Xiangting Dong

Single-component and warm-white-emitting phosphor NaGd(WO4)2:Tm3+, Dy3+, Eu3+: synthesis, luminescence, energy transfer, and tunable color.

Tm(3+), Dy(3+), and Eu(3+) codoped NaGd(WO4)2 phosphors were prepared by a facile hydrothermal process; they were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), energy-dispersive X-ray spectrometer (EDS), photoluminescence spectra, and fluorescence lifetime. The results show that the novel octahedral microcrystals with a mean side length of 2 μm are obtained. Under the excitation of ultraviolet, individual RE(3+) ion (Tm(3+), Dy(3+), and Eu(3+)) activated NaGd(WO4)2 phosphors exhibit excellent emission properties in their respective regions. Moreover, when codoping Dy(3+) and Eu(3+)/Tm(3+) in the single component, the energy migration from Dy(3+) to Eu(3+) has been demonstrated to be a resonant type via a dipole-quadrupole mechanism as well as that from Tm(3+) to Dy(3+) ions, of which the critical distance (R(Dy-Eu)) is calculated to be 11.08 Å. More significantly, in the Tm(3+), Dy(3+), and Eu(3+) tridoped NaGd(WO4)2 phosphors, the energy migration of Tm(3+)-Dy(3+)-Eu(3+), utilized for sensitizing Eu(3+) ions besides compensating the red component at low Eu(3+) doping concentration, has been discussed first. In addition, under 365 nm near-ultraviolet radiation (nUV), the color-tunable emissions in octahedral NaGd(WO4)2 microcrystals are realized by giving abundant blue, green, white, yellow, and red emissions, especially warm white emission, and could be favorable candidates in full-color phosphors for nUV-LEDs.

Direct fabrication of cerium oxide hollow nanofibers by electrospinning

Abstract Electrospinning technique was used to fabricate PVP/Ce(NO3)3 composite microfibers. Different morphological CeO2 nanofibers were obtained by calcination of the PVP/Ce(NO3)3 composite microfibers and were characterized by scanning electron microscopy (SEM), Transmission electron microscopy (TEM), X-ray diffraction (XRD), thermal gravimetric and differential thermal analysis (TG-DTA), and (FTIR). SEM micrographs indicated that the surface of the composite fibers was smooth and became coarse with the increase of calcination temperatures. The diameters of CeO2 hollow nanofibers (300 nm at 600 °C and 600 nm at 800 °C) were smaller than those of PVP/Ce(NO3)3 composite fibers (1–2 μm). CeO2 hollow nanofibers were obtained at 600 °C and CeO2 hollow and porous nanofibers formed by nanoparticles were obtained at 800 °C. The length of the CeO2 hollow nanofibers was greater than 50 μm. XRD analysis revealed that the composite microfibers were amorphous in structure and CeO2 nanofibers were cubic in structure with space group OH5 -FM3m when calcination temperatures were 600–800 °C. TG-DTA and FTIR revealed that the formation of CeO2 nanofibers was largely influenced by the calcination temperatures. Possible formation mechanism of CeO2 hollow nanofibers was proposed.

Electrospinning preparation and properties of magnetic-photoluminescent bifunctional coaxial nanofibers

Europium complex Eu(BA)3phen (BA = benzoic acid) and ferroferric oxide nanoparticles were incorporated into poly vinylpyrrolidone (PVP) and electrospun into coaxial nanofibers with Fe3O4/PVP as core and Eu(BA)3phen/PVP as the shell. The morphology and properties of the final products were investigated in detail by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM) and a fluorescence spectrometer. The core diameter was ca. 110 nm containing the magnetic nanoparticles and the shell thickness was ca. 90 nm. Fluorescence emission peaks of Eu3+ in the Fe3O4/PVP@Eu(BA)3phen/PVP coaxial nanofibers were observed and assigned to the transitions of 5D0 → 7F0 (581 nm), 5D0 → 7F1 (592 nm), 5D0 → 7F2 (617 nm), and the 5D0 → 7F2 hypersensitive transition at 617 nm was the dominant emission peak. Compared with Eu(BA)3phen/PVP nanofibers, the luminescent intensity of the Fe3O4/PVP@Eu(BA)3phen/PVP coaxial nanofibers was almost not weakened in spite of the existence of the non-luminescent Fe3O4/PVP core. The new type bifunctional magnetic-photoluminescent Fe3O4/PVP@Eu(BA)3phen/PVP coaxial nanofibers have potential applications in many fields due to their excellent magnetism and fluorescence.

Multicolor tunable luminescence and paramagnetic properties of NaGdF4:Tb3+/Sm3+ multifunctional nanomaterials

Tb(3+) and/or Sm(3+) doped NaGdF4 luminescent nanomaterials have been successfully synthesized by an SDS-assisted one-step hydrothermal method. The samples were characterized by X-ray diffraction (XRD), field-emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM), X-ray energy dispersive spectrometer (EDS), photoluminescence (PL) spectra and a vibrating sample magnetometer (VSM). The results show that the synthesized samples are all pure β-NaGdF4. The as-prepared Tb(3+) or Sm(3+) doped samples show strong green and yellow emission, originating from the allowed (5)D3→(7)F(J) (J = 5, 4, 3, 2) and (5)D4→(7)F(J) (J = 6, 5, 4, 3) transitions of the Tb(3+) ions and the (4)G(5/2)→(6)H(5/2), (6)H(7/2), (6)H(9/2) transition of the Sm(3+) ions. Based on the excitation wavelengths, multiple (yellowish green, yellow, white) emissions are obtained by Sm(3+) ion co-activated NaGdF4:Tb(3+) phosphors. Moreover, an energy transfer from Tb(3+) to Sm(3+) is observed, which is justified through the luminescence spectra and the fluorescence decay curves. Furthermore, the resonance-type energy transfer from Tb(3+) to Sm(3+) is demonstrated to occur via the dipole-dipole mechanism. In addition, the obtained samples also exhibit paramagnetic properties at room temperature. It is obvious that these multifunctional Tb(3+), Sm(3+) co-doped β-NaGdF4 nanomaterials, with tunable multicolors and intrinsic paramagnetic properties, may have potential application in the fields of full-color displays, biological labels, bioseparation and magnetic resonance imaging.

Tunable luminescence and energy transfer properties of NaGdF4:Dy3+, Eu3+ nanophosphors

A series of tunable luminescence NaGdF4:Dy3+, Eu3+ nanophosphors have been synthesized for the first time by a one-step hydrothermal method. X-ray diffraction (XRD), field emission-scanning electron microscope (FE-SEM), X-ray energy dispersive spectrometer (EDS) and photoluminescence (PL) spectroscopy were employed to characterize the samples. Under the excitation of UV light, NaGdF4:Dy3+ and NaGdF4:Eu3+ exhibit the characteristic emissions of Dy3+ (4F9/2 → 6H15/2, blue and 4F9/2 → 6H13/2, yellow) and Eu3+ (5D0 → 7F2, red), respectively. It is found that in a Dy3+, Eu3+-codoped NaGdF4 system, the energy is transferred from Dy3+ to Eu3+ by the photoluminescence spectra and faster decay times of the blue emissions from energy donors (Dy3+). Moreover, the energy transfer is efficient and certified to be a resonant type via a quadrupole–quadrupole interaction by comparing the experimental data and theoretical calculation. The corresponding luminescence and energy transfer mechanisms have been proposed in detail. Most interestingly, the white light with varied hues has been obtained in Dy3+ and Eu3+ co-activated NaGdF4 phosphors by utilizing the principle of energy transfer and properly designed activator contents. The results show that the obtained nanophosphors have a promising application in display and lighting fields.

Luminescence, energy-transfer and tunable color properties of single-component Tb3+ and/or Sm3+ doped NaGd(WO4)2 phosphors with UV excitation for use as WLEDs

Tb3+ and/or Sm3+ codoped NaGd(WO4)2 phosphors were prepared by a facile hydrothermal process, taking on flower-like microcrystal structures with a mean size of 3 μm. The NaGd(WO4)2:Sm3+ can efficiently absorb ultraviolet light and emit bright blue light from the WO42− group in the host, and has yellow, orange and red emissions from the f–f transitions of Sm3+, due to the host sensitization effect. Under ultraviolet excitation (405 nm), individual Sm3+ ion-activated NaGd(WO4)2 phosphors generate white and red light and the emission intensities reach a maximum at 0.015 equivalents of Sm3+. This is due to the concentration quenching effect, occurring via a resonant-type dipole–dipole interaction, for which the critical distance (RSm–Sm) is calculated to be 21.24 A. More significantly, in the Tb3+ and Sm3+ codoped NaGd(WO4)2 phosphors, the Tb3+–Sm3+ energy migration has been discussed to occur via a dipole–dipole mechanism. In addition, under different ultraviolet radiation, the color-tunable emissions in NaGd(WO4)2:Tb3+, Sm3+ microcrystals are realized, and this could make them good candidates to be used as full-color phosphors for nUV-LEDs.

Narrow-band red emitting phosphor BaTiF6:Mn4+: preparation, characterization and application for warm white LED devices

As a new class of non-rare-earth red phosphors for high-efficiency warm white light-emitting diodes (white LEDs), Mn4+ ion activated fluoride compounds have been extensively investigated recently and hold the potential to supersede commercial rare earth doped nitride phosphors. Herein, a series of Mn4+ ions doped BaTiF6 phosphors have been prepared via the hydrothermal route using citric acid as a surfactant. After a systematic investigation, we illustrate the effects of reaction time, nominal concentration of HF solution, and reaction temperature on the luminescence performance of the phosphor. The BaTiF6:Mn4+ phosphor generates narrow red emission, which is highly perceived by the human eyes and leads to excellent chromatic saturation of red emission spectra. Simultaneously, concentration and thermal quenching are investigated systematically, and the quenching mechanisms are elucidated in detail. Employing BaTiF6:Mn4+ as a red phosphor, we fabricate a high-performance white LED with low correlated color temperature of 3974 K, high color rendering index of 90.6 and luminous efficacy of 132.54 lm W-1. Based on the improvement in correlated color temperature and color rendering index, the BaTiF6:Mn4+ red phosphor supplements the deficiency of LEDs fabricated by combining blue chips and only YAG:Ce3+, which suggests that it is a promising commercial red phosphor in warm white LEDs.

NaGdF4:Dy3+ nanofibers and nanobelts: facile construction technique, structure and bifunctionality of luminescence and enhanced paramagnetic performances

Luminescent-magnetic bifunctional NaGdF4:Dy3+ nanofibers and nanobelts have been successfully fabricated by a combination of electrospinning followed by subsequent calcination with fluorination technology for the first time. The structure, morphologies, and luminescence and magnetic properties of the synthesized materials have been investigated by a variety of techniques. X-ray diffraction (XRD) analysis shows that as-prepared NaGdF4:Dy3+ nanostructures are pure hexagonal structures. Scanning electron microscopy (SEM) observations indicate that directly electrospinning-made PVP/[NaNO3 + Gd(NO3)3 + Dy(NO3)3] composite nanofibers and nanobelts have smooth surfaces, good dispersion and uniform size, and surfaces of NaGdF4:Dy3+ nanofibers and nanobelts become rough after calcination and fluorination processes. The mean diameters of PVP/[NaNO3 + Gd(NO3)3 + Dy(NO3)3] composite nanofibers and NaGdF4:0.5%Dy3+ nanofibers are, respectively, 402.20 ± 2.39 nm and 246.06 ± 5.84 nm at the confidence level of 95%. The mean widths and thicknesses of PVP/[NaNO3 + Gd(NO3)3 + Dy(NO3)3] composite nanobelts and NaGdF4:0.5%Dy3+ nanobelts are 4.16 ± 0.17 μm and 279 nm, and 0.83 ± 0.01 μm and 130 nm, respectively. Under the excitation of 274 nm ultraviolet light, NaGdF4:Dy3+ nanofibers and nanobelts show the predominant blue and yellow emission peaks at 478 and 570 nm corresponding to the 4F9/2 → 6HJ/2 (J = 15, 13) energy level transitions of Dy3+ ions, respectively. NaGdF4:0.5%Dy3+ nanofibers have higher photoluminescence intensity than their nanobelt counterpart. In addition, all the NaGdF4:Dy3+ nanofibers and nanobelts display superparamagnetic properties. The NaGdF4:0.5%Dy3+ nanobelts show the highest magnetization, and NaGdF4:0.5%Dy3+ nanofibers have slightly higher magnetization values than NaGdF4 nanofibers. NaGdF4:Dy3+ nanofibers and nanobelts simultaneously possess excellent luminescence and enhanced superparamagnetic properties, which make them ideally suitable for application in many fields such as solid-state lasers, lighting and displays, and magnetic resonance imaging. The design conception and construction strategy developed in this work may provide some new guidance for the synthesis of other rare earth fluoride nanostructures with various morphologies.

Up/down conversion, tunable photoluminescence and energy transfer properties of NaLa(WO4)2:Er3+,Eu3+ phosphors

Er3+ or/and Eu3+ codoped NaLa(WO4)2 down conversion (DC) and up conversion (UC) phosphors were prepared by a facile hydrothermal process. For NaLa(WO4)2:Er3+ phosphors, the WO42− group can efficiently absorb ultraviolet (UV) light, and emit bright blue and green emissions by the f–f transitions of Er3+ through the host sensitization effect. The critical distance of the Er3+ ions in NaLa(WO4)2 is calculated and the energy quenching mechanism is proven to be a resonant type dipole–dipole interaction. More significantly, in the Er3+ and Eu3+ codoped NaLa(WO4)2 phosphors, the bright green emissions of Er3+ ions and the red characteristic emissions of Eu3+ ions can be observed, and the Er3+–Eu3+ energy migration has been demonstrated to be a resonant type of a dipole–dipole mechanism. Color-tunable emissions of NaLa(WO4)2:Er3+,Eu3+ microcrystals are realized under different UV radiation, and this could make them good candidates for use as full-color DC phosphors for near UV-LEDs. More practically, under near infrared (NIR) laser excitation at 980 nm, these phosphors also exhibit intense green and red emissions from the Er3+–Eu3+ energy transfer process, which causes the observed UC of Eu3+. The mechanism of UC luminescence is proposed by the observed dependence of integral intensity on the power of the pumping laser.

BaTiF6:Mn4+ bifunctional microstructures with photoluminescence and photocatalysis: hydrothermal synthesis and controlled morphology

A series of BaTiF6:Mn4+ samples were synthesized via a facile hydrothermal route. The crystal structure and morphology of the obtained products were studied. Detailed investigations indicated that morphologies of bundle-like, microrod, and flower-like structures could be observed by varying the hydrothermal reaction time. Simultaneously, we investigated the influences of the reaction temperature and the barium source on the crystal phases and morphology of the prepared BaTiF6:Mn4+ products. The luminescence properties of the as-synthesized samples were systematically evaluated. The results clearly demonstrated that two intense excitation bands could be observed in the near-UV and the blue region, suggesting that the BaTiF6:Mn4+ samples provide a great opportunity for application in light-emitting diodes (LED) with blue-chip excitation. The prepared BaTiF6:Mn4+ samples exhibited an intense red emission under 470 nm light excitation. Additionally, the changes of the Mn4+ emissions based on different reaction temperatures and barium sources were studied in detail. Temperature-dependent luminescence experiments were also performed in order to evaluate the thermal stability of the BaTiF6:Mn4+ samples. Furthermore, a thorough study of the photocatalytic activities of the as-prepared samples demonstrated that BaTiF6:Mn4+ could promote satisfactory photocatalytic activities.