Y. F. Chan

Formation of ZnO nanostructures by a simple way of thermal evaporation

Mass production of ZnO nanowires, nanoribbons, and needle-like rods has been achieved by a simple method of thermal evaporation of ZnO powders mixed with graphite. Metallic catalysts, carrying gases, and vacuum conditions are not necessary. Temperature is the critical experimental parameter for the formation of different morphologies of ZnO nanostructures. Zn or Zn suboxide plays a crucial role for the nucleation of ZnO nanostructures. The as-prepared ZnO nanowires consist of single crystalline cores and thin amorphous shells. As determined by electron diffraction, the growth direction of ZnO nanowires is [001], which has no orientation relationship with the substrate. A strong room-temperature photoluminescence in ZnO nanostructures has been demonstrated.

Formation mechanism of TiO2 nanotubes

Transmission electron microscopic observation showed that TiO2 nanotubes synthesized via a simple hydrothermal chemical process formed a crystalline structure with open-ended and multiwall morphologies. Unlike multiwalled carbon nanotubes, the TiO2 nanotube walls were not seamless. During alkali treatment, crystalline TiO2 raw material underwent delamination in the alkali solution to produce single-layer TiO2 sheets. TiO2 nanotubes were formed by rolling up the single-layer TiO2 sheets with a rolling-up vector of [001] and attracting other sheets to surround the tubes.

Fabrication and magnetic properties of ultrathin Fe nanowire arrays

Ultrathin Fe nanowire (about 5 nm in diameter) arrays have been fabricated by electrodeposition using anodic porous alumina templates. These ultrathin nanowires exhibited uniaxial anisotropy and a quite large coercivity (4190 Oe) at 5 K. In addition, the field needed to saturate the magnetization, when the field was applied perpendicularly to the easy axis, was much larger than the shape anisotropy field (2πM S ). This saturation field increased with decreasing temperature. We believed that this enhanced saturation field was mainly due to the contribution of the surface spins.

ZnSe nanowires epitaxially grown on GaP(111) substrates by molecular-beam epitaxy

We report molecular-beam epitaxy growth of single crystalline ZnSe nanowires with uniform diameters (∼10 nm) on GaP(111) substrates. The growth process was based on the Au-catalyzed vapor-liquid-solid deposition. As determined by electron microdiffraction and high-resolution transmission electron microscopy, ZnSe nanowires grew generally along the 〈110〉 and 〈112〉 directions with the orientation relationship of (111)ZnSe wire//(111)GaP and 〈110〉ZnSe wire//〈110〉GaP. The dominant defects were found to be twins at the interface between the substrate and the nanowires along the (111) plane.

I-V characteristics of Schottky contacts of semiconducting ZnSe nanowires and gold electrodes

The I - V behaviour of a single semiconducting ZnSe nanowire with a diameter of about 20 nm has been studied by means of electric measurement under inspection with a transmission electron microscope. The experimental results showed not only an exponential relation between the current and applied bias voltage, but also the existence of a high contact resistance with characteristic features consistent with Schottky potential barriers at the gold electrodes and the semiconducting ZnSe nanowire contacts. These Schottky barriers with asymmetrical characteristics may result from different orientations of gold grains that have different work functions. The threshold bias voltages for breakdown of the Schottky barriers examined at the Au - ZnSe nanowire contacts were about - 2.5 and 1.6 V respectively.

Structural Characterization of Carbon Nanotube Arrays Formed in Ultra-Long Nanochannels of Porous Alumina Templates

The importance of carbon nanotube (CNT) alignment and size distribution in various potential applications has propelled extensive research on the syntheses of CNT arrays via different techniques. Anodic aluminum oxide (AAO) films are ideal templates for the synthesis of highly-ordered nanostructures because they are thermally and chemically stable, with pore size (hence the diameter of the nanostructures formed in the pores) that can be fully controlled [1]. The growth of CNTs based on AAO templates generally requires a tedious preparation process for metallic catalysts. Therefore, metal catalysts always cause contamination to the CNTs. In our previous papers, we demonstrated novel methods for preparing CNT arrays in AAO [2] and zeolite [3] templates without using any catalysts at low temperatures. Here, we show our current approach to the synthesis of uniform CNT arrays in the nanochannels of AAO templates (up to 100 m in length and 20nm in diameter with or without closed ends) by thermal pyrolysis of polyethylene glycol (Fig.1). Since no catalyst is needed, the resultant CNTs are highly pure. The structures of CNT arrays have been studied by highresolution transmission electron microscopy (HRTEM).

TEM investigation of nucleation and initial growth of ZnSe nanowires

Single crystalline ZnSe nanowires were fabricated on GaAs substrates by molecular beam epitaxy technique via Au-catalyzed vapor-liquid-solid reaction. The nucleation and initial growth of the nanowires were investigated by high-resolution transmission electron microscopy. It was revealed that Au catalysts initially reacted with the substrate forming binary AuGa2 alloy droplets. The sizes of the catalysts determined the growth direction of ZnSe nanowires. A model based on the surface energies of the nanowire nuclei was proposed to explain the size dependence of growth direction for ZnSe nanowires.