Weiguang Ran

A super energy transfer process based S-shaped cluster in ZnMoO4 phosphors: theoretical and experimental investigation

Efficient energy transfer from a sensitizer to an activator in phosphors is very important for white LEDs. Bi3+ and Eu3+ co-doped red phosphors are potential alternatives for white LEDs. However, energy transfer from Bi3+ to Eu3+ ions is still not efficient enough in most cases. Here, we have found that every six Zn sites form an S-shaped cluster in the ZnMoO4 crystal. Two Zn(2) sites will be occupied preferentially in ZnMoO4 according to the comparison between the calculated and experimental A band positions of Bi3+ in the ZnMoO4 host. Considering the S-shaped clusters and site occupation preference, a super energy transfer process from Bi3+ to Eu3+ ions is proposed. The distance between the Bi3+ and Eu3+ ions can be controlled by their total doping concentrations. When their total molar concentration is beyond 1/6, Bi3+ and Eu3+ begin to sit in two adjacent Zn(2) sites. Thus, a new super energy transfer from Bi3+ to Eu3+ emerges due to the adjacent Bi3+ and Eu3+ ions. When excited at 331 or 350 nm, which is assigned to the 1S0 → 3P1 transition of Bi3+, the phosphor emits intense red light. The relative intensity is about 6 times higher than that of an ordinary transfer process. It is a good example of how to utilize site occupation preference and provides a new way to design efficient phosphors.

Sr₂ZnWO₆:Eu³⁺, Bi³⁺, Li⁺: a potential white-emitting phosphor for near-ultraviolet white light-emitting diodes.

A series of Sr2ZnWO6 phosphors co-doped with Eu(3+), Bi(3+) and Li(+) were prepared using the Pechini method. The samples were tested using X-ray diffraction and luminescence spectroscopy. The results show that the samples can be effectively excited by near-ultraviolet (UV) and UV light. The introduction of Bi(3+) and Li(+) significantly enhances the fluorescence emission of Sr2ZnWO6 :Eu(3+) and changes the light emitted by the phosphors from bluish-green to white. When excited at 371 nm, Sr(2-x-z)Zn(1-y)WO6:xEu(3+), yBi(3+), zLi(+) (x = 0.05, y = 0.05, z = 0.05, 0.1 and 0.15) samples emit high-performance white light. Intense red-orange emission is also observed when excited by UV light. The obtained phosphor is a potential white-emitting phosphor that could meet the needs of excitation sources with near-UV chips. In addition, this phosphor might have promising application as a red-orange emitting phosphor for white light-emitting diodes based on UV light-emitting diodes.

Tunable Luminescence in Sr2MgSi2O7:Tb3+, Eu3+Phosphors Based on Energy Transfer

A series of Tb3+, Eu3+-doped Sr2MgSi2O7 (SMSO) phosphors were synthesized by high temperature solid-state reaction. X-ray diffraction (XRD) patterns, Rietveld refinement, photoluminescence spectra (PL), and luminescence decay curves were utilized to characterize each sample’s properties. Intense green emission due to Tb3+ 5D4→7F5 transition was observed in the Tb3+ single-doped SMSO sample, and the corresponding concentration quenching mechanism was demonstrated to be a diople-diople interaction. A wide overlap between Tb3+ emission and Eu3+ excitationspectraresults in energy transfer from Tb3+ to Eu3+. This has been demonstrated by the emission spectra and decay curves of Tb3+ in SMSO:Tb3+, Eu3+ phosphors. Energy transfer mechanism was determined to be a quadrupole-quadrupole interaction. And critical distance of energy transfer from Tb3+ to Eu3+ ions is calculated to be 6.7 Å on the basis of concentration quenching method. Moreover, white light emission was generated via adjusting concentration ratio of Tb3+ and Eu3+ in SMSO:Tb3+, Eu3+ phosphors. All the results indicate that SMSO:Tb3+, Eu3+ is a promising single-component white light emitting phosphor.

Broadly tunable emission from Ca2Al2SiO7:Bi phosphors based on crystal field modulation around Bi ions

New non-rare-earth activated phosphors were discovered and have attracted much attention in the field of energy-efficient LED lighting. For instance Bi3+, when mixed into different matrix materials, can realize multiple luminescence intensities ranging from 400 to 700 nm. The tunable emission results from the susceptibility of bismuth naked 6s electrons to the crystal field surrounding Bi3+. In this work, we report the broadly adjustable emission of Bi3+ doped Ca2Al2SiO7 phosphors by modifying the local environment around Bi3+. When M+ (M = Li, Na and K) is incorporated into the Ca2Al2SiO7 lattice, the luminescence intensity is improved greatly. In addition, the position of the emission peak of Bi3+ shifts from 382 to 416 nm upon varying the relative ratio of B3+/Al3+/Ga3+ ions. The spectral red-shift behaviour is attributed to the enhancement of crystal field strength surrounding Bi3+. These results are promising for investigation of the luminescence properties of Bi3+.

Mn2+ doped CdAl2O4 phosphors with new structure and special fluorescence properties: experimental and theoretical analysis

Mn2+-activated CdAl2O4 phosphors with the new structure of space group R (no. 148) have been prepared by a high-temperature solid-state reaction and their luminescence properties have been investigated in detail. The reduction of Mn4+ to Mn2+ in air atmosphere has been observed in CdAl2O4 powders for the first time. The structural properties including the phase purity and structural parameters were analyzed through Rietveld analysis. The typical transitions of Mn2+ ions in emission and excitation spectra were observed both in MnCO3 and MnO2 prepared CdAl2O4:0.01Mn2+ phosphors, which means that the luminescent centers of Mn2+ ions were from the Mn4+ ions which were reduced. Meanwhile, the energy band structures of CdAl2O4 and CdAl2O4:Mn2+ were measured with an ultraviolet-visible diffuse reflection spectroscopy (UV-vis DRS), the electronic structures were calculated using the plane-wave density functional theory (DFT). The Mn2+ activated CdAl2O4:Mn2+ phosphor prepared in air atmosphere is a potential blue-green emitting phosphor.

Nanotunnel Structures within Metal Oxides Induce Active Sites for the Strong Self-Reduction of MnIV to MnII

Zn2SiO4, Zn2GeO4, and CdAl2O4 possess high electron density in their six-membered-ring nanotunnels, manganese from MnO2 was successfully doped into them, and green or blue phosphors were produced in air. It is nanotunnel A with high electron density that induces active sites for the reduction of MnIV. MnIV is captured and reduced to MnII on active sites by seizing two electrons from native defect VO× (VO× + Mn4+ → VO·· + Mn2+). CdB4O7:0.02Mn2+ was also synthesized from MnO2 or MnCO3 to confirm the role of nanotunnels. Such inorganic crystals with unique nanotunnel structure may bring more amazing performances in the field of materials in the future.