Effect of Surface States on Charge-Transfer Dynamics in Type II CdTe/ZnTe Core-Shell Quantum Dots: A Femtosecond Transient Absorption Study

We have synthesized CdTe/ZnTe type II core-shell quantum dots at lower temperature by using 3-mercaptopropionic acid (SHCH2CH2COOH, thiol) as a capping agent, and they are resolution transmission electron microscopy. We observed a red shift characterized by steady-state absorption, emission studies, and high-in optical absorption and emission spectra with the increase in ZnTe shell thickness. Time-resolved emission studies indicate a longer average emission lifetime for CdTe/ZnTe core-shell QDs as compared with that for CdTe QDs. Femtosecond transient absorption spectroscopy was employed to study charge carrier relaxation in CdTe QDs and charge carrier relaxation and charge-transfer dynamics in CdTe/ZnTe core-shell QDs by monitoring the transients in the visible-to-near-IR region. After laser excitation of QDs, electron hole pairs are generated, which are confirmed by bleach at the excitonic position and induced absorption signal in the visible and near-IR regions for both QDs and core-shell QDs. In the transient absorption spectrum of CdTe/ZnTe QDs, exciton bleach (negative absorption) has been superimposed by positive absorption caused by trapped carriers, which indicates the presence of trap states. The carrier cooling time was found to be slower (250 fs) in CdTe/ZnTe core-shell QDs as compared with that in CdTe QDs (185 fs), indicating charge transfer from CdTe to the ZnTe shell. Ultrafast transient absorption studies suggest an influence of trap states in relaxation dynamics of photoexcited charge carriers in core-shell QDs.