Synthesis and characterization of BaAl2O4:Eu co-doped with different rare earth ions

Combustion method was used in this study to prepare BaAl₂O₄:Eu²⁺ phosphors co-doped with different trivalent rare-earths (Re³⁺=Dy³⁺, Nd³⁺, Gd³⁺, Sm³⁺, Ce³⁺, Er³⁺, Pr³⁺ and Tb³⁺) ions at an initiating temperature of 600 °C. The phosphors were annealed at 1000 °C for 3 h. As confirmed from the X-ray diffraction (XRD) data, both as prepared and post annealed samples crystallized in the well known hexagonal structure of BaAl₂O₄. All samples exhibited bluish-green emission associated with the 4f⁶5d¹→4f⁷ transitions of Eu²⁺ at ∼500 nm. Although the highest intensity was observed from Er³⁺ co-doping, the longest afterglow (due to trapping and detrapping of charge carriers) was observed from Nd³⁺ followed by Dy³⁺ co-doping. The traps responsible for the long afterglow were studied using thermoluminescence (TL) spectroscopy.     

Structural,optical and photocatalytic applications of biosynthesized NiO nanocrystals

NiO is one of the most important candidates for semiconductors metal oxide nanocrystals by the arrangement of photocatalytic application. However, the photocatalytic performance of biosynthesized nanocrystals using Aspalathus linearis (Burm.f.) R. Dahlgren has not been investigated yet. In this contribution, we synthesize α-Ni(OH)₂ using an A. linearis. A heat treatment of the α-Ni(OH)₂ is carried out at 300–400°C for 2?h at normal air yields. Furthermore, we have characterized the structural, optical and photocatalytic activity of the samples. The optical results indicate that biosynthesized nanocrystals exhibit UV–visible light absorption and a narrow range distribution of intense green light (518.95?nm) emission, which decreases significantly as annealing temperature increases. The bandgap energies of the sample annealed at 300–400°C shift to lower photon energy, compared to bulk NiO (～ 4?eV). Moreover, the photocatalytic experimental results reveal that NiO nanocrystals enable color switching of methylene blue.     

The discretized flow on domains of attraction: a structural stability result

It is well known that, for stepsize sufficiently small, compactattractors of ordinary differential equations persist underdiscretization. The present paper describes the structure ofthe discrete-time dynamical system obtained via discretizationon A(M_h)\M_h where M_h is the approximate attractor and A(M_h)is its domain of attraction. The existence of a smooth embeddinginto a continuous-time parallelizable flow is proved. The constructioncan be used to define sections for discretizations and can beinterpreted as a justification of the method of modified equations.     

PL and CL degradation and characteristics of SrAl2O4: Eu,Dy phosphors

Strontium aluminate phosphors are ideal for luminescent infrastructure materials. Their brightness and persistent glow time are much higher than previously used sulphide phosphors. Strontium aluminates prepared by the sol–gel and combustion methods are compared with commercially available strontium aluminate. High luminescent efficient SrAl₂O₄:Eu²⁺,Dy³⁺ pulsed laser deposited (PLD) thin films were also produced using the commercially available powder. Photoluminescence (PL) degradation studies showed that the phosphor intensity decreased about 20% over a period of 2 weeks under ultraviolet (UV) irradiation. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) showed that cathodoluminescence (CL) degradation is due to the formation of SrO due to electron stimulated surface reactions. The light output mechanism of the phosphor is also discussed in more detail.     

Thermoluminescent and stuctural properties of BaAl2O4:Eu,Nd,Gdphosphors prepared by combustion method

BaAl₂O₄:Eu²⁺,Nd³⁺,Gd³⁺ phosphors were prepared by a combustion method at different initiating temperatures (400–1200 °C), using urea as a comburent. The powders were annealed at different temperatures in the range of 400–1100 °C for 3 h. X-ray diffraction data show that the crystallinity of the BaAl₂O₄ structure greatly improved with increasing annealing temperature. Blue-green photoluminescence, with persistent/long afterglow, was observed at 498 nm. This emission was attributed to the 4f⁶5d¹–4f⁷ transitions of Eu²⁺ ions. The phosphorescence decay curves were obtained by irradiating the samples with a 365 nm UV light. The glow curves of the as-prepared and the annealed samples were investigated in this study. The thermoluminescent (TL) glow peaks of the samples prepared at 600 °C and 1200 °C were both stable at ∼72 °C suggesting that the traps responsible for the bands were fixed at this position irrespective of annealing temperature. These bands are at a similar position, which suggests that the traps responsible for these bands are similar. The rate of decay of the sample annealed at 600 °C was faster than that of the sample prepared at 1200 °C.     

Photoluminescence and phosphorescence properties of MAl2O4:Eu, Dy (M=Ca, Ba, Sr) phosphors prepared at an initiating combustion temperature of 500 °C

Eu²⁺ and Dy³⁺ co-doped calcium aluminate, barium aluminate and strontium aluminate phosphors were synthesized at an initiating combustion temperature of 500 °C using urea as an organic fuel. The crystallinity of the phosphors was investigated by using X-ray diffraction (XRD) and the morphology was determined by a scanning electron microscope (SEM). The low temperature monoclinic structure for both CaAl₂O₄ and SrAl₂O₄ and the hexagonal structure of BaAl₂O₄ were observed. The effect of the host materials on the photoluminescence (PL) and phosphorescence properties were investigated by using a He-Cd Laser and a Cary Eclipse fluorescence spectrophotometer, respectively. The broad band emission spectra observed at 449 nm for CaAl₂O₄:Eu²⁺, Dy³⁺, 450 nm (with a shoulder-peak at 500 nm) for BaAl₂O₄:Eu²⁺, Dy³⁺ and 528 nm for SrAl₂O₄:Eu²⁺, Dy³⁺ are attributed to the 4f⁶5d¹ to 4f⁷ transition in the Eu²⁺ ion in the different hosts.     

Phosphorescent and thermoluminescent properties of SrAl2O4:Eu,Dy phosphors prepared by solid state reaction method

Long persistent SrAl₂O₄:Eu²⁺ phosphors co-doped with Dy³⁺ were prepared by the solid state reaction method. The main diffraction peaks of the monoclinic structure of SrAl₂O₄ were observed in all the samples. The broad band emission spectra at 497 nm for SrAl₂O₄:Eu²⁺, Dy³⁺ were observed and the emission is attributed to the 4f⁶5d¹ to 4f⁷ transition of Eu²⁺ ions. The samples annealed at 1100–1200 °C showed similar broad TL glow curves centered at 120 °C. The similar TL glow curves suggest that the traps responsible for them are similar. The long afterglow displayed by the phosphors annealed at different temperatures, may be attributed to the Dy³⁺ ions acting as the hole trap levels, which play an important role in prolonging the duration of luminescence.