P. Benalloul

Mn2+ Concentration Effect on the Optical Properties of Zn2SiO4 : Mn Phosphors

The optical properties of manganese-activated zinc silicate Zn 2(1-x) Mn 2x SiO 4 are investigated using pulse selective photon excitation. The light response due to the 4 T 1g - 6 6A 1g transition is directly linked to the relative concentration of isolated Mn 2+ and Mn 2+ pairs in this structure. The decay curves can be fitted by a diffusion-limited relaxation mechanism by considering two types of centers with radiative lifetimes no equal to 15 and 1.75 ms, respectively

IIA‐III2‐S4 ternary compounds: New host matrices for full color thin film electroluminescence displays

Strontium thiogallate (SrGa2S4) thin films have been prepared by rf reactive magnetron sputtering. This sulfur matrix permits the acceleration of charge carriers to optical energies under high electric field and provides green electroluminescence when doped with europium. The color coordinates are better than those achieved by other known green thin film electroluminescent phosphors. Strontium thiogallates as other ternary IIA‐III2‐(S,Se)4 compounds doped with Eu2+ or Ce3+ are good candidates for thin film electroluminescence phosphors of full color displays.

Luminescence, energy transfer, and upconversion mechanisms of Y 2 O 3 nanomaterials doped with Eu 3+ , Tb 3+ , Tm 3+ , Er 3+ , and Yb 3+ Ions

Luminescence, energy transfer, and upconversion mechanisms of nanophosphors (Y2O3:Eu3+, Tb3+, Y2O3:Tm3+, Y2O3:Er3+, Yb3+) both in particle and colloidal forms were studied. The structure, phase, and morphology of the nanopowders and nanocolloidal media were determined by high-resolution TEM and X-ray diffraction. It was shown that the obtained nanoparticles have a round-spherical shape with average size in the range of 4 to 20nm. Energy transfer was observed for Y2O3:Eu3+, Tb3+ colloidal and powders, upconversion transitions were observed for both Y2O3:Er3+ and Y2O3:Er3+, Yb3+ nanophosphors. The dependence of photoluminescence (PL) spectra and decay times on doping concentration has been investigated. The infrared to visible conversion of emission in Y2O3:Er3+, Yb3+ system was analyzed and discussed aiming to be applied in the photonic technology.

SrGa2S4: RE phosphors for full colour electroluminescent displays

Abstract Ternary thiogallate sulphides doped with rare-earth ions fulfill the electrical and optical requirements for thin film electroluminescent phosphors. These materials represent a breakthrough in the achievement of full colour EL displays. SrGa 2 S 4 provides a deep blue emission when doped with Ce 3+ and a saturated green with Eu 2+ . Some luminescent properties of Ce 3+ and Eu 2+ in SrGa 2 S 4 compounds are reviewed and the various techniques used to prepare thin films of this ternary compound are presented. The behaviour and performances of EL devices are discussed.

Luminescence of Eu2 + in Calcium Thiogallate

Luminescence, decay, and thermal quenching under photonic excitation of CaGa 2 S 4 :Eu 2+ mono and polycrystals have been measured. For excitation into the lower 5d states, thermal quenching starts above 350 K, which makes this material appropriate for use as a yellow phosphor under excitation by blue-emitting diodes. Thermoluminescence after 337.1 nm irradiation is explained by photoionization of Eu 2+ ions.

Ce-doped TiO2 insulators in thin film electroluminescent devices

Cerium-doped TiO2 thin films have been prepared by reactive RF sputtering. At low Ce concentration, X-ray diffraction indicates that the films have the anatase structure. Ce concentrations higher than 1.2 at.% result in an amorphization of the film which remains stable up to 873 K. The TiO2 electrical properties have been stabilized and improved by cerium doping, resulting in a lower conductivity (10-9 Ω-1m-1), a higher electrical breakdown strength (2 ×107 V/m), and a high value of the permittivity (45±5). The implementation of amorphous TiO2:Ce thin films as insulator layers in ZnS:Mn alternating current thin film electroluminescent devices (ACTFELD) results in a significant drop in the threshold operating voltage and a notable increase in the device brightness compared with ACTFELD containing Y2O3 or BaTa2O6 insulator layers. Rapid thermal annealing further improves the performance of the electroluminescent device.

Radiative Properties of the Blue BaAl2S4 : Eu2 + Phosphor

The optical properties of BaAl 2 S 4 doped with Eu 2+ were investigated. The synthesis was performed in evacuated and sealed quartz ampul at 1000°C during 1 h, then at 840°C during 20 h. With absorption, excitation, and emission spectra we have determined the crystal-field splitting, the redshift, the influence of the lattice vibration properties, and the thermal quenching of the luminescence. The crystal-field splitting of Eu 2+ in BaAl 2 S 4 was estimated on the order of 8000 to 10 000 cm -1 and the Huang-Rhys parameter S = 5 ± 1 corresponds to an intermediate electron-phonon coupling regime. The mean phonon energy for BaAl 2 S 4 was evaluated by optical and Raman methods, and the obtained values were 32 ± 5 and 40 meV, respectively. These results confirm the very interesting properties of this blue BaAl 2 S 4 :Eu 2+ phosphor, the most important being the very slight thermal quenching of the luminescence.

Manipulating emission of CdTeSe nanocrystals embedded in three-dimensional photonic crystals

We report experimental and theoretical results on the photoluminescence of CdTeSe nanocrystals, embedded in a silica opaline structure by infiltration of a highly diluted solution. Strong modification of emission diagrams of embedded nanocrystals have been observed in good agreement with theoretical models. At macroscopic scale, we measured the difference of nanocrystals emission lifetime embedded either in an opal for which the emission is in the gap, or in an opal of smaller balls diameter for which the emission is outside the gap. The photonic bandgap effect leads to a lifetime increase of the order of 10%. These lifetime variations are shown to be in good agreement with the calculated local density of states modification due to the pseudogap.

Raman investigation of orthorhombic MII Ga2 (S, Se)4 compounds

Raman investigation of isostructural polycrystalline MII Ga 2 (S, Se)4 compounds with MII = Pb, Sr, Eu, Yb and Ca was performed. We observed that the effect of MII atomic mass on the phonon frequencies is negligible and that the vibrations are essentially influenced by the sizes of the MII cations. As the effects of cation substitution are small we concluded that the orthorhombic thiogallates MII Ga 2 S4 exhibit nearly the same phonon energies: about 280 cm−1 (~35 meV) and 360 cm−1 (~45 meV) for the most intense vibration modes. The substitution of S atoms by heavier Se atoms causes the shift of the Raman spectrum to lower frequencies. The orthorhombic MII Ga 2 Se4 compounds present phonon energy at about 185 cm−1 (~23 meV) for the most intense vibration mode. Results confirm that a molecular model is more adequate to describe the vibrations of these compounds than the factor-group analysis.

Cerium Concentration and Temperature Dependence of the Luminescence of SrGa2 S 4 : Ce , Na , a Blue‐Emitting Material for Electroluminescent and High Current Density Cathodoluminescent Displays

The cerium concentration and temperature dependence of the spectral distribution, quantum yield, and lifetime of the Ce 3+ photoluminescence has been investigated in powders having the composition Sr, 2r Ce , Na r Ga 2 S 4 (x < 0.5). With increasing cerium concentration the emission shows a strong shift to longer wavelengths caused mainly by changes in the ligand field. Despite a relatively small Stokes shift (2000 cm -1 ), the emission shows a low concentration quenching up to x = 0.10. Because of the size differences between the Ce 3+ , Na + , and Sr 2+ ions, cerium ions possess various environments. Energy migration to defects is impeded by the transfer of the excitation energy to the ions having the lowest emitting states, whose emission exhibits reduced overlap with the absorption of other ions. As a result of the small Stokes shift, at low Ce concentration the luminescence is not affected by the thermal quenching up to 500 K. The low concentration quenching and the high thermal stability are favorable characteristics for use of this phosphor in electroluminescence and high current density CL screens.