J. Y. Lao

Field-emission studies on thin films of zinc oxide nanowires

Studies on field emission (FE) from thin films of zinc oxide (ZnO) nanowires found that both the turn-on voltage and emission current density depend on the areal density of nanowires. The density of ZnO nanowires is controlled by the gold (Au) nanoparticle density deposited on the silicon substrates. The growth of ZnO nanowires was achieved by the thermal evaporation/condensation method. It is shown that the same screening effect observed on carbon nanotube field emitters also affects the FE from thin films of ZnO nanowires. Thin films with the lowest areal density of ZnO nanowires showed much better FE characteristics, comparable to that of carbon nanotubes. More importantly, the FE characteristics of ZnO nanowire thin film were further improved with annealing in hydrogen.

Self-assembly of semiconducting oxide nanowires, nanorods, and nanoribbons

We have synthesized three-dimensional self-assembled ZnO-based nanoscale heterostructures through the addition of alloying elements (In or Sn) during the vapor phase transport and condensation deposition process. Each threedimensional nanoscale heterostructure consists of a faceted nanowire core, with side branches (nanoribbons or nanorods) emanating from facets of the central nanowire core. The central nanowire core can be either ZnO, ðZnOÞ 11 In2O3 ,o r In 2O3, depending on incoming flux composition ratio. The physical mechanism of branching stems from segregation of alloying elements to facets during the growth of the nanowire core, thus creating nucleation sites for side branches. 2003 Elsevier Science B.V. All rights reserved.

Large-quantity free-standing ZnO nanowires

Large-quantity (grams) one-dimensional ZnO nanowires of different sizes have been synthesized by a simple thermal evaporation of ZnO powder in a tube furnace at a temperature controlled to 1000–1200 °C and pressure to 1–2 Torr air. A mixture of ZnO and graphite powder was used as the source. Fine graphite flakes were used to promote the growth. The graphite flakes are the key for large-quantity yield and were easily removed by oxidation in flowing O2 at about 700 °C for 1–3 h. The scanning- and transmission-electron-microscopic studies show that the diameter and length of the nanowires vary from 20 to 100 nm and 0.5 to 10 μm, respectively. Room temperature photoluminescence studies found that the luminescent intensity depends on the processing conditions. A reduced band edge ultraviolet (380 nm) and deep-band green (520 nm) emission have been observed for these nanowires. Most importantly, the method can be extended to any other oxide nanowires that will be the building block of future nanoscale devices.

Synthesis and photoluminescence studies on ZnO nanowires

ZnO nanowires were grown in gram quantities on graphite flakes (as collector) using the vapour transport and condensation approach. The yield, defined as the weight ratio of ZnO nanowires to the original graphite flakes, has been studied thoroughly by tuning the various growth parameters such as pressure and temperature inside the tube furnace, the amount of graphite powder in the original source, the source to collector ratio, etc. A yield as high as 200% has been achieved, equivalent to a 40% conversion of the ZnO powder in the original source. A study comparing the photoluminescence spectra of the ZnO nanowires grown on both graphite flakes and substrates with commercially available ZnO powder has been carried out.

Hierarchical oxide nanostructures

A variety of hierarchical oxide nanostructures, including ZnO nanonails on nanowires/nanobelts, ZnO nanorods on nanobelts, comb-like ZnO nanostructures, hierarchical MgO nanowires, etc., have been synthesized by thermal vapor evaporation and condensation. Observation of these nanostructures demonstrates the diversity of the geometry of these oxides in the nanoscale range. These nanostructures may be helpful in studies of nanostructure growth mechanisms and may find applications in nanoelectronics, opto-electronics, catalysts, displays, etc.

Boron carbide nanolumps on carbon nanotubes

Boron carbide nanolumps are formed on the surface of multiwall carbon nanotubes by a solid-state reaction between boron and carbon nanotubes. The reaction is localized so that the integrity of the structure of carbon nanotubes is maintained. Inner layers of multiwall carbon nanotubes are also bonded to boron carbide nanolumps. These multiwall carbon nanotubes with boron carbide nanolumps are expected to be the ideal reinforcing fillers for high-performance composites because of the favorable morphology.

Novel ZnO nanostructures

A variety of novel ZnO nanostructures such as nanowires, nanowalls, hierarchical nanostructures with 6-, 4-, and 2-fold symmetries, nanobridges, nanonails have been successfully grown by a vapor transport and condensation technique. Doping both In and Sn into ZnO hierarchical nanostructures can be created. The 2-fold eutectic ZnO structures can also be created without any doping in the source. It was found that the hierarchical nanostructures can be divided into two categories: homoepitaxial and heteroepitaxial where heteroepitaxy creates the multifold nanostructures. The novel ZnO nanowalls and aligned nanowires on a-plane of sapphire substrate have also been synthesized and the photoluminescence is studied. The ZnO nanowires also demonstrated very good field emission properties, comparable to carbon nanotubes. These nanostructures may find applications in a variety of fields such as field emission, photovoltaics, transparent EMI shielding, supercapacitors, fuel cells, high strength and multifunctional nanocomposites, etc. that require not only high surface area but also structural integrity.