Dependence of the Structural Phase Transition Temperature on Synthesis Route of CdS Quantum Dots

Semiconductor nanoparticles with size of less than their Bohr exciton diameter (approximately 10 nanometer), so called quantum dots(QDs), are of great interest for fundamental researches and novel applications in many fields of science and technology such as electronic, biotechnology, material and medical sciences[1-5]. These nanoparticles exhibit size-dependent optical and optoelectronic properties due to quantum confinement effect (or size quantization effect)[6-8]. Among the various QDs in II-VI and III-V component semiconductors, the binary cadmium chalcogenides such as CdS, CdSe, CdTe, … have been the most studied due to their individual electronic , optical and structural properties. As it is known, bulk CdS has stable hexagonal structure (wurtzite type) at room temperature and two meta stable cubic structure (zinc blende and rack salt types) have been found only in low dimensional system[9]. These two crystalline phases (cubic and hexagonal) may coexist in CdS nanoparticles sample which is prepared by a synthesis route [10-11]. However, it is well known that some factor such as synthesis rout, impurity, temperature of chemical reaction and post heat treatment (annealing) temperature can strongly affects on crystal structure [12,13,14 ] of CdS nanoparticles. Although there have been many studies on effect of temperature on size and crystal structure of CdS nanoparticles ,but the effect of synthesis route on the temperature of structural pha se transition has not reported yet .In this work we have synthesized CdS quantum dots by two different chemical precipitation routes and temperature of crystalline phase transition was determined by annealing at various temperature under argon atmosphere for each sample.