Photoluminescence Studies of CdS Nanoparticles - Proceedings of All India Seminar on Biomedical Engineering 2012 (AISOBE-2012) - page 80

[ 1 - 6 ]. Such particles display optical and physical properties which are interme-
diate between those of the bulk materials and those of the isolated molecules. For
example, the optical absorption of bulk cadmium sulfide (CdS) typically extends to
690 nm. When CdS is made into 40 Å nanoparticles the longest absorption band
shifts to 530 nm. In nanoparticles a large percentage of the atoms are on the surface,
rather than in the bulk phase. Consequently, the chemical and physical properties of
the material, such as the melting point or phase transition temperature, depend on
the particle size. Nanoparticles can be made from a wide variety of materials,
including CdS, ZnS, Cd 3 P 2 , and PbS, to name a few. The nanoparticles frequently
display photoluminescence and sometimes display electroluminescence [ 7 - 12 ].
Additionally, some nanoparticles can form self-assembled arrays. Because of such
favorable properties, nanoparticles are being extensively studied for use in opto-
electronic displays. The CdS nanocrystalline has become a subject of both scientific
and industrial importance in the past one decade.
Nanomaterials can be characterized using different techniques like X-ray dif-
fraction, Ultra-violets spectroscopy, Scanning electron microscopy, Atomic force
microscopy, and Tunneling electron microscopy. Many features of nanocrystals
differ from those of their bulk counterparts and are dependent on their individual
sizes. It has always been a prime goal, since the beginning of research in this field,
to prepare samples of nanocrystals as identical as possible.
As the crystalline size decreases the band gap values increase. In principle, the
band gap can be varied by changing the crystalline size. Thus, for low band gap
material, one has to control the crystalline size to obtain the required band gap.
Light emission from these nanocrystals is possible through a radiative recombi-
nation process of charge carriers generated by higher energy photon absorption.
The color of the emission can be layered by changing the crystalline size and
appropriate doping. The efficiency changes from 3 % in bulk to 18 % in nano-
crystallites, leading to a tremendous improvement in the brightness of emitted
light; because of the small size, the intensity of emitted light increases [ 13 ].
Experimental Support
It is a great challenge to synthesize particles of nanometer dimension with narrow
size distribution without any impurity. Various methods have been attempted for
the synthesis. Nanoparticles of CdS are synthesized in aqueous medium through
chemical precipitation technique starting from cadmium salt and sodium sulfide,
and using tri-ethanolamine as capping agent. These compounds are weighted in a
microbalance. The stoichiometric solution was taken in a burette and was added in
drops with continuous sitting to precipitate of CdS was formed. After complete
precipitation, the solution in conical flask was constantly stirred for about 20 h.
Then the precipitates were filtered out. The nanoparticles are separated from the
reaction medium by centrifugation, washing, and drying. Photoluminescence