Near infrared and charge transfer luminescence of Yb-doped LaPO4 at room temperature

The Yb³⁺-doped LaPO₄ was prepared by hydrothermal reaction under fine acidity control and identified by X-ray diffraction and FT-IR spectroscopy. The obtained powders crystallize in the monoclinic phase of LaPO₄. The spectroscopic study at room temperature (RT) of the Yb³⁺-doped LaPO₄ powder was investigated. Thus a wide band, characteristic of the fundamental ²F_5/2↔²F_7/2 transition in near infrared (NIR) range, has been located for La_(1−x)Yb_xPO₄ (x = 5, 10%). Four Stark levels of the ground ²F_7/2 state are located on the emission spectra between 976 nm and 1030 nm, after excitation at 925 nm. Low re-absorption of the 0-phonon transition was registered. Charge transfer band (CTB) luminescence of Yb³⁺, which is not observed in LaPO₄ in later works, was appeared under 266 nm excitation. In the UV–Visible spectra, double band typical for the CTB luminescence of Yb³⁺ are observed. The decay time dependence as a function of the concentration is also reported and compared to other works. The room temperature radiative lifetimes of the IR emission and charge transfer band luminescence are compatible with potential applications of this phosphor respectively as solid-state lasers and scintillators.     

Compositionally graded quaternary electron blocking layer for efficient deep ultraviolet AlGaN-based light-emitting diodes

Optical Review - We present the enhancement of ultraviolet (UV) light-emitting diodes (LEDs) using numerical analysis. We have employed a compositionally graded quaternary (AlInGaN) electron...     

Time-dependent density functional theory investigations on structural modification in carbazole-based organic photosensitizers to improve electron injection in dye-sensitized solar cell

The time-dependent density functional theory approach, implemented at hybrid-B3LYP, GGA-PBE, and density functional-based tight binding levels of theory, was used to model photoinjection in organic dye/TiO₂ quantum dot to explore the prospects of improvement of dye-sensitized solar cell (DSSC). The photosensitizer used in this study consisted of six carbazole-based organic dyes, a cyanoacrylic acid group as an acceptor and an oligothiophene π-bridge spacer. The modifications were made in the dyes by increasing the length of the spacer by adding thiophene and oxadiazole rings at different positions of the donor-acceptor bridge. The structural variations appeared to alter the electronic and optical properties of dyes studied via energy levels and excitation spectra. The UV-Vis spectra calculated for all the dyes in solvents exhibited a red shift in spectral peaks with an increase in the polarity of the solvents. The findings of the study pointed toward the indirect photoinjection of the dye-(TiO₂)₉₆ complex for six different dyes. The substitution of the oxadiazole ring at the center and addition of a thiophene ring at the edge of the spacer produced two dyes that exhibited the lowest injection energies of 0.11 and 0.17 eV, along with the regeneration energies of 1.18 and 1.12 eV, respectively. The dyes reported here may have promising applications in photoanode for enhancing the performance of DSSC.     

Microwave assisted non-aqueous sol–gel synthesis of LiNiPO4 and its copper doped analogues

A series of single phase LiNi_(1?x)Cu_xPO₄ (0 < x < 0.99) was successfully synthesized by non-aqueous sol–gel method followed by microwave (MW) annealing. The advantage of MW short span annealing is obvious from the nanometer sized particles compared to reported microparticles. XRD, IR and SEM studies were conducted to investigate the phase purity, crystal structure, lattice parameters and morphology respectively. It was found that 10 min of microwave annealing is a time and cost effective approach to produce single phase compositions at low temperature. X-ray studies confirmed the single phase formation of pure and the doped analogues which were found to be iso-structural with the parent LiNiPO₄ compound. The cell parameters with the addition of copper content were in accord with the Vegards law. Impedance studies pointed toward enhanced electrical properties of doped samples as compared to the undoped one.     

Synthesis and Characterization of Some Thallium Cyclopentadienide Fused Ring Pyridazines

Thallium cyclopentadiene (Cp) salts are important precursors for the synthesis of a variety of compounds having materials and commercial applications. The synthesis of 5,6-fused ring thallium Cp pyridazines 2a–d from previously reported 5,6-fused ring pyridazines is described herein. This route was quite general, and features an efficient and convenient two-step synthesis of a series of TlCp salts in a THF solution of thallium ethoxide. Full characterization of newly formed TlCp salts 3a–d are reported.     

Hölder Continuity of the Rotation Number for Quasi-Periodic Co-Cycles in $${SL(2, \mathbb R)}$$

We prove two results on the rotation number of the skew-product system \({(\omega ,A):(\theta ,y)\in\mathbb T^d\times\mathbb R^2\mapsto (\theta +\omega ,A(\theta)y)\in\mathbb T^d\times\mathbb R^2,}\) where ω is Diophantine and \({A(\theta)\in SL(2, \mathbb R)}\) is homotopic to the identity. On the one hand, we prove that this function has the behavior of a \({\frac{1}{2}-}\) Hölder function. On the other, we show that the length of the gaps has a sub-exponential estimate which depends on its label given by the gap-labeling theorem. We give also an estimate of the complement of the spectrum. These results are obtained by studying the reducibility of the quasi-periodic co-cycle (ω , A).     

Fukuyama reduction,Fukuyama coupling and Fukuyama–Mitsunobu alkylation: recent developments and synthetic applications

Fukuyama reaction for the synthesis of multifunctional aldehydes, secondary amines and ketones has gained considerable importance in synthetic organic chemistry because of mild reaction conditions. The use of thioesters in both Fukuyama aldehydes and ketones synthesis is highly attractive for organic chemists as they are easily accessible from corresponding carboxylic acids. Fukuyama–Mitsunobu reaction utilizes 2-nitrobenzenesulfonyl (Ns) for the protection/activation/deprotection of primary amines to afford secondary amines in good yields and high enantioselectivities. This review presents recent synthetic developments and applications of Fukuyama reaction for the synthesis of aldehydes, secondary amines and ketones.

Graphic abstract

     

Growth of Yb3+-doped KY3F10 concentration gradient crystal fiber by laser-heated pedestal growth (LHPG) technique

Crystal fiber of cubic Yb³⁺-doped KY₃F₁₀ has been grown. Both, mono-doped and concentration gradient fibers were grown by laser-heated pedestal growth (LHPG) method under argon atmosphere. The preparation of feed rod materials under CF₄ atmosphere prior to LHPG is efficient to obtain oxygen-free fluorides. Yb³⁺-doped KY₃F₁₀ crystals have excellent spectroscopic properties, weak self-quenching property and can be expected for solid-state laser application.     

Dual photonic band gap and reversible tuning of 3D photonic crystal fabricated by multiphoton polymerization with photoresponsive polymer

A 3D woodpile photonic crystal (PhC) composed of two parts with different periodic parameters was fabricated with a novel photoresist containing photoresponsive azobenzene moieties by multiphoton polymerization. Dual photonic band gaps (PBGs) were experimentally measured, which were attributed to two independent parts of the 3D PhC with different periodicities. Under light irradiation, PBG tuning of 36 nm was obtained, and this tuning behavior showed good reversibility.     

Convergence of Point Processes with Weakly Dependent Points

For each n≥1, let {X _j,n }_1≤j≤n be a sequence of strictly stationary random variables. In this article, we give some asymptotic weak dependence conditions for the convergence in distribution of the point process $N_{n}=\sum_{j=1}^{n}\delta_{X_{j,n}}For each n≥1, let {X _j,n }_1≤j≤n be a sequence of strictly stationary random variables. In this article, we give some asymptotic weak dependence conditions for the convergence in distribution of the point process N_n=?_j=1ⁿd_Xj,nN_{n}=\sum_{j=1}^{n}\delta_{X_{j,n}} to an infinitely divisible point process. From the point process convergence we obtain the convergence in distribution of the partial sum sequence S _n =∑ _j=1 ⁿ X _j,n to an infinitely divisible random variable whose Lévy measure is related to the canonical measure of the limiting point process. As examples, we discuss the case of triangular arrays which possess known (row-wise) dependence structures, like the strong mixing property, the association, or the dependence structure of a stochastic volatility model.