Optoelectronics with Carbon Nanotubes

ii. Electroluminescence Intensity in the Unipolar Regime
Figure III-4 shows an example of EL intensity vs. photon energy as the drain bias is
decreased stepwise from -5.0 V to -8.0 V while the device was in an “on” state by keeping VGS at
-7 V. One large, broad peak is observed around 0.7 eV that seems to be comprised of three or
more different peaks (note the values 0.66 eV, 0.71 eV and 0.88 eV in the figure). The main
peak is identified as the emission from E11 excitons, given the known diameter distribution of the
sample. It also agrees with theoretical predictions 24, 25 and previous two-photon spectroscopy
and photoconductivity experiments 26-28 that show that the dominant transition is from the E11
excitonic state.
Referring to the PL work by Weisman et al. 15 , this E11 value suggests a CNT diameter of
1.4 to 1.5 nm, which is on the small-diameter tail of the distribution for this sample which has
the average diameter of 1.8 nm. The precise (n, m) assignment is not possible from these data,
given the unknown shifts under electric field and/or high temperature, broadening and the limited
signal-to-noise ratio. Based on the comparison by Fritag et al. of PL and EL from a same tube
109 , it is also likely that this E11 peak is already red-shifted from a field-induced doping by tens of
meV with respect to the PL peak for the same tube. If this is the case, the corresponding
diameter may even be slightly smaller. Most other devices from the same sample showed the
main peak closer to 0.6 eV and even lower, sometimes not detectable because of the detection
window cut-off (~0.56 eV), which agrees with the average diameter of 1.8 nm.
According to Ref. 18 and assuming the diameter of 1.45 nm, the energy gap (Eg)and the
binding energy of E11 excitons for this SWNT are expected to be 0.95 eV and 0.23 eV,
respectively, giving the E11 excitonic transition energy of 0.72 eV 15 , which is also in agreement
with the main peak position of this device within experimental uncertainty and differences due to
dielectric environment. Note that the free-particle recombination peak at 0.95 eV is not
detectable because the oscillator strength has been transferred to the excitonic E11 peak and
because of the large broadening that obscures smaller peaks.
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