Hierarchical electrohydrodynamic structures for surface-enhanced Raman scattering (adv. Mater. 23/2012).

On page OP 175, U. Steiner and co-workers destabilise polymer trilayer films using an electric field to generate separated micrometre-sized core-shell pillars, which are further modified by selective polymer dissolution to yield polymer core columns surrounded by a rim and micro-volcano rim structures. When coated with gold and decorated with Raman active probes, all three structure types give rise to substantial enhancement in surface-enhanced Raman scattering (SERS). Since this SERS enhancement arises from each of the isolated structures in the array, these surface patterns are an ideal platform for multiplexed SERS detection.

Ultrafast dynamics of exciton formation in semiconductor nanowires.

The dynamics of free electron-hole pairs and excitons in GaAs-AlGaAs-GaAs core-shell-skin nanowires is investigated using femtosecond transient photoluminescence spectroscopy at 10 K. Following nonresonant excitation, a bimolecular interconversion of the initially generated electron-hole plasma into an exciton population is observed. This conducting-to-insulating transition appears to occur gradually over electron-hole charge pair densities of 2-4 × 10(16) cm(-3) . The smoothness of the Mott transition is attributed to the slow carrier-cooling during the bimolecular interconversion of free charge carriers into excitons and to the presence of chemical-potential fluctuations leading to inhomogeneous spectral characteristics. These results demonstrate that high-quality nanowires are model systems for investigating fundamental scientific effects in 1D heterostructures.

Phase separation induced by Au catalysts in ternary InGaAs nanowires.

We report a novel phase separation phenomenon observed in the growth of ternary In(x)Ga(1-x)As nanowires by metalorganic chemical vapor deposition. A spontaneous formation of core-shell nanowires is investigated by cross-sectional transmission electron microscopy, revealing the compositional complexity within the ternary nanowires. It has been found that for In(x)Ga(1-x)As nanowires high precursor flow rates generate ternary In(x)Ga(1-x)As cores with In-rich shells, while low precursor flow rates produce binary GaAs cores with ternary In(x)Ga(1-x)As shells. First-principle calculations combined with thermodynamic considerations suggest that this phenomenon is due to competitive alloying of different group-III elements with Au catalysts, and variations in elemental concentrations of group-III materials in the catalyst under different precursor flow rates. This study shows that precursor flow rates are critical factors for manipulating Au catalysts to produce nanowires of desired composition.

Structural and optical properties of III-V nanowires and nanowire heterostructures grown by metalorganic chemical vapour deposition

We have investigated the structural and optical properties of III-V nanowires grown by metalorganic chemical vapour deposition. Binary GaAs, InAs and InP nanowires, and ternary InGaAs and AlGaAs nanowires, have been fabricated and characterised. A variety of axial and radial heterostructures have also been fabricated, including GaAs/AlGaAs core-multishell and GaAs/InGaAs superlattice nanowires. GaAs/AlGaAs core-shell nanowires exhibit strong photoluminescence as the AlGaAs shell passivates the GaAs nanowire surface reducing the surface nonradiative recombination. © 2007 IEEE.

Spatially-resolved photoluminescence imaging of CdS and GaAs/AlGaAs nanowires

We use low temperature spatially resolved photoluminescence imaging to study optical properties and electronic states of single CdS and GaAs/AlGaAs core-shell nanowires. © 2007 American Institute of Physics.

Growth, structural and optical properties of III-V nanowires for optoelectronic applications

We investigate the growth of III-V nanowires by MOCVD and the structural and optical properties of these nanowires. Binary and ternary nanowires of GaAs, InAs, InP, AlGaAs and InGaAs are achieved. We discuss the nucleation and growth issues involved in fabricating high quality nanowires suitable for device applications. We have fabricated and characterised a variety of axial and radial heterostructures including GaAs/InGaAs superlattices, and GaAs/AlGaAs core-shell and core-multishell nanowires. © 2007 IEEE.

Evolution of InAs branches in InAsGaAs nanowire heterostructures

Branched nanowire heterostructures of InAsGaAs were observed during Au-assisted growth of InAs on GaAs nanowires. The evolution of these branches has been determined through detailed electron microscopy characterization with the following sequence: (1) in the initial stage of InAs growth, the Au droplet is observed to slide down the side of the GaAs nanowire, (2) the downward movement of Au nanoparticle later terminates when the nanoparticle encounters InAs growing radially on the GaAs nanowire sidewalls, and (3) with further supply of In and As vapor reactants, the Au nanoparticles assist the formation of InAs branches with a well-defined orientation relationship with GaAsInAs core/shell stems. We anticipate that these observations advance the understanding of the kink formation in axial nanowire heterostructures. © 2007 American Institute of Physics.

Growth of III-V nanowires and nanowire heterostructures by metalorganic chemical vapor deposition

We have investigated the structural and optical properties of III-V nanowires, and axial and radial nanowire heterostructures, fabricated by metalorganic chemical vapor deposition. In addition to binary nanowires, such as GaAs, InAs, and InP, we have demonstrated ternary InGaAs and AlGaAs nanowires. Core-shell nanowires consisting of GaAs cores with AlGaAs shells, and core-multishell nanowires with alternating shells of AlGaAs and GaAs, exhibit strong photoluminescence. Axial segments of InGaAs have been incorporated within GaAs nanowires to form GaAs/InGaAs nanowire superlattices. We have developed a two-temperature growth procedure to optimize nanowire morphology. An initial high temperature step promotes nucleation and epitaxial growth of straight (111)B-oriented nanowires. Lower temperatures are employed subsequently, to minimise radial growth. © 2007 IEEE.

III-V nanowires for optoelectronics

The optical and structural properties of binary and ternary III-V nanowires including GaAs, InP, In(Ga)As, Al(Ga)As, and GaAs(Sb) nanowires by metal-organic chemical vapour deposition are investigated, Au colloidal nanoparticles are employed to catalyze nanowire growth. Zinc blende or wurtzite crystal structures with some stacking faults are observed for these nanowires by high resolution transmission electron microscope. In addition, the properties of heterostructure nanowires including GaAs-AlGaAs core-shell nanowires, GaAs-InAs nanowires, and GaAs-GaSb nanowires are reported. Single nanowire luminescence properties from optically bright InP nanowires are reported. Interesting phenomena such as two-temperature procedure, nanowire height enhancement of isolated ternary InGaAs nanowires, kinking effect of InAs-GaAs heterostructure nanowires, and unusual growth property of GaAs-GaSb heterostructure nanowires are investigated. These nanowires will play an essential role in future optoelectronic devices.

Structural and optical properties of axial and radial heterostructure III-V nanowires grown by metalorganic chemical vapour deposition

We have investigated the structural properties and photoluminescence of novel axial and radial heterostructure III-V nanowires, fabricated by metalorganic chemical vapour deposition. Segments of InGaAs have been incorporated within GaAs nanowires, to create axial heterostructure nanowires which exhibit strong photoluminescence. Photoluminescence is observed from radial heterostructure nanowires (core-shell nanowires), consisting of GaAs cores with AlGaAs shells. Core-multishell nanowires, of GaAs cores clad in several alternating layers of thick AlGaAs barrier shells and thin GaAs quantum well shells, exhibit a blue-shifted photoluminescence peak arising from quantum confinement effects. © 2006 Crown Copyright.

Quantum dots and nanowires for photonics applications

We review our results on integrated photonic devices fabricated using InGaAs quantum-dots. Selective-area metal organic chemical vapor deposition (MOCVD) is used to grow the active region with quantum dots emitting at different wavelengths for fabrication of the integrated devices. We will also review the structural and optical properties of III-V nanowires, and axial and radial nanowire heterostructures grown by MOCVD. In addition to binary nanowires, such as GaAs, InAs, and InP, we have demonstrated ternary InGaAs and AlGaAs nanowires. Core-shell nanowires consisting of GaAs cores with AlGaAs shells, and core-multishell nanowires with several alternating shells of AlGaAs and GaAs, exhibit strong photoluminescence. Axial segments of InGaAs have been incorporated within GaAs nanowires to form GaAs/InGaAs nanowire superlattices.

Effect of size, composition, and morphology on magnetic performance: First-order reversal curves evaluation of iron oxide nanoparticles

© 2014 AIP Publishing LLC. Superparamagnetic nanoparticles are employed in a broad range of applications that demand detailed magnetic characterization for superior performance, e.g., in drug delivery or cancer treatment. Magnetic hysteresis measurements provide information on saturation magnetization and coercive force for bulk material but can be equivocal for particles having a broad size distribution. Here, first-order reversal curves (FORCs) are used to evaluate the effective magnetic particle size and interaction between equally sized magnetic iron oxide (Fe2O3) nanoparticles with three different morphologies: (i) pure Fe2O3, (ii) Janus-like, and (iii) core/shell Fe2O3/SiO2synthesized using flame technology. By characterizing the distribution in coercive force and interaction field from the FORC diagrams, we find that the presence of SiO2in the core/shell structures significantly reduces the average coercive force in comparison to the Janus-like Fe2O3/SiO2and pure Fe2O3particles. This is attributed to the reduction in the dipolar interaction between particles, which in turn reduces the effective magnetic particle size. Hence, FORC analysis allows for a finer distinction between equally sized Fe2O3particles with similar magnetic hysteresis curves that can significantly influence the final nanoparticle performance.

Spatially separated excitons in quantum-dot quantum well structures

In the framework of the effective-mass envelope-function theory, the electronic and optical properties of a spherical core-shell quantum-dot quantum well (QDQW) structure with one and two wells have been investigated. The results show that the energies of electron and hole states depend sensitively on the well thickness and core radius of quantum-dot quantum well structure. An interesting spatially separated characteristic of electron and hole in QDQW is found and enhanced significantly in the two-wells case. The normalized oscillator strength for the optical transition between the electron and hole states in QDQW exhibits a deep valley at some special well thickness. The Coulomb interaction between the electron and hole is also taken into account. [S0163-1829(98)02412-6].

核壳结构改性剂增韧聚氯乙烯的机理及性能优化

我国是世界上聚氯乙烯产量最大的国家。由于在氯乙烯分子中氯的含量大于50%，而氯又是制碱工业的必然副产物，因此聚氯乙烯工业成为氯碱平衡中的主要杠杆。随着近年来世界石油资源的日益短缺和价格的上涨及国内聚氯乙烯产能过剩，开发聚氯乙烯的新用途，发展聚氯乙烯树脂的改性具有深远的意义。本工作的目的是以开发PVC核壳结构改性剂MBS和注塑级PVC/ABS合金为背景，对核壳粒子改性的聚氯乙烯的屈服机理和改性剂性能优化进行了研究。 对于增韧剂的研究，人们总是希望用最少的增韧剂用量来达到最好的增韧效果，即提高增韧剂的增韧效率，而且MBS改性的PVC在应用时还需要保持较高的透光率和具有一定的抗应力发白的能力。为此我们成功开发了具有高增韧效率、高透明及抗应力发白的MBS增韧剂，并研究了MBS结构特性对PVC/MBS共混物力学性能及屈服机理的影响。结果表明：粒径范围在80nm~280nm的橡胶粒子均能有效地对PVC进行增韧。大粒径的MBS通过空洞化释放三维静张应力引发基体的屈服来吸收能量，而小粒径的MBS不能产生空洞，但同样能引起基体的屈服起到增韧的作用。为了兼顾PVC/MBS共混物的韧性和透明性，我们在制备MBS时将一部分苯乙烯单体与丁二烯共聚结合来提高丁苯胶乳的折光指数与PVC基体相匹配，而另一部分苯乙烯单体以接枝的方式结合，这样可以保持MBS的弹性，并且制备的PVC/MBS共混物具有高韧性和高透明；同时我们发现MBS的内包容物及多层结构都会削弱PVC/MBS共混物的透光率。研究表明橡胶粒子的空洞化是引发共混物应力发白的主要原因。为此通过制备小粒径及具有一定交联程度的MBS来抑制空洞的产生，制备了兼有抗应力发白和韧性的MBS。成功开发了多个牌号的MBS产品，并在吉化集团公司建成了年产5000吨的生产车间，实现了MBS的工业化生产。 由于PVC的熔体流动性较差，通过与熔体流动性好的ABS树脂共混，成功开发了注塑级PVC/ABS合金。ABS树脂是SAN树脂和PB-g-SAN的共混物，在PVC中加入ABS使增韧对象由PVC基体转化为PVC/SAN共混基体，并研究了基体性质的改变对增韧效果的影响。结果表明随着PVC/ABS共混体系基体中PVC含量的增加共混物的冲击强度显著增加，形变机理发生了由银纹向剪切屈服的转变。