Delivered by Ingenta to: Bhabha Atomic Research Centre IP : 59.163.30.6 Mon, 06 Jun 2011 05:04:22 RESEARCH ARTICLE Copyright © 2011 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 11, 4771–4780, 2011 Quickest Single-Step Mechanosynthesis of CdS Quantum Dots and Their Microstructure Characterization Soumitra Patra 1 , Biswarup Satpati 2 , and Swapan Kumar Pradhan 1 ∗ 1 Department of Physics, The University of Burdwan, Golapbag, Burdwan 713104, West Bengal, India 2 C. M. E.R. I. Mahatma Gandhi Avenue, Durgapur 713209, West Bengal, India CdS quantum dots (QDs) with a mixture of both cubic (Zinc-blende) and hexagonal (Wurtzite) phases have been prepared within 50 min by mechanical alloying the stoichiometric mixture of Cd and S powders at room temperature in a planetary ball mill under Ar. The Rietveld analysis of X-ray powder diffraction data collected from ball-milled samples estimates relative phase abundances of cubic and hexagonal phases and several microstructure parameters like lattice parameters, particle size, lattice strain, concentrations of different kinds of stacking faults etc. in both kinds of QDs. Initially, the hexagonal phase is formed and then simultaneously, the most dense plane (111) of minor cubic phase is formed coherently on the most dense (002) plane of hexagonal phase. In the course of milling up to 30 h, the hexagonal phase partially transforms slowly to cubic phase and the molar ratio of cubic and hexagonal phase becomes ∼0.7:0.3 and particles sizes of both phases reduce to ∼6 nm. The cubic phase contains a significant amount of lattice strain but the hexagonal phase is almost strain-free. The presence of different kinds of stacking faults is revealed clearly from the high resolution transmission electron microscope (HRTEM) images. The particle sizes of QDs and their distributions obtained from the Rietveld analysis agree well with the analysis of HRTEM images. Keywords: Mechanical Alloying, XRD, Quantum Dots, Microstructure, Stacking Faults. 1. INTRODUCTION The synthesis of binary metal chalcogenides of group II-VI semiconductors has been the focus of recent sci- entific research due to their important non-linear opti- cal properties, 1–4 photo-luminescence, 5–8 quantum size effect 9–11 and other important physical and chemical properties. 12–14 Semiconductor nanocrystals are synthe- sized by several physical and chemical routes 1–4 15–17 but there is no report, so far, on single step mechanosynthesis of CdS QDs by mechanical alloying the elemental powders at room temperature. The optical and electrical properties of QDs are significantly different from their bulk coun- terpart and strongly depends on the crystallite size. 18–20 Blue-shift of the optical absorption and enhanced nonlin- ear optical coefficient are some of interesting properties exhibited in QDs. Their size dependent properties arise from the quantum confinement due to reduction in size of crystallites. 21–23 Bulk CdS having 2.4 eV band-gap at room temperature is widely used as a photo detector in the visi- ble spectrum and window material in thin film solar cells. ∗ Author to whom correspondence should be addressed. Due to unique absorption and luminescence properties it also finds application in optoelectronic devices. 24–27 It has been observed that CdSe , CdS , PbS and ZnS QDs contain lattice strain and may also contain other kinds of lattice imperfections. 28–32 To interpret and under- stand the change in properties of QDs in a better way, quantification of these lattice imperfections in terms of microstructure characterization is very much essential as several physical as well as chemical properties can have dependence on the concentration of lattice imperfections of different kinds. In most of the earlier cases, changes in the properties of QDs were believed to be of size confine- ment alone. Because of size confinement, there is a rea- sonable possibility that QDs contain lattice imperfections. This fact is somehow overlooked or ignored in the previ- ous works but need to be explained in detail in terms of microstructure characterization for a better understanding of change in properties of QDs with size confinement. In a recent work on mechanosynthesized ZnS QDs, we have observed presence of secondary wurtzite phase as cell on the primary zinc-blende core and measured concentrations of stacking faults inside the core QDs. 31 J. Nanosci. Nanotechnol. 2011, Vol. 11, No. 6 1533-4880/2011/11/4771/010 doi:10.1166/jnn.2011.3927 4771