CdSe Spherical Quantum Dots Stabilised by Thiomalic Acid: Biphasic Wet Synthesis and Characterisation

CdSe quantum dots stabilised by thiomalic acid have been synthesised by an aqueous biphasic ligand exchange reaction in air. The materials are completely water-soluble and were found to be stable over a long time. X-ray diffraction and transmission electron microscopy reveal the formation of CdSe nanocrystals with cubic structure (a=0.6077 nm; spatial group: F-43m). The average particle size is about 5 nm. Energy dispersive X-ray analysis shows that the nanocrystals are nonstoichiometric, with a Cd/Se ratio varying between 60/40 and 70/30, and indicates the presence of Cd2+ ions at the nanocrystal surface. Diffuse reflectance infrared Fourier transform measurements suggest that thiomalic acid chelates CdSe through the thiol group and one carboxylic function, while the second COOH group is semi-free. A complex-like structure is proposed, in which thiomalic acid forms a five-membered chelate ring with the Cd2+ ions present on the nanocrystal surface. Chelate effect accounts for the easiness of ligand exchange and is expected to additionally stabilise the nanosystem.