As a novel class of inorganic phosphors, oxynitride and nitride luminescent materials have received considerable attention because of their potential applications in solid-state lightings and displays. In this review we focus on recent developments in the preparation, crystal structure, luminescence and applications of silicon-based oxynitride and nitride phosphors for white light-emitting diodes (LEDs). The structures of silicon-based oxynitrides and nitrides (i.e., nitridosilicates, nitridoaluminosilicates, oxonitridosilicates, oxonitridoaluminosilicates, and sialons) are generally built up of networks of crosslinking SiN 4 tetrahedra. This is anticipated to significantly lower the excited state of the 5d electrons of doped rare-earth elements due to large crystal-field splitting and a strong nephelauxetic effect. This enables the silicon-based oxynitride and nitride phosphors to have a broad excitation band extending from the ultraviolet to visible-light range, and thus strongly absorb blue-to-green light. The structural versatility of oxynitride and nitride phosphors makes it possible to attain all the emission colors of blue, green, yellow, and red; thus, they are suitable for use in white LEDs. This novel class of phosphors has demonstrated its superior suitability for use in white LEDs and can be used in bichromatic or multichromatic LEDs with excellent properties of high luminous efficacy, high chromatic stability, a wide range of white light with adjustable correlated color temperatures (CCTs), and brilliant color-rendering properties. r

This letter reports a β-SiAlON:Eu2+ green phosphor with the composition of Eu0.00296Si0.41395Al0.01334O0.0044N0.56528. The phosphor powder exhibits a rod-like morphology with the length of ∼4μm and the diameter of ∼0.5μm. It can be excited efficiently over a broad spectral range between 280 and 480 nm, and has an emission peak at 535 nm with a full width at half maximum of 55 nm. It has a superior color chromaticity of x=0.32 and y=0.64. The internal and external quantum efficiencies of this phosphor is 70% and 61% at λex=303nm, respectively. This newly developed green phosphor has potential applications in phosphor-converted white LEDs.

In this letter, a yellow oxynitride phosphor α-SiAlON with compositions of Ca0.625EuxSi0.75−3xAl1.25+3xOxN16−x (Ca-α-SiAlON:Eu, x=0–25) was prepared by gas pressure sintering. The diffuse reflection spectrum, photoluminescence spectrum, and chromaticity of the powder phosphors were presented. It absorbs light efficiently in the UV–visible spectral region, and shows a single intense broadband emission at 583–603nm. This phosphor may become a good candidate for creating white light, typically warm white light, when coupled to a blue light-emitting diode (λem=450nm).

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Green α-sialon:Yb2+ and red Sr2Si5N8:Eu2+ oxynitride/nitride phosphors have been demonstrated as potential downconversion luminescent materials for white light-emitting diodes (LEDs). In this letter, the authors attempt to fabricate white LEDs by combining α-sialon:Yb2+ and Sr2Si5N8:Eu2+ with a blue LED die and report their optical properties. These two phosphors lend themselves for use in 2-phosphor-converted white LEDs with promising properties: a wide range of tunable correlated color temperature (2700–6700K), acceptable color rendering index (82–83), and luminous efficacy (17–23lm∕W). These LEDs are acceptable for general lighting.

The crystal structure and photoluminescence properties of undoped and Ce3+-doped CaAlSiN3 as well as the application of white-light LEDs are reported. CaAlSiN3 and CaAlSiN3:Ce3+ have been synthesized, starting from Ca3N2, AlN, Si3N4, and CeN or CeO2 with and without Li3N, by a solid state reaction at 1700 °C for 4 h under high purity nitrogen atmosphere. Instead of an ideal CaAlSiN3, a more appropriate formula is proposed to be CaAl1−4δ/3Si1+δN3 (δ ≈ 0.3−0.4) with an Al/Si ratio of about 1:2 on the basis of the bond valence sum calculations, in which Al/Si is disorderly occupied on the 8b site within Cmc21 space group. Ce3+ can be incorporated into the host lattice of CaAlSiN3, and the estimated maximum solubility of Ce3+ is about x = 0.02 (e.g., 2.0 mol % with respect to Ca) of Ca1−2x Ce x Li x AlSiN3. CaAlSiN3:Ce3+ can be efficiently excited by blue light (450−480 nm) and yields yellow-orange emission with a broadband peaking in the range of 570−603 nm, originating from the 5d1 → 4f1 transition of Ce3+. With an increase of Ce concentration, the emission band of Ce3+ shifts to longer wavelengths due to the increased Stokes shift corresponding to structural relaxation and energy transfer of Ce3+. Upon excitation in blue light range (450−480 nm), the absorption and external quantum efficiency are about 70% and 56%, respectively, for both Ca1−2x CexLixAlSiN3 and Ca1−x CexAlSiN3−2x/3O3x/2 at x = 0.01. In addition, Ca1−2x Ce x Li x AlSiN3 and Ca1−x Ce x AlSiN3−2x/3O3x/2 show high thermal stability in air with the quenching temperature above 300 °C for x = 0.01. Using single CaAlSiN3:Ce3+ as the wavelength conversion phosphor combined with a blue InGaN LED-chip (450 nm), warm white-light LEDs can be generated, yielding the luminous efficacy of about 50 lm/W at color temperature 3722 K and the color rendering index (Ra) of 70, which demonstrates that CaAlSiN3:Ce3+ is a highly promising yellow-orange phosphor for use in white-light LEDs.

New interesting R-SiAlON:Eu-based yellow oxynitride phosphors with the formula (Ca x Eu y )Si 12-2x-3y Al 2x+3y O y N 16-y (x ) 0.2-2.2, y ) 0-0.25) have been prepared, and the effects of the activator concentration and the overall composition of the host lattice on optical properties have been investigated. It has been demonstrated that the Eu 2+ -doped R-SiAlON phosphors absorb strongly in the UV-visible part of the electromagnetic spectrum and exhibit an intense emission band with peaks at 583-605 nm. We have shown that the optical properties depend greatly on the Eu 2+ concentration and the overall composition of the host lattice. The results indicate a concentration quenching effect at y g 0.075 and a systematic red-shift in emission wavelength as the Ca 2+ or the Eu 2+ concentration increases.

Rare-earth-doped oxynitride or nitride compounds have been reported to be luminescent and may then serve as new phosphors with good thermal and chemical stabilities. In this work, we report the photoluminescence (PL) spectra of europium-, terbium-, and praseodymium-doped Ca-␣-SiAlON ceramics. The highly dense ceramics were prepared by hot pressing at 1750°C for 1 h under 20 MPa in a nitrogen atmosphere. Europium-doped Ca-␣-SiAlON displayed a single broad emission band peaking at ‫؍‬ 550 -590 nm depending on the europium concentration. The emission bands in the spectra of europium-doped Ca-␣-SiAlONs were assigned to the allowed transition of Eu 2؉ from the lowest crystal field component of 4f 6 5d to 8 S 7/2 (4f 7 ) ground-state level. The emission spectra of terbium-and praseodymium-doped Ca-␣-SiAlON ceramics both consisted of several sharp lines, which were attributed to the 5 D 4 3 7 F j (j ‫؍‬ 3, 4, 5, 6) transitions of Tb 3؉ and 3 P 0 3 3 H j (j ‫؍‬ 3, 4, 5) transitions of Pr 3؉ , respectively. In particular, the terbium-doped ␣-SiAlON ceramics showed a strong green emission among these phosphors.