CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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Sample t r(ps) t 1(ps) t 2(ps) t 3(ps)
CdSe 0.155 2.55(45) 27(16.8) >400(38.2)
CdSe/ZnTe3 0.18 2.59(30) 30(33) >400(37)
CdSe/ZnTe4 0.3 3(21.4) 50(32.5) >1000(46.1)
Table 5.2: Fit for kinetic traces of a) CdSe at 460 nm, b) CdSe/ZnTe3 at 480 nm,
and c) CdSe/ZnTe4 at 520 nm after excitation at 400 nm laser light.
To understand the influence of thickness of shell on cooling dynamics after
photoexcitation we have shown the photoinduced process in Scheme 1. As illustrated in the
scheme 1, 400nm light can either excite the carriers directly from ZnTe to CdSe by a charge
transfer or CdSe may be first excited and hole transfer occurs subsequently. However the
possibility of direct excitation from ZnTe to CdSe is very low due to forbidden nature of the
transition. Therefore, we can safely assume that the excitation initially populates the CdSe
levels and then holes are transferred to ZnTe. Assuming that holes are injected from CdSe,
there are two ways hole can be transferred from CdSe to ZnTe as depicted in Scheme 1.
In route I, photo-generated holes in CdSe can cool and then be transferred to ZnTe
shell. On the other hand in route II hot holes from CdSe can be directly transferred to ZnTe
and get cooled in the valence band of ZnTe. We can find out the most probable route by
analysis the cooling dynamics of charge carriers in both core and core-shell samples. We
have observed that cooling time as monitored by bleach formation kinetics of charge carrier
(mostly contributed from electron) increases with the thickness of the shell from 150fs for
CdSe to ~300fs for CdSe/ZnTe4 core shell.