Optoelectronics with Carbon Nanotubes

The CNTs produced for this study were all known to be single-wall nanotubes (SWNTs) from
their prior characterizations 100-102 .
The arc-discharge method was the first one used in the discovery of CNTs 2, 3, 103 . It has
since been improved to produce long, aligned bundles of SWNTs suitable for electronic and
optical experiments at the single-tube level 100 . Liu et al. successfully synthesized ropes of high-
quality SWNTs with the individual SWNT diameter of about 1.7 nm by creating a high direct-
current plasma arc between two graphite rods with appropriate metal catalysts (nickel and cobalt)
and a growth promoter (sulfur). The arc-discharge SWNTs used in this study were produced by
the same group at Shenyang National Laboratory for Materials Science, Institute of Metal
Research in China. Its diameter range was determined to be 1.8 nm to 2.2 nm by independent
TEM measurements of tubes from the same batch.
Similarly to arc-discharge, laser ablation produces bulk bundles of CNTs. This technique
utilizes continuous or pulsed high-intensity laser that ablates a carbon target containing metal
catalysts (nickel and cobalt). CNTs form as the laser-evaporated carbon condenses in a 1200 °C
furnace, and are then collected by an inert gas flow 102 . Van der Waals interactions cause the
formation of bundled ropes that consist of hundreds of SWNTs each. The laser ablation sample
used was synthesized at Rice University and consisted of the smallest diameter range of all the
samples used. Statistics using the TEM have shown the diameters to be 0.6 nm to 1.5 nm,
centered around 1.0 nm 56 . Using this sample, a large-scale study of CNT field-effect transistor
(CNTFET) transport was conducted with three different contact metals, giving us a good
electrical characterization of back-gated CNTFETs and better understanding of the role of the
work functions of contact metals 56 .
While the previous two methods produce CNT bundles which require chemical
processing and sonication to be separated into individual tubes, the CVD approach allows for a
direct growth of clean, spatially-separated CNTs on the substrate. Catalyst particles dispersed in
solution are spun on a sample surface and heated in a tube furnace through which a hydrocarbon
gas flows. CVD eliminates the need for sonication to unbundle individual CNTs, avoiding extra
chemical and/or physical stress that could contribute to unwanted defects. Defects and chemical
residues can significantly affect the properties of CNTs and could complicate the interpretation
of experimental results. The CVD tube growth parameters had been tailored to grow small-
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