title: Type-II core/shell CdS/ZnSe nanocrystals: synthesis, electronic structures, and spectroscopic properties.
abstract: We report a two-step synthesis of highly luminescent CdS/ZnSe core/shell nanocrystals (emission quantum yields up to 50%) that can produce efficient spatial separation of electrons and holes between the core and the shell (type-II localization regime). Our synthesis involves fabrication of cubic-singony CdS core particles that are subsequently overcoated with a layer of ZnSe in the presence of surfactant-ligands in a noncoordinating solvent. Studies of different growth regime of the ZnSe shell indicate that one approach to obtaining high emission efficiencies is through alloying the CdS/ZnSe interface with CdSe, which leads to the formation of an intermediate ZnCdSe layer with a graded composition. We perform theoretical modeling of these core/shell nanocrystals using effective mass approximation and applying first-order perturbation theory for treating both direct electron-hole coupling and the core/shell interface-polarization effects. Using this model we determine the range of geometrical parameters of the core/shell structures that result in a type-II localization regime. We further applied this model to evaluate the degree of electron-hole spatial separation (quantified in terms of the electron-hole overlap integral) based on measured emission wavelengths. We also discuss the potential applicability of these nanocrystals in lasing technologies and specifically the possibility of single-exciton optical gain in type-II nanostructures.
authors.affiliation: C-PCS, Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA. Ivanov@lanl.gov
authors.forename: Sergei A
authors.initials: SA
authors.lastname: Ivanov
authors.forename: Andrei
authors.initials: A
authors.lastname: Piryatinski
authors.forename: Jagjit
authors.initials: J
authors.lastname: Nanda
authors.forename: Sergei
authors.initials: S
authors.lastname: Tretiak
authors.forename: Kevin R
authors.initials: KR
authors.lastname: Zavadil
authors.forename: William O
authors.initials: WO
authors.lastname: Wallace
authors.forename: Don
authors.initials: D
authors.lastname: Werder
authors.forename: Victor I
authors.initials: VI
authors.lastname: Klimov