Photoluminescent Colloidal CdS Nanocrystals with High Quality via Noninjection One-Pot Synthesis in 1-Octadecene

High-quality colloidal photoluminescent US quantum dots (QDs) were synthesized via a noninjection one-pot approach in noncoordinating solvent 1-octadecene. This synthetic approach uses cadmium acetate dihydrate and elemental sulfur as Cd and S sources, respectively, together with one long-chain fatty acid (CH3-(CH2)(n)-COOH) as surface ligands and 2,2'-dithiobisbenzothiazole (MBTS) to increase sulfur activity. The US QDs were synthesized at elevated temperatures such as 240-300 degrees C, and the kinetics of nucleation/growth was monitored via the temporal evolution of the optical properties of the growing CdS QDs. Various synthetic parameters were investigated, such as the feed molar ratios of (0.5-8)Cd/1S and (2-64)S/1MBTS, reactant concentrations of 5-80 mmol/Kg, and growth temperature of 220-350 degrees C. The feed molar ratios of (1-2)Cd/1S and (8-32)S/1MBTS are suggested to be the optimal synthetic window, together with the S feed concentration of 10-20 mmol/Kg, and the growth temperature of 240-260 degrees C. Moreover, ligand effects such as ligand length and concentration were thoroughly investigated. With an increase of the chain length of the fatty acid, the size of the resulting CdS QDs was systematically reduced. The acids of moderate carbon-chain length (n = 10-16) bestowed CdS QDs in high quality regarding narrow size distribution (similar to 17-22 nm in full width at half-maximum), high nanocrystal yield, and high quantum yield (up to 30%). Meanwhile, the acids with longer carbon chain led to small-sized nanocrystals in low concentration, due to large steric hindrance retarding severely the nucleation and growth, as indicated by the late appearance of nanocrystal absorption and slow increase in size. The acids with shorter carbon chain resulted in large-sized nanocrystals in low concentration, due to small steric hindrance causing ready nucleation and growth, as indicated by the large and fast increase in size. Therefore, the steric hindrance of varied-length fatty acids affects the reactivity of Cd2+ with great impacts on the nanocrystal nucleation/growth and thus the nanocrystal size and surface passivation. Furthermore, with an increase in the acid concentration, the size and size distribution of the resulting CdS QDs increased, together with a decrease in nanocrystal yield, due to an enhanced solubility of the US nanocrystals and thus a hindered nucleation with a low nuclei concentration.