Quantum dots (QDs) of semiconductor have been widely studied in recent years owing to their size dependent properties and the wide range of potential applications [1]. In semiconductors with very small size, quantum confinement modulates the band structure of nano-particles and increases the band gap [2]. The particle size is dependent on the kinetics of nucleation and growth from a supersaturated solution as well as processes such as coarsening [2]. In this study, quantum dots of CdS with a hexagonal phase have been prepared at a relatively low temperature (60oC) and short time in micro-emulsion (O/W) under ultrasound [3]. The nano-particles have been characterized by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), UV-Visible spectroscopy. The study of optical properties reveals that the band-gap of the CdS nano-particle decreases with increase of sonication time and reaches an approximate constant value. The growth kinetics of the CdS nanoparticles was monitored by the red-shift in UV-visible absorbance peaks. The particle size as a function of time and temperature was approximated by using the effective mass model. The average cubed particle radius is shown the emergence of linear regions after longer times with increasing temperature. This indicates that the increase in particle size at longer times is related to the diffusion-limited coarsening. The rate constant for coarsening increases with temperature. The Arrhenius-type plot was created by using the slopes of the LSW curves for the CdS nanocrystals and the obtained activation energy for the ripening was 28.32 kJ mol-1. These results suggest that the cavitatoin lowers the activation energy for coarsening. In fact, the acoustic cavitation can strongly increase the mass transfer and therefore, the diffusion of surface atoms is rather easier than that of the inner atoms. [1] P.V. Kamat, J. Phys. Chem. C 112, 18737 (2008). [2] J. Park, K. Hyi Lee, J. F. Galloway, and P. C. Searson, J. Phys. Chem. C, 112, 17849 (2008). [3] M. H. Entezari, N. Ghows Ultrason. Sonochem. 18, 127 (2011). *Corresponding author: moh_entezari@yahoo.com

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