Debasish Banerjee

Field emission of zinc oxide nanowires grown on carbon cloth

An extremely low operating electric field has been achieved on zinc oxide (ZnO) nanowire field emitters grown on carbon cloth. Thermal vaporization and condensation was used to grow the nanowires from a mixture source of ZnO and graphite powders in a tube furnace. An emission current density of 1mA∕cm2 was obtained at an operating electric field of 0.7V∕μm. Such low field results from an extremely high field enhancement factor of 4.11×104 due to a combined effect of the high intrinsic aspect ratio of ZnO nanowires and the woven geometry of carbon cloth.

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.

Large-scale periodic patterns and their applications for growing aligned nanotube and nanowire arrays

Large-scale, two-dimensional arrays of periodic particles were prepared by nanosphere lithography. We modified the fabrication technique based on a self-assembly of latex particles on water surface in order to improve mask quality and size. Modifications of particles arrangement in an array were also practicable by using double-layered masks and mask transfer method. Such particle arrays were used for catalytic growth of aligned carbon nanotubes and ZnO nanorods with various configurations, length, and diameter. These exhibit interesting phenomena - antenna effects, photonic bandgap behavior, subwavelength lensing, and enhanced field emission. Therefore, they can be used in variety of future optoelectronic devices, such as THz and IR detectors.

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.

Nanomaterials fabrication and physics

We have studied growth of carbon nanotubes by chemical vapor deposition and zinc oxide nanowires by physical vapor deposition on carbon cloth with iron sulfate or stainless steel as a catalyst, and various combinations of carbon source and carrier gases. The field emission of these nanostructures shows a combined result of field enhancement from carbon nanotubes or Zinc oxide nanowires and carbon cloth. An emission current density of 1 mA/cm2 was obtained at 0.4 V/μm and 0.7 V/μm for nanotubes and nanowires, respectively. Field enhancement factor of ~4x104 has been observed. Moreover, electron transport characteristics and structural studies of carbon nanotubes have been investigated. Microscopic observations of electric wall-by-wall breakdown imply that transport in the nanotubes is not ballistic and that a significant scattering occurs as carriers traverse nanotubes length.