Optoelectronics with Carbon Nanotubes

iases constant at -5V and +5 V, and stepped up the VDS in the positive (a) and negative (b)
directions to induce the two different modes similar to Figure V-8. We fit Gaussian distributions
to the spectra to extract the peak positions and the widths under the assumption that the emission
comes from a collection of multiple tubes with a distribution of diameters. As we shall see in the
following, the emission is not a simple reflection of the diameter distribution; however there is
still a multiple-tube effect and the Gaussian function fits the emission profile quite adequately
(Figure V-10 (b), inset).
The device emits electroluminescence in the near infrared with a maximum spectral
intensity at about 0.58 eV in the forward direction and 0.60 eV in reverse. There was no
measurable peak shift observed as the input power was increased. Given the diameter range of
1.3 nm to 1.7 nm and referring to the Kataura plot 15 , the intensity peaks are at the lowest edge of
the E11 energy range for the sample, in agreement with Ref. 97. As Engel et al. suggests 99 , this
is expected since larger diameter tubes have smaller Schottky barriers which is critical for carrier
injection, and excitons efficiently relax into lower-energy states in large-diameter tubes 159, 160 .
Adam, et al. found a similar effect in their carbon-nanotube network FETs operated with a global
gate 97 . An efficient energy transfer mechanism, Förster Resonance Energy Transfer, has been
suggested as the major contributor because of the aligned orientation of dipoles between exciton
donors and acceptors 160 . Qian et al. obtained the maximum transfer rate of 0.5 ps -1 in their
DNA-wrapped CNT sample 159 , which is even faster than the nonradiative decay rate in our
single-tube LEDs (a few ps). As we saw in the AFM profile of our sample (Figure V-1 (c)), the
tubes are largely in bundles; furthermore, most of our tubes are in good alignment with each
other (Figures V-1 and V-2), making the Förster interaction a very likely mechanism responsible
for the observation of energy peaks at the lowest-energy edge of our diameter range.
97