The backlight technology contributes significantly to the improvements in the image quality and color saturation of liquid crystal displays used for televisions, mobile phones, computers, tablet personal computers(PCs) and car navigators. Due to the small color gamut, low efficiency and environmental issues, the conventional CCFL(cold cathode-fluorescence lamps) backlight units are progressively replaced by emerging technologies such as phosphor-converted white light-emitting diodes(wLEDs) and quantum dots(QDs). The latter promises a thinner, lighter, brighter, and more vivid display^[1-9]. QDs backlights have very wider color gamut(＞100% NTSC, NTSC stands for National Television System Committee, here refers to the full range of color that can theoretically be displayed.), but they have disadvantages of high cost, toxicity, short lifetime, and complex structures. Alternatively, phosphor-converted wLED backlights, which combine a blue LED chip with a single or multiple phosphors, are mostly used due to their large size, cost effectiveness, robustness and high efficiency. In this technology, phosphors are one of key components that make a great influence in the color saturation and brightness of liquid crystal displays(LCDs).

Phosphors for use in wLED backlights have different requirements in emission spectral position, shape, width and decay time from those used in general lighting. Generally, they should have narrow emission band with a steep shape, short green or long red emission, small decay time, and high reliability. As the green emission spectrum is located in between the blue and red ones, the green phosphor is thus extremely important to control the color gamut, color purity and luminous efficacy of the wLED backlights. As seen in Table 1, the potential green phosphors used for wLED backlights are YAG∶Ce³⁺, SrGa₂S₄∶Eu²⁺, Sr₃Si₁₃Al₃O₂N₂₁∶Eu²⁺, and etc. For example, Ito et al^[5] attempted the application of SrGa₂S₄∶Eu²⁺(green) and CaS∶Eu²⁺(red) phosphor sheets in wLED backlights, and obtained a color gamut of 90% NTSC(Commission Internationale de L′Eclairage(CIE) 1976) . Fukuda et al^{[3, 10]} reported a wLED backlight with a color gamut of 94.2% by using the green Sr₃Si₁₃Al₃O₂N₂₁∶Eu²⁺ phosphor. However, SrGa₂S₄∶Eu²⁺ has a big problem of serious luminescence degradation, making them hard to be applied in high-quality LCD displays. In addition, the large emission band width of YAG∶Ce³⁺ and Sr₃Si₁₃Al₃O₂N₂₁∶Eu²⁺ limits them to achieve wide color gamut and high luminous efficacy of the wLED backlights. The small color gamut is dominantly ascribed to the large overlap between the green and the red emission spectra after the wLED emission spectrum passes through RGB(red, green, blue) color filters used in LCDs for balancing the image quality and power consumption. For instance, the color gamut of wLED using YAG∶Ce³⁺ is only 72% NTSC, which is hard to provide clean red and rich picture quality^[1]. To achieve much larger color gamut and higher reliability wLED backlights, narrow green-emitting phosphors with high quantum efficiency and small luminescence degradation over time and temperature are continuously pursued.

表 1

Table 1

表 1(Table 1)

Table 1 Green phosphor candidates for use in wLED backlight Emission maxima/nm Full width at half maximum(FWHM)/nm Stability Ref. YAG∶Ce3+ 535 110 excellent [1] Sr2GaS4∶Eu2+ 540 47 Bad [5] Sr3Si13Al3O2N21∶Eu2+ 525 66 good [10] β-sialon∶Eu2+ 535 55 excellent [11-12]

Table 1 Green phosphor candidates for use in wLED backlight

β-Sialon∶Eu²⁺(sialon:silicon aluminum oxynitride, Si_6-zAl_zO_zN_8-z) is such a green phosphor that exhibits a narrow emission band, steep emission spectrum, very small luminescence degradation and high quantum efficiency. It is thus an interesting phosphor especially suitable for wLED backlights. In this work, the synthesis, crystal structure, luminescence and applications of β-sialon∶Eu²⁺will be overviewed.

1 Synthesis of β-sialon∶Eu²⁺

1.1 High temperature solid state reaction

Solid state reaction is a very popular way to synthesize nitride phosphors including β-sialon∶Eu^2+[11-12]. According to following chemical reaction, the raw materials for preparing β-sialon∶Eu²⁺ phosphors are usually Si₃N₄, AlN, Al₂O₃ and Eu₂O₃. The powder mixtures are fired in a gas-pressure sintering furnace at a high temperature(1900~2050 ℃) and a high gas pressure(≥1.0 MPa N₂).

$\left( 2-z/3 \right)\text{S}{{\text{i}}_{\text{3}}}{{\text{N}}_{\text{4}}}\text{+}\left( z\text{/3} \right)\text{A}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}\to \text{ }\!\!\beta\!\!\text{ -S}{{\text{i}}_{\text{6-}z}}\text{A}{{\text{l}}_{z}}{{\text{N}}_{8-z}}$

(1)

The high temperature solid state reaction method enables to produce well-crystalline and large-size powders that have less surface defects and thus high luminescence efficiency. However, the manufacturing cost is high because the process requires the use of expensive raw materials, a high-temperature and high-pressure furnace and long processing time. Therefore, facile and cost effective synthetic methods for β-sialon∶Eu²⁺ are necessary, and these include combustion synthesis, carbothermal reduction and nitridation, gas reduction and nitridation, and etc.

1.2 Combustion synthesis

Combustion synthesis, also named self-propagating high-temperature synthesis(SHS), takes advantages of the high exothermic heat to complete the chemical reaction^[13]. When a compact of powder mixtures is ignited at one end, the highly exothermic reaction spontaneously propagates through the reactant mixture in a very short time, forming a phase-pure powder material. In the process heating is accomplished by the exothermic heat of the chemical reaction, and once initiated any external energy is not required. The whole processing time is of the order of seconds instead of hours, therefore, this method promises energy efficiency and time saving.

Zhou et al^[14] reported the combustion synthesis of β-sialon∶Eu²⁺ by using high purity Si, Si₃N₄, AlN, Al₂O₃ and Eu₂O₃ as starting powders. A pellet composed of titanium and carbon powders were served as an igniter. The whole process ended in a few minutes. The synthesized phosphor powders exhibited a well-faceted rod-like morphology for small z values(0.25~0.5) , and emitted strong green light under ultraviolet(UV) or blue light irradiation. The chemical reaction of this process is given as below.

$\text{Si+}{{\text{N}}_{\text{2}}}\text{+S}{{\text{i}}_{\text{3}}}{{\text{N}}_{\text{4}}}\text{+AIN+A}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}\text{+Si}{{\text{O}}_{\text{2}}}\text{+E}{{\text{u}}_{\text{2}}}{{\text{O}}_{\text{3}}}\to \text{S}{{\text{i}}_{\text{6-z}}}\text{A}{{\text{l}}_{\text{z}}}{{\text{N}}_{\text{8-z}}}\text{:E}{{\text{u}}^{\text{2+}}}$

(2)

Both Xu et al^[15] and Niu et al^[16] prepared Eu²⁺-doped β-sialon powders by the combustion synthesis using NaCl as a diluent. The starting materials consisted of Si, SiO₂, Al, Eu₂O₃ and NaCl. This new combustion synthesis method offers many advantages, such as low cost of raw materials, uniform particle morphology, and small thermal quenching of the phosphor.

1.3 Carbothermal reduction and nitridation

The carbothermal reduction and nitridation(CRN) method is also a simple and facile route to β-sialon, which usually uses carbon as the reducing agent in the reaction. This method allows to use much cheaper raw materials(i.e., oxides), mix the powders in air, and to react at lower temperatures than the traditional solid state reaction method does. As the residual carbon in the synthesized phosphor is detrimental to photoluminescence properties, the carbon content in the starting powders should be carefully controlled. Otherwise, the as-synthesized phosphor powder needs post-treatments to remove the residual carbon.

Yan et al^[17] synthesized β-sialon∶Eu²⁺ by firing the SBA-15/C/Al₂O₃ composite powders via the carbothermal reduction and nitridation method at 1370~1450 ℃ for 6 h in a nitrogen atmosphere. The high-purity rod-like β-sialon∶Eu²⁺ phosphor powders were obtained at 1420 ℃. The authors found that the particle morphology of the phosphor was equivalent to that of the SBA-15 starting powder.

Jun et al^[18] prepared β-sialon∶Eu²⁺ phosphors by CRN using the mixture of pyrophyllite(Al₂Si₄O₁₀(OH)₂), Si₃N₄, Eu₂O₃, AlN, and carbon black powders at 1500 ℃. The secondary phases like Si₃N₄ and AlN coexisted with the dominant β-sialon∶Eu²⁺. The emission spectrum of the synthesized phosphor showed a broad band centered at 580 nm, indicating that the Eu²⁺ luminescence may not be originated not only from β-sialon∶Eu²⁺ but also from other impurity phases.

Bernardo et al^[19] reported a novel synthetic method to β-sialon∶Eu²⁺ by using a preceramic polymer(KiON) and nano-sized fillers(Al₂O₃, Si₃N₄, and Eu₂O₃). The firing of the powder mixture firing was done for 3 h at 1600 ℃ in N₂, or 1 h at 1500~1700 ℃ in N₂/H₂. The chemical reaction is given as below.

$\begin{align} & \text{3}\text{.5Si}{{\text{C}}_{\text{0}\text{.66}}}{{\text{N}}_{\text{0}\text{.6}}}{{\text{O}}_{\text{0}\text{.03}}}\text{+0}\text{.7S}{{\text{i}}_{\text{3}}}{{\text{N}}_{\text{4}}}\text{+0}\text{.2A}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}\text{+0}\text{.008E}{{\text{u}}_{\text{2}}}{{\text{O}}_{\text{3}}}\text{+1}\text{.34}{{\text{N}}_{\text{2}}}\to \\ & \text{E}{{\text{u}}_{\text{0}\text{.016}}}\text{S}{{\text{i}}_{\text{5}\text{.6}}}\text{A}{{\text{l}}_{\text{0}\text{.4}}}{{\text{O}}_{\text{0}\text{.4}}}{{\text{N}}_{\text{7}\text{.6}}}\text{+0}\text{.33CO+1}\text{.98C}\left[ \text{excess} \right] \\ \end{align}$

(3)

2 Photoluminescence properties of β-sialon∶Eu²⁺

2.1 The Eu²⁺ luminescence

The luminescence of Eu²⁺ in β-sialon was firstly reported by Hirosaki in 2005^[11]. As presented in Fig. 1, the emission spectrum of β-sialon∶Eu²⁺ shows a narrow band with the peak maximum of 535 nm and a full width at half maximum of 55 nm upon 450 nm excitation(Fig. 1(a)). The slim emission band is unusual for Eu²⁺, which is strongly related to its local environment and will be discussed later. The excitation spectrum covers a wide spectral range of 250~500 nm, showing a dominant band centered at 300 nm as well as a second intense band centered at 400 nm(Fig. 1(b)). It thus indicates that β-sialon∶Eu²⁺ can be pumped by both near UV and blue LED chips.

图 1

Fig. 1

Fig. 1 Emission(a), excitation(b) spectra and chromatic coordinates(c) of β-sialon∶Eu2+ with a composition of Eu0.00296Si0.41395Al0.01334O0.0044N0.56528. The emission spectrum was measured under 450 nm irradiation, and the excitation one was obtained by monitoring 535 nm(Reprinted from ref.[11])

β-Sialon∶Eu²⁺ has the absorption, internal and external quantum efficiencies of 66%, 50%, 33%, respectively^[11]. The luminescence properties of the commercial β-sialon∶Eu²⁺ are significantly enhanced, with the external quantum efficiency almost doubled after careful post-treatments. The decay time of β-sialon∶Eu²⁺ was measured to be 1.7 μs. As seen in Fig. 1(c), β-sialon∶Eu²⁺ exhibits a higher color saturation than the well known green-emitting Y₃Al₅O₁₂∶Ce³⁺ and ZnS∶Cu, Al, and close to that of SrGa₂S₄∶Eu²⁺. The narrow emission band and high color purity enable β-sialon∶Eu²⁺ to be a promising green phosphor for use in LED backlights.

图 2

Fig. 2

Fig. 2 CL spectrum(a) and CL mapping of β-sialon∶Eu2+phosphor(b)(Reprinted from reference [11])

It is generally accepted that metal atoms cannot stabilize the β-sialon structure so that β-sialon usually contains no metals other than Al. However, the intense green emission of Eu²⁺ in β-sialon indicates that Eu²⁺ is dissolved in the host lattice. To confirm it, the cathodoluminescence(CL) spectrum and images of β-sialon∶Eu²⁺ phosphor powders were measured and given in Fig. 2. The CL spectrum looks very similar to the photoluminescence(PL) one, showing a single emission band centered at 530 nm(Fig. 2(a))^[11]. The CL mapping of the 530 nm emission clearly reveals that each needle-like β-sialon∶Eu²⁺ phosphor(Fig. 2(b)) particle emits the green light uniformly. These localized luminescence strongly supports that Eu²⁺ is definitely accommodated in the β-sialon crystal lattice.

2.2 Composition-tuning of luminescence

The luminescence of Eu²⁺ in a solid can often be tuned by the composition tailoring^[12]. The composition of β-sialon can be varied by controlling the z value as well as the activator concentration. As seen in Fig. 3, at a fixed Eu²⁺ concentration, the emission spectrum is obviously red-shifted with increasing the z value. For example, the emission maximum shifts from 527 nm(z=0.1) to 548 nm(z=2.0) when the Eu²⁺ molar fraction is 0.3%. This red-shift is much remarkable at low Eu²⁺ molar fraction(i.e., ＜0.1%). The luminescence intensity is also composition dependent. The highest intensity is achieved for the sample with z=1.0 and an optimal Eu²⁺ molar fraction of 0.3%. The increased intensity can be interpreted by the particle coarsening as the z value increases, because a higher z value leads to a larger amount of transient liquid phase that promotes the growth of phosphor particles. This is verified by the particle morphology changing from a needle-like shape to a rounded one with increasing the z value.

图 3

Fig. 3

Fig. 3 Emission maximum(a) and luminescence intensity(b) of β-sialon∶Eu2+ with varying z values and Eu2+ concentrations(Reprinted from reference [12])

Usually, the emission spectrum is red-shifted with increasing the N/O ratio, which can be observed in Ca-α-sialon∶Eu²⁺ or Y-Si-O-N∶Ce³⁺ phosphors^[20-21]. This is attributed to the increased covalence of Eu/Ce-(O, N) chemical bonds. However, it is not the case in β-sialon∶Eu²⁺ where the red-shift occurs with reducing the N/O ratio. We consider that, the local structure and electronic band structure of β-sialon∶Eu²⁺ would be changed with the composition. As the z value increases, Eu atoms are coordinated to more oxygen ones, and the distance of Eu-O is longer than that of Eu-N, resulting in enlarged Eu(O, N)6 polyhedra. In addition, Boyko et al^[22] addressed that the band gap of β-sialon reduces with increasing z value. Therefore, the unusual red-shift can be attributable to (i)increased Stokes shift as the volume of Eu(O, N)6 polyhedra increases; and (ii)reduced band gap of the β-sialon host which may downshift the energy levels of excited states of Eu²⁺.

The blue-shift of the emission spectra of β-sialon∶Eu²⁺ is required to further increase the color saturation, with the purpose to enhance the color gamut of the LED backlights. It thus can be realized by reducing the z value or the oxygen content. Takahashi et al^[23] used Si instead of Si₃N₄ to finely control the oxygen content in β-sialon. As shown in Fig. 4, the emission spectrum becomes narrowed in band width and blue-shifted in the right wing when the oxygen concentration reduces. The mass fraction of oxygen of 0.74%, 0.40% and 0.33% corresponds to the z value of 0.13, 0.07 and 0.06, respectively(Fig. 4(a)). The emission maximum is then reduced from 528(z=0.13) to 525 nm(z=0.06) .

图 4

Fig. 4

Fig. 4 Emission spectra(a), emission maximum and band width(b), and chromatic coordinates(c) of β-sialon∶Eu2+ with reducing z values(or oxygen concentrations)(Reprinted from reference [23])

Besides the spectral position, the band width of the emission spectrum also plays a crucial role in controlling the color saturation of β-sialon∶Eu²⁺. Very interestingly, the reduction in z value also results in the decrease of the band width(Fig. 4b). A very small band width of 47 nm is achieved for β-sialon∶Eu²⁺ with z=0.06. The band narrowing is closely related to the local environment of Eu²⁺ in β-sialon. Takasa et al^[24] reported that the structural disorder of β-sialon was enhanced by increasing the oxygen concentration(i.e., z value). Conversely, by reducing the oxygen content, the structural disorder would be minimized, leading to less spectral fluctuations and hence a narrow emission band.

Both of the blue-shift and narrowing of the emission band improve the color saturation of β-sialon∶Eu²⁺, as seen in Fig. 4(c). This is extremely important to obtain a large color gamut by using short-wavelength β-sialon∶Eu²⁺.

3 Electronic and crystal structure of β-sialon∶Eu²⁺

3.1 Electronic structure

Li et al^[25] investigated the electronic crystal structure of β-sialon∶Eu²⁺(Eu_xSi_6-zAl_z-xO_z+xN_8-z-x, x=0.013 and z=0.15) by using the relativistic discrete variation Xα(DV-Xα) method, as given in Fig. 5. The conduction band is made up of the Si-3s, Si-3p and small partial N-2p orbitals, while the valence band is dominated by the Si-3s orbital. With the Eu incorporation, partial high levels of the Eu-5d orbitals are hybridized with the Si-3s, Si-3p and N-2p ones at the bottom of the conduction band, suggesting that the 5d electrons of Eu²⁺ take part in the formation of chemical bonding. The valence band mainly consists of N-2s, and N-2p hybridized with small amounts of Si-3s and Si-3p distributed in lower energies for Eu_xSi_6-zAl_z-xO_z+xN_8-z-x. The calculated 4f and 5d levels of Eu²⁺ are located in between the top of N-2p and the bottom of Si-3s3p.

图 5

Fig. 5

Fig. 5 Density of states and atomic energy levels of the [EuSi12N30]-40 cluster for β-SiAlON∶Eu2+(Reprinted from reference [25])

Yoo et al^[26] calculated the electronic structure of β-sialon∶Eu²⁺ using the density functional theory(DFT), and got the same conclusions. The energy gap between the valence band maximum and the extra level from Eu²⁺ is reduced with increasing the z value, which is 3.42, 3.12, and 2.34 eV for z=0.25, 0.5 and 1.0, respectively. The decrease in the energy gap validates the red-shift of the emission band of β-sialon∶Eu²⁺ with more Al-O bonds substituting Si-N ones.

3.2 Local structure and the Eu²⁺ location

The location of Eu²⁺ in β-sialon was argued because Eu²⁺ cannot substitute Al³⁺ or Si⁴⁺ due to the very large size mismatch. By using relativistic DV-Xα method, Li et al^[25] proposed that Eu²⁺ occupied the 2b site in a hexagonal unit cell and directly bonded to six nearest nitrogen/oxygen atoms at distances of 0.24850~0.25089 nm. Later, Kimoto et al^[27] directly observed the position of Eu²⁺ which was resided at the large voids along the [001] direction, agreeing well with the simulation results. As seen in Fig. 6(a) and Fig. 6(b), the white spot in the dark field image(arrowed) clearly indicates the location of Eu²⁺ in the hexagonal channel. The DFT calculation of the potential energy surface of β-Si₃N₄, done by Broch et al^[28], suggested that Eu would occupy the interstitial site in the center of the unit cell(see Fig. 6(c)). The extended X-ray absorption fine structure(EXAFS) result also confirmed the location of Eu²⁺ in the large void. A majority of Eu(＞90%) was in the divalent oxidation state, measured by EXAFS.

图 6

Fig. 6

Fig. 6 (Color online)Bright field(a) and dark field STEM images of β-sialon∶Eu2+(b), and the calculated potential energy surface of the (001) plane for β-Si3N4(c)(Reprinted from references [27-28])

In Ca-α-sialon, Eu²⁺ is coordinated to seven nearest nitrogen/oxygen atoms at three different distances^[29]. In β-sialon, however, Eu²⁺ is bonded to six nitrogen/oxygen atoms at an equivalent distance, which indicates a high site symmetry of Eu²⁺. Moreover, the structural disorder due to the random distribution of Si/Al and O/N at the same sites is much less in β-sialon. These results, therefore, lead to a narrower emission band of β-sialon∶Eu²⁺(i.e., 55 nm versus 92 nm for Ca-α-sialon∶Eu²⁺).

4 Reliability of β-sialon∶Eu²⁺

The lifetime of white LEDs for both general illumination and LCD backlights is greatly dependent on the reliability of phosphors. The luminescence degradation is usually caused by thermal, moisture, and/or irradiation attacks. The temperature-dependent luminescence indicated that β-sialon∶Eu²⁺ showed a small thermal quenching and maintained 84%~87% of the initial intensity at 150 ℃. Brgoch et al^[28] calculated the Debye temperature(a proxy for structural rigidity) of β-sialon∶Eu²⁺ using DFT. It showed that the Debye temperature was quite high for β-Si₃N₄(956 K), and reduced with increasing the z value(see Fig. 7a). This reduction is ascribed to phonon mode softening due to chemical disorder and the lower valences of Al³⁺ and O^2- in the crystal lattice.

图 7

Fig. 7

Fig. 7 Calculated Debye temperature as a function of x in β-Si3-xAlxOxN4-x(x=0, 0.5, 1.5, 2 and 2.5) (a), and the emission intensity as functions of exposure time when β-sialon∶Eu2+(b) aged at 50 ℃ and irradiated under intense UV light(365 nm, 120 mW/cm2)(c)(Reprinted from reference [28, 30])

Industrial aging tests were also applied to β-sialon∶Eu²⁺ to evaluate its reliability against thermal, moisture, and blue light irradiation attacks. Under the circumstance of high temperature(85 ℃) and high humidity(85%), the luminescence remains unchanged up to 6000 h^[30]. Moreover, there is no luminescence degradation when β-sialon∶Eu²⁺ is kept at 150 ℃ for 2500 h. Under an intense UV light excitation(365 nm, 120 mW/cm²), the emission intensity is stable up to 8000 h. These results imply that β-sialon∶Eu²⁺ has a very good reliability and is superior to orthosilicate Sr₂SiO₄∶Eu²⁺ and thiogallte SrGa₂S₄∶Eu²⁺ phosphors. The high reliability is attributed to the high structural rigidity of β-sialon∶Eu²⁺, enabling it to be used in electronic devices with a long lifetime.

5 LCD backlight applications

The wLED backlight is superior to the cold cathode fluorescent light(CCFL) counterpart in terms of energy saving, efficiency, color gamut, and environment friendliness. The initial wLED backlight was prepared by using a single yellow-emitting YAG, and the color gamut of it was less than 75% of NTSC^[1]. This small color saturation is obviously not enough for high resolution LCD displays, for example 4K or 8K televisions(TVs). It is therefore necessary to apply multi-phosphors to widen the color gamut. As RGB color filters are commonly used in LCD displays, phosphors are required to have narrow emission bands to minimize the luminescence loss and to enhance the color purity. β-Sialon∶Eu²⁺ is such a green phosphor showing a narrow emission band, and it is thus expected to achieve high brightness and vivid colors.

We used β-sialon∶Eu²⁺(green) and CaAlSiN₃∶Eu²⁺(red) instead of YAG∶Ce³⁺ to prepare wide color gamut wLEDs^[1]. In comparison with wLEDs using YAG∶Ce³⁺(Fig. 8(a)), the wLEDs using the phosphor blend shows a smaller spectral overlap between red and green emission(Fig. 8(b)). It thus leads to higher color purity of the red and green components. The color gamut of wLEDs using β-sialon∶Eu²⁺ and CaAlSiN₃∶Eu²⁺ is 92%(CIE 1976) and 82%(CIE 1931) NTSC, about 15% higher than that using a single YAG∶Ce³⁺. As shown in Table 2, it is also larger than that of wLEDs using Sr₂SiO₄∶Eu²⁺ and CaAlSiN₃∶Eu²⁺(75% NTSC, CIE 1931) .

图 8

Fig. 8

Fig. 8 Electroluminescence spectra of wLEDs using YAG∶Ce3+(a) and β-sialon∶Eu2++CaAlSiN3∶Eu2+(b), and the color-space coverage of wLEDs using different phosphor combinations(c)(Reprinted from reference [1])

图 9

Fig. 9

Fig. 9 Excitation and Emission spectra of K2SiF6∶Mn4+, β-sialon∶Eu2+ and CaAlSiN3∶Eu2+(a); and electroluminescence spectra of wLEDs using β-sialon∶Eu2+ and K2SiF6∶Mn4+ after filtering(b)(Reprinted from reference [1])

Further increase of the color gamut can be realized by using both narrow-band green and red phosphors^[31]. As shown in Fig. 9(a), the narrow-band K₂SiF₆∶Mn⁴⁺ red phosphor is superior to CaAlSiN₃∶Eu²⁺ in terms of (i)sharp line spectra and free of self-absorption enabling high efficiency after filtering; (ii)very low absorption of the green emission from β-sialon∶Eu²⁺; (iii)extremely small spectral overlap between the emission spectra of K₂SiF₆∶Mn⁴⁺ and β-sialon∶Eu²⁺; and (iv)almost no photons at wavelengths ＞700 nm in K₂SiF₆∶Mn⁴⁺. It thus implies that the combination of K₂SiF₆∶Mn⁴⁺ with β-sialon∶Eu²⁺ can produce large color gamut as well as higher efficiency. The emission spectrum of wLEDs using β-sialon∶Eu²⁺ and K₂SiF₆∶Mn⁴⁺ after filtering shows almost no overlaps between the green and red emissions(Fig. 9(b)), and leads to a wider color gamut of 96%(CIE 1976) or 86%(CIE 1931) NTSC(Table 2). The color-space coverage is therefore increased by 4% when K₂SiF₆∶Mn⁴⁺ is used instead of CaAlSiN₃∶Eu²⁺.

表 2

Table 2

表 2(Table 2)

Table 2 Optical properties of phosphor-converted wLEDs for LCD backlights Phosphors Color temperature/K Luminous efficacy/(lm·W-1) Color gamut(% NTSC) Ref. Green Red CIE 1931 CIE 1976 YAG∶Ce3+ 8 000 105 67.9 N/A [7] Sr2SiO4∶Eu2+ CaAlSiN3∶Eu2+ 8 000 103 74.7 N/A [7] β-sialon∶Eu2+ CaAlSiN3∶Eu2+ 8 620 38 82.1 91.9 [1] β-sialon∶Eu2+ K2SiF6∶Mn4+ 8 611 94 85.9 96.2 [23]

Table 2 Optical properties of phosphor-converted wLEDs for LCD backlights

6 Perspectives

β-Sialon∶Eu²⁺ is of great importance to enhance the color gamut, brightness and reliability of LED backlights due to its narrow emission band and high color purity. Differing from other phosphors, β-sialon∶Eu²⁺ is an interstitial type phosphor where Eu²⁺ occupies the large voids of the lattice, making it a very unique luminescent material. To control or optimize its photoluminescence properties, it is necessary to have a deep insight into the structure-property relation. The location of Eu²⁺ in the crystal lattice is clarified, but it still remains unsolved to increase the Eu concentration to further enhance the absorption efficiency. In addition, there is also a big challenge to enhance the quantum efficiency of β-sialon∶Eu²⁺ with low z values.

With rapid developments of display technologies, much higher color gamut is required to achieve high resolution and vivid images. To compete with quantum dots, the green phosphors must have much narrower and blue-shifted emission bands. It thus requires to search for new phosphor hosts with high site symmetry of the activators.

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