Structures of Au nanowires encapsulated in carbon nanotubes

Classical molecular dynamics simulations have been used to investigate the structure of Au nanowires encapsulated in single-walled carbon nanotubes (SWCNT). It was found that Au nanowires with helical multishell structures can be formed within the SWCNTs. Each shell is composed of helical rows of atoms. The distance between the tube wall and the outermost shell of the Au nanowire is about 3 Å, and the spacing between the Au intershells varies from 2.08 to 2.33 Å. The radii of the SWCNTs and the numbers of filled Au atoms play dominant roles in deciding the final structures of the Au nanowires formed within the SWCNTs over the range of diameters considered. In SWCNTs with a given diameter, the Au nanowires with helical multishell structure will be formed when enough Au atoms have filled the nanotube. Otherwise, Au nanowires with a bulky fcc structure will be formed.     

Protactinium-231: A new radionuclide for AMS

For the first time, an AMS system was used to determine concentrations of the actinide Protactinium-231. ²³¹Pa has widespread applications in the earth sciences. It can be used for U-series disequilibrium dating, and ²³¹Pa is an important tracer in Paleoceanography. The Pa isotopes were measured with the compact ETH/PSI-AMS facility TANDY operating at about 300 kV. The linearity of the AMS is documented with two standard-dilution experiments and first measurements of natural samples from very different climate archives are presented. Our results show that it is possible to determine ²³¹Pa amounts in the lower femtogram (1 fg = 10⁻¹⁵ g) range with the TANDY. In natural samples, ²³¹Pa concentrations between 1 and 4 pg/g (1 pg = 10⁻¹² g) were measured with an average total error of 3% (one sigma). The average over all reproducibility of these first measurements was better than 2% with clear potential for improvement.     

Polarization effects in non-relativistic ep scattering

The cross section which addresses the spin-flip transitions of a proton (antiproton) interacting with a polarized non-relativistic electron or positron is calculated analytically. In the case of attraction, this cross section is greatly enhanced for sufficiently small relative velocities as compared to the result obtained in the Born approximation. However, it is still very small, so that the beam polarization time turns out to be enormously large for the parameters of e^± beams available now. This practically rules out a use of such beams to polarize stored antiprotons or protons.     

Ion beam synthesis and doping of photonic nanostructures

Optically-active silica nanowires are produced by metal-induced growth on silicon substrates using ion-implantation, with two different strategies employed for their fabrication. The first is based on Er implantation of nanowires produced by a thin-film Pd catalyst layer, and the second employing implanted Er as both the catalyst and dopant. The luminescence properties of the resulting Er-doped silica nanowires are reported and compared with similarly implanted fused silica samples. Comparison shows that the luminescence lifetime of Er is increased by incorporation within the nanowires due to a reduction in the density of available optical states in these structures. Additional details of the synthesis, structure and properties of these functionalised nanowires are also presented.     

Growth of III-V semiconductor nanowires by molecular beam epitaxy

We present here the growth of GaAs, InAs and InGaAs nanowires by molecular beam epitaxy. The nanowires have been grown on different substrates [GaAs(0 0 1), GaAs(1 1 1), SiO₂ and Si(1 1 1)] using gold as the growth catalyst. We show how the different substrates affect the results in terms of nanowire density and morphology. We also show that the growth temperature for the InGaAs nanowires has to be carefully chosen to obtain homogeneous alloys.     

ZnO nanowire growth by electric current heating method: A study on the effect of substrate temperature

In this study, we report the growth of ZnO nanowire on quartz glass substrates with Au-catalyst assistance by electric current heating of ZnO ceramic bar. The effect of substrate temperature on the properties of ZnO nanostructures has been investigated systematically. Structural analysis indicates that the grown ZnO crystals belong to hexagonal phase with preferential growth along (0 0 2) orientation. Scanning electron microscopic studies reveal the aligned ZnO nanowires were grown at 800 °C. The typical length and diameter of nanowires are in the uniform ranges of 4–20 μm and 20–100 nm, respectively, showing their high aspect ratio of about 1000. We have made an attempt to discuss about the change in ZnO nanostructures with different substrate temperatures and the possible mechanism for the growth of nanowires. Optical reflectance studies show the infrared reflectivity was controlled through the substrate temperature.     

ECR plasma assisted deposition of zinc nanowires

Deposits of one dimensional nanowires of zinc with diameters of 90-120 nm have been obtained by means of dc sputtering within an electron cyclotron resonance plasma reactor. The sputtering has been made effective by using a negatively biased cylindrical target. The structure of the nanocrystalline wires deposited on glass substrates were investigated with scanning electron microscopy, transmission electron microscopy and scanning tunneling microscopy. STM revealed that the structure of the one dimensional nanowires are ensemble of nanoclusters and nanowires with diameter of 4-5 nm. The crystalline nature of the metallic nanowires was studied with X-ray and electron diffraction analysis. The native oxide present on the metallic wires was revealed by photoluminescent spectroscopy. Theoretical modeling has been used to explain the possible mechanisms operative inside the plasma which lead into deposition of zinc on the substrate starting from the precursor species.     

Tuning the dimensionality of ZnO nanowires through thermal treatment: an investigation of growth mechanism

ABSTRACT: In this study we synthesized various dimensionalities of ZnO nanowires using the Ti-grid assisted chemical vapor deposition process. Energy dispersive x-ray spectroscopic mapping technique accompanied with a lattice diffusion model was used to characterize the growth mechanism. A diffusion ratio was obtained to describe the mechanism based on short-circuit- and lattice- diffusion of Zn ions from the Zn base film is then proposed for the growth of ZnO nanowires. The tunable dimensionalities of ZnO nanowires allow us to modify the morphology of ZnO nanocrystals by developing well controlled potential applications.     

Growth of crystalline YbVO4 thin-film optical waveguides by pulsed laser deposition

Epitaxial YbVO₄ films have been grown on sapphire and Si/SiO₂ substrates by pulsed laser deposition. Films were grown over a range of temperatures from 600 to 700 °C in the presence of an oxygen pressure between 2 and 20 Pa. The films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), high-resolution scanning electron microscopy (HRSEM), and dark-mode prism coupling measurements. YbVO₄ films show epitaxial growth and display one main axis orientation of (2 0 0). The degree of crystal orientation increases with increasing depositing temperature and oxygen pressure up to 700 °C and 20 Pa, respectively. AFM and HRSEM measurements show that the prepared films are dense and homogeneous and three-dimensional-island growth mechanism is confirmed. According to prism coupling measurements, sharp dip is observed for both transverse-electric (TE) and transverse-magnetic (TM) mode, which means that the light could be well confined in the prepared film.     

Filter diagonalization: Filtering and postprocessing with prolates

A detailed account is given of a recent modification of the Filter Diagonalization technique that serves to analyze a signal spectrum within a selected energy range. Our approach employs for filtering the eigenfunctions of the Finite Fourier Transform, or prolates, which are superior to other filters due to their special properties. In particular, prolates are simultaneously band-limited and highly concentrated at a finite time-interval, producing filters with optimal accuracy. In addition both features are acquired by the convolution of a band-limited function with a prolate, that permits the latter to be interpolated via the Walter and Shen sampling formula, which essentially simplifies the supplementary computations. Rigorous filtering error estimates are obtained. Test calculations illustrate the facilities of the presented modification.