Inverted Core/Shell Nanocrystals Continuously Tunable between Type-I and Type-II Localization Regimes

We study inverted core/shell nanocrystals (NCs) in which a core of a wide gap semiconductor (ZnSe) is overcoated with a shell of a semiconductor of a narrower gap (CdSe). By monitoring radiative recombination lifetimes for a series of these NCs with a fixed core radius and progressively increasing shell thickness, we observe a continuous transition from Type-I (both electron and hole wave functions are distributed over the entire NC) to Type-II (electron and hole are spatially separated between the shell and the core) and back to Type-I (both electron and hole primarily reside in the shell) localization regimes. These observations are in good agreement with the calculated dependence of the electron−hole overlap integral on shell thickness.