crystal field splitting resulting from their surrounding ligands.^20,21 The Tb³⁺ ion is regarded as a promising green activator for showing sharp lines at 486, 542, 582 and 619 nm due to ⁵D₄ → ⁷F₆, ⁵D₄ → ⁷F₅, ⁵D₄ → ⁷F₄ and ⁵D₄ → ⁷F₃ transitions, respectively. For many materials, in order to have intense Tb³⁺ emission in the phosphor, sensitizers such as Eu²⁺ and Ce³⁺ are needed, since the f–f transitions of Tb³⁺ ion arespin-forbidden.^22,23 Moreover divalent Sm²⁺ has the 4f⁶ electron configuration, which can be excited into the 4f⁵5d¹ continuum under irradiation with UV and visible light. The Sm²⁺-doped phosphors often exhibits efficient deep red emission.^24–26 Compared with other red emission ions (Eu³⁺, Mn²⁺, Pr³⁺) which are usually used for energy transfer, the Sm²⁺ often shows a more wide broadband excitation. This may make the Sm²⁺ a potential red emission ion for energy transfer.

As for hosts, nitrides and oxynitrides such as MAlSiN₃ (M = Ca, Sr),^27–29 MYSi₄N₇ (M = Sr, Ba),^30,31 MSi₂O₂N₂ (M = Ca, Sr, Ba),^32,33 α-SiAlON,^34,35 and β-SiAlON,³⁶ are good candidates for host materials owing to several merits such as their high chemical, physical stability and low thermal quenching.³⁷ In the oxynitride or nitride phosphors, β-sialon is well known as a great green phosphor, which has a special structure, derived from β-Si₃N₄ by equivalent substitution of Al–O for Si–N, and the activators are situated into the channel of c direction. The o-sialon is a common and excellent ceramic material in Si–Al–O–N phase diagram which has a similar structure with β-sialon.³⁸ The structure is derived from Si₂N₂O by equivalent substitution of Al–O for Si–N, which can be written as Si_2−nAl_nO_1+nN_2−n. However, the luminescence properties of o-sialon have not been previously investigated. So we chose the o-sialon for the study and synthesize a series of novel oxynitride phosphors Si_2−nAl_nO_1+nN_2−n doped with Eu²⁺, Tb³⁺ and Sm²⁺. The crystal structure, thermal quenching and occupation situation of lanthanide ions for the o-sialon were investigated. Their luminescent properties under UV excitation and the energy transfer were studied.

2. Experimental

A series of Eu²⁺, Tb³⁺, Sm²⁺ singly and co-doped Si_2−nAl_nO_1+nN_2−n were synthesized by a solid state reaction. The starting materials employed are Si (A.R.), SiO₂ (A.R.), Al(OH)₃ (A.R.), Eu₂O₃ (99.99%), Tb₄O₇ (99.99%) and Sm₂O₃ (99.99%). The ingredients were well mixed and sintered in boron nitride (BN) crucibles at 1400 °C for 2 h and subsequently at 1550 °C for 4 h in a reducing atmosphere of 20% H₂–80% N₂.

The phase structures of the obtained samples were characterized by powder X-ray diffraction (XRD) using a Rigaku diffractometer with Ni-filtered Cu Kα radiation. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were obtained by a FLS-920T fluorescence spectrophotometer equipped with Xe 900 (450 W xenon arc lamp) as the light source. The PL decay curves were measured by a FLS-920T fluorescence spectrophotometer with an nF900 ns Flashlamp as the light source. All the measurements were performed at room temperature. High-temperature luminescence intensity measurements were carried out by using an aluminum plaque with cartridge heaters; the temperature was measure by thermocouples inside the plaque and controlled by a standard TAP-02 high-temperature fluorescence controller.

3. Results and discussion

3.1 Phase identification and crystal structure

To evaluate the structural parameters of Si_1.92Al_0.08O_1.08N_1.92: structural refinement was carried out by the material studio program using the Rietveld method. As displayed in Fig. 1(a), the red solid line and black crosses represent the calculated and experimental patterns, respectively. The pink short vertical lines show the Bragg reflection positions of the calculated pattern. The difference between the experimental and calculated results is plotted by the blue line at the bottom. The structural parameters of Si_1.96Al_0.04N_1.96O_1.04 are used as the initial parameters in the Rietveld analysis.³⁹ The resulting crystallographic data of Si_1.92Al_0.08O_1.08N_1.92 is summarized in Table 1. The atomic coordinates and site occupancy fraction (SOF) are presented in the ESI (Table S1†). All atomic positions and equivalent isotropic displacement parameters were refined converging to the residual factors R_wp = 13.27%, R_p = 9.17%. We can find that the Si_1.92Al_0.08O_1.08N_1.92 has orthorhombic crystal system (Cmc21(36) space group) which is similar to the crystal system of Si_1.96Al_0.04N_1.96O_1.04.

Fig. 1 (a) Rietveld refinement of the powder XRD profile of Si_1.92Al_0.08O_1.08N_1.92. (b) The crystal structure of Si_1.92Al_0.08O_1.08N_1.92 and the coordination environment of the activator ions.

Table 1 Crystal structural data and lattice parameters

Formula	Si1.92Al0.08O1.08N1.92
Crystal system	Orthorhombic
Space group	Cmc21(36)
a/Å	8.8901
b/Å	5.5002
c/Å	4.8545
α/°	90
β/°	90
γ/°	90
Rwp	13.27%
Rp	9.17%

---

As shown in Fig. 1(b), the Si_1.92Al_0.08O_1.08N_1.92 comprises a three-dimensional network structure of (Si, Al)(O, N)₄ tetrahedra with a continuous and folded channel. The channel was formed by the corner shared connection and it is composed by six-membered ring arraying along the c direction. There are six sites on the six-membered ring. Two of which are only occupied by O²⁻, the other four can be occupied by O²⁻ or N³⁻. When the rare earth ions are doped, the coordination environments of the cation sites are not appropriate for them to occupy, because Eu²⁺ (1.17 Å, CN = 6; 1.25 Å, CN = 8), Tb³⁺ (0.92 Å, CN = 6; 1.04 Å, CN = 8) and Sm²⁺ (0.96 Å, CN = 6; 1.27 Å, CN = 8) have a larger radius compared with Al³⁺ (0.39 Å, CN = 4) and Si⁴⁺ (1.17 Å, CN = 6), we think that the rare earth ions could not occupy any sites of cation ions (Al³⁺, Si⁴⁺) in the structure of o-sialon. We can not know the definite position of the rare earth ions in Si_1.92Al_0.08O_1.08N_1.92, however because the Si_1.92Al_0.08O_1.08N_1.92 have the similar structure composition of (Si, Al)(O, N)₄ tetrahedra and the analogical channel composed by six-membered ring with β-sialon, we can obtain some enlightenment from the rare earth ions in β-sialon. Lin Gan et al. describe detailedly and demonstrate the position of Ce in β-SiAlON using an atom-resolved Cs-corrected scanning transmission electron microscope. The Ce³⁺ have two sites which are in the c-projected structural channels coordinated with six and nine N(O) atoms. We can reasonably infer that the positions of rare earth ions in Si_1.92Al_0.08O_1.08N_1.92 are also in the interspace or channel and there are several coordination environments around Eu²⁺ ions.

Fig. 2 shows the XRD patterns of the obtained Si_1.92Al_0.08O_1.08N_1.92:x%Eu²⁺ (0.09 ≤ x ≤ 0.27) and Si_1.92Al_0.08O_1.08N_1.92:18%Eu²⁺,1%Tb³⁺,0.03%Sm²⁺ phosphors. When the diffraction data are compared with the simulated patterns, it is found that all the positions and relative intensities are in good agreement with the calculated and no impurity phase is detected. The results show that there is no detectable impurity phase presented when doping Eu²⁺, Tb³⁺ and Sm²⁺ ions.

Fig. 2 The XRD patterns of the Si_1.92Al_0.08O_1.08N_1.92:x%Eu²⁺ (0.09 ≤ x ≤ 0.27) and Si_1.92Al_0.08O_1.08N_1.92:18%Eu²⁺,1%Tb³⁺,0.03%Sm²⁺ samples.

3.2 Photoluminescent properties

3.2.1 Photoluminescent properties of Si_2−nAl_nO_1+nN_2−n:Eu²⁺. Considering that there is different environment in the c channel and the Eu²⁺ ions are sensitive to their surrounding ligands. The Gaussian fitting was used for analysis. The emission spectrum of Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺ under 340 nm excitation and its Gaussian fitting are shown in Fig. 3(a). The emission spectrum can be decomposed into three components with the maxima at 400(M1), 450(M2), and 500(M3). The three emission bands are assigned to the 4f⁶5d¹ → 4f⁷ transition of Eu²⁺in different coordination environments. The appearance of the three peaks results from that the different proportion of oxygen and nitrogen around the Eu²⁺ ions forms the different coordination environments. While increasing the rate of O/N, the size of the channel becomes larger, which will reduce the crystal field strength and make the emission band shift to shorter wavelength.

Fig. 3 (a) The experimental data and Gaussian fitting of the emission spectrum Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺ monitored at 340 nm. (b) The emission spectra of Si_2−nAl_nO_1+nN_2−n:0.18%Eu²⁺ (z = 0.032, 0.048, 0.064, 0.08). (b) The excitation spectra of Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺ monitored at different emissions (400 nm, 450 nm, 500 nm). (c) The emission spectra of Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺ under different excitation wavelengths (300 nm, 320 nm, 340 nm, 360 nm, 380 nm, 400 nm).

In order to further demonstrate there are different luminescent centers for Si_1.92Al_0.08O_1.08N_1.92:Eu²⁺, the excitation spectra monitored at different wavelengths and emission spectra of Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺ are shown in Fig. 3(b) and (c). When monitoring different emission peaks, we can observe different excitation peaks, and by changing the excitation wavelengths, the shapes of the emission spectra have different peak patterns. The results further prove that the Si_1.92Al_0.08O_1.08N_1.92 have more than one position for Eu²⁺ to occupy.

The influence of the Eu²⁺ concentration on the luminescent properties was investigated by varying the Eu²⁺ concentration between 0.09% and 0.4%. The emission spectra of Si_1.92Al_0.08O_1.08N_1.92 with varied Eu²⁺ concentrations under excitation at 340 nm are given in Fig. 4. As the concentration of Eu²⁺ increased, the luminescence intensity increased and reached a maximum at x = 0.007. When the Eu²⁺ concentration exceeded 0.007, concentration quenching occurred. Meanwhile, the emission peak positions of the Si_1.92Al_0.08O_1.08N_1.92:xEu²⁺ phosphors shift from 427 nm to 458 nm as the Eu²⁺ concentration increases. The red shift of the emission peak can be ascribed to two reasons. First, when the Eu²⁺ concentration was changed, the shapes of emission spectra are different. So we infer that the intensity of three sites have inequable change degree relative to the same variation of Eu²⁺ concentration, which can make different peak positions. Second, from the Fig. 2, we observe that the shift toward largeangle of the diffraction position was happened with the increasing of Eu²⁺. Due to the Bragg equation, the lattice constant become smaller with the increasing of Eu²⁺. The red-shifting behavior of the emission band can be explained in terms of the Eu²⁺ ions experiencing a strengthening of the crystal field strength caused by increasing Eu²⁺ concentration due to the shrinkage of the lattice.

Fig. 4 The emission spectra of Si_1.92Al_0.08O_1.08N_1.92:x%Eu²⁺ (0.09 ≤ x ≤ 0.4). The inset shows the relationship between the emission intensity or the emission peak of Eu²⁺ ions and the doped Eu²⁺ content.

3.2.2 Luminescence properties of Si_1.92Al_0.08O_1.08N_1.92:Eu²⁺,Tb³⁺,Sm²⁺. Fig. 5(a) shows the excitation and emission spectra of Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺, and Fig. 5(b) displays the excitation and emission spectra of Si_1.92Al_0.08O_1.08N_1.92:1%Tb³⁺. Monitored at 542 nm, the excitation spectrum of Tb³⁺ contains an intense broad band at 250 nm, which is attributed to the allowed transition of the Tb³⁺ from the ground state (⁷F₆) to excited states of the 4f⁷5d¹ configuration. There are also many peaks located at 284, 295, 303, 318, 338, 355, 379, 486 nm in the excitation spectrum, which are due to the various forbidden 4f → 4f transitions of Tb³⁺ ions.^40,41 From the emission spectrum excited at 250 nm, it can be seen that the emission spectrum is dominated by the characteristic green emission ⁵D₄ → ⁷F₅ (542 nm) transitions of Tb³⁺, and the emission also has some weaker peaks due to the ⁵D₄ → ⁷F₆ (486 nm), ⁵D₄ → ⁷F₄ (587 nm) and ⁵D₄ → ⁷F₃ (619 nm) of Tb³⁺.

Fig. 5 Emission spectra (right) and excitation spectra (left) of Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺ (a), Si_1.92Al_0.08O_1.08N_1.92:1%Tb³⁺ (b), Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,1%Tb³⁺ (c), Si_1.92Al_0.08O_1.08N_1.92:0.03%Sm²⁺ (d), and Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,1%Tb³⁺,0.0045%Sm²⁺ (e) samples.

A considerable spectral overlap between the PL spectrum of Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺ and the PLE spectrum of Si_1.92Al_0.08O_1.08N_1.92:1%Tb³⁺ were observed. This implies that the energy transfer from Eu²⁺ to Tb³⁺ would be highly expected in Eu²⁺ and Tb³⁺ co-doped Si_1.92Al_0.08O_1.08N_1.92. Fig. 5(c) shows the PL and PLE spectra of Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,1%Tb³⁺. At the excitation of 340 nm, the phosphor shows blue emission of the Eu²⁺ ions and green emission of the Tb³⁺ ions. Monitored at 542 nm, the excitation spectrum shows a broad band, which has a similar shape with the PLE spectrum of Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺. This indicates the occurrence of an energy transfer from Eu²⁺ to Tb³⁺, and the doped of Eu²⁺ enhanced the efficient of Tb³⁺ under the excitation at 340 nm.

Fig. 5(d) displays the excitation and emission spectra of Si_1.92Al_0.08O_1.08N_1.92:0.03%Sm²⁺. The emission spectrum under 340 nm excitation of Sm²⁺ doped Si_1.92Al_0.08O_1.08N_1.92 shows the sharp emission lines at 680 nm, 710 nm and 730 nm of Sm²⁺ ions overlapped on the weak broad emission between 600 and 750 nm, the sharp emission can be attributed to ⁵D₀ → ⁷F_j (j = 0, 1, 2) transition and the broad band can be attributed to 5d → 4f transition of Sm²⁺. When monitored at 680 nm the excitation spectrum shows a wide range from 250 nm to 600 nm consisting of two broad bands with maxima at 360 and 470 nm, respectively, the bands can be attributed to transitions from the 4f⁶ ground state to the 4f⁵5d¹ excited states of Sm²⁺. It can be find that the excitation spectrum of Si_1.92Al_0.08O_1.08N_1.92:0.03%Sm²⁺ has an obvious spectral overlap with the emission spectrum of Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺ and Si_1.92Al_0.08O_1.08N_1.92:1%Tb³⁺. This implies that the energy transfer from Eu²⁺, Tb³⁺ to Sm²⁺ maybe happened in the Eu²⁺, Tb³⁺ and Sm²⁺co-doped Si_1.92Al_0.08O_1.08N_1.92. Fig. 5(e) shows the excitation and emission spectra of Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,1%Tb³⁺,0.0045%Sm²⁺, which also have a wide broad band excitation from 250 to 600 nm monitoring at 680 nm. Comparing with the Si_1.92Al_0.08O_1.08N_1.92:Sm²⁺, it has a stronger absorption at 250–400 nm and the shape of this range is alike to the excitation spectra of Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,1%Tb³⁺. So we infer that the energy transfer could happen from Eu²⁺/Tb³⁺ to Sm²⁺.

In order to further understand the energy transfer between the Eu²⁺, Tb³⁺ and Sm²⁺ ions. A series of Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,y%Tb³⁺,z%Sm²⁺ samples were prepared for the investigation. Fig. 6(a) shows the emission spectra of Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,y%Tb³⁺ (0 ≤ y ≤ 1) and the variation of the emission intensity of Eu²⁺ and Tb³⁺ ions excited at 340 nm. The Eu²⁺ content was fixed at 0.18% which is the optimal content, while the Tb³⁺ content changes from 0 to 1%. As can be seen, the intensity of the emission spectra of the Eu²⁺ ions decreases monotonically with an increase in the Tb³⁺ doping content, whereas the intensity of the Tb³⁺ emission obviously increases. In addition, the emission peak positions have a blue shift with the increasing of Tb³⁺ because of the different energy transfer efficiencies for the three lattice site. These demonstrate the existence of the energy transfer from Eu²⁺ to Tb³⁺. Fig. 6(b) shows the emission spectra of Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,1%Tb³⁺,z%Sm²⁺ (0 ≤ z ≤ 0.03) and the variation of the emission intensity of Eu²⁺, Tb³⁺ and Sm²⁺ ions excited at 340 nm. As can be seen, the intensity of the emission spectra of the Eu²⁺ ions and Tb³⁺ decreases monotonically and the intensity of the Sm²⁺ emission spectra obviously increases with the increasing of Sm²⁺concentration. This proves that the energy transfer from Eu²⁺or/and Tb³⁺to Sm²⁺ is existent.

Fig. 6 (a) The emission spectra of Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,y%Tb³⁺ (0 ≤ y ≤ 1). The inset shows the relationship between the emission intensity of Eu²⁺ and Tb³⁺ ions and the doped Tb³⁺ content. (b) The emission spectra of Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,1%Tb³⁺,z%Sm²⁺ (0 ≤ z ≤ 0.03). The inset shows the relationship between the emission intensity of Eu²⁺, Tb³⁺ and Sm²⁺ ions and the doped Sm²⁺ content.

To provide further evidence of the phenomenon that the energy transfer from the Eu²⁺ to the Tb³⁺ ions and Eu²⁺/Tb³⁺to the Sm²⁺ ions occurs in the Si_1.92Al_0.08O_1.08N_1.92 host, some fluorescence decay curves were measured. Fig. 7(a) shows the PL decay curves of Eu²⁺ in the Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,y%Tb³⁺ (0 ≤ y ≤ 1). Fig. 7(b) and (c) shows the PL decay curves of Eu²⁺ and Tb³⁺ in the Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,1%Tb³⁺,z%Sm²⁺ (0 ≤ z ≤ 0.03) samples. The fluorescence of them all tends to be a non-exponential function with increasing the Tb³⁺ or Sm²⁺ concentration. The decay process of these samples are characterized by an average lifetime, τ, which can be calculated using eqn (1)^42,43 as follows:

where I(t) is the luminous intensity at time t. On the basis of eqn (1), the lifetimes are listed in the Fig. 7. We can find that the lifetimes of Eu²⁺ and Tb³⁺ in the Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,y%Tb³⁺ and Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,1%Tb³⁺,z%Sm²⁺ all have a decreasing trend with the increasing of activators. This demonstrates that the energy transfers from Eu²⁺ → Tb³⁺ and Eu²⁺, Tb³⁺ → Sm²⁺ are existent.

Fig. 7 (a) Decay curves of Eu²⁺ in Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,y%Tb³⁺ (0 ≤ y ≤ 1) displayed on a logarithmic intensity scale (excited at 340 nm, monitored at 440 nm). (b) Decay curves of Eu²⁺ in Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,1%Tb³⁺,z%Sm²⁺ (0 ≤ z ≤ 0.03) displayed on a logarithmic intensity scale (excited at 340 nm, monitored at 440 nm). (c) Decay curves of Tb³⁺ in Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,1%Tb³⁺,z%Sm²⁺ (0 ≤ z ≤ 0.03) displayed on a logarithmic intensity scale (excited at 340 nm, monitored at 542 nm). (d) The dependence of I₀/I of Eu²⁺ on C^6/3, C^8/3 and C^10/3 in Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,y%Tb³⁺ (0 ≤ y ≤ 1).

Energy transfer efficiency η between the Eu²⁺ and Tb³⁺ ions was also obtained from the decay lifetime by using the eqn (2):^43–45

The τ and τ₀ are the lifetimes of sensitizer (Eu²⁺) ion with and without the presence of activator (Tb³⁺). The η are calculated to be 0, 0.032, 0.041, 0.063, 0.096 and 0.126 with y = 0, 0.05, 0.4, 0.55, 0.85 and 0.1. In general, energy transfer from the sensitizer to the activator in a phosphor may take place via a multipolar interaction or an exchange interaction. This can be evaluated according to the Dexter's theories,^46–48 as given in the following eqn (3):

η₀/η ∝ C^n/3 (3)

where η₀ and η are the luminescence quantum of Eu²⁺ in the absence and presence of Tb³⁺ and C is the concentration of Tb³⁺. When n = 6, 8, and 10, it corresponds to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions. The value of η₀/η can be estimated approximately from the correlated lifetime ratio (τ₀/τ), thus equation can be changed as follows:

τ₀/τ ∝ C^n/3 (4)

As shown in Fig. 7(d), this clearly indicates a better fitting result for C^6/3 compared with the others through the linear fitting. This indicates that the dipole–dipole interaction is mainly responsible for the energy transfer from the Eu²⁺ to the Tb³⁺ ions.

The corresponding energy levels scheme and energy transfer in the Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,y%Tb³⁺,z%Sm²⁺ upon excitation with UV radiation is illustrated in Fig. 8. The energy transfer behavior between Eu²⁺, Tb³⁺ and Sm²⁺ ions can be ascribed to the similar value of energy level of the excited 5d state of Eu²⁺, the ⁵D_j (j = 3, 4) of Tb³⁺ions, and the excited 5d state of Sm²⁺. The energy transfer process can be divided into four parts. The first part is the excitation and the emission of Eu²⁺. When the Eu²⁺ ions are excited by the UV light, the electron is pumped to the 5d level, and then it relaxes to the lowest 5d crystal field splitting state, and then a blue emission appears owing to the transition from 5d to 4f level.

Fig. 8 The energy level diagram and the energy transition schematic diagram of Eu²⁺, Tb³⁺ and Sm²⁺ in Si_1.92Al_0.08O_1.08N_1.92:Eu²⁺,Tb³⁺,Sm²⁺.

The second part is the energy transfer from Eu²⁺ to Tb³⁺. An energy transfer process takes place from the 5d state of Eu²⁺ to ⁵D₃ of Tb³⁺, which relaxes to levels ⁵D₄ later. Then the excited Tb³⁺ relaxes to the ⁷F_j (j = 3, 4, 5, 6) levels non-radioactively and gives the green characteristic emission of Tb³⁺. The third part is the energy transfer from Eu²⁺ to Sm²⁺. An energy transfer process takes place from the 5d state of Eu²⁺ to 5d state of Sm²⁺, and then it relaxes to the lowest 5d crystal field splitting state or the ⁵D₀ of Sm²⁺. And then the sharp emission of ⁵D₀ → ⁷F₀ occurs with the weak broad band 5d → 4f emission. The fourth part is the energy transfer from Tb³⁺ to Sm²⁺. An energy transfer process takes place from ⁵D₃ or ⁵D₄ of Tb³⁺ to 5d of Sm²⁺, and then it gives the emission processes of Sm²⁺ similar to the third part.

3.3 Temperature-dependent PL properties

Thermal quenching is one of the important technological parameters for phosphors used in UV WLEDs. Fig. 9(a) shows the temperature dependence of luminescence for Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺ under 340 nm excitation. The emission intensity is about 92% at 150 °C, and 84% at 250 °C. The shape of the emission spectra shows a blue-shift with increasing temperature due to the thermally active phonon assisted excitation from a lower energy sublevel to a high energy sublevel in the excited state of Eu²⁺. As shown in Fig. 9(b), at higher temperatures, electrons at lower excited levels (M(2)) could jump to higher excited levels (M(1)), assisted by thermal phonons. This makes the result that when increasing temperature the blue shift of emission peak takes place. The energy transfer from M(2) to M(1) can occur by non-radiative relaxation. The temperature dependence of the integrated emission intensity of Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,1%Tb³⁺,0.003%Sm²⁺ is presented in the Fig. 9(c). The integrated emission intensity at 150 and 250 °C remains about 90% and 78%. It can be noted that the emission intensity of Sm²⁺ increases for 580–670 nm wavelength. The phenomenon also appeared in the β-sialon:Sm²⁺ and can be rationalized in terms of the energy transfer from ⁵D₀ to 4f⁵5d¹.⁴⁹ As shown in Fig. 9(d), in Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,1%Tb³⁺,0.003%Sm²⁺ sample, the 4f⁵5d¹ state is located at higher energy than the ⁵D₀ state, so at higher temperatures, electrons at lower excited levels (⁵D₀) could jump to higher excited levels (4f⁵5d¹), resulting in the increase of intensity of Sm²⁺ from 600 nm to 670 nm in the range of 25–150 °C.

Fig. 9 (a) Temperature dependence of emission intensity of Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺. (b) The schematic configurational coordinate diagrams that present possible mechanisms of temperature-dependent emission for Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺. (c) Temperature dependence of emission intensity of decay curves of Eu²⁺ in Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,1%Tb³⁺,0.003%Sm²⁺. (d) The schematic configurational coordinate diagrams that present possible mechanisms of temperature-dependent emission for Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,1%Tb³⁺,0.003%Sm²⁺.

3.4 CIE coordinates and quantum efficiency of Si_1.92Al_0.08O_1.08N_1.92:Eu²⁺,Tb³⁺,Sm²⁺

Fig. 10 shows the variation of the Commission International deL'Eclairage (CIE) chromaticity coordinates of the Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺ (1%Tb³⁺ or 0.03%Sm²⁺), Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,y%Tb³⁺ and Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,1%Tb³⁺,z%Sm²⁺ under excitation at 365 nm. The CIE coordinates are calculated and summarized in the ESI (Table S2†). The results indicate that we can get blue green red and even white emission light and the emission light can be modulated from blue to green with increasing the doping content of Tb³⁺ ions. And the Sm²⁺ can add the red compositions in the Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,1%Tb³⁺, making the colors of samples change from white to deep red. So the color can be tunable in a wide range for the Si_1.92Al_0.08O_1.08N_1.92:18%Eu²⁺,y%Tb³⁺,z%Sm²⁺. For an evaluation of the luminescence efficiency, the QEs of our phosphors are estimated. The QE of Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺ under 340 nm is measured to be 59.5% at room temperature and the QE of Si_1.92Al_0.08O_1.08N_1.92:0.18%Eu²⁺,1%Tb³⁺,0.003%Sm²⁺ under 340 nm is measured to be 32.8%.

Fig. 10 CIE chromaticity diagram for Si_1.92Al_0.08O_1.08N_1.92:x%Eu²⁺,y%Tb³⁺,z%Sm²⁺ excited at 340 nm.

4. Conclusions

In summary, A novel oxynitride phosphors Si_2−nAl_nO_1+nN_2−n:x%Eu²⁺,y%Tb³⁺,z%Sm²⁺ was synthesized successfully via a solid state reaction and the crystal structure was refined. There are three different crystalline sites for the lanthanide ions. The Eu²⁺, Tb³⁺ and Sm²⁺ singly-doped Si_1.92Al_0.08O_1.08N_1.92 respectively show a blue, a green and a deep red emission. It was demonstrated that the energy transfer from Eu²⁺ to Tb³⁺, Eu²⁺ to Sm²⁺ and Tb³⁺ to Sm²⁺ are subsistent in the Si_2−nAl_nO_1+nN_2−n:x%Eu²⁺,y%Tb³⁺,z%Sm²⁺. The change of the color for Si_2−nAl_nO_1+nN_2−n:x%Eu²⁺,y%Tb³⁺,z%Sm²⁺ in a wide range was successfully realized through the energy transfer. The thermal stabilities of them have a great performance, the blue shift and the increasing of the emission intensity along with the rising of temperature were also investigated. In conclusion, the novel color-tunable Si_2−nAl_nO_1+nN_2−n:x%Eu²⁺,y%Tb³⁺,z%Sm²⁺ phosphors are expected to have promising applications in UV WLEDs.

Acknowledgements

We acknowledge the financial support from the National Science Foundation under grant No. 51202099 and grant No. 51372105, and the Doctoral Program of Higher Education under grant No. 20120211130003, and the State Key Laboratory on Integrated Optoelectronics under grant No. IOSKL2013KF15.

Notes and references

1. R. J. Xie, N. Hirosaki, M. Mitomo, K. Sakuma and N. Kiumra, Appl. Phys. Lett., 2006, 89, 241103.
2. J. S. Kim, P. E. Jeon, J. C. Choi and H. L. Park, Appl. Phys. Lett., 2004, 84, 2931.
3. W. B. Im, Y. I. Kim, N. N. Fellows, H. Masui, G. A. Hirata, S. P. Den and B. R. Seshadri, Appl. Phys. Lett., 2008, 93, 091905.
4. Z. Y. Mao and D. Wang, J. Inorg. Chem., 2010, 49, 4922.
5. C. Hecht, F. Stadler, P. J. Schmidt, J. S. A. der Guenne, V. Baumann and W. Schnick, Chem. Mater., 2009, 21, 1595.
6. Z. G. Xia, R. S. Liu, K. W. Huang and V. Drozd, J. Mater. Chem., 2012, 22, 15183.
7. W. Z. Lv, Y. C. Jia, Q. Zhao, M. M. Jiao, B. Q. Shao, W. Lu and H. P. You, Adv. Opt. Mater., 2014, 2, 183.
8. N. Guo, H. You, Y. Song, M. Yang, K. Liu, Y. Zheng, Y. Huang and H. Zhang, J. Mater. Chem., 2010, 20, 9061.
9. W. R. Liu, C. H. Huang, C. W. Yeh, J. C. Tsai, Y. C. Chiu, Y. T. Yeh and R. S. Liu, Inorg. Chem., 2012, 51, 9636.
10. W. Lu, Y. C. Jia, Q. Zhao, W. Z. Lv and H. P. You, Chem. Commun., 2014, 50, 2635.
11. N. Guo, H. P. You, C. Z. Jia, R. Z. O. Yang and D. H. Wu, Dalton Trans., 2014, 43, 12373.
12. H. K. Liu, L. B. Liao and Z. G. Xia, RSC Adv., 2014, 4, 7288.
13. J. C. Zhang, Y. Z. Long, H. D. Zhang, B. Sun, W. P. Han and X. Y. Sun, J. Mater. Chem. C, 2014, 2, 312.
14. M. M. Jiao, Y. C. Jia, W. Lu, W. Z. Lü, Q. Zhao, B. Q. Shaoand and H. P. You, J. Mater. Chem. C, 2014, 2, 4304.
15. L. Wu, Y. Zhang, M. Gui, P. Lu, L. Zhao, S. Tian, Y. Konga and J. Xu, J. Mater. Chem., 2012, 22, 6463.
16. S. Zhang, H. Liang and C. Liu, J. Phys. Chem. C, 2013, 117, 2216.
17. D. Kang, H. S. Yoo, S. H. Jung, H. Kim and D. Y. Jeon, J. Phys. Chem. C, 2011, 115, 24334.
18. G. Zhu, S. Xin, Y. Wen, Q. Wang, M. Que and Y. Wang, RSC Adv., 2013, 3, 9311.
19. D. Jia, R. S. Meltzer and W. M. Yen, Appl. Phys. Lett., 2002, 80, 1535.
20. K. A. Denault, Z. Chenga, J. Brgocha, S. P. DenBaars and R. Seshadri, J. Mater. Chem. C, 2013, 1, 7339.
21. G. Zhang, J. Wang, Y. Chen and Q. Su, Opt. Lett., 2010, 35, 2382.
22. M. Jiao, N. Guo, W. Lü, Y. Jia, W. Lv, Q. Zhao, B. Shao and H. You, Inorg. Chem., 2013, 52, 10340.
23. N. Guo, Y. Song, H. You, G. Jia, M. Yang, K. Liu, Y. Zheng, Y. Huang and H. Zhang, Eur. J. Inorg. Chem., 2010, 4636.
24. N. R. J. Poolton, A. J. J. Bos and P. Dorenbos, J. Phys.: Condens. Matter, 2012, 24, 225502.
25. J. Y. Wang, Y. L. Huang and Y. D. Li, J. Am. Ceram. Soc., 2011, 95, 1494.
26. C. Kulshreshtha, S. H. Cho, Y. S. Jung and K. S. Sohn, J. Electrochem. Soc., 2007, 154, J86.
27. K. Uheda, N. Hirosaki, Y. Yamamoto, A. Naito, T. Nakajima and H. Yamamoto, Electrochem. Solid-State Lett., 2006, 9, H22.
28. H. Watanabe, H. Wada, K. Seki, M. Itou and N. Kijima, J. Electrochem. Soc., 2008, 155, F31.
29. X. Piao, K. Machida, T. Horikawa, H. Hanzawa, Y. Shimomura and N. Kijima, Chem. Mater., 2007, 19, 4592.
30. Y. Q. Li, C. M. Fang, G. de With and H. T. Hintzen, J. Solid State Chem., 2004, 171, 4687.
31. Y. Q. Li, G. de With and H. T. Hintzen, J. Alloys Compd., 2004, 385, 1–11.
32. R. S. Liu, Y. H. Liu, N. C. Bagkar and S. F. Hua, Appl. Phys. Lett., 2007, 91, 061119.
33. Y. Q. Li, A. C. A. Delsing, G. de With and H. T. Hintzen, Chem. Mater., 2005, 17, 3242.
34. T. Suehiro, N. Hirosaki, R. J. Xie and M. Mitomo, Chem. Mater., 2005, 17, 308–314.
35. K. Sakuma, N. Hirosaki, R. J. Xie, Y. Yamamoto and T. Suehiro, Mater. Lett., 2007, 61, 547.
36. R. J. Xie, K. Kimoto, T. Sekiguchi, Y. Yamamoto, T. Suehiro, M. Mitomo and N. Hirosaki, Appl. Phys. Lett., 2005, 86, 211905.
37. R. J. Xie and N. Hirosaki, Sci. Technol. Adv. Mater., 2007, 8(7), 588–600.
38. T. Ekstrom, P.-O. Olsson and M. Holmstrom, J. Eur. Ceram. Soc., 1993, 12, 165.
39. O. Lindqvist, J. Sjoberg, S. Hull and R. Pompe, Acta Crystallogr., Sect. B: Struct. Sci., 1991, 47, 672.
40. H. You, X. Wu, H. Cui and G. Hong, J. Lumin., 2003, 104, 223.
41. Y. Song, N. Guo and H. You, Eur. J. Inorg. Chem., 2011, 2327.
42. N. Guo, Y. Song, H. You, G. Jia, M. Yang, K. Liu, Y. Zheng, Y. Huang and H. Zhang, Eur. J. Inorg. Chem., 2010, 4636.
43. F. Lahoz, I. R. Martin, J. Mendez-Ramos and P. J. Nunez, Chem. Phys., 2004, 120, 6180–6190.
44. P. Paulose, G. Jose, V. Thomas, N. Unnikrishnan and M. J. Warrier, Phys. Chem. Solids, 2003, 64, 841–846.
45. K. H. Kwon, W. B. Im, H. S. Jang, H. S. Yoo and D. Y. Jeon, Inorg. Chem., 2009, 48, 11525–11532.
46. D. L. Dexter and J. H. Schulman, J. Chem. Phys., 1954, 22, 1063.
47. G. Blasse, Philips Res. Rep., 1969, 24, 131–144.
48. P. L. Li, Z. J. Wang, Z. P. Yang and Q. L. Guo, J. Mater. Chem. C, 2014, 2, 7823.
49. Z. G. Yang, Z. Y. Zhao, M. D. Que and Y. H. Wang, Opt. Mater., 2013, 01, 024.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra16603a

---