Facile Synthesis of (Sr,Ca)₂Si₅N₈:Eu²⁺-Based Red-Emitting Phosphor for Solid-State Lighting

A new facile synthetic route to produce a nitride-based red-emitting phosphor has been established. The (Sr,Ca)₂Si₅N₈:Eu²⁺-based multicomponent phosphor was successfully synthesized from the stable SrCO₃–CaCO₃–Eu₂O₃–Si₃N₄ system by simple one-step heating at 1600 °C for 4 h in an unpressurized N₂ atmosphere. The synthesized (Sr,Ca)₂Si₅N₈:Eu²⁺-based red broadband emitting phosphor exhibited the peak wavelength as long as 661 nm with a practically high external quantum efficiency of 60% under the excitation at 450 nm, while the coexisting secondary phase was inactive under the blue-light excitation, showing no detrimental effects on the photoluminescent properties. The enhanced red emission compared to the unmodified Sr₂Si₅N₈:Eu²⁺ phosphor enables further improvement of the color rendering properties of the white light-emitting diodes for solid-state lighting applications

Discovery of New Nitridosilicate Phosphors for Solid State Lighting by the Single-Particle-Diagnosis Approach

Discovery of novel luminescent materials is of fundamental importance in the advancement of solid state lighting and flat panel display technologies. In this work, we report a single-particle-diagnosis method for the discovery of new phosphors by just characterizing a luminescent crystalline particle as small as 10 μm in diameter. We explored single-particle fluorescence imaging and spectroscopy techniques to evaluate the photoluminescence of a phosphor particle distinguished from a complex powder mixture and applied a high-resolution single-crystal X-ray diffractometer to determine its crystal structure. The approach enabled us to discover two new phosphors in the Ba₃N₂–Si₃N₄–AlN ternary system: Ba₅Si₁₁Al₇N₂₅:Eu²⁺ and BaSi₄Al₃N₉:Eu²⁺. Ba₅Si₁₁Al₇N₂₅:Eu²⁺ crystallizes in the space group of <i>Pnnm</i> (no. 58) with <i>a</i> = 9.5923(2), <i>b</i> = 21.3991(5), <i>c</i> = 5.8889 (2) Å and <i>Z</i> = 2, while BaSi₄Al₃N₉:Eu²⁺ in the space group of <i>P</i>21/<i>C</i> (no.14) with <i>a</i> = 5.8465(4), <i>b</i> = 26.7255(18), <i>c</i> = 5.8386(4) Å, β = 118.897° and <i>Z</i> = 4. The single-particle photoluminescence of Ba₅Si₁₁Al₇N₂₅:Eu²⁺ shows yellow emission (λ_em = 568 nm, fwhm = 98 nm) and a quantum efficiency of 36% under the 405 nm excitation. BaSi₄Al₃N₉:Eu²⁺ shows blue emission (λ_em = 500 nm, fwhm = 67 nm) upon the 365 nm excitation

Narrow-Band Green-Emitting Phosphor Ba₂LiSi₇AlN₁₂:Eu²⁺ with High Thermal Stability Discovered by a Single Particle Diagnosis Approach

The narrow-band green-emitting phosphor Ba₂LiSi₇AlN₁₂:Eu²⁺ was discovered by analyzing a single particle in a powder mixture, which we call the single particle diagnosis approach. Single crystal X-ray diffraction analysis of the particle revealed that Ba₂LiSi₇AlN₁₂:Eu²⁺ crystallizes in the <i>Pnnm</i> space group (No. 58) with <i>a</i> = 14.0941 Å, <i>b</i> = 4.8924 Å, <i>c</i> = 8.0645 Å, and <i>Z</i> = 2. The crystal structure is composed of a corner-sharing (Si,Al)­N₄ corrugated layer and edge-sharing (Si,Al)­N₄ and LiN₄ tetrahedra. Ba­(Eu) occupies the one-dimensional channel in a zigzag manner. The luminescence properties were also measured using a single crystalline particle. Ba₂LiSi₇AlN₁₂:Eu²⁺ shows a green luminescence peak at approximately 515 nm with a narrow full-width at half-maximum of 61 nm. It shows high quantum efficiency of 79% with 405 nm excitation and a small decrease of luminescence intensity even at 300 °C

Achieving Multicolor Long-Lived Luminescence in Dye-Encapsulated Metal–Organic Frameworks and Its Application to Anticounterfeiting Stamps

Long-lived luminescent metal–organic frameworks (MOFs) have attracted much attention due to their structural tunability and potential applications in sensing, biological imaging, security systems, and logical gates. Currently, the long-lived luminescence emission of such inorganic–organic hybrids is dominantly confined to short-wavelength regions. The long-wavelength long-lived luminescence emission, however, has been rarely reported for MOFs. In this work, a series of structurally stable long-wavelength long-lived luminescent MOFs have been successfully synthesized by encapsulating different dyes into the green phosphorescent MOFs Cd­(m-BDC)­(BIM). The multicolor long-wavelength long-lived luminescence emissions (ranging from green to red) in dye-encapsulated MOFs are achieved by the MOF-to-dye phosphorescence energy transfer. Furthermore, the promising optical properties of these novel long-lived luminescent MOFs allow them to be used as ink pads for advanced anticounterfeiting stamps. Therefore, this work not only offers a facile way to develop new types of multicolor long-lived luminescent materials but also provides a reference for the development of advanced long-lived luminescent anticounterfeiting materials

Microwave-Assisted Synthesis of CdS/ZnS:Cu Quantum Dots for White Light-Emitting Diodes with High Color Rendition

High quality CdS/ZnS:Cu quantum dots (QDs) were first synthesized via a green microwave irradiation route. As-prepared core/shell doped QDs presented a strong absorption in the blue light region and highly efficient red to deep red emission with a maximum quantum yield of 40%. The composite formed by dispersing CdS/ZnS:Cu QDs into silicone resin showed an excellent photostability under blue illumination. Finally, high color rendition white light was generated from the CdS/ZnS:Cu QDs-assisted phosphor-converted white light-emitting diode (WLED) in which there was no reabsorption between quantum dots and phosphors. Under operation of 40 mA forward bias current, the fabricated WLED emitted bright natural white light with a high color rendering index of 90, a luminous efficiency of 46.5 lm/W, and the correlated color temperature of 6591 K. Simultaneously, the good color stability was accompanied by the CIE color coordinates of (0.3155, 0.3041) under different forward bias currents

Blue-Emitting Sr₃Si_8–<i>x</i>Al_<i>x</i>O_7+<i>x</i>N_8–<i>x</i>:Eu²⁺ Discovered by a Single-Particle-Diagnosis Approach: Crystal Structure, Luminescence, Scale-Up Synthesis, and Its Abnormal Thermal Quenching Behavior

The single-particle-diagnosis approach allows for the fast discovery of novel luminescent materials using powdered samples. This paper reports a new blue-emitting Sr₃Si_8–<i>x</i>Al_<i>x</i>O_7+<i>x</i>N_8–<i>x</i>:Eu²⁺ phosphor for solid state lighting and its scale-up synthesis. The structure-, composition-, and temperature-dependent luminescence were investigated and discussed by means of various analytic techniques including single-crystal XRD diffractometer, single-particle fluorescence spectroscopy, FTIR spectra, decay time, low-temperature luminescence, and computed energy level scheme. Sr₃Si_8–<i>x</i>Al_<i>x</i>O_7+<i>x</i>N_8–<i>x</i> crystallizes in the monoclinic system (space group <i>C</i>2/<i>c</i>, no. 15) with <i>a</i> = 18.1828 (13) Å, <i>b</i> = 4.9721 (4) Å, <i>c</i> = 15.9557 (12) Å, β = 115.994 (10)^ο, and <i>Z</i> = 2. The Sr atoms are coordinated to 8 and 6 O/N atoms and located in the voids along [010] formed by vertex-sharing (Si,Al)-(O,N)₄ tetrahedra. Phase-pure powder samples of Sr₃Si_8–<i>x</i>Al_<i>x</i>O_7+<i>x</i>N_8–<i>x</i>:Eu²⁺ were synthesized from the chemical composition of the single particle by controlling the <i>x</i> value. Luminescence of both a single particle and powders show a broad Eu²⁺ emission band centered at ∼465 nm and a fwhm of ∼70 nm, under the UV light irradiation. The title phosphor has a band gap of 5.39 eV determined from the UV–vis spectrum, absorption efficiency of 83%, internal quantum efficiency of 44.9%, and external quantum efficiency of 37.4% under the 355 nm excitation. An abnormal thermal quenching behavior is observed in Sr₃Si_8–<i>x</i>Al_<i>x</i>O_7+<i>x</i>N_8–<i>x</i>:Eu²⁺ that has a high activation energy for thermal quenching (0.294 eV) but a low thermal quenching temperature (∼370 K), which is ascribed to the partial overlap between the Eu²⁺ excited energy level and the conduction band of the host

Extra-Broad Band Orange-Emitting Ce³⁺-Doped Y₃Si₅N₉O Phosphor for Solid-State Lighting: Electronic, Crystal Structures and Luminescence Properties

Luminescent materials play an important role in making solid state white light-emitting diodes (w-LEDs) more affordable home lighting applications. To realize the next generation of solid-state w-LEDs with high color-rendering index (CRI), the discovery of broad band and long emission wavelength luminescent materials is an urgent mission. Regarding this, the oxonitridosilicate Y₃Si₅N₉O with a high nitrogen concentration should be a suitable host material to achieve those promising luminescent properties. In this work, a phase-pure Ce³⁺-doped Y₃Si₅N₉O was successfully synthesized through the carbothermal reduction and nitridation method. Y₃Si₅N₉O:Ce³⁺ shows an emission maximum at 620 nm and an extremely broad emission band with a full-width at half-maximum (fwhm) of 178 nm. The electronic and crystal structure calculations indicate an indirect band gap of 2.6 eV (experimental value: 4.0 eV), and identify two Ce³⁺ sites with different local environments that determine the luminescence properties. The orange-emitting phosphor has the absorption, internal and external quantum efficiencies of 89.5, 17.2, and 15.6% under 450 nm excitation, respectively. The valence state of Ce, cathodoluminescence, decay time, and thermal quenching of the phosphor were also investigated to understand the structure–property relationships

Blue-Emitting Sr₃Si_8–<i>x</i>Al_<i>x</i>O_7+<i>x</i>N_8–<i>x</i>:Eu²⁺ Discovered by a Single-Particle-Diagnosis Approach: Crystal Structure, Luminescence, Scale-Up Synthesis, and Its Abnormal Thermal Quenching Behavior

The single-particle-diagnosis approach allows for the fast discovery of novel luminescent materials using powdered samples. This paper reports a new blue-emitting Sr₃Si_8–<i>x</i>Al_<i>x</i>O_7+<i>x</i>N_8–<i>x</i>:Eu²⁺ phosphor for solid state lighting and its scale-up synthesis. The structure-, composition-, and temperature-dependent luminescence were investigated and discussed by means of various analytic techniques including single-crystal XRD diffractometer, single-particle fluorescence spectroscopy, FTIR spectra, decay time, low-temperature luminescence, and computed energy level scheme. Sr₃Si_8–<i>x</i>Al_<i>x</i>O_7+<i>x</i>N_8–<i>x</i> crystallizes in the monoclinic system (space group <i>C</i>2/<i>c</i>, no. 15) with <i>a</i> = 18.1828 (13) Å, <i>b</i> = 4.9721 (4) Å, <i>c</i> = 15.9557 (12) Å, β = 115.994 (10)^ο, and <i>Z</i> = 2. The Sr atoms are coordinated to 8 and 6 O/N atoms and located in the voids along [010] formed by vertex-sharing (Si,Al)-(O,N)₄ tetrahedra. Phase-pure powder samples of Sr₃Si_8–<i>x</i>Al_<i>x</i>O_7+<i>x</i>N_8–<i>x</i>:Eu²⁺ were synthesized from the chemical composition of the single particle by controlling the <i>x</i> value. Luminescence of both a single particle and powders show a broad Eu²⁺ emission band centered at ∼465 nm and a fwhm of ∼70 nm, under the UV light irradiation. The title phosphor has a band gap of 5.39 eV determined from the UV–vis spectrum, absorption efficiency of 83%, internal quantum efficiency of 44.9%, and external quantum efficiency of 37.4% under the 355 nm excitation. An abnormal thermal quenching behavior is observed in Sr₃Si_8–<i>x</i>Al_<i>x</i>O_7+<i>x</i>N_8–<i>x</i>:Eu²⁺ that has a high activation energy for thermal quenching (0.294 eV) but a low thermal quenching temperature (∼370 K), which is ascribed to the partial overlap between the Eu²⁺ excited energy level and the conduction band of the host

Color-Tunable and High-Efficiency Dye-Encapsulated Metal–Organic Framework Composites Used for Smart White-Light-Emitting Diodes

Luminescent metal–organic frameworks (MOFs) (typically dye-encapsulated MOFs) are considered as one kind of interesting downconversion materials for white-light-emitting diodes (LEDs), but their quantum efficiency (QE) is not sufficient and thus needs to be significantly enhanced for practical applications. In this study, we successfully synthesized a series of Rh@bio-MOF-1 (Rh = rhodamine) with an internal QE as high as ∼79% via a solvothermal reaction followed by cation exchanges. The high efficiency of the Rh@bio-MOF-1 composites was attributable to the high intrinsic luminescent efficiency of the selected Rh dyes, the confinement effect in the bio-MOF-1 host, and the uniform particle morphology. The emission maximum could be continuously tuned from 550 to 610 nm by controlling the species and concentration of encapsulated dye molecules, showing great color tunability of the dye-encapsulated MOFs. The emission lifetime of ∼7 ns was 1 or 2 magnitude orders shorter than that of Ce³⁺- or Eu²⁺-doped inorganic phosphors, allowing for visible light communication (VLC). White LEDs, fabricated by using the synthesized Rh@bio-MOF-1 composite and inorganic phosphors of green (Ba,Sr)₂SiO₄:Eu²⁺ and red CaAlSiN₃:Eu²⁺, exhibited a high color rendering index of 80–94, a luminous efficacy of 94–156 lm/W, and an excellent stability in color point against drive current. The Rh@bio-MOF-1 composites with tunable colors, short emission lifetime, and high QE are expected to be used for smart white LEDs with multifunctions of both lighting and VLC

Structure, Luminescence, and Application of a Robust Carbidonitride Blue Phosphor (Al_1–<i>x</i>Si_<i>x</i>C_<i>x</i>N_1–<i>x</i>:Eu²⁺) for Near UV-LED Driven Solid State Lighting

As an extension of nitride luminescent materials, carbidonitride phosphors are also attracting great attention due to their superior thermal stability. This paper reports a blue-emitting carbidonitride phosphor Al_1–<i>x</i>Si_<i>x</i>C_<i>x</i>N_1–<i>x</i>:Eu²⁺ suitable for near ultraviolet (UV) light emitting diodes (LEDs), which is formulated by introducing SiC into AlN:Eu²⁺. With the introduction of carbon (silicon), the lattice abnormally shrinks along both <i>a</i>- and <i>c</i>-axes at low <i>x</i> values (<i>x</i> ≤ 0.08), due to the formation of a dense interlayer for accommodating the luminescence center Eu²⁺. Both of the Raman spectra and solid state NMR spectroscopy show that both Si and C are dissolved in the AlN lattice. A single blue emission band (λ_em = 472–477 nm) is observed for compositions of <i>x</i> > 0.05 by cathodoluminescence measurements. Under the 365 nm excitation, the maximum luminescence is attained for the composition of <i>x</i> = 0.06 that has an external quantum efficiency of 61% and absorption efficiency of 74.4%, which is about 11–15% higher than the corresponding carbon-free nitride sample. The thermal quenching of Al_1–<i>x</i>Si_<i>x</i>C_<i>x</i>N_1–<i>x</i>:Eu²⁺ reduces with increasing C (SiC) content, and the sample of <i>x</i> = 0.06 shows a small loss of ∼4.0% in quantum efficiency even at 200 °C. Using this phosphor in a near UV-driven white LED, a superhigh color rendering index of Ra = 95.3 and R9 = 72 as well as a color temperature of 3533 K are achieved