Carrier relaxation dynamics in type-II ZnO/CdSe quantum dot heterostructures.

Semiconductor heterostructures with type-II band alignment are well known for their engineering property of efficient charge separation in photogenerated carriers. Herein, type-II CdSe/ZnO core/shell quantum dot heterostructures with CdSe shells of different thicknesses have been synthesized and a study of carrier dynamics is carried out using femtosecond transient absorption and picosecond emission spectroscopy. Carrier lifetime measurements by transient emission spectroscopy have revealed reduced electron-hole overlap in the type-II localization regime of ZnO/CdSe heterostructures. Femtosecond transient absorption studies have revealed hot electron transfer from CdSe shell to ZnO core prior to electron cooling in the CdSe shell. In addition, a surface channel for the hole cooling process has been identified in the transient absorption measurements. Effects of carrier trapping at interfacial defect states and type-II localization on carrier recombination have been recognized in our transient absorption and emission studies of ZnO/CdSe QDs heterostructure.