Optoelectronics with Carbon Nanotubes

etter control of injected carriers also resulted in a great improvement in emission efficiency. In
addition, we also compared emission from a p-n junction and from a CNTFET using the same
tube, which further provided evidence for broadening due to high electric fields in CNTFET.
From a practical point of view, single-tube devices have many shortcomings, such as the
non-uniform wavelength of emitted photons from different tubes, and the lack of robustness of
any single device. Since CNTs are actually a family of different structures which also come in
metallic and semiconducting varieties with different energy gaps, controlling the output
characteristics of a single-tube CNTFET or p-n junction is essentially impossible. Chapter V
presents results from carbon nanotube film LEDs made of many aligned tubes that had been
purified to be 99% semiconducting. Using such thin films made from almost-parallel CNTs has
several advantages. Firstly, electric current per tube is greatly reduced, making it unlikely that
individual tubes fail. And because of the presence of many tubes in a device, the device remains
operational even if a few tubes fail, and the overall emission characteristic changes little.
Secondly, statistical averaging leads to similar peak energy levels among devices for better
uniformity. And finally, the total light output from a film device far exceeds that from a single-
tube under similar operating voltages. For these reasons, CNT thin film devices are perhaps the
most promising in terms of practical applications.
The history of EL from CNT devices discussed in the previous section shows that there is
much we do not yet understand regarding emission mechanisms, and that there lies a great deal
of challenge before CNTs can be used in commercial optoelectronics. In the following pages,
we hope to contribute to a better understanding of EL emission from CNT devices and hence to
better techniques for employing CNTs in applications.
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