The Effect of Temperature on the Operation of Quantum Well Laser: A Simulation Study Based on Three-Level Rate Equations

A temperature-dependent quantum well laser equivalent circuit model based on three-level rate equations is presented. The model is simulated using an ORCAD PSPICE circuit simulator. The characteristic properties such as threshold current, delay, and frequency response are simulated and compared with the available results. Included are temperature effect, leakage current, and heat dissipation effect on the chip. Threshold current increases, whereas turn on delay and optical output decrease with increasing temperature. An increase in temperature also reduces the operating frequency and resonance peak shifts to higher frequency. It is also observed that the modulation response of a QW laser with shorter separate confinement heterostructure region is more sensitive to the temperature variation.     

Oxalate-bridged dirhenium(I) hexacarbonyl complexes: synthesis, reactions, and crystal structures

The complex [{Re(CO)₅}₂(μ,η¹:η¹-C₂O₄)] 1 undergoes thermal decarbonylation to give [Re₂(CO)₆(C₂O₄)]_n, which reacts with triphenylphosphine and trans-1,2-bis(diphenylphosphino)ethylene (dppene) to give anti-[Re₂(PPh₃)₂(CO)₆(μ,η²:η²-C₂O₄)] 2 and [Re₂(μ-dppene)(CO)₆(μ,η²:η²-C₂O₄)] 4, respectively. Complex 2 is oxidized on prolonged exposure to air (1 week) to form anti-[Re₂(OPPh₃)₂(CO)₆(μ,η²:η²-C₂O₄)] 3. In the presence of excess dppene, the complex [Re₂(μ-dppene)₂(CO)₆(μ,η¹:η¹-C₂O₄)] 5 is also formed alongside 4. With the chelating diphosphine 1,3-bis(diphenylphosphino)propane (dppp), the complex [(η²-dppp)Re(CO)₃(μ,η¹:η¹-C₂O₄)Re(CO)₃(η²-dppp)] 6 is formed. The structures of 3 and 4 have been determined by X-ray crystallography. The dppene ligand in complex 4 adopts an unusual “syn” conformation wherein the two phosphorus lone pairs of electrons are eclipsed, thus forming an “A-frame” type of bridge.     

Micellization phenomena of biodegradable amphiphilic triblock copolymers consisting of poly(beta-hydroxyalkanoic acid) and poly(ethylene oxide)

This paper reports the studies on micelle formation of new biodegradable amphiphilic poly(ethylene oxide)-poly[(R)-3-hydroxybutyrate]-poly(ethylene oxide) (PEO-PHB-PEO) triblock copolymer with various PHB and PEO block lengths in aqueous solution. Transmission electron microscopy showed that the micelles took an approximately spherical shape with the surrounding diffuse outer shell formed by hydrophilic PEO blocks. The size distribution of the micelles formed by one triblock copolymer was demonstrated by dynamic light scattering technique. The critical micellization phenomena of the copolymers were extensively studied using the pyrene fluorescence dye absorption technique, and the (0,0) band changes of pyrene excitation spectra were used as a probe for the studies. For the copolymers studied in this report, the critical micelle concentrations ranged from 1.3 x 10(-5) to 1.1 x 10(-3) g/mL. For the same PEO block length of 5000, the critical micelle concentrations decreased with an increase in PHB block length, and the change was more significant in the short PHB range. It was found that the micelle formation of the biodegradable amphiphilic triblock copolymers consisting of poly(beta-hydroxyalkanoic acid) and PEO was relatively temperature-insensitive, which is quite different from their counterparts consisting of poly(alpha-hydroxyalkanoic acid) and PEO.     

Investigation of the influence of hydroxy groups on the radical scavenging ability of polyphenols

Recently, O-H bond dissociation enthalpies (BDEs) have been successfully used to express the free radical scavenging ability of polyphenolic antioxidants. In this work, the BDEs of phenol, catechol, resorcinol, hydroquinone, pyrogallol, phloroglucinol, 1,2,4-benzenetriol, and 5-hydroxypyrogallol have been calculated at B3LYP/6-311G++(3df, 3pd) and used to elucidate the effect of OH groups. Increasing the number of OH groups in the adjacent (vicinal) position decreases the BDE of phenols. Increasing the number of O-H groups in the alternative position C(1,3) as in resorcinol and C(1,3,5) as in phloroglucinol does not show any notable change in the BDEs when compared to that of OH in C(1) as in phenol. 5-Hydroxypyrogallol has the smallest BDE (250.3 kJ mol(-1)) followed by pyrogallol (289.4 kJ mol(-1)), then 1,2,4-benzenetriol (294.8 kJ mol(-1)), and then catechol (312.8 kJ mol(-1)). Overall, our results indicated that the presence of ortho and para hydroxy groups reduces the BDEs. An intramolecular hydrogen bond (IHB) develops due to the ortho arrangement of OH's and plays a dominant role in decreasing the BDEs. This key study on phenols showed that the reactive order of OH position in the benzene ring is the following: 5-hydroxypyrogallol > pyrogallol > 1,2,4-benzenetriol > catechol > hydroquinone > phenol approximately resorcinol approximately phloroglucinol.     

Preparation and characterization of polypseudorotaxanes based on block-selected inclusion complexation between poly(propylene oxide)-poly(ethylene oxide)-poly(propylene oxide) triblock copolymers and alpha-cyclodextrin

A series of new polypseudorotaxanes were synthesized in high yields when the middle poly(ethylene oxide) (PEO) block of poly(propylene oxide)-poly(ethylene oxide)-poly(propylene oxide) (PPO-PEO-PPO) triblock copolymers was selectively recognized and included by alpha-cyclodextrin (alpha-CD) to form crystalline inclusion complexes (ICs), although the middle PEO block was flanked by two thicker PPO blocks, and a PPO chain had been previously thought to be impenetrable to alpha-CD. X-ray diffraction studies demonstrated that the IC domains of the polypseudorotaxanes assumed a channel-type structure similar to the necklace-like ICs formed by alpha-CD and PEO homopolymers. Solid-state CP/MAS (13)C NMR studies showed that the alpha-CD molecules in the polypseudorotaxanes adopted a symmetrical conformation due to the formation of ICs. The compositions and stoichiometry of the polypseudorotaxanes were studied using (1)H NMR, and a 2:1 (ethylene oxide unit to alpha-CD) stoichiometry was found for all polypseudorotaxanes although the PPO-PEO-PPO triblock copolymers had different compositions and block lengths, suggesting that only the PEO block was closely included by alpha-CD molecules, whereas the PPO blocks were uncovered. The hypothesis was further supported by the differential scanning calorimetry (DSC) studies of the polypseudorotaxanes. The glass transitions of the PPO blocks in the polypseudorotaxanes were clearly observed because they were uncovered by alpha-CD and remained amorphous, whereas the glass-transition temperatures increased, because the molecular motion of the PPO blocks was restricted by the hard crystalline phases of the IC domains formed by alpha-CD and the PEO blocks. The thermogravimetric analysis (TGA) revealed that the polypseudorotaxanes had better thermal stability than their free components due to the inclusion complexation. Finally, the kinetics of the threading process of alpha-CD onto the copolymers was also studied. The findings reported in this article suggested interesting possibilities in designing other cyclodextrin ICs and polypseudorotaxanes with block structures.     

Mimicking both petal and lotus effects on a single silicon substrate by tuning the wettability of nanostructured surfaces

We describe a new method of fabricating large-area, highly scalable, "hybrid" superhydrophobic surfaces on silicon (Si) substrates with tunable, spatially selective adhesion behavior by controlling the morphologies of Si nanowire arrays. Gold (Au) nanoparticles were deposited on Si by glancing-angle deposition, followed by metal-assisted chemical etching of Si to form Si nanowire arrays. These surfaces were chemically modified and rendered hydrophobic by fluorosilane deposition. Au nanoparticles with different size distributions resulted in the synthesis of Si nanowires with very different morphologies (i.e., clumped and straight nanowire surfaces). The difference in nanowire morphology is attributed to capillary force-induced nanocohesion, which is due to the difference in nanowire porosity. The clumped nanowire surface demonstrated the lotus effect, and the straighter nanowires demonstrated the ability to pin water droplets while maintaining large contact angles (i.e., the petal effect). The high contact angles in both cases are explained by invoking the Cassie-Baxter wetting state. The high adhesion behavior of the straight nanowire surface may be explained by a combination of attractive van der Waals forces and capillary adhesion. We demonstrate the spatial patterning of both low- and high-adhesion superhydrophobicity on the same substrate by the simultaneous synthesis of clumped and straight silicon nanowires. The demonstration of hybrid superhydrophobic surfaces with spatially selective, tunable adhesion behavior on single substrates paves the way for future applications in microfluidic channels, substrates for biologically and chemically based analysis and detection where it is necessary to analyze a particular droplet in a defined location on a surface, and as a platform to study in situ chemical mixing and interfacial reactions of liquid pearls.     

Determination of volatile organic compounds in water using ultrasound-assisted emulsification microextraction followed by gas chromatography

Volatile organic compounds (VOCs) are toxic compounds in the air, water and land. In the proposed method, ultrasound-assisted emulsification microextraction (USAEME) combined with gas chromatography-mass spectrometry (GC-MS) has been developed for the extraction and determination of eight VOCs in water samples. The influence of each experimental parameter of this method (the type of extraction solvent, volume of extraction solvent, salt addition, sonication time and extraction temperature) was optimized. The procedure for USAEME was as follows: 15 μL of 1-bromooctane was used as the extraction solvent; 10 mL sample solution in a centrifuge tube with a cover was then placed in an ultrasonic water bath for 3 min. After centrifugation, 2 μL of the settled 1-bromooctane extract was injected into the GC-MS for further analysis. The optimized results indicated that the linear range is 0.1-100.0 μg/L and the limits of detection (LODs) are 0.033-0.092 μg/L for the eight analytes. The relative standard deviations (RSD), enrichment factors (EFs) and relative recoveries (RR) of the method when used on lake water samples were 2.8-9.5, 96-284 and 83-110%. The performance of the proposed method was gauged by analyzing samples of tap water, lake water and river water samples.     

Improved Hessian approximation with modified secant equations for symmetric rank-one method

Symmetric rank-one (SR1) is one of the competitive formulas among the quasi-Newton (QN) methods. In this paper, we propose some modified SR1 updates based on the modified secant equations, which use both gradient and function information. Furthermore, to avoid the loss of positive definiteness and zero denominators of the new SR1 updates, we apply a restart procedure to this update. Three new algorithms are given to improve the Hessian approximation with modified secant equations for the SR1 method. Numerical results show that the proposed algorithms are very encouraging and the advantage of the proposed algorithms over the standard SR1 and BFGS updates is clearly observed.     

Effective numerical evaluation of the double Hilbert transform

In this paper, we propose two methods to compute the double Hilbert transform of periodic functions. First, we establish the quadratic formula of trigonometric interpolation type for double Hilbert transform and obtain an estimation of the remainder. We call this method 2D mechanical quadrature method (2D-MQM). Numerical experiments show that 2D-MQM outperforms the library function “hilbert” in Matlab when the values of the functions being handled are very large or approach to infinity. Second, we propose a complex analytic method to calculate the double Hilbert transform, which is based on the 2D adaptive Fourier decomposition, and the method is called as 2D-HAFD. In contrast to the pointwise approximation, 2D-HAFD provides explicit rational functional approximations and is valid for all signals of finite energy.