Keywords: quantum dots, temperature-dependent phase stability, CdS, first-principle molecular dynamic calculations.

Due to their unique optoelectronic properties and a wide range of current and potential applications, nano-sized semiconductors have attracted much attention in recent years. Among these semiconductors, cadmium sulfide (CdS), being one of the most important wide-gap semiconductors, is stable and exhibits high luminescence [1]. CdS nanostructures have found many applications in photovoltaic cells, photonic switches and optoelectronic devices. In recent years, CdS quantum dots have also been used in biolabeling, bioimaging, drug delivery and other biotechnological areas. It is due to these optoelectronic and bio-related applications, the analysis of thermal stability of CdS quantum dots becomes one of the most important avenues of research in this field. Additionally, CdS quantum dots represent an intermediate state of matter between molecular species and bulk materials. Therefore, a better understanding of their temperature dependent phase stability and phase transformations will ultimately enable us to taylor their electrical and optical properties from molecular level to bulk crystals.

It has been known for a long time that the bulk CdS is highly stable and it exists in the hexagonal wurtzite phase in a wide range of temperatures, from room temperature to its melting point 1750°C. Recently it has been shown that due to the large surface to volume ratio and reconstruction of surface crystal structure, the thermodynamic properties of the CdS nanostructure can differ significantly from bulk CdS [2]. For example, under appropriate conditions some wurtzite structure semiconductor materials can transform into a graphitic structure when they are in the form of thin nanoplates [3]. Since the internal pressure deduced from surface stress is very large for nanostructures, at ambient conditions, the stability phase in nanosize structures is always a high pressure stability phase struture in bulk [4]. Nevertheless, the temperature-dependent phase stability of the CdS quantum dots remains poorly understood to date. In particular, systematic calculations and comprehensive comparisons of their thermodynamic stability, as well as mechanical, electronic, and optical properties under different temperature conditions are still lacking. To fill this gap, in this work we carry out first-principle molecular dynamic (MD) calculations for CdS quantum dots of various phase structures and study temperature-dependent phase stability of these nanostructures. Based on our calculations, we analyze the relative stability of different CdS phases under different temperature conditions. Our results indicated that at the temperature range studied in this work, the phase stability sequence for the Cd₄₈S₄₈ nanostructure is rocksalt, wurtzite and graphitic phase, which is the same as that of bulk CdS crystal under high pressure. Although the temperature can affect the total energy of the CdS nanostructure, it cannot change the phase stability sequence of CdS nanostructures in the temperature range studied in this work.

1

K. Barnham, J.L. Marques, J. Hassard, P. O'Brien, "Quantum-dot concentrator and thermodynamic model for the global redshift", Applied Physics Letters, 76, 1197-1199, 2000. doi:10.1063/1.125981

2

A.S. Barnard, H. Xu, "First principles and thermodynamic modeling of CdS surface and nanorods", Journal of Physics Chemistry C, 111, 118112-18117, 2007. doi:10.1021/jp0761766

3

C.L. Freeman, F. Claeyssens, N.L. Allan, J.H. Harding, "Graphitic nanofilms as precursors to wurtzite films: theory", Physical Review Letters, 96, 066102, 2006. doi:10.1103/PhysRevLett.96.066102

4

C.C. Yang, S. Li, "Size-dependent temperature-pressure phase diagram of carbon", Journal of Physical Chemistry C, 112, 1423-1426, 2008. doi:10.1021/jp076049+

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