Size-and Shape-Controlled Synthesis Of Cadmium Sulfide Nanocrystals

OBJECTIVE Cadmium sulfide (CdS) is a compound semiconductor with interesting opto-electric and chemical sensing properties. As the crystal size decreases to the nanometer scale, CdS exhibits novel size-dependent electric and optical properties, which might have potential application in solar cell and other opto-electric nano-devices. In this thesis, I conducted a systematic study of the reaction process and mechanism in the synthesis of CdS nanocrystals. The research is aimed at obtaining shape-controlled and monodisperse CdS nanocrystals with a high yield.METHOD CdS nanocrystals were synthesized through a thermal dissociation of metal-oleylamine complexes in the mixture with sulfur. We studied the size and morphology of the as-produced nanocrystal as a function of reaction temperature as well as Cd/S ratio. The optimal conditions of two reaction schemes for the synthesis of CdS nanocrystals were explored in the study.RESULTS A variety of shape-controlled CdS nanocrystals with cubic, truncated-octahedron, and monoclinic hexahedron structures were produced through the thermal dissociation of metal-oleylamine complexes in the mixture with sulfur. We found that the CdS nanocrystals start nucleation at 140°C and develop a full crystal morphology at 170°C. We identified the nanocrystal structures with scanning electron microscope, atomic force microscope, and X-ray diffraction technique. The optical properties of the as-produced nanocrystals were studied using UV-vis absorption spectroscopy and fluorescent spectroscopy. CONCLUSION The experimental conditions in two different approaches to synthesizing CdS nanocrystals were systematically investigated in this thesis. The reaction temperature as well as the Cd/S precursor ratio were found to influence the nanocrystal morphology. Shape-controlled CdS nanocrystals with cubic, truncated-octahedron, and monoclinic hexahedron structures were obtained through the thermal dissociation of metal-oleylamine complexes in the mixture with sulfur at 170°C. Quantum confinement effect was observed in the UV-vis absorption spectroscopy and fluorescent spectroscopy of CdS nanocrystals, revealing the crystal size distribution and defect density in the as-produced nanomaterials.