CHEM01200604004 Shri Sanyasinaidu Boddu - Homi Bhabha ...

constant, ħ is h/2π, e is the electron charge, m e and m h are effective mass of electron and hole,
respectively.
2
ε ⎡ 1 1 ⎤
2 ⎢ ⎥
e me
mh
a = +
⎣
………………………..(4)
⎦
When the particle size is smaller than the Bohr radius, quantum confinement plays an
important role in the optical properties of semiconductors. Their absorption spectra show a
consistent blue shift as the size of the particle decreases. In some cases, up to 10 discrete
optical transitions between different discrete energy levels of the QDs have been observed in
the absorption spectrum.
In insulators, luminescence can arise through different mechanisms. The localised
centres provided by defects and dopants offer a major contribution to the luminescence in this
type of materials. The dopants are also known as activators and they can create states within
the forbidden zone of the band gap of a material and in the valance and conduction bands
[19]. Generally, two types of activator ions can be distinguished. In the first case, energy
levels of the activator ion involved in the emission process show only weak interactions with
the host lattice. Typical examples are lanthanide ions (Ln 3+ ), where the optical transitions
take place solely within 4f levels which are well shielded from chemical environment by
outer electrons. Therefore, characteristic line like emission spectra can be observed from such
materials. The second type of activator ions strongly interact with the host lattice. This is the
case when d electrons or ion with s 2 configurations are involved. Examples include Mn 2+ ,
Eu 2+ , Ce 3+ , Pb 2+ , Sb 3+ , etc. Similar situation is also observed for ions like WO 2- 4 , MoO 2- 4 ,
NbO 3- 4 , VO 3- 4 , etc. The strong coupling of the electronic states of emitting ion with
vibrational modes of the lattice leads to broad bands in the emission and absorption/
excitation spectra.
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