Zhiguo Xia

A novel single-composition trichromatic white-emitting Sr3.5Y6.5O2(PO4)1.5(SiO4)4.5 : Ce3+/Tb3+/Mn2+ phosphor: synthesis, luminescent properties and applications for white LEDs

A series of single-composition trichromatic white-emitting Sr3.5Y6.5O2(PO4)1.5(SiO4)4.5 (SYPSO)u2006:u2006Ce3+, Tb3+, Mn2+ phosphors were investigated, and the crystal structures and luminescence properties, especially the energy transfer behavior, are discussed in detail. It was found that the SYPSOu2006:u2006Ce3+, Tb3+, Mn2+ phosphors can yield three major RGB emission bands in the visible spectral region: blue emission from the 5d–4f transitions of Ce3+, green emission from the 5D4–7FJ (J = 3, 4, 5, 6) transitions of Tb3+ and red emission from the 4T1(4G)–6A1(6S) transition of Mn2+ ions. The energy transfer process between Ce3+ and Tb3+ and between Ce3+ and Mn2+ have both been demonstrated to be a resonant type via a dipole–dipole mechanism, and the energy transfer (ET) efficiency, as well as the critical distance, have also been estimated. Based on the energy transfer, the emission colors of the obtained phosphors can be tuned from blue to green/red, and eventually to white emission by controlling the doping content of the Tb3+/Mn2+ ions. A white light emitting-diodes (w-LEDs) lamp was finally fabricated using the present phosphor (with the composition of SYPSOu2006:u20060.05Ce3+, 0.25Tb3+, 0.25Mn2+), which exhibited a high color rendering index (Ra) of 89.95 at a correlated color temperature of 6189 K and CIE coordinates of (0.320, 0.318). These results suggested that the present phosphor is a potential single-component white-light phosphor for n-UV-pumped w-LED.

Structure, luminescence property and energy transfer behavior of color-adjustable La5Si2BO13:Ce3+,Mn2+ phosphors

A series of color-adjustable phosphors La5Si2BO13(LSBO):Ce3+,Mn2+ were synthesized through a high temperature solid-state method. The crystal structures of Ce3+ and Mn2+ doped La5Si2BO13 phosphors were refined by the Rietveld method, which were proved to be the apatite-type hexagonal phase (space group of P63/m). It was found that two different La3+ sites in the La5Si2BO13 phase were occupied evenly by Ce3+ and Mn2+ ions, and then the formed vacancy contributed to the charge compensation. La5Si2BO13:Ce3+,Mn2+ phosphors exhibited a broad excitation band ranging from 250 to 375 nm and two broad emission bands centred at 418 nm and 585 nm upon 345 nm excitation. It is found that the emission colors could be tuned from blue-violet (0.1629, 0.0523) to pink (0.3227, 0.1830) by changing the ratio of Ce3+/Mn2+. Moreover, the energy transfer mechanism was verified to be the dipole–dipole interaction, and the critical distance was calculated to be 10.02 A by using the concentration quenching method.

Near-infrared luminescence and energy transfer studies of LaOBr:Nd 3+ /Yb 3+

LaOBr:Nd(3+)/Yb(3+) has been prepared via a high temperature solid-state method, and near-infrared (NIR) quantum cutting (QC) luminescence in this system has been demonstrated. NIR luminescence of LaOBr:Nd(3+)/Yb(3+) has been investigated by excitation, emission spectra and lifetime measurements, respectively. After absorption of a single 363 nm photon, down-conversion (DC) occurs from the Nd(3+) 4G(9/2) level via the cross-relaxation process Nd(3+) (4G(9/2)→4F(3/2)), Yb(3+) (2F(7/2)→2F(5/2)), followed by a second energy transfer step from Nd(3+) (4F(3/2)level) to Yb(3+) (2F(5/2)level), leading to the emission of two IR photons from Yb(3+), which is a promising avenue to boost the efficiency of solar cells with a twofold increase in the photon number.

Progress in lead-based ferroelectric and antiferroelectric single crystals: composition modification, crystal growth and properties

Ferroelectric and antiferroelectric materials, with complex perovskite structures, have experienced wide interest due to their peculiar structure and properties. In this highlight, recent advances of these two types of materials are reviewed, with particular emphasis on piezoelectrics/ferroelectric and antiferroelectric single crystals, owing to their ultra-high electric properties for practical application. Relaxor ferroelectric single crystals, represented by Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN-PT) at compositions near the morphotropic phase boundary (MPB), possess a high piezoelectric constant d33 (>2000 pC N−1) and electromechanical coupling factors k33 (90–92%). However, a low ferroelectric phase transition temperature Tr-t, far below the Curie temperature Tc restricts its further application in a broad temperature range. Some work on the design of a new pseudo-ternary system of Pb(Mg1/3Nb2/3)O3–PbZrO3–PbTiO3 (PMN-PZ-PT) and Pb(Mg1/3Nb2/3)O3–Pb(Fe1/2Nb1/2)O3–PbTiO3 (PMN-PFN-PT) have been demonstrated with emphasis on the composition modification and crystal growth. Moreover, for relaxor antiferroelectrics represented by (Pb,La)(Zr,Sn,Ti)O3 (PLZST) it is very difficult to grow bulk and high quality single crystals because of its incongruent melting. Our recent investigations on the flux growth of PLZST single crystals, especially the flux selection, compositions optimization, and defect analysis of as-grown single crystals are also described in this highlight.

A novel apatite-based warm white emitting phosphor Ba3GdK(PO4)3F:Tb3+, Eu3+ with efficient energy transfer for w-LEDs

A Ba3GdK(PO4)3F:Tb3+, Eu3+ phosphor with fluoro-apatite-structure has been fabricated by a conventional high-temperature solid-state reaction. The crystal structure, component element and microstructure of the phosphor have been systematically investigated by X-ray diffraction refinement, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM), respectively. The Ba3GdK(PO4)3F:Tb3+ phosphor shows a blue-greenish emission peak at 547 nm under excitation of 276 nm, while Ba3GdK(PO4)3F:Eu3+ displays a red emission peak near 620 nm with excitation at 396 nm. Efficient energy transfer from Tb3+ to Eu3+ ions takes place in the Ba3GdK(PO4)3F host, and the energy transfer critical distance between Tb3+ and Eu3+ ions along with the resonant energy-transfer mechanism are determined. By tuning the Tb3+/Eu3+ ratio, the emission hue can be modulated from blue-green (0.238, 0.311) to white (0.341, 0.318) and eventually to orange (0.521, 0.335). Moreover, the thermal quenching property of the as-prepared samples was studied in detail, which discloses the high thermal stability.

Facile room temperature morphology-controlled synthesis of SrSO4 microcrystals

A facile precipitation reaction without using any surfactants or templates has been employed to prepare SrSO4 microcrystals from 0-dimensional (0-D) particles to 1-D rods and 3-D dendrites. Especially, flower-like SrSO4 microcrystals with sizes of about 8 μm can be obtained under ambient conditions at room temperature. Detailed studies reveals that the formation of these 3-D structures is strongly dependent on the use of ethanol and the molar ratio of [Sr2+]/[SO42−] (R). At a prolonged aging time, the dendrites have evolved into mono-pods, double-pod, multi-pod and further into flower-like hierarchical architectures, driven by both the external crystal growth environments and intrinsic crystal structures.