Electric field induced non-linear multisubband electron mobility in V-shaped asymmetric double quantum well structure

ABSTRACT

We study the effect of the external electric field F_ext on the low-temperature electron mobility μ in an asymmetrically doped Al_xGa_1-xAs based V-shaped double quantum well (VDQW) structure. We show that nonlinearity of µ occurs under double subband occupancy on account of intersubband effects. The field F_ext alters the VDQW potential leading to transfer of subband wave functions between the wells, which affects the scattering potentials and hence μ. In the VDQW structure, due to the alloy channel layer, the alloy disorder (Al-) scattering happens to be significant along with the ionised impurity (Imp-) scattering. The non-linear behaviour of μ is because of μ^Imp, while the overall magnitude of μ is mostly due to μ^Al. The increase of difference in the doping concentrations of the outer barriers increases the nonlinearity of μ. The oscillatory character of μ is amended by varying the width of the well and barrier and also the height of the VDQW. Our results can be used to study VDQW based nanoscale field effect transistor structures.     

Facile access to polymer supported zinc–salen complex: highly efficient heterogeneous catalyst for synthesizing hydantoins,thiohydantoins and Schiff bases in aqueous medium

Research on Chemical Intermediates - The synthesis of polymer supported zinc–salen complex (PS-Zn–salen) is described. The mononuclear zinc(II)–salen complex was characterized by...     

Latest Developments in the Catalytic Application of Nanoscaled Neutral Group 8–10 Metals

In the last few decades, the synthesis and catalytic application of nanoscaled particles prepared from Group 8–10 (formerly Group VIIIB) elements have been widely explored and have achieved promising results. The innovative use of these nanoparticle catalysts may provide new opportunities in the efficient combination of conventionally used homogenous and heterogeneous catalysts. Conventional homogeneous catalysts pose extraction and recycling difficulties when dealing with metal complexes and/or ligands, whereas heterogeneous catalysts generally require more pressing experimental conditions, such as high temperatures and high pressures, to be effective. Therefore, to solve these problems, the synthesis and use of nanoparticle catalysts as replacements for conventional catalytic systems is a breakthrough owing to their improved handling and environmental and economic aspects. This Focus Review primarily addresses the catalytic applications of neutral Group 8–10 nanoparticles with an average size of less than 10 nm, and also includes a discussion of commonly used synthetic methodology.     

Soluble and processable conjugated polyazines with oligo(p-phenylene vinylene)s

An efficient polycondensation reaction between α,ω-diformyl functional aromatics, namely 2,5-diheptyloxy-1,4-diformylbenzene and side-chain substituted α,ω-diformyl oligo(p-phenylene vinylene) (OPV) with hydrazine afforded novel soluble and processable conjugated polymers, P1 and P2, respectively. The reaction conditions were investigated and structural analysis of the polymers was carried out by means of ¹H, ¹³C, ¹⁵N NMR, indicating all-trans configured CN-NC linkages. The molecular weights M_n were observed at ∼1100-1400 g/mol for P1 and ∼8700-10,500 g/mol for P2. The optical properties showed absorption maxima at ∼455 nm and ∼487 nm for P1 and P2 (CHCl₃ solutions), respectively, red shifted by 31-60 nm relative to the monomer aromatics due to conjugation through the azine linkage. The emission maxima are observed at ∼515 nm and ∼560 nm for P1 and P2 (CHCl₃ solutions), respectively. Thin films of P2 readily undergo n-doping.     

The Nephelauxetic Effect Becomes an Important Design Factor for Photoactive First-Row Transition Metal Complexes

The expansion of d-orbitals as a result of metal-ligand bond covalence, the so-called nephelauxetic effect, is a well-established concept of coordination chemistry, yet its importance for the design of new photoactive complexes based on first-row transition metals is only beginning to be recognized. Until recently, much focus has been on optimizing the ligand field strength, coordination geometries, and molecular rigidity, but now it becomes evident that the nephelauxetic effect can be a game changer regarding the photophysical properties of 3d metal complexes in solution at room temperature. In Cr^III and Mn^IV complexes with the d³ valence electron configuration, the nephelauxetic effect was exploited to shift the well-known ruby-like red luminescence to the near-infrared spectral region. In Fe^II and Co^III complexes with the low-spin d⁶ electron configuration, charge-transfer excited states were stabilized with respect to detrimental metal-centered excited states, to improve their properties and to enhance their application potential. In isoelectronic (3d⁶) isocyanide complexes of Cr⁰ and Mn^I, the nephelauxetic effect is likely at play as well, enabling luminescence and other favorable photoreactivity. This minireview illustrates the broad applicability of the nephelauxetic effect in tailoring the photophysical and photochemical properties of new coordination compounds made from abundant first-row transition metals.     

Ruthenium-catalyzed azide-alkyne cycloaddition: scope and mechanism

The catalytic activity of a series of ruthenium(II) complexes in azide-alkyne cycloadditions has been evaluated. The [Cp*RuCl] complexes, such as Cp*RuCl(PPh 3) 2, Cp*RuCl(COD), and Cp*RuCl(NBD), were among the most effective catalysts. In the presence of catalytic Cp*RuCl(PPh 3) 2 or Cp*RuCl(COD), primary and secondary azides react with a broad range of terminal alkynes containing a range of functionalities selectively producing 1,5-disubstituted 1,2,3-triazoles; tertiary azides were significantly less reactive. Both complexes also promote the cycloaddition reactions of organic azides with internal alkynes, providing access to fully-substituted 1,2,3-triazoles. The ruthenium-catalyzed azide-alkyne cycloaddition (RuAAC) appears to proceed via oxidative coupling of the azide and alkyne reactants to give a six-membered ruthenacycle intermediate, in which the first new carbon-nitrogen bond is formed between the more electronegative carbon of the alkyne and the terminal, electrophilic nitrogen of the azide. This step is followed by reductive elimination, which forms the triazole product. DFT calculations support this mechanistic proposal and indicate that the reductive elimination step is rate-determining.     

Modulated Binary–Ternary Dual Semiconductor Heterostructures

A generic modular synthetic strategy for the fabrication of a series of binary‐ternary group II‐VI and group I‐III‐VI coupled semiconductor nano‐heterostructures is reported. Using Ag₂Se nanocrystals first as a catalyst and then as sacrificial seeds, four dual semiconductor heterostructures were designed with similar shapes: CdSe‐AgInSe₂, CdSe‐AgGaSe₂, ZnSe‐AgInSe₂, and ZnSe‐AgGaSe₂. Among these, dispersive type‐II heterostructures are further explored for photocatalytic hydrogen evolution from water and these are observed to be superior catalysts than the binary or ternary semi‐conductors. Details of the chemistry of this modular synthesis have been studied and the photophysical processes involved in catalysis are investigated.     

Review of non-reactive and reactive wetting of liquids on surfaces

Wettability is a tendency for a liquid to spread on a solid substrate and is generally measured in terms of the angle (contact angle) between the tangent drawn at the triple point between the three phases (solid, liquid and vapour) and the substrate surface. A liquid spreading on a substrate with no reaction/absorption of the liquid by substrate material is known as non-reactive or inert wetting whereas the wetting process influenced by reaction between the spreading liquid and substrate material is known as reactive wetting. Young's equation gives the equilibrium contact angle in terms of interfacial tensions existing at the three-phase interface. The derivation of Young's equation is made under the assumptions of spreading of non-reactive liquid on an ideal (physically and chemically inert, smooth, homogeneous and rigid) solid, a condition that is rarely met in practical situations. Nevertheless Young's equation is the most fundamental starting point for understanding of the complex field of wetting. Reliable and reproducible measurements of contact angle from the experiments are important in order to analyze the wetting behaviour. Various methods have been developed over the years to evaluate wettability of a solid by a liquid. Among these, sessile drop and wetting balance techniques are versatile, popular and provide reliable data. Wetting is affected by large number of factors including liquid properties, substrate properties and system conditions. The effect of these factors on wettability is discussed. Thermodynamic treatment of wetting in inert systems is simple and based on free energy minimization where as that in reactive systems is quite complex. Surface energetics has to be considered while determining the driving force for spreading. Similar is the case of spreading kinetics. Inert systems follow definite flow pattern and in most cases a single function is sufficient to describe the whole kinetics. Theoretical models successfully describe the spreading in inert systems. However, it is difficult to determine the exact mechanism that controls the kinetics since reactive wetting is affected by a number of factors like interfacial reactions, diffusion of constituents, dissolution of the substrate, etc. The quantification of the effect of these interrelated factors on wettability would be useful to build a predictive model of wetting kinetics for reactive systems.     

Efficient and color-tunable Mn-doped ZnSe nanocrystal emitters: control of optical performance via greener synthetic chemistry

Formation of Mn-doped ZnSe quantum dots (Mn:ZnSe d-dots) using nucleation-doping strategy was studied systematically and optimized through greener approaches. The resulting d-dots were with high ( approximately 50%) photoluminescence (PL) quantum yield (QY), which was achieved by the controlled formation of small-sized MnSe nanoclusters as the core and a diffused interface between the nanocluster core and the ZnSe overcoating layers. Synthesis of the d-dots under high temperatures (240-300 degrees C) was achieved by varying the structure of the metal carboxylate precursors, concentration of the inhibitors, free fatty acid, and concentration of the activation reagents, fatty amines. Highly emissive d-dots synthesized under desired conditions were found to be extremely stable upon thermal treatment up to the boiling point of the solvent (about 300 degrees C), which was quantitatively studied using in situ measurements. The PL peak of the d-dots was controllably tuned in a surprisingly large optical window, from 565 to 610 nm. These highly emissive and stable d-dots possess characteristics of practical emissive materials, especially for applications requiring high power, high concentration of emitters, and under tough conditions.