Jiahua Zhang

Tunable full-color emitting BaMg2Al6Si9O30:Eu2+,Tb3+,Mn2+phosphors based on energy transfer

A series of single-phase full-color emitting BaMg(2)Al(6)Si(9)O(30):Eu(2+), Tb(3+), Mn(2+) phosphors has been synthesized by solid-state reaction. Energy transfer from Eu(2+) to Tb(3+) and Eu(2+) to Mn(2+) in BaMg(2)Al(6)Si(9)O(30) host matrix is studied by luminescence spectra and energy-transfer efficiency and lifetimes. The wavelength-tunable white light can be realized by coupling the emission bands centered at 450, 542, and 610 nm ascribed to the contribution from Eu(2+) and Tb(3+) and Mn(2+), respectively. By properly tuning the relative composition of Tb(3+)/Mn(2+), chromaticity coordinates of (0.31, 0.30), high color rendering index R(a) = 90, and correlated color temperature (CCT) = 5374 K can be achieved upon excitation of UV light. Thermal quenching properties reveal that BaMg(2)Al(6)Si(9)O(30): Eu(2+), Tb(3+), Mn(2+) exhibits excellent characteristics even better than that of YAG:Ce. Our results indicate our white BaMg(2)Al(6)Si(9)O(30):Eu(2+), Tb(3+), Mn(2+) can serve as a key material for phosphor-converted light-emitting diode and fluorescent lamps.

Blue-Emitting K2Al2B2O7:Eu2+ Phosphor with High Thermal Stability and High Color Purity for Near-UV-Pumped White Light-Emitting Diodes

Novel blue-emitting K2Al2B2O7:Eu(2+) (KAB:Eu(2+)) phosphor was synthesized by solid state reaction. The crystal structural and photoluminescence (PL) properties of KAB:Eu(2+) phosphor, as well as its thermal properties of the photoluminescence, were investigated. The KAB:Eu(2+) phosphor exhibits broad excitation spectra ranging from 230 to 420 nm, and an intense asymmetric blue emission band centered at 450 nm under λex = 325 nm. Two different Eu(2+) emission centers in KAB:Eu(2+) phosphor were confirmed via their fluorescence decay lifetimes. The optimal concentration of Eu(2+) ions in K2-xEuxAl2B2O7 was determined to be x = 0.04 (2 mol %), and the corresponding concentration quenching mechanism was verified to be the electric dipole-dipole interactions. The PL intensity of the nonoptimized KAB:0.04Eu(2+) phosphor was measured to be ∼58% that of the commercial blue-emitting BaMgAl10O17:Eu(2+) phosphor, and this phosphor has high color purity with the CIE coordinate (0.147, 0.051). When heated up to 150 °C, the KAB:0.04Eu(2+) phosphor still has 82% of the initial PL intensity at room temperature, indicating its high thermal stability. These results suggest that the KAB:Eu(2+) is a promising candidate as a blue-emitting n-UV convertible phosphor for application in white light emitting diodes.

Efficient Near-Infrared Downconversion and Energy Transfer Mechanism of Ce3+/Yb3+ Codoped Calcium Scandate Phosphor

An efficient near-infrared (NIR) downconversion has been demonstrated in CaSc2O4: Ce(3+)/Yb(3+) phosphor. Doping concentration optimized CaSc2O4: 1%Ce(3+)/5%Yb(3+) shows stronger NIR emission than doping concentration also optimized typical YAG: 1%Ce(3+)/5%Yb(3+) under 470 nm excitation. The NIR emission from 900 to 1100 nm is enhanced by a factor of 2.4. In addition, the main emission peak of Yb(3+) in the CaSc2O4 around 976 nm matches better with the optimal spectral response of the c-Si solar cell. The visible and NIR spectra and the decay curves of Ce(3+): 5d → 4f emission were used to demonstrate the energy transfer from Ce(3+) ions to Yb(3+) ions. The downconversion phenomenon has been observed under the direct excitation of Ce(3+) ions. On analyzing the dependence of energy transfer rate on Yb(3+) ion concentration, we reveal that the energy transfer (ET) from Ce(3+) ions to Yb(3+) ions in CaSc2O4 occurs mainly by the single-step ET process. Considering that the luminescence efficiency of CaSc2O4: Ce(3+) is comparable to that of commercial phosphor YAG: Ce(3+), the estimated maximum energy transfer efficiency reaches 58% in the CaSc2O4: 1%Ce(3+)/15%Yb(3+) sample, indicating that CaSc2O4: Ce(3+)/Yb(3+) sample has the potential in improving the conversion efficiency of c-Si solar cells.

Highly Efficient Green-Emitting Phosphors Ba2Y5B5O17 with Low Thermal Quenching Due to Fast Energy Transfer from Ce3+ to Tb3+

This paper demonstrates a highly thermally stable and efficient green-emitting Ba2Y5B5O17:Ce3+, Tb3+ phosphor prepared by high-temperature solid-state reaction. The phosphor exhibits a blue emission band of Ce3+ and green emission lines of Tb3+ upon Ce3+ excitation in the near-UV spectral region. The effect of Ce3+ to Tb3+ energy transfer on blue to green emission color tuning and on luminescence thermal stability is studied in the samples codoped with 1% Ce3+ and various concentrations (0-40%) of Tb3+. The green emission of Tb3+ upon Ce3+ excitation at 150 °C can keep, on average, 92% of its intensity at room temperature, with the best one showing no intensity decreasing up to 210 °C for 30% Tb3+. Meanwhile, Ce3+ emission intensity only keeps 42% on average at 150 °C. The high thermal stability of the green emission is attributed to suppression of Ce3+ thermal de-excitation through fast energy transfer to Tb3+, which in the green-emitting excited states is highly thermally stable such that no lifetime shortening is observed with raising temperature to 210 °C. The predominant green emission is observed for Tb3+ concentration of at least 10% due to efficient energy transfer with the transfer efficiency approaching 100% for 40% Tb3+. The internal and external quantum yield of the sample with Tb3+ concentration of 20% can be as high as 76% and 55%, respectively. The green phosphor, thus, shows attractive performance for near-UV-based white-light-emitting diodes applications.

Importance of Suppression of Yb3+ De-Excitation to Upconversion Enhancement in β-NaYF4: Yb3+/Er3+@β-NaYF4 Sandwiched Structure Nanocrystals

Nanosized Yb(3+) and Er(3+) co-doped β-NaYF4 cores coated with multiple β-NaYF4 shell layers were synthesized by a solvothermal process. X-ray diffraction and scanning electron microscopy were used to characterize the crystal structure and morphology of the materials. The visible and near-infrared spectra as well as the decay curves were also measured. A 40-fold intensity increase for the green upconversion and a 34-fold intensity increase for the red upconversion were observed as the cores are coated with three shell layers. The origin of the upconversion enhancement was studied on the basis of photoluminescence spectra and decay times. Our results indicate that the upconversion enhancement in the sandwiched structure mainly originates from the suppression of de-excitation of Yb(3+) ions. We also explored the population of the Er(3+4)F9/2 level. The results reveal that energy transfer from the lower intermediate Er(3+4)I13/2 level is dominant for populating the Er(3+4)F9/2 level when the nanocrystal size is relatively small; however, with increasing nanocrystal size, the contribution of the green emitting Er(3+4)S3/2 level for populating the Er(3+4)F9/2 level, which mainly comes from the cross relaxation energy transfer from Er(3+) ions to Yb(3+) ions followed by energy back transfer within the same Er(3+)-Yb(3+) pair, becomes more and more important. Moreover, this mechanism takes place only in the nearest Er(3+)-Yb(3+) pairs. This population route is in good agreement with that in nanomaterials and bulk materials.

Zinc titanium glycolate acetate hydrate and its transformation to zinc titanate microrods: synthesis, characterization and photocatalytic properties

A heterobimetallic complex, zinc titanium glycolate acetate hydrate (Zn2Ti3–GAH), tentatively formulated as Zn2Ti3(OCH2CH2O)4(OCH2CH2OH)5(CH3COO)3·2HOCH2CH2OH·H2O, was synthesized by a room-temperature homogeneous precipitation in ethylene glycol solution. Its chemical composition, crystal structure, morphology, growth mechanism and thermal behaviors were characterized in detail. The precipitated Zn2Ti3–GAH was of a highly crystalline monoclinic phase and porous microrod morphology. As the single source precursor (SSP), Zn2Ti3–GAH was transformed into different phases of zinc titanate via thermal decomposition. With the remaining shape of the microrods, cubic phases of Zn2Ti3O8 and rutile TiO2 (r-TiO2) supported hexagonal phases of ZnTiO3 (h-ZnTiO3) were obtained by calcination at 500 and 700 °C, respectively; while r-TiO2 supported Zn2TiO4 were yielded in the form of dispersed particles or chains at higher temperature (950 °C). Benefiting from the SSP route and the confinement in the specific microrod domains of precursors, the heterostructures of r-TiO2–ZnTiO3 and r-TiO2–Zn2TiO4 were formed during programmable calcination. The studies on photocatalysis by degrading methylene blue (MB) under ultraviolet (UV) irradiation indicated that the as-transformed zinc titanate exhibited enhanced activity. In particular, r-TiO2 supported h-ZnTiO3 displayed the photodegradation reaction rate constant of 0.00163 s−1, which was comparable to that of commercially available Degussa P25 TiO2. This probably related to the more effective charge separation in the r-TiO2–ZnTiO3 heterostructure packed in the microrods.

Investigation of the Energy-Transfer Mechanism in Ho3+- and Yb3+-Codoped Lu2O3 Phosphor with Efficient Near-Infrared Downconversion

A high-temperature solid-state method was used to synthesize the Ho3+- and Yb3+-codoped cubic Lu2O3 powders. The crystal structures of the as-prepared powders were characterized by X-ray diffraction. The energy-transfer (ET) phenomenon between Ho3+ ions and Yb3+ ions was verified by the steady-state spectra including visible and near-infrared (NIR) regions. Beyond that, the decay curves were also measured to certify the existence of the ET process. The downconversion phenomena appeared when the samples were excited by 446 nm wavelength corresponding to the transition of Ho3+: 5I8→5G6/5F1. On the basis of the analysis of the relationship between the initial transfer rate of Ho3+: 5F3 level and the Yb3+ doping concentration, it indicates that the ET from 5F3 state of Ho3+ ions to 2F5/2 state of Yb3+ ions is mainly through a two-step ET process, not the long-accepted cooperative ET process. In addition, a 62% ET efficiency can be achieved in Lu2O3: 1% Ho3+/30% Yb3+. Unlike the common situations in which the NIR photons are all emitted by the acceptors Yb3+, the sensitizers Ho3+ also make contributions to the NIR emission upon 446 nm wavelength excitation. Meanwhile, the 5I5→5I8 transition and 5F4/5S2→5I6 transition of Ho3+ as well as the 2F5/2→2F7/2 transition of Yb3+ match well with the optimal spectral response of crystalline silicon solar cells. The current research indicates that Lu2O3: Ho3+/Yb3+ is a promising material to improve conversion efficiency of crystalline silicon solar cell.

Inhomogeneous broadening and trap effect in hole burning of Sm2+-doped mixed crystals

Abstract We report the results of the inhomogeneous broadening and high temperature hole burning in M y M 1− y FCl 0.5 Br 0.5 :Sm 2+ system (M, M = Ca, Sr, Ba; y = 0–1). The influence of Sm 3+ as electron trap on hole burning is discussed and new trap is introduced in Yb-doped SrFCl:Sm 2+ crystals.

The Inductive Effect of Neighboring Cations in Tuning Luminescence Properties of the Solid Solution Phosphors

Forming solid solutions through cation substitution is an efficient way to improve the luminescence properties of Ce3+ or Eu2+ activated phosphors and even to develop new ones, which is badly needed for phosphor-converted white LEDs. Here, we report new color tunable solid solution phosphors based on Eu2+ activated K2Al2B2O7 as a typical case to demonstrate that, besides crystal field splitting of 5d levels, centroid shift and Stokes shift can be dominant in tuning excitation and emission spectra as well as thermal stability of solid solution phosphors, both of which were previously considered to be negligible. Moreover, a general model involving the inductive effect of neighboring cations is proposed to explain the obvious variations in centroid shift and Stokes shift with cation substitution. Our work is propitious for the construction of more reasonable structure-property relations and thus offers theoretical guidance for designing solid solution phosphors.

Improvement of Green Upconversion Monochromaticity by Doping Eu3+ in Lu2O3:Yb3+/Ho3+ Powders with Detailed Investigation of the Energy Transfer Mechanism

The monochromaticity improvement of green upconversion (UC) in Lu2O3:Yb3+/Ho3+ powders has been successfully realized by tridoping Eu3+. The integral area ratio of green emission to red emission of Ho3+ increases 4.3 times with increasing Eu3+ doping concentration from 0 to 20 mol %. The energy transfer (ET) mechanism in the Yb3+/Ho3+/Eu3+ tridoping system has been investigated carefully by visible and near-infrared (NIR) emission spectra along with the decay curves, revealing the existence of ET from the Ho3+5F4/5S2 level tothe Eu3+5D0 level and ET from the Ho3+5I6 level to the Eu3+7F6 level. In addition, the population routes of the red-emitting Ho3+5F5 level in the Yb3+/Ho3+ codoped system under 980 nm wavelength excitation have also been explored. The ET process from the Yb3+2F5/2 level to the Ho3+5I7 level and the cross-relaxation process between two nearby Ho3+ ions in the 5F4/5S2 level and 5I7 level, respectively, have been demonstrated to be the dominant approaches for populating the Ho3+5F5 level. The multiphonon relaxation process originating from the Ho3+5F4/5S2 level is useless to populate the Ho3+5F5 level. As the energy level gap between the Ho3+5I7 level and Ho3+5I8 level matches well with that between Eu3+7F6 level and Eu3+7F0 level, the energy of the Ho3+5I7 level can be easily transferred to the Eu3+7F6 level by an approximate resonant ET process, resulting in a serious decrease in the red UC emission intensity. Since this ET process is more efficient than the ET from the Ho3+5F4/5S2 level to the Eu3+5D0 level as well as the ET from the Ho3+5I6 level to the Eu3+7F6 level, the integral area ratio of green emission to red emission of Ho3+ has been improved significantly.