OVERVIEW ON THE MAJOR RESEARCH ACTIVITIES ON CARBON NANOTUBES BEING DONE IN AMERICA, EUROPE AND ASIA - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 248

On the other hand, the group has also considered the intersubband
decay of E 22 excitons in semiconducting CNTs, through both theoretical and
experimental evaluations. 31,33 Photoluminescence spectra showed that the
global behaviour of E 22 line widths is strongly associated with E 11 and E 12 exciton
bands, while it is independent of their chiral indices ( n , m ) normally used to
describe the nanotube structure. Conversely, the tubes' chirality affected the
optical properties of CNTs exposed to an external static electric ield applied
along the tubes' axis. 34 Theoretical calculations predicted different effects,
including characteristic Franz−Keldysh oscillations, quadratic Stark effects
and ield dependence of the bound exciton ionisation rate.
In a recent investigation, 35 the group showed that the photoluminescence
of a partially suspended, semiconducting CNT-based FET was quenched and
red-shifted upon application of a longitudinal electric ield. The authors
explained the quenching by a loss of oscillator strength and by an increase
in the non-radiative decay of the E 11 exciton; the shifts towards lower
frequencies were instead attributable to ield-induced doping (i.e., change
of electronic properties), most probably associated with the screening and
heating of CNTs.

9.2.1.3 CNT nanophotonics

In addition to electronics, semiconducting SWCNTs have shown a promising
future in nanophotonics. To that purpose, Avouris and collaborators have
integrated a single, electrically excited, semiconducting nanotube transistor
with a planar

λ

/2 microcavity towards nanotube-based nanophotonic

devices. 36
The device was based on a single semiconducting single-walled carbon
nanotube-based FET combined with a planar optical λ /2 microcavity. 37-40
A three-dimensional schematic representation of the device is depicted in
Fig. 9.4 . The photonic microcavity consisted of three dielectric layers: 250
nm of polymethyl methacrylate (PMMA), 22 nm of aluminium oxide (Al 2 O 3 )
and 250 nm of silicon oxide (SiO 2 ) sandwiched between a top gold mirror (20
nm) and a parallel bottom silver mirror (100 nm). Moreover, an individual
nanotube was placed above the Al 2 O 3 layer, parallel to the silver and the
gold mirrors, to provide optimum coupling conditions. The applied current
typically ranged between 5 and 10 μA, and electrons represented the main
carrier injected. Subsequently, the radiative decay of the produced excitons
determined light emission. Interestingly, the electroluminescence spectra
obtained from this nanotube-based system involved the same E 11 excitonic
state as observed in the photoluminescence of nanotubes in suspension.
In other words, little difference in the effective dielectric constants for a
nanotube on SiO 2 and embedded in PMMA was observed in comparison with
tubes suspended in solution. Therefore, it was suggested that similar devices