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Dr. C. Vijayan, Professor, |
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Dept. of Physics, IIT Madras |
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Links to other introductory articles: photonics , photonic materials, photoacoustics Quantum Confinement Effects in Practical realization of particle-in-a box Spatial confinement of electrons in semiconductor nanostructures
leads to remarkable changes in their quantum states. This results in considerable
modifications in their electronic properties and consequently their linear
and nonlinear optical response, thus making them very attractive candidate
materials from the points of view of both fundamental Physics as well as
their Technological potential. Quantum wells, which are two-dimensional confined systems, have
been studied and utilized for practical applications to a large extent these
days whereas not that much attention has been being paid to the interesting
properties of quantum wires and dots. Quantum dots are quasi zero-dimensional
systems which are practical realizations of the familiar text book concept of
particle-in-a-box. One way of realizing this is by preparing semiconductor
islands of nanometer dimension in dielectric matrices such as glasses and
polymers. The dielectric provides the confining potential and also acts as a
convenient embedding matrix for applications. Nanostructures of several
materials such as PbS, CdS and CdSe have been prepared in glass and polymer
matrices in this manner and several interesting results have been obtained.
As an example, some recent results on CdS quantum dots in Nafion polymer are
presented. CdS quantumm dots are prepared in Nafion polymer by an
ion-exchange reaction. Starting with a cadmium salt solution, the undoped
polymer is kept immersed in it to facilitate the exchange of cadmium ions
into Nafion. The initial concentration of cadmium ions in the solution and
the duration of treatment determine the mean size of the nanoclusters thus
formed. The polymer sheets are then exposed to hydrogen sulphide gas for
appropriate duration, when CdS forms in the form of nanoclusters in the
naturual pores of the polymer structure. The size can be determined by X-ray
Diffraction and electron microscopy. Spectroscopic study of bandgap engineering Quantum confinement effects become pronounced in the region of
strong confinement, where the size of the quantum dot is less than the
exciton Bohr diameter of the semiconductor. This parameter has a value of 6
nm for CdS, The band picture is no longer strictly valid in the case of
quantum confinement and transitions between excitonic levels gain in
oscillator strength even at room temperature. These effects can be easily
seen in optical absorption and photoluminescence spectra. The band edge
exhibits a signature blue shift with decreasing dot size. Also, excitonic
transitions show up as multiple peak structures in optical absorption and
luminescence excitation spectra. this is 6 nm. The emission spectrum shows
the characteristic exciton peak even at room temperature. Since optical
densities are quite large, the excitonic features are often masked by the
strong absorption in the optical absorption spectra. However, photoacoustic
spectroscopy can help to reveal the features of excitonic transitions and to
probe the energy level structure. Potential as photonic materials One of the main motivations of studying nanostructures is their
potential as materials for Photonic applications. Frontier research in
Photonics revolves around development and characterization of materials with
large and fast nonlinear optical susceptibilites. Quantum dot structures have
been known as potential candidate materials in this regard though much has
not been done about systematic characterization of the size dependence of
nonlinearity especially in the strongly confined regime. Optical phase
conjugation studies on CdS quantum dots reveal strong enhancement in third
order susceptibilities with decreasing dot size. Moreover, it appears that
the Links to other introductory articles: nanostructures , photonic materials, photoacoustics |