CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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size of the crystal is much larger than the Bohr radius of the exciton. Bohr radius will become
a function of the crystal size for sizes lower than this. In other words we can say that the
wave function of the exciton will also be a function of radius of a nanoparticle. Additionally,
as one reduces the radius the energy level spacing between valence and conduction band
becomes larger. Additionally formation of discrete bands (valence and conduction bands) can
be seen.
The discreteness of the individual bands leads to not only significant alteration of steady state
properties like absorption and luminescence but also dynamical properties like carrier
cooling, carrier relaxation etc. In fact it was proposed by Nozik et al [1.12] that in small
nanostructures within size quantization regime carrier cooling will be hindered due to what is
called as phonon bottleneck effect. The cooling times for size quantized structures were
expected to be several ns. However ultrafast dynamics studies in small nanocrystals showed
that cooling dynamics of carriers was still much faster (~few ps) which was explained on the
basis electron-hole energy transfer and surface mediated cooling [1.13]. Apart from cooling
of carriers, recombination also is significantly influenced by surface due to very large
number of surface sites which might trap carriers. These microscopic variables like cooling
and trapping times are ultimately related to macroscopic properties like photovoltaic
efficiency, luminescence quantum yield etc. Therefore understanding the dynamics is
imperative. In view of above mentioned factors and motivated by potential applications of
nanomaterials it is necessary to understand what governs the dynamics, so that a better
control over the properties of nanomaterials can be achieved. To control dynamics it is
imperative to know how lower dimensionality affects electronic structure of a nanoparticle.