Probing carrier separation in a novel series of infrared type-II nanoheterostructures

Three-dimensional confinement of carriers within semiconductor nanocrystals gives rise to novel characteristics such as size-controlled energy gaps, but also increased Coulomb interactions between charge carriers that can hamper spatial separation of electrons and holes, such as in photovoltaic devices. One approach to efficient ``splitting'' of photogenerated excitons is through introduction of appropriate energy gradients within a heterostructured nanocrystal. Specifically, a type-II staggered alignment of electronic states can provide nearly complete separation of electrons and holes between different spatial regions of a nanoheterostructure. In this work, we use static and transient photoluminescence spectroscopies to examine the evolution of type-II behavior in PbSe/CdSe/CdS core/shell/shell heterostructured nanocrystals. We observe a marked red shift and a dramatic increase in radiative lifetimes (to $>$10 $\mu $s) with increasing CdS shell thickness, indicative of type-II carrier separation, at effective band gaps of 0.95 eV, the narrowest band gap of any such system to date. We will compare these measurement to those of a homologous ``tetrapod'' system that exhibits surprisingly high emission quantum yields, to further elucidate the effect of structure on carrier separation.