Ultrafast Charge-Carrier Dynamics and Defect-Mediated Recombination in ZnO Quantum Dots from fs-Pulse Ablation

Understanding the effect of surface defects on the carrier dynamics of n-type ZnO quantum dots (QDs) at ultrafast temporal resolution is crucial for various optoelectronic and photocatalytic applications. These potential applications of ZnO QDs fundamentally rely on the relaxation and recombination studies of photogenerated charge carriers, which are very limited for ZnO QDs. In this work, the trapping phenomenon of photoexcited carriers in ZnO QD samples S1 and S2, synthesized via femtosecond pulsed-laser ablation, is studied by using an ultrafast pump–probe spectroscopy. The sample S2, synthesized at a 254 J/cm2 laser ablation fluence, shows faster relaxation than S1, synthesized at a 380 J/cm2 laser ablation fluence, due to the reduced trapping of photoexcited carriers in the S2 sample resulting from minimal surface defects. The reduction of surface traps decreases the number of separated electron–hole pairs, leading to enhanced near-band-edge (NBE) emission in the S2 sample because a smaller number of holes have been captured within the traps. Our study also investigates the charge-carrier relaxation and recombination dynamics of the ZnO QDs with an above bandgap pump excitation by varying the pump fluence from 5.3 mJ/cm2 to 31.6 mJ/cm2. The decay kinetics involves the two processes: the first is the energy relaxation via carrier–phonon scattering in the valence band at a faster time scale, and the second arises from the trapping and recombining of photoexcited charge carriers at a slower time scale. The results presented in this study offer a comprehensive understanding of photogenerated charge-carrier dynamics and recombination pathways for the advancement of ZnO-based photovoltaic and photocatalytic applications.