CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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of 3.2ps (~90%) which can be attributed to de-population time constant of second exciton.
The recovery kinetics at 610 nm can be fitted with time constants (7ps, 35ps, >400ps)
attributed to de-population time constant of second exciton which basically the charge
recombination dynamics between electron and hole pairs.
We have observed that on excitation of CdTe QD, bleach appears at the
excitonic positions. By monitoring the excited state dynamics at those excitonic wavelengths,
we get information of both cooling and recombination dynamics of the charge carriers. It is
reported in the literature that on size quantization both valence and conduction band becomes
discrete and shows interesting properties like slow carrier cooling dynamics in QD material
as compared to that in bulk. The cooling based on Frohlich interaction by electron-LO
phonon interaction increases with respect to radius because in smaller quantum dot the
energy separation between the exciton states are much larger as compared to larger quantum
dot due to increase in confinement energy as given below
-1
= exp( E/k
LO
BT)
(4.1)
Where ΔE is the energy difference between the exciton states and ω LO is the energy of LO
phonon [4.2]. Typically in a QD this is expected to give a cooling time of several hundreds of
picoseconds. However in our investigation we have observed that the cooling time is sub 1 ps
for the charge carriers. Again according to the above equation, cooling time of QD should
increases with decreasing the size of the QD. In the present investigation we have also
synthesized different sizes QDs and carried out transient absorption studies to monitor the
cooling time for different size particles by following the bleach at the excitonic wavelengths.
Panel A Figure 4.3 shows the kinetic traces (bleach kinetics) at 1S 3/2 -1S e excitonic position
for different QD sizes namely 3.3 nm, 3.5 nm and 3.8 nm. It is very interesting to see that