Kohsei Takahashi

Local analysis of Eu2+ emission in CaAlSiN3

Abstract We have investigated the local luminescence properties of Eu-doped CaAlSiN3 by using low-energy electron beam (e-beam) techniques. The particles yield broad emission centered at 655 nm with a shoulder at higher wavelength under light excitation, and a broad band around 643 nm with a tail at 540 nm under e-beam excitation. Using cathodoluminescence (CL) in a scanning electron microscope (SEM), we have observed small and large particles, which, although with different compositions, exhibit Eu2+-related emissions at 645 and 635 nm, respectively. Local CL measurements reveal that the Eu2+ emission may actually consist of several bands. In addition to the red broad band, regularly spaced sharp peaks have been occasionally observed. These luminescence variations may originate from a variation in the composition inside CaAlSiN3.

Visible tunable lighting system based on polymer composites embedding ZnO and metallic clusters: from colloids to thin films

Abstract The development of phosphor devices free of heavy metal or rare earth elements is an important issue for environmental reasons and energy efficiency. Different mixtures of ZnO nanocrystals with Cs2Mo6I8(OOC2F5)6 cluster compound (CMIF) dispersed into polyvinylpyrrolidone matrix have been prepared by very simple and low cost solution chemistry. The resulting solutions have been used to fabricate highly transparent and luminescent films by dip coating free of heavy metal or rare earth elements. The luminescence properties of solution and dip-coated films were investigated. The luminescence of such a system is strongly dependent on the ratios between ZnO and CMIF amounts, the excitation wavelength and the nature of the system. By varying these two parameters (ratio and wavelength), a large variety of colors, from blue to red as well as white, can be achieved. In addition, differences in the luminescence properties have been observed between solutions and thin films as well as changes of CMIF emission band maximum wavelength. This may suggest some possible interactions between the different luminophore centers, such as energy transfer or ligands exchange on the Mo6 clusters. Graphical Abstract

Ce-Doped La3Si6.5Al1.5N9.5O5.5, a Rare Highly Efficient Blue-Emitting Phosphor at Short Wavelength toward High Color Rendering White LED Application

Phase pure nondoped and Ce doped La3Si6.5Al1.5N9.5O5.5 (Al containing La N-phase) samples have been obtained by solid-state reaction synthesis for the first time. 1% Ce-doped La3Si6.5Al1.5N9.5O5.5 phosphor displays a broad excitation band ranging from UV to 410 nm, with a maximum at 355 nm. UV light excitation results in a narrow Ce3+ 5d-4f emission band (fwhm = 68 nm) centered at 418 nm. The emission can be tuned from 417 nm at 0.5% Ce to 450 nm at 50% Ce. A high internal quantum efficiency up to 84% is achieved for a 1% Ce doped sample, which has CIE chromaticity coordinates of x = 0.157 and y = 0.069, close to the NTSC blue standard (x = 0.155; y = 0.070). Compared to La3Si8O4N11:Ce phosphor, the quantum efficiency and thermal stability have been enhanced for La3Si6.5Al1.5N9.5O5.5:Ce phosphor without shifting the emission peak wavelength. La3Si6.5Al1.5N9.5O5.5:Ce shows less thermal quenching than La3Si8O4N11:Ce and no shift or change in the shape of emission spectra with increasing the temperature from 4 to 573 K. These results show that La3Si6.5Al1.5N9.5O5.5:Ce is more efficient than any other (oxy-)nitride phosphor with an emission in the short wavelength blue region (400-450 nm). A white LED was fabricated using the La3Si6.5Al1.5N9.5O5.5:5%Ce as a blue phosphor. The high color rendering index (Ra = 93.2, R9 = 91.4, and R12 = 89.5) obtained shows that the phosphor is a very promising conversion phosphor for white LEDs.

Synthesis and photoluminescence properties of a phase pure green-emitting Eu doped JEM sialon (LaSi6−zAl1+zN10−zOz, z ∼ 1) phosphor with a large red-shift of emission and unusual thermal quenching behavior

A green-emitting phosphor, Eu2+ doped LaAl(Si6−zAlz)(N10−zOz) (denoted as JEM, z ∼ 1), was synthesized via a solid-state reaction method. By the careful control of the sintering temperature and chemical composition of starting materials (z value, N/O ratio and Si/Al ratio), a single phase JEM phosphor was achieved for the first time. The JEM:Eu2+ phosphor displays a broad 4f–5d excitation band from the UV region up to 525 nm, with a maximum at 355 nm. Excitation at 355 nm results in a Eu2+ 5d–4f emission band, that shows a large red-shift (490 → 564 nm) and a change in shape with increasing Eu concentration. The thermal quenching behavior of JEM:Eu2+ has been investigated in the temperature range from 4 K to 573 K. An abnormal nearly linear relation is observed between temperature and luminescence intensity. These phenomena indicate that there are two types of Eu2+ sites, although only one crystallographic site can be found in the JEM structure. A non-linear decay behavior with a bi-exponential curve and time-resolved photoluminescence emission spectra with two distinctive Gaussian peaks further give strong evidence for two emission sites in the Eu2+ doped JEM phosphor.

Optical properties of solid-state laser lighting devices using SiAlON phosphor–glass composite films as wavelength converters

In this work, SiAlON phosphor–glass films were investigated as wavelength converters in solid-state laser lighting. The phosphor–glass composite films were prepared by dispersing phosphor powders into a silica precursor solution and sintering at 500 °C. Both simulation and experiment were carried out to evaluate the optical properties of solid-state lighting devices using SiAlON:Eu or YAG:Ce–glass films. The device using SiAlON:Eu phosphors initially has lower brightness than that of the device using YAG:Ce at lower laser powers, but the latter has an illuminance saturation at 1000 lx whereas the SiAlON-based device is free of saturation even at higher laser powers. The device using SiAlON phosphor–glass composite films has a maximum illuminance 15% higher than that of the device using YAG when the temperature exceeds 250 °C. These better optical properties are ascribed to the higher thermal stability of SiAlON phosphors that are able to achieve high luminance and thermally robust solid-state lighting.

Influence of Si on the particle growth of AlN ceramics

Aluminum nitride (AlN) powders doped with various concentrations of Si were investigated to understand the effects of Si on the growth and luminescence of AlN. Certain Si-doping concentrations enhanced the particle growth of AlN and affected the UV emission bands at 280 and 350 nm. Cross-sectional cathodoluminescence images showed that SiNx coated AlN particles on the surface and formed grain boundaries. These grain boundaries may play a critical role in the solid-state reaction and crystal growth of AlN.

Mo6 cluster-based compounds for energy conversion applications: comparative study of photoluminescence and cathodoluminescence

Abstract We report the photoluminescence (PL) and cathodoluminescence (CL) properties of face-capped [Mo6Xi8La6]2− (X = Cl, Br, I; L = organic or inorganic ligands) cluster units. We show that the emission of Mo6 metal atom clusters depends not only on the nature of X and L ligands bound to the cluster and counter-cations, but also on the excitation source. Seven members of the AxMo6Xi8La6 series (A = Cs+, (n-C4H9)4N+, NH4+) were selected to evaluate the influence of counter-cations and ligands on de-excitation mechanisms responsible for multicomponent emission of cluster units. This study evaluates the ageing of each member of the series, which is crucial for further energy conversion applications (photovoltaic, lighting, water splitting, etc.).

Optical properties of green-emitting β-sialon:Eu phosphor-containing silica glasses and their deterioration mechanism

β-sialon:Eu phosphor-containing silica glass was synthesized as a wavelength converter in solid-state lighting. The optical properties and pore density of the sample strongly depended on the sintering temperature. Sintering at a high temperature of 1050 °C led to a severe deterioration of β-sialon:Eu phosphor owing to N2 evaporation, whereas a high conversion efficiency could be achieved by lowering the sintering temperature down to 600 °C. The β-sialon:Eu phosphor-containing silica glass sintered at 600 °C showed a high durability against high-power excitation, making it possible to be used in high-luminosity and high-power solid-state laser lighting.

Achieving superwide-color-gamut display by using narrow-band green-emitting γ-AlON:Mn,Mg phosphor

The display backlight generated using sharp β-sialon:Eu (green) and K2SiF6:Mn (red) phosphors shows a very wide color gamut that mostly covers the whole National Television System Committee (NTSC) triangle. In this work, an alternative green phosphor is investigated to further improve the display color gamut. γ-AlON:Mn,Mg is a green phosphor that shows a shorter and narrower emission spectrum than sharp β-silaon:Eu. The display color gamut in the blue-green region is thus widened greatly by substituting γ-AlON:Mn,Mg for sharp β-sialon:Eu. The color gamut of displays with γ-AlON:Mn,Mg and K2SiF6:Mn exceeds 100% of the NTSC standard both in the CIE 1931 and the CIE 1976 color spaces. Furthermore, the white LEDs with γ-AlON:Mn,Mg have excellent stability that is comparable to those of LEDs with sharp β-sialon:Eu.

Low-energy Cathodoluminescence for (Oxy)Nitride Phosphors

Nitride and oxynitride (Sialon) phosphors are good candidates for the ultraviolet and visible emission applications. High performance, good stability and flexibility of their emission properties can be achieved by controlling their composition and dopants. However, a lot of work is still required to improve their properties and to reduce the production cost. A possible approach is to correlate the luminescence properties of the Sialon particles with their local structural and chemical environment in order to optimize their growth parameters and find novel phosphors. For such a purpose, the low-voltage cathodoluminescence (CL) microscopy is a powerful technique. The use of electron as an excitation source allows detecting most of the luminescence centers, revealing their luminescence distribution spatially and in depth, directly comparing CL results with the other electron-based techniques, and investigating the stability of their luminescence properties under stress. Such advantages for phosphors characterization will be highlighted through examples of investigation on several Sialon phosphors by low-energy CL.