CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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constants of τ 1 = 7ps (53%), τ 2 = 35 ps (25%) and τ 3 => 400 ps (22%) gives an idea of
charge recombination dynamics of both hot and trapped charge carriers. In Figure 2 we can
observe a positive absorption beyond 670 nm after photoexcitation. This wavelength region
(670 nm -1000 nm) is suitable for monitoring the trapping dynamics of the charge carriers as
it does not overlap with excitonic position of the QDs. The kinetic decay trace at 1000 nm
which is attributed to positive absorption of the charge carriers (both electrons and holes) can
be fitted multi-exponentially with time constants τ 1 = 1ps (72.5%), τ 2 = 25 ps (22%) and τ 3
> 400 ps (5.5)%. The kinetic decay trace at 1000 nm gives us information about trapping
dynamics of the photo-excited charge carriers. Here we can see a fast component (1ps) which
dominates the decay kinetics at 1000 nm. So the extra ultrafast component can be attributed
to trapping dynamics due to charge carriers in the QD. Klimov et al [4.2, 4.19] reported a 1.5
ps dynamics in this spectral range and assigned it to the relaxation of the hole. On the other
hand, Guyot-Sionnest et al [4.6,21] reported intraband electron transitions in the IR range
from 2.5 to 5 m. Therefore it was debated whether the features observed at even shorter
wavelength (1900-2000 nm) could be due to the hole transitions. However in the present
studies we are monitoring in the visible to near IR (below 1000 nm), so the trapping
dynamics involve in this wavelength region might not be due single carrier (electron or hole).
To disentangle the trapping dynamics of individual charge carrier we have realized that it is
very important to quench individual carrier (both electron and hole) with suitable quencher
and monitor the kinetics, where we can separate the trapping contributions from each of the
other charge carrier. As mentioned earlier, addition of these quenchers lead to a pulse width
limited separation of charge carriers, therefore they compete with the trapping of charge
carriers. The concentrations of the quenchers used in the present study were ~10 -3 M which is