Optoelectronics with Carbon Nanotubes

emission observed at higher current hints at an annihilation of these localized excitons, possibly
by exciton-exciton interaction.
Finally, CNT film diodes were fabricated using a thin film of purified and aligned CNTs.
Although the efficiency is limited in these devices because they rely on percolation for carrier
transport, it showed a clear rectifying behavior and a very strong threshold-less emission that is
tunable by the top gate. The polarization of the emission from film diodes was comparable to
that of single tube CNTFETs.
Going forward, there is much that still needs to be addressed in electroluminescence from
carbon nanotubes to understand its fundamental physics. For example, temperature
measurements of phonons by Raman spectroscopy during transport could yield more precise
information on relative populations of different phonons, although Raman spectroscopy from a
single tube on a substrate is notoriously difficult. The situation can be improved by suspending
CNTs across a trench to decrease signal quenching by the substrate, but the device would in turn
be far more likely to fail from high temperature and oxidation. Another problem with this
method is that the dielectric environment would no longer be the same (i.e., it changes the
bandgap, excitonic binding energy and the phonon relaxation pathways), so the results would
need to be interpreted with care to be applicable to on-substrate devices. Nonetheless, suspended
CNTs are a promising structure for investigating their basic physics, especially in optics, because
of the CNT’s extreme sensitivity to extrinsic effects.
Another area that needs further exploration is the effect of external electric fields on the
luminescence process. Electric fields all along the channel can be measured directly using
photocurrent, which would provide a much more accurate estimate of its effects on exciton
production and on broadening. The challenge is that the spatial resolution of such measurement
is limited by the spot size of the excitation laser 70 , so a significant improvement in the technique
is needed to map out the details of Schottky barriers and other potential fluctuations along the
channel.
From the applications point of view, the CNT film diode is perhaps the most promising
type of device. The averaging effects that arise from using many tubes lead to reliable, robust
and consistent performance. Advances in CNT purification techniques can create raw materials
with even lower percentage of metallic nanotubes and perhaps even a very narrow range of
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