Controlling the spatial location of photoexcited electrons in semiconductor CdSe/CdS core/shell nanorods

Abstract

It is commonly assumed that after an electron-hole pair is created in a semiconductor by absorption of a photon the electron and hole rapidly relax to their respective lowest-energy states before recombining with one another. In semiconductor heterostructure nanocrystals, however, intraband relaxation can be inhibited to the point where recombination occurs primarily from an excited state. We demonstrate this effect using time-resolved optical measurements of CdSe/CdS core/shell nanorods. For nanorods with large CdSe cores, an electron photoexcited into the lowest-energy state in the core remains in the core, and an electron photoexcited into an excited state in the CdS shell remains in the shell, until the electron recombines with the hole. This provides a means of controlling the spatial location of photoexcited electrons by excitation energy. The control over electron localization is explained in terms of slow relaxation into the lowest-energy electron state in the nanorods, on time scales slower than electron-hole recombination. The observation of inhibited relaxation suggests that a simple picture of band alignment is insufficient for understanding charge separation in semiconductor heterostructures.