Penternary Wurtzitic Nitrides Li1-xZnxGe2-xGaxN3: Powder Synthesis, Crystal Structure, and Potentiality as a Solar-Active Photocatalyst.

We developed a new penternary wurtzitic nitride system Li1-xZnxGe2-xGaxN3 (0 ≤ x ≤ 1) by hybridizing LiGe2N3 and ZnGeGaN3. Fairly stoichiometric fine powder samples were synthesized by the reduction-nitridation process at 900 °C. While the end member LiGe2N3 possessed a relatively large band gap of 4.16 eV, the band gap of the developed penternary system varied in a broad range of 3.81 to 3.10 eV, showing promising responsivity to the solar spectrum. The crystal structure of LiGe2N3 was precisely determined by time-of-flight neutron powder diffraction for the first time, revealing the complete ordering of Li and Ge in the Cmc21 structure. The structural evolution from completely ordered LiGe2N3 to fully disordered ZnGeGaN3 was quantitatively analyzed by Rietveld refinement based on a partially disordered Cmc21 model, and the obtained results were also supported by 71Ga solid-state NMR spectroscopy. The synthesized Li1-xZnxGe2-xGaxN3 powder samples exhibited photocatalytic activities for the water reduction and oxidation reactions under solar light irradiation, with the H2 evolution rate of 0.3-59.0 µmol/h and the O2 evolution rate of 3.1-296.2 µmol/h, depending on the composition. Stable solar hydrogen generation of up to 48 h was demonstrated by the x = 0.80 sample, with the total amount of H2 evolved over 1.6 mmol and an external quantum efficiency of 2.1%.

Quaternary Wurtzitic Nitrides (1 - x)ZnGeN2-2xGaN (x = 0.02, 0.05): Disorder-Induced Band-Gap Narrowing and Potentiality as a Solar-Active Photocatalyst.

We examined the ZnGeN2-GaN solid-solution system (Zn1-xGe1-xGa2xN2) in the unexplored compositional region of x < 0.10 to reveal the transitional structural and optical properties caused by the introduction of Ga. Fairly stoichiometric fine powder specimens with compositions of x = 0.02 and 0.05 were prepared by the gas-reduction-nitridation method, and their partially ordered Pna21 structure was identified by solid-state 71Ga NMR spectroscopy and time-of-flight neutron powder diffraction. The Rietveld refinement results of the neutron diffraction data showed that the introduction of 2 atom % Ga readily retards the cation ordering in ZnGeN2, and this composition-induced transition to the wurtzite disordered phase proceeds mostly in the range of x < 0.10. The synthesized samples showed gradual red shifts of the absorbance and photoluminescence excitation spectra with their x value, consistent with their degree of disorder, indicating that the narrowing of the band gap achieved in the current system results primarily from the disorder of the cation sublattice accompanied by octet-rule violation, as has been predicted theoretically. The test reactions for photocatalytic water splitting resulted in improved H2 evolution rates of 6.1-72.6 µmol/h under UV-visible-light irradiation, and stable solar H2 evolution of up to 5 days was demonstrated.

Down-Conversion Nitride Materials for Solid State Lighting: Recent Advances and Perspectives.

Advances in solid state white lighting technologies witness the explosive development of phosphor materials (down-conversion luminescent materials). A large amount of evidence has demonstrated the revolutionary role of the emerging nitride phosphors in producing superior white light-emitting diodes for lighting and display applications. The structural and compositional versatility together with the unique local coordination environments enable nitride materials to have compelling luminescent properties such as abundant emission colors, controllable photoluminescence spectra, high conversion efficiency, and small thermal quenching/degradation. Here, we summarize the state-of-art progress on this novel family of luminescent materials and discuss the topics of materials discovery, crystal chemistry, structure-related luminescence, temperature-dependent luminescence, and spectral tailoring. We also overview different types of nitride phosphors and their applications in solid state lighting, including general illumination, backlighting, and laser-driven lighting. Finally, the challenges and outlooks in this type of promising down-conversion materials are highlighted.

Constancy of the quadrupolar interaction product in nanocrystalline gallium nitride revealed by 71Ga MAS NMR shift distribution.

The question of whether the broad 71,69Ga nuclear magnetic resonance (NMR) signal of hexagonal gallium nitride (h-GaN) at 530-330 ppm is related to the Knight shift (caused by the presence of carriers in semiconductors) is the subject of intense debate. The intensity increase observed for the narrower 71Ga magic angle spinning (MAS) NMR signals above 1050 °C suggests that the broader signals do not reflect the decomposition of h-GaN. Herein, we utilized 71Ga multi-quantum (MQ) MAS NMR spectroscopy to reveal that the quadrupolar interaction products for the broad signal of nanocrystalline h-GaN are almost constant in the entire shift range that we investigated, equaling 1.7 ±â€¯0.1 MHz or similar values. Since the above parameter is sensitive to the local chemical symmetry around the Ga atom, the NMR shift distribution is considered not to be related to that of the chemical environment. Consistent with the most recent reports, including those on double-resonance 15N{71Ga} measurements, the Knight shift may be ascribed to defects serving as shallow donors and populating the conduction band. Thus, MQMAS measurements performed using a low-field NMR instrument or by choosing half-integer quadrupole nuclei with a large quadrupole constant such as 69Ga are expected to provide important information for each Knight shift value and for analyzing the nature of semiconductors other than GaN.

Na-α'-GeGaON Solid Solution Analogous to α'-SiAlON: Synthesis, Crystal Structure, and Potentiality as a Photocatalyst.

We demonstrated, for the first time, formation of the Na-α'-GeGaON (NamGe12-(m+n)Gam+nOnN16-n) solid solution, an analogue of the well-established α'-SiAlON system. We successfully synthesized single-phase powder samples by reduction-nitridation of Na2CO3-GeO2-Ga2O3, in the solubility range of m ≈ 0.8-1.7 with n ≈ 0.2-0.3. The Rietveld refinement of powder X-ray diffraction data for Na-α'-GeGaON was conducted on the basis of the space group P31c of α'-SiAlON, and the refinement converged with RB = 1.78% and RF = 1.02% for the composition of Na1.26(1)Ge10.50Ga1.50O0.24(1)N15.76(1), indicating reliably the isomorphism between the SiAlON and GeGaON systems. The results of (23)Na solid-state nuclear magnetic resonance (NMR) spectroscopy clearly showed a single peak at the chemical shift of ∼ 16 ppm, further proving the accommodation of Na in the α'-GeGaON matrix with the expected coordination environment. The synthesized Na-α'-GeGaON exhibited stable photocatalytic activity for the evolution of H2 from water under ultraviolet irradiation, which was comparable to that attained by ß-Ge3N4.

Europium(ii)-activated oxonitridosilicate yellow phosphor with excellent quantum efficiency and thermal stability - a robust spectral conversion material for highly efficient and reliable white LEDs.

Knowing the physicochemical properties of a material is of great importance to design and utilize it in a suitable way. In this paper, we conduct a comprehensive survey of photoluminescence spectra, localized cathodoluminescence, temperature-dependent luminescence efficiency, and applications of Eu(2+)-doped Sr0.5Ba0.5Si2O2N2 in solid-state lighting. This phosphor exhibits a broad emission band with a maximum at 560-580 nm and a full-width at half maximum of 92-103 nm upon blue light excitation, whereas a dual-band emission (i.e., 470 nm and 550 nm) is observed under electron beam irradiation due to perhaps the intergrowth of BaSi2O2N2:Eu(2+) and Sr0.5+σBa0.5-σSi2O2N2:Eu(2+) in each phosphor particle. Under 450 nm blue light irradiation, this yellow phosphor exhibits excellent luminescence properties with absorption, internal and external efficiencies of 83.2, 87.7 and 72.6%, respectively. Furthermore, it also possesses high thermal stability, with the quantum efficiency being decreased by only 4.2% at 150 °C and a high quenching temperature of 450 °C. High-efficiency white LEDs using the title phosphor have a luminous efficacy, color temperature and color rendition of ∼120 lm W(-1), 6000 K and 61, respectively, validating its suitability for use in solid-state white lighting.

Local analysis of Eu2+ emission in CaAlSiN3.

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.

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.

A novel and high brightness AlN:Mn2+ red phosphor for field emission displays.

Mn(2+) doped-AlN red phosphors were prepared by the solid-state reaction method. X-ray diffraction, SEM-EDS, photoluminescence and cathodoluminescence were utilized to characterize the prepared phosphor. Under UV light or electron beam excitation, the AlN:Mn(2+) phosphors exhibit a strong red emission centered at 600 nm, which is ascribed to the characteristic (4)T1((4)G)-(6)A1((6)S) transition of Mn(2+). Energy level diagrams were constructed to discuss the photoluminescence and cathodoluminescence processes of the AlN:1% Mn(2+) phosphor. The oxygen-related defects in AlN have great influence on the photoluminescence and cathodoluminescence properties of the AlN:1% Mn(2+) phosphor. The dependence of brightness on accelerating voltage or electric current, the decay behavior of CL intensity under the electron bombardment, and the stability of CIE chromaticity coordinates were investigated in detail. The results indicate that the AlN:Mn(2+) phosphor exhibits a higher brightness, higher color purity, and lower saturation compared to the red Y2O3:Eu(3+) phosphor, which gives the AlN:Mn(2+) phosphor great potential as a red phosphor for full color FEDs.

Synthesis and photoluminescent properties of (La,Ca)3Si6N11:Ce³+ fine powder phosphors for solid-state lighting.

We have developed a new Ce(3+)-activated nitride phosphor, (La,Ca)(3)Si(6)N(11):Ce(3+), using the gas-reduction-nitridation method. The synthesized (La,Ca)(3)Si(6)N(11):Ce(3+) possesses tunable yellow broadband emission with the dominant wavelength of 577-581 nm and the external quantum efficiency up to ∼42%, under an excitation of 450 nm. Precise steady-state and time-resolved photoluminescence analyses revealed that the only one type of Ce(3+) center is active under the blue-light excitation. By combining the synthesized (La,Ca)(3)Si(6)N(11):Ce(3+) phosphors with the 450-nm InGaN chip, a broad range of white light with the correlated color temperatures of ∼2600-3800 K can be created, demonstrating their promising applicability to the warm-white light-emitting diodes.

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.