On the formation of well-aligned ZnO nanowall networks by catalyst-free thermal evaporation method

Two-dimensional ZnO nanowall networks were grown on ZnO-coated silicon by thermal evaporation at low temperature without catalysts or additives. All of the results from scanning electronic spectroscope, X-ray diffraction and Raman scattering confirmed that the ZnO nanowalls were vertically aligned and c-axis oriented. The room-temperature photoluminescence spectra showed a dominated UV peak at 378 nm, and a much suppressed orange emission centered at similar to 590 nm. This demonstrates fairly good crystal quality and optical properties of the product. A possible three-step, zinc vapor-controlled process was proposed to explain the growth of well-aligned ZnO nanowall networks. The pre-coated ZnO template layer plays a key role during the synthesis process, which guides the growth direction of the synthesized products. (C) 2007 Elsevier B.V. All rights reserved.

On the formation of well-aligned ZnO nanowall networks by catalyst-free thermal evaporation method

Two-dimensional ZnO nanowall networks were grown on ZnO-coated silicon by thermal evaporation at low temperature without catalysts or additives. All of the results from scanning electronic spectroscope, X-ray diffraction and Raman scattering confirmed that the ZnO nanowalls were vertically aligned and c-axis oriented. The room-temperature photoluminescence spectra showed a dominated UV peak at 378 nm, and a much suppressed orange emission centered at similar to 590 nm. This demonstrates fairly good crystal quality and optical properties of the product. A possible three-step, zinc vapor-controlled process was proposed to explain the growth of well-aligned ZnO nanowall networks. The pre-coated ZnO template layer plays a key role during the synthesis process, which guides the growth direction of the synthesized products. (C) 2007 Elsevier B.V. All rights reserved.

Metal-free growth of Si/SiO2 nanowires by annealing SiOx (x < 2) films deposited by PECVD

A new metal catalysis-free method of fabricating Si or SiO2 nanowires (NWs) compatible with Si CMOS technology was proposed by annealing SiOx (x < 2) films deposited by plasma -enhanced chemical vapor deposition (PECVD). The effects of the Si content (x value) and thickness of SiOx films, the annealing process and flowing gas ambient on the NW growth were studied in detail. The results indicated that the SiOx film of a thickness below 300 rim with x value close to 1 was most favorable for NW growth upon annealing at 1000-1150 degrees C in the flowing gas mixture of N-2 and H-2. NWs of 50-100nm in diameter and tens of micrometers in length were synthesized by this method. The formation mechanism was likely to be related to a new type of oxide assisted growth (OAG) mechanism, with Si nanoclusters in SiOx films after phase separation serving as the nuclei for the growth of NWs in SiOx films > 200nm, and SiO molecules from thin SiO, film decomposition inducing the NW growth in films < 100nm. An effective preliminary method to control NW growth direction was also demonstrated by etching trenches in SiOx films followed by annealing.

Large-Scale and Template-Free Growth of Free-Standing Single-Crystalline Dendritic Ag/Pd Alloy Nanostructure Arrays

Large-scale arrays consist of dendritic single-crystalline Ag/Pd alloy nanostructures are synthesized for the first time. A simple galvanic replacement reaction is introduced to grow these arrays directly on Ag substrates. The morphology of the products strongly depended on the reaction temperature and the concentration of H2PdCl4 solution. The mechanism of the formation of alloy and the dendritic morphology has been discussed. These alloy arrays exhibit high surface-enhanced Raman scattering (SERS) activity and may have potential applications in investigation of "in situ" Pd catalytic reactions using SERS. Moreover, electrocatalytic measurements suggest that the obtained dendritic Ag/Pd alloy nanostructures exhibit electrocatytic activity toward the oxidation of formic acid.

Micron-granula polyolefin with self-immobilized nickel and iron diimine catalysts bearing one or two allyl groups

Self-immobilized nickel and iron diimine catalysts bearing one or two allyl groups of [ArN=C](2)(C10H6)NiBr2 [Ar = 4-allyl-2,6-(i-Pr)(2)C6H2] (1), [ArN=C(Me)[Ar'N=C(Me)]C5H3NFeCl2 [Ar = Ar' = 4-allyl-2,6-(i-Pr)(2)C6H3, Ar = 2,6-(i-Pr)(2)C6H3, and Ar' = 4-allyl-2,6-(i-Pr)(2)C6H3] were synthesized and characterized. All three catalysts were investigated for olefin polymerization. As a result, these catalysts not only showed high activities as the catalyst free from the allyl group, such as [ArN=C](2)C10H6,NiBr2 (Ar = 2,6-(i-Pr)(2)C6H2)], but also greatly improved the morphology of polymer particles to afford micron-granula polyolefin. The self-immobilization of catalysts, the formation mechanism of microspherical. polymer, and the influence on the size of the particles are discussed. The molecular structure of self-immobilized nickel catalyst 1 was also characterized by crystallographic analysis.

Carboxyl-cored dendrimer and toluene-assisted fabrication of uniform platinum nanodendrites at a water/oil interface and their potential application as a catalyst

Uniform platinum nanodendrites have been prepared at a water/oil interface by a facile catalyst-free method at room temperature. This is carried out by introducing NaBH4 into the platinum precursor solution in the presence of the second generation of carboxyl-cored dendrimer ([G-2]-CO2H dendrimer) and toluene to act as a protective agent and a linker, respectively. The average fractal dimension of 1.61 of the obtained platinum nanodendrites is calculated by analysing the transmission electron micrographs using the programs Fractal Dimension Version 1.1 and Fractal Dimension Calculator. Control experiments show that the fabrication of platinum nanodendrites can be operated with a wide parameter window, which undoubtedly raises the degree of control of the synthesis process. The potential application of such a nanostructure as a catalyst is investigated, and the results reveal that they show highly efficient catalytic properties for the typical redox reaction between hexacyanoferrate (III) and thiosulfate ions at 301 K.

Enhanced performance of p-GaN by Mg delta doping

The electrical and structural properties of Mg delta-doped GaN epilayers grown by MOCVD were investigated. Compared to uniform Mg-doping GaN layers, it has been shown that the delta-doping (delta-doping) process could suppress the dislocation density and enhance the p-type performance. The influence of pre-purge step on the structural properties of GaN was also investigated. The hole concentration of p-GaN decreases when using a pre-purge step. These results can be explained convincingly using a simple model of impurity incorporation under Ga-free growth condition. (C) 2007 Elsevier B.V. All rights reserved.

Growth and production of free-living heterotrophic nanoflagellates in a eutrophic lake - Lake Donghu, Wuhan, China

In situ growth of heterotrophic nanoflagellates (HNF) in Lake Donghu, a eutrophic shallow lake in mainland China, was studied from January 1999 to March 2000 using a modified Weisse protocol. The study results indicated that the growth rates of HNF showed pronounced seasonal variation (-0.37-1.25 d(-1)), reaching the maximum during spring to early summer. When the water temperature was higher than 25.5 degreesC, HNF growth was inversely proportional to water temperature. There was an effect by bacterial abundance and autotrophic picoplankton on HNF growth that depended on location. HNF biomass was the highest in late spring, and the HNF production ranged from -2.25 to 35.45 mg l(-1) d(-1) with mean of 3.17 mg l(-1) d(-1). When considered in the context of biomass and production data for zooplankton in Lake Donghu, it was evident that HNF contributed significantly to the total zooplankton production in Lake Donghu. These in situ studies indicate that temperature and food supply are the major determinants of HNF abundance and productivity.

High-temperature AlN interlayer for crack-free AlGaN growth on GaN

This paper presents a study of the transformation of high-temperature AlN (HT-AlN) interlayer (IL) and its effect on the strain relaxation of Al0.25Ga0.75N/HT-AlN/GaN. The HT-AlN IL capped with Al0.25Ga0.75N transforms into AlGaN IL in which the Al composition increases with the HT-AlN IL thickness while the total Ga content keeps nearly constant. During the HT-AlN IL growth on GaN, the tensile stress is relieved through the formation of V trenches. The filling up of the V trenches by the subsequent Al0.25Ga0.75N growth is identified as the Ga source for the IL transformation, whose effect is very different from a direct growth of HT-AlGaN IL. The a-type dislocations generated during the advancement of V trenches and their filling up propagate into the Al0.25Ga0.75N overlayer. The a-type dislocation density increases dramatically with the IL thickness, which greatly enhances the strain relaxation of Al0.25Ga0.75N. (c) 2008 American Institute of Physics.

Growth of crack-free GaN films on Si(111) substrate by using Al-rich AlN buffer layer

GaN epilayers were grown on Si(111) substrate by metalorganic chemical vapor deposition. By using the Al-rich AlN buffer which contains Al beyond stoichiometry, crack-free GaN epilayers with 1 mum thickness were obtained. Through x-ray diffraction (XRD) and secondary ion mass spectroscopy analyses, it was found that a lot of Al atoms have diffused into the under part of the GaN epilayer from the Al-rich AlN buffer, which results in the formation of an AlxGa1-xN layer at least with 300 nm thickness in the 1 mum thick GaN epilayer. The Al fraction x was estimated by XRD to be about 2.5%. X-ray photoelectron spectroscopy depth analysis was also applied to investigate the stoichiometry in the Al-rich buffer before GaN growth. It is suggested that the underlayer AlxGa1-xN originated from Al diffusion probably provides a compressive stress to the upper part of the GaN epilayer, which counterbalances a part of tensile stress in the GaN epilayer during cooling down and consequently reduces the cracks of the film effectively. The method using the Al diffusion effect to form a thick AlGaN layer is really feasible to achieve the crack-free GaN films and obtain a high crystal quality simultaneously. (C) 2004 American Institute of Physics.

Growth of crack-free AlGaN film on thin AlN interlayer by MOCVD

We have studied the effect of low-temperature-deposited (LT) and high-temperature-deposited (FIT) AlN interlayer with various thickness on AlGaN film grown on GaN using c-plane sapphire as substrate. All the Al0.25Ga0.75N films thicker than 1 mum with LT-AlN interlayer or with HT-AlN interlayer were free of cracks, however, their surfaces were different: the Al0.25Ga0.75N films with LT-AlN interlayer showed smooth surface, while those with HT-AlN interlayer exhibit rough surface morphology. The results of X-ray double crystal diffraction and Rutherford backscattering showed that all of the AlGaN films were under compressive strain in the parallel direction. The compressive strain resulted from the effect of interlayer-induced stress relieving and the thermal mismatch for the samples with LT-AlN interlayer, and it was due to the thermal mismatch between AlGaN and the underlying layers for those with HT-AlN interlayer. (C) 2004 Elsevier B.V. All rights reserved.

Growth of aligned single-walled carbon nanotubes under ac electric fields through floating catalyst chemical vapour deposition

Through floating catalyst chemical vapour deposition(CVD) method, well-aligned isolated single-walled carbon nanotubes (SWCNTs) and their bundles were deposited on the metal electrodes patterned on the SiO2/Si surface under ac electric fields at relatively low temperature(280 degrees C). It was indicated that SWCNTs were effectively aligned under ac electric fields after they had just grown in the furnace. The time for a SWCNT to be aligned in the electric field and the effect of gas flow were estimated. Polarized Raman scattering was performed to characterize the aligned structure of SWCNTs. This method would be very useful for the controlled fabrication and preparation of SWCNTs in practical applications.

Metal catalysis-free, direction-controlled planar growth of single-crystalline alpha-Si3N4 nanowires on Si(100) substrate

Single-crystalline alpha-Si3N4 nanowires are controlled to grow perpendicular to the wet-etched trenches in the SiO0.94 film on the plane of the Si substrate without metal catalysis. A detailed characterization is carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The photoluminescence at 600 nm from alpha-Si3N4 nanowires is attributed to the recombination at the defect state formed by the Si dangling bond N3 equivalent to Si-center dot. The growth mechanism is considered to be related to the catalysis and nitridation of SiO nanoclusters preferably re-deposited around the inner corner of the trenches, as well as faster Si diffusion along the slanting side walls of the trenches. This simple direction-controlled growth method is compatible with the CMOS process, and could facilitate the fabrication of alpha-Si3N4 nanoelectronic or nanophotonic devices on the Si platform.