Biomedical Engineering Reference
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showed activated behaviour, as expected for a decay into optical phonons.
29
On the whole, the reported data accounted for an anharmonic phonon decay,
i.e., a non-equilibrium phonon distribution, in order to justify the observed
electronic transport behaviour of CNTs.
Once exposed to photo-excitation, CNTs reach high-energy E
22
state;
subsequently they relax (decay) by means of phonons (luorescence) to
lower-energy E
11
states, with a radiative lifetime at room temperature of
about 1-10 ns.
30
Recent work has suggested that only about 10% of the E
22
excitons decay frees electrons.
31
It must therefore be concluded that a non-
radiative-decay process controls the E
11
exciton lifetime. In other words,
another intrinsic energy dissipation channel must inluence the relaxation
of the lowest exciton state. To explain this phenomenon, the authors have
reported a theoretical study on the eficiency of decay pathways of nanotubes
involving purely multiphonon decay (MPD) as well as other electronic decay
mechanisms.
32
The calculations indicated that the MPD of free excitons is too
slow to be responsible for the experimentally observed lifetimes; conversely,
the combination of localised exciton MPD and the phonon-assisted indirect
exciton ionization (PAIEI) allow explaining the range of available experimental
data. The results suggested a mechanism in which PAIEI involved exciton
decay into both an optical phonon and an
intraband
electron-hole pair. In
fact, in the presence of the free carriers in CNTs, an exciton could decay fast
by creating a phonon and an intraband electron-hole (
e
-
h
) pair, as shown in
Fig. 9.3. The resulting lifetime was in the range of 5 to 100 ps.
Figure 9.3
Schematic representation of the exciton decay mechanisms: (a)
multiphonon decay (MPD) and (b) phonon-assisted indirect exciton ionisation (PAIEI)
in CNTs. Figure redrawn from Perebeinos and Avouris.
32
See also Colour Insert.
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