H.L. Li

Fine magnetic properties obtained in FeCo alloy nanowire arrays

Anodic aluminum oxide (AAO) was used as a template to prepare highly ordered FeCo alloy nanowire arrays. The as-deposited samples were annealed at 300, 400, 450, 500, 550 and 600 C, respectively. The structure and magnetic properties of FeCo nanowire arrays dependence on different annealing temperature were analyzed by XRD and vibrating sample magnetometer. We found that the annealing samples had large coercivity (as high as 2800 Oe) and high squareness (Mr=Ms) (about 0.9), the value of which was much larger than that of as-deposited samples (coercivity about 1500 Oe, squareness about 0.7). The temperature dependence of coercivity field Hc and squareness of the FeCo nanowire arrays associated with the microstructure has been investigated and correlated with microstructure features, such as stress relaxation, structure ordering, second phase, and grain size. 2002 Elsevier Science B.V. All rights reserved.

Silicon quantum-wires arrays synthesized by chemical vapor deposition and its micro-structural properties

Well-aligned arrays of silicon nanowires (SiNWs) have been synthesized by a chemical vapor deposition (CVD) template method without catalyst. The micro-structures of the SiNWs were studied by high-resolution transmission electron microscopy (HRTEM). Selected-area electron diffraction (SAED) and X-ray diffraction (XRD) indicate that each nanowire is essentially a single crystal and has a different orientation in an array. According to VLS mechanism, the growth of SiNWs without catalyst is related to the structure of template. The superior field emission behavior is believed to result from the oriented growth and the sharp tips of SiNWs.

Modulated multiferroicity of Cr-doped orthorhombic polycrystalline YMnO3

We synthesize a series of orthorhombic polycrystalline YMn(1-x)CrxO(3) (o-YMCO, x < 0.3) samples using high-pressure sintering, and investigate the multiferroicity by various characterization techniques. It is identified that Cr doping remarkably enhances the ferromagnetism, and the ferroelectric polarization is also significantly modulated. The enhanced ferromagnetism is argued to originate from the ferromagnetic interactions between neighbouring Cr3+ spins and Mn3+ spins. While the microscopic mechanism for polarization generation remains to be clarified, it may be argued, based on the response of polarization against external magnetic field, that Cr doping induces a partial transition from the symmetric exchange striction mechanism in o-YMnO3 to the asymmetric exchange striction mechanism in o-YMCO for ferroelectricity generation.

Morphology and thermal stability of Ti-doped copper nitride films

A weakly Ti-doped copper nitride (Cu3N) film was prepared by cylindrical magnetron sputtering. The XPS results indicate that Ti atoms do not substitute for the Cu atoms but probably locate at the grain boundaries. The columnar grains size is about half of that of the undoped Cu3N film and the surface is smoother. For weakly Ti-doped Cu3N films, a dense layer appears on top of the columnar crystals. The RMS of the Cu film formed by annealing of the weakly Ti-doped Cu3N film is more than twice larger than that of the film before annealing. Compared with the undoped Cu3N film, it possesses fine thermal stability both in vacuum and in atmosphere.

Synthesis of carbon nanotubes/Si nanowires core-sheath structure arrays and their field emission properties

Abstract A new composite structure of carbon nanotubes (CNTs)/Si nanowires (SiNWs) arrays have been synthesized by chemical vapor deposition (CVD) within the pores of microporous alumina template. The results of scanning electron microscopy (SEM) and transmission electron microcopy (TEM) reveal that the obtained well-aligned composite structure has a core-sheath structure and the deposited material in the carbon sheath is polycrystalline silicon. Field emission from these CNTs-sheathed SiNWs exhibits significant enhancement compared to the pure SiNWs in turn-on field, total emission current and stability. The field emission characteristics of the composite structure are analyzed based on Fowler–Nordheim theory. The electron field emission increased with decreasing diameter of such structure. The well-aligned core-sheath structure provides an important means to fabricate emitter devices with chemically inert surface as well as with superior performance of field emission properties.

Aqueous gold sols of rod-shaped particles prepared by the template method

Abstract Aqueous gold sols of rod-shaped particles are obtained by electrochemical deposition of gold into porous aluminum oxide membranes followed by dissolution of aluminum oxide and stabilization of the rods with different polymers. The visible absorption spectra of the sols show one absorption maximum, λmax, at around 530 nm. With increasing aspect ratio, the maximum shifts to short wavelength, which is in agreement with theoretical predictions. Absorption spectra of the sols show that the presence of polyvinyl alcohol or poly(vinylpyrrolydon) as stabilizer can improve the dispersity of aqueous gold sols, and then stable sols of the rods are obtained.

Effect of catalyst nanoparticle size on growth direction and morphology of InN nanowires

Au-assisted growth of InN nanowires (NWs) was accomplished by a simple chemical vapor deposition system. The as-prepared InN NWs exhibit two morphologies with different growth directions: periodic NWs (PNWs) and smooth NWs (SNWs) along and , respectively. The PNWs with crinoids morphology resulted when larger Au particles (∼40 nm in diameter) were used, while the SNWs with smooth sidewalls were obtained when smaller Au particles (∼10 nm in diameter) served as the collector. Furthermore, the mechanism of this growth behavior was discussed in terms of the effect of catalyst nanoparticle size.

Structure and optical investigation of faceted hexagonal aluminum nitride nanotube arrays

Arrays of single-crystalline aluminum nitride nanotubes (AlNNTs) were successfully synthesized at 780 °C by a facile CVD method without templates or catalysts. Faceted hexagonal AlNNTs with diameters from tens to hundreds of nanometers were observed. 264 nm deep-ultraviolet emission associated with Al vacancies was detected by photoluminescence. Raman characterization indicates that the lattice of the AlNNTs is notably defective, and disorder-activated silent B1 (low) and B1 (high) were detected at 583.0 and 730.0 cm−1 respectively, which were allowed by the breakdown of the translational crystal symmetry. The redshift of E2 (high) Raman modes indicates that biaxial tensile stress exists in the samples.