Charge Carrier Localization Impact on the Spectral–Temporal Photoluminescence Separation in Type II CdTe/CdSe Nano-Heterostructures

Abstract

Peculiarities of exciton dynamics in tetrapod-shaped CdTe/CdSe nanocrystals (nanotetrapods) are revealed under various optical excitation conditions. The photoluminescence (PL) spectra exhibit a pair of well resolved excitonic bands originating from CdTe and CdSe parts of the tetrapod arms as well as PL caused by the indirect transitions of spatially separated charge carriers (CT band). We demonstrate the pump intensity dependent giant blue shift of the indirect transition emission band caused by the exciton's space filling effect and the modification of energy level structure by the induced internal electric field in both domains. The first-principles calculations reveal that the ground states of both electrons and holes are localized in the CdSe domain, but the charge carrier spatial separation is still present, as holes are localized at the interface. This picture is proven by the time-resolved PL measurements from the sub-picoseconds to tens of nanoseconds range, which demonstrate the presence of a picosecond time scale corresponding to the indirect excitons being much smaller than the previously observed value.