Cr doping optimization in CaAl2O4:Eu blue phosphor

A new composition of phosphor material CaAl₂O₄:Eu²⁺ co-doped with Cr³⁺ was investigated. Various compositions with Eu²⁺ (1 and 2 mol%) and Cr³⁺ (0.05–0.1 mol%) were prepared by solid-state reaction method. These compositions show high brightness and longer persistent luminescence. Phase and crystallinity were investigated by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Excitation and emission spectra were taken to investigate the luminescence characteristics. Broad band UV excited luminescence of the CaAl₂O₄:Eu²⁺, Cr³⁺ was observed in the blue region (λ_max = 440 nm) due to transitions from 4f⁶5d¹ to the 4f⁷ configuration of the Eu²⁺ ion. Cr³⁺ ion co-doping generates deep traps which results in longer afterglow phosphorescence compared to parent phosphor.     

Spectroscopic properties of Yb in NaYbP2O7 diphosphate single crystals

Yb³⁺ absorption and fluorescent emissions were investigated for NaYbP₂O₇ diphosphate single crystals. The interpretation of electronic energy level positions has been done by using the comparison of absorption and emission spectra with those of vibronic sideband energy positions from Raman and IR absorption spectroscopies. The Yb³⁺ energy levels scheme in this host was drawn. The decay time for infrared Yb³⁺ (⁵F_5/2 excited state) fluorescence was in the range of 1–2 ms. The interest feature is leading to broad emission band at room temperature from 940 nm to 1100 nm, which seems of high interest for ultra-short laser pulse production.     

Effects of sucrose concentration on morphology and luminescence performance of Gd2O3:Eu nanocrystals

Red Gd₂O₃:Eu nanophosphors were prepared by novel sucrose-assisted combustion method. The sucrose hydrolysis and complexing procedures were discussed by the Fourier transform infrared spectroscopy (FTIR) analysis. The effect of sucrose-to-metal ratio (S:M) on morphology and luminescence intensity of Gd₂O₃:Eu nanocrystals were investigated. And the optimum ratio was found to be S:M = 7:1, as confirmed by the results of scanning electron microscope (SEM), transmission electron microscope (TEM) and photoluminescence (PL) measurements. The highest photoluminescence emission from the particles obtained at S:M = 7:1 was about 85% of the commercial red phosphors.     

Syntheses,structures, and luminescent properties of cadmium (II) complexes: 3D supramolecular [Cd(phen)(NO3)(NO2)(H2O)]n and Cd(phen)2(NO3)(NO2) constructed by π–π stacking interactions

Two novel complexes [Cd(phen)(NO₃)(NO₂)(H₂O)]_n (1) and Cd(phen)₂(NO₃)(NO₂) (2) (phen = 1,10-phenanthroline) have been synthesized by the reductive reaction of metal source Cd(NO₃)₂·4H₂O with phen and benzidine in the mixed solution of DMF, ethanol and water. Crystal structures of complexes 1 and 2 were determined by single-crystal X-ray diffraction. Complex 1 is a one-dimensional (1D) zig-zag infinite chain in which (phen)Cd^(II) units were bridged by two O atoms of NO₂⁻. The three-dimensional (3D) supramolecular structure of 1 is constructed through hydrogen-bond and aromatic π–π stacking interactions between adjacent metal–organic polymeric coordination chains. Complex 2 is a mononuclear structure, and self-assembled through π–π stacking interactions to form a three-dimensional (3D) supramolecular structure. The complex 1 exhibits luminescent property in the near-UV at room temperature in solutions of DMSO and DMF with the emission energy following the order DMF < DMSO, which might be ascribed to the presence of a highly polarized ground state. Complexes 1 and 2 have blue-purple luminescence at room temperature in the solid state. The blue-purple luminescence of the complexes is due to π* → π transition of phen.     

Luminescence properties of Eu ions in K2YF5 crystals

Luminescence properties of Eu³⁺ ions (0.1, 1.0 and 6.0 mol%) in K₂YF₅ crystals are investigated by the site-selective laser-excitation spectroscopy. The excitation spectrum of the ⁵D₀ emission exhibits two strong ⁷F₀ → ⁵D₀ excitation lines together with four weaker lines. Six different emission spectra are obtained under the excitation at each excitation line indicating that there existed at least six different sites for Eu³⁺. The relative intensity of the excitation lines is dependent on the Eu³⁺ concentrations. The ⁵D₀ emissions of Eu³⁺ show two types of decay behavior with lifetimes of 0.5 and 5.0 ms at 15 K. It is suggested that the fast and slow decays correspond to isolated and paired Eu³⁺ ions in K₂YF₅. The results are discussed in relation with one dimensional chain structure of the K₂YF₅ lattice.     

Broad band photoluminescence studies of diamond layers grown by hot-filament CVD

Photoluminescence and Raman spectroscopy were employed to investigate the broad band luminescence in thin diamond films grown on a silicon substrate by the HF CVD technique. The broad band luminescence with a maximum emission at 1.8–2 eV observed for CVD diamonds is characteristic for amorphous carbon with sp²-hybridized carbon bonds. As was shown by the Raman spectroscopy our diamond layer contained certain amounts of amorphous carbon phase and diamond nanocrystals which were the source of an additional energy state within the diamond energy gap. The experimental results precluded the possibility of broad band luminescence being due to the electron–lattice interaction. The amorphous carbon and diamond nanocrystals admixture in polycrystalline diamond layer introduced a defect state in the energy gap not in the form of point defects but rather in the form of a line or extended defects. In consequence these extended defects were responsible for the broad PL spectrum in the CVD diamond films.     

Co-precipitation synthesis of Nd:YAG nanopowders II: The effect of Nd dopant addition on luminescence properties

Nanopowders of Yttrium Aluminium Garnet (Y₃Al₅O₁₂, YAG) doped with neodymium (Nd:YAG, 0.2–24.0 at.%) were prepared using the co-precipitation method followed by an annealing treatment up to 950 °C. For a concentration of neodymium lower than 3.2 at.% the materials were found constituted by the garnet phase according to X-ray diffraction investigations. However, at higher neodymium loading the hexagonal and monoclinic forms of yttrium aluminium oxides were found together with the garnet phase.For Nd quantity lower than 0.8% the luminescence emission spectra appear to be nearly the same, indicating that in the examined range of composition the immediate surrounding of the emitting Nd³⁺ ions is independent of its concentration. For Nd quantity higher than 0.8%, the emission lines have nearly the same peak position, but the band-widths are larger and the relative line intensity increases with neodymium content. A dramatic decrease of two magnitude orders of the luminescence lifetimes occurs with the Nd quantity increase. This behaviour is ascribable to the presence of a secondary phase together with the quenching effects.     

Copolymers containing pendant styryltriphenylamine and carbazole groups: Synthesis, optical, electrochemical properties and its blend with Ir(ppy)3

A series of vinyl copolymers (PVKST12-PVKST91) and homoploymer PVST containing pendant hole-transporting 4-(4-oxystyryl)triphenylamine (12-100 mol%) and carbazole chromophores were synthesized by radical copolymerization and employed as host for Ir(ppy)₃ phosphor to tune emission color. They were characterized using the ¹H NMR, FT-IR, absorption and photoluminescence spectra, elemental analysis, GPC, cyclic voltammetric and thermal analysis (DSC, TGA). Their weight-average molecular weights (M_w) and decomposition temperatures (T_d) were 1.46-5.68 × 10⁴ and 356-399 °C, respectively. The HOMO levels of PVKST12-PVKST91 and PVST, estimated from the onset oxidation potentials in cyclic voltammograms, were −5.40 to −5.14 eV, which are much higher than −5.8 eV of the conventional host poly(9-vinylcarbazole) (PVK) owing to high hole-affinity of the 4-(4-oxystyryl)triphenylamine groups. Therefore, copolymers PVKST are effective in reducing hole-injection barrier between the PEDOT:PSS and emitting layer. Electroluminescent devices [ITO/PEDOT:PSS/PVKST:Ir(ppy)₃:PBD/BCP/Ca/Al] using the hole-transporting PVKST as host were fabricated to tune the emission color. Their EL spectra showed a major emission at 515 nm and a minor peak at 435 nm attributed to Ir(ppy)₃ and 4-(4-oxystyryl)triphenylamine, respectively. The C.I.E. 1931 coordinates shift from (0.29, 0.61) for PVK to (0.33, 0.42) for PVST with an increase in 4-(4-oxystyryl)triphenylamine content.     

Luminescence properties of Ca10K(PO4)7:RE (RE = Ce, Tb, Dy, Tm and Sm) under vacuum ultraviolet excitation

A series of RE³⁺ (RE = Ce, Tb, Dy, Tm and Sm) activated Ca₁₀K(PO₄)₇ were synthesized by conventional state reaction and their photoluminescence properties under vacuum ultraviolet excitation were investigated. The PO₄³⁻ absorption lies within the range from 125 to 180 nm in all the excitation spectra of Ca₁₀K(PO₄)₇:RE³⁺. The first f-d transition of Ce³⁺ is observed at 316 nm, and the Ce³⁺ emission is located at about 350 nm. Both the first spin-allowed and spin-forbidden f-d transitions of Tb³⁺ are situated at 232 and 263 nm, respectively. The emission spectrum of Ca₁₀K(PO₄)₇:Tb³⁺ exhibits typical Tb³⁺ emissions with the predominant peak at 544 nm. The O²⁻-Dy³⁺ charge transition band was calculated and identified around 173 nm, the CIE chromaticity coordinates of Dy³⁺ were calculated to be 0.364 and 0.392. The Ca₁₀K(PO₄)₇:Tm³⁺ demonstrates the strongest excitation at about 182 nm assigned to O²⁻-Tm³⁺, and gives the predominant emission at 453 nm. The ⁴G_5/2-⁶H_7/2 transition of Sm³⁺ at 601 nm is the most intensive in the emission spectrum.     

Photoluminescence,thermoluminescence and reversible photoluminescence modulation of multifunctional optical materials Pr3+ doped KxNa1-xNbO3 ferroelectric ceramics

It is highly significant to develop multifunctional optical materials to meet the huge demand of modern optics. Usually, it is difficult to realize multiple optical properties in one single material. In this study, we choose ferroelectric (K_xNa_1-x)NbO₃:Pr³⁺ (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) as hosts, and the rare earth ions Pr³⁺ are doped in them. For the first time, the integration of photoluminescence, photochromism, luminescence modulation and thermoluminescence and has been achieved in the Pr³⁺ doped (K_xNa_1-x)NbO₃:Pr³⁺ ferroelectric ceramics. Upon 337- or 448-nm light irradiation, all samples show strong red emissions centered at 610 nm. The photochromic reaction increases with the increasing K⁺ content in the (K_xNa_1-x)NbO₃:Pr³⁺ ceramics. A strong photochromic reaction has been found in the (K_0.5Na_0.5)NbO₃:Pr³⁺ ceramics. Accordingly, a large and reversible photoluminescence modulation (ΔR_t = 50.71%) is achieved via altering 395-nm-light irradiation and 200 °C thermal stimulus. All the prepared ceramics show a visible thermoluminescence when stimulated at 200 °C. The mechanisms of luminescence modulation and thermoluminescence are discussed. Present study could provide a feasible paradigm to realize multiple optical properties in one single material.