Optoelectronics with Carbon Nanotubes

found multiple tubes bound together in numerous AFM measurements and some Raman
measurements of our devices. In contrast, very large widths are always observed and multiple
peaks are observed very commonly in back-gated devices, suggesting that these are basic
features that characterize single-tube, global-gate devices.
The small peak at 0.88 eV in Figure III-4, is observed in some devices, though not all.
This peak is interpreted to be the signature of an exciton-optical phonon complex. The peak
from optical phonon scattering has been directly observed in photoconductivity 27, 118 and
photoluminescence 29, 30, 46, 119 experiments, and provides evidence for the excitonic nature of the
dominant lowest-energy peak observed from SWNTs. The energy separation from the main
peak (i.e., 0.88 eV – 0.71 eV = 0.17 eV) is somewhat smaller than the 0.18 eV observed in PL
studies. However, the main peak is a combination of 0.66 eV and 0.71 eV peaks and the
sideband is broadened from 0.88 eV toward lower energy (with possibly another peak around
0.85 eV), suggesting that the latter is also a combination of two optical phonon sidebands. Since
we observe two main excitonic transitions at 0.66 eV and 0.71 eV, each one can have its own
sideband due to optical phonon scattering.
The optical phonon sideband is also expected to be significantly broader than the main
peak because finite-momentum phonons are allowed to contribute to the final excitonic zero-
momentum state. The broad width of the absorption spectrum of optical-phonon sideband was
indeed observed in a photoconductivity experiment 27 , which is the opposite process of
electroluminescence. This peak also adds to the asymmetry of the spectral peak as the broad
“bump” between 0.8 eV and 1.0 eV.
Aside from optical phonons, exciton dephasing (i.e., changing of the k-state by inelastic
scattering with phonons) by acoustic phonons can lead to broadening, as in the case of
photoluminescence. Lefebvre et al. measured FWHM of PL spectrum on single SWNTs as a
function of temperature, and obtained the linear relationship FWHM = 2 + 0.025T where the
width is in meV and T is in Kelvin 115 . 0.025 meV K -1 is the coupling constant between acoustic
phonons and excitons, which can be also expressed as 0.29 kB where kB is the Boltzmann
constant. This value reflects a FWHM of the PL spectrum that is significantly below the thermal
energy, kBT, at least down to 50 K, which is expected from the expected weak coupling between
acoustic phonons and excitons.
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