CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...
CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...
CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
120<br />
dynamics of hot electron. The presence of a hole quencher decouples the electron from the<br />
hole as the hole is localized on the surface therefore the interaction between the electron and<br />
the hole is low. Therefore the cooling dynamics exclusively contains information about the<br />
electron relaxation. It is interesting to see that the cooling dynamics of hole in the valence<br />
band is faster as compared to electron in the conduction band. Density of states in valence<br />
band is much higher as compared to the conduction band and also the energy gap between<br />
two quantized states in the valence band is much lower as compared to the energy gap<br />
between two quantized states in the conduction band. Therefore as expected the electron<br />
cooling takes place at a slower rate.<br />
Figure 4.4 also depicted the bleach recovery kinetics at first exciton position (at 610<br />
nm) from which we can estimate the charge recombination (CR) dynamics between the<br />
charge carriers. The bleach recovery kinetics of CdTe QD without any quenchers (Figure 4a,<br />
Panel A) can be fitted tri-exponentially with time constants of τ 1 = 7ps (53%), τ 2 = 35 ps<br />
(25%) and τ 3 => 400 ps (22%). It is interesting to see that majority of the charge carriers<br />
recombined with in 100 ps. This might be due to the QDs are smaller in dimension so it is<br />
expected that there will be reasonable wave function overlap between photo-generated<br />
electrons and holes. The longer components can be attributed to the CR dynamics between<br />
trapped charge carriers (both trapped electron and trapped holes). However we can see the<br />
bleach recovers at 610 nm much faster in presence of electron quencher (BQ) as shown in<br />
Figure 4.4 b. The kinetic decay trace can be fitted tri-exponentially with time constants τ 1 =<br />
5.5ps (56.5%), τ 2=60ps (38.7%) and τ 3 = >400 ps (4.8%). It was shown before by Burda et<br />
al [4.17] that the addition of benzoquinone (BQ) to a colloidal solution of CdSe rapidly<br />
removes the electrons from the conduction band of the photoexcited NP and forms a short