Role of Alcohol in the Synthesis of CdS Quantum Dots

The proper choice of the solvent is an important parameter in wet-chemical syntheses. Here, we highlight the use of 1-tetradecanol, a fatty alcohol, as an excellent solvent for the synthesis of high-quality zinc blende CdS quantum dots from cadmium laureate and tri-n-octylphosphine sulfide, the organometallic precursors. The as-prepared CdS quantum dots exhibit a defect-free (<1%) narrow blue emission at 452 nm with a full width at half-maximum of 18 nm (size distribution <5%) and photoluminescence quantum yield of 52% in comparison with quinine sulfate (reference dye). Our approach is simple and can be conducted as a one-pot reaction in air at temperatures below 200 °C. The key synthetic challenge is the preparation of the cadmium precursor. In the presence of strong ligands like tri-n-octylphosphine, the particle yield is reduced and no CdS forms, when alcohol is replaced by typical coordinating solvents, such as tri-n-octylphosphine oxide and hexadecylamine. This implies that under the given conditions, solvents with a stronger coordination power than alcohol inhibit the precursor conversion into monomers. Furthermore, the particle growth in alcohol is widely independent from the initial precursor concentration, indicating that our synthesis follows the LaMer model. Finally, a 1H NMR study and control experiments with different alcohols and non-coordinating solvents prove that fatty alcohols are good surface ligands for CdS quantum dots. Our results demonstrate that the sulfur reactivity or the cleavage of the P═S bond in tri-n-alkylphosphine sulfide is not the bottleneck in the synthesis of CdS quantum dots, if a solvent with an appropriate coordination power is used.