Phosphor-converted white light-emitting diodes (pc-WLEDs) are emerging as an indispensable solid-state light source for the next generation lighting industry and display systems due to their unique properties including but not limited to energy savings, environment-friendliness, small volume, and long persistence. Until now, major challenges in pc-WLEDs have been to achieve high luminous efficacy, high chromatic stability, brilliant color-rending properties, and price competitiveness against fluorescent lamps, which rely critically on the phosphor properties. A comprehensive understanding of the nature and limitations of phosphors and the factors dominating the general trends in pc-WLEDs is of fundamental importance for advancing technological applications. This report aims to provide the most recent advances in the synthesis and application of phosphors for pc-WLEDs with emphasis specifically on: (a) principles to tune the excitation and emission spectra of phosphors: prediction according to crystal field theory, and structural chemistry characteristics (e.g. covalence of chemical bonds, electronegativity, and polarization effects of element); (b) pc-WLEDs with phosphors excited by blue-LED chips: phosphor characteristics, structure, and activated ions (i.e. Ce 3+ and Eu 2+ ), including YAG:Ce, other garnets, non-garnets, sulfides, and (oxy)nitrides; (c) pc-WLEDs with phosphors excited by near ultraviolet LED chips: single-phased white-emitting phosphors (e.g. Eu 2+ –Mn 2+ activated phosphors), red-green-blue phosphors, energy transfer, and mechanisms involved; and (d) new clues for designing novel high-performance phosphors for pc-WLEDs based on available LED chips. Emphasis shall also be placed on the relationships among crystal structure, luminescence properties, and device performances. In addition, applications, challenges and future advances of pc-WLEDs will be discussed.

Phosphors in phosphor-converted white light-emitting diodes: Recent advances in materials, techniques and properties

Metal–organic frameworks (MOFs) are intrinsically porous extended solids formed by coordination bonding between organic ligands and metal ions or clusters. High electrical conductivity is rare in M...

Electrically Conductive Metal-Organic Frameworks.

Rise of silicene: A competitive 2D material

The synthesis of lanthanide-activated phosphors is pertinent to many emerging applications, ranging from high-resolution luminescence imaging to next-generation volumetric full-color display. In particular, the optical processes governed by the 4f-5d transitions of divalent and trivalent lanthanides have been the key to enabling precisely tuned color emission. The fundamental importance of lanthanide-activated phosphors for the physical and biomedical sciences has led to rapid development of novel synthetic methodologies and relevant tools that allow for probing the dynamics of energy transfer processes. Here, we review recent progress in developing methods for preparing lanthanide-activated phosphors, especially those featuring 4f-5d optical transitions. Particular attention will be devoted to two widely studied dopants, Ce3+ and Eu2+. The nature of the 4f-5d transition is examined by combining phenomenological theories with quantum mechanical calculations. An emphasis is placed on the correlation of hos...

Lanthanide-Activated Phosphors Based on 4f-5d Optical Transitions: Theoretical and Experimental Aspects.

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 ...

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

We perform first-principles calculations of mechanical and electronic properties of silicene under strains. The in-plane stiffness of silicene is much smaller than that of graphene. The yielding strain of silicene under uniform expansion in the ideal conditions is about 20%. The homogeneous strain can introduce a semimetal-metal transition. The semimetal state of silicene, in which the Dirac cone locates at the Fermi level, can only persist up to tensile strain of 7% with nearly invariant Fermi velocity. For larger strains, silicene changes into a conventional metal. The work function is found to change significantly under biaxial strain. Our calculations show that strain tuning is important for applications of silicene in nanoelectronics.

First-principles calculations of mechanical and electronic properties of silicene under strain

3+ have been synthesized using solid-state reactions and characterized by X-ray diffraction, Raman spectroscopy, and photoluminescence measurement. Raman spectra are used to identify the Aand B-site substitutions, because specific Raman peaks corresponding to different ions motion are sensitive to each situation. Raman data reveal that both the A- and B-site substituted solid solutions are formed. The photoluminescence intensity of the B-site substituted Sr2CaMoO6 is evidently higher than that of the A-site substituted phosphor. The WO6 group introduced into Sr2CaMoO6:Eu 3+ ,Li + acts as an energy obstacle to prevent energy transfer among MoO6 groups, leading to more energy being trapped by Eu 3+ , and a higher photoluminescence intensity is obtained. The phosphor with optimized composition Sr2Ca0.80Li0.10Eu0.10Mo0.10W0.90O6 shows a better luminescence intensity than the commercial phosphor Y2O2S:Eu under 395 nm excitations.

https://iopscience.iop.org/article/10.1149/1.2898897/pdf

Photoluminescence and Raman Spectra of Double-Perovskite Sr2Ca ( Mo / W ) O6 with A- and B-Site Substitutions of Eu3 +

The samples of complex perovskite Ba(Mg1/3Ta2/3)O3 (BMT) were synthesized at various sintering temperatures and the X-ray diffraction (XRD) patterns and Raman spectra of the samples were collected. Both the structure refinements using the XRD data and the Raman spectra show that the samples sintered above 1500°C with high pellet relative density (>95%) take almost-perfect B-site-ordered structure; while the samples sintered below 1400°C with low pellet relative density (<70%) take the more disordered structure at B-sites. The vibrational modes of 1:2 ordered BMTwere obtained and illustrated by using first-principle calculations. With the assistance of the calculation results, Raman peaks of BMT were assigned as A1g(1) (107 cm−1), A1g(2) (212 cm−1), A1g(3) (433 cm−1), A1g(4) (798 cm−1), and Eg(1) (104 cm−1), Eg(2) (160 cm−1), Eg(3) (264 cm−1), Eg(4) (386 cm−1), Eg(5) (576 cm−1). The highly ordered BMT sample at B-site shows longer phonon lifetime and weaker coupling among phonons than the disordered BMT, which probably are the intrinsic reasons for the highly ordered BMT sample to have the low dielectric loss.

XRD and Raman Studies on the Ordering/Disordering of Ba(Mg1/3Ta2/3)O3

A novel oxonitridosilicate phosphor host Sr(3)Si(2)O(4)N(2) was synthesized in N(2)/H(2) (6%) atmosphere by solid state reaction at high temperature using SrCO(3), SiO(2), and Si(3)N(4) as starting materials. The crystal structure was determined by a Rietveld analysis on powder X-ray and neutron diffraction data. Sr(3)Si(2)O(4)N(2) crystallizes in cubic symmetry with space group Pa 3, Z = 24, and cell parameter a = 15.6593(1) A. The structure of Sr(3)Si(2)O(4)N(2) is constructed by isolated and highly corrugated 12 rings which are composed of 12 vertex-sharing [SiO(2)N(2)] tetrahedra with bridging N and terminal O to form three-dimensional tunnels to accommodate the Sr(2+) ions. The calculated band structure shows that Sr(3)Si(2)O(4)N(2) is an indirect semiconductor with a band gap ≈ 2.84 eV, which is close to the experimental value ≈ 2.71 eV from linear extrapolation of the diffuse reflection spectrum. Sr(3-x)Si(2)O(4)N(2):xEu(2+) shows a typical emission band peaking at ~600 nm under 460 nm excitation, which perfectly matches the emission of blue InGaN light-emitting diodes. For Ce(3+)/Li(+)-codoped Sr(3)Si(2)O(4)N(2), one excitation band is in the UV range (280-350 nm) and the other in the UV blue range (380-420 nm), which matches emission of near-UV light-emitting diodes. Emission of Sr(3-2x)Si(2)O(4)N(2):xCe(3+),xLi(+) shows a asymmetric broad band peaking at ~520 nm. The long-wavelength excitation and emission of Eu(2+) and Ce(3+)/Li(+)-doped Sr(3)Si(2)O(4)N(2) make them attractive for applications in phosphor-converted white light-emitting diodes.

Promising Oxonitridosilicate Phosphor Host Sr3Si2O4N2: Synthesis, Structure, and Luminescence Properties Activated by Eu2+ and Ce3+/Li+ for pc-LEDs

Ba(Mg1/2W1/2)O3 ceramic was synthesized using a conventional solid-state reaction method at 1500°C for 4 h. The face-centered cubic crystal structure of the material was confirmed by Rietveld refinement of X-ray diffraction (XRD) data, and vibrational modes were obtained by Raman and Fourier transform far-infrared (FTIR) reflection spectroscopies. First-principle calculations based on density functional theory with local density approximation were used to calculate Gamma-point modes and dielectric properties of Ba(Mg1/2W1/2)O3. The Raman spectrum with nine active modes can be fitted with Lorentzian function, and the modes were assigned as F2g(1) (126 cm−1), F2g(2) (441 cm−1), Eg(O) (538 cm−1), and A1g(O) (812 cm−1). Far-infrared spectrum with 12 infrared active modes was fitted using both the Lorenz three-parameter classical and four-parameter semiquantum models. Consequently, the modes were assigned as F1u(1) (144 cm−1), F1u(2) (284 cm−1), F1u(3) (330–468 cm−1), and F1u(4) (593–678 cm−1). The active modes were represented by linear combinations of symmetry coordinates that were obtained by group theory analyses. The Raman mode A1g, which has the highest wave number (812 cm−1) is dominated by the breath vibration of the MgO6 octahedron. The infrared modes F1u(2), that can be described as the inverted vibrations of Mg atoms in the MgO6 octahedron along the xi, yi, and zi axes have the most contributions to the microwave permittivity and dielectric loss.

Chun-Hai Wang

Papers

First-principles calculations of mechanical and electronic properties of silicene under strain

Photoluminescence and Raman Spectra of Double-Perovskite Sr2Ca ( Mo / W ) O6 with A- and B-Site Substitutions of Eu3 +

XRD and Raman Studies on the Ordering/Disordering of Ba(Mg1/3Ta2/3)O3

Promising Oxonitridosilicate Phosphor Host Sr3Si2O4N2: Synthesis, Structure, and Luminescence Properties Activated by Eu2+ and Ce3+/Li+ for pc-LEDs

First‐Principle Calculation and Assignment for Vibrational Spectra of Ba(Mg1/2W1/2)O3 Microwave Dielectric Ceramic