Bing-Ming Cheng

Versatile phosphors BaY2Si3O10:RE (RE = Ce3+, Tb3+, Eu3+) for light-emitting diodes.

Rare-earth-activated BaY(2)Si(3)O(10) (BYSO) phosphors were synthesized via a solid-state reaction. BaY(2)Si(3)O(10):Ce(3+) yields an indigo-blue emission peak at 404 nm according to excitation at 334 nm attributed to the Ce(3+) 4f-->5d transition. BaY(2)Si(3)O(10):Tb(3+) typically generates green emission peaks resulting from the 5D4-->7FJ transition. BaY(2)Si(3)O(10):Eu(3+) exhibits red emission peaks upon excitation at 393 nm. The quantum efficiency of these phosphors was found to be 53%, 55%, and 63% of commodity. The results in this work demonstrate that these phosphors with new compositions are good candidate luminescent materials for use in plasma display panels and light-emitting diodes, excited from VUV to UV.

Facile Atmospheric Pressure Synthesis of High Thermal Stability and Narrow-Band Red-Emitting SrLiAl3N4:Eu2+ Phosphor for High Color Rendering Index White Light-Emitting Diodes

Red phosphors (e.g., SrLiAl3N4:Eu(2+)) with high thermal stability and narrow-band properties are urgently explored to meet the next-generation high-power white light-emitting diodes (LEDs). However, to date, synthesis of such phosphors remains an arduous task. Herein, we report, for the first time, a facile method to synthesize SrLiAl3N4:Eu(2+) through Sr3N2, Li3N, Al, and EuN under atmospheric pressure. The as-synthesized narrow-band red-emitting phosphor exhibits excellent thermal stability, including small chromaticity shift and low thermal quenching. Intriguingly, the title phosphor shows an anomalous increase in theoretical lumen equivalent with the increase of temperature as a result of blue shift and band broadening of the emission band, which is crucial for high-power white LEDs. Utilizing the title phosphor, commercial YAG:Ce(3+), and InGaN-based blue LED chip, a proof-of-concept warm white LEDs with a color rendering index (CRI) of 91.1 and R9 = 68 is achieved. Therefore, our results highlight that this method, which is based on atmospheric pressure synthesis, may open a new means to explore narrow-band-emitting nitride phosphor. In addition, the underlying requirements to design Eu(2+)-doped narrow-band-emitting phosphors were also summarized.

Structure and Electronic Configuration of an Iron(II) Complex in a LIESST State: A Pump and Probe Method

Two polymorphs of mononuclear six-coordinate iron(II) spin-crossover complex trans-[Fe(tzpy)(2)(NCS)(2)] (tzpy = 3-(2-pyridyl)[1,2,3]triazolo[1,5-a]pyridine) (1) were isolated and structurally characterized. According to the thermally dependent magnetic measurements, polymorph A undergoes a gradual spin transition from a paramagnetic high-spin state ((5)T(2), S = 2, HS-1) above 200 K to a diamagnetic low-spin state ((1)A(1), S = 0, LS-1) below 120 K, whereas polymorph B shows an abrupt spin transition with T(1/2) at 102 K. Molecular and crystal structures of polymorph A in the HS-1 and LS-1 states were studied at 300 and 40 K, respectively. Significant differences in Fe-N distances and coordination geometries of Fe were found between the two spin states, as expected. Light-induced excited spin state trapping (LIESST) was observed upon irradiating the crystal with 532 nm laser light at 40 K, whereupon a metastable high-spin state (HS-2) was formed; the molecular and crystal structure of this metastable state were investigated by a pump and probe method because of its relatively fast relaxation. The electronic configuration of the Fe center in the HS-1, LS-1, and LIESST (HS-2) states were further confirmed by Fe K- and L-edge absorption spectroscopy. In addition, the C[triple bond]N stretching frequency on the ligand can also be followed through the spin transition. The excitation and relaxation process concerning such metastable state were followed by the C[triple bond]N stretching frequency and magnetic susceptibility measurements in the temperature ranges 15-55 K and 5-80 K, respectively. The structure and electronic configuration of the LIESST state of polymorph A were firmly established by X-ray diffraction, X-ray absorption, infrared absorption, and magnetic measurements. A single-crystal-to-single-crystal transition through irradiation was demonstrated. The changes in structure and electronic configuration as a result of the spin transition are believed to occur concurrently.

UV/VUV switch-driven color-reversal effect for Tb-activated phosphors

The remarkable narrow-band emission of trivalent lanthanide-doped phosphors excited by the vacuum ultraviolet (VUV) radiation lines of Xe atoms/Xe2 molecules at 147/172 nm are extensively investigated in the development of plasma display panels and Hg-free fluorescent lamps, which are frequently used in our daily lives. Numerous solid materials, particularly Tb3+-doped oxides, such as silicates, phosphates and borates, are efficient green/blue sources with color-tunable properties. The excitation wavelength and rare earth concentration are usually varied to optimize efficiency and the luminescent properties. However, some underlying mechanisms for the shift in the emission colors remain unclear. The present study shows that a UV/VUV switch systematically controls the change in the phosphor (Ba3Si6O12N2:Tb) photoluminescence from green to blue, resulting in a green emission when the system is excited with UV radiation. However, a blue color is observed when the radiation wavelength shifts to the VUV region. Thus, a configurational coordinate model is proposed for the color-reversal effect. In this model, the dominant radiative decay results in a green emission under low-energy UV excitation from the 5D4 state of the f–f inner-shell transition in the Tb system. However, under high-energy VUV excitation, the state switches into the 5D3 state, which exhibits a blue emission. This mechanism is expected to be generally applicable to Tb-doped phosphors and useful in adjusting the optical properties against well-known cross-relaxation processes by varying the ratio of the green/blue contributions.

A long-lived photo-induced metastable state of linkage isomerization accompanied with a spin transition.

In addition to the generally observed LIESST phenomenon, polymorph D of trans-[Fe(II)(abpt)(2)(NCS)(2)] exhibits a long-lived photo-induced metastable state through linkage isomerization accompanied with a spin crossover transition, which is stable up to 108 K.

Vacuum ultraviolet and visible spectra of ZnO:Eu3+ prepared by combustion synthesis

Zinc oxide doped with 1 at.% Eu3+ has been prepared by combustion synthesis using several different reductants. Samples sintered at 800 °C were ~30 nm in size and Fourier transform IR spectra demonstrated that they were relatively free of contaminants. Ultraviolet and near-ultraviolet laser excited emission spectra showed that Eu3+ ions are disordered and not situated at discrete lattice sites in ZnO and consequently no evidence for energy transfer from the host to Eu3+ was found. Vacuum ultraviolet (VUV) excitation produced defect site emission in addition to near-band-edge emission but the intensity of the Eu3+ visible emission was very weak. Bands between 6.2 and 9.1 eV in the VUV excitation spectra have been assigned to electric dipole allowed transitions, 3d–4p.

PHOTOLYSIS OF ETHYNE IN SOLID NEON AND SYNTHESIS OF LONG-CHAIN CARBON CLUSTERS WITH VACUUM-ULTRAVIOLET LIGHT

The absorption spectrum of ethyne, C2H2, in solid Ar was measured in the wavelength region 107-220 nm with light from a synchrotron. Based on that absorption, irradiation of samples of ethyne dispersed in neon with vacuum-ultraviolet (VUV) radiation yielded various products that were identified through their infrared absorption spectra including C n (n = 3-12), C2H, C2H3, C4H, C4H2, C8H–, and C8H2. The efficiency of photolysis of ethyne and the nature of photoproducts depend on the selected wavelength of VUV light. Information about the photodissociation of C2H2 with various photon energies and the formation and identification of large carbon clusters and hydrides at low temperature might be useful in photochemical models to simulate the composition of the atmosphere of Titan and as a source of aerosols.

FORMATION AND IDENTIFICATION OF INTERSTELLAR MOLECULE LINEAR C5H FROM PHOTOLYSIS OF METHANE DISPERSED IN SOLID NEON

Photolysis of methane dispersed (1/1000) in solid Ne at 3 K with vacuum-ultraviolet light from a synchrotron produced infrared absorption lines of several products, including new lines at 3319.3 and 1955.5 cm–1. Based on experiments with isotopic labeling and results of quantum-chemical calculations, these lines are assigned to the C-H stretching and C=C stretching modes, respectively, of interstellar molecule linear C5H radicals.

Production of N3 upon Photolysis of Solid Nitrogen at 3 K with Synchrotron Radiation

The photodissociation of gaseous molecular nitrogen has been investigated intensively, but the corresponding knowledge in a solid phase is lacking. Irradiation of pure solid nitrogen at 3 K with vacuum-ultraviolet light from a synchrotron produced infrared absorption lines of product l-N3 at 1657.8 and 1652.6 cm(-1). The threshold wavelength to generate l-N3 was determined to be (143.7±1.8) nm, corresponding to an energy of (8.63±0.11) eV. Quantum-chemical calculations support the formation of l-N3 from the reaction N2 +N2, possibly through an activated complex l-N4 upon photoexcitation with energy above 8.63 eV. The results provide a possible application to an understanding of the nitrogen cycle in astronomical environments.

Excitation and Emission Spectra of Cs2NaLnCl6 Crystals Using Synchrotron Radiation

ABSTRACT The visible emission and vacuum ultraviolet excitation spectra of the series Cs2NaLnCl6 (Ln = Y, Nd, Sm, Eu, Tb, Er, Yb) and Cs2NaYCl6:Ln3+ (Ln = Sm, Er) have been recorded using synchrotron radiation at room temperature, and in some cases at 10 K. The excitation spectra comprise features associated with charge transfer, excitation from the valence to conduction band, and impurity bands. No d–f emissions were observed for these Ln3+ ions, so that the emission bands comprise intraconfigurational 4f N –4f N transitions and impurity bands, whose natures are discussed. Theoretical simulations of the f–d absorption spectra have been included. The comparison with data from the synchrotron at Desy enables a comprehensive account of the ground (or vibrationally excited ground for Ln2+) states of the Ln3+ 4f N , Ln3+ 4f N−15d, and Ln2+ 4f N+1 configurations relative to the valence and conduction bands of Cs2NaLnCl6, for which the band gaps are between 6.6 and 8.1 eV.