CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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1.5.2. Semiconductor-Dye Nanostructures
There are several approaches in such a nanostructure, one of the approaches being use
of dye for separation of carries from quantum dot. We mentioned earlier in section 1.3.4
possibility of generating multiexcitons in a single quantum dot via impact ionization. If one
needs to extract these carriers we need another acceptor group which has significant affinity
for one of the carriers, thereby leading to faster separation of carriers. In fact such a structure
has been shown to exhibit charge separation in picoseconds time scale in CdSe and PbS with
methylene blue as electron acceptor [1.37, 1.38]. Additionally other approaches include use
of QD as acceptor and dye as donor similar to DSSC. One of the added advantage a quantum
dot has is open circuit potential can be tuned by just varying the size of quantum dot.
1.5.3. Metal-Semiconductor Nanostructures
Several studies have shown ability of metal nanoparticles to act as electron sink in
metal semiconductor composites due to positioning of Fermi level of metal below conduction
band of semiconductor as shown in fig. 1. 10.
Quantitative IPCE measurement show good improvement in case of TiO 2 -Au
composites [1.39]. Studies on actual charge separation by Mulvaney et al [1.40] revealed that
different metal nanoparticles with ZnO behave differently. This study clearly showed that a
complete transfer of electrons is not present in silver, gold. In these cases electrons are shared
between ZnO and metal. However in the case of Pt a complete charge transfer is achieved.
Though metal as an electron sink shows promise, further quantitative analysis of charge
separation yield are necessary to better understand efficiency of charge separation process.