Optoelectronics with Carbon Nanotubes

Finally, we note the observation of EL from carbon nanotube networks. An array of
many CNTs forming a thin film offers greater robustness than single-tube devices, and is more
reproducible and consistent because of statistical averaging among the tubes. Thin CNT films
and transistors made from such structures have garnered attention as a viable material in flexible
and transparent electronics component with high mobility 90-96 . EL from a random network of
SWNTs as a transistor device was first observed by Adam, et al. by ambipolar recombination of
carriers 97 . The spectral analysis of the light emission revealed that it was preferentially
generated in the largest diameter tubes in the sample; they found that convoluting the density of
states and the Fermi-Dirac distribution of carriers reproduced the red-shift observed in the main
EL peak relative to the absorption peaks. Their calculations show that the largest diameter tubes,
though comprising only 4 % of their sample, dominate the EL spectrum.
The quality of CNTFETs was greatly improved by Engel et al., when they combined 99%
purely semiconducting tubes produced by density gradient ultracentrifugation 98 and a CNT self-
assembly technique from solution 99 ; they succeeded in fabricating FETs from highly aligned
CNT thin films with a high on/off ratio and a very low sheet resistance. The devices operated in
ambipolar transport regime, and the EL from this material was also found to emit mainly from
large-diameter tubes even at higher source-drain biases (VDS). They attribute this to the fact that
the current is carried preferentially in large-diameter tubes, and that excitons created in smaller-
diameter tubes can decay into large-diameter tubes before decaying radiatively. This same
material was used in the film LED experiment in this work that will be discussed in much more
detail later.
EL has also been observed from an array of aligned SWNTs grown on a quartz substrate.
It is possible to grow very straight and long carbon nanotubes that are aligned along a
crystallographic plane of quartz. Zaumseil et al. achieved an efficient injection of carriers by
using electrolytes instead of oxide dielectric, which enable a high capacitive coupling between
the gate and the nanotube array grown on quartz 10 . They observed many ambipolar emission
spots at relatively low source drain voltage (|VDS| < 3 V) thanks to the efficient gating provided
by ionic liquids. However, the calculated light emission efficiency is quite low (~10 -9 ), although
this may be partly due to the limits of detection. Another problem is the presence of metallic
tubes in the array, resulting in high off-currents and poor on/off ratios. There is little control
over the metallic/semiconducting ratio and the diameter distribution of aligned nanotubes grown
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