Wen Kuang Hsu

Tungsten oxide tree-like structures

Abstract An interesting micrometer scale tree-like structure has been generated by heating a W foil, partly covered by a SiO 2 plate, in an Ar atmosphere at ca. 1600°C. Upon sonication, the trees are broken into nanoneedles (ca. 5–50 nm wide and 20–200 nm long) and planar polyhedral nanoparticles (ca. 10–50 nm cross-section). Structural analysis, using ED, EDX, XRD, and HRTEM, showed that: (1) the trees consist of well-crystallised WO x phases ( x =0–3); (2) the nanoneedles are composed mainly of monoclinic W 18 O 49 phases; and (3) the nanoparticles consist primarily of WO 3 . The tree growth is thought to arise from the intrinsic crystalline feature of WO x , the planar defect or the shear structure of which is responsible for the breakdown of the trees.

Nanotubes: A Revolution in Materials Science and Electronics

Nanotube theoretical and experimental research has developed very rapidly over the last seven years, following the bulk production of C60 and structural identification of carbon nanotubes in soot deposits formed during plasma arc experiments. This review summarises achievements in nanotube technology, in particular various routes to carbon nanotubes and their remarkable mechanical and conducting properties. The creation of novel nanotubules, nanowires and nanorods containing other elements such as B, N, Si, O, Mo, S and W is also reviewed. These advances are paving the way to nanoscale technology and promise to provide a wide spectrum of applications.

A SIMPLE ROUTE TO SILICON-BASED NANOSTRUCTURES

.Powders of the nanoparticles were obtained from the colloidal solutions by removing the water with a rotary evaporator (bath temperature 50 C). Transmission electron micrographs of the samples were taken using a Philips CM 300 UT electron microscope, working at 300 kV acceleration voltage. A Philips Xpert system was used to measure the X-ray diffraction pattern of powder samples. UV-vis absorption spectra of the colloidal solutions were recorded with a Lambda 40 spectrometer (Perkin‐Elmer). Photoluminescence spectra were recorded with a Spex Fluoromax 2 spectrometer having a spectral resolution of 0.5 nm.

3D Silicon oxide nanostructures: from nanoflowers to radiolaria

Novel flower-like nanostructures consisting of silicon oxide nanofibers, radially attached to a single catalytic particle, were generated by solid-solid and gas-solid reactions under a temperature gradient. In this process, a mixture of SiC and Co powders, deposited on silica substrates and heated under an Ar/CO atmosphere at ca. 1500C, produced material with unusual three-dimensional (3D) networks of nanofibers of uniform diameter (ca. 20-120nm) and length (ca. 10-250mu;m). Scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), X-ray powder diffraction and energy dispersive X-ray (EDX) analyses reveal that the nanofibres are amorphous and consist only of silicon oxide, generated from the reaction of CO with SiC. Nanostructure formation is catalyzed by Co particles, which act as nucleation sites and templates for 3D growth. Experiments using Si3N4 and Si in conjunction with other catalysts (e.g. Fe, Ni and CoO) yield similar results and confirm that the resulting SiOx fibres display virtually unique and remarkable radial growth starting from single metal particles. These structures exhibit morphologies comparable to radiolarian and diatom skeletons and may provide insight into the formation of microbiological systems.

A low resistance boron-doped carbon nanotube–polystyrene composite

A composite made from boron-doped carbon nanotubes and polystyrene exhibits relatively low electrical resistance and minor variations in conduction when mechanically loaded. Carbon nanotubes form a network within the plastic film, thereby establishing electrical conduction uniformly throughout the composite. Individual carbon nanotubes behave as intrinsic resistors, therefore the film resistance obeys Ohms law.

SiC–SiOx heterojunctions in nanowires

Novel 2-D nanoscale SiC networks have been generated by heating SiC–Fe–Co mixtures under a CO atmosphere. Examination of the products by EDX and HRTEM showed that most of the nanowires consist of β-SiC elongated single crystals, wrapped in amorphous SiOx sheaths. Intriguing crystalline features and defects associated with SiC nanowires were observed. We believe that a binary Fe–Co catalyst is responsible for the SiC network: Fe catalyses the formation of the SiC inner cores and Co the SiOx (x = 1–2) outer shell. A two-step growth mechanism, involving a vapour–liquid–solid (V–L–S) step, is thought to account for SiC nanowire creation.

Synthesis of nanotubes via catalytic pyrolysis of acetylene: A SEM study

Pyrolysis of acetylene over quartz plates coated with various metal catalysts resulted in the formation of all-carbon nanostructures which were observed directly by scanning electron microscopy (SEM). The nanotubes were studied in detail using transmission electron microscopy (TEM), and appear to grow as ultra-thin tubes with a central hollow core and considerable thickening due to secondary pyrolytic deposition. In nearly all samples the nanotube diameter was uniform. In some cases catalytic particles were entrapped within the tubes. The efficiency of the catalysts was evaluated semi-quantitatively.

Tungsten Disulphide Sheathed Carbon Nanotubes

An insulated nanotube wire is formed by the binary phase of layered tungsten disulphide and carbon nanotubes (shown in the HRTEM image) generated by the sulphidization of tungsten oxide coated multiwalled carbon nanotubes at 900 °C. Thermogravimetric analysis shows that the tungsten disulphide coat acts as an antioxidant.

Self-assembly of Si nanostructures

Abstract Flower-like Si nanostructures are formed in high yield, by heating an SiO 2 plate at ca. 1600 °C (Ta heater) under Ar (100 Torr). The product consists of metal-free cubic phase Si nanowires surmounted by bulbous Si tips. HRTEM observations show that the nanowires contain kink and twinning defects, whereas the tips are generally well-crystallized and covered with a thin layer of amorphous SiO x ( x = 1–2). A growth model is proposed to account for these observations.

Tungsten–niobium–sulfur composite nanotubes

Novel W–Nb–S composite inorganic nanotubes have nbeen generated by replacing W in the S–W–S sandwich layers of nWS2 by Nb. The tubes are uniform, with the Nb concentration 10 natom%. Raman spectral features of the tubes are described.