High-Density Excitons in a Diluted Magnetic Semiconductor: Cd 1 − x Mn x Te

Time-resolved magneto photoluminescence in a diluted magnetic semiconductor Cd 0.9 Mn 0.1 Te has been carried out with varying exciton density from 10 14 to 10 19 cm m 3 . The reduction of the Zeeman shift and that of the magnetic polaron energy was found under strong photoexcitation. The spectral feature is interpreted in terms of the heating of the manganese spin subsystem. Polarization dependence of the spin heating is observed for the first time, revealing the contribution of the spin flip between excitons and magnetic ions to the heating process.     

Vacancy-like defects in nanocrystalline SnO2: influence of the annealing treatment under different atmospheres

The study of electronic and chemical properties of semiconductor oxides is motivated by their several applications. In particular, tin oxide is widely used as a solid state gas sensor material. In this regard, the defect structure has been proposed to be crucial in determining the resulting film conductivity and then its sensitivity. Here, the characteristics of vacancy-like defects in nanocrystalline commercial high-purity tin oxide powders and the influence of the annealing treatment under different atmospheres are presented. Specifically, SnO₂ nanopowders were annealed at 330 °C under three different types of atmospheres: inert (vacuum), oxidative (oxygen) and reductive (hydrogen). The obtained experimental results are discussed in terms of the vacancy-like defects detected, shedding light to the basic conduction mechanisms, which are responsible for gas detection.     

Efficient polymer passivation of ligand‐stripped nanocrystal surfaces

Water‐dispersible, polymer‐wrapped nanocrystals are highly sought after for use in biology and chemistry, from nanomedicine to catalysis. The hydrophobicity of their native ligand shell, however, is a significant barrier to their aqueous transfer as single particles. Ligand exchange with hydrophilic small molecules or, alternatively, wrapping over native ligands with amphiphilic polymers is widely employed for aqueous transfer; however, purification can be quite cumbersome. We report here a general two‐step method whereby reactive stripping of native ligands is first carried out using trialkyloxonium salts to reveal a bare nanocrystal surface. This is followed by chemically directed immobilization of a hydrophilic polymer coating. Polyacrylic acids, with side‐chain grafts or functional end groups, were found to be extremely versatile in this regard. The resulting polymer‐wrapped nanocrystal dispersions retained much of the compact size of their bare nanocrystal precursors, highlighting the unique role of monomer side‐chain functionality to serve as effective, conformal ligation motifs. As such, they are well poised for applications where tailored chemical functionality at the nanocrystal's periphery or improved access to their surfaces is desirable. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Carbon dioxide adsorption studies on delafossite CuFeO2 hydrothermally synthesized

Delafossite CuFeO₂ oxide was synthesized by a hydrothermal technique using Cu₂O and FeOOH as precursors with the addition of fused NaOH as mineralizer. The amount of rhombohedral and hexagonal delafossite phase formed depends on the synthesis time lapses between 2 and 5 days and on the NaOH concentration. The compounds obtained were analyzed with Raman Spectroscopy, X-Ray Diffraction (XRD), X-Ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) in order to obtain their morphological and structural properties. Optical behavior was studied by UV–vis Spectroscopy and gas adsorption measured with a Quartz-Crystal Microbalance (QCM). Our results show that this type of hydrothermal synthesis is capable of recreating the delafossite-type structure of copper-iron oxide and produces a high yield of material with the right stoichiometry. The highest uptake of carbon dioxide is observed on the sample with the highest ratio between rhombohedral and hexagonal delafossite phase.     

Modelling of high-temperature dark current in multi-quantum well structures from MWIR to VLWIR

In this paper, a model to calculate the dark current of quantum well infrared photodetectors at high-temperature regime is presented. The model is derived from a positive-definite quantum probability-flux and considers thermionic emission and thermally-assisted tunnelling as mechanisms of dark current generation. Its main input data are the wave functions obtained by time-independent Schrodinger equation and it does not require empirical parameters related to the transport of carriers. By means of this model, the dark current of quantum well infrared photodetectors at high-temperature regime is investigated with respect to the temperature, the barrier width, the applied electric field and the position of the first excited state. The theoretical results are compared with experimental data obtained from lattice-matched InAlAs/InGaAs, InGaAsP/InP on InP substrate and AlGaAs/GaAs structures with rectangular wells and symmetric barriers, whose absorption peak wavelengths range from MWIR to VLWIR. The corresponding results are in a good agreement with experimental data at different temperatures and at a wide range of applied electric field.     

Polyoxometalate‐Mediated One‐Pot Synthesis of Pd Nanocrystals with Controlled Morphologies for Efficient Chemical and Electrochemical Catalysis

Polyoxometalates (POMs), as inorganic ligands, can endow metal nanocrystals (NCs) with unique reactivities on account of their characteristic redox properties. In the present work, we present a facile POM‐mediated one‐pot aqueous synthesis method for the production of single‐crystalline Pd NCs with controlled shapes and sizes. The POMs could function as both reducing and stabilizing agents in the formation of NCs, and thus gave a fine control over the nucleation and growth kinetics of NCs. The prepared POM‐stabilized Pd NCs exhibited excellent catalytic activity and stability for electrocatalytic (formic acid oxidation) and catalytic (Suzuki coupling) reactions compared to Pd NCs prepared without the POMs. This shows that the POMs play a pivotal role in determining the catalytic performance, as well as the growth, of NCs. We envision that the present approach can offer a convenient way to develop efficient NC‐based catalyst systems.     

Thermoelectric properties of stannite-phase CuZn2AS4 (CZAS; A=Al,Ga and In) nanocrystals for solar energy conversion applications

The current study aimed to comprehensively investigate structural, electronic, optical and transport properties of quaternary semiconductor CuZn₂AS₄ (CZAS; A=Al, Ga and In) nanocrystals (NCs). Based on energy considerations, the stannite structure (I-42m; No. 121) is found to be more stable than the kesterite (I-4; No.82) and wurtzite (P6₃mc; No.186) type structures. By means of hybrid functional calculations, these nanocrystals have direct band gap of 0.81–1.71 eV with a high absorption coefficient of >10⁴ cm^?1, which are well-suited for use in solar energy-conversion applications. Some of the latest advances in applications of these nanocrystals in thermoelectric applications are also highlighted in the current study. It is observed that transport coefficients of these materials are found to be nearly direction independent and isotropic. All three samples are p-type conductors at room temperature. Especially, the Seebeck coefficient of CuZn₂AlS₄ is even larger than that of CuZn₂GaS₄ and CuZn₂InS₄ under the studied carrier concentration and temperature region. The maximum figure of merit (ZT) reaches 0.982 (0.977), 0.984 (0.974) and 0.53 (0.955) for p-type (n-type) CuZn₂AlS₄, CuZn₂GaS₄, and CuZn₂InS₄, respectively, at 300 K. The high Seebeck coefficients, high figure of merit and low thermal conductivities make these systems good candidates for high-efficiency thermoelectric conversion applications.     

Micro‐/Nanostructured Multicomponent Molecular Materials: Design,Assembly, and Functionality

Molecule‐based micro‐/nanomaterials have attracted considerable attention because their properties can vary greatly from the corresponding macro‐sized bulk systems. Recently, the construction of multicomponent molecular solids based on crystal engineering principles has emerged as a promising alternative way to develop micro‐/nanomaterials. Unlike single‐component materials, the resulting multicomponent systems offer the advantages of tunable composition, and adjustable molecular arrangement, and intermolecular interactions within their solid states. The study of these materials also supplies insight into how the crystal structure, molecular components, and micro‐/nanoscale effects can influence the performance of molecular materials. In this review, we describe recent advances and current directions in the assembly and applications of crystalline multicomponent micro‐/nanostructures. Firstly, the design strategies for multicomponent systems based on molecular recognition and crystal engineering principles are introduced. Attention is then focused on the methods of fabrication of low‐dimensional multicomponent micro‐/nanostructures. Their new applications are also outlined. Finally, we briefly discuss perspectives for the further development of these molecular crystalline micro‐/nanomaterials.