Lili Wang

Energy transfer from Bi3+ to Eu3+ triggers exceptional long-wavelength excitation band in ZnWO4:Bi3+, Eu3+ phosphors

Color-tunable phosphors of ZnWO4:x mol% Bi3+, 3 mol% Eu3+ (x = 0.5, 1, 2, 3, 4, 5) were prepared through a precipitation method and their luminescence properties were investigated as a function of Bi3+ concentration. The most intense 616 nm emission indicates that Eu3+ occupies the Zn2+ sites without inversion symmetry. The spectral overlap between the emission band of Bi3+ and the excitation band of Eu3+ supports the energy transfer from Bi3+ to Eu3+, which has been demonstrated to be of a resonant type via a dipole–dipole mechanism. Energy transfer from Bi3+ to Eu3+ triggers a long-wavelength excitation band at 340 nm originating from the Bi3+ 1S0 → 3P1 transition, which makes the phosphors fit for long-wavelength radiation. The decay curves of Bi3+ and Eu3+ emission were measured to understand the energy transfer processes. Interestingly, the critical concentration of Bi3+ for Eu3+ 616 nm emission in ZnWO4:Bi3+, Eu3+ was greatly increased by 400 times compared with that for Bi3+ 560 nm emission in ZnWO4:Bi3+. Color-tunable emission in ZnWO4:Bi3+, Eu3+ phosphors can be obtained by the modulation of the excitation wavelength and the ratio of Bi3+ and Eu3+. Our work provides a novel approach to develop phosphors which can be excited effectively under long-wavelength radiation.

Luminescence properties of nano and bulk ZnWO4 and their charge transfer transitions

We here investigated the photoluminescence properties of nano and bulk ZnWO4 phosphors with the aim to understand the observed altered properties of nanoparticles as compared to their bulk counterparts. Their excitation bands were decomposed into three components by Gaussian fitting. The influence of crystalline defects that are present due to the low synthesis temperature of ZnWO4 nanoparticles on the excitation spectra was studied. The structures of ZnWO4 samples were refined with the General Structure Analysis System. Making use of the estimated refractive indices and the refined lattice parameters, four chemical bond parameters of W–O bonds were calculated and integrated to give the environmental factor he, which can to some extent explain the broadened excitation band in bulk ZnWO4. A new factor H of every W–O bond was calculated, and based on the results, three components of the excitation band at about 250, 280 and 300 nm were assigned to be from W–O1, W–O2(2) and W–O2(1) charge transfer transitions, respectively.

Applications oriented design of Bi3+ doped phosphors

Based on the relationships between Bi3+ energy levels and environmental factor he in phosphors established before, positions of A band of Bi3+ in ZnWO4 and CdWO4 are predicted to lie in near ultraviolet region. ZnWO4:Bi3+ and CdWO4:Bi3+ were prepared via a precipitation method. By examining their photoluminescence, it was confirmed that Bi3+ 1S0-3P1 transition are located around 340 and 350 nm, matching well with predicted results, 357 nm and 377 nm, respectively. The emission spectra indicate that they can be effectively excited by 340 and 350 nm ultraviolet light, exhibiting a satisfactory green performance (560 and 537 nm), according with ultraviolet chip.

Remote Control Effect of Li+, Na+, K+ Ions on the Super Energy Transfer Process in ZnMoO4:Eu3+, Bi3+ Phosphors

Luminescent properties are affected by lattice environment of luminescence centers. The lattice environment of emission centers can be effectively changed due to the diversity of lattice environment in multiple site structure. But how precisely control the doped ions enter into different sites is still very difficult. Here we proposed an example to demonstrate how to control the doped ions into the target site for the first time. Alkali metal ions doped ZnMoO4:Bi3+, Eu3+ phosphors were prepared by the conventional high temperature solid state reaction method. The influence of alkali metal ions as charge compensators and remote control devices were respectively observed. Li+ and K+ ions occupy the Zn(2) sites, which impede Eu and Bi enter the adjacent Zn(2) sites. However, Na+ ions lie in Zn(1) sites, which greatly promoted the Bi and Eu into the adjacent Zn(2) sites. The Bi3+ and Eu3+ ions which lie in the immediate vicinity Zn(2) sites set off intense exchange interaction due to their short relative distance. This mechanism provides a mode how to use remote control device to enhance the energy transfer efficiency which expected to be used to design efficient luminescent materials.

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.

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.

Structure and photoluminescence properties of novel Sr6Ca4(PO4)6F2:Re (Re = Eu2+, Mn2+) phosphors with energy transfer for white-emitting LEDs

A series of Sr6Ca4(PO4)6F2:Eu2+,Mn2+ phosphors were synthesized through traditional solid state reaction. The structural and spectroscopic properties of the series samples along with the energy transfer from Eu2+ to Mn2+ ions have been investigated in detail. Under excitation at 295 or 365 nm, Eu2+ singly doped phosphors exhibit a broad emission band centered at 450 nm. When Eu2+ and Mn2+ were co-doped in Sr6Ca4(PO4)6F2, a broad blue emission band peaking at 450 nm and an orange emission band at 570 nm appeared simultaneously upon ultraviolet (365 nm) excitation, which result from the 4f–5d transition of Eu2+ ions and the 4T1–6A1 transition of Mn2+ ions, respectively. By controlling the doping concentration of Mn2+ ions, a series of tunable luminescent colors including white light are obtained with a wide band emission and a correlated color temperature of 6662 K. The possible energy transfer mechanism was proposed to be dipole–quadrupole, with regard to the analysis of photoluminescence spectra and decay curves experimental results of the series samples. The critical distance between the Eu2+ and Mn2+ ions has been calculated by both the concentration quenching method (11.53 A) and the spectral overlap method (9.43 A). Overall, this research indicated that the prepared samples may be potentially applied in white-light-emitting diodes under UV-pumping.

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.