Dual quantum systems, 0-dimensional quantum dot, and 2-dimensional quantum wells were constructed in one II-VI semiconductor nanocrystal by the epitaxial growth of a barrier (ZnS) layer between the systems in solution. By alteration of the thickness of the barrier layer, the two quantum systems were controlled to either electronically coupled or decoupled. Evidence of optical coupling between the two band gap emissions was also observed. The position and relative intensity of the two emissions can be independently tuned by reaction conditions. Total photoluminescence quantum efficiency of the dual emitting bands reached as high as 30% at room temperature under synthetic conditions not optimized for high emission.

Highly photoluminescent CdSe/CdTe/ZnSe type-II/type-I composite nanocrystals, both dot-and peanutshaped, were prepared via the modified successive ionic layer adsorption and reaction (SILAR) techniques, straight SILAR for peanut-shaped ones and SILAR coupled with thermal-cycling (SILAR-TC) for dotshaped ones. The CdSe/CdTe type-II heterojunction offered the nanocrystals with near-infrared emission and the CdTe/ZnSe type-I heterojunction helped to confine the photogenerated charges away from the ligands and solution environment. This structural feature makes the photoluminescence quantum yield of the CdSe/CdTe/ZnSe core/shell/shell type-II/type-I dots that have a uniformly grown ZnSe shell retain as high as 60% after replacing the original amine ligands with mercaptopropionic acid (MPA). Conversely, the emission of the corresponding CdSe/CdTe core/shell dots (CdSe/CdTe/ZnSe composite peanuts) was completely (almost completely) quenched by the same ligand treatment. The emission properties of the MPA-coated CdSe/CdTe/ZnSe core/shell/shell dots were stable in water in the buffer solutions with their pH in a range between about 5 and 9.

Type II CdSe/CdTe core/shell nanocrystals with a dot shape were synthesized using a modified SILAR technique that incorporates cycling of the reaction temperature (thermal cycling). Conversely, experimental results revealed that the standard SILAR alone produced type II core/shell nanocrystals in a peanut shape (1D). Despite their differences in shape, the optical properties observed for the type II dot- and peanut-shaped core/shell nanocrystals were similar. The dot-shaped nanocrystals were confirmed as core/shell structures with an abrupt type II heterojunction within the experimental accuracy, and the peanut-shaped ones were found to be consistent with CdSe and CdTe separated on the two ends of the rods. Similar techniques were used for the synthesis of CdS/CdSe/CdTe type II colloidal quantum well heterostructures with dot and peanut shapes. For these type II colloidal quantum well structures, the PL peak positions were shown to be readily tunable by varying the CdSe and CdTe shell thickness, something not typically seen for the quantum dots. The PL quantum yield of these nanocrystals were found to range between 30 and 60%.