3-Mercaptobutyric Acid as an Effective Capping Agent for Highly Luminescent CdTe Quantum Dots: New Insight into the Selection of Mercapto Acids

CdTe quantum dots (QDs) were synthesized by a modified hydrothermal method with Na2TeO3, as the Te source. Thioglycolic acid (TGA), thiolactic acid (TLA), 3-mercaptopropionic acid (MPA), 3-mercaptobutyric acid (3MBA), 4-mercaptobutyric acid (MBA), and 5-mercaptovaleric acid (MVA) were used. Their effects on the growth and fluorescence of as-synthesized QDs were investigated: Detrimental precursor aggregation when using TGA-like molecules (TGA and L-cysteine) due to their special secondary coordination was observed and further confirmed by comparing with DL-homocysteine and N-acetyl-L-cysteine. The aggregation could be suppressed by using a bulky methyl group as a side chain to confine the carboxyl group, that is, TLA. Moreover, MPA was confirmed as a better stabilizer compared with linear TGA, MBA, and MVA probably because of its suitable balance between hydrophilicity and hydrophobicity. Accordingly, we designed and synthesized 3MBA that has the same main chain length as MPA but with a side methyl group as TLA. 3MBA-modified CdTe QDs exhibited an excellent optical property with a quantum yield of 71%, much higher than that of MPA and TGA. Preliminary results of CdSe QDs confirmed the versatility of 3MBA. Our results uncovered the possible origin of the advantages of mercapto acids with a methyl side chain and a suitable chain length.