Down-Conversion Nitride Materials for Solid State Lighting: Recent

Review pubs.acs.org/CR

Cite This: Chem. Rev. 2018, 118, 1951−2009

Down-Conversion Nitride Materials for Solid State Lighting: Recent Advances and Perspectives Le Wang,† Rong-Jun Xie,*,‡ Takayuki Suehiro,§ Takashi Takeda,§ and Naoto Hirosaki§ †

College of Optical and Electronic Technology, China Jiliang University, Hangzhou, Zhejiang 310018, China College of Materials, Xiamen University, Simingnan-Road 422, Xiamen 361005, China § Sialon Group, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡

ABSTRACT: 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.

CONTENTS 1. Introduction 2. Selection of Down-Conversion Luminescent Materials for Solid State Lighting 2.1. Phosphor-Converted White LEDs 2.2. Selection Criteria for Luminescent Materials 2.2.1. Strong Absorption of the LED Light 2.2.2. Suitable Emission Spectrum 2.2.3. High Quantum Efficiency 2.2.4. Small Thermal Quenching/Degradation 2.2.5. High Chemical Stability 2.2.6. Small Luminance Saturation 2.3. Current Status of LED Luminescent Materials 2.4. Choice of Luminescent Nitride Materials 2.4.1. Red-Shifted Excitation and Emission Spectra 2.4.2. Small Stokes Shifts 2.4.3. High Thermal Stability 2.4.4. Abundant Emission Colors 3. Survey of Different Types of Down-Conversion Nitride Materials 3.1. Eu2+-Doped Down-Conversion Nitride Materials 3.1.1. Binary Nitride 3.1.2. Nitridosilicate 3.1.3. Nitridogallate 3.1.4. Nitridoaluminosilicate 3.1.5. Nitridomagnesosilicate 3.1.6. Nitridomagnesoaluminate 3.1.7. Nitridomagnesogallate 3.1.8. Nitridolithoaluminate © 2018 American Chemical Society

3.1.9. Nitridolithosilicate 3.1.10. Nitridolithoalumosilicate 3.1.11. Nitridoborate 3.1.12. Oxonitridosilicate 3.1.13. Oxonitridoalumosilicate 3.1.14. Carbidonitride 3.1.15. Carbodiimide 3.1.16. Nitridoborosilicate 3.1.17. (Oxo)nitridophosphate 3.2. Ce3+-Doped Down-Conversion Nitride Materials 3.2.1. Aluminum Nitride 3.2.2. Nitridosilicate 3.2.3. Oxonitridosilicate 3.2.4. Nitridoaluminosilicate 3.2.5. Oxonitridoaluminosilicate 3.2.6. Nitridomagnesosilicate 3.2.7. Nitridomagnesoaluminate 3.2.8. Nitridolithoalumosilicate 3.2.9. Carbidonitride 3.2.10. Carbodiimide 3.3. Down-Conversion Nitride Materials Doped with Other Activators 3.4. Activator-Free Down-Conversion Nitride Materials 3.4.1. GaZnON 3.4.2. g-C3N4 3.4.3. BCNO

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Received: May 21, 2017 Published: February 2, 2018 1951

DOI: 10.1021/acs.chemrev.7b00284 Chem. Rev. 2018, 118, 1951−2009

Chemical Reviews 3.4.4. Nitridoborate 4. Discovery of New Down-Conversion Nitride Materials 4.1. Discovery of New Materials with Known Crystal Structures 4.1.1. Solid State Combinatorial Chemistry 4.1.2. Incorporation of Nitrogen into Oxides 4.1.3. Chemical Unit Substitution 4.2. Discovery of New Materials with Unknown Crystal Structures 4.2.1. Single Crystal Growth and Structure Determination 4.2.2. Solid State Combinatorial Chemistry 4.2.3. Single-Particle-Diagnosis Approach 5. Crystal Structure of Down-Conversion Nitride Materials 5.1. Two-Dimensional Layer Structures 5.2. Three-Dimensional Structures with Corner (Vertex)-Sharing Tetrahedra 5.2.1. Nitridosilicate 5.2.2. Nitridoaluminosilicate 5.2.3. Oxonitridoalumosilicate 5.2.4. Nitridophosphate 5.3. Three-Dimensional Structures with Edgeand Corner-Sharing Tetrahedra 5.3.1. Nitridogallate 5.3.2. Nitridoaluminosilicate 5.3.3. Nitridomagnesosilicate 5.3.4. Nitridolithoalumosilicate 5.3.5. Oxonitridoaluminosilicate 5.4. Three-Dimensional Structures with Isolated Tetrahedra 6. Structure-Related Luminescence and Spectral Tuning of Down-Conversion Nitride Materials 6.1. Structure-Related Spectral Position of the Emission Band 6.1.1. Effect of the Structure Type 6.1.2. Effect of the Cation Size 6.1.3. Effect of the Anion Type 6.2. Structure-Related Spectral Width of the Emission Band 6.2.1. Narrow-Band Nitride Phosphors 6.2.2. Broad-Band Nitride Phosphors 6.3. Spectral Tuning of Down-Conversion Nitride Materials 6.3.1. Cationic Substitution 6.3.2. Anionic Substitution 6.3.3. Chemical Unit Substitution 6.3.4. Energy Transfer Strategy 7. Temperature-Dependent Luminescence of Down-Conversion Nitride Materials 7.1. Thermal Quenching 7.1.1. Effect of Activator Concentration 7.1.2. Effect of Activator Species 7.1.3. Effect of the Chemical Composition 7.2. Thermal Quenching Mechanisms 7.3. Thermal Degradation 7.4. Thermal Degradation Mechanisms 7.5. Strategies for Enhancing Thermal Stability 8. Applications of Down-Conversion Nitride Materials in Solid State Lighting 8.1. Blue LED-Driven White LEDs 8.1.1. One-Phosphor-Converted White LEDs

Review

8.1.2. Two-Phosphor-Converted White LEDs 8.1.3. Multiple-Phosphor-Converted White LEDs 8.2. Near UV-LED-Driven White LEDs 8.3. High Density Blue Light-Driven White LEDs/ LDs 8.3.1. White LEDs Using Phosphor-in-Glass (PiG) 8.3.2. White LEDs using Phosphor Ceramics 8.3.3. Blue LD-Driven Laser Lighting 8.4. Wide Color Gamut Backlights for LCDs 8.4.1. White LED Backlights Using a NarrowBand Green Phosphor 8.4.2. White LED Backlights Using Both Narrow-Band Green and Red phosphors 9. Summary and Outlook Author Information Corresponding Author ORCID Notes Biographies Acknowledgments List of Acronyms and Abbreviations References

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1. INTRODUCTION Along with the progress of human civilization, the lighting technology has gone through a diversified process of development. The current lighting in the public domain is dominated by conventional incandescent filament bulbs or fluorescent tubes, and the lighting technology lasts for more than 100 years. Although most people are comfortable with them, the cost, efficiency and environmental concern of traditional lighting is now being questioned more broadly because people become more aware of the alternative ways of lighting. In 2014, the Nobel Prize in Physics was awarded jointly to Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura “for the invention of efficient blue light-emitting diodes (LEDs) which has enabled bright and energy-saving white light sources”. InGaN-based white LEDs (wLEDs) were therefore debuted and appeared as a new generation and alternative of solid state lighting (SSL) after highly efficient InGaN-based blue LEDs were invented in 1993, which are now ushering in the next lighting revolution and offering many opportunities not only to give people much visual and aesthetic enjoyment but also to make substantial energy savings and a significant reduction in environmental impact over conventional power generation.1−4 There are several types of wLEDs using InGaN- or GaN-based semiconductor materials and technologies. The first one is multichip wLEDs by combining three individual red, green, and blue LED chips.5−7 The multichip wLEDs exhibit high luminous efficiency and color rendering/color gamut but have disadvantages of high cost, complex drive circuits, and the low wall-plug efficiency of green LEDs. The second one is named as phosphorconverted wLEDs (pc-wLEDs), which is composed of phosphors as down-conversion luminescent materials and blue- or ultraviolet InGaN LED chip as a primary light source.3−5,8−12 Although the pc-wLEDs suffer from large Stokes loss (due to wavelength down conversion), and reliability issues (aging of phosphors and packaging materials), they have become the mainstream in the market as they promise simple structure, high luminous efficiency, cost effectiveness, and spectral design

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DOI: 10.1021/acs.chemrev.7b00284 Chem. Rev. 2018, 118, 1951−2009

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enhance the color rendering of wLEDs and failed to find alternative red color converters in traditional material systems, nitridosilicate (e.g., Sr2Si5N8:Eu2+) and nitridoaluminosilicate (e.g., CaAlSiN3:Eu2+) phosphors with excellent photoluminescence properties and reliability appeared timely.33,34 When one complained that the emission of CaAlSiN3:Eu2+ covered a large part of spectral component longer than 700 nm, the nitridolithoaluminate phosphor (Sr[LiAl3N4]:Eu2+) debuted as a narrow-band red down-conversion material with less NIR emission, which enables the increase of luminous efficiency of wLEDs by 14%.35 In addition, when people were not satisfied with the green orthosilicate phosphor (Sr,Ba)2SiO4:Eu2+ for use in wide color gamut backlights due to its low reliability and color saturation,36,37 the narrow-band oxynitride phosphor βsialon:Eu2+ with excellent stability against moisture and thermal attacks was discovered and was soon used as a representative green color converter in wLED backlights.38,39 This review starts with the guidelines on luminescent material requirements for SSL, followed by the survey of different types of down-conversion nitride materials. Then, we highlight the methodologies for materials discovery of nitrides with new crystal structures and luminescence properties and overview the crystal structure and its relationship with luminescent properties. Following this, we discuss the spectral tuning and thermal stability (thermal quenching and thermal degradation) of nitride phosphors. Finally, we sum up their practical applications in InGaN-based SSL for general illumination and liquid crystal display (LCD) backlights.

flexibility. The third one is monolithic phosphor-free wLEDs based on InGaN nanostructures, such as InGaN/GaN multiple quantum wells (MQWs), InGaN quantum dots (QDs), nanowires, nanorods, nanopyramids, etc.13−22 Compared to the former two, the third type of wLEDs is highly efficient, costeffective and highly reliable, but it is involved in complicated materials growth conditions, tricky device fabrication process, and dependency of color coordinates on injection current. In this review, we concentrate solely on nitride phosphor materials used in the second type of wLEDs. Although there are III−V nitride semiconductor materials (i.e., GaN, InN, AlN, InGaN, InAlGaN) applied in the first and third types of wLEDs, they are usually not considered as down-conversion materials for solid state lighting. Therefore, these nitrides are beyond the scope of this article and will not be covered and discussed. As mentioned above, phosphors (inorganic luminescent materials), also named as color/light converters and downconversion luminescent materials, are recognized as one of the key components in wLEDs, which convert the ultraviolet or blue light emission from InGaN- or GaN-based chips into appropriate visible light.3−5,8−12 The mixture of the transmitted blue and converted visible light (green, yellow, or red) mimics white light to human eyes. The first commercialized wLED was manufactured in 1996 by combining a blue LED chip and a yellowemitting cerium-doped yttrium aluminum garnet (YAG:Ce) phosphor.12 Although luminescent materials have already been utilized in lighting and display technologies (i.e., fluorescent lamps, backlight, electroluminescence displays, field emission displays, plasma display panels, etc.) for a long period of time and are now still playing an indispensable role in those applications,23−25 they are required differently in many aspects when used for wLEDs, such as spectral position and shape, conversion efficiency, luminance saturation, thermal quenching, reliability, refractive index, and form of usage.3 The traditional luminescent materials used for fluorescent lamps or CRTs are thus hardly used for wLEDs, as they have quite a low conversion efficiency of blue light and some reliability problems. The newly developed wLED technologies therefore trigger the research and development of new luminescent materials and material systems. Up to date, a huge number of down-conversion luminescent materials have been explored and suggested for use in InGaNbased SSL. They have a broad range of material systems, including quantum dots (semiconductor nanocrystals, lead halide perovskites), metal−organic frameworks (MOFs), fluorides, aluminates, silicates, phosphates, halophosphates, vanadates, molybdates, tungstates, oxynitrides, nitrides, oxysulfides, sulfides, etc.4−9,26−30 Several review papers have already been devoted to down-conversion luminescent materials, covering a variety of topics like material compositions, selection rules, luminescence tuning, structural design, and applications.3−5,8−11 In these reviews, nearly all kinds of luminescent materials used for wLEDs are broadly reported, especially the oxide phosphors. On the other hand, down-conversion nitride material are either sketchily described or totally omitted. Recently, nitride phosphors with new crystal structures and promising properties are emerging in an endless stream, making it quite necessary to provide a comprehensive overview of this interesting family of luminescent materials. Although nitride phosphors have come into view for about 20 years,31,33 they always bring us a pleasant surprise and play critical roles in SSL, keeping scientists encouraged to explore and discover new materials. For example, when people felt disappointed with the moisture-sensitive (Ca,Sr)S:Eu2+ used to

2. SELECTION OF DOWN-CONVERSION LUMINESCENT MATERIALS FOR SOLID STATE LIGHTING 2.1. Phosphor-Converted White LEDs

As aforementioned, InGaN-based SSL is an emerging green lighting technology that promises to save huge energy and significantly reduce carbon emissions from fossil fuel consumption.1−5 The ultimate goal of SSL is therefore to replace conventional incandescent and fluorescent lamps for general illumination. Besides this, it is also proposed to substitute traditional cold cathode fluorescent lamp (CCFL) backlights, xenon/halogen headlights for vehicles, and to replace ceiling surgical halogen lamps for surgical procedures. Although there are tremendous efforts devoted to monolithic phosphor-free wLEDs, pc-wLED are superior in luminous efficiency, flexible design of spectra, cost and mass production. Now, pc-wLEDs are everywhere in our daily life. As shown in Figure 1, there are three types of pc-wLEDs. The first type is the simplest one, which combines a blue LED chip with a yellow phosphor (usually Ce3+-activated yttrium aluminum garnet, Y3Al5O12:Ce3+). As the red component is not sufficient in the emission spectra, the color rendering index (CRI) of this type wLEDs is usually smaller than 75, which is not accepted for general illumination (CRI > 85). Moreover, the color temperature of the device is usually higher than 4500 K. To improve the color rendering properties and obtain warm white, multiple phosphors are utilized to produce a much broader and tunable emission spectrum. The second type of pc-wLEDs is still to use a blue LED to pump the phosphor blend consisting of green/red or green/yellow/red, and the third type is to combine a UV or near-UV LED with phosphor mixtures of blue/green/red or blue/green/yellow/red. 1953

DOI: 10.1021/acs.chemrev.7b00284 Chem. Rev. 2018, 118, 1951−2009

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absorption of these emissions. A stronger absorption indicates that a larger number of photons can be involved in the sequential luminescence process, which is beneficial to improving the luminescence efficiency of phosphors. 2.2.2. Suitable Emission Spectrum. The position and shape of the emission spectrum, such as peak and dominant emission wavelengths and full width at half-maximum (fwhm), are very important as they determine optical properties of wLEDs. For general lighting, phosphors with much broader emission spectra are desired to achieve high color rendering indices, whereas for backlighting the phosphors are required to have narrow emission bands to obtain wide color gamut. In both types of wLEDs, there are inevitable trade-offs between luminous efficacy and color rendition, making it extremely important to specially tackle the selection of phosphors with suitable emission spectra. 2.2.3. High Quantum Efficiency. The quantum efficiency indicates the ability of phosphors to convert the absorbed photons into luminescence, and higher efficiency means less energy loss during the luminescence process. It can be divided into internal and external ones; the difference between them is that the external quantum efficiency is the ratio of the number of emitted photons to that of the incident ones, whereas the internal one is the ratio of emitted photons to the absorbed ones.3,43 The quantum efficiency is largely affected by the composition, microstructure, defects, and impurities of luminescent materials. 2.2.4. Small Thermal Quenching/Degradation. The small temperature-dependent luminescence of phosphors is required to maintain the brightness and color coordinates of wLEDs. The thermal stability of phosphors is evaluated by the thermal quenching/degradation behaviors, which is determined by the electronic/crystal structure, structural rigidity, chemical composition, surface state of phosphor materials. 2.2.5. High Chemical Stability. The phosphor should be stable for handling, storage, and usage under the ambient atmosphere and will not chemically react with CO, CO2, H2O, air, etc. 2.2.6. Small Luminance Saturation. The phosphors must retain their luminance under high power or high flux density irradiations. This is quite important for them to be used in highpower and high-brightness SSL.

Figure 1. Three types of phosphor-converted white LEDs. ght (a) blue LED + yellow phosphor, (b) blue LED + red/green phosphors, and (c) UV LED + blue/green/red phosphors.

Differing from wLEDs for general illumination, wLED backlights need to pass through RGB color filters to attain high color saturation of three primary colors aiming to realize vivid images and brilliant displays.39,40 This difference in device structures requires three distinct and sharp emission bands from wLED backlights, which makes different requests for luminescent materials. In addition, in backlights color gamut instead of color rendering index is used to describe the coverage area in CIE1976 or CIE1931 coordination diagram.41 2.2. Selection Criteria for Luminescent Materials

Phosphors play a vital role in determining optical properties of wLEDs, such as spectral distribution, luminous efficacy, color rendering, color temperature, and lifetime. To pursue better performance of lighting devices, the luminescent materials must be carefully chosen to meet the requirements for different applications.3,5,42 Although there is a huge amount of luminescent materials, only those matching well with the selection rules can be considered as candidates for practical applications, as described in the following. 2.2.1. Strong Absorption of the LED Light. UV (350−410 nm) or blue (440−480 nm) LEDs are the primary light source to excite phosphors, so the phosphors should have a strong

Figure 2. (a) Radar graph highlighting the superiority of down-conversion nitride materials and (b) research topics on luminescent materials for SSL. 1954

DOI: 10.1021/acs.chemrev.7b00284 Chem. Rev. 2018, 118, 1951−2009

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tetrahedra.3,11,35,47,90−96 Large voids or channels are thus formed by these connected tetrahedral building blocks, which can accommodate a variety of cations. As nitrogen has a smaller electronegativity, larger formal charge, and higher polarizability than oxygen, the metal-N bonds in nitride compounds are usually more covalent than the corresponding metal-O ones, which give rise to greater nephelauxetic effect and stronger crystal field acting on rare earth activators (i.e., Eu2+ and Ce3+). This finally results in the lowering of the 5d energy levels and thus of excitation and emission energies (Figure 3). Dorenbos surveyed the lumines-

2.3. Current Status of LED Luminescent Materials

Luminescent materials discovered so far can be categorized into different groups, depending on their crystal structure and chemical compositions. On the other hand, as seen in Figure 2, when considering some key quantitative parameters of luminescent properties and production cost (i.e., efficiency, absorption of blue light, color tunability, reliability, cost, etc.), luminescent materials in various groups differ greatly, and some fatal shortcomings, such as low quantum efficiency and stability, hinder them from being used in wLEDs practically. Although to discuss materials other than nitrides is beyond the scope of this review, luminescent nitrides are highlighted as the most promising and interesting color converters for SSL when compared to other material systems. In addition, the research on luminescent materials for wLEDs is still one of the hottest topics in solid state chemistry and materials science. This covers several important issues: (i) materials discovery, to search for emerging luminescent materials with new crystal structures and promising luminescent properties by applying developed methodologies;26,35,44−50 (ii) synthetic strategies, to develop appropriate synthetic routes to luminescent materials with controlled particle size, morphology, and crystallinity, enhanced luminescence or low cost;3,10,11,51−59 (iii) structural analysis, to clarify the overall crystal structure, local coordination environment, covalence state of activators, structure distortion, structure ordering, and disordering;59−68 (iv) materials computation, to understand the electronic/band structure, Debye temperature, and dielectric constants of luminescent materials and link them to luminescent properties, thermal stability, and structural rigidity of luminescent materials by using the first-principle calculation, density function theory, molecular dynamics, etc.;69−74 (v) luminescent property tailoring, to enhance and tune the luminescent properties by controlling the local structure/coordination environment via substitutions and to improve the reliability of luminescent materials via surface engineering;9,11,50,75−81 (vi) applications, to testify the suitability of luminescent materials in SSL, and apply them in white LEDs/LDs for use in general lighting, backlighting, vehicle headlighting, projectors, and even agricultural lighting.3,5,11,82−84 In this paper, we will focus on most of these issues and carry out a comprehensive review of luminescent nitride materials.

Figure 3. Schematics of 5d energy levels of a free Eu2+ ion and Eu2+ ions in nitride compounds. Both the nephelauxetic effect and crystal-field splitting control the Eu2+ emission.

cence of Eu2+ in a variety of inorganic compounds and demonstrated that Eu2+-doped (oxy)nitride materials generally showed longer emission wavelengths than others, owing to the large centroid shift of 5d levels caused by the large polarizability of N3− ligand (Figure 4).10,97 In addition, Mikami et al. proposed

2.4. Choice of Luminescent Nitride Materials

Nitrogen has a Pauling’s electronegativity of 3.04 (3.44 for oxygen, 2.55 for carbon) and a trivalent state with a formal charge greater or less than -3.85 Nitride compounds are usually formed by the combination of nitrogen with elements which have less electronegativities, such as nonmetal elements (Si, B, and P), alkaline-earth and alkali metals, transition metals, and lanthanide elements.86−89 In addition, nitrogen is located between carbon and oxygen in the Periodic Table, yielding two major classes of binary nitrides that exhibit similarities in chemical bonding characters either with carbides (metallic bonding) or with oxides (ionic−covalent bonding). Nitride compounds, based on their dominant chemical bonding nature, can be simply grouped into ionic, transition metal, and covalent nitrides. Among these nitrides, only the covalent (or ionic−covalent) one can be considered as hosts for luminescent materials because they are insulative, transparent, and have large band gaps. In general, covalent nitride materials are structurally built up on highly condensed three-dimensional framework consisting of edgeand/or vertex-sharing SiN4, AlN4, LiN4, MgN4, GeN4, or GaN4

Figure 4. 4f → 5d emission wavelengths of Eu2+ in different hosts. Reprinted with permission from ref 10. Copyright 2014 WileyBlackwell.

to use dielectric constant or refractive index (calculated by density function perturbation theory) to discuss the red-shifted luminescence of (oxy)nitride phosphors and suggested that (oxy)nitride phosphors exhibited large dielectric constants and short metal-N bonds that contribute to the long-wavelength emission.69 1955

DOI: 10.1021/acs.chemrev.7b00284 Chem. Rev. 2018, 118, 1951−2009

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Table 1. Crystal Structure, Space Group, Lattice Parameters, Coordination Number (cn), Condensation Degree (k), and Photoluminescence Properties of Eu2+-Doped Luminescent (Oxo)Nitride Materialsa lattice parameters nitride compounds

crystal structure

space group

a

b

c

AlN

hexagonal

P63mc

3.11

3.11

4.98

Mg3N2 Ca2Si5N8 Sr2Si5N8 Ba2Si5N8 CaSiN2 SrSiN2 BaSiN2 LiSi2N3 BaSi7N10 SrSi6N8 SrYSi4N7 BaYSi4N7 Ba3Ga3N5 Mg3GaN3 CaAlSiN3 SrAlSiN3 SrAlSi4N7 Ba2AlSi5N9 Ba5Al7Si11N25 BaAl3Si4N9 Sr[Mg3SiN4] Ba[Mg3SiN4] Li2Ca2[Mg2Si2N6] Li2Ca1.88Sr0.12[Mg2Si2N6] Ca[Mg2Al2N4] Sr[Mg2Al2N4] Ba[Mg2Al2N4] Ba[Mg2Ga2N4] Ca[LiAl3N4] Sr[LiAl3N4] Ca18.75Li10[Al39N55] Ca3Mg[Li2Si2N6] Ba[Li2(Al2Si2)N6] Ba2LiAlSi7N12 Mg6Al(BN2)2N4 Mg7Si(BN2)2N4 Mg3(BN2)N

cubic monoclinic orthorhombic orthorhombic orthorhombic monoclinic orthorhombic orthorhombic monoclinic orthorhombic hexagonal hexagonal monoclinic trigonal orthorhombic orthorhombic orthorhombic triclinic orthorhombic monoclinic tetragonal triclinic monoclinic monoclinic tetragonal tetragonal tetragonal tetragonal tetragonal triclinic cubic monoclinic tetragonal orthorhombic rhombohedral rhombohedral hexagonal

9.955 14.352 5.710 5.783 5.123 5.975 5.605 9.208 6.873 7.855 6.0160 6.0123 16.801 3.394 9.802 9.843 11.742 9.860 9.5923 5.8465 11.495 3.451 5.5472 5.5744 8.066 8.169 8.260 8.3654 11.160 5.866 22.415 5.966 7.8282 14.0942 3.4318 3.4384 3.540

9.955 5.610 6.822 6.959 10.207 7.283 11.361 5.301 6.713 9.259 6.0160 6.0123 8.3301 3.394 5.651 5.760 21.391 10.320 21.3991 26.7255 11.495 6.069 9.844 9.8439 8.066 8.169 8.260 8.3654 11.160 7.511 22.415 9.806 7.8282 4.8924 3.4318 3.4383 3.540

Mg3Ga(BN2)N2 CaSi2O2N2 SrSi2O2N2

rhombohedral monoclinic triclinic

Ia3̅ Cc Pmn21 Pmn21 Pbca P21/c Cmca Cmc21 Pc Imm2 P63mc P63mc C2/c R3m Cmc21 Cmc21 Pna21 P1 Pnnm P21/c I41/a P1 C2/m C2/m I4/m I4/m I4/m I4/m I41/a P1 Fd3m C2/m P4/ncc Pnnm R3̅m R3̅m P63/ mmc R3m P21 P1

3.456 15.036 11.320

BaSi2O2N2 Sr0.25Ba0.75Si2O2N2 Sr0.5Ba0.5Si2O2N2 La4‑xCaxSi12O3+xN18‑x Ce4‑xCaxSi12O3+xN18‑x Ca1.5Ba0.5Si5O3N6 Ca15Si20O10N30 Ca1.62Eu0.38Si5O3N6 La2.5Ca1.5Si12O4.5N16.5 Sr3Si2O4N2 Ba3Si6O12N2 BaSi6N8O Ca-α-sialon Sr-α-sialon Li-α-sialon

orthorhombic orthorhombic triclinic monoclinic monoclinic monoclinic cubic monoclinic monoclinic cubic trigonal orthorhombic trigonal trigonal trigonal

Cmcm Pna21 P1 C2 C2 Cm Pa3̅ Cm C2 Pa3̅ P-3 Imm2 P31c P31c P31c

Si6‑zAlzOzN8‑z

hexagonal

P63

κ

cn

λem (nm)

λex (nm)

472−477

365

52

589 605 630 580 630 670−685 600−630 572−584 475 450 548−570 503−527 638 578 650 610 632 584 568 500 615 670 638 634 607 612 666 649 668 650 647 734 532 515 570 560 710

365 450 450 450 450 400 400 310 300 370 390 385 365 365 450 455 450 450 405 365 450 450 450 450 440 440 440 440 470 440 450 450 395 400 330 320 420

147

fwhm (nm)

12

1

9.955 9.686 9.341 9.391 14.823 5.497 7.585 4.778 9.633 4.801 9.7894 9.7869 11.623 25.854 5.063 5.177 4.966 10.346 5.8889 5.8386 13.512 6.101 5.9978 6.0170 3.286 3.354 3.432 3.4411 12.865 9.965 22.415 11.721 9.9557 8.0645 31.436 31.107 17.402

6 7 8, 9 8, 9 6 8 8 5 12 10 12 12 4, 6, 8 6 5 5 6, 8 6−10 11,10, 8 11 8 8 6 6 8 8 8 8 8 8 5, 6 6 8 11

0.625 0.625 0.625 0.5 0.5 0.5 0.67 0.7 0.75 0.57 0.57 0.6 0.6 0.67 0.67 0.71 0.67 0.72 078 1 1 0.67 0.67 1 1 1 1 1 1 0.71 0.67 1 0.75

3.456 15.450 14.107

31.792 6.851 7.736

4, 5 7 7

0.5 0.5

600 560 540

340 450 450

180 98 86

14.3902 5.470 7.206 18.5427 18.5427 7.070 15.4195 7.0595 18.535 15.6593 7.5218 8.105 7.7928 7.7919 7.8360

5.3433 14.277 7.389 4.8404 4.8404 23.867 15.4195 23.7504 4.833 15.6593 7.5218 9.678 7.7928 7.7919 7.8360

4.8326 4.791 7.334 10.7007 10.7007 4.825 15.4195 9.6345 10.689 15.6593 6.4684 4.837 5.6533 5.6538 5.6870

8 8 7 6 6 6, 12 6,7,8 6, 12 6 6, 7, 8 6, 8 6 7 7 7

0.5 0.5 0.5 0.57 0.57 0.56 0.5 0.56 0.57 0.33 0.43 0.67 0.75 0.75 0.75

492 472 565 565 581 570−590 641 592−600 495−575 600 525 500 585 575 573−577

450 400 450 460 450 460 450 400 460 450 303 400 400 460

37 36 92 80 85 103 145 105 69−92 80 68 102 94 93 95

7.6090

7.6090

2.9115

6

0.75

535

450

55

1956

85 90 110 120 100 180 79 44 120 80 85 132 90 72 75 100 98 67 43 78 62 62 67 82 104 60 50 54 124 57 61

280

ref 101, 103, and 104 105 33 and 109 33 and 110 33 and 110 112 and 114 112 and 113 112 and 113 115 116−119 120 and 121 122 123 94 94 34 128 45 129 46 46 130 131 132 and 133 133 95 95 95 95 134 35 136 137 138 47 139 139 140 141 142 and 145 142, 143, and 145 142−144 146 147 and 148 48 149 150 152 153 154 155 157−159 160 and 161 163−167 168 169, 170, and 172 171−176

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Table 1. continued lattice parameters

a

nitride compounds

crystal structure

Sr14Si68‑sAl6+sOsN106‑s Sr5Al5+xSi21‑xN35‑xO2+x Sr3Si13Al3O2N21 SrSiAl2O3N2 SrSi9Al19ON31 Sr3Si8−xAlxO7+xN8−x Sr(Al0.3Si0.7)4(N0.8O0.2)6 Sr2Si7Al3ON13 α-SrNCN Sr2B2−2xSi2+3xAl2−xN8+x CaP2N4 SrP2N4 BaP2N4 BaSr2P6N12 Ca2PO3N Ba3P5N10Cl Ba3P5N10Br Ba3P5N10I

monoclinic orthorhombic orthorhombic orthorhombic rhombohedral monoclinic orthorhombic orthorhombic orthorhombic hexagonal hexagonal hexagonal cubic cubic orthorhombic orthorhombic orthorhombic orthorhombic

space group P21 Pmn21 Pmn21 P212121 R3̅ C2/c Fdd2 Pna21 Pnma P62c P63 P63 Pa3̅ Pa3̅ Pnma Pnma Pnma Pnma

a

b

c

67.789 23.614 9.037 4.9198 5.335 18.1828 5.8061 11.8033 12.422 4.799 16.847 17.111 10.233 10.087 6.794 12.5182 12.5660 12.6311

9.042 7.487 14.734 7.8973 5.335 4.9721 37.762 21.589 3.966 4.799 16.847 17.111 10.233 10.087 5.439 13.1798 13.2240 13.2565

7.469 9.059 7.464 11.3494 79.1 15.9557 9.5936 5.0131 5.392 9.780 7.8592 8.107 10.233 10.087 9.416 13.7676 13.8030 13.8689

cn 9 7, 10 9 12 6, 8 10 6, 8 6 9 5−12 9−12 12 12 9, 10 8,10,11 8,10, 11 8,10,11

κ

λem (nm)

0.70 0.70 0.70 0.6 0.875 0.53 0.67 0.71

508 510 515−525 472 450−490 465 490 615 603 598 575 529 460 450 528 439,597 472, 582 494, 585

0.51 0.5 0.5 0.5 0.5 0.25 0.5 0.5 0.5

λex (nm) 376 450 305 290 365 334 450 427 400 400 400 400 400 400 390 400 40

fwhm (nm) 60 69 66 88 90 70 97 106 157 147 76 70 47 47 125 30, 85

ref 179 180 181 182 183−185 186 187 189 193 194 and 195 196 196 and 197 196 and 198 196 and 199 200 201 202 201

(κ = M/A, M = Si, Al, Li, Mg, Al, Ge, Ga, A = O, N; FWHM = fwhm).

Figure 5. (a) Excitation and emission spectra of AlN:Eu,Si. (b) Chromatic coordinates of AlN:Eu, Si and other representative blue phosphors. (c) Lifetime of field emission displays (FEDs) using AlN:Eu,Si and Y2SiO5:Ce. (d) Absorption (α), internal (ηi), and external (η0) quantum efficiency of AlN:Eu, Si. (e) Cathodoluminescence (CL) images of (i) 360 nm and (ii) 550 nm emission of Si-free AlN:Eu, and CL images of (iii) 360 nm and (iv) 470 nm emissions of AlN:Eu doped with 2.9 mol % Si, showing that Eu is uniformly dissolved in the AlN lattice associated with the Si-doping. (f) Transmission electron microscope (TEM) images of AlN:Eu,Si (i-ii) and Si-free AlN:Eu (iii) in the [100] direction, (iv) electron diffraction patterns of AlN:Eu,Si. Reprinted with permission from ref 65. Copyright 2010 Royal Society of Chemistry. Reprinted with permission from ref 101. Copyright 2007 American Institute of Physics. Reprinted with permission from 102. Copyright 2009 Wiley-Blackwell. Reprinted from ref 103. Copyright 2009 American Chemical Society.

Thus, nitride phosphors have the following merits over their counterparts, and these merits enable them to be the most suitable luminescent materials for SSL applications. 2.4.1. Red-Shifted Excitation and Emission Spectra. Strong covalent chemical bonds of Eu(Ce)-N and relatively short distance of the bonds result in the large nephelauxetic effect and strong crystal field strength, then yielding redshifts of both excitation and emission bands. The red-shifted spectra enable

phosphors to absorb a wide range of light wavelengths, including near UV, blue or even green, and to emit longer emission wavelengths (i.e., green, yellow, and red). 2.4.2. Small Stokes Shifts. Stokes shift is defined as the difference between positions of the band maxima of absorption and emission spectra of the same electronic transition. A small Stokes shift indicates less energy loss during the luminescence process, which is a necessary requirement for obtaining high 1957

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(Figure 5f). Wang et al. further improved the quantum efficiency up to 61%, by using SiC as the Si source. At the same time, the thermal stability of carbon-substituted AlN:Eu,Si is enhanced owing to the stronger covalent bond of Si−C (compared to Si− N).104 These promising photoluminescence properties of AlN:Eu,Si allow for great potentials in UV-LED-driven wLEDs. Long et al. reported the luminescence of Eu2+-doped Mg3N2 (Mg3−xEuxN2, x = 0−0.08).105 Under near UV light excitation, Mg3N2:Eu2+ exhibits a broad emission band with a maximum of 612 nm and a fwhm of 180 nm. The excitation spectrum shows a wide band in a range of 250−450 nm and a dominant peak at 428 nm. An anomalous blueshift with increasing Eu is seen in Mg3N2, which is ascribed to the weakening of crystal fields caused by lattice relaxation. The internal quantum efficiency of Mg3N2:Eu2+ is 21.2% at x = 0.05 under 428 nm excitation. 3.1.2. Nitridosilicate. Alkaline earth or alkali metal nitridosilicates (M−Si−N, where M = Li, Mg, Ca, Sr, and Ba) can be considered as an extension of oxosilicates by replacing all O by N, yielding more or less interconnected SiN4 tetrahedra. The N → O substitution leads to significant structural diversities in oxosilicates, making them possible to become interesting phosphor hosts. The first nitridosilicate phosphor was reported by Gaido in 1974, which is green-emitting Eu2+-doped MgSiN2 (λem = 517 nm).106 Till now, a number of luminescent nitridosilicates have been discovered and investigated. Eu2+-doped M2Si5N8 (M = Ca, Sr, Ba) phosphors are considered as important red-emitting down-conversion luminescent materials for wLEDs due to their excellent photoluminescence properties, such as very broad excitation band, tunable emission, and high quantum efficiency.31,33,52,107,108 These phosphors can be synthesized by firing the powder mixture of Si3N4 and MNx (M = Ca, Sr, Ba, and Eu) or metal M and Si(NH)2 at 1500−1700 °C in nitrogen atmosphere.107−110 Hoppe et al. reported the luminescence of Ba2Si5N8:Eu2+ that emits at 600 nm and has a strong absorption peak at ∼540 nm.31 Both Li et al.33 and Piao et al.107 discussed the photoluminescence of (Ca, Sr, Ba)2Si5N8:Eu2+ as LED conversion phosphors. The emission maximum of M2Si5N8:Eu2+ (1 mol %) is 605, 610, 574 nm for M = Ca, Sr, and Ba, respectively. With increasing Eu concentration, the emission band redshifts in the range of 605−615, 609−680, and 570−680 nm for M = Ca, Sr, and Ba, owing to the energy transfer from Eu(I) to Eu(II). The substitution of Sr by Ca also redshifts the emission band of Sr2Si5N8:Eu2+ from 624 to 643 nm. The quantum efficiency of M2Si5N8:Eu2+ increases going from Ca to Ba and Sr under the blue light irradiation.33 The absorption, internal and external quantum efficiency of Sr2Si5N8:Eu2+ (2 mol %) are 82, 87, and 71%, respectively.3,108 Hao et al. reported an enhancement of the luminescence intensity of Sr2Si5N8:Eu2+ by ∼25% by coupling with the localized surface plasmon oscillation of Ag nanoparticles.111 Nanosized (Sr, Ba)2Si5N8:Eu2+ (100−200 nm) phosphors were produced at low temperature (150 °C) by using metal amides M(NH2)2 as precursors.54 MSiN2:Eu2+ (M = Ca, Sr, and Ba) is also a kind of red-emitting luminescent materials.112−114 The optical band gap of MSiN2 is 4.8, 4.2, and 4.1 eV for M = Ca, Sr, Ba, respectively.113 Duan et al. synthesized a series of M1−xEuxSiN2 phosphors through the solid-state reaction method. The emission maximum shifts in the range of 670−685 (M = Sr) and 600−630 nm (M = Ba) with varying Eu concentrations.113 Li et al. reported that the Eu2+doped CaSiN2 emitted at 630 nm and had a low external quantum efficiency of 24.5% under 450 nm excitation.114 The low quantum efficiency and high thermal quenching rate of

quantum efficiencies. Nitride phosphors usually have significantly red-shifted excitation spectra with larger crystal field splitting and therefore show smaller Stokes shifts when compared to oxidic phosphors. 2.4.3. High Thermal Stability. The structure of nitride phosphors consists of highly condensed edge- and/or vertexsharing MN4 (M = Si, Al, Li, Mg, Ge, Ga, etc.) tetrahedra, which offers high structural stiffness. The small Stokes shift also leads to a high position of the crossover point between the ground state and excited state parabolas of Eu2+ or Ce3+ and therefore creates a large energy barrier for thermal quenching. 2.4.4. Abundant Emission Colors. The structural diversity of nitride compounds enables nitride phosphors to emit light in a very large spectral range spanning the ultraviolet, visible, to infrared. Moreover, the emission color can thus be tuned broadly by compositional tailoring or band gap engineering.

3. SURVEY OF DIFFERENT TYPES OF DOWN-CONVERSION NITRIDE MATERIALS Extensive investigations have been made on the search of new luminescent nitride materials and on the study of their photoluminescence properties as well as applications in SSL. A variety of nitride luminescent materials have been synthesized, the structure of which contains AlN4, SiN4, MgN4, LiN4, GaN4, GeN4, CN4, and PN4 tetrahedral unit or their combinations. Accordingly, it might be straightforward and convenient to make a comprehensive investigation of nitrides based on different tetrahedral building blocks. In addition, considering different luminescent centers in these luminescent materials, we classify nitride phosphors into four categories: Eu2+-doped, Ce3+-doped, other activator-doped, and activation-free luminescent materials. 3.1. Eu2+-Doped Down-Conversion Nitride Materials

Eu2+ is one of the mostly used activators in luminescent materials, owing to its intense and broad photoluminescence spectra derived from its parity-allowed 4f7↔4f65d1 electronic transitions leading to a high oscillator strength (10−2).10 As 5d electrons are not shielded from their surroundings, the energy levels depend largely on local coordination environments. The emission color of Eu2+ can therefore be varied in a very broad range from ultraviolet, blue, green, yellow, to deep red. The latest survey of Eu2+-doped nitride materials, their crystal structure and photoluminescence properties is summarized in Table 1. 3.1.1. Binary Nitride. AlN is well-known as an excellent optoelectronic material that can emit deep ultraviolet light (220 nm),98 as well as a promising thermal sink material that has a thermal conductivity as high as 280 W/(m K).99,100 Hirosaki et al. synthesized Si- and Eu2+-codoped AlN powders by firing the powder mixture of Si3N4, AlN, and Eu2O3 at 2050 °C under 1.0 MPa N2 and observed very intense blue cathodoluminescence at 470 nm (Figure 5a).101 As shown in Figure 5 (panels b and c), the high color purity and stability of AlN:Eu,Si enables it to be used in long-lived full color field emission displays (FEDs). Dierre et al. discussed the role of Si playing in the cathodoluminescence of AlN:Eu,Si, which enhances the solubility of Eu in the lattice by forming some structure defects such as stacking faults (Figure 5d).102 Takeda et al. clarified the local structure of AlN:Eu,Si by using HAADF-STEM and suggested a layered structure where Eu forms a single layer with the Si condensation between the AlN wurtzite blocks (Figure 5e).65 Later, Inoue et al. investigated photoluminescence properties of AlN:Eu,Si for use in wLEDs.103 It has a symmetric emission band centered at 465 nm and an external quantum efficiency of 46% under the 365 nm excitation 1958

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Figure 6. Excitation and emission spectra of (a) Ba[Mg3SiN4]:Eu2+ and (b) Sr[Mg3SiN4]:Eu2+. (c) Diffuse reflectance spectra of nondoped and Eu2+doped Ba[Mg3SiN4]. (d) Schematic configurational-coordinate diagrams of Eu2+ in Ba[Mg3SiN4] (left) and Sr[Mg3SiN4] (right). The emission process of Ba[Mg3SiN4]:Eu2+ consists of typical Eu2+ 4f65d1 → 4f7 emission and ETE over the whole investigated temperature region. In contrast, Sr[Mg3SiN4]:Eu2+ only shows Eu2+ 4f65d1 → 4f7 emission. (e) Excitation and emission spectra and (f) thermal quenching of M[Mg2Al2N4]:Eu2+ (M = Ca, Sr, and Ba). Reprinted from ref 95. Copyright 2014 American Chemical Society. Reprinted from ref 130. Copyright 2014 American Chemical Society. Reprinted from ref 131. Copyright 2015 American Chemical Society.

Ga, and EuF3 in a sodium flux at 760 °C utilizing weld shut tantalum ampules. Under 365 nm excitation, Ba3Ga3N5:Eu2+ displays a symmetric emission band centered at 638 nm (fwhm = 84.5 nm), and the red emission is attributed to those Eu2+ in octahedral coordination. Eu2+-doped Mg3GaN3 shows a yellowish emission under UV or blue light excitation, having an emission maximum of 578 nm and fwhm of 132 nm. The calculated band gap is 1.46 and 3.0 eV for Ba3Ga3N5 and Mg3GaN3, respectively. 3.1.4. Nitridoaluminosilicate. Nitridoaluminosilicates are structurally built up on highly condensed framework containing either solely XN4 (X = Si or Al) or mixed XN4 tetrahedra. Uheda et al. synthesized the CaAlSiN3:Eu2+ phosphor by using gas pressure sintering of Ca3N2, Si3N4, AlN, EuN raw powders at 1600−1800 °C under 1.0 MPa N2.34 CaAlSiN3:Eu2+ is a promising red-emitting phosphor for use in high color rendering wLEDs, which exhibits a broad emission band (fwhm = 92 nm) centered at 660 nm, high thermal quenching temperature (>600 K) and high quantum efficiency (>90%) under the blue light excitation. The superior thermal and chemical stability of CaAlSiN3:Eu2+ enables it to replace the moisture-sensitive CaS:Eu2+ phosphor for practical applications. Watanabe et al. prepared SrAlSiN3:Eu2+ single crystals using hot-isostatic pressing (HIP) method (2000 °C, 190 MPa N2), which is isostructural with CaAlSiN3.128 SrAlSiN3:Eu2+ shows a shorter (λem = 610 nm) and narrower (fwhm = 81 nm) emission band than CaAlSiN3:Eu2+, owing to the enlarged N2-centered octahedron as well as the elongated Eu−N2 bond length. Hecht et al.45 and Kechele et al.129 in Schnick’s group synthesized single crystals of SrAlSi4N7:Eu2+ and Ba2AlSi5N9:Eu2+. Both crystals were obtained from a reaction of Si3N4, AlN, metals Eu, Sr, or Ba at 1630−1725 °C in a radio frequency furnace. SrAlSi4N7:Eu2+ shows a broad emission band centered at 635 nm (2 mol % Eu2+) and a fwhm of 75 nm under the 450 nm excitation, which is originated from Eu2+ on the six-coordinated Sr1 site. Ba2AlSi5N9:Eu2+ (2 mol %) exhibits a broad emission band with the maximum emission of 584 nm and the bandwidth of 100 nm.

CaSiN2:Eu2+ is ascribed to the hybridization between the Eu4f5d orbital and N-2p orbital that leads to the photoionization effect. Eu2+-doped LiSi2N3 emits a yellow emission.115 The calculated and experimental band gaps are 5.0 and 6.4 eV, respectively. Li et al. reported that it had an emission peak at 580 nm under the 310 nm excitation and a low quantum efficiency due to the photoionization process.115 A very low concentration quenching (0.5 mol %) was observed for LiSi2N3:Eu2+, which is caused by the significantly shorter distance of LiEu-LiEu. The crystal structure of BaSi7N10 was reported by Huppertz et al.116 It has a band gap of 4.8 eV, calculated from the diffuse reflectance spectrum.117 Although originally BaSi7N10:Eu2+ was reported to be a red-emitting phosphor (perhaps Ba2Si5N8:Eu2+ impurity phase was included), Li et al.117 and Anoop et al.119 addressed that it actually emitted a cyan emission (λem = 482− 500 nm) under UV excitation. Stadler et al. reported the structure of SrSi6N8 which is a reduced nitridosilicate with direct Si−Si bonds.120 Shioi et al. synthesized SrSi6N8:Eu2+ by firing the powder mixture of Si3N4, SrSi2, and Eu2O3 at 1900 °C under 0.9 MPa nitrogen atmosphere.121 A blue emission (λem = 450 nm) is observed under 370 nm excitation. Li et al. investigated the luminescence of Eu2+-doped MYSi4N7 (M = Sr and Ba).122,123 SrYSi4N7:Eu2+ shows a yellow emission at 548−570 nm under 390 nm excitation, while BaYSi4N7:Eu2+ exhibits a blue-green emission at 505−537 nm under 385 nm excitation. The short emission in the Ba compound is due to longer Ba−N bonds (i.e., small crystal field splitting) and large ionic size of Ba (i.e., small Stokes shift). 3.1.3. Nitridogallate. Several alkaline earth metal nitridogallate compounds, such as Sr3Ga2N4,124 Ca3Ga2N4,124,125 Sr3Ga3N5,94,124 Ba3Ga2N4,126 and Ba6Ga5N,127 have been reported, but they are not investigated as phosphor hosts. The first two luminescent nitridogallate single crystals (Ba3Ga3N5:Eu2+ and Mg3GaN3:Eu2+) were grown by Hintze et al.94 They were obtained from a reaction of NaN3 and Ba or Mg, 1959

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Both BaAl3Si4N9:Eu2+ and Ba5Si11Al7N25:Eu2+ single crystals were discovered by Hirosaki et al. using the single-particle diagnosis approach.46 The single crystals were synthesized by firing the powder mixture of Ba3N2, Si3N4, AlN, and EuN at 1900 °C in 1.0 MPa N2 using a gas-pressure sintering furnace. BaAl3Si4N9:Eu2+ is a blue-emitting phosphor that has an emission maximum of 500 nm and a fwhm ∼67 nm under 365 nm excitation, which stems from Eu2+ residing on one single 11coordinated Ba site. In Ba5Si11Al7N25:Eu2+, there are three different crystallographic sites for Ba atoms which are coordinated to 8, 10, and 11 nitrogen atoms, respectively. A yellow emission (λem = 570 nm, fwhm = 98 nm) is obtained for Ba5Si11Al7N25:Eu2+ under the 405 nm excitation, which is originated from Eu2+, occupying the eight-coordinated Ba site. 3.1.5. Nitridomagnesosilicate. Schmiechen et al. synthesized single crystals of alkaline earth metal nitridomagnesosilicates (M[Mg3SiN4], M = Ca, Sr, Ba) and discussed their crystal structure and luminescence properties.130,131 M[Mg3SiN4] (M = Ca, Sr, and Ba) was synthesized by firing the mixture of MF2, Mg3N2, EuF3 (0.5−2.5 mol %), and Si(NH)2 in a tantalum ampule at 900−1050 °C for 24 h. A small amount of LiN3 was added as the nitrogen source. Ba[Mg3SiN4]:Eu2+ (2 mol %) shows a deep red emission color with the maximum emission of 670 nm and a quantum efficiency of 32% under blue irradiation (Figure 6a). Sr[Mg3SiN4]:Eu2+, however, has a blueshifted and narrowed emission band (λem = 615 nm, fwhm = 43 nm) (Figure 6b). Both Ba[Mg3SiN4]:Eu2+ (4.0 eV) and Sr[Mg3SiN4]:Eu2+ (3.9 eV) have the identical band gap (Figure 6c). From the viewpoint of the volume of the MN8 polyhedron (the Ba−N and Sr−N distances are 2.88−3.04 and 2.65−2.95 Å, respectively), Ba[Mg3SiN4]:Eu2+ would be expected to have a shorter emission than Sr[Mg3SiN4]:Eu2+. The anomalous emission is caused by the additional trapped-exciton emission, besides the normal 5d → 4f electronic transitions of Eu2+ (Figure 6d). The smaller thermal quenching of Ba[Mg3SiN4]:Eu2+ with a large Stokes shift (3475 cm−1) implies that there are some other loss mechanisms (i.e., photoionization) for Sr[Mg3SiN4]:Eu2+ with a small Stokes shift (772 cm−1). Schmiechen et al.132 analyzed the crystal structure of Li2Ca2[Mg2Si2N6], and Strobel et al.133 developed another narrow-band red phosphors of Li2Ca2[Mg2Si2N6]:Eu2+ and Li2(Ca1−xSrx)2[Mg2Si2N6]:Eu2+. Under the blue light excitation, Li2Ca2[Mg2Si2N6]:Eu2+ and Li2(Ca1.88Sr0.12)2[Mg2Si2N6]:Eu2+ show an emission maximum at 638 and 634 nm and a same fwhm of 62 nm, respectively (Figure 7a). The band gap determined from the diffuse reflectance spectrum is about 4.6 eV. 3.1.6. Nitridomagnesoaluminate. Pust et al.95 synthesized alkaline earth metal nitridomagnesoaluminate single crystals and investigated their structure and luminescence. M[Mg2Al2N4] (M = Ca, Sr, and Ba) crystals were obtained by a solid-state reaction of MF2, EuF3, AlF3, Mg3N2, metal Li, and LiN3 in a Ta ampule at 900 °C for 24 h. The bulk powders were synthesized by firing the mixture of Mg3N2, AlN, and MH2 at 1450 °C using a hot-isostatic pressure sintering. The onset of fundamental absorption edge of nondoped M[Mg2Al2N4] hosts varies in the range of 3.65−3.8 eV. A doping level of 2 mol % Eu2+ in M[Mg2Al2N4] yields an emission band centered at 607, 612, and 666 nm for M = Ca, Sr, and Ba, respectively (Figure 6e). The nontypical redshift in emission band with increasing cation size (Ca → Ba) indicates anomalous luminescence in these nitridomagnesoaluminates. For M = Ca and Sr, the normal Eu2+ emission from the 4f65d1 excited state is seen at high temperature, whereas the emission from the trapped exciton state is observed at low temperature

Figure 7. Excitation and emission spectra narrow-band red phosphors of (a) Li2(Ca1.88Sr0.12)2[Mg2Si2N6]:Eu2+, (b) Sr[LiAl3N4]:Eu2+ (pink), compared with CaAlSiN3:Eu2+ (gray), (c) Ca[LiAl3N4]:Eu2+, and (d) Ca18.75Li10.5[Al39N55]:Eu2+. Reprinted with permission from ref 35. Copyright 2014 Nature Publication Group. Reprinted from ref 132. Copyright 2015 Wiley-VCH. Reprinted from ref 133. Copyright 2017 American Chemical Society. Reprinted from ref 134. Copyright 2014 American Chemical Society. Reprinted from ref 136. Copyright 2016 American Chemical Society.

(120 115 110 95 120 100 70 80 116 153 72 115 116 130 58 108 117 80 72−105 167 178 130 75 110 104 110 130 120 105 80 120 >120

204 205 205 207 206 207 207 208 123 124 209 210 214 32 32 32 32 218 218 218 218 219 220 221 222 223 226 228 229 130 132 230 138 231 233

3.2.1. Aluminum Nitride. Liu et al. synthesized the Ce3+doped AlN phosphor (Al0.95N:Si0.05,xCe3+, x = 0.01−0.15%) by gas-pressure sintering of AlN, α-Si3N4, and CeO2 at 2050 °C for 4 h.204 It shows a blue emission band with the maximum of 418 nm and a fwhm of 90 nm under the 340 nm excitation. The color coordinates of AlN:Ce3+ are (0.15, 0.07), which are closer to the NTSC blue (0.15, 0.08) than those of AlN:Eu2+ (0.13, 0.12). 3.2.2. Nitridosilicate. As overviewed above, alkaline earth nitridosilicates are shown to be promising hosts for Eu2+. Moreover, upon doping with Ce3+, these compounds can also possess interesting luminescence which enables them to be 1964

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Krevel et al. reported the luminenscence of Ce3+ in Y−Si−O−N compounds, including YSiO2N, Y5(SiO4)3N, Y2Si3O3N4, and Y4Si2O7N2.32 The emission maximum is 442, 475, 493, and 504 nm for YSiO2N, Y5(SiO4)3N, Y2Si3O3N4, and Y4Si2O7N2 under UV excitation (350−390 nm), respectively. The Ce 3+ luminescence in these compouds is dominantly effected by the coordination environment such as the coordination number and the ratio of N/O. The similar trend is also observed in Ce3+doped La−Si−O−N materials, such as LaSiO2N (λem = 416 nm), La3Si8O4N11(λem = 424 nm), La5(SiO4)3N (λem = 478 nm), and La4Si2O7N2 (λem = 488 nm).218 Durach et al. reported the crystal structure and luminescence of a yellow-emitting La3BaSi5N9O:Ce3+.220 Although the degree of condensation (κ) is less than 0.5, the structure of La3BaSi5N9O consists of a three-dimensional network of vertex-sharing Q4and Q2-type SiN4/SiN2O2 tetrahedra, forming sechser rings that are condensed to zehner rings which are interconnected by dreier rings containing two SiN4 and one SiN2O2 tetrahedra.220 This unprecedented tetrahedra network structure in a compound with κ < 0.5 leads to significantly red-shifted luminescence. Under the 440 nm excitation, La3BaSi5N9O:Ce3+ shows a very broad emission band with a maximum of 578 nm and the fwhm of 167 nm. The extremely wide emission band is owing to the presence of 12 different crystallographic sites for La/Ba atoms which are coordinated to 8 or 9 N/O atoms. Zhu et al. reported an orange-emitting oxonitridosilicate, Y3Si5N9O:Ce3+.221 The structure of Y3Si5N9O is characterized by a highly condensed three-dimensional network of vertex-sharing SiN4 and SiON3 tetrahedra, forming five-membered ring channels that accommodate two types of Y atoms. The Y1 atoms are coordinated to 5 N and 1 O, and Y2 to 6 N and 1 O. The excitation spectrum consists of several broad bands centered at 288, 358, 436, and 506 nm, respectively. The emission band has a maximum at 620 nm and a fwhm of 178 nm under the 435 nm excitation. The absorption, internal quantum efficiency, and external quantum efficiency of Y3Si5N9O:Ce3+ (1 mol %) are 89.5, 17.2, and 15.6% under 450 nm excitation, respectively. 3.2.4. Nitridoaluminosilicate. Two Ce3+-doped nitridoaluminosilicate luminescent materials, CaAlSiN3:Ce3+ and SrAlSi4N7:Ce3+, have been reported, both of which show highly efficient yellow emissions.222−225 Li et al. reported that CaAlSiN3:Ce3+ showed a maximum emission at 580 nm (1 mol % Ce3+) under the 460 nm excitation, and the emission could be red-shifted to 603 nm with increasing Ce 3+ concentration.222 The excitation band covers a wide spectral range of 250−550 nm, showing five sub-bands centered at 259, 313, 370, 421, and 483 nm, respectively. The external quantum efficiency reaches the maximum of 54% under the 450 nm excitation for 1 mol % Ce3+. The thermal quenching temperature is ∼300 °C (1 mol % Ce3+) and decreases with increasing Ce3+ concentration. Both high efficiency and thermal stability of CaAlSiN3:Ce3+ enables it to be a promising yellow phosphor for creating warm wLEDs. Zhang et al.223 and Ruan et al.225 synthesized a phase pure SrAlSi4N7:Ce3+ by using excess AlN to minimize impurity phases. Upon doping of 5 mol % Ce3+, SrAlSi4N7 shows a broad emission band with a maximum at 555− 565 nm and a fwhm of 115 nm under the 450 nm excitation. The maximum external quantum efficiency of 38.3% is reported. 3.2.5. Oxonitridoaluminosilicate. Ce3+-doped oxonitridoaluminosilicates, such as α-sialon, β-sialon and JEM [LaAl(Si6−zAlz)(N10−zOz)], are reported to be potential blue or bluegreen phophors for near UV LEDs.226−229 Xie et al.226 and Li et al.227 investigated the effect of composition on the luminescence

of Ce3+-doped Ca-α-sialon. Ca-α-sialon:Ce3+ shows a broad emisison band centered at 500−518 nm and a fwhm of 100 nm under ∼385 nm excitation. It has a strong absorption between 350−440 nm, suggesting that it is suitable for use in combination with near UV LEDs. The external quantum efficiency is 35% for 8 mol % Ce3+ under the 380 nm excitation. Liu et al.228 reported that β-sialon:Ce3+ exhibited a maximum emission at 486 nm and a fwhm of 104 nm. Compared to αsialon:Ce3+, β-sialon:Ce3+ has a red-shifted excitation spectrum but a blue-shifted emission spectrum. The absorption and external quantum efficiencies of β-sialon:Ce3+ are 53.4 and 25% under the 410 nm excitation. It also shows a small thermal quenching and maintains 88% of the initial emission intensity at 150 °C. Takahashi et al.229 synthesized a blue-emitting phosphor, JEM:Ce3+, with the aim of developing an alternative robust blue phosphor to BaMgAl10O17:Eu2+. The excitation spectrum shows a broad band with a maximum at 365 nm. The maximum redshifts and expands to the blue spectral region with increasing the Ce3+ concentration, thus making it to be excited efficiently at 405 nm. JEM:Ce3+ has an emission band with a maximum at 460−495 nm as well as a fwhm of 110 nm. Under the 405 nm excitation, the internal and external quantum efficiencies are 62 and 50%, respectively. 3.2.6. Nitridomagnesosilicate. Schmiechen et al. reported the luminescence of Ce3+-doped Ca[Mg3SiN4].130 The excitation spectrum shows a very intense band with a maximum of 480 nm and two weak bands centered at 310 and 350 nm, respectively. The emission band consists of two maxima at 530 and 585 nm with a fwhm of 130 nm. Strobel et al. reported a broadband green emission in Li2Ca2[Mg2Si2N6]:Ce3+ under the blue light irradiation.133 The emission band exhibits a peak at 540 nm and a shoulder at 600 nm, leading to a fwhm of ∼120 nm. Two distinct absorption bands centered at ∼400 and ∼480 nm are seen. 3.2.7. Nitridomagnesoaluminate. The Ce3+-doped Sr[Mg2Al2N4] shows a very broad excitation spectrum.230 The emission spectrum covers the spectral range of 500−700 nm with the maxima of 580 and 620 nm. At 150 °C, the luminescence of Sr0.985[Mg2Al2N4]:Ce0.0153+ phosphor remains ∼65% of its initial intensity measured at room temperature. 3.2.8. Nitridolithoalumosilicate. Strobel et al. reported a Ce3+-doped nitridolithoalumosilicate phosphor, Ba[Li2(Al2Si2)N6]:Ce3+.138 Compared to Ba[Li2(Al2Si2)N6]:Eu2+ that exhibits a green emission at 540 nm, the Ce3+-doped Ba[Li2(Al2Si2)N6] shows the blue-shifted emission with two maxima at 468 and 507 nm under the 420 nm excitation. 3.2.9. Carbidonitride. Zhang et al. reported the Ce3+ luminescence in Y2Si4N6C which shows yellow emissions.231 The structure of Y2Si4N6C consists of a three-dimensional network of [C(SiN3)4] starlike units. Two crystallographically independent Y atoms occupy the cavity of the network, with Y1 coordinated to 5 N and Y2 to 6 N atoms. Under the 425 nm excitation, Y2Si4N6C:Ce3+ (3 mol %) shows a broad emission band with a maximum of 560 nm and a fwhm of ∼120 nm. The luminescence intensity of Y2Si4N6C:Ce3+ is about 25% of that of a commercial YAG:Ce3+. Hsu et al. investigated the effect of the La → Y substitution on the luminescence of (Y0.95−xLaxCe0.05)2Si4N6C (0 ≤ x ≤ 0.95).232 With increasing the La content, the emission band blue-shifts, and the emission maximum changes from 538 to 442 nm. 3.2.10. Carbodiimide. Wu et al. reported the luminescence of Ce3+ in a carbodiimide Y2(CN2)3.233 It was synthesized by 1965

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Figure 10. (a) Excitation and emission spectra of γ-alon:Mn2+, Mg2+ (5 mol % Mn2+ and 10 mol % Mg2+). (b) Chromatic coordinates of γ-alon:Mn2+, Mg2+, showing a high color purity. (c) Absorption and quantum efficiency of γ-alon:Mn2+, Mg2+ as a function of excitation wavelengths. (d) Excitation and emission spectra of γ-alon:Mn2+, Mg2+, Eu2+ (green: 5 mol % Mn2+, 10 mol % Mg2+ and 2 mol % Eu2+; red: 5 mol % Mn2+ and 10 mol % Mg2+; blue: 5 mol % Mn2+, 10 mol % Mg2+). (e) Absorption and quantum efficiency of γ-alon:Mn2+, Mg2+ (5 mol % Mn2+, 10 mol % Mg2+, and 2 mol % Eu2+). (f) Thermal quenching of γ-alon:Mn2+, Mg2+, Eu2+ (red: 5 mol % Mn2+ and 10 mol % Mg2+; black: 5 mol % Mn2+, 10 mol % Mg2+ and 2 mol %). Reprinted with permission from ref 241. Copyright 2008 American Institute of Physics. Reprinted from ref 243. Copyright 2015 American Chemical Society.

firing the powder mixture of YF3, Li2(CN2), and CeF3 in a silica ampule at 500−700 °C for 8−12 h. The structure of Y2(CN2)3 consists of one layer of Y3+ ions and another layer of [NCN]2− anions within the c-axis. The Y atoms are coordinated to 7 N atoms, forming a capped trigonal prismatic polyhedron (YN7). Y2(CN2)3:Ce3+ shows three bands with the maxima of 250, 310, and 410 nm in the excitation spectrum, respectively. Under the 410 nm excitation, the carbodiimide shows a broad emission band with a maximum of 570−577 nm and a fwhm of >120 nm. The optimal Ce3+ concentration is 3 mol %.

dual-emission consisting of green and red was observed in Eu2+Yb2+ codoped SrSi2O2N2 by Ruan and Kirakosyan et al., which enables one to create high color rendering warm wLEDs.237,238 A red emission from Yb2+-doped CaAlSiN3 was reported by Zhang et al., which is 629 nm (fwhm = 75 nm) under the 450 nm excitation.239 Liu et al. reported the red luminescence of Pr3+ in β-sialon.240 The excitation spectrum consists of five peaks in the visible light region (460, 470, 488/, 498, and 509 nm) that are due to the 3H4 → 3PJ (J = 2, 1, and 0) transitions, respectively. Under 460 nm excitation, the emission spectrum shows several spectral lines with peaks at 613, 624, and 641 nm, which can be assigned to 1D2 → 3H4, 3P0 → 3H6, and 3P0 → 3F2 transitions of Pr3+ ions, respectively. Xie et al. reported the narrow-band green emission of Mn2+and Mg2+-codoped γ-alon.241 γ-alon has a defect spinel structure with the composition of Al(64+x)/3□(8−x)/3O32−xNx (3.5 ≤ x ≤ 5.8, □ = cation defect).242 γ-alon:Mn2+, Mg2+ (5 mol % Mn2+ and 10 mol % Mg2+) was synthesized by a solid-state reaction of Al2O3, AlN, MgO, and MnCO3 at 1800 °C for 2 h under a 0.5 MPa N2 atmosphere. It exhibits several excitation peaks at 340, 358, 381, 424, and 445 nm, which are attributed to electronic transitions from the ground state of 6A1 to the excited states of 4T2 (4P), 4E (4G), 4T2, [4E (4G), 4A (4G)] and 4T2 (4G), respectively (Figure 10a). Under the 445 nm excitation, the emission spectrum shows a single narrow band with a maximum of 520 nm and a fwhm of 44 nm, which is assigned to the 4T1 (4G) → 6A1 transition. The narrow emission band of γ-alon:Mn2+, Mg2+ results in a higher color purity than other green phosphors, which makes it possible to be used in LCD backlight (Figure 10b). The internal quantum efficiency is 61%, but the absorption efficiency of Mn2+ is as low as 20% due to the spin-forbidden transitions, which leads to a low external quantum efficiency of 12% (Figure 10c). To solve this problem of low absorption of Mn2+, Liu et al. applied the Eu2+ → Mn2+ energy transfer strategy to increase the efficiency by 700% (Figure 10d).243 Under the 365 nm excitation, 2 mol % Eu2+doped γ-alon:Mn2+,Mg2+ has the absorption, internal, and external quantum efficiencies of 65, 75, and 49% respectively, enabling it to match well with near-UV LEDs (Figure 10e). γ-

3.3. Down-Conversion Nitride Materials Doped with Other Activators

Besides the most commonly used Eu2+ and Ce3+, other ions such as Yb2+, Pr3+, Mn2+, and Be2+ are sometimes used as activators in nitride materials.234−242 Xie et al. reported a blue-light excitable green-emitting Yb2+doped Ca-α-sialon with the composition of (Ca1−xYbx)m/2Si12−m−nAlm+nOnN16−n (0.002 ≤ x ≤ 0.10, 0.5 ≤ m = 2n ≤ 3.5).234 The excitation spectrum consists of several distinct bands centered at 445, 342, and 307 nm, which can be assigned to the 5d4f13 to 5d4f14 electron transitions. Under the 445 nm excitation, the emission spectrum shows a broad band with a maximum at 549 nm and a fwhm of 78 nm. The composition is optimized at x = 0.005 (0.5 mol %), m = 2 and n = 1. Liu et al. investigated the luminescence of Yb2+ in β-sialon (Si8−zAlzOzN6−z:Yb2+).235 Similar to α-sialon:Yb2+, β-sialon:Yb2+ exhibits three main excitation bands centered at 305, 355, and 480 nm, respectively. The emission spectrum displays a broad band peaking at 540 nm and has a fwhm of 66 nm for z = 0.5 and 0.4 mol % Yb2+ under the 480 nm excitation. The external quantum efficiency is about 9%, which is much lower than that of Eu 2+ -doped β-sialon. Bachmann et al. studied the Yb 2+ luminescence in SrSi2O2N2 which emits an anomalous red emission at 615 nm under the 450 nm excitation.236 This strongly red-shifted luminescence (compared to 540 nm for Eu2+) is characterized by a large Stokes shift and a low thermal quenching temperature, owing to the Yb2+-trapped exciton luminescence. A 1966

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Figure 11. (a) Excitation and emission spectra of the sample with a molar ration of PEG/B = 4.0 × 10−3 and heated at 700 °C, the inset illustrating the closed-shell [BO2]− luminescence center. (b) Total and partial densities of state of BCNO (upper) with the inset showing the configuration of BCNO, and the schematic energy level diagram of BCNO (down). (c) Tunable emission colors of BCNO phosphors with different compositions (PEG/B ratio) and processing parameters (temperature and holding time) (from left to right, 2.0 × 10−3, 900 °C, 30 min; 2.0 × 10−3, 800 °C, 30 min; 4.0 × 10−3, 700 °C, 60 min; 4.0 × 10−3, 700 °C, 45 min; and 4.4 × 10−3, 700 °C, 30 min. Reprinted with permission from ref 259. Copyright 2008 Wiley-VCH. Reprinted with permission from ref 260. Copyright 2009 Electrochemical Society. Reprinted with permission from ref 263. Copyright 2014 Wiley-Blackwell.

alon:Mn2+, Mg2+ has a small thermal quenching, and its luminescence is maintained 88% of the initial intensity at 150 °C. With the Eu codoping, the thermal quenching becomes a little bit large (Figure 10f). Teisseyre et al. reported an interesting light converter based on yellow-emitting beryllium-doped GaN (GaN:Be) single crystals.244 The single crystal was grown at 1450 °C under 1 GPa of nitrogen pressure by the high nitrogen pressure solution method. Under the 400 nm excitation, GaN:Be shows a very broad emission band with the maximum at 610 nm and a fwhm of ∼150 nm. The origin of the yellow emission is still argued, which could be related to defects formed by the substitution of Ga by Be.245−247 By growing violet-blue GaN diodes on the GaN:Be substrate using the metal organic chemical vapor deposition (MOCVD) technique, Teisseyre et al. successfully fabricated high color rendering monolithic wLEDs (Ra ∼ 89.7).244

due to transitions between lone pair (LP) states in the valence band and the π* antibonding states in the conduction band. Zhang et al. reported the tunable emission colors of g-C3N4 (from 400 to 510 nm) by controlling the temperature of thermal condensation of melamine. The tunable color is owing to changes in the optical band gap that is closely linked to the size of the sp2 clusters.255 Guo et al. improved the quantum yield of g-C3N4 from 5.9 to 11.8% by treating it in nitric acid.256 wLEDs were prepared by pumping the phosphor mixture of blue g-C3N4 and red coppercysteamine Cu3Cl(SR)2 at a weight ratio of 1:1.67 by a 365 nm near UV LED, showing the color coordinates (x, y), correlated color temperature, and CRI of (0.352, 0.320), 4523 K, and 94.3, respectively. Han et al. also reported the white light emission with the color coordinates of (0.337, 0.249) and the color temperature of 5468 K from the phosphor blend of g-C3N4/ Y2MoO6:Eu3+ under near UV excitation.257 Wang et al. prepared g-C3N4/silica gels by using a one-pot one-step solvothermal method and observed a broad band with emission maxima at 430, 480, 580, and 630 nm, respectively.258 The emissions of 480, 580, and 630 nm are ascribed to the self-excitation effect. White light emission with a quantum yield of 27% is realized for g-C3N4/ silica gels under the 365 nm excitation. 3.4.3. BCNO. Luminescent carbon-based boron oxynitride (BCNO) was first reported by Ogi et al.259 BCNO phosphors were synthesized by a one-step low-temperature liquid process that heated the mixed solution of H3BO3, (NH2)2CO, and polyethylene glycol [H(CH2CH2O)nOH, PEG] with a MW of 20000 at 700−900 °C for 30−60 min.260 For the sample with a molar ratio of PEG/B = 4.0 × 10−3 heated at 700 °C, the excitation spectrum covers a broad spectral range from UV to visible region, and the emission spectrum exhibits a single broad band with a maximum of 540 nm and a fwhm of >110 nm (Figure 11a). The luminescence is originated from the closed-shell [BO2]− anions that act as a luminescence center, with the electron transitions from the first excitation state of 2Πg to the ground state of 1∑+g·259−264 By combining the experimental results with the first-principle calculations, Zhang et al. addressed that the carbon and oxygen impurities could lead to additional Cand O-related energy levels in between the valence and conduction band of BCNO, as shown in Figure 11b.263 The

3.4. Activator-Free Down-Conversion Nitride Materials

3.4.1. GaZnON. Chouhan et al. selected β-Ga2O3 (Eg = 4.40 eV) as a host and introduced elements of Zn and N into its crystal lattice to form solid solutions. The band gap is reduced to 3.53− 3.21 eV with the introduction of Zn and N.248 The solid solution GaZnON nano phosphor was synthesized by combustion using a unimolecular mixture of metal oxides and urea as a flux/combustible agent. The structural analysis evidence the (Ga/Zn)-N bonds and O deficiency in the compounds, and the centrally occupied Ga/Zn is tetrahedrally coordinated to O/ N at apex positions. Under UV excitation (350 nm), GaZnON exhibits blue luminescence with a broad emission band (fwhm ∼100 nm) covering the spectral range of 300−550 nm. The blue emission is originated from donor−acceptor pairs or the chargetransfer luminescence. This blue phosphor is suggested for use in near UV LED-driven wLEDs. 3.4.2. g-C3N4. Polymeric graphic carbon nitride (g-C3N4), a novel organic and metal-free semiconductor and an appealing material for a variety of applications including photocatalysis, bioimaging, and sensing, has a two-dimensional structure and a band gap of 2.7 eV.249−254 It can be viewed as graphite with the carbon lattice being partially substituted by nitrogen atoms in a regular fashion. Strong blue emission at ∼460 nm (fwhm ∼80 nm) under near UV excitation is observed in g-C3N4, which is 1967

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already known phosphors with slight variations in structure and composition; type III, novel phosphors having known crystal structures but never considered as luminescent materials before; and type IV, novel phosphors having new crystal structure. Only type III and type IV are considered as really “new” because there are no IP problems. In this review, we still treat type II phosphors as new luminescent materials for academic purposes.

emission color and bandwidth of BCNO can be tuned from 387 to 571 nm by controlling the PEG/B ratio and the heating regime (Figure 11c).259,261,262 High quantum efficiency (53−79%) can be achieved for phosphors with blue, green, and yellow colors. 3.4.4. Nitridoborate. Nitridoborates containing [BN]n−, [BN2]3−, [BN3]6−, [B2N4]8−, and [B3N6]9− ions are regarded as one of the most interesting group of nitride compounds, which can be synthesized by conventional solid state method at relatively low temperatures.140,141 Some of these compounds, such as Mg3(BN2)N and Mg3Ga(BN2)N2, have been reported as broad-band luminescent materials, leading to a significant extension of nitride phosphors.140,141 Mg3(BN2)N was synthesized by Scholch et al. using a solid state reaction of MgCl2, Mg3N2, and Li3BN2 in a silica ampule under vacuum at 700 °C for 24 h.140 The precursor Li3BN2 was prepared by the reaction between Li3N and BN at 650 °C for 3 h in Ar. The undoped Mg3(BN2)N shows a yellow body color and a bandgap of 2.7 eV, indicative of its strong absorption of blue light. Under the 420 nm excitation, Mg3(BN2)N exhibits a very wide emission band with a maximum of 700 nm and a fwhm of 173 nm. The excitation spectrum covers a broad range of 250− 500 nm and has a maximum at 420 nm. Doping of Eu2+ in Mg3(BN2)N does not change excitation and emission spectra of the undoped sample, indicating that defects (e.g., nitrogen/ magnesium vacancies or oxygen-related) rather than Eu2+ contribute to the luminescence. Dutczak et al. reported the synthesis and luminescence of Mg3Ga(BN2)N2.141 The nitridoborate was obtained by firing the powder mixture of MgCl2, GaN, Li3N, and Li3BN2 in a niobium ampule at 1100 °C for 48 h. The optical bandgap of Mg3Ga(BN2)N2 is about 3.1 eV. It exhibits a narrow excitation band centered at 340 nm and an extremely broad emission band with a maximum of 605 nm as well as a fwhm of 203 nm under the 340 nm excitation. The large fwhm value implies multiple emission centers in Mg3Ga(BN2)N2. The luminescence is ascribed to lattice defects, such as Mg/Ga vacancies and oxygen substituting nitrogen, which induces additional energy levels between the valence and conduction bands.

4.1. Discovery of New Materials with Known Crystal Structures

Starting from a known crystal structure, luminescent nitride materials with new compositions and interesting luminescence can be developed and synthesized by means of (i) solid state combinatorial chemistry; (ii) incorporation of nitrogen into oxides; and (iii) chemical unit substitution. 4.1.1. Solid State Combinatorial Chemistry. The combinatorial chemistry approach has been reported for highthroughput screening and optimizing luminescent materials.264−267 The conventional combinatorial chemistry is usually designed for liquid solution- or thin film-based high-throughput experimentation but not feasible for nitride-based luminescent materials because some of the raw materials are extremely susceptible to an ambient atmosphere. Sohn et al. developed a solid-state high-throughput powder-dispensing synthesis technique for screening nitride phosphors with promising photoluminescence and enhanced color chromaticity.268−271 In the ARSi4N7:Eu2+ (A = Sr, Ca, Ba; R = Y, La, Lu) system consisting of 3 alkali earth elements and 3 rare earth elements (Figure 12), a double-ternary combinatorial chemistry library is visualized in terms of structure, luminescent intensity, and chromatic coordinates, leading to the discovery of new green-emitting SrLuSi4N7:Eu2+ and (Sr,Ba)YSi4N7:Eu2+.269 In the Ce3+-doped

4. DISCOVERY OF NEW DOWN-CONVERSION NITRIDE MATERIALS As mentioned in section 3, luminescent nitrides have shown very promising photoluminescence properties for use in wLEDs and attracted significant attention for both chemists and materials scientists. From the academic viewpoint, as a new type of luminescent materials, there is a growing demand to find new materials with interesting crystal structure and properties. In addition, from the industrial viewpoint, the continuous development of emerging SSL technologies and devices also motivate the search for novel luminescent materials with promising and desired properties. Traditionally, luminescent nitride materials, such as AlN:Eu2+, α-sialon:Eu2+, β-sialon:Eu2+, CaAlSiN3:Eu2+, MSiN2:Eu2+ (M = Ca, Sr, Ba), MSi2O2N2:Eu2+ (M = Ca, Sr, Ba), LaSi3N5:Ce3+, and La3Si6N11:Ce3+, are discovered by screening the database and doping them with activators. However, there is a limitation for such a trial and error method because nitride compounds in the database will not be updated without discovering new ones. Moreover, this method is tough and time-consuming. Therefore, it is highly required to develop effective methodologies for discovering new luminescent nitride materials. Sohn et al.268−271 classified LED phosphors into four types: type I, already known phosphors with new applications; type II,

Figure 12. Double-ternary combinatorial chemistry library in terms of (a) photographs under UV irradiation, (b) photoluminescence intensity, and chromaticity coordinates (c) x and (d) y. Reprinted from ref 269. Copyright 2012 American Chemical Society. 1968

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substitution of B−O by Si−N in La5Si2BO13, Xia et al. developed oxynitride phosphors of La5(Si2+xB1−x)(O13−xNx):0.05Ce3+ (x = 0−1) which show the redshift of emission from 421 to 463 nm (Figure 13a).219 The apatite-type La5Si3O12N (x = 1) can be

YN-AlN-Si3N4 system, new phosphors of Y6+x/3Si11−yAlyN20+x−yO1−x+y:Ce3+ (x = 0−3, y = 0−3) are discovered, which show green-to-yellow colors under blue or UV excitation.271 4.1.2. Incorporation of Nitrogen into Oxides. Starting from oxide phosphors, oxynitride luminescent materials can be derived by introducing nitrogen into the host lattice (see Table 3). These oxynitride phosphors are also considered as new Table 3. Examples of Oxynitride Compounds Derived from Oxides by Incorporating Nitrogen (with Eu2+ Doping) oxide host

derived oxynitride

ref

Ba0.83Al11O17.33 MAl2O4 (M = Ca, Sr, Ba) SrAl2Si2O8 Y3Al5O12 Sr2Al2SiO7 (Sr,M)2SiO4 (M = Mg,Ca,Ba) BaMgAl10O17 La5Si2BO13 Sr2SiO4 Ba2Si3O8 β-Sr2−yMgySiO4 Sr3B2O6 Ca14Mg2[SiO4]8

BaAl11O16N MAl2−xSixO4−xNx SrAl2−xSi2+xO8−xNx Y3Al5−xSixO12−xNx SrAl2−xSi1+xO7−xNx (Sr,M)2Si(O1−xNx)4 BaMgAl10−xSixO17−xNx La5Si2+xB1−xO13−xNx Sr2SiNzO4−1.5z Ba4Si6O16−3/2xNx β-Sr2−yMgySiO4−1.5xNx Sr3B2O6−3/2xNx Ca14Mg2Si8O30+δN2−δ

272 273 274 275 276 277 278 219 279 280 281 282 283

Figure 13. (a) Formation of La5Si3O12N from La5Si2BO13 via chemical unit cosubstitution of B−O by Si−N. Normalized emission spectra of La5(Si2+xB1−x)(O13−xNx):0.05Ce3+ (x = 0−1) phosphors upon excitation at 365 nm showing the red-shift with the cosubstitution. (b) Discovery of M2PO3N (M = Ca, Sr) compounds with the PO3N tetrahedral cosubstitution of SiO4 in M2SiO4. (c) Schematics of structural evolutions of Ca1−xLixAl1−xSi1+xN3 with the chemical unit substitution of [LiSi]5+ → [CaAl]5+ in CaAlSiN3. Reprinted from ref 201. Copyright 2015 American Chemical Society. Reprinted from ref 219. Copyright 2015 American Chemical Society. Reprinted from ref 288. Copyright 2016 American Chemical Society.

materials with different compositions and photoluminescence properties from their hosts (type II). The oxide precursor phosphors are usually nitridized by reacting them with Si3N4 or BN at high temperature. A variety of techniques are adopted to confirm the accommodation of nitrogen in the lattice, including XRD, EDS, FT-IR, EXAFS, XPS, and EPR.13,278,281,283−285 The incorporation of nitrogen significantly changes the local coordination environment or local structure of Eu2+ (Ce3+) ions, yielding both higher covalence and polarizability of Eu2+ (Ce3+)N3− bonds versus Eu2+ (Ce3+)-O2− bonds, and finally leading to the redshift of both excitation and emission bands. Ma et al. observed an additional red emission peak at ∼616 nm in βSr2SiO4:Eu2+ with introducing N3−, which is caused by the splitting of Eu(II) emission.281 Lee et al. found that the emission color was not tunable, but the photoluminescence intensity was increased by 148% associated with the partial nitridation of Ca13.7Eu0.3Mg2Si8O32.283 Moreover, the thermal stability is generally improved by nitridation of the corresponding oxide luminescent materials, owing to the increased structural stiffness. A detailed discussion on the spectral tuning with the nitrogen substitution can be seen in section 6.3. 4.1.3. Chemical Unit Substitution. Xia et al. has summarized the luminescent materials discovered by the chemical unit substitution strategy which is feasible for the heterovalent substitution.50 The idea behind the strategy lies on the fact that the structure of each compound is built up of various polyhedral units, and these polyhedra can be replaced by others. With the chemical unit substitution, the crystal structure generally remains unchanged. This approach has been also applied to tune the optical properties of perovskite oxynitrides.93 For luminescent materials developed by using the chemical unit substitution, the notable materials are Ca-α-sialon:Eu2+ and β-sialon:Eu2+. Ca-α-sialon:Eu2+ is formed by replacing [SiN]+ with [AlO]+ and [AlN]0 in α-Si3N4 and introducing Ca2+ for charge balance. β-sialon:Eu2+ is derived from β-Si3N4 by substituting [SiN]+ by [AlO]+. With the

considered to be derived from La5Si2BO13 (x = 0) with the chemical unit substitution. Marchuk et al. discovered a new oxonitridophosphate with the β-K2SO4 structure, M2PO3N (M = Ca, Sr). It can also be regarded to be formed by replacing Si−O by P−N in M2SiO4 (Figure 13b).50,201 Wang et al. synthesized solid solution nitride phosphors, Ca1−xLixAl1−xSi1+xN3:Eu2+, which are derived by the [LiSi]5+ → [CaAl]5+ substitution in CaAlSiN3:Eu2+ (Figure 13c).286−289 The emission color can be tailored in a wide range by controlling the substitution degree. Other examples can be found in Ca1−xLaxAl1+xSi1−xN3:Eu2+ formed by the [LaAl] 6+ → [CaSi] 6+ substitution, 286 Ca1−xAl1−xSi1+xOxN3−x:Eu2+ by the [SiO]2+ → [AlN]0 substitution and reducing one Ca2+ for charge neutrality,289 and Y2Si4N6C derived from SrYSi4N7 by the [YC]− → [SrN]− substitution.290 Recently, Schnick et al. discovered a series of new nitride luminescent materials which are isotypic to UCr4C4.35,95,130 These novel compounds have the general formula of AB2C2X4 and ABC3X4, allowing for a high substitutional variability.95 The pairs of Ga/Mg, Al/Mg, Li/Al, and Mg/Si can reside on the tetrahedrally coordinated sites, leading to the formation of Ba[Mg2Ga2N4], M[Mg2Al2N4] (M = Ca, Sr, Ba), Sr[LiAl3N4], and Sr[Mg3SiN4], respectively.35,95,130 1969

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Figure 14. (a−c) Schematics of the single crystal growth method for determining the crystal structure and photoluminescence of Ca[LiAl3N4]:Eu2+. (d−f) Schematics of the combined method (TEM and synchrotron microfocus diffraction) for determining the crystal structure and photoluminescence of La3BaSi5N9O2:Ce3+ microcrystallines. (e) The antiphase boundary highlighted with green lines and domain positions visualized by red and blue lines, analyzed by STEM-HAADF. (f) Excitation (blue) and emission (red) spectra of La3BaSi5N9O2:Ce3+ showing a yellow emission under the 400 nm excitation. Reprinted from ref 134. Copyright 2014 American Chemical Society. Reprinted from ref 220. Copyright 2015 American Chemical Society.

4.2. Discovery of New Materials with Unknown Crystal Structures

utilized. Durach et al. used the technique that combines transmission electron microscopy (TEM) and synchrotron microfocus diffraction to solve the structure of La3BaSi5N9O2 microcrystallines (Figure 14, panels d−f).220 This approach enables one to analyze particles with a volume even smaller than 1 μm3.295 It can also promise a much more accurate determination of structural parameters such as bond lengths, mixed occupancies, and displacement parameters, compared to the electron crystallography.296,297 Real structures of luminescent nitride materials, such as intergrowth, microtwinning, stacking disorder, and vacancy ordering, are hardly seen by single-crystal X-ray diffraction but can be elucidated by the combination of powder X-ray diffraction and TEM.143,144,146,147,180 Seibald et al. analyzed the real structures of Sr0.5Ba0.5Si2O2N2:Eu2+ and Sr0.25Ba0.75Si2O2N2:Eu2+ by using the above hybrid techniques and explored the structureluminescence relationship.146,147 Simulations of powder X-ray diffraction patterns reveal a disorder model with many antiphase and few twin boundaries in Sr0.5Ba0.5Si2O2N2:Eu2+. The emission shifting to longer wavelengths in Sr0.5Ba0.5Si2O2N2:Eu2+ is ascribed to a less restricted local lattice relaxation at the Eu2+ sites because of the shorter Eu−O bond length compared to that in SrSi2O2N2:Eu2+. In Sr0.25Ba0.75Si2O2N2:Eu2+, the intergrowth of nanodomains with SrSi2O2N2 and BaSi2O2N2-type structure is clarified by high-resolution TEM (HRTEM), resulting in the pronounced diffuse scattering. The HRTEM image simulations also reveal the cation ordering in the structure model, which enables one to explain the unexpected narrow emission band and intense blue-green emission in Sr0.25Ba0.75Si2O2N2:Eu2+. 4.2.2. Solid State Combinatorial Chemistry. As addressed in section 4.1.1, the luminescent materials discovered in the literature by the combinatorial chemistry approach cannot be considered as real “novel” phosphors but new or optimized compositions because the crystal structure of end members are already known. Sohn et al. proposed a solid state combinatorial chemistry method and carried out the pioneer work on the discovery of novel nitride luminescent materials.48,149,150,152 This strategy is named the “heuristic optimization involving a parametrization of the material novelty”, which applies a nondominated-sorting genetic algorithm (NSGA) for a preliminary screening of materials libraries and the particle swarm

4.2.1. Single Crystal Growth and Structure Determination. It is quite hard to analyze and determine the crystal structure of powders by X-ray powder diffraction (XRPD), but it becomes easier to use a single crystal for structure determination. The crystal growth method has already been used to discover other optical materials including nonlinear optical crystals, laser crystals, and scintillator crystals.50 For phosphor materials, this approach is adopted at Schnick’s group, leading to the development of a variety of luminescent nitride crystals with new crystal structures and promising photoluminescence properties. The crystal growth method is schematically shown in Figure 14a. First, it is the first step to grow high quality single crystals for structural determination, and the method of which includes the flux crystal growth and high-pressure crystal growth. Nitride compounds, such as nitridosilicates, nitridoaluminates, nitridogallates, nitridoaluminosilicates, nitridolithosilicates, nitridolithoaluminates, nitridomagnesosilicates, are grown by the flux crystal growth method.33,35,46,94,95,109,110,129,131−133,135−138 The fluxes used are usually sodium or lithium metals. The highpressure crystal growth is utilized to prepare crystals of nitridophosphates, oxonitridophosphates, and nitridophosphate chlorides.196−202 Second, the single crystals are picked, sealed in a glass capillary, and checked for quality on a Buerger precession camera. The crystal structure is determined by means of singlecrystal X-ray diffraction and then solved using direct method with SHELXS.291 The structure refinement is done by the leastsquares method using SHELXL (Figure 14b).292 To confirm the electrostatic consistency of the crystal structure, lattice energy calculations are carried out by Madelung Part of Lattice Energy (MAPLE).293−295 Third, the doped single crystals are sealed in silica glass capillaries, and their luminescence is then characterized at a spectrofluorimeter connected to a digital microscope by optical fibers (Figure 14c). Large-size and homogeneous single crystals suitable for structural determination with conventional single-crystal X-ray diffraction are hardly grown, and in most cases inhomogeneous and microcrystalline particles with crystal sizes of several micrometers are obtained. Some analytic methods other than the single-crystal X-ray diffraction need to be considered and 1970

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optimization (PSO) for fine-tuning in a converged composition space to narrow the possibility of discovery. As shown in Figure 15a, the approach consists of three sequential steps. Step I is to design and choose a composition

scale up processing (Figure 15, panels b and c). Several novel nitridosilicate phosphors have been discovered by this approach, which include Ca15Si20N30O10:Eu2+, Ba1.5Ca0.5Si5N6O3:Eu2+, BaAlSi 4 O 3 N 5 :Eu 2 + , La 4 − x Ca x Si 2 4 O 3 + x N 1 8 − x :Eu 2 + and Ce4−xCaxSi24O3+xN18−x:Eu2+.48,49,149,150,152 4.2.3. Single-Particle-Diagnosis Approach. As mentioned in Section 4.2.1, the conventional single-crystal X-ray diffraction is not suitable for solving the structure of crystals with sizes below several micrometers. In addition, if the synthesized powder shows mixed phases and the powder diffraction suggests a new compound with a very complex structure, the X-ray powder diffraction is also not feasible for structural determination. To solve these problems, Xie et al. proposed a singleparticle-diagnosis approach for rapidly discovering novel luminescent nitride materials with new crystal structures (Figure 16).46 The principle of this approach is to consider an individual luminescent particle in a complex powder mixture as a tiny single crystal and then to identify it by structural determination and property measurements using microscale characterization tools. The luminescent particles can be distinguished and separated by its emission color, crystalline morphology, growth habit, and size. In this method, a state-of-the-art fine-focus single-crystal X-ray diffractometer with a superhigh resolution is used for structural determination. The superhigh resolution is realized by using a charge-coupled devices (CCD) area detector and multilayer focusing mirrors, allowing one to analyze the structure of a very small single microcrystal that has a size 2 orders of magnitude lower than the single crystals usually used in the conventional single-crystal X-ray diffraction. The single-particle diagnosis method consists of four steps. The first step is to prepare the powder library in random material systems by using a traditional solid state reaction (Figure 16b). Eu2+ or Ce3+ is doped in the powders and regarded as

Figure 15. (a) Overall description of the proposed heuristic optimization involving a parametrization of the material novelty. (b) Schematics of phase purity and luminescence intensity enhanced with the screening process. (c) Photoluminescence spectra and photograph of the discovered BaAlSi4O3N5:Eu2+ showing blue emissions. Reprinted from ref 48. Copyright 2012 Wiley. Reprinted from ref 49. Copyright 2014 American Chemical Society.

space (materials libraries); step II is to pinpoint a novel phosphor by utilizing an NSGA- and PSO-assisted combinatorial materials search, complemented by the parametrization of the material novelty, and step III is to identify the new compounds by means of compositional analysis, structural determination, and powder

Figure 16. (a) Schematics of the single-particle-diagnosis approach consisting of four steps. (b) Powder library in the ternary system of Si3N4−Ba3N2− AlN. (c) Small single crystals pinpointed from samples 1−3 with different compositions. Two new nitride luminescent materials (cyan and yellow emissions) are discovered. Reprinted from ref 46. Copyright 2014 American Chemical Society. 1971

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degree. Oxosilicates have the maximal degree of condensation of 0.5 in SiO2, whereas (oxo)nitride(alumino)silicates have the degree of condensation varying in the range of 0.25 ≤ κ ≤ 1 (Table 1). The enhanced cross-linking at the N atoms together with higher condensation degrees in nitridosilicates, therefore, leads to the excellent thermomechanical properties and chemical stability. In classical (oxo)silicates, oxygen can only be terminally connected to Si (denoted as O[1]) or simply bridging two Si (O[2]), whereas nitrogen in nitride materials has a variety of coordination architectures, including N[1], N[2], and N[3] or even ammonium-type N[4], as shown in Figure 17. Moreover, the MX4

luminescent probes. The second step is to visually inspect the powder mixtures by a digital optical microscope coupled with an ultraviolet lighting source, pinpoint each crystal with distinct emission colors and morphologies, and then mount the crystal on the tip of a fine glass fiber using an epoxy for single-crystal Xray diffraction (Figure 16c). The third step helps one to identify if the crystal is an unknown phase or not by using the preliminary lattice parameters obtained in the second step. The last step is to finally determine the structure and composition of the new phase, as well as to carry out photoluminescence measurements of a single crystal. As seen in Figure 16 (panels b and c), Hirosaki et al. prepared about 20 compositions in the system of Ba3N2− Si3N4−AlN in a single experiment, selected three compositions (samples 1−3) from them, and finally discovered two new nitridosilicate phosphors: Ba5Si11Al7N25:Eu2+ (orange) and BaSi4Al3N9:Eu2+ (cyan).46 Other nitride phosphors, such as Ba2LiAlSi7N12:Eu2+, Ca1.62Eu0.38Si5O3N6:Eu2+, Sr2B2−2xSi2+3xAl2−xN8+x:Eu2+, and La2.5Ca1.5Si12O4.5N16.5:Eu2+ are also found by this method.47,153,154,194 Differing from traditional methods, the single-particlediagnosis approach promises a one-pot synthesis, use of just a few micrograms of powders, and simultaneous discovery of multiple compounds in one single experiment. With this approach, there is no need to synthesize phase pure powders or large-size single crystals any more, thus enabling one to substantially reduce the cost, energy, and labor intensity.

Figure 17. Schematics showing nitrogen coordinated to Si atoms with numbers of (a) 2, (b) 3, and (c) 4.

(M = Si, Al, Mg, Ga, Li, P; X = O, N) tetrahedra can be linked by common corners and/or edges, while the SiO4 tetrahedra in (oxo)silicates are exclusively corner-shared. Therefore, numerous architectures can be established in nitride materials. In this section, we describe crystal structures of luminescent nitride materials and simply group them into two- and threedimensional structures according to the dimensionality of the substructure.

5. CRYSTAL STRUCTURE OF DOWN-CONVERSION NITRIDE MATERIALS Similar to oxides, nitride compounds display great versatility in structure, including one-dimensional chain type, two-dimensional layerlike structure, three-dimensional framework, zeolitelike structure, etc.201,202,298 The crystal chemistry of nitrides with d i ff e r e n t t y p e s h a s b e en o v er v i ew ed by se v er al groups.86,88,89,298−304 Recently, with developing a unique high temperature synthetic route by the use of metals and silicon diimide, Schnick et al. has carried out extensive investigations of the solid state chemistry of nitridosilicates and sialons that can be considered as hosts for luminescent materials.298,299,304 In nitridosilicates and sialons, the construction and arrangement of basic tetrahedral building blocks (i.e., [Si/Al][O/N]4) are flexible and diverse. They can either be isolated, corner-sharing, edge-sharing, corner-/edge-sharing, or even face-sharing, enabling one to generate many kinds of structural configurations. In addition, these building blocks can be substituted partially or totally by other types, forming new materials. For example, based on the structure of UCrC4 or Na[Li3SiO4], Schnick et al. has developed a lot of new nitride phosphors with amazing photoluminescence properties (such as narrow-band red emissions).35,95,130,131,134 These new compounds will not only make a great supplement to the material family of nitrides but also allow us to have a deep insight into the crystal chemistry of nitrides. A great amount of nitride phosphors with structural diversity and luminescence versatility have been reported or discovered so far. Nitride phosphors surveyed in this work, such as (oxo)nitridosilicates and (oxo)nitrodoaluminosilicates, can be considered as a significant extension of silicate chemistry by introducing Si−N and/or Al−N bonds into the structure to substitute Si−O bonds. The condensation degree (κ) in these nitrides, which are constructed from three-dimensional and highly condensed MN4 (M = Si, Al) tetrahedral networks, can be evaluated by the molar ratio M:N. The higher M:N ratio is, the higher condensation

5.1. Two-Dimensional Layer Structures

Although MSi2O2N2 (M = Ca, Sr, and Ba) compounds have the same chemical formula, they exhibit different crystal structures.3,143,144 CaSi2O2N2 crystallizes in the P21 monoclinic structure, SrSi2O2N2 in the P1 triclinic system, and BaSi2O2N2 in the Pbcn orthorhombic system. The crystal structure of these compounds is closely related, containing silicate layers built up of highly condensed and corner-sharing SiON3 tetrahedra with the Q3 type. The silicate layers are separated by alkaline earth metal layers, as seen in Figure 18 (panels a−c). In this structure, each nitrogen atom is linked to three neighboring silicon, and the oxygen atom is exclusively bonded terminally to silicon atoms, forming a corrugated layer of anion [Si2O2N2]2−. As shown in Figure 18 (panels d−f), in CaSi2O2N2 and SrSi2O2N2, Ca/Sr atoms are coordinated to six O atoms, forming a distorted

Figure 18. Crystal structure of (a) CaSi2N2O2 viewed along [100], (b) SrSi2O2N2 viewed along [100], and (c) BaSi2O2N2 viewed along [001]. Polyhedra of (d) Ca[O6N] in CaSi2O2N2, (e) Sr[O6N] in SrSi2O2N2, and (f) Ba[O6N2] in BaSi2O2N2. 1972

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SrSi9Al19ON31, with the Eu-containing layer instead of the Srcontaining one being formed.47 Figure 20c shows that Eu2+ in AlN is located at the stacking faults, revealed by HAADF. A layered structure is thus proposed by Takeda (Figure 21d), where the Eu layer is sandwiched between the AlN wurtzite blocks.

trigonal prism that is capped by a single N atom (Figure 18, panels d and e), whereas in BaSi2O2N2, Ba atoms are surrounded by six atoms, forming a cuboid that is additionally capped by two N atoms (Figure 18f). BaSiN2 crystallizes in Cmca orthorhombic structure, containing pairs of edge-sharing SiN4 tetrahedra to form bow-tie shaped Si2N6 dimers sharing vertexes to form layers puckered twodimensional sheets separated by Ba ions. SrSiN2 has a distorted form of this structure, having a P21/c monoclinic structure (Figure 19). In such structures, there is only one crystallographic

5.2. Three-Dimensional Structures with Corner (Vertex)-Sharing Tetrahedra

5.2.1. Nitridosilicate. Differing from MSiN2 (M = Sr and Ba), CaSiN2 does not show the two-dimensional layer structure. It is isostructural to KGaO2 and has a Pbca orthorhombic structure.112 The structure of CaSiN2 consists of exclusively vertex-sharing SiN4 tetrahedra that are linked to form a threedimensional stuffed-cristobalite type framework (Figure 22a). The Ca atoms are hosted in the channels running along [100] that are formed by the tetrahedral network. There are two different crystallographic positions for Ca atoms. The Ca1 atom is surrounded by 4 N atoms located between 2.40 and 2.49 Å and by 2 further N atoms with distances of 2.79 and 3.02 Å. The Ca2 atom is in a highly distorted octahedral environment, again connected to 4 N atoms with shorter distances (between 2.43 and 2.48 Å) and to 2 N atoms with longer distances (between 2.79 and 2.83 Å). MgSiN2 has a wurtzite-derived structure, consisting of a threedimensional framework of vertex-sharing SiN4 tetrahedra (Figure 22b).3,112 These SiN4 tetrahedra form condensed sechser-ring channels along [010] that accommodate Mg atoms. The orientation of the [SiN4] tetrahedra in MgSiN2 leads to a noncentrosymmetric structure. M2Si5N8 (M = Ca, Sr, and Ba) is a very promising host for redemitting phosphors doped with Eu2+.33 The crystal structure of these nitridosilicates is different for Ca and Sr/Ba, respectively. Ca2Si5N8 has a monoclinic crystal system with the space group of Cc, whereas both Sr2Si5N8 and Ba2Si5N8 crystallize in a Pmn21 orthorhombic structure.109,110 Ca2Si5N8 has a highly condensed three-dimensional framework consisting of corner-sharing SiN4 tetrahedra in which half of the nitrogen atoms connect two Si neighbors (N[2]) and the other half of the nitrogen atoms have three Si neighbors (N[3]).109 The SiN4 tetrahedra form corrugated layers of condensed dreier rings that are interconnected by further SiN4 tetrahedra (green in color) (Figure 23, panels a and c). There are two different crystallographic sites for Ca atoms that are situated in the sechser-ring channels running along [010]. Each Ca atom in Ca2Si5N8 is coordinated to seven nitrogen atoms. Sr2Si5N8 and Ba2Si5N8 have the similar threedimensional networks of corner-sharing SiN4 tetrahedra with the same local coordination of N−Si bonding.110 Compared to Ca2Si5N8, M2Si5N8 (M= Sr, Ba) has significantly corrugated layers (Figure 23, panels b and d). The Sr or Ba atoms are coordinated to eight or nine nitrogen atoms. The average bond length between alkaline earth metals and nitrogen is about 2.880 Å. MYSi4N7 (M = Sr, Ba) is isostructural to MYbSi4N7, consisting of a three-dimensional network structure formed by cornersharing SiN4 tetrahedra (Figure 24a).122 Large channels along [100] and [010] are created by Si6N6 rings, which accommodate both Sr and Y atoms that are coordinated to 12 (SrN12) and 6 (YN6) nitrogen atoms, respectively. In the structure, one N atom (N3) unusually connects four Si atoms [N[4]], whereas other two N atoms (N1 and N2) are linked to two Si atoms (N[2]) as usual (Figure 24b).

Figure 19. Crystal structure of SrSiN2 viewed along (a) [001] and (b) [100]. Sr atoms are in red.

site for Ba or Sr atoms, which are both connected to eight N atoms. The Ba-N bond lengths in BaSiN2 are in the range of 2.777−3.350 Å, and the Sr−N distances vary in the range of 2.564−3.027 Å. Ba3Si6O12N2:Eu2+ is a layerlike oxonitridosilicate, and its structure is constructed by layers of vertex-sharing SiO3N tetrahedral forming vierer- and sechser-rings as fundamental building blocks (Figure 20).156,157 The nitrogen atoms are

Figure 20. Crystal structure of Ba3Si6O12N2 viewed along (a) [010] and (b) [001]. Ba atoms are green, and the SiO3N tetrahedra are blue.

connected to three silicon atoms (N[3]), while the oxygen atoms are either terminally bound (O[1]) or bridge two silicon atoms (O[2]). Two crystallographically independent Ba2+ atoms are situated between the silicate layers. The Ba1 atoms are coordinated to 6 O with an equal distance of 2.760 Å, forming a trigonal antiprism. The Ba2 atoms are coordinated to 6 O with distances of 2.812−2.959 Å and to 2 N with distances of 3.054− 3.499 Å, creating a capped distorted octahedron. SrSi9Al19ON31 crystallizes in a R3̅ rhombohedral structure, reported by Grins.185 It shows a layered structure, having a common chemical formula of (SrM4X5)(M6X9)(M3X3)n−2, with n being the number of layers between the SrM4X5 regions (n = 8 for SrSi9Al19ON31) and M = (Si, Al) and X = (O, N) (Figure 21a). The Sr atoms are situated in layers formed by vertexsharing MX4 tetrahedra. The layers are in between polarity inverted and face-sharing MX4 tetrahedral blocks or alternating with AlN type blocks into which (Si,Al)3(O,N)4.5 layers are incorporated. The Sr atoms are coordinated to 12 nearest (O, N) atoms, forming a cuboctahedron (Figure 21b). The Sr-(O, N) distances of ∼3 Å are equal. The neighboring Sr atoms are separated by a distance of 5.3350 Å. Takeda et al. addressed that the Eu- and Si-codoped AlN exhibited a similar structure of 1973

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Figure 21. Crystal structure of SrSi9Al19ON31 viewed along [010], showing the Sr-containing layers formed by vertex-sharing MX4 tetrahedra. (b) 12 coordination of Sr, forming a SrN12 cuboctahedron. (c) HAADF-STEM image of (1) Eu, Si codoped AlN along [110] and (2) the highest resolution image. (d) Schematic layered structure of AlN:Eu,Si (2) derived from the structure of SrSi9Al19ON31 (1), consisting of three parts: Eu layer (filled part), the wurtzite block (hatched part), and the inversion layer (open part). Reprinted with permission from ref 65. Copyright 2010 The Royal Society of Chemistry.

Figure 22. Crystal structure of (a) CaSiN2 along [100] and (b) MgSiN2 along [010]. [SiN4] tetrahedra are blue, Ca orange and pink spheres, and Mg yellow spheres.

Figure 24. (a) Crystal structure of SrYSi4N7 viewed along [010], and (b) the star-like [N(SiN3)4] building groups.

Figure 25. Crystal structure of (a) LaSi3N5 viewed along [100], vierer ring pink, interconnecting [SiN4] tetrahedra blue; and (b) La3Si6N11 viewed along [001], showing vierer- and achter-rings running along [001].

atoms connected to three silicon atoms in a way similar to the Si3N4 structure; and the remaining three-fifths of nitrogen atoms are linked two silicon atoms. La3Si6N11 has a P4bm tetrahedral structure, consisting of highly condensed three-dimensional networks of corner-sharing SiN4 tetrahedra forming layers of vierer- and achter-rings running along [001] (Figure 25b).210 There are two crystallographically independent La atoms with La1 situated in the vierer-ring channels and coordinated to nine nitrogen atoms and with La2 captured by the achter ring and connected to eight nitrogen atoms. The La1−N distances are 2.70 Å in average, and the average La2−N distances are 2.62 Å. 5.2.2. Nitridoaluminosilicate. Eu2+-doped CaAlSiN3 is an important red phosphor for use in high color rendition white

Figure 23. Crystal structure of (a) Ca2Si5N8 along [010] and (b) Sr2Si5N8 along [100], showing blue corrugated [SiN4] layers, and green interconnecting [SiN4] tetrahedra (Ca, orange spheres; Sr, red spheres), and layers of condensed dreier rings in (c) Ca2Si5N8 and (d) Sr2Si5N8.

LaSi3N5 crystallizes in the orthorhombic crystal system with the space group of P212121.3,209 The structure is built up of vertex-sharing SiN4 tetrahedra (pink in color) forming vierer rings, which are interconnected by chains of SiN4 tetrahedra (blue in color) (Figure 25a). The La atoms are situated in pentagonal voids along [001] and are coordinated to nine nitrogen atoms with an average distance of 2.78 Å. There are two types of nitrogen environments, with two-fifths of nitrogen 1974

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LEDs.34 CaAlSiN3 has a wurtzite-related structure with the Cmc21 orthorhombic system, which is closely related to LiSi2N3 and Si2ON2 (Figure 26, panels a−c).34 The structure is

supertetrahedral networks is occupied by Ca and Li atoms. There are three different Ca sites in the structure. Ca1 and Ca3 atoms are coordinated to N atoms with distances of 2.39−2.73 Å, forming trigonal antiprismatic (distorted octahedral) coordination. Ca2 has the trigonal prismatic coordination by N, and the Ca2−N distances vary in the range of 2.49−2.73 Å. The degree of condensation (i.e., atomic ratio Al:N) is 0.71. Li has two different crystallographic positions, forming rings around the N1 position. The Li−N distance is in the range of 1.94−2.52 Å. 5.2.3. Oxonitridoalumosilicate. M-α-Sialon is structurally derived from α-Si3N4 by the partial substitution of Si−N bonds by Al−O and Al−N bonds.163 Metals M (i.e., alkaline earth metals, alkali metals, or rare earth metals) need to be introduced into the structure to maintain the charge balance with the substitutions. Similarly, β-sialon is derived from β-Si3N4 by partially substituting Si−N bonds by Al−O bonds. The α- and βsialons have respectively the P31c trigonal and P63 hexagonal structures which are built up of corner-sharing (Si,Al)(O,N)4 tetrahedra.163,172 The stacking sequence for the layers of Si/Al and O/N atoms is different for α- and β-sialon, which is ABAB··· and ABCDABCD··· in β- and α-sialon, respectively (Figure 28).163 The CD layer in the α phase is related to AB by a c-glide

Figure 26. Crystal structure of (a) CaAlSiN3 along [001], (b) LiSi2N3 along [100], (c) Si2ON2 along [100], (d) CaAlSiN3 along [010], (e) LiSi2N3 along [100], (f) Si2ON2 along [100], and (e) AlN along [100].

composed of highly condensed three-dimensional network of vertex-sharing XN4 (X = Si, Al) tetrahedra in which one-third of the nitrogen atoms are coordinated to two Si (N[2]) and the remaining two-thirds connect to three Si (N[3]).34 The disordering Si and Al atoms randomly occupy the same 8b site, forming sechser-ring (X6N6) channels running along [001]. The Ca atoms are situated in these channels and are connected to five N atoms with distances of 2.420−2.703 Å.34 The structure of CaAlSiN3, LiSi2N3, and Si2ON2 can also be considered as a superstructure of variant of the wurtzite-type AlN, with the tetrahedra forming corrugated layers of highly condensed dreier rings (Figure 26, panels e−g). The similarity in structure of CaAlSiN3, LiSi2N3, and Si2ON2 makes it possible to form solid solutions of CaAlSiN3−LiSi2N3 and CaAlSiN3−Si2ON2, which can tune the luminescent properties of CaAlSiN3:Eu2+. Wagatha et al. reported the crystal structure of Ca18.75Li10.5[Al39N55].136 It crystallizes in a cubic structure with the Fd3̅m space group. Ca18.75Li10.5[Al39N55]:Eu2+ exhibits similar structural motifs with Na26Mn39O55, which comprises two interpenetrating diamond-like frameworks built up of vertexsharing sphalerite-like T5 supertetrahedra that are connected by bridging AlN4 tetrahedra (Figure 27).136 Each supertetrahedron consists of 35 AlN4 units. The interstitial space between the

Figure 28. (a) AB (left) and CD (right) Si−N layers in silicon nitride. The stacking sequence in the α-phase is ABCD and in the β-phase is ABAB. Crystal structure of (b) β-sialon and (c) α-sialon viewed along [001]. Reprinted from ref 163. Copyright 1991 American Chemical Society.

plane. In β-sialon, the corner-sharing (Si,Al)(O,N)4 tetrahedra form large sechser-ring channels running along [001], whereas in α-sialon, the structure contains cages instead of channels owing to the c-glide plane. The M atoms in α-sialon are coordinated to 7 O/N anions at three different distances. In β-sialon, the Eu atoms are linked to 6 O/N atoms at equal distances.178 5.2.4. Nitridophosphate. Karau et al. reported the crystal structure of alkaline earth nitridophosphates, MP2N4 (M = Ca, Sr, Ba).197−199 MP2N4 (M = Ca and Sr) is isotypic with BaGa2O4, crystallizing in the P63 hexagonal structure. It exhibits a stuffed framework structure containing sequences of tetrahedral layers in the a−b plane which are built up from sechser rings formed by corner-sharing PN4 tetrahedra (Figure 29a). There are six different crystallographic sites for M = Ca and Sr.196,197 Ca atoms are coordinated to 5−12 N atoms with distances of 2.36 to 3.21 Å, whereas Sr atoms are linked to 9−12 N atoms with distances

Figure 27. Sketched structure of Ca18.75Li10.5[Al39N55]:Eu2+ showing the characteristic structural feature of T5 supertetrahedra: (a) AlN4tetrahedra network, (b) three different Ca sites (Ca1, yellow; Ca2, green; Ca3, orange), and (c) two Li sites (red). Reprinted from ref 136. Copyright 2016 American Chemical Society. 1975

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Figure 30. (a) Crystal structure of Ba3Ga3N5 viewed along [010]. (b) Infinite strands formed by edge-and corner-sharing GaN4 tetrahedra. (c) Coordination of four Ba atoms. (d) Crystal structure of Mg3GaN3 viewed along [010]. (e) Mg/Ga ordering in Mg3GaN3, with Mg atoms orange and Ga atoms pale green.

Figure 29. Crystal structure of nitridophosphates (a) MP2N4 (M = Ca, Sr) and (b) BaP2N4 viewed along [001]. There are two Ba crystallographic sites in the structure, with Ba2 (IIb) to be substituted by Sr. Crystal structure of nitridophosphate halides (c) Ba3P5N10X (X = Cl, I) and (d) Ba3P5N10Br viewed along [001]. PN4 tetrahedra are green, N atoms blue, Ba atoms gray, and halide ions X magenta. Reprinted with permission from ref 197. Copyright 2007 Wiley-VCH. Reprinted with permission from ref 201. Copyright 2015 American Chemical Society. Reprinted with permission from ref 202. Copyright 2015 Wiley-VCH.

the structure of Mg3GaN3 can be described based on anioncentered polyhedra (Figure 30, panels d and e). There are two different nitrogen sites in the structure: N1 is coordinated to six Mg atoms in edge-sharing MgN4 tetrahedra, and N2 is linked to five metal atoms in one MgN4 tetrahedron and four (Mg/Ga)N4 tetrahedra. 5.3.2. Nitridoaluminosilicate. Nitridoaluminosilicates are structurally built up on highly condensed framework containing either solely XN4 (X = Si or Al) or mixed XN4 tetrahedra. In SrAlSi4N7, the network structure is composed of corner-sharing SiN4 tetrahedra incorporating infinite one-dimensional chains of edge-sharing AlN4 tetrahedra running along [001] (Figure 31,

from 2.50 to 3.38 Å. BaP2N4 has a cubic structure, which is isostructural to a high-pressure phase of CaB 2 O 4 and BaGa2S4.197,199 In BaP2N4, Ba atoms occupy two different crystallographic sites and are coordinated to 12 N atoms with distances of 2.78−3.48 Å (Figure 29b). The crystal structure of nitridophosphate halides is built up of all-side vertex-sharing PN4 tetrahedra, forming a zeolite-like framework with three-dimensional achter-ring channels (Figure 29, panels c and d).201,202 The achter-ring channels contain alternately Ba and halide atoms, respectively. There are five independent crystallographic sites for Ba atoms. Ba1, Ba4, and Ba5 are coordinated to 8 N and 2 X atoms, whereas Ba2 and Ba3 are linked to 6 N and 2 X atoms forming slightly distorted hexagonal bipyramids.201,202 The Ba−N bond lengths are in good agreement with those usually observed in other barium nitridophosphates. However, the Ba−X distances are dependent on the coordination number, with the distance of the coordination sphere of Ba2 and Ba3 significantly shorter than that of the coordination sphere of Ba1, Ba4, and Ba5.201,202 5.3. Three-Dimensional Structures with Edge- and Corner-Sharing Tetrahedra

5.3.1. Nitridogallate. Hintze et al. reported the crystal structure of barium nitridogallate (Ba3Ga3N5) and magnesium double nitride (Mg3GaN3).94 Ba3Ga3N5 crystallizes in a C2/c monoclinic structure, consisting of infinite strands running along [010] which are made up of edge- and corner-sharing GaN4 tetrahedra (Figure 30, panels a and b).94 There are four different crystallographic sites of Ba with Ba1, Ba2, Ba3, and Ba4 coordinated to six, eight, four, and six nitrogen atoms, respectively (Figure 30c). The bond distances of Ba1−N, Ba2−N, Ba3−N, and Ba4−N are in the range of 2.767−2.8982, 2.867−3.119, 2.677−2.841, and 2.719−3.038 Å, respectively.94 Ba3Ga3N5 has a high degree of condensation of 0.6 owing to the large portion of edge-sharing GaN4 tetrahedra. Mg3GaN3 has a trigonal R3̅m crystal structure containing (Mg/Ga)N4 and MgN4 tetrahedra that are edge-shared with the same kind and corner-shared with the other kind.94 Alternatively,

Figure 31. Crystal structure of SrAlSi4N7 viewed along (a) [001] and (b) [010], showing infinite chains of all edge-sharing AlN4 tetrahedra (green) parallel to the c axis. (c) Crystal structure of Ba2AlSi5N9 viewed along [100], showing (d) highly condensed dreier-ring layers formed by corner-sharing (Si,Al)N4 tetrahedra.

panels a and b).45 There is an ordered distribution of Si and Al in this structure. On the other hand, the Si and Al atoms are randomly distributed and disordered in other nitridoaluminosilicates, such as CaAlSiN3, Ba2 AlSi5N 9, BaAl3Si4N 9, and Ba5Si11Al7N25.34,46,129 In CaAlSiN3, the (Si/Al)N4 tetrahedra are all edge sharing, forming a six-membered ring. Two-thirds of the nitrogen atoms are linked to three Si atoms and the rest connected to two Si atoms, creating a more rigid structure than CaSiN2 where all nitrogen atoms are coordinated to two Si 1976

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Figure 32. Crystal structure of (a) BaMg3SiN4 along [100], (b) SrMg3SiN4 along [001], (c) Sr[LiAl3N4] along [011], and (d) Ca[LiAl3N4] along [001]. Strands of (e) MgN4 (cyan) and SiN4 (blue) tetrahedra in SrMg3SiN4, and (e′) LiN4 (pink) and AlN4 (green) tetrahedra in Ca[LiAl3N4]. Cuboid-like polyhedron of (f) BaN8 in BaMg3SiN4, and (f′) CaN8 in Ca[LiAl3N4].

Figure 33. Crystal structures of Ba[Li2(Al2Si2)N6] viewed along (a) [001] and (b) [010] and Ba2LiAlSi7N12 viewed along (c) [010] and (d) [001]. (e) Bisphenoidal arranged tetrahedra of Li2N6. (f) Truncated square pyramid of BaN8. (e′) BaN11 polyhedra. Reprinted from ref 138. Copyright 2015 American Chemical Society.

a three-dimensional framework built up of infinite strands of edge- and corner-sharing (Mg, Si)N4 tetrahedra (Figure 32, panels a and b). The Ba atoms are centered inside vierer ring channels running along [100] and coordinated to eight nitrogen atoms. Tetrahedrally coordinated Mg2+ and Si4+ are disordered in the structure. On the other hand, Ca[Mg3SiN4] and Sr[Mg3SiN4] are isotypic to Na[Li3SiO4] with ordering of Si and Mg atoms, and their structure consists of strands along [001] formed by SiN4 tetrahedra linked to MgN4 ones via two edges and one corner (Figure 32, panels e and e′).131,132 The M atoms (M = Ca, Sr) are centered inside the vierer ring created by cornersharing SiN4 and MgN4 tetrahedra. In all these compounds, there is only one crystallographic site for Ba that is coordinated to eight nitrogen atoms, forming a cuboidal polyhedron (Figure 31, panels f and f′). In addition, their structures all have a maximum degree of condensation (k = 1), indicative of a highly rigid framework. 5.3.4. Nitridolithoalumosilicate. Pust et al. reported the crystal structure of nitridolithoaluminates, M[LiAl3N4] (M = Ca and Sr).35,134 Although they show the similar chemical formula, Ca[LiAl3N4] and Sr[LiAl3N4] have different crystal structures. Ca[LiAl3N4] is isotypic to Na[Li3SiO4] and Sr[LiAl3N4] to

atoms. In Ba2AlSi5N9, the unusual framework, consisting of both vertex- and edging-sharing (Si/Al)N4 tetrahedra, is built up on highly condensed tetrahedral sheets which are interconnected by dreier and vieier rings (Figure 31, panels c and d). 5.3.3. Nitridomagnesosilicate. At Schnick’s group, several nitridomagnesosilicates (M[Mg3SiN4], M = Ca, Sr, and Ba), nitridomagnesoaluminates [M[Mg2Al2N4], M = Ca, Sr, and Ba), nitridolithoaluminates (M[LiAl3N4] (M = Ca and Sr)] and nitridomagnesogallates (Ba[Mg2Ga2N4]) were reported to exhibit the UCr4C4-type structure.35,95,130,131,134 The common feature of the structure is that it contains a highly condensed three-dimensional network built up of strands of edge- and corner-sharing MN4 (M = Si, Al, Li, Mg, and Ga) tetrahedra which form vierer ring channels (Figure 32, panels a−d).131,132 The metal atoms are situated in these channels and coordinated to eight nitrogen atoms forming a cuboidal polyhedron (Figure 32, panels f and f′). With Eu doping, these compounds usually show promising luminescent properties, such as red emissions and narrow emission bands, enabling them to be amazing phosphors used for backlights. Schmiechen et al. reported the crystal structure of M[Mg3SiN4] (M = Ca, Sr, and Ba).131,132 M[Mg3SiN4] contains 1977

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Figure 34. (a) Crystal structure of Eu3Si15−xAl1+xOxN23−x (x ≈ 5/3) projected along [100] and projections of the first (middle) and second (right) framework units viewed along [001]. (b) Crystal structure and dreier ring layers of Sr14Si68−sAl6+sOsN106−s (s ≈ 7) viewed along [001]. (c) Crystal structure and dreier ring layers of Sr5Al5+xSi21−xN35−xO2+x (x ≈ 0) viewed along [010]. Reprinted with permission from ref 305. Copyright 2009 International Union of Crystallography.

periodicities along the a axis. The first and second substructures are expressed as AM2X and M2X4 (M = Si, Al, X = O, N), respectively. The chemical composition of such a crystal is generally considered as (AM2X)m(M2X4)n, where n/m represents the ratio of dimensions for the a axis in the two substructures (a1/a2). The [n/m] value is [23/14], [8/5], and [5/3] for Sr 1 4 Si 6 8 − s Al 6 + s O s N 1 0 6 − s :Eu 2 + (s ≈ 7) (Figure 34b), Sr5Al5+xSi21−xN35−xO2+x:Eu2+ (x ≈ 0) and Sr3Si13Al3O2N21:Eu2+, respectively.179−181 In these compounds, the host structure usually consists of highly condensed sechser or dreier ring layers that are formed by vertex- and edge-sharing (Si,Al)(O,N)4 tetrahedra (Figure 34, panels b and c). Sr5Al5+xSi21−xN35−xO2+x crystallizes in the orthorhombic Pmn21 structure, containing three independent Sr atoms which are coordinated to 9 (O, N) anions at distances of 2.45−3.44 Å (Figure 34c). 180 Sr3Si13Al3O2N21 has an orthorhombic P212121 structure. There is only one crystallographic site for Sr atoms in the structure that are coordinated to 10 (O, N) anions at distances of 3.0 Å.181

Cs[Na3PbO4], both can also be considered as an ordered variant of the UCr4C4- structure type.134 In both cases, a highly condensed three-dimensional framework is built up of edge- and corner-sharing AlN4 and LiN4 tetrahedra (Figure 32, panels c and d). Vierer ring channels along [001] for Ca[LiAl3N4]:Eu2+ and [011] for Sr[LiAl3N4]:Eu2+ are therefore formed, where the alkaline earth metals are centered. The MN8 (M = Ca and Sr) polyhedra are connected to each other by common faces, forming infinite strands. The degree of condensation of both compounds is 1.0. The crystal structure of Ba[Li2(Al2Si2)N6] was reported by Strobel et al.138 It contains a highly condensed threedimensional network of corner- and edge-sharing disordered (Al/Si)N4 tetrahedra that form two opposite and corner-sharing vierer ring layers (Figure 33, panels a and b). Both vierer ring channels runs along [001], with the small one linked to each other by common corners in an up−down sequence and the large one containing equally aligned (Al/Si)N4 tetrahedra.137 The LiN4 tetrahedra are centered in the small vierer ring channels and connected to each other by common edges, forming bow-tie units of Li2N6.138 These units are further linked to produce a tetragonal Li4N12-bisphenoid inside the small vierer ring channels (Figure 32f). The Ba atoms are centered in the large vierer ring channels, which are coordinated to eight nitrogen atoms at distances of 2.93−3.10 Å (Figure 33f).138 Takeda et al. reported the crystal structure of Ba2LiAlSi7N12.47 It consists of a three-dimensional network structure built up of vertex-sharing (Si/Al)N4 tetrahedra forming a corrugated layer along [001] as well as alternatively aligned edge-sharing (Si/ Al)N4 and LiN4 tetrahedra forming a pillar running along [010] (Figure 33, panels c and d). Both the corrugated layer and the pillar layer create a large one-dimensional channel running along [010], where Ba atoms are accommodated in a zigzag manner and coordinated to 11 nitrogen atoms (Figure 33e′). The BaN11 polyhedra are connected to each other by common faces. The distances of Ba−N and Ba−Ba are 2.93−3.32 Å (3.12 Å in average) and 3.49 Å, respectively.47 5.3.5. Oxonitridoaluminosilicate. Michiue et al. reported an interesting commensurate composite crystal structure in Eu3Si15−xAl1+xOxN23−x (x ≈ 5/3).305 The closely related structure is also observed in Sr14Si68−sAl6+sOsN106−s:Eu2+ (s ≈ 7), Sr5Al5+xSi21−xN35−xO2+x:Eu2+ (x ≈ 0) and Sr3Si13Al3O2N21:Eu2+ that emit intense green emissions. As seen in Figure 34a, the commensurate composite crystal model is composed of two substructures with different

5.4. Three-Dimensional Structures with Isolated Tetrahedra

As described in previous sections, nitride luminescent materials usually consist of (i) two-dimensional layer structures with metals ions sandwiched between the edge- and/or cornersharing tetrahedral layers or (ii) highly condensed threedimensional framework with metal ions accommodated in the voids formed by edge- and/or corner-sharing tetrahedra. However, in some nitride compounds (such as calcium germanium nitride, nitridomolybdates, nitridosilicates, nitridophosphates, etc.), the tetrahedral units are not connected to each other but isolated.306−309 Isolated tetrahedra are found in several nitride hosts for phosphors including lanthanum (yttrium) oxonitridosilicates and alkaline earth metal oxonitridophosphates (M2PO3N, M = Ca, Sr).32,200,218 As shown in Figure 35 (panels a and b), M5Si3O12N (M = La, Y) has the apatite structure consisting of isolated Si(O, N)4 tetrahedra. Two M sites are present in the structure, with M1 being coordinated to 7 O/N atoms and M2 to 9 O/N atoms. In Y4Si2O7N2, every two Si(O, N)4 tetrahedra share one N atom at the center, forming isolated Si2(O, N)7 ditetrahedra.32,310 M2PO3N (M= Ca, Sr) is isostructural to β-K2SO4, which has a structure consisting of discrete or isolated PO3N tetrahedra (Figure 35, panels c and d).200 There are two distinct crystallographic sites for metal atoms. M1 is coordinated to 7 O and 3 N atoms and M2 has 9 nearest anions of 7 O and 2 N atoms. 1978

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where Reff is defined as 1 1 = 6 Z R eff

(3)

(4)

Figure 36. Effects of local structure/coordination environments of the 5d energy levels of Eu2+. The centroid shift εc(2+, A), crystal field splitting εcfs(2+), redshift D(2+, A), the lowest energy 4f-5d transition of Eu2+ Eabs(Eu2+, A), valence band top (Ev) and conduction band bottom (Ec) of host A, exciton level (Eex), and activation energy from lowest 5d level to Ec (ΔE) are indicated. Reprinted with permission from ref 69. Copyright 2010 Elsevier.

6.1. Structure-Related Spectral Position of the Emission Band

Wang et al. calculated the centroid shift, spectroscopic polarizability (αsp), and cation electronegativity (χav) of some Ce3+-doped nitride phosphors and proposed a linear relationship between αsp and χav shown as below314

The luminescence of a dopant with allowed 4 f n → 4 f n−15d transitions is largely affected by the crystal structure of the hosts, especially the local structure or coordination of the dopant. When a free ion of Eu2+ or Ce3+ is doped into a compound, the average energy position of the 5d levels (i.e., the centroid or barycenter) will be lowered relative to that of the free ion. This is known as the spectroscopic redshift. Dorenbos summarized spectroscopic data of Ce3+- and Eu2+-doped inorganic compounds and proposed a relation between the redshift D(A) and crystalline environment of Ce3+ [the centroid shift εc(A) and the total crystal field splitting εcfs(A)] via310−313

αsp = 0.87 +

18.76 χav2

(5)

By comparing these data of nitride compounds to those of fluoride and oxide counterparts, Wang et al. addressed that the average anion polarizability exhibited an increased tendency with the average cation polarizability (b) in the sequence of bF < bO < bN. It thus indicates that the large centroid shift can be expected in nitride materials. Similarly, van Uitert proposed an empirical equation to correlate the emission color of Eu2+ or Ce3+ with structural parameters of the host via315

(1)

where 1/r(A) is the fraction of the crystal field splitting that adds to the redshift. The centroid shift εc (cm−1), based on the ligand polarization model, can be related to the Z coordination anion ligands at distance Ri (pm) via 1.44 × 1017αsp εc = 6 Z R eff

6

(R i − 12 ΔR)

D(Eu 2 +, A) = 0.64D(Ce 3 +, A) − 0.233 eV

6. STRUCTURE-RELATED LUMINESCENCE AND SPECTRAL TUNING OF DOWN-CONVERSION NITRIDE MATERIALS Photoluminescence properties of materials are significantly dependent on the crystal structure of host compounds. The structural parameters, such as covalence of chemical bonds, type and size of cations/anions, volume and symmetry of the coordinated polyhedron, coordination number, bond length, and ordering/disordering, play important roles in the spectral position, shape, and width of excitation and emission bands. In addition, the structural rigidity and the electronic structure of host compounds are closely tied to the thermal quenching temperature of luminescent materials, as will be discussed in section 7. Therefore, in order to design/discover new luminescent materials with promising properties or to enhance/modify the photoluminescence properties of existing materials, it is of great importance to understand the structure− property relationship. Furthermore, with this relationship in mind, it is able to modulate the emission spectrum and tune the emission color of luminescent materials in a broad range.

εcfs(A) − 1890 cm−1 r(A)

i=1

1

αsp is the spectroscopic polarizability of anions, and ΔR is the ionic size difference between Ce3+ and the cation for which it substitutes. The centroid shift depends on the coordination number, bond length, and anion polarizability, whereas the crystal field splitting is greatly controlled by the nearest anion ligands, such as the shape and size of the coordinating anion polyhedron. For Eu2+, the redshift D(Eu2+, A), schematically presented in Figure 36, can be related to that of Ce3+ [D(Ce3+, A)] via the following equation313

Figure 35. Crystal structures of La5Si3O12N viewed along (a) [001] and (b) [100] and Ca2PO3N viewed along (c) [001] and (d) [100], showing isolated Si(O/N)4 (blue) and P(O/N)4 (green) tetrahedra.

D(A) = εc(A) +

Z

∑

E = Q [1 − (V /4)1/v 10−(cn·ea·r)/8]

(6)

where E is the position in energy of the lower d-band edge for Eu2+ or Ce3+ (in cm−1), Q is the position in energy for the lower d-band edge for the free ion, V is the valence of the dopant, cn is the coordination number, ea is the electron affinity of the atoms that form anions (in eV), and r is the radius of the host cation

(2) 1979

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substituted by Eu2+ are all coordinated to eight nitrogen atoms, leading to the similarity in polyhedral configuration and volume. This finally results in the nearly equivalent crystal field splitting, therefore yielding red emissions in these emerging nitride phosphors. 6.1.2. Effect of the Cation Size. The size of cations which are substituted by Eu2+ or Ce3+ has a great influence in emission colors of nitride phosphors. It is generally regarded that the metal−ligand bond length and the polyhedron volume of the coordinating cation decrease when the ionic size of cations reduces, which leads to a large crystal field splitting and therefore the redshift of luminescence. With decreasing the ionic size from Ba, Sr, to Ca, the emission band redshifts and the emission wavelength increases. This trend has already been observed in a number of inorganic compounds such as orthosilicate (M2SiO4:Eu2+), thiogallates (MGa2S4:Eu2+), binary sulfides (MS:Eu2+), and phosphates.321−325 It also applies for nitride phosphors, as shown in Figure 37. It is seen that the emission

substituted by the dopant (in nanometers). Using this equation, Xie et al. demonstrated that the predicted luminescence of nitride phosphors was in line with the experimental one.192 As described above, nitride compounds exhibit a tremendous versatility in chemical composition, crystal structure, and local coordination. This leads to great variations in the spectral position and shape of photoluminescence spectra for divalent europium or trivalent cerium (also including other ions such as Yb2+), as seen in Tables 1 and 2. According to van Uitert and Dorenbos, the luminescence of Eu2+ or Ce3+ in nitrides is significantly controlled by the nephelauxetic effect and crystal field splitting which are exclusively related to the crystal structure of the host crystals.311−315 In other words, the spectral position and shape are dominantly determined by structural parameters, such as the crystallographic site to be occupied, coordination number, bond length, symmetry, covalence, the ionic size of the substituted cations, etc. By using the crystallographic data of the crystal hosts, the spectral position of the emission band of Eu2+ or Ce3+ could be predicted, which enables one to search for new luminescent materials and to provide guidelines to design luminescent materials. 6.1.1. Effect of the Structure Type. Different emission colors have already been observed in polymorphic orthosilicate Ca2SiO4 due to the changes in crystal structure, with βCa2SiO4:Ce3+ exhibiting an intense blue emission at 425 nm under the 360 nm excitation whereas γ-Ca2SiO4:Ce3+ showing a strong yellow emission at 575 nm under the 450 nm excitation.316−318 This also occurs for nitrides with polymorphic phase transitions. α- and β-Sialons are derived from two polymorphs of Si3N4 (i.e., α-Si3N4 and β-Si3N4, respectively). As described previously, the structure differs from each other in crystal system and local coordination. In α-sialon, Eu2+ is situated in a cage and coordinated to 7 (O, N) atoms at different distances, while in β-sialon, Eu2+ is centered in a hexagonal channel and coordinated to 6 (O, N) atoms at the same distance. Therefore, α-sialon:Eu2+ emits a yellow color (585 nm), while βsialon:Eu2+ shows an intense green emission (535 nm). For CaSiN2, there are also two polymorphs reported in the literature: low-temperature orthorhombic phase and high-temperature cubic phase.206,208 The orthorhombic phase consists of two crystallographically independent Ca atoms which are respectively coordinated to 6 and 8 N, showing a yellow emission at 530 nm.208 The cubic phase emits at a longer wavelength of 625 nm.206 On the other hand, polytypoid nitride phosphors or layerstructured nitride phosphors exhibit very similar luminescence. SrSi10−xAl18+xN32−xOx (x ≈ 1) has an AlN polytypoid structure that is similar to many AlN polytypoids such as 15R, 12H, 21R, and 27R (a general composition is MmXm+1, M = (Al, Si), X = (N, O), m = 4, 5, 6, and 7).319,320 The polytypoid structure enables SrSi10−xAl18+xN32−xOx (x ≈ 1) to have identical luminescence properties to AlN:Eu,Si, both of which show a strong blue emission at about 470 nm under UV excitation. Another case is seen in layer-structured oxonitridoaluminosilicates, including Sr14Si68−sAl6+sOsN106−s:Eu2+ (s ≈ 7), Sr5Al5+xSi21−xN35−xO2+x:Eu2+ (x ≈ 0) and Sr3Si13Al3O2N21:Eu2+.179−181 These materials show short-wavelength green emissions at 508−515 nm and relatively narrow emission bands under blue light excitation. In addition, nitride phosphors exhibiting the UCr4C4-type structure, such as nitridomagnesosilicates, nitridomagnesoaluminates, nitridolithoaluminates, and nitridomagnesogallates, show quite similar structure arrangements. The cations that are

Figure 37. Emission maximum versus the ionic radius of alkaline earth metals for Eu2+-doped nitride phosphors.

band of nitridosilicates, nitridophosphates, nitridolithoaluminates, and oxonitridosilicates shifts to longer wavelengths in the sequence of Ba < Sr < Ca. For example, MSi2O2N2:Eu2+ shows yellow (560 nm), green (540 nm), and blue-green (495 nm) for M = Ca, Sr, and Ba, respectively (Table 1). This is also true of La and Y, and the Y-compounds show a little bit longer emission than the La-ones (Table 2). The opposite case is seen in nitridomagnesosilicates (M[Mg3SiN4], M = Sr and Ba) and nitridomagnesoaluminates (M[Mg2Al2N4], M = Ca, Sr, and Ba).95,130,131 These nitrides exhibit emission bands redshifting with increasing the ionic size of alkaline earth metals, indicative of the anomalous luminescence. The nontypical redshift in emission is originated from two emission processes for Ba[Mg3SiN4]:Eu2+ and Ba[Mg2Al2N4]:Eu2+ (i.e., the trapped exciton emission and the normal 4f65d1 → 4f7 electron transitions). For Sr[Mg3SiN4]:Eu2+ and (M[Mg2Al2N4]:Eu2+ (M = Ca and Sr), the emission process consists exclusively of the 4f65d1 → 4f7 electron transitions. Another exceptional situation is observed in CaSiN2:Eu2+ and Ca2Si5N8:Eu2+, which show unusual shorter emissions than the Sr and Ba series (Table 1). The reason is unclear yet but perhaps due to the different crystal structure between Ca and Sr (Ba) compounds. As the crystal structure is the same for compounds of M = Sr and Ba, it obeys the rule that the Sr series have longer luminescence than the Ba ones. 1980

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6.1.3. Effect of the Anion Type. The type of ligand (i.e., anion species) is closely related to the covalence of chemical bonding in the host and the bond length of metal−ligands, which determines the centroid shift. As addressed by Dorenbos, the spectroscopic polarizability of anions can be used instead of crystallographic parameters to estimate the centroid shift. In going from Cl-, Br-, to I-, the polarizability increases due to the large ionic size of halide anions.311−313 This leads to redshift in emission band with increasing the ionic size of halide ions. In Eu2+-doped nitridophosphate halides, Ba3P5N10X (X = Cl, Br, I), two emission bands are displayed due to the presence of two crystallographic sites for Ba.201,202 The blue emission, assigned to Eu2+ occupying the CN = 10 site, redshifts for larger halide ions, whereas the orange emission, originated from the CN = 8 site, shows an opposite trend with increasing the size of halide ions. This difference is attributed to the fact that for the former site the Ba(Eu)−X (X = Cl, Br, and I) bonds mainly contribute to the nephelauxetic effect while for the latter site the short Ba(Eu)−N bonds dominate (i.e., the halide ions have less effects on the nephelauxetic effect). The emission color is also controlled by the N/O ratio, and a higher ratio leads to the redshift of the emission band. This is ascribed to the higher formal charge of N versus O and the higher covalent nature of the metal−N bonds versus the metal−O bonds. It will be discussed in the following sections. The strategy of the substitution of N by O or vice versa is used to tune the emission color of the existing nitride phosphors or to develop new oxynitride phosphors, as addressed in section 4.

Figure 38. Emission spectra of (a) BaSi2O2N2:Eu2+ vs SrSi2O2N2:Eu2+, (b) β-sialon:Eu2+ vs Ca-α-sialon:Eu2+, (c) β-sialon:Eu2+ with varying z, and (d) Sr[LiAl3N4]:Eu2+ vs CaAlSiN3:Eu2+, showing narrow emission bands for nitride phosphors with highly symmetric coordination configurations.

crystallographic site, (iii) high ordering of (O, N) and/or (Si, Al), and (iv) equal or approximately equal metal-N bond lengths. In β-sialon, Eu2+ is coordinated to 6 (O, N) atoms at an equal distance, forming an octahedral coordination (Figure 9, panels e and f). In BaSi2O2N2, Eu2+ is connected to nearest 6 O and 2 N atoms, creating a capped cuboid (Figure 18f). In Ba[Li2(Al2Si2)N6], Eu2+ is coordinated to 8 N atoms, resulting in a truncated square pyramid (Figure 33f). The high symmetry of the coordination configuration leads to uniform crystal fields around the dopant site, thus enabling one to reduce the inhomogeneous line broadening. For β-sialon:Eu2+, the bandwidth is further reduced with decreasing z value (i.e., the oxygen content). As the z value decreases, the degree of substitution of Si−N by Al−O also reduces. Therefore, the structural ordering in β-sialon enhances, which results in further narrowing of the emission band. The narrow-band red phosphors can be found in Eu2+-doped nitrides with the UCr 4 C 4 -type structure, including M[LiAl3N4]:Eu2+ (M = Ca, Sr),35,134 M[Mg3SiN4]:Eu2+ (M = Sr and Ba),130,131 and M[Mg2Al2N4]:Eu2+ (M = Ca, Sr, and Ba).95 In all cases, Eu2+ atoms are coordinated to 8 N atoms, and then highly symmetric cuboidal polyhedra around Eu2+ form, which reduces the fluctuations in crystal field strength (Figure 32, panels f and f′). Furthermore, the degree of condensation is quite high (κ = 1) in the [LiAl3N4]2−, [Mg3SiN4]2−, and [Mg2Al2N4]2− frameworks, which reduces phonon energies and structural relaxation around Eu2+ in its excitation state. It also applies for Ca18.75Li10.5[Al39N55]:Eu2+ (fwhm = 54 nm) as the linked sphalerite-type T5 supertetrahedra result in a framework with a high degree of condensation (κ = 0.71) (Figure 27). The nontypical broader bands observed in Ba[Mg3SiN4]:Eu2+ (fwhm ∼78 nm vs 43 nm for Sr[Mg 3 SiN 4 ]:Eu 2+ ) and Ba[Mg 2 Al 2 N 4 ]:Eu 2+ (fwhm ∼104 nm vs 69 nm for Sr[Mg2Si2N4]:Eu2) are ascribed to the additional trapped exciton emission. 6.2.2. Broad-Band Nitride Phosphors. Broad-band emissions can be realized in materials with the following natures of local structure: (i) multiple crystallographic sites for activators enabling the superimposed emission spectra, (ii) enhanced structural disordering of (O, N) and/or (Si, Al) leading to spectral broadening, and (iii) low symmetry of coordination polyhedra resulting in variations in crystal field strength.

6.2. Structure-Related Spectral Width of the Emission Band

The bandwidth of the emission band, usually expressed by fwhm, is a key factor determining the color purity of phosphors. Narrow-band phosphors are required to produce wider color gamut wLED backlights for liquid crystal displays (LCDs), whereas broad-band phosphors are pursued to create high color rendering wLEDs for general lighting. Trivalent rare earth ions except Ce3+, such as Eu3+, Tb3+, and Pr3+, show sharp emission lines owing to the 4f → 4f electron transitions, and the bandwidth of several nanometers does not change with the crystal structure of crystal hosts. This also holds true for transient metal ions like Mn4+ which exhibits sharp line red emissions in K2(Si, Ti)F6.326,327 On the other hand, for rare earth ions with 4f → 5d electron transitions, such as Eu2+ and Ce3+, they usually show broad emission bands, and the bandwidth is significantly influenced by the local structure (symmetry, coordination, structure ordering/disordering, etc.). 6.2.1. Narrow-Band Nitride Phosphors. Both narrowband green and red phosphors are urgently required for LCD backlights because the loss in emission intensity and color saturation of phosphors passing through color filters can be minimized, and therefore, both high luminous efficacy and wide color gamut will be achieved. Moreover, if the red-emitting narrow-band phosphors have less luminescence at wavelengths >700 nm that is not sensitive to human eyes, the luminous efficacy of wLEDs will be enhanced. Several narrow-band blue and green phosphors have been discovered, including BaSi2O2N2:Eu2+ (fwhm ∼36 nm),146 βsialon:Eu2+ (fwhm ∼55 nm),171 Ba[Li2(Al2Si2)N6]:Eu2+ (fwhm ∼57 nm),138 Ba2LiAlSi7N12:Eu2+ (fwhm ∼61 nm),47 etc. The band widths are much smaller than those usually observed for Eu2+ in other hosts, as shown in Figure 38. The common features of the local structure in these hosts consist of (i) only one crystallographic site for dopant Eu2+, (ii) highly symmetric 1981

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Moreover, as Ce3+ has a doublet ground state, it thus enables the achievement of broad-band emissions. La3BaSi5N9O2:Ce3+ exhibits an extremely broad emission band with a fwhm of 167 nm.220 In this phosphor host, there are 12 crystallographically independent sites for La3+/Ba2+ (Ce3+), leading to superposition of emissions from different sites. In addition, the asymmetric coordination of these sites causes structural relaxation which broadens the emission band. Y3Si5N9O:Ce3+ shows an extra-broad emission band with a fwhm of 178 nm, which is attributed to (i) two different crystallographic sites for Ce3+ with CN = 7 and 8, respectively, and (ii) low symmetry of the Ce1 site.221 Unusual broad-band emissions are also observed in nitridoborates, which is due to multiple emission centers.140,141 In Ca3Mg[Li2Si2N6]:Eu2+, Poesl et al. reported a wide emission band of 124 nm, which is again originated from Eu2+ residing on two crystallographically different Ca sites with the slight distortion of the octahedra.137 Wang et al. developed broadband red-emitting CaAlSiN3:Eu2+ by incorporating LiSi2N3 into the lattice and forming solid solutions Ca1−xLixAl1−xSi1+xN3:Eu2+ (x = 0−0.22).286,288 The fwhm increases from 88 nm (x = 0) to 117 nm (x = 0.22). The structure symmetry is lowered (structural distortion) by the introduction of LiSi2N3 into the CaAlSiN3 lattice, evidenced by the peak splitting of XRD patterns.286 In addition, the (Si, Al) disordering increases as LiSi2N3 is incorporated. Both structural distortion and structural disordering can cause inhomogeneous crystal fields around Eu2+, leading to the spectral broadening. Similarity, Li et al. introduced Si2ON2 into CaAlSiN3:Eu2+ to form solid solutions Ca1−xAl1−xSi1+xOxN3−x:Eu2+ (x = 0− 0.22).289 The bandwidth increases from 90 (x = 0) to 122 nm (x = 0.22), owing to the enhanced structural distortion as well as the structural disordering of (Si, Al) and (O, N). The broadband red phosphors are expected to produce high color rendering wLEDs.286,289

either by Ca or Ba.145 Sr1−xCaxSi2O2N2:Eu2+ shows continuous redshifts in emission as the Ca content increases, with the emission color changing from 538 nm (x = 0) to 555 nm (x = 1). Although the luminescence efficiency can remain as high as 90%, the thermal quenching of Ca-substituted SrSi2O2N2 :Eu2+ increases, indicative of the reduction in thermal stability. The Ba substitution in SrSi2O2N2:Eu2+ (Sr1−xBaxSi2O2N2:Eu2+) also redshifts the emission maximum from 538 nm (x = 0) to 564 nm (x = 0.75). At x = 0.5, both high quantum efficiency (91%) and high thermal quenching temperature (560 K) are achieved. Li et al. carried out a detailed study of Ba-substituted SrSi2O2N2:Eu2+ and addressed that the emission color could be divided into three groups depending on the Ba concentration (phase I, x = 0−0.63; phase II, x = 0.65−0.77; and phase III, x = 0.78−1.0),63 as shown in Figure 39. In phase I, a single broad

6.3. Spectral Tuning of Down-Conversion Nitride Materials

Figure 39. Emission spectra of Sr1−xBaxSi2O2N2:Eu2+ (2 mol %) with (a) x = 0−0.63, (b) x = 0.65−0.77, and (c) x = 0.78−0.98. (d) Chromatic coordinates and (e) photographs of Sr1−xBaxSi2O2N2:Eu2+ (2 mol %) with varying x. The samples were excited under 405 nm. Reprinted from ref 63. Copyright 2014 American Chemical Society.

Spectral tuning in luminescent materials is an ever-lasting topic that is of both scientific and technical importance. It enables one to (i) discuss and clarify the structure−property relationship of luminescent materials; (ii) provide guidelines for designing luminescent materials with controllable emission colors and spectral configurations as well as enhanced luminescence efficiency and thermal stability, and (iii) develop luminescent materials with optimized and desired properties suitable for practical applications. As discussed in sections 6.1 and 6.2, the spectral position and shape of Eu2+ and Ce3+ emissions are dominantly controlled by local coordination environments, indicating that the spectral tuning can be realized by tailoring the local structure. In this part, we will discuss the strategies for color tuning of nitride phosphors, which include cationic substitution, anionic substitution, chemical unit substitution, and energy transfer strategy. 6.3.1. Cationic Substitution. The type of cations (i.e., alkaline earth metals, alkali metals, and lanthanide metals) largely affect the crystal fields around activator ions by changing the bond length of metal−ligand and the volume of coordination polyhedra, leading to the shift in emission colors. Therefore, the strategy of the cationic substitution is often used to modify the emission color of nitride phosphors. The luminescence of MSi2O2N2:Eu2+ (M = Ca, Sr, and Ba) lies on the type of alkaline earth metals and shows yellow, green, and blue colors for Ca, Sr, and Ba, respectively. Bachamann et al. investigated the color tuning in SrSi2O2N2:Eu2+ by replacing Sr

emission band is seen for all compositions. In addition, an anomalous redshift with the admixture of Ba occurs, which is due to the shrinkage of Sr(Eu)-O/N bond lengths in local structure. In phase II, a dual emission band with emission maxima at 565 and 494 nm is observed, and the luminescence intensity of the blue band is enhanced with increasing Ba. This leads to a blueshift of the emission color (from white to blue). A similar double emission band also happens to phase III, with the emission color changing from yellowish green to cyan with increasing Ba. These two emissions are attributed to Eu2+ occupying both Sr2+ and Ba2+ crystallographic sites. The emission shift obeys the law that the substitution of smaller Sr2+ by larger Ba2+ leads to a small crystal field. Seibald et al.147 and Wang et al.148 developed an intense yellow oxonitridosilicate phosphor for wLEDs, Sr0.5Ba0.5Si2O2N2:Eu2+. Both of the cathodoluminescence mapping and spectra indicate two kinds of emissions from Sr0.5+δBa0.5‑δSi2O2N2:Eu2+ (λem = 550 nm) and BaSi2O2N2:Eu2+ (λem = 470 nm) that intergrow and form a wovenlike structure. Seibald et al. observed small antiphase domains within larger twin domains in the same phosphor.147 Under the 450 nm excitation, only a single broad emission band with a maximum of 570 nm and a fwhm of 94 nm 1982

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(4 mol % Eu2+) is seen. This yellow phosphor exhibits a high external quantum efficiency of 72%, enabling it to be a very promising phosphor for wLEDs. CaAlSiN3:Eu2+ exhibits a deep red emission at 650 nm and high quantum efficiency under blue light irradiation, which is regarded as an amazing red phosphor for high color rendition wLEDs.34 However, its deep red color, far from the eye’s sensitive curve, greatly reduces the luminous efficacy of lighting devices. Watanabe et al. demonstrated a blueshift of the emission band of Ca1−xSrxAlSiN3:Eu2+ with replacing Ca by Sr.328,329 For 0.8 ml% Eu2+, the emission maximum largely shifts from 650 nm (x = 0) to 610 nm (x = 1), as seen in Figure 40. The blueshift is ascribed

coordination polyhedra as the result of lattice expansion, and thus the blueshift occurs.332 6.3.2. Anionic Substitution. The anionic substitution (O → N) strategy has already been used in photocatalytic materials (e.g., TiO2−xNx) to enhance the visible light harvesting by bandgap engineering.333 Differences in the formal charge, electronegativity, bond length of metal−ligand, and the size of coordination polyhedra with varying anions make it possible to tune the emission color by the anionic substitution. As nitrogen has a higher formal charge and a smaller electronegativity than oxygen, doping of oxidic hosts with nitrogen is thus a simple way to redshift the luminescence. This can also be interpreted by using the anion polarizability. Nitrogen has a higher polarizability than oxygen, which thus accounts for a larger centroid shift. Yang et al. reported the color tuning in Y2Si2O7:Ce3+ by introducing nitrogen into the lattice.217 As seen in Figure 41a,

Figure 40. (a) Emission spectra of Ca1−xSrxAlSiN3:Eu2+ (0.8 mol %). (b) Schematics of structural ordering and charge variations associated with the cationic substitution. Reprinted with permission from ref 330. Copyright 2015 American Chemical Society.

Figure 41. (a) Color tuning in Y2Si2O7:Ce3+ by changing the nitridation degree. (b) Emission spectra of nitrogen-doped Sr2SiO4:Eu2+ (NSSO) and nitrogen-free Sr2SiO4:Eu2+ (SSO) under 453 and 320 nm excitations, respectively. (c) Photographs of SSO and NSSO under UV and natural light irradiations. Reprinted with permission from ref 217. Copyright 2008 Elsevier. Reprinted with permission from ref 284. Copyright 2015 Royal Society of Chemistry.

to increased Sr(Eu)-N bond lengths and enlarged coordination polyhedra. Later, Tsai et al. studied the structural ordering and charge variation induced by the cationic substitution and correlated to the blueshift of emission and thermal quenching of Ca1−xSrxAlSiN3:Eu2+.330 With the substitution of Ca by Sr, the structural ordering of SiN4 and AlN4 tetrahedra is enhanced, resulting in a rigid framework and therefore smaller thermal quenching and the narrowing of the emission band. Furthermore, the Eu3+ concentration in CaAlSiN3:Eu2+ is minimized by the cationic substitution, which improves the luminescence efficiency.330 The emission of Sr2Si5N8:Eu2+ can be tuned by substituting Sr by Ca or Ba. With the Ca substitution, Sr2−xCaxSi5N8:Eu2+ shows a redshift from 620 nm (x = 0) to 643 nm (x = 1) under the 450 nm excitation. This redshift is attributed to (i) the relaxation of 5d electrons of Eu2+ as Ca atoms occupy a larger Sr site and (ii) energy transfer between two Eu2+ due to the shortening of the (Sr, Ca, Eu)I−(Sr, Ca, Eu)II distance.331 When Ba is incorporated into Sr2Si5N8:Eu2+, a blueshift of emission from 610 nm (x = 1) to 585 nm (x = 2) in Sr2−xBaxSi5N8:Eu2+ is observed. The Ba substitution leads to the enlargement of the

with increasing the nitridation degree, the material composition changes from Y2Si2O7 to YSi3N5, and the emission color shifts from purple, blue, cyan, to yellow. Both of the excitation and emission bands are red-shifted significantly, and finally the yellow-emitting YSi3N5:Ce3+ (λem = 552 nm) can be excited by blue light. Sr2SiO4:Eu2+ is a promising green phosphor for use in combination with near UV LEDs. Ju et al. addressed that the emission color of Sr2SiO4:Eu2+ could be tuned from green to red by incorporating nitrogen in the lattice,284 as shown in Figure 41, panels b and c. The local coordination environment around Eu2+ is greatly altered by the nitrogen substitution for oxygen. The excitation spectrum of nitrogen-doped Sr2SiO4:Eu2+ (NSSO) is red-shifted and broadened, enabling it to be excited by blue light. Under the 453 nm excitation, NSSO exhibits a symmetric emission band with a maximum of 625 nm (similar to Sr2Si5N8:Eu2+), which is originated from the nitrogen-coordinated Eu2+ ions. Under the 320 nm excitation, NSSO shows a full color emission with 480 and 555 nm emissions assigned to Eu2+ ions occupying the Sr(I) and Sr(II) sites in Sr2SiO4, respectively. 6.3.3. Chemical Unit Substitution. Kim et al. used the double substitution method to synthesize perovskite oxynitrides from oxides and to tune their optical properties (i.e., bandgap and 1983

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absorption band).334 For example, staring from white SrTiO3 powders, brown LaTiO2N can be produced by replacing Sr−O by La−N, and red LaTaON2 by substations of Sr−O and Ti−O by La−N and Ta−N, respectively. Double substitution, also termed as chemical unit substitution, is a very powerful tool to modify the luminescence of the existing phosphors or to discover new luminescent materials.50 The mostly used double substitutions in nitride phosphors include Al−O ↔ Si−N and MIII−N ↔ MII−O. Black et al. investigated the color tuning in Sr2SiO4:Eu2+ and Sr 2 SiO 4 :Ce 3 + by substituting Sr−O by La−N. 3 3 5 Sr2−xLaxSiO4−xNx:Eu2+ shows the emission band broadening and redshifting with the double substitution. The emission color changes from yellow-green (x = 0) to orange-red (x = 1) for the Eu2+-doped samples and from blue-green (x = 0.2) to orangeyellow (x = 1) for the Ce3+-doped ones. Xia et al. reported the similar tunable emission in Sr2SiO4:Eu2+ by the Lu−N → Sr−O substitution.336 Under the 365 nm excitation, the emission maximum of LuxSr1.97−xSiNxO4−x:0.03Eu2+ shifts from 563 nm (x = 0) to 583 nm (x = 0.005), and the emission color turns from yellow-green to deep yellow accordingly. The redshift associated with the double substitution in Sr2SiO4:Eu2+ is due to the enhanced covalence and crystal field splitting. Setlur et al. prepared a red-emission enhanced YAG phosphor (Y3Al5−xSixO12−xNx:Ce3+) for warm wLEDs by incorporating Si−N to the garnet lattice (partially replacing Al−O).275 The emission spectra of nitride-YAG gets broadening and shows an additional red emission with a maximum of 620−630 nm under the 470 nm excitation. This additional red emission is assigned to Ce3+ ions with N3− in its nearest neighbor coordination. The notable examples of color tuning via the substitution of Si−N by Al−O in nitride luminescent materials are α- and βsialon:Eu2+ phosphors. Either increasing or decreasing the substitution in both materials will change the local coordination environments of activator ions and therefore their luminescence. Ca-α-sialon:Eu2+ is reported to be an orange phosphor (λem = 585 nm) for the composition of m = 2, n = 1, and 7 mol % Eu2+, which thus enables one to produce warm white by combining with blue LEDs.163−167 To realize high color temperature or high color rendition of wLEDs using Ca-α-sialon:Eu2+, the emission band needs to be blueshifted to obtain yellow or yellow green colors or red-shifted to achieve red colors. As seen in Figure 42a, the emission maximum redshifts continuously with increasing the m value (i.e., the number of Al−N substituting for Si−N). The redshift is owing to the increase of the Stokes shift caused by the lattice expansion as longer Al−N bonds (1.78 Å) replace shorter Si−N ones (1.74 Å). In addition, by increasing the n value (i.e., the number of Al−O substituting for Si−N), one can find that the blueshift occurs, which is ascribed to the reduced crystal field splitting and centroid shift. Accordingly, the emission can be tuned in a wide range of 569−603 nm by varying both m and n values, as seen in Figure 42b. As to β-sialon:Eu2+, shorter wavelengths ( 0.2), which is attributable to the increased Stokes shift as a result of structural relaxation. The emission maximum redshifts from 665 nm (x = 0.2) to 738 nm (x = 0.96). The similar blueshift also happens to Eu2+ or Ce3+-doped Ca1−xAl1−xSi1+xOxN3−x.289 The emission maximum shifts from 650 nm (x = 0) to 638 nm (x = 0.22) for the Eu2+-doped composition and from 606 nm (x = 0) to 575 nm (x = 0.22) for the Ce3+-doped case. 6.3.4. Energy Transfer Strategy. Energy transfer can occur between two identical activators on different crystallographic sites or between two dissimilar activators on the same site. For phosphors with two or more crystallographic sites for activator ions (Eu2+ and Ce3+), the activator ions can act as both donor and acceptor and energy transfer between them then takes place. Since the probability of energy transfer mainly depends on the distance between activators, the redshift is therefore often observed when the activator concentration increases. Recently, Sato et al. reported color-tunable Ca2−xEuxSiO4 by using the crystal-site engineering approach.77 Two crystallographically independent Ca atoms with different coordination environments are present in Ca2SiO4. As the Eu2+ concentration increases from x = 0.2 to 0.8, Eu2+ tends to occupy the smaller Ca(2n) sites, which leads to strong interactions between Eu2+ and O2− and therefore a significant spectral redshift from yellow-green (λem = 550 nm, x = 0.2) to deep-red (λem = 650 nm, x = 0.8). This redshift can be considered as an emission and radiative energy transfer from high-energy Eu2+ sites to low-energy Eu2+ sites. In addition, many kinds of LED phosphors have overlapping emission and excitation spectra, inevitably leading to the selfabsorption effect through either nonradiative or radiative energy transfer that contribute to the spectral shift. The self-absorption effect will lead to either homogeneous or inhomogeneous spectral band broadening, whereas the radiative one can result in both homogeneous and inhomogeneous band broadening which also causes a redshift in emission. Such a redshift associated with the activator concentration has also been seen in nitride phosphors, such as Ca-α-sialon:Eu2+, CaAlSiN3:Eu2+, SrAlSi 4 N 7 :Eu 2+ , M 2 Si 5 N 8 :Eu 2+ , SrSi 2 O 2 N 2 :Eu 2+ , JEM:Ce 3+ , JEM:Eu2+, La3Si6N11:Ce3+, etc. Li et al. observed the large redshift in M2Si5N8:Eu2+ (M = Sr and Ba) with increasing Eu2+ concentration (Figure 43a) and assigned it to the large Stokes shift.33 This is supported by the DFT calculation which shows that the structural rigidity decreases with increasing Eu concentration, and the occupied 5d state in the excited Eu2+ becomes more delocalized. On the other hand, Sohn et al. analyzed the time-resolved photoluminescence of Sr2Si5N8:Eu2+ and addressed that redshifts with increases Eu concentration was ascribed to the energy transfer between two Eu2+ situated in two different crystallographic sites but not to the Stokes shift (configuration coordination model).338,339 The broadening of emission bands with increasing Eu2+ concentration, especially for Ba2Si5N8:Eu2+, further confirms that energy transfer occurs.

Figure 43. (a) Emission spectra of Ba2Si5N8:Eu2+ with varying Eu Excitation. (b) Excitation spectra of JEM:Ce3+ (La1−xCexAl2Si5N9O) with varying x. (c) Actual (upper) and normalized (lower) emission spectra and (d) photographs of JEM:Eu 2+ with varying Eu concentrations. Reprinted with permission from ref 33. Copyright 2006 Elsevier. Reprinted with permission from ref 188. Copyright 2016 Royal Society of Chemistry. Reprinted with permission from ref 229. Copyright 2007 American Institute of Physics.

Takahashi et al. reported a blue-emitting JEM:Ce3+ (LaAl(Si6−zAlz)N10−zOz:Ce3+) phosphor for ultraviolet LEDs.229 To enhance the absorption at 405 nm, one of the strategies is to increase the Ce3+ concentration in the lattice. As seen in Figure 43b, the excitation spectra broaden and redshift with increasing the Ce3+ concentration (x), leading to a dramatic increase in the absorption of 405 nm light. The spectral shift in excitation band is clearly indicative of changes in local coordination surrounding Ce3+. The emission maximum increases from 460 to 500 nm when the Ce3+ concentration increases from 0.28 to 5.5 at%. With doping of Eu2+, JEM also shows a significant redshift in both excitation and emission bands when the activator concentration increases, and the emission color changes from blue-green (λem = 490 nm) to yellow (λem = 564 nm) under the 355 nm excitation (Figure 43, panels c and d).188 In JEM, La (Ce or Eu) has only one crystallographic site and is coordinated to 7 (O, N) atoms. The redshift in JEM is thus ascribed to the energy transfer from the oxygen-rich emission site to the nitrogen-rich one. Another example is La2.5Ca1.5−yEuySi12O4.5N16.5 which shows a dramatic redshift with increasing Eu concentration. The emission maximum increases from 495 nm for y = 0.002 to 575 nm for y = 0.7, and the emission color correspondingly shifts from cyan to orange.154 Moreover, the excitation band also exhibits a spectral broadening, and the absorption maximum shifts from 388 to 511 nm with increasing the Eu concentration. In La2.5Ca1.5Si12O4.5N16.5, there are two independent La/Ca atoms with La1/Ca1 being coordinated to 2 N and 4 (O, N) atoms and La2/Ca2 to 3 N and 3 (O, N) atoms. The redshift is thus attributable to the energy transfer from Eu2+ occupying the La1/ Ca1 site to that situated at the La2/Ca2 site. The energy transfer is evidenced by the reduction of the decay time with increasing the Eu2+ concentration. Besides the energy transfer between the same activator ions, it can also occur between different ions and leads to spectral tuning. Differing from the first case, in the second case, two distinct emission bands from different activator ions are seen, which can tune the emission color in a very broad range and make it possible to realize full colors in one crystal host by carefully selecting the type of activator ions. The strategy of energy 1985

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7.1. Thermal Quenching

transfer between different activators is quite often applied in oxidic phosphors, which dominantly consists of energy transfer between Mn2+ and Eu2+ or between Mn2+ and Ce3+.340−342 On the other hand, only a few examples can be found in nitride phosphors to tune emission colors by energy transfer between dissimilar activator ions. Ruan et al. observed the enhanced quantum efficiency, tunable emission color, and band broadening in SrSi2O2N2 through the energy transfer between Eu2+ and Yb 2 + . 2 3 7 With increasing the Yb 2 + concentration, Sr0.98−xEu0.02YbxSi2O2N2 shows the emission color changing from green (x = 0) to yellow (x = 0.02), owing to the energy transfer from Eu2+ to Yb2+. It is a weak orange-red emission for Eu-free Sr0.98Yb0.02Si2O2N2. For the codoped yellow phosphor (Sr0.96Eu0.02Yb0.02Si2O2N2), the fwhm of the emission band is significantly increased to 170 nm (vs 125 nm for Sr0.98Yb0.02Si2O2N2, and 81 nm for Sr0.98Eu0.02Si2O2N2). Under the 450 nm excitation, the codoped yellow phosphor has an external quantum efficiency of 16%, which is much higher than that of 3.4% for Sr0.98Yb0.02Si2O2N2. Liu et al. reported the energy transfer from Eu2+ to Mn2+ in γ-alon and achieved a dramatic enhancement of luminescence and quantum efficiency.243 The emission color shifts from blue (λem = 471 nm) for γ-alon:Eu2+ (2 mol %) to green (λem = 520 nm) for codoped γ-alon:Eu2+, Mn2+ (2 mol % Eu2+ and 5 mol % Mn2+). Under the 365 nm excitation, the external quantum efficiency is increased from 7% (γalon:Mn2+) to 49% (γ-alon:Eu2+, Mn2+). In addition, although there are no obvious contributions to the spectral tuning, the energy transfer between dissimilar ions takes effect in improving the luminescence efficiency or afterglow properties of the acceptor ions. Lin et al. reported the enhancement of the red emission in Sr2Si5N8:Eu2+, Mn2+ by the energy transfer from Mn2+ to Eu2+.343 Zhang et al. synthesized a reddish orange Ca2Si5N8:Eu2+, Tm3+, Dy3+. It has a stronger afterglow intensity and a longer duration time than Ca2Si5N8:Eu2+, Tm3+.344

When a phosphor is heated, its luminescence usually declines due to the interaction with host crystal energy levels and lattice vibrations with subsequent phonon quenching. This decline is known as thermal quenching, which is thus an indicator to evaluate how stable the luminescent intensity and color coordinates of phosphor are against thermal attacks. The temperature-dependent luminescence data are plotted using the following equation to describe thermal quenching of luminescence intensity I(T) with temperature T313 I0

I (T ) = 1+

Γ0 Γu

(

ΔE

exp − kT

)

(7)

where I0 is the initial intensity at 25 °C, ΔE is the energy barrier for thermal quenching, k is the Boltzmann’s constant, Γ0 is the attempt rate for thermal quenching at T = ∞, and Γυ is the radiative decay rate of the 5d state of Eu2+ or Ce3+. In addition, thermal quenching temperature (T0.5) is defined as the temperature at which the luminescence intensity is decreased by 50%. Both ΔE and T0.5 are applied to characterize the thermal quenching of luminescent materials. For phosphors doped with Eu2+ or Ce3+, thermal quenching is strongly related to the structure rigidity of the host, the electronic/band structure of the host, and the position of the lowest 5d excited states with respective to the bottom of the conduction band. 7.1.1. Effect of Activator Concentration. The thermal quenching of a phosphor generally increases when the activator concentration increases. Bachmann et al. investigated the thermal quenching behavior of YAG:Ce3+ with varying Ce3+ concentrations, and addressed that for diluted samples the thermal quenching was caused by temperature-dependent oscillator strength, whereas for heavily doped samples thermally activated concentration quenching dominated.345 In addition, the Stokes shift usually becomes larger when the activator concentration increases, which leads to the increase of thermal quenching. This also holds true for nitride phosphors, such as αsialon:Eu2+/Ce3+, β-sialon:Eu2+/Ce3+, CaAlSiN3:Ce3+, SrAlSi4N7:Eu2+/Ce3+/Yb2+, Sr0.5Ba0.5Si2O2N2:Eu2+, etc.148,169,170,174,222,224,225,227,228,239,346 By following this rule, one may suggest that a low doping level would be preferred to maintain high luminescence at elevated temperature. However, there is a trade-off between thermal quenching and luminescence efficiency. When further considering the luminescence efficiency, one should choose an appropriate activator concentration but not a low concentration. Therefore, the balance between the thermal quenching and the intrinsic luminescence efficiency needs to be taken into account when determining the optimized doping concentration. 7.1.2. Effect of Activator Species. As mentioned before, several rare-earth ions including Eu2+, Ce3+, and Yb2+ are usually used as activators in nitride phosphors. These activators exhibit different thermal quenching behaviors, even in a same host lattice, which are dominantly due to the position of their 5d excited states. Bachmann et al. investigated the luminescence and thermal quenching of Eu2+- and Yb2+-doped SrSi2O2N2.236 Yb2+ shows an anomalous luminescence with a larger Stokes shift and a lower thermal quenching temperature than Eu2+, which is attributed to the impurity trapped-exciton emission. The large difference in thermal quenching of Eu2+ and Yb2+ is originated from the fact that the lowest position of the 5d excited state of Eu2+ is below the bottom of the conduction band of SrSi2O2N2, whereas it is

7. TEMPERATURE-DEPENDENT LUMINESCENCE OF DOWN-CONVERSION NITRIDE MATERIALS Although the SSL technology has the superior conversion efficiency from electricity to light emission and thus has less heat generation than traditional lighting technologies such as incandescent bulbs and Xeon lamps, it still has a big thermal issue problem, typically for high-power devices. In addition to applying some thermal management (i.e., heat sink configuration) to maintain the temperature of devices as low as possible, an alternative option is to utilize thermally stable luminescent materials. Therefore, it is quite essential to explore the temperature-dependent luminescence, which is used to evaluate the reliability of a phosphor under thermal attacks. In most cases, the temperature-dependent luminescence is characterized by thermal quenching (i.e., thermally induced luminescence quenching) which is usually caused by large Stokes shifts or photoionization. On the other hand, thermal degradation occasionally occurs in some phosphors, which is originated from the change of valence of activator ions and/or oxidation-induced phase transition. Thermal degradation can yield a permanent damage to luminescent materials, which means the luminescence cannot be recovered even after the thermal load is released. In this part, both thermal quenching and thermal degradation of nitride phosphors are discussed. 1986

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[Mg2Al2N4]:Eu2+ has the highest quenching temperature.93 This anomalous thermal quenching agrees well with the unusual luminescence, both caused by the trapped-exciton emission of the Ba isomorphic compound. The substation of Al−O by Si−N or Si−O by Al−N in oxidic phosphors leads to smaller thermal quenching. Wang et al. observed the reduction in thermal quenching of BaMgAl10O17:Eu2+ by introducing Si−N into the lattice.278 Zhu et al. addressed that the thermal quenching of Sr2Si1−xAlxO4−2xNx:Eu2+ was reduced continuously by substituting Si−O by Al−N.347 The exceptional case is observed in the Si−N-substituted YAG:Ce3+. Setlur et al. reported that with the substitution of Al−O by Si−N, nitride-YAG:Ce3+ resulted in a redshift of the emission band but also exhibited a lower thermal quenching temperature than typical YAG:Ce3+.275 The large thermal quenching is perhaps due to the nonradiative level crossing between the excited 5d level and the low energy 4f1 levels because the 5d energy levels of Ce3+ coordinated with N are lowered and then the energy barrier is reduced for the nonradiative crossovers. On the other hand, the substitution of Si−N by Al−O or Al−N usually leads to higher thermal quenching. Xie et al. and Liu et al. reported that in α-sialon:Eu2+ (MxSi12−m−nAlm+nOnN16−n:Eu2+, M = Li, Ca), the thermal quenching was largely dependent on the m value (i.e., the number of Si−N substituted by Al−N) rather than the n value (i.e., the number of Si−N substituted by Al−O), which increases with increasing m.169,170,348 The increased thermal quenching is caused by the enhanced Stokes shift. Chen et al. found that the thermal quenching energy of M1.98Eu0.02Si2−xAlxOxN8−x decreased linearly with increasing x for M = Ca and Sr.75 The Ca isomorphic compound has the largest thermal quenching, whereas the Ba one shows the smallest thermal quenching. The cation size-mismatch effect is used to explain the difference in thermal quenching. With the large dispersion of size between Eu2+ and Ca2+ in the Ca compound, the lattice strain is relaxed by connecting more oxygen anions with Eu2+, which thus decreases the thermal stability with increasing the Al−O substitution. Eu2+ is smaller than Ba2+ and prefers to connect nitrogen anions in the Ba isomorphic compound, which results in the gradual increase of activation energy with increasing the Al−O substitution. The introduction of carbon into the lattice can enhance the thermal stability of nitride phosphor. Huang et al. prepared carbon-doped CaAlSiN3:Eu2+ by partially replacing Al−N by Si− C (Ca0.99Al1−4δ/3−xSi1+δ+xN3−xCx:Eu2+0.01, δ = 0.345, x = 0− 0.2).349 The luminescence intensity at 150 °C maintains about 92, 96, and 98% of the initial intensity for x = 0, 0.05, 0.1, and 0.2, respectively. The increased thermal stability is attributed to the second-sphere shrinkage effect. The similar results are found in carbon-doped AlN (Al1−xSixN1−xCx:Eu2+) and Sr2Si5N8.104,190 Thermal quenching of phosphors can also be affected by forming solid solutions. Solid solution phosphors of CaAlSiN3:Eu2+-LiSi2N3 (Ca1−xLixAl1−xSi1+xN3:Eu2+) and CaAlSiN3:Eu2+-Si2ON2 (Ca1−xAl1−xSi1+xOxN3−x:Eu2+) were synthesized with aims to tune the emission color.286−289 Wang et al. reported that the thermal quenching was progressively reduced with increasing LiSi2N3 content.286 At 150 °C, the emission intensity declines by 12% for x = 0 but only by 6% for x = 0.20. The difference in thermal stability is more pronounced at higher temperatures. At 250 °C, the emission intensity looses about 29% for x = 0 and 18% for x = 0.20. Li et al. reported that the activation energy for thermal quenching increased from 0.20 eV for x = 0 to 0.24 eV for x = 0.20 in

located within the conduction band for Yb2+. The low thermal quenching temperature of Yb2+ is also seen in SrAlSi4N7 and CaAlSiN 3 . 224,346 In SrAlSi 4 N 7 (1 mol % doping), the luminescence intensity at 150 °C declines by 15.4, 20, and 60% for Eu2+, Ce3+, and Yb2+, respectively (Figure 44, panels a

Figure 44. Thermal quenching of SrAlSi4N7 doped with (a) Eu2+ and (b) Ce3+. (c) Thermal quenching of CaAlSiN3 doped with Yb2+. (d) Effect of the energy difference (ΔE5d‑CB, eV) between the lowest 5d excited state and the bottom of the conduction band on the thermal quenching ratio (I150 °C/IRT) of various nitride phosphors. Reprinted with permission from ref 225. Copyright 2013 Elsevier. Reprinted with permission from ref 239. Copyright 2012 Royal Society of Chemistry. Reprinted with permission from ref 346. Copyright 2011 WileyBlackwell.

and b). In CaAlSiN3 (2 mol % doping), the luminescence loss of Yb2+ (45%) is also much larger than that of Eu2+ (∼10%) and Ce3+ (∼20%) (Figure 44c). Zhang et al. calculated the energy difference (ΔE5d‑CB) between the lowest 5d excited state and the bottom of the conduction band of several Eu2+- and Yb2+-doped nitride phosphors and correlated it with the thermal quenching (Figure 44d).239 He addressed that ΔE5d‑CB was a universal parameter to evaluate the thermal quenching. A large energy difference indicates a low thermal quenching loss. For example, ΔE5d‑CB is 0.56 and 0.13 eV, and the luminescence loss is 10 and 45% for Eu2+- and Yb2+-doped CaAlSiN3, respectively. On the other hand, there are no big differences in thermal quenching when these activators are doped in Ca-α-sialon and β-sialon. It thus indicates that the electron/band structure of hosts plays a dominant role in thermal quenching, as will be discussed later. 7.1.3. Effect of the Chemical Composition. For a same activator ion, the chemical composition of the host has a significant impact on the thermal quenching of the phosphor. The thermal quenching can be affected by the cation type, cationic/anionic substitution, the chemical unit substitution, etc. As described before, alkaline earth metal (oxo)nitride phosphors show a great cationic size-dependent luminescence, and a redshift is seen with decreasing the cationic size. As to thermal quenching, it has somewhat deviated from that rule. The quenching rate usually decreases in the sequence of Sr > (Sr, Ca) ∼ (Sr,Ba) > Ba > Ca. This is true in M 2Si5N 8:Eu 2+, MSi 2 O 2 N 2 :Eu 2 + , MYSi 4 N 7 :Eu 2 + (M = Ca, Sr, and Ba).33,145,122,123 For example, the quenching temperature is about ∼300, 220, and 130 °C for M = Sr, Ba, and Ca in M2Si5N8:Eu2+, respectively. In the case of MSi2O2N2:Eu2+, the quenching temperature is 400, 600, and 600 K for M = Ca, Sr, and Ba, respectively. An exception is found in nitridomagnesoaluminates, where Ba[Mg 2 Al 2 N 4 ]:Eu 2+ rather than Sr1987

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Ca1−xAl1−xSi1+xOxN3−x:Eu2+.289 The enhanced thermal stability in these solid solutions is owing to (i) the second-sphere shrinkage effect and (ii) the increase in energy difference between the lowest 5d energy level and the bottom of the conduction band. On the other hand, Wang et al. found that the introduction of La into CaAlSiN3:Eu2+ (Ca1−xLaxAl1+xSi1−xN3:Eu2+) exhibited an opposite trend to the Li case, which degrades the photoluminescence at high temperatures.287 A remote-control effect that guides Eu2+ activators in selective Ca2+ sites is proposed to explain the thermal quenching behavior. When Ca−Si is substituted by La− Al in CaAlSiN3:Eu2+, Eu2+ is therefore surrounded by nitrogen anions which connect La3+ and Si4+/Al3+. It thus leads to a weak covalent coordination environment. However, when Ca−Al is substituted by Li−Si in CaAlSiN3:Eu2+, Eu2+ is then bound to nitrogen anions which link Li+ and Si4+. Correspondingly, a strong covalent local coordination is obtained.

al. reported that in the La−Si−ON:Ce3+ series, LaSiO2N:Ce3+ had a smaller Stokes shift (4100 cm−1) than La5Si3O12N:Ce3+ (6800 cm−1) but exhibited a low thermal quenching temperature than the latter.218 This means another mechanism other than the 4f-5d crossing model is at work. The autoionization model correlates the distance (energy barrier) between the lowest 5d excited state and the conduction band minimum (CBM) with the thermal quenching. When the temperature is raised, the 5d electrons would be thermally activated to the conduction band, delocalized, and the energy of which is dissipated through nonradiative energy transfer to traps or killer centers. The smaller the distance the Eu5d-CBM gap is, the larger the thermal quenching. Dorenbos proposed a simplified relationship between the energy barrier (ΔE) and the quenching temperature T0.5 as

7.2. Thermal Quenching Mechanisms

The autoionization model is applied to qualitatively explain the large thermal quenching of CaSiN2:Eu2+ and enhanced thermal stability of Al1−xSixN1−xCx:Eu2+, Ca1−xLixAl1−xSi1+xN3:Eu2+, and Ca1−xAl1−xSi1+xOxN3−x:Eu2+.104,114,286,289 To quantitively analyze the energy barrier for thermal quenching, the first-principles method can be used to calculate the electronic structure and the absolute location of Eu2+ or Ce3+ in the band gap of the host crystals. For Ba3Si6O12N2:Eu2+ and Ba3Si6O9N4:Eu2+ with the same trigonal crystal symmetry, they have quite different thermal quenching behaviors. Ba3Si6O9N4:Eu2+ only shows blue-green emission at low temperature and quenches completely at room temperature. Mikami et al. reported that Ba3Si6O9N4 (6.46 eV) had a smaller band gap than Ba3Si6O12N2 (6.79 eV), calculated by the GW method (Figure 46a). From the thermal quenching data, Mikami et al. estimated that the Eu5d-CBM gap was 0.6 and 0.2 eV for Ba3Si6O12N2:Eu and Ba3Si6O9N4:Eu, respectively.350 Later, Poncé et al. calculated the electronic structure of Ba3Si6O9N4 and Ba3Si6O12N2 from the first principles.73 He addressed that only BaI (EuI) contributed to the luminescence, and the Eu5d-

ΔE = T0.5/680 (eV)

The thermal quenching property greatly determines the practical applications of phosphors. In some phosphors, they exhibit very promising photoluminescence properties, but the photoluminescence quenching starts at low temperature and even no luminescence is found at room temperature, while in others luminescence quenches rapidly so that they cannot meet the requirements for usage. Therefore, the understanding of quenching mechanisms is quite important for selecting appropriate phosphors or modifying existing phosphors. Two dominant models have been proposed to explain thermal quenching, i.e., (i) crossover between the ground and excited states in the configuration coordinate diagram (4f-5d crossing model) and (ii) thermal excitation of 5d electrons to the conduction band (the autoionization model), as illustrated in Figure 45.

(8)

Figure 45. Comparison between the 4f−5d crossing and the autoionization models for explaining the thermal quenching. Reprinted from ref 73. Copyright 2016 American Chemical Society.

The 4f-5d crossing model indicates that the large atomic geometry changes result in crossovers between the 4f and 5d configurational energy curves. When the excited electron of the 5d orbital is thermally activated, it would overcome the energy barrier EA and return to the ground state nonradiatively. This model allows one to use the Stokes shift to predict the thermal quenching. A larger Stokes shift usually leads to higher thermal quenching. The effect of the activator concentration on the thermal quenching can be explained by this model. Liu et al. applied this mechanism to the thermal quenching of Ca-αsialon:Eu2+ with varying m.348 However, in some cases, the 4f-5d crossing model can be ruled out because a small Stokes shift is seen to yield large thermal quenching. For example, Benjamin et

Figure 46. Schematic illustration of electronic structure of Ba3Si6O12N2:Eu and Ba3Si6O9N4:Eu. Note that energy scale is not accurate, and Eu 4f level positions are neither experimentally nor theoretically determined. Autoionization process (Eu2+ → Eu3++e−) is denoted for Ba3Si6O9N4:Eu (right). Reprinted from ref 73. Copyright 2016 American Chemical Society. 1988

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temperature and the luminescence loss indicates that (Ca,Sr)AlSiN3:Eu2+ is more easily attacked by the high-pressure steam.

CBM gap was 0.52 and 0.37 eV for EuIBa8Si18O36N6 and EuIBa8Si18O27N12, respectively (Figure 46b). Thus, the Eu5dCMB gap of Ba3Si6O12N2:Eu2+ is 0.09 eV larger than that of Ba3Si6O9N4:Eu2+. Tolhurst et al. reported the electronic structure and the position of Eu 5d states in the band gap of Sr[LiAl3N4]:Eu2+ (SLA) and Sr[Mg3SiN4]:Eu2+ (SMS) by density functional theory calculations.351 SMS has the extremely larger thermal quenching than SLA, and its luminescence is totally quenched at room temperature. This is due to the fact that the energetic separation between the lowest Eu2+ 5d excited state and the bottom of the conduction band of SMS (0.13 eV) is smaller than that of SLA (0.28 eV). It should be noted that a small change in energy barrier for thermal quenching (0.1−0.2 eV) can be the difference for practical applications of a phosphor, making it difficult for computational predictions of phosphor properties.

7.4. Thermal Degradation Mechanisms

Thermal degradation is caused by some changes made in activator, surface state, and host crystal and therefore has different mechanisms from thermal quenching. It requires clarification of the mechanisms by using a variety of analytical techniques, such as EXAFS, XPS, XRD, SEM, and HRTEM to understand those changes. In general, the thermal degradation of LED phosphors is explained by (i) the oxidation of activations (i.e., valence change), (ii) the formation of surface layers that reduce the radiative energy transfer, and (iii) oxidation and decomposition of host crystals. Yeh et al. analyzed the valence of Eu2+ in both untreated and treated Sr2Si5N8:Eu2+ by using ESCA (electron spectroscopy for chemical analysis) and XANES.357 The increase of the Eu3+/Eu2+ ratio is confirmed in treated phosphors (typically in highly concentrated material), indicating that Eu2+ is partially oxidized into Eu3+ which usually acts as killers for luminescence. The oxidation of Eu2+ occurs via

7.3. Thermal Degradation

A lot of phosphors tend to lose their luminescence efficiency by several mechanisms, including the valence change of activators, decomposition of host crystals, and chemical reactions at surfaces under thermal-, moisture-, electron-, and/or irradiation attacks. Thermal degradation means that phosphors undergo the luminescence loss during the baking process in the manufacture or during the aging process in operation. The thermal degradation of phosphors used in plasma display panels (PDPs) and field emission displays (FEDs) have been investigated extensively,352−356 but there are quite a few reports on LED phosphors. The red-emitting Sr2Si5N8:Eu2+ phosphor has the excellent photoluminescence properties such as the useful red emission, high quantum efficiency, and small thermal quenching. However, the thermal degradation dramatically hinders its practical applications in wLEDs. Yeh et al. investigated the thermal quenching of M2Si5N8:Eu2+ (M = Ca, Sr, and Ba) and found that the luminescence of Sr2Si5N8:Eu2+ was irreversible during the heating−cooling cycle.357 Sr2Si5N8:Eu2+ degrades by 45% in one cycle, whereas Ca2Si5N8:Eu2+ and Ba2Si5N8:Eu2+ do not have such a loss in luminescence. Xie et al. reported that the luminous efficiency of the monochromatic LED using Sr2Si5N8:Eu2+ degraded by 35% when aged for 1000 h at room temperature, and the thermal degradation could be reduced by the Basubstitution.192 Similar to Sr2Si5N8:Eu2+, the green-emitting SrSi2O2N2:Eu2+ shows very promising photoluminescence for wLEDs, but the large thermal degradation makes it difficult for practical use. Cho et al. observed the slight thermal degradation of SrSi2O2N2:Eu2+ when baking at temperatures below 400 °C.37 But rapid thermal degradation is seen when the wLEDs using SrSi2O2N2:Eu2+ are treated at 85 or 85 °C/85% humidity. Wang et al. studied the thermal degradation of SrSi2O2N2:Eu2+ by baking it at varying temperatures.358 SrSi2O2N2:Eu2+ gradually degrades in luminescence (less than 10%) below 500 °C but rapidly at 600 °C (by 30%). Moreover, the thermal degradation is largely dependent on the phase purity, crystallinity, particle size, etc. Zhu et al. investigated the thermal degradation of (Ca,Sr)AlSiN3:Eu2+ by treating it at varying temperatures with or without steam.359 It shows that (Ca,Sr)AlSiN3:Eu2+ loses its luminescence only by 2% in dry air at 200 °C but starts to degrade at 150 °C in steam and loses 30% at 200 °C. The activation energy for the degradation is calculated to be 66.32 kJ mol−1. The great difference in both the initial degradation

2Eu 2 + + 1/2O2 (g) + VN → 2Eu 3 + + ON 2 −

(9)

ON2−

with VN being the nitrogen vacancy and the oxygen vacancy. An amorphous phase is also identified at the surface layer of the treated phosphor by HRETM, implying the oxidation of the phosphor host. Wang et al. also confirmed the amorphous layer with a thickness of 6 nm when Sr2Si5N8:Eu2+ was treated at 200 °C for 24 h.358 The oxidation of host lattice causes the charge imbalance and damages to the phosphor surface, which further induces amorphization. Yeh et al. addressed the fact that the Ba− N bond in Ba2Si5N8:Eu2+ was less covalent than Sr−N in Sr2Si5N8:Eu2+, therefore Ba (Eu) preferred to connect with nitrogen, which leads to quite small thermal degradation of Ba2Si5N8:Eu2+.357 Wang et al. analyzed the valence of Eu2+ in SrSi2O2N2 by XPS, and the phase assemblage by high-temperature in situ XRD.358 The Eu3+ signal in XPS spectra was intensified after baking the phosphor at 600 °C, indicating the oxidation of Eu2+. Moreover, both amorphous and crystalline SrSiO3 were detected on the phosphor surface by HRTEM, which means that the SrSi2O2N2 host undergoes oxidation during baking. Therefore, the oxidation of both activator and host lattice accounts for the thermal degradation of SrSi2O2N2:Eu2+ (Figure 47a). Zhu et al. applied the XPS and cathodoluminescence (CL) spectra to investigate the valence change of Eu2+ after (Ca,Sr)AlSiN3:Eu2+ was treated in a sealed autoclave at 200 °C for 100 h.359 Several line emission peaks at 600, 610, 650, and 700 nm are seen from CL spectra, implying the oxidation of Eu2+ into Eu3+. The XRD spectra of (Ca,Sr)AlSiN3:Eu2+ treated at 200 °C for different times show the formation of an amorphous phase (32 h) at first and finally of an impurity phase (Ca,Sr)Al2Si2O8, which indicates the decomposition of the host. Phosphor particles also undergo significant evolutions in morphology, changing from the initial rounded shape to a cracked and loosely layered structure. Zhu et al. demonstrated that the thermal degradation of (Ca,Sr)AlSiN3:Eu2+ was caused by the oxidation of both the phosphor host and activator (Eu2+) via an oxidant-gas penetration mechanism (Figure 47b).359 7.5. Strategies for Enhancing Thermal Stability

For practical applications, phosphors must have good thermal stability (i.e., small thermal quenching and thermal degradation) to retain high efficiency, stable chromaticity, and high reliability 1989

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the Eu2+ from oxidation.360 Similarly, thermal treatment of Sr2Si5N8:Eu2+ at 300−1200 °C in the N2−H2 atmosphere also results in the formation of a passivation SrSiO3 layer with a thickness of 200 nm.361 The passivation layer blocks the penetration of oxidizing species into the lattice, thus protecting the activators and host from oxidation. By developing a hydrophobic nanolayer (8 nm) of amorphous SiO2 that contains CH3 groups in the surface, Zhang et al. reported that Sr2Si5N8:Eu2+ shows superior oxidation and hydrolysis resistance as well as thermal stability in high-pressure water steam conditions at 150 °C.362 In addition to the thermal issue of nitride phosphors, the moisture/hydrolytic stability of some nitride materials needs to be concerned and improved.362,363 As mentioned in section 3.1.8, Sr[LiAl3N4]:Eu2+ (SLA) is a very interesting narrow-band red phosphor that has great potential for use in general illumination and backlights. Although it has a small thermal quenching, SLA is quite sensitive to moisture, which makes it impossible for practical applications without any surface treatments. Tsai et al. prepared the waterproof SLA phosphor by coating it with an organosilica layer using tetraethylorthosilicate (TEOS). The coated sample showed excellent moisture resistance while retaining an external quantum efficiency of 70% of the initial efficiency after aging for 5 days under harsh conditions.363

Figure 47. Schematic thermal degradation mechanism of (a) SrSi2O2N2:Eu2+ backed in a dry air and (b) (Sr,Ca)AlSiN3:Eu2+ treated in steam at 200 °C. Reprinted with permission from ref 358. Copyright 2014 Royal Society of Chemistry. Reprinted with permission from ref 359. Copyright 2015 Royal Society of Chemistry.

of wLEDs. Therefore, it is necessary to enhance the thermal stability of those phosphors having promising photoluminescence properties to make them revive. Thanks to the understanding of the mechanisms for thermal quenching or thermal degradation, some methods can be applied to enhance thermal stability effectively. Thermal quenching relies greatly on the activation concentration and the electronic/band structure of phosphor hosts. With this in mind, the first method to reduce the thermal quenching is to control the activator concentration in a low level, thus avoiding the nonradiative energy transfer between neighboring activators. Below the critical doping level for concentration quenching, there is a trade-off between the thermal quenching and quantum efficiency. Hence, the activator concentration should be carefully considered to achieve small thermal quenching without sacrificing the luminescence efficiency. The second option to reduce thermal quenching is bandgap engineering, through which the distance between the lowest Eu2+ 5d excited state and the bottom of the conduction band needs to be widened. The bandgap engineering can be realized by forming solid solutions between the phosphor host and a compound with an equivalent crystal structure or chemical composition, which can be found in MSi 2 O 2 N 2 :Eu 2+ , M2Si5N8:Eu2+, M[Mg2Al2N4]:Eu2+, CaAlSiN3−LiSi2N3, CaAlSiN3−Si2ON2, etc.95,145,286,289,357 The third method is to introduce covalent chemical bonds into the host lattice through chemical unit substitutions, such as Si−N → Al−O, Al−N → Si− O, Si−C → Al−N, etc. The substitution leads to enhanced structural rigidity and reduced lattice vibrational frequency, thus increasing the quenching temperature. Thermal degradation occurs through chemical reactions of activators and phosphor hosts with oxidizing species, which thus make the activators or phosphor surface inactive. Therefore, thermal degradation can be minimized by enhancing the oxidation resistance of activator and phosphor hosts through surface passivation. Zhang et al. reported that a hydrophilic layer was formed on phosphor particles when they were heat-treated at 300−500 °C in N2, which plays a passivating role in preventing

8. APPLICATIONS OF DOWN-CONVERSION NITRIDE MATERIALS IN SOLID STATE LIGHTING The structural diversity of luminescent nitride materials enables them to exhibit very interesting photoluminescence. Typically, their photoluminescence spectra match well with the InGaN LED chips, which making them very suitable for use as color converters in SSL. In this review, their applications in SSL will be highlighted. Except for SSL applications, luminescent nitride materials can also find applications in other fields using their cathodoluminescence, persistent luminescence, deep UV emissions, and even catalytic properties. Hirosaki et al. fabricated FEDs using the blue-emitting AlN:Eu2+ and demonstrated that the FEDs exhibited a lifetime 10 times longer than those prepared by using Y2SiO5:Ce3+.101 Watanabe et al. reported that the hexagonal BN showed a dominant far-UV (FUV) emission at 215 nm and produced a FUV plane-emission device.364,365 The deep-UV emission is also observed in AlN, enabling it to be used in optoelectronic devices.98 ten Kate et al. suggested that Yb3+doped LaSi3N5 was suitable for applications as a spectral conversion material for infrared LEDs to solar cells.366 Cho et al. reported the UV emission of the Si-doped AlN (λem = 350 nm), which can be used for water cleaning and skin disorder treatment.367 Smet et al. overviewed the persistent luminescence of nitride phosphors and suggested that they could also be used for security paints, vivo bio imaging and surveillance applications.368 Nitride materials are also developed as photocatalytic material for water splitting.369−372 SSL devices can be applied in a broad range of fields including general illumination, LCD backlights, surgical lighting, vehicle headlamps, data/cinema projectors, etc. In these devices, downconversion luminescent materials (powers, plates or films) are used jointly with a primary light source and other phosphors. According to the type of the primary light source, there are several options to realize white light, which are outlined below. In addition, to apply white SSL (white LEDs) in those applications, some key optical properties of the device should be measured and characterized, such as luminous efficacy (or luminous 1990

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Figure 48. White LEDs fabricated by combining (a) a single Ca-α-sialon+Eu2+ yellow phosphor; (b) LuAG:Ce3+ + Ca1−xLixSi1+xAl1−xN3:Eu2+; (c) BaSi2O2N2:Eu2+ + β-sialon:Eu2+ + Ca-α-sialon+Eu2+ + CaAlSiN3:Eu2+ (different phosphor blend ratios) with a blue LED. (d) White LEDs prepared by pumping the phosphor blend of JEM:Ce3+ + β-sialon:Eu2+ + CaAlSiN3:Eu2+ (different phosphor blend ratios) by a 405 nm LED. Reprinted with permission from ref 229. Copyright 2007 American Institute of Physics. Reprinted with permission from ref 286. Copyright 2016 Nature Publishing Group. Reprinted with permission from 381. Copyright 2007 American Institute of Physics.

high CCTs, Xie et al. developed a shorter-wavelength Li-αsialon:Eu2+ phosphor and achieved tunable yellow emission with the peak wavelength of 563−586 nm, by varying the Si/Al and O/N ratios in the host lattice, together with the concentration of Eu2+.169,170 A wide range of white light is thus attained, having the CCT of 3000−5200 K and Ra of 60−65. Wang et al. reported the fabrication of a daylight color wLED by using the Sr0.5−xEuxBa0.5Si2O2N2 (x = 0.04) phosphor.148 By combining a 450 nm LED chip, the chromaticity coordinates and CCT of wLEDs are (0.323, 0.328) and 5970 K, respectively. The low CRI of wLEDs using Eu2+-doped nitride phosphors significantly hinders their applications in many aspects. To improve the CRI of 1-pc wLEDs, broadband yellow phosphors need to be developed. This leads to the discovery of several Ce3+doped nitride phosphors with broad emission bands, including CaAlSiN3:Ce3+, La3Si6N11:Ce3+, and SrAlSi4N7:Ce3+. Li et al. examined photoluminescence properties of the Ce3+-activated CaAlSiN3 phosphor and its applicability to wLEDs.222 The CCT of the fabricated wLEDs using the two optimized compositions of Ca1−2xCexLixAlSiN3 (x = 0.01) and Ca1−xCexAlSiN3−2x/3O3x/2 (x = 0.01), pumped by a 450 nm LED, are 3492 and 3722 K, respectively. The Ra value is around 70 and appreciably better than those attained by using Eu2+-activated α-sialon phosphors. Suehiro et al. developed the Ca-doped La3Si6N11:Ce3+ phosphor and reported its application to 1-pc warm-white LEDs.211 Under the excitation at 450 nm, the (La,Ca)3Si6N11:Ce3+ phosphors possess tunable yellow emission with the dominant wavelength of 577−581 nm and the fwhm of 130−143 nm. By combining (La,Ca)3Si6N11:Ce3+ with a 450 nm LED chip, a broad range of warm-white light with CCTs of 3704 K (class WW) and 2911 K (class L) as well as Ra of 65 and 69 can be produced. Ruan et al. reported the application of the Ce 3+-activated SrAlSi 4N7 phosphor.225 The daylight color wLED was fabricated by combining the Sr1−xCexAl1+xSi4−xN7 (x = 0.03) phosphor with a blue LED chip, which has the chromaticity coordinates of (0.317, 0.318), CCT of 6300 K, and Ra of 78, respectively.

efficiency, lumens, correlated color temperature (CCT), color rendering index (CRI), color gamut, and chromaticity coordinates (or color point). The definitions of these parameters can be found elsewhere.3,41,373 Please remember that, these optical properties are dominantly dependent on what type and what combination of down-conversion luminescent materials used in the SSL devices. 8.1. Blue LED-Driven White LEDs

8.1.1. One-Phosphor-Converted White LEDs. It is the simplest way to combine a yellow phosphor with a blue LED to create white light, which is also named as one-phosphorconverted (1-pc) wLEDs. This method promises high luminous efficacy because there is no reabsorption between dissimilar phosphors. However, the emission spectra of 1-pc wLEDs cannot cover all the spectral range of visible light, leading to a relatively low color rendering index. Currently, YAG:Ce3+ is still the first choice to fabricate 1-pc wLEDs dueto its excellent quantum efficiency and broad emission band. The IP problem of such a combination triggers the search for alternative yellow phosphors. Fortunately, several highly efficient yellow nitride phosphors have been discovered and applied, which are grouped into Eu2+ and Ce3+-doped luminescent materials, respectively. There are two interesting Eu 2+-doped yellow nitride phosphors suitable for wLEDs: α-sialon:Eu 2 + and Sr0.5Ba0.5Si2O2N2:Eu2+. The fabrication of 1-pc wLEDs using the most representative orangish-yellow emitting nitride phosphor, Ca-α-sialon:Eu2+, was first reported by Sakuma et al. (Figure 48a).374 The CIE 1931 chromaticity coordinates and the CCT of wLEDs are (0.458, 0.414), respectively, and 2750 K, corresponding to the incandescent lamp color. Such a warm white could not be attained by using a single YAG:Ce3+ phosphor. The Ra value was restricted to 57, which is attributable to the narrower emission band of Ca-α-sialon:Eu2+. The evaluated chromaticity variation from room temperature to 200 °C is only 0.006, which is significantly smaller than 0.021 observed for wLEDs using YAG:Ce3+. It indicates the excellent thermal stability of Ca-α-sialon:Eu2+. To realize wLEDs with 1991

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8.1.2. Two-Phosphor-Converted White LEDs. As the 1-pc wLEDs have the color rendering index smaller than 80 because of the deficiency in enough green and red spectral components, they are usually used as flashlight, footlamps, and signals. To use in general illuminations such as exhibition, food market, medical operations, and photographic rooms, wLEDs should have a high color rendering index (Ra > 85). It is thus required to use two phosphors (i.e., green and red) instead of one to achieve much boarder emission spectra. Yamada et al. fabricated a red-enhanced trichromatic wLED by using an Eu-doped strontium calcium silicon nitride phosphor which shows an emission peak at 655 nm and the fwhm of 110 nm under the 460 nm excitation.375 By combining the red phosphor and a short-wavelength YAG:Ce3+ with a 460 nm InGaN chip, the Ra value of 87.7 is attained at 4670 K. The special CRI R9 (strong red) is also remarkably improved from −2.5 (wLEDs using a single YAG:Ce3+ phosphor) to 62.6. Mueller-Mach et al. demonstrated all-nitride wLEDs consisting of an InGaN-based blue chip, a yellow-green MSi2O2N2:Eu2+ and an orange-red M2Si5N8:Eu2+ (M: alkaline earth).376 A high Ra value of 90 is obtained at 3200 K, together with R9 of 56 and R13 (skin tone) of 93. The CCT and CRIs are very stable against changes in temperature and driven current, owing to the high thermal quenching temperature (>200 °C) of both phosphors. Xie et al. attempted to fabricate trichromatic wLEDs by employing Ca-α-sialon:Yb2+ (green) and Sr2Si5N8:Eu2+ (red) phosphors.377 The wLEDs cover the whole CCT range of ∼6500−2700 K and Ra of 82−83. Yang et al. reported a high color rendering wLED by combining a blue LED (λem = 455 nm) with a green SrSi2O2N2:Eu2+ (λem = 538 nm) and a red CaSiN2:Ce3+ (λem = 642 nm).378 The three-band wLED has the luminous efficacy of 30 lm/W, chromaticity coordinates of (0.339, 0.337), CCT of 5206 K, and Ra of 90.5, respectively. In addition, compared to the wLED prepared by using sulfide phosphors, the wLED using two nitride phosphors exhibit small variations in optical properties as the forward-bias current increases from 5 to 60 mA. Watanabe et al. reported the spectral tuning of the SrxCa1−xAlSiN3:Eu2+ (SCASN:Eu2+) solid solution system and the fabrication of a trichromatic wLED using the developed SCASN:Eu2+ and a green-emitting Ca3Sc2Si3O12:Ce3+ phosphor, pumped by a 460 nm LED chip.328 The Ra value is 90, and the power efficiency is improved by 14%, compared to the system using an unmodified CASN:Eu2+ phosphor. Fukuda et al. fabricated trichromatic wLEDs by using the green-emitting Sr3Si13Al3O2N21:Eu2+ phosphor with a blue LED and a red-emitting (Ca,Sr)2SiO4:Eu2+. The wLEDs show CCTs in the range of 6450−3230 K and Ra of 82−88.181 Li et al. used the green phosphor Ba3Si6O12N2:Eu2+ to prepare trichromatic wLEDs by combining it with Sr2Si5N8:Eu2+ and a 460 nm blue LED. The CCTs of 6440−3120 K and relatively high Ra of 88−94 are obtained.159 Wang et al. developed a blue-shifted and broadband red Ca1−xLixSi1+xAl1−xN3:Eu2+ to minimize the trade-off between the color rendering index and luminous efficacy.286 The wLED, fabricated by combining Ca 1−x Li x Si 1+x Al 1−x N 3 :Eu 2+ and Lu3Al5O12:Ce3+ with a blue LED, exhibits superhigh color rendering indices of Ra = 95 and R9 = 96, and luminous efficacy of 101 lm/W (CCT ∼ 3036 K) (Figure 48b). 8.1.3. Multiple-Phosphor-Converted White LEDs. The emission spectra of wLEDs can be further modified by using three (green, yellow, and red) or more phosphors to realize much higher color rendering properties including the average color

rendering index (Ra) and the ninth color rendering index (R9).379 Sakuma et al. reported high CRI quadchromatic wLEDs that combine Ca-α-sialon:Eu2+, β-sialon:Eu2+, and CaAlSiN3:Eu2+ phosphors with a 450 nm LED chip.380 The spectral deficiencies observed for the aforementioned Ca-α-sialon:Eu2+-based dichromatic system are effectively compensated by employing the green and red phosphors, and high Ra values of 81−88 can be achieved in the whole CCT range of 6600−2800 K. The special CRIs, R9 of 78−99 and R15 (skin tone of Japanese women) of 93−99, are also significantly high, owing mainly to the long emission wavelength of the CaAlSiN3:Eu2+ phosphor used (∼650 nm). Kimura et al. further improved the color rendering properties of the above wLEDs, by adding a blue-green BaSi2O2N2:Eu2+ phosphor possessing an emission peak wavelength of 496 nm.381 The Ra value of the developed pentachromatic wLEDs reaches as high as 95−98 in the whole CCT range of ∼6400−2900 K. Such a superhigh color rendering index is hardly achieved by using blue LEDs, which is owing to the compensation of the spectral deficiency between the blue and green regions by using the bluelight excitable BaSi2O2N2:Eu2+ (Figure 48c). Pust et al. developed a new narrow-band red-emitting phosphor, SrLiAl3N4:Eu2+ (SLA), which exhibits an emission peak wavelength of 654 nm and the fwhm of only ∼50 nm.35 The fabrication of a prototype warm wLED employing SLA, Lu3Al5O12:Ce3+ (green), and (Ba,Sr)2Si5N8:Eu2+ (orange) phosphors pumped by a blue LED is demonstrated, and high CRIs of Ra = 91 and R9 = 57 are attained at the CCT of 2700 K. The developed quadchromatic system shows an increase of 14% in luminous efficacy, compared with a commercial high-CRI LED utilizing a deep-red CaAlSiN3:Eu2+ phosphor. 8.2. Near UV-LED-Driven White LEDs

An alternative option to generate white light is to pump phosphor blends (RGB, red + green + blue phosphors) by near UV LEDs. This method usually allows one to produce a higher color rendering index (Ra > 90) than that using blue LEDs, because (i) the blue phosphor always has a larger emission band than blue LEDs, and (ii) the spectral gap between the blue (from LED) and green (from phosphors) spectral regions can be filled by selecting appropriate phosphors. In addition, there are many choices of phosphors that can be excited by UV light. On the other hand, some trade-offs for this type of wLEDs are also obvious, such as (i) the reabsorption of high energy phosphor emissions by other phosphors and (ii) large Stokes shifts leading to energy loss. Since most green-, yellow-, and red-emitting nitride phosphors developed so far can also be excited by near UV efficiently, it is thus important to find robust blue phosphors that can absorb near UV light. Although numerous blue phosphors have been discovered and reported (Tables 1 and 2), only a few of them show excellent luminescence efficiencies and durability, which include JEM:Ce3+, LaSi3N5:Ce3+, and AlN:Eu2+. Takahashi et al. reported an interesting blue-emitting LaJEM:Ce3+ (LaAl(Si6−zAlz)(N10−zOz):Ce3+) phosphor.228 It has a higher external quantum efficiency (50%) than the commercial BAM:Eu2+ (46%) under 405 nm excitation. The pentachromatic wLEDs fabricated by combining JEM:Eu2+, β-sialon:Eu2+, Ca-αsialon:Eu2+, and CaAlSiN3:Eu2+ phosphors with a 405 nm LED chip show a high Ra value of 95−96, CCT of 2830−4350 K, and luminous efficacy of 19−20 lm/W, respectively (Figure 48d). 1992

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laser irradiation and heavy thermal attack, the color converter in white LDs must also be PiG or phosphor ceramics. 8.3.1. White LEDs Using Phosphor-in-Glass (PiG). The phosphor-in-glass (PiG) is usually prepared by firing the powder mixture of phosphor and glass frit at a middle temperature in air. To avoid the oxidation of Eu2+ or Ce3+ and the reaction between phosphor particles and glass, low-melting-point glass compositions are chosen. YAG-PiGs have been extensively investigated and demonstrated to exhibit excellent thermal stability and optical properties in high power wLEDs,385−392 but the deficiency of the red components in YGA-PiGs results in low color rendering properties. It is thus required to prepare redemitting or other PiGs to enhance the color rendition. The packaging configuration has a great influence in optical properties of wLEDs using multicolor PiGs. Lee et al. prepared PiG disks by firing Lu3Al5O12:Ce3+ and CaAlSiN3:Eu2+ with a low melting point glass with the composition of 25%SiO2-25% B2O335%ZnO-5%Al2O3-15%K2O at 600 °C.388 Remote-phosphor wLEDs were then fabricated by a smart design of cutting and reassembling of PiGs, which have Ra of 79−80, CCT of 3016 K (2-PiG), and 2045 K (4-PiG), and the luminous efficacy of 8 lm/ W. This configuration leads to excellent thermal and chemical stability together with high luminous efficiency. Xiang et al. reported a stacking geometric configuration of PiG by screenprinting a red CaAlSiN3:Eu2+ phosphor (R) layer (43 μm in thickness) on the Lu3Al5O12:Ce3+-PiG (Lu-PiG) substrate with the aim to enhance the red component (Figure 49).400 Using this

Yaguchi et al. attempted the spectral tuning of the LaSi3N5:Ce3+ phosphor by doping Ca into the La site, for optimizing its applicability to near UV-driven wLEDs.382 Both the excitation and emission spectra of (La,Ca)Si3(O,N)5:Ce3+ phosphors are effectively red-shifted and broadened by the Cadoping, and the optimized composition exhibits an improved external quantum efficiency of 45% and a luminous efficacy of radiation (LER) of the blue emission up to 152 lm/W, measured under the 380 nm excitation. The spectral simulation of trichromatic wLEDs using (La,Ca)Si 3 (O,N) 5 :Ce 3 + , SrSi2O2N2:Eu2+, and Sr2Si5N8:Eu2+ phosphors demonstrates significantly higher Ra values of 93−95 in the whole CCT range of 3000−6500 K, compared to the devices using the BAM:Eu2+ phosphor or InGaN LED as a blue component. The high Ra is mainly attributed to the improved R5 (97.3) and R6 (97.9) values in the fabricated wLEDs. Wang et al. developed a thermally robust Al1−xSixCxN1−x:Eu2+ blue phosphor (λem = 470 nm, fwhm = 56 nm) by partially substituting Al−N by Si−C.104 Trichromatic wLEDs were produced by combining Al1−xSixCxN1−x:Eu2+, β-sialon:Eu2+, and (Sr,Ca)AlSiN3:Eu2+ phosphors with a UV LED (λem = 380 nm). The Ra value is 90.1 (CCT ∼ 3015 K) and further increased to 95.3 (CCT ∼ 3533 K) by using a broader green-emitting LuAG:Ce3+ phosphor. 8.3. High Density Blue Light-Driven White LEDs/LDs

High-power and/or high-brightness wLEDs are getting more and more attention for use in some special cases such as projectors, vehicle’s headlamps, landscape lighting, airport lighting, and so on. In these lighting devices, the excitation energy or density of the primary blue light is usually higher than that applied in traditional low-power wLEDs; the downconversion luminescent materials used for high-power wLEDs therefore must be more thermally robust and have less luminance saturation. In traditional wLEDs, the phosphor blend (mixed with organic binders such as silicone) is coated on LED chips tightly; it would thus have suffered from thermal attacks by the heat generated from LED chips. This is more serious when the phosphor materials are excited by high power density LED chips. Moreover, the yellowing of organic binders under excitation usually happens, which would result in the decrease of the transparency of the binder and thus the degradation of wLEDs. To overcome these problems, the phosphor powders must be dispersed into a glass matrix (phosphor-in-glass, PiG) or sintered into ceramics (phosphor ceramics). These bulk luminescent materials exhibit larger thermal conductivities and higher stabilities against light irradiation than the phosphor-in-silicone (PiS), enabling one to produce highly reliable and highly bright white light, when they are combined with blue LED chips in a remote-phosphor configuration.382−392 On the other hand, although the state-of-the-art wLEDs are realized via a combination of blue- or UV-LED chips, the commercial LEDs have the well-known efficiency droop (i.e., the decline in efficiency with increasing input current density). It is thus very hard to achieve superhigh brightness white light using LEDs unless a large-size LED chip array is used. Laser diodes, however, do not have such droops in efficiency, allowing one to generate superhigh brightness white light using only a smaller number of LD chips.393 Therefore, blue LD chips instead of LED chips combined with a phosphor converter have become an emerging technology for a diverse range of high brightness applications including displays, projectors, automotive headlights, and general illumination.394−399 To survive from high-flux

Figure 49. (a) The schematic of the fabrication process of a red phosphor stacked Lu3Al5O12:Ce3+-PiG (red and Lu-PiG) plate, (b) SEM cross-sectional image of red and Lu-PiG plates, (c) EL spectra, and (d) CIE color coordinates of wLEDs constructed by the combination of a blue chip and a red and Lu-PiG plate with different concentrations of (wt %) CaSiAlN3:Eu2+ phosphor. Insets are photographs of wLEDs in operation. Reprinted with permission from ref 400. Copyright 2016 Elsevier.

stacking color converter with an appropriate amount of CaAlSiN3:Eu2+ (15 wt %), a warm wLED was prepared. Driven under the 20 mA current, the warm wLED shows a luminous efficacy of 102.1 lm/W, CCT of 3410 K, and Ra of 76.5. Peng et al. proposed a packaging structure consisting of a multilayered red (CaAlSiN3:Eu2+), green (Ba2MgSi2O7:Eu2+), and blue ((Sr,Ba)3MgSi2O8:Eu2+) phosphor-in-glass (PiG) to improve 1993

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the luminous efficacy of near UV driven wLEDs.401 wLEDs packaged with the multilayered PiG with the order of R-G-B have the luminous efficacy increased by 8.2% (driven under 500 mA) compared to those packaged with randomly mixed RGB PiG. The color rendering index and correlated color temperature are 86.8 and 2984 K, respectively. Using the multilayered PiG with the order of G-R-B, wLEDs achieve the highest luminous efficacy of 29.2 lm/W, CCT of 3326, and Ra of 84.2. Several nitride phosphor PiGs have been synthesized, including Ca-α-sialon:Eu2+, β-sialon:Eu2+, and CaAlSiN3:Eu2+, which are demonstrated to have potential applications in wLEDs or solid state laser lighting. Segawa et al. prepared the Ca-αsialon:Eu2+ yellow PiG by dispersing the phosphor in a series of zinc phosphate glasses, xZnO-(100-x)P2O5 (x = 50, 55, 60, and 65) and melting the powder mixture at 1200 °C for 10 min.402 The PiG with a glass composition of x = 60 and a phosphor content of 3 wt % is found to achieve a comparable quantum efficiency and thermal quenching behavior to the phosphor powder. They also explored other metaphosphate glass systems (50MO-50P2O5; mol %, M = Zn, Ca, and Ba) and concluded that the barium phosphate glass containing 3−4 wt % Ca-αsialon:Eu2+ phosphor was optimized as a host material.403 Zhu et al. synthesized a green-emitting PiG material by cofiring βsialon:Eu2+ phosphor powders with ZnO−B2O3−BaO−Al2O3 glass frits at 630−660 °C for 20−80 min in air.404 The PiG sample containing 5 wt % β-sialon:Eu2+ phosphor fired at 650 °C for 40 min exhibits the optimized optical properties, possessing a transmittance and an external quantum efficiency (EQE) of 29% and 23% at 441 nm, respectively. The thermal conductivity of the fabricated PiG material is ∼10 times higher than that of silicone, which will benefit the rapid heat-release when pumped by laser diodes. It is demonstrated that the luminous flux (brightness) of the PiG sample excited by a 441 nm solid-state laser diode increases linearly with the incident laser power, up to the laser flux density of 0.7 Wmm−2. Zhu et al. also reported a fully densified translucent CaAlSiN3:Eu2+ PiG material by using the same method.405 The red-emitting PiG has an EQE of 43%, transmittance of 30% at 640 nm, and a thermal conductivity of 1.12 W m−1 K−1. Under the excitation of a blue laser, the maximum luminous flux of CaAlSiN3:Eu2+ PiG sample is 39 lm at the laser flux density of 0.5 W/mm2. 8.3.2. White LEDs using Phosphor Ceramics. Phosphor ceramics are the most suitable color converters for use in highpower SSL because (i) they show excellent stability and reliability of photoluminescence properties under high flux excitation and thermal attacks, (ii) their microstructures can be easily tailored, allowing for freedom in controlling the scattering and absorption, and (iii) they are freely manipulated and engineered. Raukas et al. reviewed the phosphor ceramics as a light converter in LEDs.406 He suggested that the M2Si5N8:Eu2+ (M = Ca, Sr, Ba) amber ceramic converter disk and platelet could be utilized in full conversion LEDs (fc-LEDs) for automotive and industrial signaling applications (Figure 50, panels a and c). A direct amber LED (InGaAlP) loses more than 50−60% of the luminous flux when warming up to 80−100 °C, whereas the observed decrease in light output for the blue-pumped amber fcLED is ∼35% even at 150 °C, obviously validating the high thermal stability of phosphor ceramics (Figure 50b). Joshi et al. reported transparent Mg-α/β-sialon:Eu2+ yellow phosphor ceramic plates, obtained by hot-press sintering under a uniaxial compression of 30 MPa at 1850 °C for 1 h.407 The phosphor ceramics with a thickness of 0.1 mm shows the

Figure 50. (a) As-prepared (polished) amber ceramic converter disk (left) and diced platelets (right) of M2Si5N8:Eu (M = Ca,Sr,Ba). (b) Applications of phosphor ceramics in vehicle lamps including headlamp, signal and DRL (daytime running lamp). (c) Temperature-dependence of the normalized light output from blue pumped fc-amber LED (red ■) at up to 150 °C, compared to the same from direct InGaAlP chip at 350 mA (red ▲). Reprinted with permission from ref 406. Copyright 2013 The Electrochemical Society.

transmittance of 10−50% in the spectral region of 500−700 nm and an emission maximum of 570 nm. Wieg et al. prepared AlN:Ce3+ ceramics by using the CAPAD (current activated pressure assisted densification) technique.408 The phosphor ceramics show a quite broad emission band centered at 430, 490, and 612 nm under the 375 nm excitation, respectively. The photoluminescence is attributed to both the intrinsic AlN defect complexes and the 4f → 5d electronic transitions of Ce3+. The high thermal conductivity (∼90 W m−1 K−1) together with the white light emission of AlN:Ce3+ enables the phosphor ceramics to be used for LED and laser-driven SSL. 8.3.3. Blue LD-Driven Laser Lighting. YAG:Ce3+ and YAG:Ce3+-Al2O3 luminescent ceramics have often been used in laser-driven SSL.393−398 For luminescent nitride materials, Yoshimura et al. developed a laser lighting device using sialon phosphor-glass composite films as the wavelength converter (Figure 51a).409 The phosphor-glass composite films adhered on a glass substrate were prepared by dispersing α- and/or βsialon:Eu2+ phosphor powders into a silica precursor solution and subsequent dip-coating and sintering at 500 °C. As shown in Figure 51b, the phosphor-glass films exhibit comparable quantum efficiencies with corresponding phosphor powders and show very intense luminescence under UV light irradiation.

Figure 51. (a) Schematic diagram of the SSL device. (b) Photos of SiAlON phosphor−glass composite films under UV light irradiation. (c) Temperature of the phosphor−glass composite film and YAG phosphor as a function of the excitation light power (blue LD). (d) Luminance of the phosphor−glass composite film and YAG phosphor as a function of the excitation light power (blue LD). Reprinted with permission from ref 409. Copyright 2016 The Japan Society of Applied Physics. 1994

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The temperature of YAG:Ce3+ increases suddenly from 150 to 250 °C when the blue light irradiation power exceeds 2 W, whereas that of the sialon phosphor films remains a steady increase (Figure 51c). The brightness of the composite film using sialon phosphors is not saturated until the power is up to 4 W, leading to a maximum luminance of the sialon-based device 15% higher than that of the YAG:Ce3+-based system (Figure 51d). Li et al. reported a translucent red-emitting CaAlSiN3:Eu2+ ceramic for solid state laser lighting (Figure 52).410 The ceramic

Standard Committee (NTSC) standard]. Although QD backlights exhibit a large color space, they have drawbacks of small size, high cost, and toxicity (containing Cd or Pb).411−415 wLEDs are therefore considered as the most suitable alternative to CCFLs to achieve vivid and brilliant displays. The first generation wLED backlight is the 1-pc-converted wLED by using a broadband YAG:Ce3+, which has a small color gamut of ∼75%. With advances in phosphor materials, 2-pc-converted wLEDs with a much wider color space can be attained. Differing from those used for general lighting, phosphors in wLED backlights are required to have their emission spectra matching well with the color filters. Therefore, to realize a large color gamut, phosphors should have both a narrow emission band and an appropriate emission color. 8.4.1. White LED Backlights Using a Narrow-Band Green Phosphor. β-sialon:Eu2+ is considered as one of the most suitable candidates of green phosphors for wLED backlights because it has a steep and narrow band with an emission maximum of 535 and a fwhm of 55 nm. In addition, βsialon:Eu2+ has an extremely high stability, showing no shifts in luminescence efficiency and color points under the reliability test (85 °C and 85% RH) for 6000 h.171 Xie et al. first reported a three-band wide color gamut wLED backlight by combining βsialon:Eu and CaAlSiN3:Eu2+ phosphors with a blue LED (Figure 53a).39 This 2-pc-converted wLED shows a discrete

Figure 52. (a) A reflection mode is used to measure optical properties of phosphor ceramics under blue LD irradiation. (b) The emission spectra of the CASN:Eu2+ ceramic excited by a blue LD. Inset shows the photo of the emitting ceramic. (c) Luminous flux of CaSiAlSiN3:Eu2+ ceramic plates with different thicknesses as a function of incident power density. Reprinted with permission from ref 410. Copyright 2016 Royal Society of Chemistry.

converter has the enhanced thermal stability (15% increase) and higher thermal conductivity (4 W1− K−1) compared to the corresponding phosphor powder (∼0.5 W1− K−1). The external quantum efficiency of the ceramic is 60% (87% of the powder) under the 450 nm excitation. In a reflection mode under the blue LD irradiation, the CaAlSiN3:Eu2+ ceramic exhibits the same strong red emission at 650 nm with the powder (Figure 52b). The phosphor ceramics show no luminance saturation when the incident power density increases from 20 to 150 W/cm2 (Figure 52c). The luminous efficacy of the ceramic with the thickness of 150 μm is 42.2 lm/W. It thus can be applied in low-power laser lighting devices, aiming to improve the color rendition or color gamut.

Figure 53. Electroluminescence spectra of wLEDs using (a) βsialon:Eu2+ + CaAlSiN3:Eu2+, (c) sharp β-sialon:Eu2+ + K2SiF6:Mn4+, and (e) sharp β-sialon:Eu2+ + γ-alon:Mn2+. Color space area (color gamut) of (b) β-sialon:Eu2+ + CaAlSiN3:Eu2+, (d) sharp β-sialon:Eu2+ + K2SiF6:Mn4+, and (f) sharp β-sialon:Eu2+ + γ-alon:Mn2+. Reprinted with permission from ref 39. Copyright 2009 The IOP Publishing Ltd. Reprinted with permission from ref 416. Copyright 2016 John Wiley & Sons. Reprinted with permission from ref 417. Copyright 2017 The Japan Society of Applied Physics.

spectrum with distinct separation of red, green, and blue primary colors, owing to a narrow and asymmetric emission band of βsialon:Eu2+. The color gamut is 91.9% of the NTSC standard in the CIE 1976 color space, much higher than 71.6% for a conventional 1-pc wLED using YAG:Ce3+ (Figure 53b). 8.4.2. White LED Backlights Using Both Narrow-Band Green and Red phosphors. Great efforts have been made to

8.4. Wide Color Gamut Backlights for LCDs

Much concern has been expressed regarding the backlights for LCDs with wide color gamut, high brightness, low power consumption, and mercury-free. The traditional cold cathode fluorescent lamps (CCFL) not only contain mercury but also have a small color gamut [65−75% of the National Television 1995

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compete with QD backlights by using thermally robust and reliable nitride phosphors.

discover and develop both narrow-band green- and red-emitting phosphors for getting simultaneous large color gamut and high luminous efficacy of wLED backlights. As to green phosphors, Takahashi et al. developed a sharp β-sialon:Eu2+ with an emission maximum of 525 nm and a fwhm of 47 nm, enabling one to make the color gamut more wider.176,416 Yoshimura et al. simulated the relation between the color gamut and display brightness by using green phosphors with varying emission maxima and band widths and proposed that the phosphor having an emission peak longer than 520 nm and the bandwidth narrower than 45 nm would be highly favorable for superwide color gamut displays (Figure 54).417 To this end, γ-alon:Mn,Mg could be the most suitable

9. SUMMARY AND OUTLOOK Among all inorganic luminescent materials for SSL, nitride phosphors are considered as the most encouraging materials as they possess great diversity, rigidity, and stability in structure that enable the full color emission, strong absorption of blue light, high thermal stability, small Stokes shift, and high quantum efficiency. The peculiar photoluminescence properties enable luminescent nitrides to be applied in SSL and display shortly after their discovery. Some of them have already been commercialized, such as MSi2O2N2:Eu2+ (M = Sr, Ba), M2Si5N8:Eu2+ (M = Ca, Sr, Ba), MAlSiN3:Eu2+ (M = Ca, Sr), β-sialon:Eu2+, α-sialon:Eu2+, and La3Si6N11:Ce3+, playing key roles in lighting devices with superior optical properties and reliability including superhigh color rendering properties, tunable color temperatures, wider color gamut, and longer longevity. This paper highlights the recent research and development of luminescent nitride materials, covering a broad range of topics, such as materials discovery, photoluminescence properties, crystal structure and structure-luminescence relationship, spectral tuning strategies, thermal stability, and their applications in wLEDs. As a new kind of luminescent materials, nitride phosphors must be paid much more attention, and the discovery of novel nitride host compounds is still an endless mission for chemists and materials scientists. It is thus of significant importance to develop efficient approaches for searching for and screening of new crystals, such as single-crystal growth, solid state combinatorial chemistry, and single-particle diagnosis methods. To discover luminescent nitrides with desired spectral position and shape, the design and selection of appropriate prototypes of crystal structure is extremely important. Schnick et al. discovered a series of emerging luminescent nitrides with narrow-band red emissions based on the UCr4C4-type structure, which opens up a new direction for the “top-down” structural design of luminescent nitrides.35,131,132,134 The single-particle diagnosis approach, combining with both combinatorial screening and single-crystal growth methods, simplifies the search and identification of nitride compounds with new crystal structures. In addition, the structure building blocks of nitride hosts can be extended from SiN4 and AlN4 tetrahedra to MgN4, LiN4, GaN4, GeN4, PN4, or BN3, which greatly enriches the family of luminescent nitrides. Although it is not fully outlined in this review, the synthesis approach is only the key to obtaining luminescent nitride materials with high luminescence intensity, high quantum efficiency, high phase purity, controllable size, and morphology. The methods for synthesizing nitride materials can be classified into solid state reaction, gas reduction and nitridation, carbothermal reduction and nitridation, nitridation of alloys, and the ammonothermal method, mainly depending on the used raw materials. Differing from the synthesis of oxidic phosphors, the preparation of nitride phosphors usually requires high nitrogen gas pressure (1−100 MPa) or sometimes superhigh pressure (1−10 GPa) by using the Walker-type multianvil technique. For phosphors, it is a must to achieve single-phase materials for maximizing the intrinsic luminescence of the target composition and minimizing the negative effects from impurity phases. Especially, it is a great challenge for the scale-up synthesis of phase-pure phosphors starting from the stoichiometric composition of single crystals. To this end, the selection of an appropriate synthetic method is very critical. In addition, to

Figure 54. Simulated effects of the peak emission and bandwidth on the color gamut and display brightness of wLED backlights. Reprinted with permission from ref 405. Copyright 2017 The Japan Society of Applied Physics.

candidate for such a purpose because it emits at 520 nm and has a fwhm of 44 nm. Then, Yoshimura et al. greatly improved the external quantum efficiency of the narrow band γ-alon:Mn,Mg green phosphor up to 30% under 445 nm excitation by optimizing the composition and synthetic conditions.417 With regard to red phosphors, CaAlSiN3:Eu2+ has deep red spectral components that significantly reduces the luminous efficacy of wLEDs. Moreover, a large spectral overlap between CaAlSiN3:Eu2+ and β-sialon:Eu2+ leads to strong reabsorption and hence a low luminous efficacy. To solve these problems, an alternative narrow-band K2SiF6:Mn4+ (KSF) red phosphor showing line spectra is suggested for use in wLED backlights. By using both narrow-band green and red phosphors superlarge color gamut wLEDs can be achieved. Wang et al. reported production of a wide color gamut wLEDs by using β-sialon:Eu2+ and KSF phosphors. The color gamut of the fabricated wLEDs, defined in the CIE 1931 and CIE 1976 color spaces, is respectively ∼86% and ∼94−96% of the NTSC standard (CIE 1976).418 Yoshimura et al. attempted the combination of Sharp β-sialon:Eu2+ and KSF to realize a large color space (Figure 53, panels c and d). Both high color gamut (107% of NTSC in CIE 1976) and luminous efficacy (114 lm/ W) are finally realized.416 Later, Yoshimura et al. proposed the application of the combination of γ-alon:Mn,Mg and KSF and reported a superwide color gamut with a world record of 110.8% of NTSC (CIE 1976) (Figure 53, panels e and f).417 The reliability test demonstrates that wLEDs using γ-alon:Mn,Mg + KSF exhibit high stabilities in both luminous efficacy and color point when in operation for 1000 h, which is quite similar to those using sharp β-sialon:Eu2+. It thus makes it possible to 1996

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AUTHOR INFORMATION

obtain single-phase nitride phosphors, it is also very essential to choose the nonstoichiometric composition, appropriate raw materials, atmosphere, and suitable crucibles. Photoluminescence properties (absorption, emission, and quantum efficiency) and thermal quenching of luminescent nitrides are closely related to the local structure and electronic structure of host crystals. It is quite important to understand the local coordination environment and electronic structure by using both experimental techniques and computations. The valence state of activators, structural ordering/disordering, and defects (antiphase, twins, intergrowth, stacking faults, etc.) revealed by a variety of analytical techniques including EXAFS, HRTEM, EPR, XPS, and HAADF enable one to clarify the luminescence of nitrides, helping to get deep insights into the structureluminescence relations. With this information, it is also able to modify and tune the emission color as well as the thermal stability of nitrides by cationic/anionic substation or chemical unit substitution, developing useful materials for practical applications such as blueshifted Ca1−xSrxAlSiN3:Eu2+, narrow-band βsialon:Eu 2 + (with small z values) and broadband Ca1−xLixSi1+xAl1−xN3:Eu2+.176,286,328,329,337 The dielectric constant calculated by using DFT enables one to explain the redshifted emission of nitrides reasonably, and the Debye temperature computed by DFT is an indicator of the structural rigidity, both of which need to be strengthened and applied to a broad range of luminescent nitride materials. The calculated electronic/band structure allows us to understand the band gap and the position of 5d excited state of Eu2+/Ce3+ relative to the CBM of the host and then to evaluate the thermal quenching behavior of nitride materials. These structural analyses and computations are thus powerful tools to predict and interpret luminescence and thermal quenching of nitride materials, and they in turn provide guidelines for material design and property tailoring. Luminescent nitride materials play an indispensable role in wLEDs for lighting and display, enabling one to produce solid state devices with high efficiency, high color rendition, wide color gamut, and great reliability. Currently, they are widely used in wLEDs for general illumination, vehicle headlamps, medical operations, exhibition lighting, and LCD backlighting. Moreover, luminescent nitrides can also be applied as emissive materials for FED and FELs (field emission lamps) due to their high reliability against electron bombardment.101,418−419 The afterglow properties of luminescent nitrides make them possible to be used in security ink, optical storage media, and in vivo bioimaging.344,367,420 For these applications, the particle size of nitrides should be significantly reduced from several micrometers to several nanometers, which requires using special synthetic routes such as the ammonothermal synthesis, laser ablation, etc.54,55,421−426 With the discovery of new nitride compounds together with the deep understanding of luminescence, crystal/electronic structure, and structure-luminescence relations, nitride materials with promising and unique luminescent properties will be developed, and they will definitely find a broad range of application fields, not only limited to solid state lighting but also including optoelectronic devices, health care, medical/biological imaging, security, optical storage, plant growth, photocatalysis, etc.

Corresponding Author

*E-mail: [email protected]. ORCID

Rong-Jun Xie: 0000-0002-8387-1316 Takayuki Suehiro: 0000-0001-9444-9738 Notes

The authors declare no competing financial interest. Biographies Le Wang received her Ph.D. degree of Optical Engineering in 2012 from Zhejiang University, China. She is now a Professor at College of Optical and Electronic Technology of China Jiliang University. She was a visiting researcher at National Institute for Materials Science (NIMS, Japan) in 2015 and a research associate at Hongkong Polytech University in 2017. Her research interests cover solid state lighting technologies, optics of LEDs, nitride luminescent materials, and their applications. Rong-Jun Xie received his B.S. (1992), M.S. (1995), and Ph.D. (1998) degrees from North University of China, Xi’an Jiaotong University, and Shanghai Institute of Ceramics, Chinese Academy of Sciences, respectively. He worked as a postdoctoral researcher at National Institute for Inorganic Materials (NIRIM, Japan) from 1998−2000, at National Institute of Advanced Science and Technology (AIST, Japan) from 2001−2002, and as an Alexander von Humboldt Research Fellow at Darmstadt University of Technology (TUD, Germany) from 2002− 2003. He joined National Institute for Materials Science (NIMS, Japan) as a Senior Researcher since 2003. He was a Chief Researcher at Sialon Group at NIMS till the end of 2017. Since 2018, he is a full professor at Xiamen University, China. His research interests include the synthesis, properties, and applications of luminescent nitride materials for solid state lighting and displays. To date, he has published over 200 scientific papers on luminescent nitride materials. Takayuki Suehiro received his B.S. degree in 1997 and Ph.D. degree in 2002 from Yokohama National University. He worked as a postdoctoral researcher at National Institute for Materials Science from 2002−2006 and as an assistant professor at Tohoku University from 2006−2012. Since 2012, he joined NIMS as a senior researcher. His research interests involve synthesis of novel nitride materials and their optical and photocatalytic applications. To date, he has 46 scientific publications. Takashi Takeda received his B.S. degree in 1995 and Ph.D. degree in 2000 from Kobe University. He worked as a postdoctoral researcher at Sumitomo Chemical from 2000−2001. He was an assistant professor at Tohoku University from 2001−2002 and Hokkaido University from 2002−2007. He is currently working as a principal researcher at National Institute for Materials Science (NIMS). His current research interests involve nitride and oxynitride functional materials. Naoto Hirosaki received his B.S. (1978), M.S. (1980), and Ph.D. (1990) from Kyoto University (Japan), respectively. In 1980, he joined Nissan Motor Co., Ltd., and he was a Senior Researcher at National Institute for Research in Inorganic Materials (NIRIM) of the Science Technology Agency. He joined the NIMS as a Senior Researcher in 2001, Chief Researcher from 2002 to 2006, Group Leader at the Nitride Particle Group of the NIMS from 2006 to 2011. Now, he is the leader of the Sialo Group at NIMS. In 2015, he was awarded NIMS Research Fellow and Medal with Purple Ribbon by the Japanese Prime Minister’s Office for the innovation of phosphor materials for wLEDs. He has published more than 230 papers in the areas of nonoxide ceramics and nitride phosphor materials. 1997

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ACKNOWLEDGMENTS We greatly acknowledge financial support of this work from the National Natural Science Foundation of China (Grants 61575182, 61405183, 5157223, and 51561135015), the National Key Research and Development Program (MOST, 2017YFB0404301), the JSPS KAKENHI (Grant 15K06448), and the Natural Science Foundation of Zhejiang Province (Grant LY16F050004).

wLED XANES XPS XRD XRPD YAG

white light-emitting diode X-ray absorption near edge structure X-ray photoelectron spectroscopy X-ray diffraction X-ray powder diffraction Y3Al5O12

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LIST OF ACRONYMS AND ABBREVIATIONS BAM BaMgAl10O17 BCNO carbon-based boron oxynitride CAPAD current-activated pressure-assisted densification CASN CaAlSiN3 CBM conduction band minimum CCD charge-coupled device CCFL cold cathode fluorescence lamp CCT correlated color temperature CIE Commission International del’Eclairge CL cathodoluminescence CRI color rendering index CRT cathode ray tube DFT density function theory ECSA electron spectroscopy for chemical analysis EDS energy dispersive X-ray spectrometry EQE external quantum efficiency EPR electron paramagnetic resonance EXAFS extended X-ray absorption fine structure FED field emission display FEL field emission lamp fwhm full width at half-maximum FT-IR Fourier-transform-infrared spectroscopy FUV far-ultraviolet HAADF high-angle annular dark field HIP hot isostatic pressure HRTEM high-resolution TEM IP intellectual property JEM LaAl(Si6−zAlz)(N10−zOz) LCD liquid crystal display LD laser diode LED light-emitting diode LP long pair MAPLE Madelung part of lattice energy MOF metal−organic frame MQW multiple quantum well MW molecular weight NIR near-infrared NSGA nondominated-sorting genetic algorithm NSSO nitrogen-doped Sr2SiO4:Eu2+ NTSC National Television System Committee PEG H(CH2CH2O)nOH PiG phosphor-in-glass PDP plasma display panel PSO particle swarm optimization QD quantum dot STEM scanning transmission electron microscopy SLA Sr[LiAl3N4] SMS Sr[Mg2Si2N4] SCASN (Sr, Ca) AlSiN3 SSL solid state lighting TEM transmission electron microscopy UV ultraviolet 1998

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