Microdynamics behavior of the photogenerated carriers in ligand-capped CdTe quantum dots

A microdynamics study of high-quality CdTe/ligand self-assembled quantum dots (QDs) is significant because it is closely related to the energy transformation among specific quantum states, such as the electron and vibration levels of an elementary reaction. Transient photovoltaic (TPV) technology was used to probe the microdynamics behavior of the photogenerated carriers in the QDs. The TPV polarity of the nanocrystalline CdTe particles depended on the type of capping ligand used [3-mercaptopropionic acid (MPA) or β-mercaptoethylamine (MA)] because of a quantum tunneling effect. This resulted in a negative TPV polarity of the MPA samples upon illumination with a laser pulse of 532 nm and excitation intensity of 50 μJ, the opposite to the situation for the MA sample. The effect of the concentration of Cd2+ cations on the separation and recombination rates of the photo-induced free charge carriers of the CdTe/MPA self-assembled QDs was negligible. The different chemical bonds that formed at the interface between the core-CdTe nanoparticles and the capping ligand led to the change in the intensity of the photoacoustic signal in the 550–800 nm region caused by the non-radiative de-excitations as compared to the CdTe QDs capped by MA with that capped by MPA, and to the differences in the Raman scattering between the two samples. The property of those lattice vibration quantum states locating at the Brillouin zone boundary or near the Brillouin zone boundary was associated to a great extent with the formation of the shell-CdS in between the core-CdTe and the capping ligand according to the computer simulation results.