Quasi-Type II Carrier Distribution in CdSe/CdS Core/Shell Quantum Dots with Type I Band Alignment

Band alignments are essential for understanding the optical properties and carrier transfer of core/shell QDs. As CdSe/CdS core/shell QDs with increasing shell thickness represent red-shifted absorption and luminescence spectra, weakened oscillator strength of the lowest electronic transition, and elongated luminescence lifetime, they are assigned to quasi-type II band alignment. However, femtosecond transient absorption spectroscopy with state-selective excitation revealed a type I band alignment of the CdSe/CdS QDs with a thin CdS shell, in which the excited electron is localized in the CdSe core with core excitation while delocalized in the whole QDs with shell excitation, even though a quasi-type II carrier distribution was observed with steady-state spectroscopy. In the type I core/shell QDs, the CdS shell acts as an energy barrier in surface electron and hole-trapping processes. The time constant of the hole-trapping process of the CdSe core (∼10 ps) was elongated 10 times owing to a tunnel effect through the high energy barrier of the CdS shell, which was estimated from the decay related to the biexcitonic induced spectral shift. The biexcitonic spectral shift induced by a ∼100 ps hole-trapping process was also observed at the 1S­(e)–2S_3/2(h) transition. Our results from transient absorption spectroscopy with state-selective excitation are useful to clarify band alignment and carrier distribution of hetero-nanostructures, which could help to objectively extract charge carriers in photovoltaic applications