CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

146
However, due to staggered band alignment between CdSe and ZnTe, photoexcited
hole which was generated in CdSe core will migrate to ZnTe shell. Now to understand charge
carrier cooling dynamics and charge transfer dynamics we have monitored the kinetics at the
bleach wavelengths for CdSe core and CdSe/ZnTe core-shell of different thickness and
shown in Figure 8. In the inset of Figure 8 we have shown the bleach kinetics at shorter time
scale.
We can clearly see that the growth time of the bleach from core to core-shell and
further on increasing shell thickness. The growth time can be fitted with time constants of
155fs for CdSe core, 180 fs for CdSe/ZnTe3 and 300 fs for CdSe/ZnTe4 core-shell (Table 2).
This growth time of the bleach can be attributed cooling of the charge carriers [5.8]. In a bulk
semiconductor the cooling of carriers is expected to be pulse width limited. This can be
rationalized as follows, on photoexcitation the carriers are initially populated in the higher
exciton level as 400nm laser light populates the higher excitons which finally populates the
lowest excitonic state with the formation of 1S bleach [5.20]. In QD bleach formation is
delayed due to quantized nature of the individual bands [5.1]. The main contribution to
cooling of the carriers comes from Auger mediated or surface mediated processes as reported
earlier and not by multi-phonon emission. Since density of states (DOS) of holes are much
larger compared to electrons because of higher effective mass and degenerate valence band,
faster cooling of hole is achieved by of electron-hole (e-h) mediated energy transfer and
subsequent electron cooling [5.8, 5.22-5.23]. So in the present investigation we attribute that
the cooling of the charge carrier mainly to electron cooling.