Naoto Hirosaki

Silicon-based oxynitride and nitride phosphors for white LEDs—A review

Abstract 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 SiN4 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.

Luminescence Properties of a Red Phosphor, CaAlSiN3 : Eu2 + , for White Light-Emitting Diodes

We developed a new red phosphor, CaAlSiN 3 :Eu 2+ , which has a broad excitation band extended from UV region to 590 nm. At the optimum Eu concentration of 1.6 mol %, quantum output is seven times higher than for a conventional red phosphor, La 2 O 2 S:Eu 3+ , under 405 nm excitation. The phosphor is also more efficient than CaSiN 2 :Eu 2+ or Ca 2 Si 5 N 8 :Eu 2+ at any excitation wavelength. One reason of the high room-temperature efficiency is small thermal quenching, which is probably related to a rigid network of [SiN 4 ] and [AlN 4 ] tetrahedra. The phosphor is chemically stable as well. Accordingly, it is a promising material for warm-white light-emitting diodes.

Characterization and properties of green-emitting β-SiAlON:Eu2+ powder phosphors for white light-emitting diodes

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.

Eu2+-doped Ca-α-SiAlON: A yellow phosphor for white light-emitting diodes

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).

2-phosphor-converted white light-emitting diodes using oxynitride/nitride phosphors

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.

Wavelength-tunable and thermally stable Li-α-sialon:Eu2+ oxynitride phosphors for white light-emitting diodes

Eu2+-activated Li-α-sialon is a promising yellow phosphor for white light-emitting diodes (LEDs). This letter reports that the emission of Eu2+ in Li-α-sialon can be tuned widely (563–586nm) by tailoring the composition or controlling the Eu2+ concentration. The thermal stability of Li-α-sialon:Eu2+, relying a little on the composition and the Eu2+ concentration, remains high in a wide temperature range (25–200°C). Moreover, the chromaticity of Li-α-sialon:Eu2+ does not shift with changes in temperature. Using a single Li-α-sialon:Eu2+ phosphor, highly efficient white LEDs (46–55lm∕W) with different color temperatures (3000–5200K) can be fabricated.

Extrahigh color rendering white light-emitting diode lamps using oxynitride and nitride phosphors excited by blue light-emitting diode

The blue-light-excitation-type white light-emitting diode (LED) lamps are considered to be very suitable for lighting for art objects, shop window displays, and medical applications because they do not give infrared ray and ultraviolet ray. But their color rendering indices are needed to be improved for such applications. In this letter, the authors have fabricated white LED lamps with a broad range of color temperatures, and realized extrahigh color rendering index Ra values of 95–98 in them, using four oxynitride/nitride phosphors and a blue LED die. It means UV LED die is not always necessary for high color rendering white LED lamps. The luminous efficacies of white LED lamps are 28–35lm∕W, which are sufficiently high for extremely high color rendering white LED lamps.

Warm-white light-emitting diode with yellowish orange SiAlON ceramic phosphor

A warm-white light-emitting diode (LED) without blending of different kinds of phosphors is demonstrated. An approach that consists of a blue LED chip and a wavelength-conversion phosphor is carried out. The phosphor is a newly developed yellowish orange CaEuSiAlON ceramic phosphor with high efficiency. The CIE1931 chromaticity coordinates (x, y) are (0.458, 0.414), the color temperature is 2750 K, and the luminous efficacy of this LED is 25.9 lm/W at room temperature and with a forward-bias current of 20 mA. The chromaticity of the assembled LED is more thermally stable than that of a LED with a conventional oxide phosphor (YAG:Ce) because of the better thermal stability of the oxynitride phosphor.

Blue-emitting AlN: Eu2+ nitride phosphor for field emission displays

An Eu2+-activated AlN phosphor was synthesized by firing the powder mixture of AlN, α-Si3N4, and Eu2O3 at 2050°C for 4h under 1.0MPa N2. This nitride phosphor emits a strong blue color with the chromaticity coordinates of x=0.139 and y=0.106 at an accelerating voltage of 3kV. The cathodoluminescence properties of AlN:Eu2+ was evaluated by utilizing it in the Spindt-type field emission display panel. It shows that the nitride phosphor exhibits higher brightness, higher color purity, lower saturation, and longer lifetime than the currently used Y2SiO5:Ce3+, indicative of the suitability of the AlN:Eu2+ blue phosphor in field emission displays.

Piezoelectric Properties of Spark-Plasma-Sintered (Na0.5K0.5)NbO3–PbTiO3 Ceramics

In the search for high-performance piezoelectrics, (1-y)(Na0.5K0.5)NbO3–yPbTiO3 (y≤0.50) solid solution has been prepared using the spark-plasma-sintering method. Dielectric investigation reveals that both the ferroelectric phase transition temperatures and the peak value of the dielectric constant are strongly suppressed in the low y range. However, improved electromechanical coupling constants are observed at the same y range. These properties are explained by considering factors such as composition, crystal structure, ferroelectric domain microstructure, and poling conditions.