Optoelectronics with Carbon Nanotubes

In the previous study which examined global bottom-gated transistor behavior from the
identical raw material 99 , EL and PL intensity maximum of another device was found at 0.64 eV.
Referring to the empirical Kataura plot by Weisman et al. 15 , we see that different chiralities for
the same diameter could easily differ in emission energy by about 100 meV in this diameter
range. If only a limited number of tubes are involved in emission, as discussed above, statistical
averaging between samples is limited; the difference of 40 meV in peak positions between
devices is entirely possible. Since we do not have enough spatial resolution to identify the tubes
participating in emission (let alone determine diameters and chiralities of those tubes), it is not
possible to make any meaningful conclusions based on the comparison between the two unipolar
emission peaks, other than suggesting that that number of tubes involved in conduction and
emission may be small.
On the other hand, the ambipolar and unipolar emissions in our data are from the same
device and from the same measurement session, which makes them more directly comparable.
Figure V-9 (c) compares normalized intensities of the two modes. In addition to the small
difference in the maximum intensity energies, we see that in forward bias, there is a greater
spectral weight of higher-energy emission. This was not observed in reverse bias, even when the
intensity was comparable (when the intensity is increased, the broadening obscures the
difference somewhat, but there is no feature around 0.65 eV). This would not be the signal from
exciton-optical phonon coupling, since the optical phonon energy is about 180 meV, much larger
than the signal-main peak separation observed. The difference in energy suggests the separation
between the defect-bound and the “free exciton” states in single-tube CNT diodes. While it may
seem unlikely that one observes spectral features in electroluminescence signal from such an
ensemble of CNTs with a range of diameter and various chiralities which undoubtedly lead to a
wide range of E11 transition energies 15 . However, existing literature on tube-tube energy
transfer suggests that the long channel length ensures that lower energy states dominate, since
energy relaxation to tubes with lower-energy E11 states happens very fast, within 3 ps. In that
case, it is reasonable that the defect-bound states and free-exciton states can show up as two
peaks at least to some extent.
We have seen during single-tube LED measurements that the lower-energy, defect-bound
state sometimes appears after the tube has been stressed with higher levels of current. The films
devices have already gone through higher-current measurements and “recycling” from previous
99