CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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dynamical spectrum is negligible [5.5]. The broad feature beyond the exciton bleach feature
can arise from trapped carriers or intraband transitions [5.18]. The assignment of this feature
to intraband transition is highly unlikely as the intraband transition show very fast decay (few
ps) and highly wavelength dependent kinetics which is not observed in the present
investigation. So from the above observation we can safely conclude that the feature most
likely comes from trapped carriers.
To monitor the charge carrier dynamics in CdSe QD we have monitored the bleach
recovery kinetics at 460 nm and transient decay kinetics at 720 nm. Figure 6A show the
bleach recovery kinetics of CdSe QD at 460 nm (1S Excitonic position). The growth kinetics
of the bleach at 460 nm can be fitted 155 fs and the recovery kinetics can be fitted multiexponentially
with time constants 2.55 ps (45%), 27 ps (17%) and > 400 ps (38%). Here 155
fs time constant can be attributed to state filling to 1S excitonic level [5.8]. The bleach
recovery time constants suggest the charge recombination time constants between photogenerated
electrons and holes. We have also monitored decay kinetics of excited state
absorption (ESA) of photo excited CdSe at 690 nm (Figure 5.4 inset) which can be fitted
multi-exponentially 1.3ps (30%), 10 ps (45%) and >200 ps (25%). It is interesting to see that
growth time of the signal can be fitted single exponentially with pulse-width limited time
constant (< 100 fs). Earlier investigations suggest that the positive absorption band in NIR
region is mainly due to intraband transition of the photo excited holes [5.21]. In our early
studies of CdTe QD we have observed that positive absorption can be due to both the trapped
charge carriers (electron and hole).