Guan Yow Chen

Carbon nanotubes grown on In2O3:Sn glass as large area electrodes for organic photovoltaics

The authors report the growth of multiwall carbon nanotubes directly onto indium tin oxide glass via chemical vapor deposition as large area semitransparent electrodes for organic solar cell applications. The rate of nanotube growth on this ternary oxide is greatly reduced as compared to that of silicon dioxide and glass substrates enabling a high degree of control over nanotube height. The strong potential of this nanostructured semitransparent substrate as an interpenetrating hole-extracting electrode in bulk-heterojunction organic solar cells is also demonstrated.

Growth and field emission properties of vertically aligned carbon nanofibers

Vertically aligned carbon nanofibers (VACNFs) were synthesized on Ni-coated Si substrates using a dc plasma-enhanced chemical-vapor deposition system. The size of the Ni islands used as catalyst to grow the VACNFs was formed by both thermal annealing and laser processing on thin metal layers. It was observed that the diameter of the carbon nanofibers is strongly dependent on the initial Ni island dimension. By varying the laser power from 228 to 279mJ∕cm2, the size of these Ni islands can be controlled independent of the initial Ni film thickness. Electron field-emission results show that the emission threshold field is dependent on both the height and radius of these VACNFs and also field shielding effects. Threshold fields as low as 2V∕μm was obtained from the sample with the largest height over radius ratio.

The role of the gas species on the formation of carbon nanotubes during thermal chemical vapour deposition

In this paper, we investigate the several roles that hydrogen plays in the catalytic growth of carbon nanotubes from the point of view of gas species, catalyst activation and subsequent interaction with the carbon nanotubes. Carbon nanotubes and nanofibres were grown by thermal chemical vapour deposition, using methane and a mixture of hydrogen and helium, for a range of growth temperatures and pre-treatment procedures. Long, straight carbon nanotubes were obtained at 900 °C, and although the growth yield increases with the growth temperature, the growth shifts from nanotubes to nanofibres. By introducing a helium purge as part of the pre-treatment procedure, we change the gas chemistry by altering the hydrogen concentration in the initial reaction stage. This simple change in the process resulted in a clear difference in the yield and the structure of the carbon nanofibres produced. We find that the hydrogen concentration in the initial reaction stage significantly affects the morphology of carbon fibres. Although hydrogen keeps the catalyst activated and increases the yield, it prevents the formation of graphitic nanotubes.

Deployment of titanium thermal barrier for low-temperature carbon nanotube growth

Chemical vapor-synthesized carbon nanotubes are typically grown at temperatures around 600°C. We report on the deployment of a titanium layer to help elevate the constraints on the substrate temperature during plasma-assisted growth. The growth is possible through the lowering of the hydrocarbon content used in the deposition, with the only source of heat provided by the plasma. The nanotubes synthesized have a small diameter distribution, which deviates from the usual trend that the diameter is determined by the thickness of the catalyst film. Simple thermodynamic simulations also show that the quantity of heat, that can be distributed, is determined by the thickness of the titanium layer. Despite the lower synthesis temperature, it is shown that this technique allows for high growth rates as well as better quality nanotubes.

Growth of tungsten oxide nanowires using simple thermal heating

Tungsten oxide nanowires are grown directly on tungsten wires and plates using thermal heating in an acetylene and nitrogen mixture. By heating the tungsten in nitrogen ambient, single crystal tungsten oxide nanowires can be synthesized via a self-assembly mechanism. It was found that the yield can be significantly increased with the addition of acetylene, which also results in thinner nanowires, as compared to nanowires synthesized in an oxidizing ambient. The tungsten oxide nanowires are 5 to 15nm in diameter and hundreds of nanometers in length. In some cases, the use of acetylene and nitrogen process gas would result in tungsten oxide nanowires samples that appear visually transparent. Comparison of the growth using the acetylene/nitrogen or then air/nitrogen mixtures is carried out. A possible synthesis mechanism, taking into account the effect of hydrocarbon addition is proposed.

In situ Observation of the Growth of Tungsten Oxide Nanostructures

This paper describes a method for producing tungsten oxide nanostructues in an environmental scanning electron microscope. The growth is observed in real time and offers direct observation of the morphology of the tungsten oxide condensed onto the cooler part of a typical W scanning electron microscope filament. We also find that the growth of nanostructures occurs on timescales much shorter than reported thus far. Corresponding to increasing temperatures along the tungsten filament, we find that the tungsten oxide deposits successively as nano-clusters, nano-multirods, ‘pine-tree’-like structures and, ultimately, single nanowires.

Growth kinetics changes of vertically aligned carbon nanostructures synthesised at low substrate temperatures

Carbon nanotubes and nanofibres are typically synthesised under substrate temperatures above 600°C. Here we investigate the influence of the substrate temperature and the plasma conditions on the growth of vertically aligned carbon nanostructures using Direct Current plasma Chemical Vapour Deposition, at temperatures below 550°C. These nanostructures are produced using a C 2H2 based plasma and nickel thin film as the catalyst. We found that preferential deposition of amorphous carbon takes place as the synthesis temperature is lowered below 500°C. However, lowering the carbon concentration in the gas feedstock (<2% cone.) allows for the nucleation of nanofibre-like structures, whilst balancing the buildup of amorphous carbon. This method allows for the synthesis of vertically aligned structures at low temperatures (around 230°C) without intentional heating, while still achieving reasonable average growth rates up to 27 nm/min. The only heating was provided by the plasma, which typically consumes ∼ 4 W/cm2. It was found that by varying the applied plasma bias during high temperature synthesis, we increased the growth rate up to 165 nm/min. Based on the observations of experimental process variations and the morphology of the synthesised structures, we propose a growth mechanism for such low temperature growth and examine the resulting morphology changes.

Direct catalytic growth of high-density carbon nanotubes on nanoclusters at low temperatures

Carbon nanotubes (CNTs) have received extensive attention due to their one-dimensional structure and ability to demonstrate many novel physical and chemical phenomena in the quantum scale. However, the application of CNTs in electronics is hindered due to their higher growth temperatures which are usually in excess of 500 °C, which is not compatible with current semiconductor technology in industry. Low temperature growth is necessary for integrating CNTs into standard semiconductor devices such as CMOS and large-scale integrated circuits. To date, various techniques have been utilised to lower the CNT growth temperature by: 1. using various carbon sources with lower dissociation temperature; 2. exploring metal catalyst films of the low melting point or metal nanoparticles as catalysts; and, 3. introducing a plasma during deposition to increase the dissociation and ionization of feed gases. In this study, we report the low temperature growth of vertically aligned high-density CNTs by a DC plasma chemical vapour deposition method, using Ni nanoclusters as catalysts. The Ni nanoclusters are free from a high-temperature formation process compared to the film based catalysts and directly demonstrate catalytic growth of CNTs at substrate temperatures as low as 390 °C. The density of as-grown CNTs is up to 10 /cm , as shown in Figure 1. Transmission electron microscopy studies show the CNTs are made of crystalline graphene shells and have a uniform diameter distribution. The field electron emission properties of the samples are investigated.

Gas Sensing Properties of Vapour-Deposited Tungsten Oxide Nanostructures

Tungsten oxide nanostructures of the type WO3−x, where x=0..1, show excellent promise for gas-sensing and electrochromic applications. Here we determine the morphology and chemical composition of the deposited nanostructures using transmission electron microscopy and x-ray photoelectron spectroscopy. We also show the gas sensing properties of the deposited nanostructures for the different structures and relate the observed behaviour to the oxygen vacancies present in the respective tungsten oxide allotrope.

Influences of hydrogen gas on carbon nanotube growth

For practical deployment of carbon nanotubes, an understanding of their growth mechanism is required in order to obtain better control over their crystallinity, chirality and other structural properties. In this study, we focus on the influences of gas species on carbon nanotube synthesis using thermal chemical vapour deposition. The influence of methane, hydrogen, and helium gases was investigated from the viewpoint of gas chemistry in relation to the nanotube structural change, by varying the growth pressure, the gas-flow ratio and the growth temperature. Simple changes in the hydrogen gas concentration during different growth stages have been found to induce surprising changes to the nanotube formation. The structure of the tubular carbon growth changed from amorphous to graphitic as the growth temperature and the concentration of hydrogen in the initial periods of growth decreases. The excess hydrogen tends to give rise to poor crystalline carbon nanofibres but has the effect of increasing the yields. Hydrogen gas is typically used in reducing metal catalyst particles during the pre-treatment and the carbon nanotube growth periods. We show that while hydrogen species can improve yield, it can also result in the degradation of the nanotubes crystallinity. The use of hydrogen in the growth process is one of the key parameters for enhanced control of carbon nanotube/nanofibre growth and their resulting crystallinity.