Photo-induced absorption spectra of a poly(p-phenylenevinylene) polymer with fluorinated double bonds

We report on the optical and photo-physical properties of a fluorinated poly(p-phenylenevinylene) derivative, namely poly(2-methoxy-5-[(2-ethylhexyl)oxy]-1,4-phenylenedifluorovinylene) (MEH-PPDFV) and we compare them with those of the well-known non fluorinated reference system, poly(2-methoxy-5-[(2-ethylhexyl)oxy]-1,4-phenylenevinylene) (MEH-PPV) synthesized with the same procedure.Remarkable blue-shifts of absorption and emission spectra of MEH-PPDFV with respect to MEH-PPV are observed, due to the larger distortion from planarity of the fluorinated species. In addition, the blue-shift for optical transitions in absorption spectra is more pronounced than in emission ones, since the molecule conformation in the excited states is more planar than in the ground one.Microsecond photo-induced absorption spectra of MEH-PPDFV reveal three bands possessing spectral properties and/or kinetics different from those observed for MEH-PPV. The assignment of these transitions is provided in terms of free and trapped polarons as well of triplet excitons. Free-polarons have an intrinsic decay constant of 3 μs, while the trapping dynamics has a longer kinetics of 40 μs. The triplet exciton, instead, shows a monoexponetial decay behavior of constant about 10 μs.     

High-efficiency phosphor-in-glass with ultra-high color rendering indexing for white laser diode lighting

Stable color converters exhibiting high color rendering index have drawn researchers’ attention for their applications in high-quality white laser lighting. In this study, we develop the multi-color phosphor-in-glass (PIG) with the weight ratio of green-emitting (Y₃Al_3.08Ga_1.92O₁₂:Ce³⁺) to red-emitting (CaAlSiN₃:Eu²⁺) phosphor powders (10/1–18/1) by low temperature co-sintering method. The obtained composite material displays an outstanding optical and thermal performance, including a high internal quantum efficiency of 84.2%, a high transparency of 45% in the visible region and a low thermal quenching (it remains 86% at 448 K). By integrating 450 nm blue laser diodes with optimized multi-color PIG, the white light with a maximum luminous flux of 258 lm and a luminous efficiency of 137 lm/W is achieved for the first time. Additionally, considering the white balance, by tailoring the weight ratio of green-emitting to red-emitting phosphor and the thickness of PIG, the 14/1 PIG at fixed thickness of 0.75 mm produces pure white light with ultra-high color rendering index of 95.2 and a high luminous efficiency of 120.9 lm/W under power density of 2.39 W/mm² irradiation. The above superior characteristics imply that the multi-color PIG is an ideal candidate for high-quality white laser lighting applications.     

Preparation and characterization of nano-porous silica aerogel from rice husk ash by drying at atmospheric pressure

Silica aerogel, a mesoporous material, was prepared from rice husk ash by sol–gel method and dried under atmospheric pressure. In this method, rice husk ash, which is rich in silica, was extracted with sodium hydroxide solution to produce a sodium silicate solution. This solution was neutralized with acid to form a silica hydrosol, and a small amount of tetraethyl silicate (TEOS) to form a gel. The aged gel was washed carefully by distilled water and ethanol and finally dried under atmospheric air. The prepared silica aerogels were characterized by XRF, FT-IR, TG, DTA, DTG, XRD, BET and SEM measurements. The synthesized TEOS-doped silica aerogel was a light solid with specific surface area of 315 m²/g, pore volume 0.78 cm³/g, average pore size 9.8 nm, bulk density 0.32 g/cm³ and porosity 85%.     

Energy transfer and control of color luminescence in Dy3+/Tb3+ co-doped transparent oxyfluorosilicate glass ceramics containing LiYF4 nanocrystals

Dy³⁺/Tb³⁺ co-doped oxyfluoride glass ceramics (GC) containing lithium tetrafluoroyttrium (LiYF₄) and yttrium fluoride (YF₃) nanograins were prepared by a traditional melt-quenching approach. The crystalline phases and morphologies of the GCs were investigated. The GCs showed enhanced green emission from Tb³⁺ ions and the quenching of Dy³⁺ emission because of the efficient energy transfer from Dy³⁺ to Tb³⁺ ions. The electric dipole-dipole interactions were responsible for the energy transfer mechanism between Dy³⁺ and Tb³⁺ ions. The energy transfer efficiencies of these glass ceramics were calculated from their decay curves. The Dy³⁺/Tb³⁺ co-doped oxyfluoride GCs with various visible emissions were found to be potential candidates for white-light emitting diode applications.     

Effect of sputtering power on the electrical and optical properties of Ca-doped ZnO thin films sputtered from nanopowders compacted target

In the present work, we have deposited calcium doped zinc oxide thin films by magnetron sputtering technique using nanocrystalline particles elaborated by sol–gel method as a target material. In the first step, the nanoparticles were synthesized by sol–gel method using supercritical drying in ethyl alcohol. The structural properties studied by X-ray diffractometry indicates that Ca doped ZnO has a polycrystalline hexagonal wurzite structure with a grain size of about 30 nm. Transmission electron microscopy (TEM) measurements have shown that the synthesized CZO is a nanosized powder. Then, thin films were deposited onto glass substrates by rf-magnetron sputtering at ambient temperature. The influence of RF sputtering power on structural, morphological, electrical, and optical properties were investigated. It has been found that all the films deposited were polycrystalline with a hexagonal wurtzite structure and preferentially oriented in the (0 0 2) crystallographic direction. They have a typical columnar structure and a very smooth surface. The as-deposited films show a high transmittance in the visible range over 85% and low electrical resistivity at room temperature.     

Effects of vacancies on electronic and optical properties of GaN nanosheet: A density functional study

In this work, electronic and linear optical properties of GaN nanosheet are investigated through density functional theory and the dielectric tensor is derived within the random phase approximation (RPA). Electronic band structure calculations indicate that Ga atom vacancies in the 2D GaN system leads to n-type semiconductor, whereas the N atom vacancies change the electronic structure to a p-type semiconductor. Dielectric function, absorption coefficient, optical conductivity, extinction index, reflectivity, loss function and the refraction index of the GaN nanosheet and the Ga-defected or N-defected GaN systems are studied for both parallel (E∥x) and perpendicular (E∥z) applied electric field polarizations. Our results confirm that GaN nanosheet is a semiconductor which is sensitive to the type of vacancies and shows optical anisotropy at both polarizations states.     

Progress in discovery and structural design of color conversion phosphors for LEDs

Phosphor materials enable the optical frequency conversion to realize the full-color white emission light-emitting diodes (LEDs). So far much effort has been devoted to the design and discovery of novel LED phosphors for solid state lighting. In this review, firstly, we briefly describe several representative families of LED phosphors. Secondly, we propose the design methodology aimed at discovery of new phosphors with focus on the crystal structural considerations. Thirdly, we review the results of our work and other researchers on the recent advances in discovery and structural design of LED phosphors that exemplify the adopted strategies, including (1) design of the novel phosphors from the existed structural models, (2) discovery of novel phosphors from new crystal materials by doping and (3) structural modification of the known phosphors. The importance on the structure-property relations and recently reported methodologies involved in the crystal chemistry analysis for the discovery of LED phosphors, including mineral-inspired structural model design, exploratory crystal growth via single particle diagnostic approach, chemical unit cosubstitution, and so on, have been summarized in this review. We finally discuss the topics of structure-related active investigations and future opportunities for new and improved host materials for the color conversion applied in LEDs.     

Study of tunnel pavements behaviour in fire by using coupled cone calorimeter – FTIR analysis

In recent years there has been a growing interest in analyzing the contribution of pavements to fire growth for improving safety in tunnels. However, only few analyses take into account or quantifying toxic gases emitted during the pavement burn out. In this study, simultaneous cone calorimeter and FTIR analyses were conducted to evaluate the contribution to fire growth of two different types of fireproof pavements (concrete and asphalt) obtaining averaged values of heat release rate per unit area of 0 and 50 kW/m² respectively. The CO released was monitored as a valuation of how complete is the combustion taking place and also to compare the toxic potential of such materials. Further approximated ignition temperatures of asphalt in the range of 420–450 °C were also obtained. The results indicate that concrete pavement do not contribute to fire growth since no ignition was observed while asphalt pavement contributes similarly to other components generally found in vehicles. Very opaque fumes with significant concentrations of CO were detected during asphalt pavement combustion. Severe thermal degradation was observed in the asphalt pavement samples, including calcination and the detachment of aggregates while on the surface of concrete pavement samples just some minor cracks were reported.     

Operating conditions of OOK signal regenerator with feed-forward control

The optimum operating conditions for a signal regenerator employing a hybrid optical-electrical scheme for intensity-modulated signals are experimentally investigated. The scheme comprises an optical coupler, a photo-detector, and a Mach–Zehnder intensity modulator, and can suppress the intensity noise, accompanying extinction ratio degradation. It is shown that the operating conditions that achieve the lowest bit error rate (BER) after regeneration depend on the extinction ratio and the optical signal-to-noise ratio of the input signal. We also investigate the noise suppression effect under conditions that do not change the extinction ratio.     

Amphiphilic light-emitting dendrons with oligo(phenylene vinylene) branches and oligo(ethylene oxide) terminal chains

We have recently synthesized new amphiphilic light-emitting dendrons with oligo(phenylene vinylene) core–branches and oligo(ethylene oxide) terminal chains (designated as: OPPV-d-OPEO). Due to the amphiphilic properties arising from the large difference in solubility between the two components, unique concentration- and temperature-dependent photoluminescence emissions were observed for aqueous solutions of the amphiphilic conjugated dendrons. The synthesis and amphiphilic behaviour of the OPPV-d-OPEO in solution are reported in this paper. Effects of the amphiphilic behavior on fluorescence emissions of the dendrons are discussed.