Zhen Song

The synthesis of narrow-band red-emitting SrLiAl3N4:Eu2+ phosphor and improvement of its luminescence properties

In this paper, narrow-band red-emitting SrLiAl3N4:Eu2+ phosphor has been successfully prepared using a solid-state reaction method. The effects of sintering temperatures, times and fluxes on phase formation and luminescence properties are investigated, respectively. The addition of BaF2 flux not only enhances room-temperature emission intensity, but also improves the luminescence thermal stability, which is ascribed to the increase of crystallinity. Under blue light excitation, the as-prepared SrLiAl3N4:Eu2+ phosphor has a narrow emission band with a peak wavelength at ∼648 nm and a full-width at half-maximum of ∼1177 cm−1 (∼50 nm). The critical quenching concentration of Eu2+ is about 1 mol%. White light-emitting-diode (w-LED) devices have been fabricated which are obtained by combining a 455 nm chip with the commercial yellow phosphor and the present red phosphor. The results exhibit a potential application for phosphor-converted LEDs (pc-LEDs).

Luminescence Tuning, Thermal Quenching, and Electronic Structure of Narrow-Band Red-Emitting Nitride Phosphors

Exploring high-performance narrow-band red-emitting phosphor is an important challenge for improving white light LEDs. Here, on the basis of three interesting nitride phosphors with similar vierer rings framework structure, two phosphor series, Eu2+-doped Sr(LiAl)1-xMg2xAl2N4 and Sr(LiAl3)1-y(Mg3Si)yN4 (x, y = 0-1), are successfully synthesized by a solid state reaction. They show narrow-band red emission with tunable emission peaks from 614 to 658 nm and 607 to 663 nm. The varying luminescence behaviors with composition and structure are discussed based on centroid shift, crystal field splitting and Stokes shift. On the basis of experimental data, we construct the host referred binding energy (HRBE) and vacuum referred binding energy (VRBE) schemes of divalent/trivalent lanthanide-doped end-member compounds, and further give thermal quenching mechanism of these series phosphors.

Synthesis and luminescence properties of europium activated Ca3Al2O6-Sr3Al2O6 system

Abstract Divalent europium activated tristrontium dialuminum hexaoxide phosphor, (Sr 1- x Eu x ) 3 Al 2 O 6 , was obtained by solid state reaction. Crystal structure and luminescence properties of synthesized (Sr 1- x Eu x ) 3 Al 2 O 6 were investigated. The major excitation band of (Sr 1- x Eu x ) 3 Al 2 O 6 located in blue light region, the photoluminescence spectrum showed red light emission peaked at 618 nm which could be attributed to the d-f transition of the Eu 2+ . The influence of Ca 2+ substitution for Sr 2+ on structural and luminescence properties of Eu 2+ doped Sr 3 Al 2 O 6 was also studied. The photoluminescence peak position of (Sr 1- y Ca y ) 2.94 Eu 0.06 Al 2 O 6 varied from 618 to 655 nm with increasing y value. The reason for redshift in the emission band of (Sr 1- y Ca y ) 2.94 Eu 0.06 Al 2 O 6 phosphor was also discussed.

Control of Luminescence in Eu2+-Doped Orthosilicate-Orthophosphate Phosphors by Chainlike Polyhedra and Electronic Structures

A series of Eu2+-doped orthosilicate-orthophosphate solid-solution phosphors, KxBa1.97-x(Si1-xPx)O4:0.03Eu2+, have been synthesized via the conventional solid-state reaction. Using varying compositions, the lowest-energy excitation can be tuned from 470 to 405 nm, with an emission from 515 to 423 nm. We determined how chainlike cation polyhedra controlled excitation- and emission-band features by introducing in-chain characteristic length d22 and outside-chain characteristic length d12 and that there was a nearly linear relationship between the lowest-energy-excitation position and the ratio of d22 to d12. This influence of chainlike polyhedra on luminescence can be understood through the inductive effect. Luminescent thermal properties are improved remarkably by the cosubstitution of K+ and P5+ ions for Ba2+ and Si4+ ions with a T1/2 over 200 °C. We have established the host-referred-binding-energy (HRBE) and vacuum-referred-binding-energy (VRBE) schemes for the electronic structure of the series of lanthanide-doped phosphors according to the Dorenbos model and given a thermal-quenching mechanism for this series of phosphors.

Peak wavelength selection guides of chip and phosphors for phosphor-converted white light-emitting diodes

The dependences of light efficiency of radiation (LER) and color-rendering index (CRI) of trichromatic white light-emitting diode (wLED), composed of blue LED die, green/yellow, and red phosphors, on the peak wavelength of each primary were investigated by theoretical calculations, at correlative color temperature (CCT) from 2,700 to 6,500xa0K. The peak wavelength of InGaN based blue LED chip ranges from 450 to 471xa0nm, while those of Ca3Sc2Si3O12:Ce3+, b-SiAlON:Eu2+, and Y3Al5O12:Ce3+ based green/yellow phosphors range from 511 to 572xa0nm, and those of Sr2Si5N8:Eu2+ and CaAlSiN3:Eu2+ red phosphors range from 620 to 650xa0nm, which cover almost all the practically used, commercially available wave bands until now. Then, based on the results, selection guides of peak wavelengths for blue LED chip and phosphors to obtain tradeoff LER >280xa0lm·W−1 as well as CRI >80 in all CCTs are proposed. The favorable wave bands of each primary are suggested.

The Inductive Effect in Nitridosilicates and Oxysilicates and Its Effects on 5d Energy Levels of Ce3

The inductive effect exists widely in inorganic compounds and accounts well for many physicochemical properties. However, until now this effect has not been characterized quantitatively. In this work, we collected and analyzed the structural data of more than 100 nitridosilicates and oxysilicates, whose structures typically consist of [SiN4] or [SiO4] tetrahedra. We introduce a new parameter, the inductive effect factor μΔχ, related to the difference of electronegativity between constituent metal elements and silicon. Then, a linear relationship is established between average length of Si-N/Si-O bonds and the inductive factor with the help of statistical method, that is, l̅ = 1.7313 + 0.0166 μΔχ (Å) with adjusted (adj) R2 = 0.800 for Si-N and l̅ = 1.6221 + 0.0035 μΔχ(Å) with adj R2 = 0.240 for Si-O. Furthermore, our research shows that the distinct positive correlation does exist between the inductive factor and the centroid shift of 5d levels of Ce3+. This work will help us understanding the inductive effect deeply and quantitatively.

Consequence of Optimal Bonding on Disordered Structure and Improved Luminescence Properties in T-Phase (Ba,Ca)2SiO4:Eu2+ Phosphor

T-phase (Ba,Ca)2SiO4:Eu2+, showing excellent luminescent thermal stability, has a positionally disordered structure with the splitting of five atom sites, but until now the reason has remained unclear. Herein, we investigate the coordination environments of each cation site in detail to understand the origins of the atom site splitting. We find that the three cation sites in the split-atom-site model are optimally bonded with ligand O atoms compared to the unsplit-atom-site model. This atom site splitting results in larger room and smaller room for each splitting cation site, which just accommodates larger Ba2+ ions and smaller Ca2+ ions, respectively, leading to more rigid structure. Based on the X-ray diffraction data refinement, the boundary of the T-phase for (Ba1- xCa x)2SiO4 is redetermined. The Eu2+-doped T-phase (Ba,Ca)2SiO4 phosphors show excellent luminescent thermal stability, which can be attributed to optimal bonding and more rigid structure with atom site splitting. These results indicate that T-phase (Ba,Ca)2SiO4:Eu2+ phosphors have promise for practical applications.

Correlation between the energy level structure of cerium-doped yttrium aluminum garnet and luminescent behavior at varying temperatures

Luminescent spectra of cerium-doped yttrium aluminum garnet are measured at varying temperatures. It is found that the two excitation peaks demonstrate a reverse trend as the temperature rises, and the breadth of the high-energy emission peak experiences an abrupt widening. These effects could be directly linked to the energy level scheme of Ce3+ under the crystal field of local symmetry. Moreover, an alternative fitting function is provided which could effectively resolve the emission curve.

Charge Transfer, Local Structure, and the Inductive Effect in Rare-Earth-Doped Inorganic Solids

The charge transfer (CT) process is widely present in inorganic compounds. However, the explanation of this process accounting the inductive effect was not reported. In this work, through the analysis of local structure about the second nearest cations (SNCs) in some compounds doped with trivalent lanthanide, we verify successfully the important role of the inductive effect in the CT process. By introducing electronegativity factor ∑ iχ i( Ai)/ N - x( M), and ionic radius factor ∑ i ri( Ai)/ N, the semiquantitative model is proposed. Strong positive correlation between the electronegativity factor and CT energy and strong negative correlation between the ionic radius factor and CT energy are given. At last, the interrelationship among these two inductive factors, the CT process, the change of local coordination structure, and the chemical composition is revealed. This work will facilitate our understanding of the CT process and the delicate role of the local structure and the inductive effect.