Showing papers in "Journal of Luminescence in 2007"

Enhancement of red spectral emission intensity of Y3Al5O12:Ce3+ phosphor via Pr co-doping and Tb substitution for the application to white LEDs

Abstract: We have enhanced color-rendering property of a blue light emitting diode (LED) pumped white LED with yellow emitting Y3Al5O12:Ce3+ (YAG:Ce) phosphor using addition of Pr and Tb as a co-activator and host lattice element, respectively. Pr3+ addition to YAG:Ce phosphor resulted in sharp emission peak at about 610 nm through 1D2→3H4 transition. And when Tb3+ substituted Y3+ sites, Ce3+ emission band shifted to a longer wavelength due to larger crystal field splitting. Y3Al5O12:Ce3+, Pr3+ and (Y1−xTbx)3Al5O12:Ce3+ phosphors were coated on blue LEDs to fabricate white LEDs, respectively, and their color-rendering indices (CRIs, Ra) were measured. As a consequence of the addition of Pr3+ or Tb3+, CRI of the white LEDs improved to be R a = 83 and 80, respectively. Especially, blue LED pumped (Y0.2Tb0.8)3Al5O12:Ce3+ white LED showed both strong luminescence and high color-rendering property.

Interaction of the flavonoid hesperidin with bovine serum albumin: A fluorescence quenching study

Abstract: The interaction between the flavonoid hesperidin and bovine serum albumin (BSA) was investigated by fluorescence and UV/Vis absorption spectroscopy. The results revealed that hesperidin caused the fluorescence quenching of BSA through a static quenching procedure. The hydrophobic and electrostatic interactions play a major role in stabilizing the complex. The binding site number n , and apparent binding constant K A , corresponding thermodynamic parameters Δ G o , Δ H o , Δ S o at different temperatures were calculated. The distance r between donor (BSA) and acceptor (hesperidin) was obtained according to fluorescence resonance energy transfer. The effect of Cu 2+ , Zn 2+ , Ni 2+ , Co 2+ , and Mn 2+ on the binding constants between hesperidin and BSA were studied. The effect of hesperidin on the conformation of BSA was analyzed using synchronous fluorescence spectroscopy and UV/Vis absorption spectroscopy.

Photoluminescence and absorption in sol gel-derived ZnO films

Abstract: Highly c-axis-oriented ZnO films were obtained on corning glass substrate by sol–gel technique. The characteristics of photoluminescence (PL) of ZnO, as well as the exciton absorption in the absorption (UV) spectra are closely related to the post-annealing treatment. The difference between PL peak position and the absorption edge, designated as Stokes shift, is found to decrease with the increase of annealing temperature. The minimum Stokes shift is about 150 meV. The decrease of Stokes shift is attributed to the decrease in carrier concentration in ZnO film with annealing. X-ray diffraction, surface morphology and refractive index results indicate an improvement in crystalline quality with annealing. Annealed films also exhibit a green emission centered at ∼520 nm with activation energy of 0.89 eV. The green emission is attributed to the electron transition from the bottom of the conduction band to the antisite oxygen OZn defect levels.

Reduction from Eu3+ to Eu2+ in BaAl2O4:Eu phosphor prepared in an oxidizing atmosphere and luminescent properties of BaAl2O4:Eu

Abstract: A reduction phenomenon of Eu3+→Eu2+ was observed for the first time when Eu3+ ions were doped into an AlO4-tetrahedron-containing compound BaAl2O4 in an oxidizing atmosphere of air by high-temperature solid-state reaction. X-ray powder diffraction patterns and photoluminescent spectra are used to confirm the compound structure and detect the simultaneous existence of both divalent and trivalent europium ions, respectively. The abnormal Eu3+→Eu2+ reduction is explained by a charge compensation model. Spectroscopic properties of BaAl2O4:Eu are discussed and Eu2+ emission spectrum shows consistence with the results reported by Katsumata et al. [J. Cryst. Growth 198/199 (1999) 869.] and Lin et al. [Mater. Chem. Phys. 70 (2001) 156.]

Optical absorption and photoluminescence studies of Eu3+-doped phosphate and fluorophosphate glasses

Abstract: Phosphate (P2O5+K2O+BaO+Al2O3+Eu2O3) and fluorophosphate (P2O5+K2O+BaO+BaF2+Al2O3+Eu2O3) glasses with different Eu3+ ion concentrations have been prepared and characterized through optical absorption, photoluminescence and decay times. An intense red luminescence is observed from the 5D0 emitting level of Eu3+ ions in these glasses. The relative luminescence intensity ratio of 5D0→7F2→5D0→7F1 transitions has been evaluated to estimate the local site symmetry around the Eu3+ ions. The emission spectra of these glasses show a complete removal of degeneracy for the 5D0→7F1 and 5D0→7F2 transitions. Second and fourth rank crystal-field (CF) parameters have been calculated together with the CF strength parameter by assuming the C2v symmetry for the Eu3+ ions in both the phosphate and fluorophosphate glasses. Judd–Ofelt parameters have been evaluated from the luminescence intensity ratios of 5D0→7FJ (J=2, 4 and 6) to 5D0→7F1 transitions. These parameters have been used to derive radiative properties such as transition probabilities, branching ratios, radiative lifetimes and peak stimulated emission cross-sections for the 5D0→7FJ transitions. Decay curves of the 5D0 level of Eu3+ ions in these two Eu3+:glass systems have been measured by monitoring the 5D0→7F2 transition (611 nm) at room temperature. The experimental lifetime of the 5D0 level in the title glasses is found to be higher than Eu3+-doped niobium phosphate glasses. The analysis indicates that the lifetime of the 5D0 level is found to be less sensitive to the Eu3+ ion concentration and addition of BaF2 has no significant effect on the optical properties of Eu3+-doped phosphate glasses.

Luminescence and scintillation properties of YAG:Ce single crystal and optical ceramics

Abstract: We use various techniques to study optical and scintillation properties of Ce-doped yttrium aluminum garnet, Y 3 Al 5 O 12 (YAG:Ce), in the form of a high-quality industrial single crystal. This was compared to optical ceramics prepared from YAG:Ce nanopowders. We present experimental data in the areas of optical absorption, radioluminescence, scintillation decay, photoelectron yield, thermally stimulated luminescence and radiation-induced absorption. The results point to an interesting feature—the absence of antisite (Y Al , i.e. Y at the Al site) defects in optical ceramics. The scintillation decay of the ceramics is faster than that of the single crystal, but its photoelectron yield (measured with 1 μs integration time) is about 30–40% lower. Apart from the photoelectron yield value the YAG:Ce optical ceramic is fully comparable to a high quality industrial YAG:Ce single crystal and can become a competitive scintillator material.

Thermoluminescence of nanocrystalline LiF:Mg, Cu, P

Abstract: Nanocrystalline LiF:Mg, Cu, P of rod shape (about 30–40 nm in diameter and 0.3–0.5 μm in length) has been prepared by the chemical co-precipitation method. Thermoluminescence (TL) and dosimetric characteristics of the nanocrystalline phosphor are studied and presented here. The formation of the material was confirmed by the X-ray diffraction (XRD). Its shape and size were also observed by transmission electron microscope (TEM). The TL glow curve of the nanocrystalline powder shows a single peak at 410 K along with four overlapping peaks of lesser intensities at around 570, 609, 638 and 663 K. The observed TL sensitivity of the prepared nanocrystalline powder is less than that of the commercially available “Harshaw TLD-700H hot-pressed chips” at low doses but it still around three times more than that of LiF:Mg, Ti (TLD-100) phosphor. The 410 K peak of the nanomaterial phosphor shows a very linear response with exposures increasing up to very high values (as high as 10 kGy), where all the other thermoluminesent dosimeters (TLD) phosphors show saturation. This linear response over a large span of exposures (0.1 Gy–10 kGy) along with negligible fading and its insensitivity to heating treatments makes the nanocrystalline phosphor useful for its application to estimate high exposures of γ-rays. The ‘tissue equivalence’ property of this material also makes it useful over a wide range of high-energy radiation.

Electron–phonon and vibronic couplings in the FMO bacteriochlorophyll a antenna complex studied by difference fluorescence line narrowing

Abstract: The electron–phonon and vibronic couplings governing the spectral properties have been studied in the Fenna–Matthews–Olson (FMO) bacteriochlorophyll a (BChl a )–protein complex at 4.5 K using a spectrally selective difference fluorescence line-narrowing technique. The complex is a part of the light-harvesting system of green photosynthetic bacteria. Its lowest-energy absorption band, peaking at 826 nm and responsible for the fluorescence, is believed to be due to Q y transitions of largely isolated molecules. One of the main merits of the used method compared with the more common fluorescence line narrowing is that the zero-phonon lines (ZPL) resonant with the excitation laser can be accurately measured, allowing precise determination of the Huang–Rhys (HR) factors, the main characteristics of the linear electron–phonon and vibronic coupling strengths. Over 60 individual vibrational modes of intra- and intermolecular origin have been resolved in the energy range of 45–1600 cm −1 . The small HR factors for these modes, ranging between 0.001 and 0.018, add up to a value of S vib =0.38±0.07. The effective HR factor for the phonons, S ph , was found clearly wavelength-dependent, varying from ∼0.7 at short wavelengths to ∼0.3 at the long-wavelength tail of the absorption spectrum. Coupling between the BChl a molecules is likely responsible for this wavelength dependence.

Size-controlled synthesis of ZnO nanoparticles and their photoluminescence properties

Abstract: Nanocrystalline ZnO powders have been synthesized by means of a novel gel-network precipitation method using gelatin as a template at lower temperature condition. The products were characterized by using TG-DTA, XRD and TEM techniques. The products appeared to be regularly spherical or elliptical and their sizes range from 20 to 40 nm. The average particle size increased with increasing calcining temperature, and decreased with increasing gelatin concentration. Furthermore, the photoluminescence (PL) spectra of the ZnO nanopowders were also investigated. The relative luminescent intensities for the ultraviolet band and green emission band showed a dependence on preparation conditions. The probable mechanism was discussed.

Excited state intramolecular proton transfer in amino 2-(2′-hydroxyphenyl)benzazole derivatives: Effects of the solvent and the amino group position

Abstract: The excited-state intramolecular proton transfer (ESIPT) mechanism in six amino 2-(2′-hydroxyphenyl)benzazole derivatives were investigated in different solvents by means of UV–vis absorption and steady-state fluorescence. The amino benzazoles are fluorescent in the blue-orange region under UV radiation. Changes in the absorption, emission and excitation spectra were analyzed and correlated to the position of the amino group and the solvent polarity. The equilibrium between the conformers in solution in the ground state, confirmed by the solvatochromic effect, reflects the dual fluorescence emission presented by these dyes.

Red-shift of emission wavelength caused by reabsorption mechanism of europium activated Ca-α-SiAlON ceramic phosphors

Abstract: The optical properties of divalent europium activated Ca-α-SiAlON were investigated and the excitation and emission processes were discussed. The emission wavelength is influenced by the activator concentration of Eu more than the codopant concentration of Ca, in case of the same matrix composition of α-SiAlON. The red-shift of emission increases when the concentration of Eu increases. In addition, it was revealed that the red-shift of emission would be caused by the construction of the package of optical device. The correlated color temperature becomes low by the red-shift for the white light-emitting diode (LED) lamp using Ca-α-SiAlON:Eu phosphor when the phosphor powder concentration increases in the transparent resin which coats the primary light source of a blue LED die. The additional red-shift can be observed when the excessive amount of phosphor coating is intentionally poured to the white LED lamps. These red-shifts of emission wavelength could be explained by the classical configurational coordinate model.

Rare earth ions doped full-color luminescence glasses for white LED

Abstract: Silicate and phosphate glasses co-doped with rare-earth ions (REIs)(Ce 3+ , Tb 3+ , Eu 3+ ) are presented in the present work. Their photoluminescence properties were studied by excitation and emission spectra. A combination of blue, green and red bands is shown for both silicate and phosphate glasses that allows the observation of white light when the glass is excited by UV light. The relative emission intensity ratios of the green to the red can be tuned by varying the concentrations of activator and/or sensitizer as well as the composition of glass matrices.

Preparation of water-soluble CdTe/CdS core/shell quantum dots with enhanced photostability

Abstract: CdTe/CdS core/shell quantum dots (QDs) have been synthesized in an aqueous phase using thioacetamide as a sulfur source. The quantum yield was greatly enhanced by the epitaxial growth of a CdS shell, which was confirmed by X-ray photoelectron spectroscopy (XPS) results. The quantum yield of as-prepared CdTe/CdS core/shell QDs without any post-preparative processing reached 58%. The experimental results illustrate that the QDs with core/shell structure show better photostability than thioglycolic acid (TGA)-capped CdTe QDs. The cyclic voltammograms reveal higher oxidation potentials for CdTe/CdS core/shell QDs than for TGA-capped CdTe QDs, which explains the superior photostability of QDs with a core/shell structure. This enhanced photostability makes these QDs with core/shell structure more suitable for bio-labeling and imaging.

Light emitting CdS quantum dots in PMMA: Synthesis and optical studies

Abstract: Freshly prepared CdS-quantum dots (QDs) in DMF (clear pale solution) when loaded in polymethylmethacrylate (PMMA) lead to excellent optical properties. The tuning of the absorption and emission wavelengths via experimentally control parameters is considered novel and significant. The absorption band for CdS was observed at about 370 nm in polymeric matrix. The blue, green and orange light emissions from such composite solution were tuned and stabilized by simply varying the concentration of CdS, cadmium and sulphur in the final product. Photoluminescence (PL) measurement with 2% CdS loading showed band-edge emissions from the composite with only about 20–25 nm Stokes shift in emission wavelength. Observation of such optical properties indicated that the composite has narrow particle size distribution and particle diameter may well be below 10 nm. X-ray diffraction (XRD) patterns of the film with higher loading of CdS showed broad pattern for hexagonal CdS. Thermo-gravimetric analysis (TGA) of CdS/PMMA composite film revealed that it has better thermal stability than PMMA alone. Transmission electron microscopy (TEM) showed agglomerated tiny dots in nano-meter regime.

White-light long-lasting phosphor Sr2MgSi2O7:Dy3+

Abstract: A Sr 2 MgSi 2 O 7 :Dy 3+ phosphor emitting long-lasting white light was prepared and investigated. Under the ultraviolet excitation, the emission spectra of Sr 2 MgSi 2 O 7 :Dy 3+ are composed of a broad band peaking at 441 nm from the host and the characteristic emission of Dy 3+ peaking at 480, 575 and 668 nm, originating from the transitions of 4 F 9/2 → 6 H 15/2 , 4 F 9/2 → 6 H 13/2 and 4 F 9/2 → 6 H 11/2 in the 4f 9 configuration of Dy 3+ . After stopping the excitation, the afterglow spectra consist of only emissions from the transitions of Dy 3+ and the emission band from the host does not contribute to the afterglow spectra. The red-light emission peaking at 668 nm from the transitions of 4 F 9/2 → 6 H 11/2 is rather weak. Therefore, the afterglow spectra are composed of intense blue-light emission peaking at 480 nm and yellow-light emission peaking at 575 nm, showing white-light for human eyes. The white-light afterglow can last about 40 min in darkness after excitation with 254 nm for 10 min (250 μW/cm 2 ). The afterglow decay curves can be fitted into hyperbolic curve. The thermoluminescence curves show two thermoluminescence bands peaking at about 295 and 330 K, which are responsible for the long-lasting afterglow emission.

Red luminescence induced by intervalence charge transfer in Pr3+-doped compounds

Abstract: The excited state dynamics of the Pr 3+ in oxide lattices is revisited in the light of the ‘virtual recharge’ model. The connection between the quenching of the blue-green emission from the 3 P 0 level and the formation of a Pr 3+ to metal intervalence charge-transfer state (IVCTS) is discussed on the basis of new experimental evidences. Updated information is presented, concerning the energy position of the IVCTS in various materials and its relation with the host properties. Finally, the limits of the proposed model are outlined and the perspectives for future work are pointed out.

Optical properties of Ho3+-doped novel oxyfluoride glasses

Abstract: Novel oxyfluoride glasses SiO 2 –Al 2 O 3 –Na 2 O–ZnF 2 doped with Ho 3+ and Ho 3+ /Yb 3+ were fabricated. The optical properties of the synthesized glasses were experimentally and theoretically investigated in detail. The experimental and calculated oscillator strengths of Ho 3+ were determined by measurement of absorption spectrum of Ho 3+ -singly doped glass. According to the Judd-Ofelt theory, the Judd-Ofelt parameters were calculated, by which the radiative transition probabilities, fluorescence branching ratios and radiative lifetimes were obtained. Visible upconversion luminescence was observed under 980 nm diode laser excitation and the influence of Yb 3+ concentration on the emission bands were also investigated. The dependence of the upconversion emission intensity upon the excitation power was examined, and the upconversion mechanisms were discussed.

Photoluminescence of hexagonal boron nitride: Effect of surface oxidation under UV-laser irradiation

Abstract: We report on the UV laser-induced fluorescence of hexagonal boron nitride (h-BN) following nanosecond laser irradiation under vacuum and in different environments of nitrogen gas and ambient air. The observed fluorescence bands are tentatively ascribed to impurity and mono (VN) or multiple (m-VN with m¼ 2 or 3) nitrogen vacancies. A structured fluorescence band between 300 and 350 nm is assigned to impurity-band transition and its complex lineshape is attributed to phonon replicas. An additional band at 340 nm, assigned to VN vacancies on surface, is observed under vacuum and quenched by adsorbed molecular oxygen. UV-irradiation of h-BN under vacuum results in a broad asymmetric fluorescence at� 400 nm assigned to m-VN vacancies; further irradiation breaks more B–N bonds enriching the surface with elemental boron. However, no boron deposit appears under irradiation of samples in ambient atmosphere. This effect is explained by oxygen healing of radiation-induced surface defects. Formation of the oxide layer prevents B–N dissociation and preserves the bulk sample stoichiometry.

Luminescent materials and spectroscopic properties of Dy3+ ion

Abstract: Dy 3+ ion can be used as a dopant for electron trapping materials such as long-lasting phosphorescence (LLP) or solid dosimeter. Due to the special electron trapping and thermoluminescent properties of the materials doped with Dy 3+ , LLP such as SrAl 2 O 4 : Eu 2+ ,Dy 3+ or glass ceramic SrO–Al 2 O 3 –B 2 O 3 : Eu 2+ ,Dy 3+ and solid dosimeter such as BaB 4 O 7 :Dy 3+ and SrB 4 O 7 :Dy 3+ were prepared. Dy 3+ ion can also be used for lighting by mixing its yellow 4 F 9/2 → 6 H 13/2 and blue 4 F 9/2 → 6 H 15/2 emission bands as in high-pressure mercury lamp phosphor Y 0.79 Gd 0.20 V 0.5 P 0.5 O 4 : 0.01Dy. In this paper, the VUV excitation spectra and visible emission spectra of Sr 3 Gd 0.94 (PO 4 ) 3 :0.06Dy, Ba 3 Gd 0.94 (PO 4 ) 3 :0.06Dy and Ca 4.99 (PO 4 ) 3 F: 0.01Dy with chromaticity coordinates in the white-light region are described. They have potential application for mercury-free fluorescence lamp.

Photoluminescence and cathodoluminescence properties of Y2O3:Eu nanophosphors prepared by combustion synthesis

Abstract: Eu-doped Y 2 O 3 nanophosphors were prepared using combustion synthesis. In this method, urea was employed as a fuel. The particle size was estimated to be in the range of 10–20 nm as determined by X-ray diffractometry and transmission electron microscopy. The photoluminescent and cathodoluminescent spectra are described by the well-known 5 D 0 → 7 F J transition ( J =0, 1, 2, etc.) of Eu 3+ ions with the strongest emission for J =2. The optical properties of nanophosphors were compared with commercial with an order of micrometer size. The effects of urea-to-metal nitrate molar ratio and the other synthesis conditions on the particle size and luminescent properties will be discussed in detail. The red emission of Eu-doped Y 2 O 3 nanophosphors is promising materials not only in high-resolution screen but also in telecomunication as well as in biosensor.

Synthesis and luminescence properties of nanocrystalline Gd2O3:Eu3+ by combustion process

Abstract: The nanocrystalline Gd 2 O 3 :Eu 3+ powders with cubic phase were prepared by a combustion method in the presence of urea and glycol. The effects of the annealing temperature on the crystallization and luminescence properties were studied. The results of XRD show pure phase can be obtained, the average crystallite size could be calculated as 7, 8, 15, and 23 nm for the precursor and samples annealed at 600, 700 and 800 °C, respectively, which coincided with the results from TEM images. The emission intensity, host absorption and charge transfer band intensity increased with increasing the temperature. The slightly broad emission peak at 610 nm for smaller particles can be observed. The ratio of host absorption to O 2− –Eu 3+ charge transfer band of smaller nanoparticles is much stronger compared with that for larger nanoparticles, furthermore, the luminescence lifetimes of nanoparticles increased with increasing particles size. The effects of doping concentration of Eu 3+ on luminescence lifetimes and intensities were also discussed. The samples exhibited a higher quenching concentration of Eu 3+ , and luminescence lifetimes of nanoparticles are related to annealing temperature of samples and the doping concentration of Eu 3+ ions.

The influence of different conditions on the luminescent properties of YAG:Ce phosphor formed by combustion

Abstract: Powder phosphor yttrium aluminum garnet (YAG), activated with trivalent cerium (Ce 3+ ) is synthesized by combustion from mixed metal nitrate reactants and urea with ignition temperature of 500–550 °C. After repeated calcinations at a given temperature for 5 h, pure-phase phosphor was obtained. The experiment result shows the appropriate temperature for repeated calcinations is 1000 °C. Luminescent intensity at various quantity of urea is studied in detail and the maximum value shown in the result in fact is 2.5 times larger than the theoretical value. In addition, the characteristics of particles such as crystallinity, morphology and photoluminescence were investigated. There are two peaks in its excited spectrum and the major one is a broad band around 470 nm, which matches the blue emission of GaN LED very well. The emission peak locates at about 532 nm, which can combine the blue light of GaN LED to yield white light.

SnO2:Eu nanoparticles dispersed in silica: A low-temperature synthesis and photoluminescence study

Abstract: SnO2:Eu and SnO2:Eu nanoparticles dispersed in silica matrix were prepared at a relatively low temperature of 185 °C in ethylene glycol medium. For as-prepared SnO2:Eu nanoparticles there exists a weak energy transfer from the SnO2 host to the Eu3+ ions. However, the energy transfer can be significantly improved by dispersing the Eu3+-doped SnO2 nanoparticles in silica matrix. Effective shielding of surface Eu3+ ions in SnO2:Eu nanoparticles from the stabilizing ligand by silica matrix is the reason for the improved extent of energy transfer. Increase in asymmetric ratio of luminescence (ratio of the intensity of the electric dipole allowed transition, 5D0→7F2, to magnetic dipole allowed transition, 5D0→7F1) for SnO2:Eu nanoparticles dispersed in silica compared to that of SnO2:Eu nanoparticles, has been attributed to the distorted environment around surface Eu3+ ions brought about by the presence of both tin and silicon structural units. 119Sn and 29Si MAS NMR studies on this sample confirmed that there is no interaction between the tin and silicon structural units even after heating the samples at 900 °C.

Synthesis and photoluminescence of CeO2:Eu3+ phosphor powders

Abstract: The crystalline structure and photoluminescence (PL) properties of europium-doped cerium dioxide synthesized by the solid-state reaction method were analyzed. CeO 2 :Eu 3+ phosphor powders exhibit the pure cubic fluorite phase up to 10 mol% doping concentration of Eu 3+ . With indirect excitation of CeO 2 host at 373 nm, the PL intensity quickly increases with increasing Eu 3+ concentration, up to about 1 mol%, and then decreases indicating the concentration quenching. While with direct excitation (467 nm), much more stronger PL emissions, especially the electric dipole emission 5 D 0 – 7 F 2 at 612 nm, are observed and no concentration quenching occurs up to 10 mol% doping concentration of Eu 3+ . The nature of this behavior and the cause of the concentration quenching were discussed.

Luminescent and thermal properties of full-color emitting X3MgSi2O8:Eu2+, Mn2+ (X=Ba, Sr, Ca) phosphors for white LED

Abstract: The X 3 MgSi 2 O 8 :Eu 2+ , Mn 2+ (X = Ba, Sr, Ca) phosphors show three emission peaks: the blue and green bands of Eu 2+ transitions, and the red band of Mn 2+ transitions. Three emission peaks are shifted toward longer-wavelength side with substituting X cations with smaller cations in the order of Ba, Sr, and Ca cations. This behavior is discussed in terms of two factors of the crystal-field strength and the covalence. As an increase of temperature, the red and green emission peaks show anomalous blueshift along with broadening bandwidth and decreasing intensity. This phenomenon is described in terms of the phonon-assisted back tunneling from the excited state of low-energy emission band to the excited state of high-energy emission band.

Enhanced luminescent properties of Y3Al5O12:Tb3+,Ce3+ phosphor prepared by spray pyrolysis

Abstract: Yttrium aluminum garnet (YAG) particles doped with Tb 3+ or double doped with Tb 3+ and Ce 3+ were prepared by spray pyrolysis and characterized by photo- and cathode-luminescence. It was tried to incorporate a broad band of Ce 3+ activator into the line peaks of Tb 3+ in YAG host without the reduction of emission intensity. Ce-codoped YAG:Tb particles showed a broad band emission due to the d–f transition of Ce 3+ and a reduction in the intensity of emission peaks due to 5 D 3 – 7 F j ( j =3, 4, 5, 6) transition of Tb 3+ when they were excited by the ultraviolet light of 270 nm. These results supported that an effective energy transfer occurs from Tb 3+ to Ce 3+ in YAG host. Codoping Ce 3+ ions greatly intensified the excitation peak at 270 nm for the emission at 540 nm of Tb 3+ , which means that more lattice defects, involving in the energy absorption and transfer to Tb 3+ , are formed by the Ce 3+ codoping. The finding gives a promising approach for enhancing the luminescence efficiency.

Upconversion luminescence and kinetics in Er3+:YAlO3 under 652.2 nm excitation

Abstract: Ultraviolet and visible upconversion properties of Er3+ in YAlO3 were investigated following 652.2 nm excitation of the multiples 4F9/2. The luminescence and excitation spectra were recorded. Ultraviolet (326–342 and 354–359 nm), violet (405–420 nm), blue (436–442 nm) and green (525–575 nm) upconversion and infrared downconversion luminescence were simultaneously observed. The intense green luminescence corresponds to the emissions from the thermal coupled 4S3/2 and 2H11/2 bands and 2G9/2 level. Energy transfer upconversion processes were proposed to explain the upconversion phenomena. The luminescence kinetics was discussed in detail by the analyses of fluorescence decay curves.

Preparation and luminescent properties of europium-doped yttria fibers by electrospinning

Abstract: Sub-micrometer-sized fibers of europium-doped yttria (Y 2 O 3 :Eu 3+ ) were prepared by electrospinning followed by high-temperature calcinations for the first time. The fibers were with diameters of 200–400 nm and lengths of several 10 μm and cubic in phase. The spectral properties of the Y 2 O 3 :Eu 3+ fibers were studied, in contrast with those of bulk powders. The results indicated that in the present Y 2 O 3 :Eu 3+ fibers the excited charge transfer band had slightly blue shift in comparison with that in the bulk due to weaker covalence of Eu–O bonds. In addition, both of the lifetimes of the 5 D 1 and 5 D 0 states in the fibers became shorter than that in the bulk due to improved nonradiative transition rates.

Photoluminescence properties of samarium-doped TiO2 semiconductor nanocrystalline powders

Abstract: In this work, the luminescence properties of samarium ions-doped titanium dioxide prepared by the sol–gel process were studied. A strong orange red emission (4G5/2−6H7/2 (orange) and 4G5/2–6H9/2 (red)) ascribed to the electron transitions in 4f5 configuration of Sm3+ ions was observed upon excitation into TiO2 host. The energy transfer from TiO2 to Sm3+ was verified and the relevant mechanism was discussed. In addition, the impacts of metal ion codopants upon the TiO2:Sm3+ luminescence properties were studied. The results indicated that the central excitation band shifted to blue in the bismuth-codoped materials (0.5–4% in molar ratio), while it shifted to red in the zirconium-codoped materials. Such materials may find applications in white light-emission diode (LED) and tunable solid lasers.

Absolute location of lanthanide energy levels and the performance of phosphors

Abstract: Methods to construct the absolute position of the energy levels of divalent and trivalent lanthanides with respect to the valence band and conduction band of a host compound are available and can be applied to many different compounds. With the diagrams of lanthanide levels in SrAl 2 O 4 , LiYSiO 4 , and CaGa 2 S 4 it will be demonstrated how they can be used to understand phosphor performance. The quenching of Ce 3+ and Eu 2+ 5d–4f luminescence, the electron and hole trapping ability of the lanthanides, and the valence stability of lanthanide ions are discussed.