Effect of the Reaction Temperature on the Optical Properties of CdSTe Quantum Dots Synthesized Under Microwave Irradiati
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Effect of the Reaction Temperature on the Optical Properties of CdSTe Quantum Dots Synthesized Under Microwave Irradiation Glorimar Rivera-Rodriguez1; Oscar Peralez-Perez2; Yi-Feng Su3 and Luis Alamo-Nole1 1 Chemistry Department, Pontifical Catholic University of Puerto Rico, Ponce, Puerto Rico, USA 2 Department of Engineering Science & Materials, University of Puerto Rico, Mayaguez, Puerto Rico, USA 3 National High Magnetic Laboratory, Florida State University, Tallahassee, Florida, USA ABSTRACT Water-stable CdSTe quantum dots were synthesized under microwave irradiation heating conditions. An aqueous telluride solution (produced by reducing metallic Te with NaBH4), cadmium sulphate and thioglycolic acid were mixed together in an oxygen-free atmosphere to prevent oxidation of the telluride species. The reaction temperature varied from 60o C to 180o C and was controlled by using a microwave reactor (1,000 W) to control nucleation rate and tune the size of the quantum dots. Photoluminescence analyses of resulting quantum dots evidenced a red shift (from 490 nm to 640 nm, using an excitation wavelength of 380 nm) when the reaction temperature was increased, which suggested crystal growth. The variation in size was also evidenced by the color of the quantum dot suspensions that changed from blue to red, when excited with a 405 nm, 5mW diode laser. The highest quantum yield was observed for quantum dots synthesized from 120o to 150o C. X-ray diffraction analyses suggested the formation of a solid solution of CdSTe with average crystallite size ranging from 1.4 nm to 3.2 nm. FT-IR spectroscopy evidenced the presence of residual thioglycolic functional groups onto the crystals surface, whereas HRTEM confirmed the nanometric size of the quantum dots. INTRODUCTION Quantum dots (QDs) are semiconductor nanomaterials with electrical and luminescent properties that enable them to be applied in engineering, chemistry, and biology [1]. Moreover, the optical properties of QDs can be modified by changing their composition and crystal size at the nanoscale [2,3]. Highly crystalline QDs can be synthesized via an organometallic route that uses organic solvents and hydrophobic organic surface-coating species [4, 5]. Despite of the high quality of QDs produced by this route, their hydrophobicity limited their use in most of the envisioned applications [6]. On the other hand, the aqueous synthesis routes, e.g. hydrothermal reflux and the use of microwave heating, are conducive to water stable nanocrystals. QDs produced through the hydrothermal approach are usually polydisperse and exhibit low intensity and broad emission peaks (i.e. low quantum yield) that can be attributed to the heterogeneous heating conditions. The synthesis of QDs via microwave heating leads to the formation of highly monodispersed and water-stable nanocrystals with excellent optical properties; the more stable and homogeneous heating conditions attained by microwave irradiation favor a rapid and uniform nucleation of nanocrystals with less concentration of defects o
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