BASIC CONCEPTS AND AN OVERVIEW - Foundations of Nanotechnology: Pore Size in Carbon-Based Nano-Adsorbents - page 18

by hotwire CVD using a mixture of C 2 H 2 and NH 3 gases. They used crystalline
Si and SiO 2 substrates coated with Ni films of 30 nm in thickness as a catalyst for
nanotube formation. SEM images showed that micron sized grains were present
in the deposit on the Si substrate, whereas nanosized grains were evident in the
deposit on the SiO 2 substrate. Nanotube formation could not be confirmed by
SEM, but there was evidence of the possible formation of nanotubes on the Ni-
coated SiO 2 substrate. Two novel nanostructures, which are probably the missing
link between onion like carbon particles and nanotubes have also been obtained.
Low synthesis temperature <520°C due to the non-equilibrium characteristics
of microwave plasma operated at low pressure is also reported, which is cru-
cial for some fascinating applications. Some researchers employed a high-density
plasma chemical vapor deposition (PECVD) to grow high quality carbon nano-
tubes at low temperatures. High density, aligned CNTs can be grown on Si and
glass substrate. The CNTs were selectively deposited on the patterned Ni catalyst
layer, which was sputtered on Si substrate. Some others synthesized pure carbon
nanotubes at very low temperature using MW-PECVD with methane/hydrogen
gas. Others prepared the massive carbon nanotubes on silicon, quartz and ce-
ramic substrates using MW-PECVD. The nanotubes, ranging from 10 to 120 nm
in diameter and a few tens of micron in length, were formed under hydrocarbon
plasma at 720°C with the aid of iron-oxide particles. Morphology of the nano-
tubes is strongly influenced by the flow ratio of methane to hydrogen. Defectless
nanotubes with small diameters are favorably produced under a small flow ratio.
To date, many methods for synthesizing carbon nanotubes have been developed
and most of which operated at high temperature over 4000°Cfor graphite arc dis-
charge and laser ablation. These temperatures are too high and unsuitable for the
fabrication of electronic devices because most electric connections are made of
aluminum with the melting point below 700°C. These challenges have promoted
the current exploration of low temperature synthesis of carbon nanotubes such as
CVD, PECVD and MWPECVD. Recently, these low temperature methods have
been successful in growing highly aligned and large quantities of carbon nano-
tubes. It is also possible to control over length, diameter and structure of carbon
nanotubes grown by CVD techniques. Therefore, CVD techniques are the most
popular methods to synthesize the carbon nanotubes [56].
Recently have grown straight carbon nanotubes, carbon nanotubes “knees,”
Y-branches of carbon nanotubes and coiled carbon nanotubes on a graphite sub-
strate held at room temperature by the decomposition of fullerene under moderate
heating at 450°C in the presence of 200-nm Ni particles. The grown structures
were investigated without any further manipulation by STM. The formation of the
carbon nanostructures containing non-hexagonal rings is attributed partly to the
templating effect of the high pyrolitic graphite (HOPG), partly to the growth at
room temperature, which enhances the probability of quenching-in for non-hex-
agonal rings. Similar coiled nanotubes were found after several steps of chemical