CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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size dependent optical properties in a glass matrix. Later Brus et al proved size dependent
effects in CdS colloidal solutions [1.5]. The size dependent effect in semiconductors was
attributed to what is now called size quantization effect. Size quantization effect refers to the
increase in band gap of semiconductors with decreasing size. Brus et al [1.6] and Efros et al
[1.7] independently provided theoretical framework for this observation. It was immediately
realized that size quantization effect provided us a way to control the electronic property of
materials, which is immensely beneficial in different applications eg. photovoltaics [1.8],
biological tagging [1.9], optoelectronics [1.10] and Quantum computing [1.11].
It was realized from UV-Visible absorption spectroscopy that size quantization effect was
seen only below a particular size regime [1.5]. Size quantization effect can be explained on
the basis of factors determining the electronic structure of bulk crystals. In a bulk crystal,
electron and holes move in a potential created by the lattice ions. This potential is unique not
only due to local bonding arrangement but also to lattice potential created due to periodicity
of the lattice. Therefore the motion of charge carriers in the crystalline solid will be governed
by this lattice potential. The lattice potential not only imparts a unique electronic dispersion
but also governs its transport property like coherence length. Coherence length is the distance
to which a carrier can travel before it scatters. In a semiconductor solid similar to charge
carriers the exciton (electron-hole pair) behaves as a composite particle with a reduced mass
arising from electron and holes. Therefore an exciton also has coherence length or mean free
path in a particular crystal lattice. This characteristic length is also called Bohr radius of the
exciton. Size quantization effect is observed for crystallite size below Bohr radius. Bohr
Radius of exciton is a material property it varies from fraction of nanometers to several tens
of nanometers. In bulk crystals size does not control electronic characteristics because the