A General Formulation Encompassing the Effects of Salts and Micelles (Direct and Reverse) on Ionic Reactions

Given the common behavior of ionic reactions in micellar and salt solutions and in microemulsions, a general approach has been developed for the interpretation of kinetic results in these media. This approach takes as a starting point the Brønsted equation. It has been checked by employing kinetic results for cation/cation \(([\mathrm{Ru}(\mathrm{NH}_{3})_{5}\mathrm{py}^{2+}] + [\mathrm{Co}(\mathrm{NH}_{3})_{4}\mathrm{pzCO}_{2}^{2+}])\), anion/anion \((\mathrm{I}^{-}+ \mathrm{IrCl}_{6}^{2-})\) and cation/anion \(([\mathrm{Ru}(\mathrm{NH}_{3})_{5}\mathrm{py}^{2+}] + \mathrm{S}_{2}\mathrm{O}_{8}^{2-})\) reactions. The approach can be easily generalized to cases in which more than two pseudophases (or more than one receptor) are present in the reactive system, as well as cases in which the reaction can follow more than two reaction paths. The approach is consistent with (but more general than) the Pseudophase and related models, such as the Pseudophase Ion Exchange Model.     

Free-carrier absorption modulation in silicon nanocrystal slot waveguides

Free-carrier absorption (FCA) has proven to be an important obstacle in the development of a silicon-based laser; however, FCA may serve as a potential advantage in active silicon-based switches or modulators. In this work, we present FCA modulation in slot waveguides with silicon nanocrystals (Si-ncs) embedded in SiO(2) as the low-index slot material. Slot waveguides were fabricated with and without Si-ncs, and the presence of Si-ncs was shown to increase the pump-induced FCA loss in the waveguides by a factor of 4.5. We modeled the Si-nc material using a four-level rate equation analysis to estimate the excited population of Si-ncs, allowing us to extract a value of 2.6 × 10(-17) cm(2) for the FCA cross section of the Si-nc material.     

A note on sequences lying in the range of a vector measure valued in the bidual

Let be a Banach space. It is unknown if every subset of lying in the range of an -valued measure is actually contained in the range of an -valued measure. In this paper we solve this problem in the case when we consider only vector measures of bounded variation.

     

Thermal evolution of (Zr,Ti)O2 gels synthesized at different basicpH

(Zr, Ti)O₂ gels as precursors of zirconium titanate (ZT) powders were prepared by the sol-gel method, which involves hydrolysis of ZrClO₂+TiCl₄ in the presence or absence of hydrogen peroxide, in twopH ranges, 8–9 or 11–12. Thermal evolution of these precursors has been studied by DTA, BET surface area, XRD and SEM. Differences in DTA curves, surface area and morphology were observed. In particular, ZT prepared atpH=8–9 with hydrogen peroxide was purer, more homogeneous and presented a different morphology compared to ZT processed atpH=11–12; hydrated/hydroxylated Zr?Ti species were formed to a greater extent in the latter case. Addition of hydrogen peroxide during chemical processing of these gels plays a key role in controlling the crystallization temperature of ZT.     

Preparation and Characterization of Amorphous ZrO2-SiO2 Composite Powders Processed by Sol-Gel Chemistry

Composite ZrO₂-SiO₂ powders, with different ZrO₂ contents, including pure ZrO₂ powders, were prepared by precipitation in SiO₂ suspensions, of zirconia gels from solutions of zirconyl chloride at pH = 11. These products were investigated in connection with the phase changes in ZrO₂ caused by heat-treatments. ZrO₂-SiO₂ mixtures containing 0–100% mol ZrO₂, were studied by differential thermal analysis (DTA), X-ray powder diffraction (XRD), temperature programmed desorption combined with mass spectroscopy (TPD-MS), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX), to obtain information on the morphological and structural features of the particles before and during the heat treatment up to 1200°C. Specific surface areas were determined using nitrogen adsorption by the BET method. The results offer an explanation about some of the factors which can be influencing on the stabilization of metastable-cubic/tetragonal (C/T) phase of ZrO₂ and the evolution of surface areas (vulcano profile) observed in the composites.     

Design and characterization of silicon nanocrystal microgear resonators

We report on the design, optimization and characterization of silicon nanocrystal microgear resonators. We present three-dimensional finite-difference time-domain simulations to optimize the gear geometry and guide our experimental study. We fabricated a series of microgears with varying geometry and compared their photoluminescence spectra to that of a reference microdisk. The microgears exhibited a single dominant mode in the photoluminescence spectrum with quality factors as high as 10³. We further demonstrated the ability to tune the wavelength of the dominant mode by changing the number of gear teeth and thereby selecting a mode with a different azimuthal order.     

Femto to millisecond observations of indole-based squaraine molecules photodynamics in solution

We present femto-to-millisecond studies of the photodynamics of seven types of indole-based squaraine molecules (SQs) in solvents of different H-bonding ability and viscosity. These SQs can be classified into two families: SQs with two carboxylic groups in the side indole groups (symmetrical SQs) and with only one carboxylic group (asymmetrical SQs). Steady-state absorption and fluorescence techniques show narrow absorption and emission bands, with a small Stokes shift (about 300 cm(-1)). The femtosecond transient absorption spectra give a very short (～100 fs) dynamics (assigned to IVR) and the associated spectra show two excited species assigned to two stereoisomers. A trans-cis photoisomerization occurs in a very fast time through a conical intersection. Pico-to-nanosecond emission experiments also reveal the presence of two fluorescing trans stereoisomers whose lifetimes show similar sensitivities to the nature of solvent. For example, lifetimes of 1.72, 0.46 and 0.29 ns were determined for the trans photoisomer of the SQ 41 in triacetin, dichloromethane and acetonitrile, respectively, reflecting the short decay of the S(1) state in highly polar and low viscous solvents. Flash photolysis experiments gave the transient absorption signals of the cis photoisomer that is formed after the twisting process at S(1). The cis-to-trans photoisomerization at the ground state happens in the μs time scale (1-4 μs), and it depends on the H-bonding ability and viscosity of the solvent. Thus, combining fs-ns and ns-μs experiments suggests that in the conical intersection region, only a small fraction of the twisted trans isomers are converted to the cis ones in the excited states. These results bring detailed and global insight into the large time window photodynamics of this family of SQs in solution.     

Tunable photoluminescence and electroluminescence of size-controlled silicon nanocrystals in nanocrystalline-Si/SiO2 superlattices

We present photoluminescence and electroluminescence of silicon nanocrystals deposited by plasma-enhanced chemical vapor deposition (PECVD) using nanocrystalline silicon/silicon dioxide (nc-Si/SiO₂) superlattice approach. This approach allows us to tune the nanocrystal emission wavelength by varying the thickness of the Si layers. We fabricate light emitting devices (LEDs) with transparent indium tin oxide (ITO) contacts using these superlattice materials. The current-voltage characteristics of the LEDs are measured and compared to Frenkel-Poole and Fowler-Nordheim models for conduction. The EL properties of the superlattice material are studied, and tuning, similar to that of the PL spectra, is shown for the EL spectra. Finally, we observe the output power and calculate the quantum efficiency and power conversion efficiency for each of the devices.     

Detours in the Development of the Organocatalytic Diels-Alder Reactions of Vinylboronic Compounds. A Dead End?

Attempts to improve the organocatalytic Diels-Alder reactions of vinylboronic compounds are described. Further experiments, theoretical calculations and NMR studies were performed to optimize the existing procedure and analyze the reactivity of the reactive intermediate and the ligand-exchange process. In particular, the relationship between the structure and the performance of the organocatalysts was studied theoretically and experimentally. Although the catalytic activity of the α-hydroxy acids could not be improved much, a deeper insight has been gained, which might be useful for the development of other organocatalytic reactions of organoboron compounds.