Effect of a solid solution of AlN on the crystal structure and optical properties of LiSi₂N₃:Eu phosphors

Abstract

The solid solutions of nitridosilicate phosphors LiSi₂N₃:Eu-xAlN (0 ≤ x ≤ 0.35) were synthesized using a gas-pressed sintering in this work. The effects of a solid solution of AlN on crystal structure, morphology, thermal stability and luminescence properties were studied. The luminescence intensity of LiSi₂N₃:Eu phosphor is significantly improved by the solution of AlN, but the emission position is unchanged. The emission intensity of LiSi₂N₃:Eu_0.01-0.30AlN is about 2.17 times that of LiSi₂N₃:Eu_0.01. The results show that these solid solutions could be the most useful way to improve the luminescence intensity of LiSi₂N₃:Eu phosphors.

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1. Introduction

In recent years, white light-emitting diodes (W-LEDs) are revolutionizing an increasing number of applications due to their high efficiency, energy saving qualities, and environmentally friendliness.^1–7 In phosphor converted W-LED technologies, the emission of LED chips (blue or near-ultraviolet (n-UV) light) is down-converted into useful green, yellow, or red light by phosphors. Therefore, phosphors are one of the key materials in solid state lighting. Among the phosphors used to combine with blue or n-UV LEDs, nitride/oxynitride have received remarkable attention as host lattices for phosphors due to their high chemical stability and good thermal quenching, and they exhibit intense luminescence when activated with Ce³⁺/Eu²⁺ such as in M₂Si₅N₈ (M = Ca, Sr, Ba),^8–10 CaAlSiN₃ (ref. 11) Ca-α-SiAlON,^12,13 and MSi₂N₂O₂ (M = Ca, Sr, Ba).^14–16

Although a large number of phosphors have been discovered or developed for use in solid state lighting, a very limited number of them can be applied to W-LEDs practically. Therefore, it is necessary to research on unexplored compositional regions beyond the traditional phosphor materials.

Recently, there has been a growing focus on research into lithium nitrides for use as luminescent materials such as Sr[LiAl₃N₄],¹⁷ Ca[LiAl₃N₄],¹⁸ and LiSi₂N₃.¹⁹ Li and Si (Al) can be combined with N to form [LiSi(Al)_xN_y] frameworks, which have a high density.^17,20 The higher degree of condensation leads not only to increased stability of the materials, but also to high rigidity of the host lattice and thus to limited local structural relaxation of the Eu²⁺/Ce³⁺ site in its excited state – a prerequisite for the desired small stokes-shifted, narrow-band emission.²¹

Among Li₃N–Si₃N₄ systems, LiSi₂N₃ is a thermostable compound with a well-defined crystal structure. The photoluminescent properties of LiSi₂N₃:Eu phosphor were first reported by Y. Q. Li et al.²² and they exhibit yellow emission (550–595 nm) upon excitation in the UV to blue spectral region (300–450 nm). However, the emission intensity of LiSi₂N₃:Eu must be further improved. Moreover, in the previous report,²³ we find that LiSi₂N₃ and AlN can form a solid solution as both AlN and LiSi₂N₃ have the wurtzite-type crystal structure. The second-phase dispersions (AlN) are introduced into a host matrix (LiSi₂N₃), allowing a fine tuning of the crystal structure, the grains size, surface defect and provides higher uniformity of their distribution. The photoluminescence properties of phosphors included in white LEDs are correlated with the crystal structure, the morphology and grain size of the phosphor powder. In this case, the solid solutions seem to be the most promising way to improve the luminescence intensity of LiSi₂N₃:Eu. Therefore, in this work, we report the effect of a solid solution of AlN on the crystal structure and optical properties of LiSi₂N₃:Eu phosphors. The morphology and thermal stability were also investigated in this study. The results indicate that the solid solutions could possibly improve the luminescence intensity of LiSi₂N₃:Eu.

2. Experimental section

2.1 Materials and synthesis

A series of nitridosilicate phosphors, LiSi₂N₃:Eu_0.01-xAlN (0 ≤ x ≤ 0.35), was prepared by gas-pressed sintering. Stoichiometric amounts of powdered Li₃N (Aldrich, >99.50%), Si₃N₄ (Aldrich, 99.90%), AlN (Aldrich, >98.0%) and EuCl₃ (Aldrich, 99.999%) were ground in an agate mortar for 30 min in a glovebox to form a homogeneous mixture. The concentrations of both moisture and oxygen in the glovebox were <1 ppm. Thereafter, the powder mixtures were transferred into BN crucibles and heated at 1500 °C for 2 h in high purity nitrogen (99.9995%) atmosphere at a pressure of 0.5 MPa. Before nitrogen was imported into the furnace, the pressure condition was changed to a 10⁻² Pa vacuum state. The sintered products were ground again, yielding crystalline powder.

2.2 Characterization

All measurements were made using finely ground powders. The phase purity of samples was analyzed by X-ray powder diffraction (XRD) (D2 PHASER X-ray diffractometer, Germany) with a graphite monochromator using Cu Kα radiation (λ = 1.54056 Å), operating at 30 kV and 15 mA. The crystal structure was refined by the Rietveld method using the GSAS (General Structure Analysis System) program.²⁴ Photoluminescence (PL) and 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. Diffuse reflectance ultraviolet-visible (UV-vis) absorption spectra were measured using a Perkin-Elmer 950 spectrometer, whereas BaSO₄ was used as a reference. The quantum efficiency (QE) was measured using a spectrofluorometer (HORIBA JOBIN YVON Fluorlog-3) equipped with a 450 W xenon lamp. High temperature luminescence intensity measurements were carried out using an aluminum plaque with cartridge heaters; the temperature was measured by thermocouples inside the plaque and controlled by a standard TAP-02 high temperature fluorescence controller. The powder morphology was investigated by scanning electron microscopy (SEM; S-3400, Hitachi, Japan). The particle size distribution was measured using the Nano Measurer program based on the SEM micrographs. Chemical composition was determined with a scanning electron microscope (SEM; S-4800, Hitachi, Japan) equipped with an energy dispersive spectrometer (EDS) system.

3. Results and discussion

3.1 Phase identification

Fig. 1 shows the Rietveld refinement for the LiSi₂N₃:Eu_0.01-xAlN (x = 0, 0.10) phosphors. All of the observed XRD peaks are obtained with the goodness of fit parameters (shown in Table 1). These results indicate that the LiSi₂N₃:Eu_0.01-xAlN (x = 0, 0.10) host is a single-phase structure. The crystal structure data of LiSi₂N₃:Eu_0.01-xAlN (x = 0, 0.10) are also shown in Table 1. LiSi₂N₃ has a wurtzite-type structure, which is isostructural with Si₂N₂O and Li₂SiO₃. Similar to Li₂SiO₃, the framework of [Si₂N₃] in LiSi₂N₃ is built up by two-dimensional, infinite, parallel layers of condensed [Si₆N₆] twelve-membered rings formed by the corner sharing of the nitrogen atoms of the tetrahedral SiN₄. Li⁺ occupies the 4a site and directly connects with five nitrogen atoms, as shown in Fig. 2.

Fig. 1 Rietveld refinement of LiSi₂N₃:Eu_0.01-xAlN (a) x = 0 and (b) x = 0.10. (Bragg reflections are indicated with tick marks).

Table 1 Crystal structure data of LiSi₂N₃:Eu_0.01-xAlN (x = 0, 0.10)

Crystal system: orthorhombic; spacegroup: Cmc2₁(36); Z = 4

Formula: LiSi₂N₃:Eu_0.01

Lattice parameters: a = 9.1670(4) Å, b = 5.2918(1) Å, c = 4.7733(1) Å, V = 231.56(4) ų

R_wp: 8.81%

R_p: 5.92%

χ²: 1.414

Formula: LiSi₂N₃:Eu_0.01-0.10AlN

Lattice parameters: a = 9.1963(4) Å, b = 5.3120(2) Å, c = 4.7871(2) Å, V = 233.85(2) ų

R_wp: 8.58%

R_p: 5.82%

χ²: 1.733

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Fig. 2 (a) The crystallographic structure of LiSi₂N₃ and (b) the local coordination of Li with nitrogen atoms (N).

For LiSi₂N₃:Eu_0.01-xAlN (0 ≤ x ≤ 0.30), there is no detectable impurity phase presented. This result indicates that the LiSi₂N₃:Eu_0.01-xAlN (0 ≤ x ≤ 0.30) retains a single phase with the addition of AlN, as shown in Fig. 3. When x > 0.30, the phases of AlN and Si₃N₄ were also identified. The intensity of the diffraction peaks of LiSi₂N₃:Eu_0.01-xAlN (0.05 ≤ x ≤ 0.30) is gradually increased with the increase of AlN, which indicates the enhancement of the crystallinity of the grain. Moreover, the diffraction peaks of LiSi₂N₃:Eu_0.01-xAlN (0.05 ≤ x ≤ 0.30) slightly shifted to lower angles with respect to the position of LiSi₂N₃:Eu_0.01 due to the lattice expansion. The solution of AlN makes the LiSi₂N₃ lattice expand. As shown in Fig. 4, the lattice constants of a, b, c and V increase with increasing AlN. A consistent shift of the lattice parameters, with increasing AlN content, confirms the formation of solid solutions.

Fig. 3 XRD patterns of LiSi₂N₃:Eu_0.01-xAlN (0 ≤ x ≤ 0.35).

Fig. 4 Lattice constants of LiSi₂N₃:Eu_0.01-xAlN (0 ≤ x ≤ 0.30) (the I represents the error bars).

A typical low-magnification TEM image and a HRTEM image are shown in Fig. 5a–d. The interplanar spacing was measured to be 4.50 Å and 4.66 Å for LiSi₂N₃:Eu_0.01 and LiSi₂N₃:Eu_0.01-0.10AlN, respectively, which matches well with the (110) interplanar distance of orthorhombic LiSi₂N₃. The larger interplanar spacing of LiSi₂N₃:Eu_0.01-0.10AlN is due to the lattice expansion, which is consistent with the shift of XRD. The corresponding EDS spectrum analysis (Fig. 5e and f) indicates that LiSi₂N₃:Eu_0.01 and LiSi₂N₃:Eu_0.01-0.10AlN both have a chemical composition of Li, Si, Eu and N and that no impurity elements are present. The EDS spectrum of LiSi₂N₃:Eu_0.01-0.10AlN more than the LiSi₂N₃:Eu_0.01 of an aluminum element.

Fig. 5 TEM image, HRTEM image and EDX spectrum of LiSi₂N₃:Eu_0.01 (a), (b), (e) and LiSi₂N₃:Eu_0.01-0.10AlN (c), (d), (f).

3.2 Photoluminescence properties

Fig. 6 shows the reflection spectra, PLE and PL spectra of the as-prepared LiSi₂N₃:Eu_0.01 powders. The undoped sample has a white body color and has an absorption in the range of 200–230 nm. For the Eu²⁺-doped samples, strong absorption bands are presented in the region from 230 to 450 nm. The PLE spectrum shows a broadband profile covering the range from the near-UV to visible part centered at ∼355 nm, which is assigned to the 4f–5d transition of Eu²⁺. The emission band is located in the range of 475–800 nm, resulting in a yellow emission centered at ∼595 nm.

Fig. 6 Reflection spectra, PLE and PL spectra of the as-prepared LiSi₂N₃:Eu_0.01.

The emission spectra of LiSi₂N₃:Eu_0.01-xAlN (0 ≤ x ≤ 0.30) with varied AlN concentrations under excitation at 355 nm is given in Fig. 7. With the doping of AlN, the PL intensity significantly improves, and the PL intensity gradually improves with increasing AlN concentration. However, the peak positions and peak shapes are unchanged with doping AlN. These indicate that the local environment of Eu²⁺ is almost unchanged with the addition of AlN. When the AlN concentration is x = 0.30, the emission intensity is about 2.17 times that of the undoped AlN sample. The increase in the intensity of the emission due to the addition of AlN can be attributed to the growth of the particle size, the narrowing of the particle-size distribution, and the increase of crystallinity.

Fig. 7 Emission spectra of LiSi₂N₃:Eu_0.01-xAlN (0 ≤ x ≤ 0.30) with varied AlN concentrations under excitation at 355 nm.

Observation of the microstructure, presented in the SEM images in Fig. 8, supports the abovementioned discussion of increasing crystallinity. The powder produced without AlN (Fig. 8a) consists of small particles whose loose edges suggest poor crystallization. The addition of AlN apparently favors the production of uniform particles, generally bigger in size (compared to those shown in Fig. 8a) and with compact edges (suggesting high crystallinity, which is in accordance with the previously discussed XRD results in Section 3.1), as shown in Fig. 8b and c. Fig. 9 shows the particle size distribution of LiSi₂N₃:Eu_0.01-xAlN (x = 0, 0.1, and 0.3). The particle size of the synthesized sample without AlN is 4.96 μm (Fig. 9a), whereas the particle sizes of the samples with x = 0.1 and x = 0.3 are 8.63 μm (Fig. 9b) and 11.86 μm (Fig. 9c), respectively. These confirm that the solution of AlN could promote grain growth, narrow grain size distribution and improve crystallinity, resulting in an improvement in the emission intensity.

Fig. 8 SEM micrographs of LiSi₂N₃:Eu_0.01-xAlN (a) x = 0, (b) x = 0.1, and (c) x = 0.3.

Fig. 9 Particle size distribution of LiSi₂N₃:Eu_0.01-xAlN (a) x = 0, (b) 0.1, and (c) 0.3.

Fig. 10 shows the reflection spectra of LiSi₂N₃:Eu_0.01-xAlN (x = 0, 0.1, 0.2, and 0.3). With increasing AlN concentration, the absorption becomes stronger and the QE of LiSi₂N₃:Eu_0.01-xAlN (x = 0, 0.1, 0.2, and 0.3) become larger. The QE of LiSi₂N₃:Eu_0.01-xAlN (x = 0, 0.1, 0.2, and 0.3) was 23%, 28%, 30% and 33%, respectively. The high crystallinity would decrease the light scattering by small particles, reduce the defects in the lattice and surface as well as decrease the nonradiative transition probability,²⁵ resulting in enhanced absorption and quantum efficiency. Thus, the reflection spectra and the QE could further demonstrate that the solution of AlN could enhance the crystallinity of LiSi₂N₃:Eu.

Fig. 10 The reflection spectra of LiSi₂N₃:Eu_0.01-xAlN (x = 0, 0.1, 0.2, and 0.3).

3.3 Temperature-dependent PL properties

Thermal quenching is one of the important technological parameters for phosphors used in W-LEDs. The temperature dependence of the PL spectra of LiSi₂N₃:Eu_0.01-xAlN (x = 0 and 0.3) under excitation at 355 nm is shown in Fig. 11, which show a relatively poor thermal stability. The quenching temperature, T_q, (the temperature at which the emission intensity is half of the initial intensity at room temperature, ∼25 °C) is about 100 °C. It is believed that thermal ionization is mainly responsible for the quenching of the luminescence of Eu²⁺ at high temperatures in the LiSi₂N₃ host,²⁶ because the excited 5d electrons are easily ionized by the absorption of thermal energy and entrance into the bottom of the conduction band of the host through the top of the Eu²⁺ excitation levels. Moreover, the emission exhibits a blue-shift with increasing temperature, and it could be considered that thermally active phonon-assisted tunneling from the excited states of low-energy emission band to the excited states of high-energy emission band in the configuration coordinate diagram occurs.^27,28 The thermal quenching of LiSi₂N₃:Eu_0.01 and LiSi₂N₃:Eu_0.01-0.30AlN is at the same rate, as shown in Fig. 11c. This suggests that the solution of AlN is unchanged the local environment of Eu²⁺, which is consistent with the emission spectra.

Fig. 11 The temperature dependence of the emission spectra ofLiSi₂N₃:Eu_0.01-xAlN (a) x = 0, (b) x = 0.30.

4. Conclusions

In summary, solid solutions of nitridosilicate phosphors LiSi₂N₃:Eu-xAlN (0 ≤ x ≤ 0.30) were successfully prepared by gas-pressed sintering. The expansion of the host lattice indicates the formation of LiSi₂N₃:Eu-xAlN solid solutions. With the addition of AlN, the emission intensity of LiSi₂N₃:Eu is significantly improved. The emission intensity of LiSi₂N₃:Eu_0.01-0.30AlN is about 2.17 times than that of LiSi₂N₃:Eu_0.01. The increase in the intensity of the emission due to the addition of AlN can be attributed to the growth of the particle size, the narrowing of the particle-size distribution, and the increase of crystallinity. These results indicate that the formed solid solution could be a useful method to improve the luminescence intensity of phosphors.

Acknowledgements

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

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