Synthesis of Type-I CdTe Core and Type-II CdTe/CdS Core/Shell Quantum Dots by a Hydrothermal Method and Their Optical Properties

CdTe and CdTe/CdS quantum dots (QDs) were synthesized via hydrothermal method in one-step processes just by controlling reaction time. A gradual thermal decomposition of thiol ligand N-acetyl-L-cysteine results in formation of a CdS shell on a CdTe core. The experimental results of X-ray diffraction and energy dispersive X-ray spectroscopy indicated that growth of the CdS shell started in a longer reaction time than 30 minutes and that the thickness of the CdS shell increased with increasing the reaction time. The CdTe/CdS QD exhibited a longer photoluminescence-decay time than the CdTe core QD due to the type-II band alignment, and the decay time in the CdTe/CdS QDs increased with an increase in the reaction time. The increase in the decay time in the CdTe/CdS QDs was qualitatively reproduced by a decrease in an overlap integral of electron and hole wave functions caused by an increase of the CdS shell thickness.