Synthesis and Characterization of Type II ZnSe/CdS Core/Shell Nanocrystals

Abstract

High-quality ZnSe/CdS core/shell nanocrystals, exhibiting a type II carrier localization regime, were fabricated via a traditional pyrolysis of organometallic precursors. The two-step synthesis involved fabrication of 4.5-6 nm ZnSe seeds followed by a subsequent deposition of the CdS shell. An efficient spatial separation of electrons and holes between the core and the shell was observed for heterostructures containing more than three monolayers of CdS, which was primarily evidenced by the spatially indirect emission tunable from 480 to 610 nm for a fixed core diameter. Because of a large (type II) offset of band edges at the core/shell interface, fabricated nanocrystals exhibited a relatively low spectral overlap between emission and absorption profiles, with associated Stokes shifts of up to 110 nm. The quantum yield of as-prepared samples was 12-18% and was further improved to 20% after purification of nanocrystals through multiple hexane/methanol extractions. Novel properties of synthesized ZnSe/CdS nanocrystals as well as their applicability to practical realizations in areas of biomedical imaging, solar sells, and quantum dot-based lasers are discussed.

Type-II CdS/ZnSe core/shell heterostructures: UV–vis absorption, photoluminescence and Raman scattering studies

“…6(a) shows UV-vis absorption and PL spectra of ZnSe-coated CdS nanoparticles obtained under the conditions as described. Comparing with CdS cores, the absorption and PL peaks strongly shift toward longer wavelengths, which is usually considered as a necessary token to assert a fabricated core/shell heterostructure to be type-II [4,9,20]. A reaction-time increase also results in a slight change in the PL peak position and FWHM, the inset of Fig.…”

“…The verification of the successful fabrication of type-II CdS/ZnSe core/shell heterostructures is usually based on three main criteria: (i) the strong red shift of exciton-related PL peak, (ii) the broadening of the UV-vis absorption tail toward long wavelengths, and (iii) long lifetimes of photo-excited electrons and holes. However, we think that only these criteria are not enough to prove that a fabricated CdS/ZnSe core/shell structure belong to type-II, because if observing carefully the absorption spectra of type-II CdS/ZnSe-related (and ZnSe/CdS well as) heterostructures reported previously [2][3][4][5]9,12,16,19,20,22,42], one can see remarkable differences in the spectral feature and shape near the UV-vis absorption edge. To illustrate this judgment, we introduce four typical sets of UV-vis absorption and PL spectra (denoted as Ab 1-4 ) of CdS/ZnSe core/shell structures that were ascribed to be characteristic of type-II in previous studies [2][3][4][5]16,19,20,22], Fig.…”

“…There is no hump in the second group (Ab 3 , Ab 4 ), however, their absorption tail broadens toward long wavelengths, which can be found in Refs. [5,16,19,20]. We believe that the magnitude and position of the humps appearing in the group (Ab 1 , Ab 2 ), and the broadening of the absorption tail in (Ab 3 , Ab 4 ) toward long wavelengths are related to the nature of the fabricated core/shell nanocrystals.…”

“…By engineering semiconductors with suitable band-gap energies and band alignment, one can produce indirectly excitonic emissions as expected. Type-II core/shell materials have been widely studied to be known as Cu 2 S/CdS [21], CdTe/CdSe [1,[33][34][35], (Zn,Cd)Te/CdSe [18,36], CdSe/ZnTe [33], CdTe/ZnSe [37], CdTe/CdS [38], (ZnSe/CdS)/CdS [16], ZnSe/CdS/ZnSe [12], and ZnSe/CdS (or CdS/ZnSe) [2,4,5,19,20,22]. Besides the type-I (or inverse type-I) and type-II core/shell heterostructures, it has been also introduced quasi-type-II (equivalent to type-I 1/2 ) core/shell structures with many intriguing optical properties [17,18,[39][40][41].…”

“…In reference to type-II CdS/ZnSe (or ZnSe/CdS) core/shell nanocrystals (for example, nanorods [9,16], nanobarbells [12,13], and nanoparticles/nanodots [2][3][4][5]9,20,22,42]), almost previous works used ultraviolet-visible (UV-vis) absorption and steadystate/time-resolved PL spectrometers to monitor sample fabrications, and to study their optical properties. The verification of the successful fabrication of type-II CdS/ZnSe core/shell heterostructures is usually based on three main criteria: (i) the strong red shift of exciton-related PL peak, (ii) the broadening of the UV-vis absorption tail toward long wavelengths, and (iii) long lifetimes of photo-excited electrons and holes.…”